Bulletin of the American Physical Society
57th Annual Meeting of the APS Division of Plasma Physics
Volume 60, Number 19
Monday–Friday, November 16–20, 2015; Savannah, Georgia
Session CP12: Poster Session II (C-Mod, MST, RFP, Stellarator; Waves, Shocks, and Instabilities; Shock Wave and Discontinuity)Poster
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Room: Exhibit Hall A |
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CP12.00001: C-MOD, MST, RFP AND STELLARATOR |
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CP12.00002: ADX -- Advanced Divertor and RF Tokamak Experiment Martin Greenwald, Brian LaBombard, Paul Bonoli, Jim Irby, Jim Terry, Greg Wallace, Rui Vieira, Dennis Whyte, Steve Wolfe, Steve Wukitch, Earl Marmar The Advanced Divertor and RF Tokamak Experiment (ADX) is a design concept for a compact high-field tokamak that would address boundary plasma and plasma-material interaction physics challenges whose solution is critical for the viability of magnetic fusion energy. This device would have two crucial missions. First, it would serve as a Divertor Test Tokamak, developing divertor geometries, materials and operational scenarios that could meet the stringent requirements imposed in a fusion power plant. By operating at high field, ADX would address this problem at a level of power loading and other plasma conditions that are essentially identical to those expected in a future reactor. Secondly, ADX would investigate the physics and engineering of high-field-side launch of RF waves for current drive and heating. Efficient current drive is an essential element for achieving steady-state in a practical, power producing fusion device and high-field launch offers the prospect of higher efficiency, better control of the current profile and survivability of the launching structures. ADX would carry out this research in integrated scenarios that simultaneously demonstrate the required boundary regimes consistent with efficient current drive and core performance. [Preview Abstract] |
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CP12.00003: Impact of the Pedestal on Global Performance and Confinement Scalings in I-mode John Walk, Jerry Hughes, Amanda Hubbard, Dennis Whyte, Anne White The I-mode is a novel high-confinement regime pioneered on Alcator C-Mod, notable for its strong temperature pedestal without the accompanying density pedestal found in conventional H-modes. This separation in transport channels gives the desired improved energy confinement while maintaining low particle confinement, avoiding excessive impurity accumulation. Moreover, I-mode operation is naturally free of deleterious Edge-Localized Modes (ELMs). Recent experiments on Alcator C-Mod have characterized the pedestal structure in I-mode. The impact of the pedestal response (particularly to fueling and heating power) and core profile stiffness on global performance and confinement have demonstrated confinement metrics competitive with H-mode operation on Alcator C-Mod, and consistent with concepts for I-mode access \& operation on ITER. Following the practice of the ITER89 and ITER98 scaling laws for L- and H-mode energy confinement, an initial, illustrative attempt at an I-mode confinement scaling has also been developed. The initial characterization from C-Mod data is consistent with the observed pedestal properties in I-mode, particularly the weak degradation of energy confinement with heating power, and comparatively strong positive response to fueling and increased magnetic field. [Preview Abstract] |
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CP12.00004: Access to high-confinement regimes on Alcator C-Mod and the complex influence of divertor geometry J.W. Hughes, B. LaBombard, D. Brunner, A. Hubbard, J. Terry, J. Rice, J. Walk, I. Cziegler, E. Edlund, C. Theiler Placement of X-points and strike points in a diverted tokamak can have a remarkable impact on plasma properties, including thermal and particle confinement. The distinctive divertor of Alcator C-Mod allows substantial variation of divertor leg length, field line attack angle and divertor baffling, allowing us to induce changes in both L-mode confinement and access to both H-mode and I-mode. With the ion $\nabla B$ drift directed toward the divertor, scanning the strike point can induce $\sim 2 \times$ reductions in H-mode power threshold, and can produce a window for I-mode operation with $H_{98}>1$. Detailed high-resolution measurements, spanning the last closed flux surface, provide profiles of key quantities ($n$,$T$,$\phi$) and their gradients, which are of likely importance in determining whether a discharge evolves an edge transport barrier, or remains in an L-mode state. Advances in Langmuir probes have enabled characterization of both radial profiles and fast ($< 1$MHz) fluctuations in L-mode as the L-H threshold power is approached. These data allow new tests of models for H-mode access, especially those attempting to explain the non-monotonic density dependence of the H-mode power threshold through changes in transport and/or turbulence. [Preview Abstract] |
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CP12.00005: Helium ELMy H-modes in Alcator C-Mod in Support of ITER Helium Operating Phases C.E. Kessel, S.M. Wolfe, M.A. Chilenski, J.W. Hughes, Y. Lin, M.L. Reinke, S.J. Wukitch ITER will operate helium majority plasmas in its earlier phases to shakedown the facility and provide plasmas in both L-mode and H-mode for commissioning and preparation for DT burning plasma operation. Part of this activity is to produce ELMy H-modes to test ELM mitigation schemes and observe the ELM impacts on the plasma facing components. It is of interest to characterize helium ELMy H-modes on present experiments to provide some basis to project to ITER and anticipate the plasma performance and ability to obtain H-modes with sufficient performance. ELMy H-mode is accessed in C-Mod by using LSN with an elongation of about 1.55, and with high lower triangularity and low upper triangularity. These regimes were produced with 1.5-4.0 MW of ICRF heating, and with H-mode line average densities of 2.0-3.2x10$^{20}$ /m$^{3}$, producing higher frequency repetitive to large infrequent ELMs, respectively. The infrequent ELM regime showed a cross between EDA and ELMy H-mode, with the EDA signature of a quasi-coherent mode at about 200 kHz. Tungsten laser blow-off was done. The pedestal features, energy confinement, ELM character, L-H threshold (1.7-2.5 MW) and W confinement will be discussed. Comparisons with deuterium ELMy H-modes will be made. [Preview Abstract] |
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CP12.00006: Validation of MMM7.1 and TGLF anomalous transport models for predicting the evolution of Alcator C-Mod temperature profiles A.H. Kritz, T. Rafiq, A.Y. Pankin, J. Hughes, M. Greenwald The Multi-Mode MMM7.1 [T.~Rafiq, {\em et al.} {\em Phys. Plasmas}, {\bf 20}, 032506, 2013] and the Trapped Gyro-Landau Fluid (TGLF)[G.M.~Staebler, {\em et al.}, {\em Phys. Plasmas} {\bf 14}, 055909, 2007] anomalous transport models are validated employing experimental data for Alcator C-Mod discharges that represent a plasma density scan. The MMM7.1 and the TGLF models compute the anomalous transport driven by the ITG, TEM, ETG, KBM and collisional drift modes. The validation study is carried out with simulations that employ the new numerical solver PT-SOLVER in the PTRANSP code and that utilize Alcator C-Mod experimental boundary and initial conditions. The predicted evolving temperature profiles are compared with corresponding Alcator C-Mod experimental data. The comparison is quantified by calculating the RMS deviations and Offsets. [Preview Abstract] |
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CP12.00007: Search for Correlation Between Plasma Rotation and Electron Temperature Gradient Scale Length in LOC/SOC Transition at Alcator C-Mod Saeid Houshmandyar, William L. Rowan, Perry E. Phillips, John R. Walk, John E. Rice Understanding the mechanism governing the linear ohmic confinement (LOC) and the transition to saturated ohmic confinement (SOC) has long been a focus of tokamak research. It is commonly accepted that at low density, the confinement is dominated by electron-scale turbulence while at high density, the turbulence is dominated by ion temperature gradient. At Alcator C-Mod, the core rotation reversal was shown to be consistent with this \textit{ansatz} [Rice \textit{et al}, Nucl. Fusion \textbf{53}, 033004 (2013)]. However a recent study at AUG suggests that the intrinsic rotation behavior is rather determined by local plasma parameters regardless of the heating method or the confinement regime [McDermott \textit{et al}., Nucl. Fusion \textbf{54}, 043009 (2014)]. Here, we follow this idea and search for dependence of intrinsic rotation on electron temperature gradient scale length, a quantity with a pivotal role in plasma transport. The high-resolution (1 $\mu $s, 7mm) electron cyclotron emission diagnostic at C-Mod (FRCECE) coupled with the B$_{T}$ jog technique allows direct $L_{Te}$ measurements. In the B$_{T}$ jog technique, a 1.5{\%} change in the toroidal magnetic field shifts the viewing volume of the ECE by $\sim$ 1 cm, and the ratio of the average of the signal to the change in the signal during its ramp-up yields $L_{Te}$. [Preview Abstract] |
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CP12.00008: Impact of dilution of deuterium on ion thermal diffusivity and turbulence in C-Mod Ohmic plasmas Miklos Porkolab, P. Ennever, E. Edlund, J. Rice, J.C. Rost, D. Ernst, C. Fiore, A. Hubbard, J. Hughes, J. Terry, M.L. Reinke, G. Staebler, J. Candy Past experiments on C-Mod and gyrokinetic studies indicated that dilution of the deuterium ion species decreases the ion diffusivity in Ohmically heated deuterium plasmas. Comparison of recent controlled seeding experiments to TGLF and GYRO simulations shows that main ion dilution reduces the ion transport in low density (LOC) plasmas by increasing the critical gradient, while in high density (SOC) plasmas ion dilution primarily decreased the stiffness (1). Meanwhile, there is still a deficit in the predicted electron transport in simulations that are restricted to wavenumbers k$\rho_s \leq 1$. Importantly, measurements of the turbulent spectrum were also carried out with a Phase Contrast Imaging (PCI) diagnostic with a new detector array with an improved frequency response (now up to 1 MHz), and the results are in good agreement with synthetic diagnostic predictions. References: (1) Paul Ennever, Invited Talk at this meeting. [Preview Abstract] |
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CP12.00009: Improved analysis of impurity transport coefficient profiles M.A. Chilenski, M. Greenwald, Y. Marzouk, N.T. Howard, J. Rice, A.E. White Work is underway on the development of a novel technique to estimate impurity transport coefficient profiles and their uncertainties. Inference of impurity transport coefficient profiles using x-ray imaging crystal spectroscopy measurements of laser blow-off impurity injections has played a key role in the validation of gyrokinetic simulations of impurity transport in L-mode (Howard et al.\ 2012, \emph{Nucl.\ Fusion} \textbf{52}, 063002). Recent attempts to apply the existing methodology for interpreting such measurements to H-mode have failed to yield reliable estimates, however. This failure exposes key issues regarding the uniqueness of the solution and the rigorous estimation of the uncertainty. A new approach is under development which uses a combination of Markov chain Monte Carlo (MCMC) and global optimization techniques to estimate impurity transport coefficient profiles even when there are multiple possible solutions. This poster will present the new methodology in detail and will show preliminary results from applying it to Alcator C-Mod data. This new approach will enable us to test the existing understanding of L-mode impurity transport and to move towards multichannel validation of gyrokinetic simulations of H-modes. [Preview Abstract] |
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CP12.00010: Mean Flows and Blob Velocities in Scrape-Off Layer (SOLT) Simulations of an L-mode discharge on Alcator C-Mod D.A. Russell, J.R. Myra, D.A. D'Ippolito, B. LaBombard, J.L. Terry, S.J. Zweben Two-dimensional scrape-off layer turbulence (SOLT) code simulations are compared with an L-mode discharge on Alcator C-Mod. Density and temperature profiles for the simulations were obtained by smoothly fitting Thomson scatter and mirror Langmuir probe (MLP) data from the shot. Simulations differing in turbulence intensity were obtained by varying a dissipation parameter. Mean flow profiles and density fluctuation amplitudes are consistent with those measured by MLP in the experiment. Blob velocities in the simulations were determined from the correlation function for density fluctuations, as in the analysis of gas-puff-imaging (GPI) blobs in the experiment. In the \textit{simulations}, it was found that larger blobs moved poloidally with the ExB flow velocity, v$_{\mathrm{E}}$ , in the near-SOL, while smaller fluctuations moved with the group velocity of the dominant linear (interchange) mode, v$_{\mathrm{E}} + $ 1/2 v$_{\mathrm{di}}$, where v$_{\mathrm{di}}$ is the ion diamagnetic drift velocity. Comparisons are made with the \textit{measured} GPI correlation velocity. The saturation mechanisms operative in the simulation of the discharge are explored. [Preview Abstract] |
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CP12.00011: Performance Assessment of the C-Mod Multi-Spectral Line Polarization MSE (MSE-MSLP) Diagnostic Steven Scott, Robert Mumgaard, Matthew Khoury The accuracy of the Alcator C-Mod Motional Stark Effect (MSE) diagnostic is limited primarily by partially polarized background light that varies rapidly both in time (1 ms) and space -- factor 10 variations are observed between adjacent spatial channels.~ ITER is likely to operate in a similar regime. Visible Bremsstrahlung, divertor molecular D2 emission, and glowing invessel structures generate unpolarized light that becomes partially polarized upon reflection.~ Because all three sources are broadband, the background light can be measured in real-time at wavelengths close to the MSE spectrum, thereby allowing the background to be interpolated in wavelength rather than in time.~ A 10-spatial-channel, 4-wavelength MSE-MSLP system has been developed using polarization polychromators that measure simultaneously the MSE pi- and sigma- lines as well as two nearby wavelengths that were chosen to avoid both the MSE spectrum and all known impurity lines on each sightline.~ Initial performance evaluation indicates that the background channel measurements faithfully track the background light in the pi- and sigma- lines.~ The improvement in accuracy of pitch-angle measurements and increased diagnostic flexibility over a wide range of plasma conditions will be reported. [Preview Abstract] |
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CP12.00012: Performance of spectral MSE diagnostic on C-Mod and ITER Ken Liao, William Rowan, Robert Mumgaard, Robert Granetz, Steve Scott, Oleksandr Marchuk, Yuri Ralchenko Magnetic field was measured on Alcator C-mod by applying spectral Motional Stark Effect techniques based on line shift (MSE-LS) and line ratio (MSE-LR) to the H-alpha emission spectrum of the diagnostic neutral beam atoms. The high field of Alcator C-mod allows measurements to be made at close to ITER values of Stark splitting ($\propto Bv_\bot$) with similar background levels to those expected for ITER. Accurate modeling of the spectrum requires a non-statistical, collisional-radiative analysis of the excited beam population and quadratic and Zeeman corrections to the Stark shift. A detailed synthetic diagnostic was developed and used to estimate the performance of the diagnostic at C-Mod and ITER parameters. Our analysis includes the sensitivity to view and beam geometry, aperture and divergence broadening, magnetic field, pixel size, background noise, and signal levels. Analysis of preliminary experiments agree with Kinetic+(polarization)MSE EFIT within $\sim 2^{\circ}$ in pitch angle and simulations predict uncertainties of 20 mT in $|B|$ and $<2^{\circ}$ in pitch angle. [Preview Abstract] |
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CP12.00013: First operation of a high-heat flux, flush mounted ``rail'' Langmuir probe array on Alcator C-Mod Adam Q. Kuang, Dan Brunner, Brian LaBombard, Rick Leccacorvi, Rui Vieira Divertor Langmuir probes are typically built proud of the divertor surface for an accurate measurement of the plasma flux. However, under the high heat flux conditions seen in Alcator C-Mod, proud tungsten probes that present a 10 degree attack angle to the incident heat flux can experience melt damage with less then 1 second plasma exposure time. A similar situation is anticipated for ITER. It is therefore desirable to develop a flush probe system that can both survive reactor-level fluxes and take accurate measurements. A poloidal array of 21 flush-mounted ``rail'' probes have been installed in the C-Mod outer divertor plate, which are toroidally-extended and field-aligned to minimize sheath expansion effects. Initial results indicate that the ``rail'' probes have a well-defined ion saturation current, reporting similar density and temperature measurements as proud probes. However, uncertainty in the projected area becomes significant when the incident magnetic field angle becomes less than $\sim$0.5 degrees. Additionally, because the flush probes are conformal to the divertor surface, they are ideally suited to measure the poloidal distribution of halo currents during disruptions. [Preview Abstract] |
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CP12.00014: Development of a reciprocating probe servomotor control system with real-time feedback on plasma position for the Alcator C-Mod tokamak D. Brunner, A.Q. Kuang, B. LaBombard, W. Burke Reciprocating probe drives are one of the diagnostic workhorses in the boundary of magnetic confinement fusion experiments. The probe is scanned into an exponentially increasing heat flux, which demands a prompt and precise turn around to maintain probe integrity. A new linear servomotor controlled reciprocating drive utilizing a commercial linear servomotor and drive controller has been developed for the Alcator C-Mod tokamak. The quick response of the controller (able to apply an impulse of 50A in about 1ms) along with real-time plasma measurements from a Mirror Langmuir Probe (MLP) allows for real-time control of the probe trajectory based on plasma conditions at the probe tip. Since the primary concern for probe operation is overheating, an analog circuit has been created that computes the surface temperature of the probe from the MLP measurements. The probe can be programmed to scan into the plasma at various times and then turns around when the computed surface temperature reaches a set threshold, maximizing the scan depth into the plasma while avoiding excessive heating. Design, integration, and first measurements with this new system will be presented. [Preview Abstract] |
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CP12.00015: Upgrades to and Recommissioning of the C-Mod FIR Polarimeter R. Watterson, J.H. Irby, D. Garnier, R. Vieira, R. Leccacorvi, R. Murray, E. Marmar The Alcator C-Mod 3-chord FIR polarimeter, with a 2 MHz bandwidth, is capable of responding to both fast changes in the plasma equilibrium and high frequency fluctuations. It operates under ITER-like plasma conditions and magnetic fields, and uses an optical layout similar to that proposed for ITER. After a brief discussion of this system, the design of the upgrade as installed on Alcator C-Mod will be presented. The laser table has been relocated from the cell to a shielded and climate controlled location, and improvements have been made to its acoustic isolation. New collimation optics, and an enclosed, humidity controlled beam-line needed to convey the FIR beams across the C-Mod cell, have been designed and installed. Results from phase calibrations and initial testing of the laser system and reference detectors during C-Mod operation will be presented. [Preview Abstract] |
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CP12.00016: Measurements of electron temperature profiles on Alcator C-Mod using a novel energy-resolving x-ray camera J. Maddox, L. Delgado, N. Pablant, K.W. Hill, M. Bitter, P. Efthimion, J. Rice The most common electron temperature diagnostics, Thomson Scattering (TS) and Electron Cyclotron Emission (ECE), both require large diagnostic footprints and expensive optics. Another electron temperature diagnostic is the Pulse-Height-Analysis (PHA) system, which derives the electron temperature from the x-ray bremsstrahlung continuum. However, the main disadvantage of the PHA method is poor temporal resolution of the Si(Li) diode detectors. This paper presents a novel x-ray pinhole camera, which uses a pixilated Pilatus detector that allows single photon counting at a rate 2MHz per pixel and the setting of energy thresholds. The detector configuration is optimized by Shannon-sampling theory, such that spatial profiles of the x-ray continuum intensity can be obtained simultaneously for different energies, in the range from 4 to 16 keV. The exponential-like dependence of the x-ray intensity with photon energies is compared with a model describing the Be filter, attenuation in air, and detector efficiency, as well as different sets of energy thresholds. Electron temperature measurements are compared with TS and ECE measurements. This work was supported by the US DOE Contract No.DE-AC02-09CH11466 and the DoE Summer Undergraduate Laboratory Internship (SULI) program. [Preview Abstract] |
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CP12.00017: Locked-mode avoidance and recovery without momentum input L. Delgado-Aparicio, J.E. Rice, S. Wolfe, I. Cziegler, C. Gao, R. Granetz, S. Wukitch, J. Terry, M. Greenwald, L. Sugiyama, A. Hubbard, J. Hugges, E. Marmar, P. Phillips, W. Rowan Error-field-induced locked-modes (LMs) have been studied in Alcator C-Mod at ITER-B$_{\phi}$, without NBI fueling and momentum input. Delay of the mode-onset and locked-mode recovery has been successfully obtained without external momentum input using Ion Cyclotron Resonance Heating (ICRH). The use of external heating in-sync with the error-field ramp-up resulted in a successful delay of the mode-onset when $P_{ICRH}>$1 MW, which demonstrates the existence of a power threshold to ``unlock'' the mode; in the presence of an error field the L-mode discharge can transition into H-mode only when $P_{ICRH}>$2 MW and at high densities, avoiding also the density pump-out. The effects of ion heating observed on unlocking the core plasma may be due to ICRH induced flows in the plasma boundary, or modifications of plasma profiles that changed the underlying turbulence. This work was performed under US DoE contracts including DE-FC02-99ER54512 and others at MIT, DE-FG03-96ER-54373 at University of Texas at Austin, and DE-AC02-09CH11466 at PPPL. [Preview Abstract] |
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CP12.00018: The effect of ICRF and LHCD waveguide and launcher location on tritium breeding ratio and radiation damage J.M. Sierchio, D.G. Whyte, G.M. Wallace, S.J. Wukitch In most tokamak fusion reactor designs, ion cyclotron radio frequency (ICRF) and lower hybrid (LH) waves used to heat the plasma and drive current are launched from the low magnetic field side where there is more access space. It has been recently proposed to launch these waves from the high-field side [Podpaly et al. FED 87, 215 (2012), LaBombard et al. Nuc. Fus. 55, 053020 (2015), and Sorbom et al. FED (2015)], which increases efficiency, allows for better wave penetration, and has favorable scrape-off-layer characteristics [Wallace RFPPC (2015)]. We investigate whether there are added benefits to high-field side launch compared with low-field side launch, including a smaller reduction in the tritium breeding ratio (TBR) and less radiation damage as measured by displacements per atom (DPA) and He retention. We present results obtained with MCNP for TBR, DPA, and He retention for several geometries including two liquid immersion blanket designs and two solid blanket designs, while varying the location of a generic waveguide and antenna. [Preview Abstract] |
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CP12.00019: Current Profile Measurements from Moderate to Strong Lower Hybrid Single-Pass Damping on Alcator C-Mod R.T. Mumgaard, G.M. Wallace, S.D. Scott, S. Shiraiwa, I. Faust, R.R. Parker Lower Hybrid Current Drive (LHCD) is an effective tool to non-inductively drive up to 100{\%} of the plasma current on Alcator C-Mod. Measurements with an upgraded MSE diagnostic show that the fast-electron current profile is broader than the Ohmic current profile but still located the plasma core in agreement with strongly centrally peaked fast electron bremsstrahlung (FEB) measurements. Scans in a regime of high current drive efficiency across a range of density, LHCD power, launched n\textbar \textbar, and plasma current show the driven current profile, FEB profile shapes, and current drive efficiency are sensitive only to total plasma current. Simulations using ray-tracing Fokker Planck codes show that the rays make 1-3 bounces through the plasma edge to bridge the spectral gap. Although in agreement with the total current, the simulations qualitatively disagree with experiment regarding current and FEB profiles as well as sensitivity to power and density. Simulations at higher plasma temperature and current predict stronger single-pass damping and preliminary experiments show increased current drive efficiency. Experiments to determine if the profile discrepancies persist when the ray bounces play a reduced role are planned, including companion experiments in D and He resulting in different edge plasma conditions. This work was performed on the Alcator C-Mod tokamak, a DoE Office of Science user facility, and is supported by USDoE awards DE-FC02-99ER54512 and DE-AC02-09CH11466. [Preview Abstract] |
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CP12.00020: Absorption and Modification of Lower Hybrid Waves in the Scrape Off Layer R. Parker, G. Wallace, S. Shiraiwa, S-G. Baek, I. Faust Loss of current drive efficiency of lower hybrid waves at high density in Alcator C-Mod current drive experiments has been attributed, at least in part, to interactions in the SOL. While ray-tracing calculations indicate that collisional absorption and modification of n$_{\mathrm{\vert \vert }}$ during reflections in the SOL can be significant, their validity can be called into question owing to steep SOL gradients. In order to further quantify these losses, full-wave calculations using a plane-stratified SOL model have been carried out. The results show that the loss resulting from reflections in the SOL can be substantial, with collisional losses accounting for a loss of up to 50{\%} per bounce of the incident wave power. The loss is sensitive to the SOL parameters with the strongest collisional absorption occurring in the case of steep temperature and weak density gradients. Modification of n$_{\mathrm{\vert \vert }}$ can also be significant when the density gradient and normal to the flux surfaces are not aligned. These effects are less severe for the fast wave since its penetration into the SOL is significantly less than that of the slow wave. [Preview Abstract] |
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CP12.00021: Improved LHCD simulation model and implication for future experiments S Shiraiwa, G Wallace, S Baek, P Bonoli, I Faust, R Parker, B LaBombard, A White, S Wukitch The simulation model for LHCD using the raytracing/FokkerPlanck (GENRAY/CQL3D) code has been improved. Including realistic 2D SOL profiles resolves the discrepancy previously observed at high density ($n_{\rm e} > 1 \times 10^{20}\,\rm m^{-3}$). Impact of nonlinear interaction in front of the launcher is investigated. It is shown that the distortion of launch $n_{||}$ spectrum is rather small (up to 10\% of injected power). These simulation results suggest that improvement of current drive observed on Alcator C-Mod is indeed caused by realizing preferable SOL plasma profiles. Implication of these results to future experiments will be discussed. In order to minimize edge parasitic losses, realizing high single pass absorption and reducing prompt losses in front of launcher are both crucial. The advantage of LH launch from low field side (LFS) and high field side (HFS) is compared in this regards. A compact LH launcher suitable to test LH wave launch from HFS on a small scale device is designed and its plasma coupling characteristic will be presented. [Preview Abstract] |
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CP12.00022: Wavenumber measurement of lower hybrid waves using multiple RF magnetic probes on Alcator C-Mod T. Shinya, S.G. Baek, G.M. Wallace, S. Shiraiwa, R.R. Parker, D. Brunner, B. LaBombard, Y. Takase RF magnetic probe was designed to measure parallel wavenumber ($k_{||} = n_{||}\omega/c$) of lower hybrid wave (LHW) on Alcator C-Mod. Experimental data from $k_{||}$ measurements provides useful information for understanding the $k_{||}$ up-shift/down-shift, mode conversion between LHW and fast wave, wave power loss mechanisms, etc. The probe was optimized using 3D electromagnetic simulation software, and had a flat sensitivity and a linear phase variation around 4.6 GHz. An array of the probes allows $k_{||}$ measurement up to 578 m$^{-1}$ ($n_{||}$ up to 6). Propagation of the LHW from the LH launcher to the probes can be examined using GENRAY with a 2D SOL model. The rays with $n_{||}$ = 1.75 or 1.7 (initial $n_{||}$) propagate nearly along the last closed flux surface, and reach probes. If the GENRAY calculation is correct, $n_{||}$ measured at probes should be 1.5-2.0. It would be physically interesting if the measured $n_{||}$ were much larger than 2. The experiment is scheduled on August, and the results will be presented. [Preview Abstract] |
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CP12.00023: ICRF mode conversion flow drive study with enhanced wave detection by phase contrast imaging on Alcator C-Mod Y. Lin, E. Edlund, P. Ennever, A.E. Hubbard, M. Porkolab, J.E. Rice, S.J. Wukitch Applying ICRF power in D(He3) plasmas has been found to drive plasma rotation in the mode conversion (MC) regime at a moderate He3 level. With the help of ICRF wave simulation, MC induced symmetry-breaking in momentum distribution is thought to be the likely cause of the observed flow drive effect. However, the detailed mechanism of how the waves generate rotation is unclear due to the involvement of three waves in the MC region: the MC ion Bernstein wave, MC ion cyclotron wave, and fast wave. Recently, the phase contrast imaging system on Alcator C-Mod has been upgraded, and it has been shown to have much higher sensitivity in detecting RF waves. Further MC flow drive experiments at 8 T will be carried out in the 2015 campaign. We will study the dependence of the rotation vs. the measured wave amplitude, k spectrum, location, and relative amplitude among the three waves. This study will shed lights on the flow drive mechanism and help assess the roles played by the different waves in the process. [Preview Abstract] |
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CP12.00024: ICRF Experiments in Alcator C-Mod S.J. Wukitch, Y. Lin, J. Terry, J. Wright, A. Hubbard, P. Ennever, Ye.O. Kazakov, J. Ongena, D. Van Eester One challenge to ion cyclotron range of frequency (ICRF) utilization is its interaction with the edge plasma. With a field aligned antenna, the impurity sources at the antenna are dramatically reduced, but the enhanced plasma potentials are yet present. An emerging explanations is that the ICRF convective cell creates a local source at the RF antenna and modifies the impurity transport/source away from the antenna. To test the importance of ICRF convective cell, the impurity source at the antenna is compared from 2-8 T. Furthermore, experiments have performed to investigate changes in impurity penetration/source by puffing known amounts of N impurity from four locations that have different mappings to an ICRF antenna energized as a function of RF power. In C-Mod, overall plasma performance has been observed to be dependent on RF absorption efficiency and a new three-ion ICRF scenario has been identified that has efficient RF power absorption. Initial experiments will investigate the heating and RF flow drive effectiveness. Latest results and analysis will be presented. [Preview Abstract] |
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CP12.00025: Overview of MST Research J.S. Sarff MST progress in advancing the RFP for (1) fusion plasma confinement with ohmic heating and minimal external magnetization, (2) predictive capability in toroidal confinement physics, and (3) basic plasma physics is summarized. Validation studies for nonlinear resistive MHD employ MST's advanced diagnostics and modeling using the NIMROD and DEBS codes. Major diagnostic improvements include an upgraded FIR system, improved DNB for CHERS and MSE, and Thomson scattering (TS) upgrades. Deep-insertion probes measure the dynamo emf associated with two-fluid MHD and Hall effects. Integrated data analysis of SXR and CHERS yields the best measurement of $Z_{eff}$=2 to date. X-ray spectra reveal formation of an energetic electron tail during reconnection events, evidence that particle energization occurs for both ions and electrons. New theoretical work identifies an island-induced Alfven eigenmode consistent with modes seen in NBI-heated plasmas. A resonant magnetic perturbation technique controls the locked-phase for quasi-single-helicity plasmas, allowing improved diagnosis and 3D equilibrium reconstructions. Small-scale density fluctuations are consistent with density-gradient-driven trapped electron modes, also predicted in GENE modeling. Supported by US DoE and NSF. [Preview Abstract] |
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CP12.00026: Advanced control of MST's poloidal field with a programmable power supply B.E. Chapman, D.J. Holly, K.J. McCollam, J.C. Morin, J.S. Sarff, A. Squitieri, J.K. Anderson, A.H. Seltzman One thrust of the MST program is to advance inductive control for the development of both the RFP's fusion potential and the predictive capability of fusion science. This entails programmable power supplies (PPS's) for the Bt and Bp circuits. A Bt PPS is in place, and a Bp PPS is being designed. Together, these supplies will provide inductive capability rivaling that of any fusion device in the world. To better inform the design of the Bp PPS, and to demonstrate some of the new capabilities that will be provided, the existing Bt PPS has been connected to MST's Bp circuit. While limited to lower voltage and current than the planned Bp PPS, this has already more than quadrupled the Ip flattop duration. It has also allowed access to very low Ip, down to 20 kA, substantially increasing MST's range of Lundquist number, important for the validation of MHD computational models. Low Ip has also allowed electron energization by high-harmonic EBW. At higher Ip, work has begun on self-similar ramp-down of Ip, a potential route to improved confinement. Work supported by U.S.D.O.E. [Preview Abstract] |
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CP12.00027: Evidence for density-gradient-driven trapped-electron modes in improved confinement RFP plasmas James Duff, Brett Chapman, John Sarff, Paul Terry, Zach Williams, Weixing Ding, David Brower, Eli Parke Density fluctuations in the large-density-gradient region of improved-confinement MST RFP plasmas exhibit features characteristic of the trapped-electron-mode (TEM), strong evidence that drift wave turbulence emerges in RFP plasmas when magnetic transport is reduced. In standard RFP plasmas, core transport is governed by magnetic stochasticity stemming from current-driven tearing modes. Using inductive control, these tearing modes are reduced, improving confinement. The improved confinement is associated with substantial increases in the density and temperature gradients, and we present evidence for the onset of drift wave instability. Density fluctuations are measured with a multi-chord, laser-based interferometer. These fluctuations have wavenumbers k$_\phi * \rho_s$ \textless 0.14, frequencies characteristic of drift waves (\textgreater 50 kHz), and are clearly distinct from residual global tearing modes. Their amplitudes increase with the local density gradient, and require a critical density gradient. Gyrokinetic analysis provides supporting evidence of microinstability in these plasmas, in which the density-gradient-driven TEM is most unstable. The experimental threshold gradient is close to the predicted critical gradient for linear stability. Work supported by DOE. [Preview Abstract] |
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CP12.00028: Gyrokinetic Studies of Microturbulence in the Madison Symmetric Torus Zachary Williams, James Duff, M.J. Pueschel, Paul Terry Reversed-field pinches operating with Pulsed Poloidal Current Drive (PPCD) exhibit microturbulence that contributes to heat and particle transport. This work focuses on the analysis of high-frequency fluctuations in a recent 200 kA PPCD discharge in the Madison Symmetric Torus, for which strong experimental evidence of microturbulence exists. Local gyrokinetic simulations were performed at multiple radial positions outside the reversal surface using the {\sc Gene} code. Linear analysis identifies the dominant instability at all positions to be a density-gradient-driven trapped electron mode. An accurate description of turbulence requires the inclusion of residual tearing mode fluctuations: though reduced in PPCD, large-scale tearing modes introduce non-negligible levels of magnetic perturbations. In simulations, they can be seen to weaken zonal flows and degrade confinement, increasing transport to experimentally observed levels. Importantly, imposed fluctuations appear to be self-consistently reinforced, contrary to the usual island-healing picture in tokamaks. Simulations also include $B_\parallel$ fluctuations, which provide finite contributions to transport, particularly when artificially zeroing out tearing modes entirely. [Preview Abstract] |
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CP12.00029: Initial Studies of Validation of MHD Models for MST Reversed Field Pinch Plasmas C.M. Jacobson, A.F. Almagri, D. Craig, K.J. McCollam, J.A. Reusch, J.P. Sauppe, C.R. Sovinec, J.C. Triana Quantitative validation of visco-resistive MHD models for RFP plasmas takes advantage of MST's advanced diagnostics. These plasmas are largely governed by MHD relaxation activity, so that a broad range of validation metrics can be evaluated. Previous nonlinear simulations using the visco-resistive MHD code DEBS at Lundquist number $S=4\times10^{6}$ produced equilibrium relaxation cycles in qualitative agreement with experiment, but magnetic fluctuation amplitudes $\tilde{b}$ were at least twice as large as in experiment. The extended-MHD code NIMROD previously suggested that a two-fluid model may be necessary to produce $\tilde{b}$ in agreement with experiment. For best comparisons with DEBS and to keep computational expense tractable, NIMROD is run in single-fluid mode at low $S$. These simulations are complemented by DEBS at higher $S$ in cylindrical geometry, which will be used to examine $\tilde{b}$ as a function of $S$. Experimental measurements are used with results from these simulations to evaluate validation metrics. Convergence tests of previous high $S$ DEBS simulations are also discussed, along with benchmarking of DEBS and NIMROD with the SPECYL and PIXIE3D codes. [Preview Abstract] |
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CP12.00030: Comparing MHD simulations of RFP plasmas to RELAX experiments K.J. McCollam, D.J. Den Hartog, C.M. Jacobson, J.P. Sauppe, S. Masamune, A. Sanpei Standard reversed-field pinch (RFP) plasmas provide a nonlinear dynamical system as a validation domain for numerical MHD simulation codes, which can be applied to general toroidal confinement scenarios including tokamaks. Using the NIMROD code, we calculate linear stability and simulate the nonlinear evolution of plasmas similar to those in the RELAX RFP experiment, whose relatively modest Lundquist numbers of order $10^4$ make the simulations tractable given present computing resources. The chosen RELAX cases cover a broad range of RFP reversal parameters and have also been previously simulated with the MIPS code (N.~Mizuguchi {\it et al}., TH/P3-26, IAEA FEC, 2012). Experimental diagnostics that can be used for validation purposes include Thomson scattering for electron temperature, interferometry for electron density, SXR imaging, and external and internal magnetic probes. RELAX's small aspect ratio ($\approx2$) motivates a comparison study using toroidal and cylindrical geometries in NIMROD. [Preview Abstract] |
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CP12.00031: Analysis of Helicities and Hall and MHD Dynamo Effects in Two-Fluid Reversed-Field Pinch Simulations Joshua Sauppe, Carl Sovinec Relaxation in the RFP is studied numerically with extended-MHD modeling that includes the Hall term and ion gyroviscous stress. Previous results show significant coupling between magnetic relaxation and parallel flow evolution [King PoP 19, 055905]. Computations presented here display quasi-periodic relaxation events with current relaxation through MHD and Hall dynamo drives. The MHD dynamo always relaxes currents while the Hall dynamo may add or subtract from it, but the total dynamo drive is similar to single-fluid MHD computations. Changes in plasma momentum are due to viscous coupling to the wall and fluctuation-induced Maxwell stresses transport momentum radially inward when two-fluid effects are included. The magnetic helicity and hybrid helicity, a two-fluid extension of magnetic helicity that includes cross and kinetic helicity [Turner, 1986], are well-conserved relative to magnetic energy at each event. The cross helicity is well-conserved in single-fluid MHD but is significantly affected by both two-fluid effects and ion gyroviscosity. The plasma parallel current evolves towards the predicted flat profile; however, the plasma flow does not. [Preview Abstract] |
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CP12.00032: Behavior of m$=$0 Modes in DEBS Modeling and MST Plasmas D. Craig, R. Hesse, D. Martin, D.J. Den Hartog, C.M. Jacobson, K.J. McCollam, M.D. Nornberg, J.A. Reusch In the reversed field pinch (RFP), poloidal mode number m$=$0 fluctuations are driven through nonlinear coupling with unstable m$=$1 tearing modes. Many relaxation processes are strongly linked to the behavior of the m$=$0 modes and hence understanding and controlling them has high leverage for many physics studies. We explore the dependence of m$=$0 modes on several key parameters in both MST experiments and visco-resistive MHD simulations using the DEBS code. In both experiment and code, m$=$0 modes are suppressed by removing their resonant surface from the plasma though the suppression is more complete in the experiment. Reduced m$=$0 magnetic mode amplitudes are correlated with a reduction in the m$=$1 mode velocity fluctuations in both experiment and code. The time scale for m$=$0 mode amplitudes to rise and fall during relaxation events does not depend strongly on the degree of magnetic field reversal in the experiment or in the code. Systematic variation of the Lundquist number and magnetic Prandtl number in the code shows that both resistivity and viscosity affect the temporal evolution of the m$=$0 modes during relaxation events. The effect of the edge resistivity profile and the electric field boundary condition on m$=$0 modes is also examined in the code. These observations are discussed in relation to the nonlinearly driven reconnection paradigm for m$=$0 mode evolution. This work has been supported by the US DOE and NSF. [Preview Abstract] |
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CP12.00033: Measurements of the Hall Dynamo in MST Plasmas J.C. Triana, A.F. Almagri, K.J. McCollam, J.S. Sarff, J.P. Sauppe, C.R. Sovinec Fluctuation-induced emfs correlated with tearing mode activity govern the relaxation process in RFP plasmas. Previous radial profile measurements in the edge of MST plasmas $(\frac{r}{a}>0.8)$ revealed a competition of the Hall, $\frac{1}{ne}\langle\tilde{\bf{j}}\times\tilde{\bf{b}}\rangle_{||}$, and MHD, $\langle\tilde{\bf{v}}\times\tilde{\bf{b}}\rangle_{||}$, terms in Ohm's law. A robust magnetic probe allows measurements of the Hall-dynamo profile much deeper in the plasma $(\frac{r}{a}>0.4)$ for low current conditions. The mode composition of the dynamo emf is computed using pseudospectral (cross-correlation) analysis with the spectrum measured from a toroidal magnetic array at the plasma surface. Extended MHD simulations with parameters comparable to the experiment have been performed using NIMROD. They predict complex variation of the Hall and MHD dynamo profiles across the plasma radius. Measurements of the Hall-dynamo profile can inform future computational work in addition to directing future experimental measurements of the MHD term. [Preview Abstract] |
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CP12.00034: Measurements of Electric Field Fluctuations Using a Capacitive Probe on the MST Reversed Field Pinch Mingsheng Tan, A.F. Almagri, J.S. Sarff, K.J. McCollam, J.C. Triana, H. Li, W.X. Ding, W. Liu Experimental measurements and extended MHD computation reveal that both flow and current density fluctuations are important for the magnetic relaxation of RFP plasmas via tearing fluctuations. Motivated by these results, we have developed a multi-electrode capacitive probe for radial profile measurements of the electrostatic potential deep in the plasma. The capacitive probe measures the ac plasma potential via electrodes insulated from the plasma using an annular boron nitride dielectric (also the particle shield), provided the secondary emission is sufficiently large (T$_{\mathrm{e}}$\textgreater 20 eV). The probe has ten sets of four capacitors with 1.5 cm radial separation. At each radius, four capacitors are arranged on a 1.3 cm square grid. This probe has been inserted up to 15 cm from the wall in 200 kA deuterium plasmas. The fluctuation amplitudes increase during the sawtooth crash and the power spectrum broadens (similar to the behavior of magnetic field fluctuations). The frequency bandwidth allows measurements of the radial coherence and phase of the fluctuations associated with rotating tearing modes up to the Alfv\'{e}nic range. A next-step goal is measurement of the total dynamo emf, $\left\langle {\widetilde{{\rm {\bf v}}}_{e} \times \widetilde{{\rm {\bf B}}}} \right\rangle \approx \left\langle {\widetilde{{\rm {\bf E}}}\cdot \widetilde{{\rm {\bf B}}}} \right\rangle /B_{0} $, to complement ongoing measurements of the Hall dynamo emf, $\left\langle {\widetilde{{\rm {\bf J}}}\times \widetilde{{\rm {\bf B}}}} \right\rangle /ne$, using a deep-insertion magnetic probe. M. Tan is supported by ITER-China Program. Work is supported by US DOE. [Preview Abstract] |
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CP12.00035: Improvements in electron temperature measurements from soft x-rays in MST plasmas L.M. Reusch, P. Franz, M.E. Galante, J. Goetz, D.J. Den Hartog, M.B. McGarry, H.D. Stephens The MST is equipped with a two-color soft x-ray tomography (SXT) diagnostic that is capable of making electron temperature measurements via the double-foil technique. Discrepancies between the double-foil temperature and Thomson scattering (TS) have been confirmed to be due to impurities present in the Be filters used to block visible light and select the energy range for soft x-ray detection. Namely, contamination from Zr led to a larger effective thickness for all filters. Furthermore, the distribution of Zr particles was highly non-uniform, making accurate accounting of the contaminated filters impossible and leading to different effective thicknesses between different probes in the SXT system. We have installed new confirmed 99.9\% purity Be filters and assessed their effect on the brightness profiles and on the two-foil temperature measurements. Results show consistent amplitudes for brightness profiles from all four probes, and the double-foil temperature measurement from SXT matches TS within uncertainty, both spatially and temporally. In addition, empirical measurements of the transmission function versus energy for the Be filters contaminated with Zr will allow us to accurately characterize data using the contaminated Be filters. This work was supported by the US DOE. [Preview Abstract] |
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CP12.00036: Combining impurity X-ray and impurity density measurements to determine $Z_{\rm eff}$ M.D. Nornberg, M.E. Galante, L.M. Reusch, D.J. Den Hartog, P. Franz, H.D. Stephens Determining the resistive dissipation of hot plasmas requires knowledge of the effective charge $Z_{\rm eff}$. Typically $Z_{\rm eff}$ is determined from visible bremsstrahlung emission. In limiter plasmas with relatively high core and edge neutral density, the neutrals likely contribute as much emission to the visible spectrum as do the impurities. By using sufficiently thick Be filters, detected soft x-ray emission can be limited to a region of the spectrum dominated by bremsstrahlung and impurity recombination. Modeling this emission requires good constraints on the impurity density profiles and charge state balance. This information can be supplied by charge exchange recombination measurements (CHERS). Combining these two different diagnostic measurements within a Bayesian framework enables the self-consistent determination of $Z_{\rm eff} = 1.9 \pm 0.1$ in the core of MST RFP plasmas with tearing mode suppression. This integrated data analysis (IDA) has the additional benefit of helping identify systematic uncertainties in the individual measurements and facilitates constraining the densities of other impurities for which there are no CHERS measurements. [Preview Abstract] |
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CP12.00037: Electron Bernstein Wave Studies in MST Andrew Seltzman, Jay Anderson, Cary Forest, Paul Nonn, Mark Thomas, Abdulgader Almagri, Brett Chapman, Ami DuBois, John Goetz, Karsten McCollam The RFP plasma is inaccessible to ECRH, requiring the electron Bernstein wave (EBW) for edge localized heating and current drive. MST is capable of generating RFPs or overdense tokamaks with Bt(0) $\sim$ 0.08-0.14T in which a 5.55 GHz RF source (450kW, 2ms pulse) can heat at fundamental and harmonic EC resonances. The design of a suitable antenna is challenging in the RFP due to a magnetic field geometry that requires a low-field-side launch. The small vacuum gap between the close-fitting conducting shell and plasma leads to substantial antenna-plasma interaction. A minimized port hole size is required to limit error fields. Even so the port hole induced magnetic field perturbation in the antenna near-field that affects the mode conversion process and introduces EC resonances. A 5cm diameter cylindrical antenna centered in 5cm and 11cm diameter portholes is used. A multi-chord time-resolved x-ray detector and GENRAY ray tracing verifies EBW heating at higher harmonics in an MST tokamak with 10-40keV detected x-ray energies. Evidence of RF-induced emission from absorption at higher harmonics (4th / 5th) in low current RFP discharges has been observed. Simultaneous reflected power changes correspond to termination of x-ray emission indicating power limits. Work supported by USDOE. [Preview Abstract] |
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CP12.00038: Particle Heating and Energization During Magnetic Reconnection Events in MST Plasmas Ami M. DuBois, A.F. Almagri, J.K. Anderson, D.J. Den Hartog, C. Forest, M. Nornberg, J.S. Sarff Magnetic reconnection plays an important role in particle transport, energization, and acceleration in space, astrophysical, and laboratory plasmas. In MST reversed field pinch plasmas, discrete magnetic reconnection events release large amounts of energy from the equilibrium magnetic field, resulting in non-collisional ion heating. However, Thomson Scattering measures a decrease in the thermal electron temperature. Recent fast x-ray measurements show an enhancement in the high energy x-ray flux during reconnection, where the coupling between edge and core tearing modes is essential for enhanced flux. A non-Maxwellian energetic electron tail is generated during reconnection, where the power law spectral index ($\gamma )$ decreases from 4.3 to 1.8 and is dependent on density, plasma current, and the reversal parameter. After the reconnection event, $\gamma $ increases rapidly to 5.8, consistent with the loss of energetic electrons due to stochastic thermal transport. During the reconnection event, the change in $\gamma $ is correlated with the change in magnetic energy stored in the equilibrium field, indicating that the released magnetic energy may be an energy source for electron energization. Recent experimental and computational results of energetic electron tail formation during magnetic reconnection events will be presented. [Preview Abstract] |
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CP12.00039: Identification of an Island-induced Alfv\'{e}n Eigenmode in MST plasmas J.K. Anderson, C.R. Cook, C.C. Hegna, J. Boguski, R. Feng, K.M. McCollam, S.H. Sears, D.A. Spong, S.P. Hirshman Recent theoretical work analytically computes the effect of a magnetic island on the shear Alfv\'{e}n continuum and may explain unresolved Alfv\'{e}nic activity observed in neutral beam-heated MST plasmas. Consideration of the previously-ignored core-localized n=5 island leads to theoretical Alfv\'{e}n continua that provide a gap in which the observed n=4 Alfv\'{e}nic bursts reside. Numerical simulations using the STELLGAP/AE3D codes, as well as a new code called SIESTAlfv\'en have identified the bursts as the first observation of an Island-induced Alfv\'{e}n Eigenmode (IAE). The IAE arises from a helical coupling of mode numbers, similar to the helicity-induced Alfv\'{e}n eigenmode, but occurs in the core of an island. The observed frequency of bursting n=4 Alfv\'{e}nic modes fall within the island-induced gap over a wide range of MST operating parameters. Characteristics such as mode frequency, width and damping rate are measured as a function of experimentally-varied magnetic island width. Coincident bursts with toroidal mode number n=1 may exhibit frequency scaling of an Alfv\'{e}nic eigenmode; the possibility of an island induced extremum mode is explored as an explanation. Work supported by US DoE under grants DE-FG02-99ER54546, DE-SC0006103 and DE-FC02-05ER54814. [Preview Abstract] |
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CP12.00040: Damping of Energetic-Particle-Driven Alfven Eigenmodes in Different Magnetic Equilibria in the MST Reversed-Field Pinch Stephanie Sears, Jay Anderson, William Capecchi, Phillip Bonofiglo, Jungha Kim Alfven wave dissipation is an important mechanism behind anomalous ion heating, both in astrophysical and reversed-field pinch (RFP) plasma systems. Additionally, the damping rate has implications for the stability of energetic particle driven modes (EPMs) and their associated nonlinear dynamics and fast ion transport, which are crucial topics for any burning plasma reactor. With a 1 MW neutral beam injector on the MST RFP, a controlled set of EPMs and Alfvenic eigenmodes can be driven in this never-before-probed region of strong magnetic shear and weak externally applied magnetic field. The decay time of the average of 100s of reproducible bursts is computed for different equilibrium profiles. In this work, we report initial measurements of Alfvenic damping rates with varied RFP equilibria (including magnetic shear and flow shear) and the effects on fast ion transport. This research is supported by DOE and NSF. [Preview Abstract] |
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CP12.00041: Measuring Fast Ion Losses in a Reversed Field Pinch Plasma P.J. Bonofiglo, J.K. Anderson, A.F. Almagri, J. Kim, J. Clark, W. Capecchi, S.H. Sears The reversed field pinch (RFP) provides a unique environment to study fast ion confinement and transport. The RFP's weak toroidal field, strong magnetic shear, and ability to enter a 3D state provide a wide range of dynamics to study fast ions. Core-localized, 25 keV fast ions are sourced into MST by a tangentially injected hydrogen/deuterium neutral beam. Neutral particle analysis and measured fusion neutron flux indicate enhanced fast ion transport in the plasma core. Past experiments point to a dynamic loss of fast ions associated with the RFP's transition to a 3D state and with beam-driven, bursting magnetic modes. Consequently, fast ion transport and losses in the RFP have garnered recent attention. Valuable information on fast-ion loss, such as energy and pitch distributions, are sought to provide a better understanding of the transport mechanisms at hand. We have constructed and implemented two fast ion loss detectors (FILDs) for use on MST. The FILDs have two, independent, design concepts: collecting particles as a function of $v_\perp$ or with pitch greater than 0.8. In this work, we present our preliminary findings and results from our FILDs on MST. This research is supported by US DOE. [Preview Abstract] |
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CP12.00042: Fast ion beta limit measurements by collimated neutron detection in MST plasmas William Capecchi, Jay Anderson, Phillip Bonofiglo, Jungha Kim, Stephanie Sears Fast ion orbits in the reversed field pinch (RFP) are well ordered and classically confined despite magnetic field stochasticity generated by multiple tearing modes. Classical TRANSP modeling of a 1MW tangentially injected hydrogen neutral beam in MST deuterium plasmas predicts a core-localized fast ion density that can be up to 25{\%} of the electron density and a fast ion beta of many times the local thermal beta. ~However, neutral particle analysis of an NBI-driven mode (presumably driven by a fast ion pressure gradient) shows mode-induced transport of core-localized fast ions and a saturated fast ion density. The TRANSP modeling is presumed valid until the onset of the beam-driven mode and gives an initial estimate of the volume-averaged fast ion beta of 1-2{\%} (local core value up to 10{\%}). A collimated neutron detector for fusion product profile measurements will be used to determine the spatial distribution of fast ions, allowing for a first measurement of the critical fast-ion pressure gradient required for mode destabilization. Testing/calibration data and initial fast-ion profiles will be presented. Characterization of both the local and global fast ion beta will be done for deuterium beam injection into deuterium plasmas for comparison to TRANSP predictions. [Preview Abstract] |
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CP12.00043: Investigation of interaction between fast ions and tearing modes in MST plasmas using full orbit tracing Jungha Kim, Jay Anderson, William Capecchi, Phillip Bonofiglo, Stephanie Sears, Yuri Tsidulko Under proper conditions, global reconnection events generate an anisotropic runaway ion distribution in MST plasmas. Full orbit tracing with time-dependent fluctuating fields, calculated by the nonlinear resistive MHD code DEBS, is used to inform a refined model of ion heating to explain this phenomenon, where tearing modes and ions interact on two distinct scales. There is anisotropic heating of thermal ions (T$_{\bot}$\textgreater T$_{\parallel}$), likely through a stochastic heating mechanism that requires high diffusivity and a tearing mode induced radial electric field with correlation length of a few cm. This process does not, however, continuously energize ions into the runaway regime. At sufficient energy, the ion guiding center deviates from the background magnetic field, which reduces the effective diffusivity to classical levels even in a stochastic magnetic field. These ``fast'' ions are accelerated by a parallel electric field (length scale of meters) induced by the equilibrium change accompanying tearing modes. This process relies on multiple global tearing modes; here we focus on a single tearing mode. This is compared to an experimental state where a transition to a single, dominant tearing mode is observed to accelerate fast ions and alter their confinement properties. Work supported by US DOE. [Preview Abstract] |
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CP12.00044: Recent Ion Energy Distribution Observations on MST RFP Plasmas Jerry Clark, J.B. Titus, E.D. Mezonlin, J.A Johnson III, A.F. Almagri, J.A. Andeson Ion energy distribution and temperature measurements have been made on the Madison Symmetric Torus (MST) using the Florida A{\&}M University compact neutral particle analyzer (CNPA). The CNPA is a low energy (0.34-5.2 keV), high energy resolution (25 channels) neutral particle analyzer, with a radial view on MST. Recently, a retarding potential system was built to allow CNPA measurements to ensemble a complete ion energy distribution with high-energy resolution, providing insight into the dynamics of the bulk and fast ion populations. Recent work has also been done to improve the analysis techniques used to infer the ion temperature measurements, allowing us to understand temperature dynamics better during global magnetic reconnection events. [Preview Abstract] |
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CP12.00045: Optimizing 50kV hydrogen diagnostic neutral beam performance for active spectroscopy in MST X. Feng, J. Boguski, D. Craig, D.J. Den Hartog, S. Munaretto, M.D. Nornberg, S. Olivia The 50 kV hydrogen diagnostic neutral beam on MST provides local measurements of impurity ion emission through charge exchange recombination spectroscopy (CHERS) and of core-localized magnetic field through the motional Stark effect (MSE). The beam, which was designed to provide 5A of neutral current at 50 kV to meet these needs, is currently on a test stand to accommodate diagnosis, in order to increase the reliability of beam formation, sustain a steady current of 5 amps for 20ms, and optimize the primary energy fraction. The reliability of arc formation was increased from 40{\%} to 80{\%} success rate with increase of cathode gas pressure from 150kPa to 200kPa, and the stability of the arc current is improved with a decrease of the insulation magnetic field. A calorimeter with 5 thermocouples is installed to measure the horizontal and vertical beam profiles as well as beam divergence. Beam energy components are quantified through Doppler-shift spectroscopy. Preliminary simulation results of the beam using the ALCBEAM code as well as a description of how changes to the beam performance can affect CHERS and MSE measurements are presented. [Preview Abstract] |
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CP12.00046: Upgrade of a CHERS diagnostic system for fast-ion and drift-instability measurements Takashi Nishizawa, D. Craig, D.J. Den Hartog, M.D. Nornberg Energetic particle modes and drift instabilities have fluctuation frequencies above the 100 kHz design specification for the current Charge Exchange Recombination Spectroscopy (CHERS) diagnostic on MST. Upgrading the CHERS system to detect fluctuations at these frequencies requires an optimization of all the light detection stages including the photomultiplier tubes (PMTs), the transimpedance amplifiers, and the data acquisition system. The PMTs need to have a linear response to the photon flux and be protected against abnormal events with much brighter light than ordinary plasmas. For this purpose, the resistor- divider network for the PMTs has been optimized based on the results of circuit-simulations and gain and linearity measurements. The pulse outputs of the PMTs corresponding to a single photoelectron are about 7.5 ns long. Therefore, the raw PMT signals require transimpedance amplifiers with shaping capabilities that will allow practical digitization rates. This digitization intrinsically causes errors in photon counts. We modeled each stage involved in the diagnostic using a Poisson process, circuit-simulations, and the superposition theorem to estimate those errors. We will discuss the details of the measurements and simulations and how parameters are optimized. [Preview Abstract] |
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CP12.00047: Neutral dynamics and ion energy transport in MST plasma Zichuan Xing, Mark Nornberg, Daniel Den Hartog, Santosh Kumar, Jay Anderson Neutral dynamics can have a significant effect on ion energy transport through charge exchange collisions. Whereas previously charge exchange was considered a direct loss mechanism in MST plasmas, new analysis indicates that significant thermal charge exchange neutrals are reionized. Further, the temperatures of the neutral species in the core of the plasma are suspected to be much higher than room temperature, which has a large effect on ion energy losses due to charge exchange. The DEGAS2 Monte Carlo simulation code is applied to the MST reversed field pinch experiment to estimate the density and temperature profile of the neutral species. The result is then used to further examine the effect of the neutral species on ion energy transport in improved confinement plasmas. This enables the development of a model that accounts for collisional equilibration between species, classical convective and conductive energy transport, and energy loss due to charge exchange collisions. The goal is to quantify classical, stochastic, and anomalous ion heating and transport in RFP plasmas. [Preview Abstract] |
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CP12.00048: Identification of Neutral Particle Sources in MST Plasmas Ryan Norval, Stefano Munaretto, John Goetz, Oliver Schmitz The plasma wall interaction (PWI) in the MST RFP has yet to be studied systematically to determine the effects of the edge plasma on overall plasma performance. Two imaging views of the MST plasma currently exist. The first views the outboard toroidal and poloidal belt limiters at the main poloidal gap limiter. The second views the inboard poloidal limiter, as well as a section of the outboard toroidal limiter away from the man gap limiter. Data from viewing outboard limiters reveals PWI structures correlate with the plasma conditions. In standard RFP plasmas at lower plasma currents the PWI is dominated by non-axisymmetric radiation belts. As the RFP plasma current rises, increasing axisymmetry is seen from the edge. When in the 3D equilibria of the quasi-single helicity (QSH) state the PWI correlates with the main magnetic mode of the plasma. The dominant source of light observed from the MST edge is from hydrogen recycling. This will be used to inform neutral particle sourcing in the EIRENE neutral transport code. EIRENE will be used to compare how variations in fueling could affect the neutral profile in MST. This work is supported by the U.S. Department of Energy. [Preview Abstract] |
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CP12.00049: Effect of Resonant Magnetic Perturbations on 3D equilibria in the MST RFP Stefano Munaretto, B.E. Chapman, A.F. Almagri, J. Boguski, M. Cianciosa, D.J. Den Hartog, A.M. DuBois, J.A. Goetz, J.D. Hanson, D.J. Holly, K.J. McCollam, T. Nishizawa, M.D. Nornberg, R.J. Norval, J.S. Sarff The orientation of 3D equilibria in the MST RFP can now be controlled with application of a resonant magnetic perturbation (RMP). This control has led to improved diagnosis revealing enhancements in both the central electron temperature and density. Coupled to a recent advance in the V3FIT code, reconstructions of the 3D equilibria have also been improved. The RMP also inhibits generation of high-energy (\textgreater 20keV) electrons, which are otherwise produced with the 3D state. This state occurs when the normally broad spectrum of core-resonant m $=$ 1 tearing modes condenses, with the innermost resonant mode growing to large amplitude $\sim$ 8{\%} of the axisymmetric field. As the dominant mode grows, eddy current in MST's conducting shell slows the mode's rotation, eventually leading to locking of the 3D structure. An m $=$ 1 RMP with an amplitude br/B $\sim$ 10{\%} can force the 3D structure into any desired orientation relative to MST's diagnostics. Reduced stochasticity and improved confinement of high-energy electrons during the formations of the 3D structure are observed. This work is supported by the US DOE. [Preview Abstract] |
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CP12.00050: Modeling the Time Evolution of QSH Equilibria in MST Plasmas Using V3FIT J. Boguski, M. Nornberg, S. Munaretto, B.E. Chapman, M. Cianciosa, P.W. Terry, J. Hanson High current and low density RFP plasmas tend towards a 3D configuration, called Quasi-Single Helicity (QSH), characterized by a dominant core helical mode. V3FIT utilizes multiple internal and edge diagnostics to reconstruct the non-axisymmetric magnetic equilibrium of the QSH state. Performing multiple reconstructions at different stages in the QSH cycle is used to learn about the time dynamics of the QSH state. Recent work on modeling a shear-suppression mechanism for QSH formation has produced a predator-prey model of the time dynamics that reproduces the observed behavior, in particular the increased persistence of the QSH state with increased plasma current. Either magnetic or flow shear can facilitate QSH formation. The magnetic shear dependence of QSH is analyzed using V3FIT reconstructions of magnetic equilibrium constrained by internal measurements of density and temperature as well as soft x-ray emission. Fluctuations in the flux surface structure are compared against the measured temperature and density fluctuations and the reconstructed temperature and density profiles are examined to look for evidence of barriers to particle and heat transport. This material is based upon work supported by the U.S. DOE. [Preview Abstract] |
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CP12.00051: Thermal Transport Barrier Behavior in a Dynamic Model of the Quasi Single Helicity State I.J. Mckinney, P.W. Terry The relaxation oscillations observed in quasi single helicity (QSH) RFP plasmas have been modeled by a reduced MHD mode-coupling model that accounts for the effect of strong shear in the inner most resonant tearing mode (dominant) on tearing modes that are resonant at larger radial positions (secondary). This model predicts a transition threshold in plasma current from a multiple helicity (MH) steady state to a dynamical state that dithers between MH and QSH. It also correctly predicts the scaling of the persistence of the QSH state with plasma current [P.W.~Terry, G.G. Whelan, Plasma Phys.~Contol.~Fusion {\bf 56}, 094002 (2014)]. Here, the model is extended to include the evolution of mean temperature by anomalous magnetic-flutter-induced heat transport. The suppression of secondary modes by the strong magnetic shear of the dominant mode creates a thermal transport barrier in QSH. The behavior of this model is compared to experimental observations, including the scaling of secondary mode amplitude with plasma current, temperature profile evolution in the relaxation oscillation, and the scaling of transport barrier thermal structure size with dominant mode amplitude. Good qualitative agreement with experiment indicates that QSH is a type of shear-induced transport barrier. [Preview Abstract] |
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CP12.00052: Observation of Hot Remnant Islands using Fast Thomson Scattering L.A. Morton, W.C. Young, D.J. Den Hartog, C.C. Hegna, E. Parke, J.A. Reusch, C.M. Jacobson The MST Fast Thomson Scattering Laser, operating with repetition rates of up to 100 kHz for up to 25 laser pulses, has allowed direct observation of temperature structures produced by tearing modes rotating at 10 - 20 kHz. A hot spot observed by Fast TS coincides with the O-point of the dominant m/n $=$ 1/6 mode reconstructed by MHD modeling from edge magnetic measurements. The electron thermal conductivity inside the island is estimated from power balance to be 75 m$^{\mathrm{2}}$/s. However, MHD modeling also predicts overlap between the n$=$6 and n$=$7 islands, producing chaotic field lines and total loss of the island flux surfaces. Ensemble-averaged data from the slower burst laser (25 kHz for 8 pulses) also indicates overlap between the temperature fluctuations associated with these modes. These temperature fluctuation also exhibits the large higher-harmonic content that characterizes the hot island in the single-shot cases. DEBS finite-beta MHD simulations qualitatively reproduce MST temperature structures in certain cases. This work is supported by the US DoE and the NSF. [Preview Abstract] |
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CP12.00053: A Flexible Master Oscillator for a Thomson Scattering Pulse-Burst Laser System D.J. Den Hartog, W.C. Young A new master oscillator will be installed in the pulse-burst laser system used for high-rep-rate Thomson scattering on the MST experiment. This new master oscillator will enable pulse repetition rates up to 1 MHz, with the ability to program a burst of pulses with arbitrary and varying time separation between each pulse. In addition, the energy of each master oscillator pulse can be adjusted to compensate for gain variations in the power amplifier section of the laser system. This flexibility is accomplished by chopping a CW laser source with a high-bandwidth acousto-optic modulator (AOM). The laser source is a 1064 nm diode-pumped solid-state laser with continuous output power variable from 100 to 500 mW. The 2 mm diameter polarized beam is focused into the gallium phosphide crystal of the AOM, which deflects the beam by approximately 60 mrad. Beam deflection is controlled by a simple digital input pulse, and is capable of producing laser pulses of less than 20 ns width at repetition rates much greater than 1 MHz. These pulses from the output of the AOM will be collimated and propagated into the laser amplifier system, where they will be amplified to $\sim$ 2 J/pulse and injected into the MST plasma. [Preview Abstract] |
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CP12.00054: Thomson Scattering at 250 kHz William Young, D. J. Den Hartog, L. A. Morton The fast Thomson scattering diagnostic on the MST Reversed-Field Pinch experiment now measures electron temperature at rates of up to 250 kHz, allowing for single shot analysis of phenomena that previously required ensembles of measurements from many shots. Recent laser upgrades include the addition of a second Nd:glass amplifier (giving a total of six amplifiers including four Nd:YAG stages) and optimization of neodymium doping levels within the glass amplifier stages to reduce thermal defocusing. The master-oscillator power-amplifier laser system operates in a pulse-burst mode where the laser generates multiple pulses per flashlamp firing and these bursts of laser pulses are repeated multiple times. When optimizing for the largest number of laser pulses, the laser produces up to 30 pulses at a rate of 100 kHz per burst repeated up to 4 times every 2 ms for a total of 120 temperature measurements per MST discharge. When optimizing for fastest pulsing rate, the laser can produce 8 pulses at 250 kHz within a single burst. A laser system upgrade currently underway is replacement of the diode-pumped pulsed Nd:YVO$_4$ master oscillator with a CW laser chopped by an acoustic-optic modulator; this upgrade may enable pulsing rates faster than 250 kHz. [Preview Abstract] |
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CP12.00055: Upgrades to the MST Thomson scattering diagnostic S.Z. Kubala, M.T. Borchardt, D.J. Den Hartog, D.J. Holly, C.M. Jacobson, L.A. Morton, W.C. Young The Thomson scattering diagnostic on MST records both equilibrium and fluctuating electron temperature with a range capability of 10 eV to 5 keV. Standard operation with two modified commercial Nd:YAG lasers allows measurements at rates of 1-25 kHz. A new laser system is being commissioned to enable measurements up to 250 kHz. Other subsystems of the diagnostic are also being improved. The power supplies for the avalanche photodiode detectors (APDs) that record the scattered light are being updated to improve safety, reliability, and maintainablity. Each of the 144 APDs will have an individual rack-mounted switched supply with bias voltage adjustable to match the APD. Long-wavelength filters (1140 nm center, 80 nm bandwidth) are being added to the polychromators to improve capability to resolve non-Maxwellian distributions and to enable electron-velocity measurement. A supercontinuum pulsed white-light source is being implemented to improve spectral calibration of the polychromators. This work is supported by the US DOE and NSF. [Preview Abstract] |
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CP12.00056: Development of a magnetic vector potential profile measurement using a Heavy Ion Beam Probe P.J. Fimognari, D.R. Demers, T.P. Crowley Measurement of the plasma current density profile remains a fundamental need in toroidal confinement. Establishing this unique capability within the fusion program is invaluable to stability and transport studies. Inference of localized values of the magnetic vector potential, which will enable current density profile studies, can be accomplished through measurement of the toroidal velocity of secondary ions produced through electron-impact ionization of a heavy ion beam in an axisymmetric plasma. We are developing a specialized detector to measure particle velocity and the techniques necessary to unfold the magnetic vector potential profile, and hence the poloidal flux and current density profiles. Initial modeling of the velocity detector has been performed. Simulations are enabling estimation of anticipated sensitivity to, and resolution of, equilibrium and fluctuating quantities. Results of this work and forward looking plans will be presented. [Preview Abstract] |
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CP12.00057: Characterization and initial results from the upgraded MST interferometer-polarimeter E. Parke, D.L. Brower, W.X. Ding, J.R. Duff The FIR interferometer-polarimeter diagnostic on MST is a high-bandwidth system with unique capabilities for measuring high-frequency density and internal magnetic fluctuations. Installation of new planar-diode mixers improves both the signal strength and the noise floor compared to the corner-cube mixers previously used. The new mixer technology also offers a simpler detection configuration that eliminates the need for additional amplifiers. We characterize the bandwidth capabilities of the upgraded heterodyne receiver system and present initial measurements in reversed-field pinch (RFP) plasmas. High wavenumber resolution becomes possible when operating without focusing elements, using only the 2-3 mm aperture on the mixer to determine the sampled chord width. This configuration will provide better resolution of small-scale fluctuations observed in the RFP during periods of improved, tokamak-like confinement. Finally, cross-correlation techniques between two mixers viewing the same chord further reduce measurement noise and improve the resolution of high-frequency, small-amplitude magnetic and density fluctuations. Initial tests of this technique in neutral-beam heated plasmas will be presented. [Preview Abstract] |
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CP12.00058: Effects of Thin Shell on the Resistive Wall Mode in Keda Torus eXperiment Wei Bai, Ping Zhu, Tao Lan, Hong Li, Jin-lin Xie, Ah-di Liu, Chi-jin Xiao, Wei-xing Ding, Wan-dong Liu Keda Torus eXperiment (KTX) is a new reversed field pinch (RFP) with a copper thin shell. The stability of resistive wall modes (RWMs) is crucial for the sustained operation of KTX. The standard formulation of dispersion relation for RWM based on the MHD energy principle has been evaluated for a cylindrical $\alpha-\Theta_{0}$ model of KTX plasma equilibrium, in an effort to investigate the effects of thin shell on the RWM in KTX. Full MHD calculations of the linear RWM in KTX using the NIMROD code are also being developed. The detailed comparisons between theoretical analyses and NIMROD calculation results, for KTX equilibria with and without rotation, are to be presented and discussed. [Preview Abstract] |
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CP12.00059: Current drive in toroidal geometry driven by external helical electrostatic perturbations Richard Nebel, Daniel Barnes, John Finn Simulation results are shown for a plasma with an initial magnetic field $B_z=const.$ when helical external electrostatic fields are applied with zero radial magnetic field at the edge. These simulations, with the DEBS code, show that the external electrostatic perturbations $\epsilon=\phi(r_{wall})$ with ($m=1,n=1$) have little effect inside the plasma below a certain threshold in $\epsilon$; however, above this bifurcation they lead to a novel single helicity nonlinear state with low shear $q\approx 1$ in the interior, in spite of having zero net helicity injection. These states have net parallel current due to the lower electrical conductivity near the plasma edge. This steady state operation has elliptic field lines (O-lines), allowed by helical symmetry (stellarator transform.) Similar states have been observed with the application of helicities $(2,1)$,$(1,2)$, and $(1,4)$. These simulation results are compared with experimental results recently found on the Tibbar Technologies device. [Preview Abstract] |
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CP12.00060: Generation of rotational transform in a toroidal confinement device with tilted coils Lucas Zeppetello, Michel Doumet, Kenneth Hammond, Ben Israeli, Justin Mann, Francesco Volpe, Anthony Clark, Donald Spong, Samuel Lazerson Experimental evidence was obtained, by means of an electron beam, that rotational transform can be generated in a toroidal configuration constructively similar to a tokamak, but solenoid-free and featuring six tilted toroidal-field coils. The coils are planar and, in fact, circular, hereby the device name CIRCUS [1]. In addition, the coils are interlinked to each other, which helps reducing the aspect ratio but is not strictly required. Comparisons between calculations and field-line mapping measurements will be presented, as well as predictions for devices featuring more coils, resulting in more axisymmetric plasmas. These are expected to operate at lower plasma current than a tokamak of comparable size and magnetic field, which might have interesting implications for disruptions and steady-state operation. Additionally, the toroidal magnetic ripple is less pronounced than in an equivalent tokamak in which the coils are not tilted.\\[4pt] [1] A. W. Clark et al., \textit{Fusion Eng.~Des.~} \textbf{89}, 2014 [Preview Abstract] |
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CP12.00061: Experimental characterization and equilibrium reconstructions of first electron cyclotron heated plasmas in the low-aspect ratio CNT stellarator Kenneth Hammond, Alek Anichowski, Francesco Volpe, Yumou Wei, Samuel Lazerson Neutral plasmas started up and sustained by electron cyclotron resonance heating are a current topic of study in the CNT stellarator. Langmuir probe measurements suggest that the microwave heating maintains a bi-Maxwellian electron distribution, and that the plasma density decays on a millisecond time scale when heating ceases. Furthermore, a Langmuir probe mounted on an electronic moving stage measures profiles of plasma temperature and density with very high spatial resolution. These profiles show evidence of magnetic islands, in agreement with electron-beam mapping of the vacuum magnetic field. Previous results suggest that the vacuum islands result from error fields due to coil misalignments [1]. We present ongoing work to reproduce these field errors with Biot-Savart calculations that account for coil misalignments. We also present results of VMEC [2] free- and fixed-boundary calculations of CNT equilibria and ongoing work to upgrade the ECRH system from 1 to 16 kW.\\[4pt] [1] P. Traverso et al., \textit{Bull.~Amer.~Phys.~Soc.~}\textbf{55}, Poster CP9-11, 2010.\\[0pt] [2] S. P. Hirshman and J. C. Whitson, \textit{Phys.~Fluids} \textbf{12}, 1983. [Preview Abstract] |
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CP12.00062: Filterscope edge plasma diagnostic for the W7-X stellarator Jeffrey Harris, Ezekial Unterberg, Jeremy Lore, Laurie Stephey, Oliver Schmitz, Glen Wurden, Christoph Biedermann, Maciej Krychowiak, Ralf Koenig W7-X is a large (R $=$ 5.5m, a $=$ 0.5m, B \textless 3T, P\textgreater 10 MW) superconducting stellarator at the Max-Planck Institut f\"{u}r Plasmaphysik in Greifswald, Germany, which will begin plasma operations in the last quarter of 2015. We describe here the first measurements with a 24-channel filterscope diagnostic [E. A. Unterberg, et al, Rev. Sci. Instrum. 83, 10D722, (2012)] of edge plasma characteristics and spectral emission from impurities near the test limiters installed for initial plasma experiments. These measurements, together with high resolution IR thermography imaging of the limiter, will be used as inputs for edge transport modeling using the EMC3 code [J. D. Lore, et al, Nucl. Fusion 52, 0540 (2012)]. [Preview Abstract] |
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CP12.00063: Expected performance and recent results from the X-ray Imaging Crystal Spectrometer on the W7-X stellarator Novimir A. Pablant, Andreas Langenberg, Manfred Bitter, Luis Delgado-Aparicio, David A. Gates, Kenneth W. Hill, Michael Mardenfeld, George H. Neilson A new high resolution x-ray imaging crystal spectrometer diagnostic (XICS) has recently been installed on W7-X stellarator. This diagnostic will contribute to the study of ion and electron thermal transport and the evolution of the radial electric field by providing high resolution temperature and rotation measurements. The XICS diagnostic will provide spatially resolved profile measurements of the ion temperature ($T_i$), electron temperature ($T_e$), poloidal flow velocity ($V_P$) and impurity ion density for the Ar16+, Ar17+ and Fe24+ charge states. This system will have a maximum time resolution of 5ms, a spatial resolution of 2cm, and spatial coverage from the core to a normalized minor radius of $\rho \approx 0.8$. The system is fully installed and will be in operation for the initial W7-X experimental campaign (OP1.1). For this initial experimental campaign the XICS diagnostic will be the primary diagnostic for measurement of the core ion temperature and poloidal rotation. The design, expected performance and analysis techniques will be presented, along with any recent measurement results. Research supported by the U.S. DOE under Contract No. DE-AC02-09CH11466 with Princeton University. [Preview Abstract] |
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CP12.00064: Visible/IR Observations of the Poloidal Limiter in W7-X G.A. Wurden, C. Biedermann, M.W. Jakubowski, S.A. Bozhenkov We have prepared a high resolution view of the poloidal graphite limiter in W7-X for the first operational period in 2015. Magnetically shielded visible (400-700 nm) and mid-band infrared (3-5 micron) cameras share a nearly identical view through a large sapphire window mounted on the AEA30 port. Both systems achieve sub-mm spatial resolution and 10 millisecond time resolution while viewing three of the limiter tiles. We will compare heat flux patterns actually observed on the limiter with numerical predictions [1] corresponding to different plasma diffusivities.\\[4pt] [1] S. A. Bozhenkov, F. Effenberg, et al, ``Limiter for the early operation phase of W7-X,'' 41$^{\mathrm{ST}}$ EPS Conference on Plasma Physics, P1.080, Berlin, June 23-27, 2014. [Preview Abstract] |
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CP12.00065: Optimizing Stellarators for Energetic Particle Confinement using BEAMS3D Peter Bolgert, Michael Drevlak, Sam Lazerson, David Gates, Roscoe White Energetic particle (EP) loss has been called the ``Achilles heel of stellarators,'' (Helander, Rep.~Prog.~Phys.~\textbf{77} 087001 (2014)) and there is a great need for magnetic configurations with improved EP confinement. In this study we utilize a newly developed capability of the stellarator optimization code STELLOPT: the ability to optimize EP confinement via an interface with guiding center code BEAMS3D (McMillan et al., Plasma Phys.~Control.~Fusion \textbf{56}, 095019 (2014)). Using this new tool, optimizations of the W7-X experiment and ARIES-CS reactor are performed where the EP loss fraction is one of many target functions to be minimized. In W7-X, we simulate the experimental NBI system using realistic beam geometry and beam deposition physics. The goal is to find configurations with improved neutral beam deposition and energetic particle confinement. These calculations are compared to previous studies of W7-X NBI deposition. In ARIES-CS, we launch 3.5 MeV alpha particles from a near-axis flux surface using a uniform grid in toroidal and poloidal angle. As these particles are born from D-T reactions, we consider an isotropic distribution in velocity space. [Preview Abstract] |
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CP12.00066: Radial electric field computations with DKES and neoclassical models in TJ-II stellarator Julio Martinell, Cesar Gutierrez-Tapia, Daniel Lopez-Bruna Radial electric fields arise due to the non-ambipolar transport in stellarator plasmas and play an important role in determining some improved confinement regimes. In order to calculate this electric field it is necessary to take all particle fluxes that are not ambipolar. The most important contribution to these fluxes comes from neoclassical transport. Here we use particle fluxes obtained from kinetic equation computations using the code DKES to evaluate the radial electric field profiles for certain discharges of the heliac TJ-II. Experimental profiles for the density and temperatures are used together with the diffusion coefficients obtained with DKES. A similar computation of the electric field is performed with three analytical neoclassical models that use an approximation for the magnetic geometry. The ambipolar electric field from the models is compared with the one given by DKES and we find that they are all qualitatively similar. They are also compared with experimental measurements of the electric field obtained with HIBP. It is shown that, although the electric field is reasonably well reproduced by the neoclassical computations, especially in high temperature regimes, the particle fluxes are not. Thus, neoclassical theory provides good $E_r$ estimates in TJ-II. [Preview Abstract] |
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CP12.00067: Global gyrokinetic models for energetic particle driven Alfv\'{e}n instabilities in 3D equilibria Don Spong, Ihor Holod The GTC global gyrokinetic PIC model has been adapted to 3D VMEC equilibria and provides a new method for the analysis of Alfv\'{e}nic instabilities in stellarators, 3D tokamaks, and helical RFP states. The gyrokinetic orderings (k$_{\vert \vert }$/k$_{\bot }$ \textless \textless 1, $\omega $/$\Omega _{ci}$ \textless \textless 1, $\rho_{EP}$/L \textless \textless 1) are applicable to a range of energetic particle driven instabilities that have been observed in 3D configurations. Applications of this model to stellarators have indicated that a variety of different Alfv\'{e}n instabilities can be excited, depending on the toroidal mode number, fast ion average energy and fast ion density profile. Both an LHD discharge [1] where bursting n $=$ 1 Alfv\'{e}n activity in the TAE gap was observed and a W7-X case [2] have been examined. TAE,/EAE/GAE modes have been found in the simulations, depending on the mode family and fast ion profiles used. The dynamical evolution of the instabilities shows the field period coupling between n and n $+$ Nfp expected for a stellarator. The development of gyrofluid reduced models that can capture relevant physics aspects of the gyrokinetic models will also be discussed. \\[4pt] [1] M. Osakabe, et al., Nuclear Fusion 46, S911 (2006).\\[0pt] [2] A. Mischenko, A. K\"{o}nies, et al. Nuclear Fusion 54, 104003 (2014). [Preview Abstract] |
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CP12.00068: High-beta extended MHD simulations of stellarators with Spitzer resistivity Torrin Bechtel The nonlinear, extended MHD code NIMROD is used to study high-beta, 3D magnetic topology evolution of a toroidal stellarator. The configurations under investigation derive from the geometry of the Compact Toroidal Hybrid (CTH) experiment. However, the vacuum rotational transform profile is artificially raised by modifying applied magnetic fields in an effort to examine the sensitivity of low order rational surfaces and/or magnetic islands. Finite beta plasmas are created using a volumetric heating source and temperature dependent anisotropic heat conduction and resistivity. Flux surface dependent temperature and density profiles are used for the initial condition so that Spitzer resistivity can be applied. The onset of MHD instabilities and nonlinear consequences are monitored as a function of beta as well as the fragility of the magnetic surfaces. [Preview Abstract] |
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CP12.00069: Overview of recent results and future plans on the Compact Toroidal Hybrid experiment D.A. Maurer, M.C. ArchMiller, M.R. Cianciosa, D.A. Ennis, J.D. Hanson, G.J. Hartwell, J.D. Hebert, J.L. Herfindal, S.F. Knowlton, X. Ma, S. Massidda, M.D. Pandya, N.A. Roberds, P.J. Traverso Goals of the Compact Toroidal Hybrid (CTH) experiment are to: (1) investigate the dependence of plasma disruptive behavior on the level of applied 3D magnetic shaping, (2) test and advance 3D computational modeling tools in strongly shaped plasmas, and (3) study the implementation of a new island divertor. Progress towards these goals and other developments are summarized. The disruptive density limit is observed to exceed the Greenwald limit as the vacuum transform is increased, but a threshold for disruption avoidance is not observed. Low q operation is routine, with low q disruptions avoided when the vacuum transform is raised to the value of 0.07 or above. Application of vacuum transform has been demonstrated to reduce and eliminate the vertical drift of elongated discharges that would otherwise be vertically unstable. Current efforts at improved equilibrium reconstruction and diagnostic development will beoverviewed. NIMROD is used to model the current ramp phase of CTH and 3D shaped sawtooth behavior. An island divertor design has begun with connection length studies and initial EMC3-Eirene results to model energy deposition on divertor plates located in an edge 1/3 island. [Preview Abstract] |
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CP12.00070: Operation in low edge safety factor regime and passive disruption avoidance due to stellarator rotational transform in the Compact Toroidal Hybrid M.D. Pandya, D.A. Ennis, G.J. Hartwell, D.A. Maurer \DeclareRobustCommand*{\g}{\raisebox{-.7ex}{$\mathchar'26$} \mkern-8mu \iota} Low edge safety factor operation at a value less than two ($q(a)=1/\g_{tot}(a)<2$) is routine on the Compact Toroidal Hybrid device. Presently, the operational space of this current carrying stellarator extends down to $q(a)=1.2$ without significant $n=1$ kink mode activity after the initial plasma current rise of the discharge. The disruption dynamics of these low $q(a)$ plasmas depend upon the fraction of rotational transform produced by external stellarator coils to that generated by the plasma current. We observe that when about 10\% of the total rotational transform is supplied by the stellarator coils, low $q(a)$ disruptions are passively suppressed and avoided even though $q(a)<2$. When the plasma does disrupt, the instability precursors measured and implicated as the cause are internal tearing modes with poloidal, $m$, and toroidal, $n$, mode numbers of $m/n=3/2$ and $4/3$ observed by external magnetic sensors, and $m/n=1/1$ activity observed by core soft x-ray emissivity measurements. Even though $q(a)$ passes through and becomes much less than two, external $n=1$ kink mode activity does not appear to play a significant role in the observed disruption phenomenology. [Preview Abstract] |
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CP12.00071: Non-axisymmetric equilibrium reconstruction on the Compact Toroidal Hybrid Experiment using external magnetic and soft x-ray inversion radius measurements X. Ma, M. Cianciosa, J.D. Hanson, G.J. Hartwell, S.F. Knowlton, D.A. Maurer, D.A. Ennis, J.L. Herfindal Non-axisymmetric free-boundary equilibrium reconstructions of stellarator plasmas are performed for discharges in which the magnetic configuration is strongly modified by the driven plasma current. Studies were performed on the Compact Toroidal Hybrid device using the V3FIT reconstruction code [1] incorporating a set of 50 magnetic diagnostics external to the plasma, combined with information from soft X-ray (SXR) arrays. With the assumption of closed magnetic flux surfaces, the reconstructions using external magnetic measurements allow accurate estimates of the net toroidal flux within the last closed flux surface, the edge safety factor, and the outer boundary of these highly non-axisymmetric plasmas. The inversion radius for sawtoothing plasmas is used to identify the location of the $q = 1$ surface, and thus infer the current profile near the magnetic axis. With external magnetic diagnostics alone, we find the reconstruction to be insufficiently constrained.\\[4pt] [1] J. D. Hanson et al., Nucl. Fusion \textbf{49}, 075031 (2009) [Preview Abstract] |
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CP12.00072: NIMROD Modeling of CTH Current Rise Dynamics Jonathan Hebert, James Hanson The 3D extended MHD code NIMROD [1] has been modified to model the Compact Toroidal Hybrid (CTH), a five-field period torsatron/tokamak hybrid device located at Auburn University. In many shots with inductively driven current in CTH, hesitations in the plasma current are observed as the plasma current is ramped. V3FIT reconstructions of the current rise demonstrate the edge rotational transform is near a low order rational, suggesting that island formation at or near the edge may be responsible for the current hesitations. The initial stages of the current drive were self-consistently modeled using NIMROD with experimentally relevant vacuum fields, loop voltages, initial temperatures and initial densities. Results show the formation of field-period-symmetry-preserving islands near the plasma edge as well as the coalescence of these islands into larger, symmetry-breaking island chains which modify the distribution of the current in the plasma. Modeling investigating the effect of CTH physical limiters will be presented. \\[4pt] [1] C.R. Sovinec et al Journal of Computational Physics, 195, 355 (2004). [Preview Abstract] |
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CP12.00073: First Results from a Coherence Imaging Diagnostic for the Compact Toroidal Hybrid D.A. Ennis, G.J. Hartwell, C.A. Johnson, D.A. Maurer, S.L. Allen An optical coherence imaging diagnostic is being commissioned for time-resolved measurements ($\sim10$ ms) of ion emissivity, velocity, and temperature in the Compact Toroidal Hybrid (CTH) experiment. The Coherence Imaging (CI) technique measures the spectral coherence of an emission line with an imaging interferometer of fixed delay. CI has a number of advantages when compared to dispersive Doppler spectroscopy, including higher throughput and the capability to provide 2D spectral images, making it advantageous for investigating the non-axisymmetric geometry of CTH plasmas. A spectral survey of the visible and ultraviolet emission for a range of CTH discharges has identified helium and carbon impurity lines that will be utilized for CI measurements in CTH. First CI measurements of He II (468.6 nm) emission from CTH plasmas will be presented along with interferograms from a calibration light source and details of the instrument design. Results from this diagnostic will aid in characterizing the equilibrium ion parameters in both the edge and core of CTH plasmas for planned island divertor and MHD mode-locking experiments. [Preview Abstract] |
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CP12.00074: Sawtooth Instability in the Compact Toroidal Hybrid J.L. Herfindal, D.A. Maurer, G.J. Hartwell, D.A. Ennis, S.F. Knowlton Sawtooth instabilities have been observed in the Compact Toroidal Hybrid (CTH), a current-carrying stellarator/tokamak hybrid device. The sawtooth instability is driven by ohmic heating of the core plasma until the safety factor drops below unity resulting in the growth of an $m=1$ kink-tearing mode. Experiments varying the vacuum rotational transform from 0.02 to 0.13 are being conducted to study sawtooth property dependance on vacuum flux surface structure. The frequency of the sawtooth oscillations increase from 2 kHz to 2.8 kHz solely due the decrease in rise time of the oscillation, the crash time is unchanged. CTH has three two-color SXR cameras, a three-channel 1mm interferometer, and a new bolometer system capable of detecting the signatures of sawtooth instabilities. The new bolometer system consists of two cameras, each containing a pair of diode arrays viewing the plasma directly or through a beryllium filter. Electron temperature measurements are found with the two-color SXR cameras through a ratio of the SXR intensities. Impurity radiation can drastically affect the electron temperature measurement, therefore new filters consisting of aluminum and carbon were selected to avoid problematic line radiation while maximizing the signal for a 100 eV plasma. [Preview Abstract] |
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CP12.00075: Simulations of Sawtooth Oscillations In CTH Nicholas Roberds, Luca Guazzotto, James Hanson, David Maurer Sawteeth are driven relaxation oscillations seen in tokamaks. Experimentally, they can be reproduced reliably. They affect the confinement of the plasma core, and in some circumstances can trigger disruptions. Sawtoothing has been observed in the Compact Toroidal Hybrid (CTH), a tokamak-stellarator hybrid having a non-axisymmetric equilibrium field. We present novel numerical simulations of sawtooth oscillations in this tokamak-stellarator hybrid. Results are contrasted and compared with simulations of a small ohmic tokamak that resembles CTH without the helical stellarator field. We have used NIMROD [1] to conduct these extended-MHD simulations in toroidal geometry. Sawtooth simulations are obtained by starting with a stable ideal MHD equilibrium from VMEC [2], and driving the central safety factor below unity with an applied loop voltage. The challenges of sawtooth simulations with 3D equilibrium fields are discussed. \\[4pt] [1] C.R. Sovinec, A.H. Glasser, T.A. Gianakon, D.C. Barnes et al, J. Comput. Phys., 355 (2004).\\[0pt] [2] S. P. Hirshman and J. C. Whitson, Phys. Fluids 3553 (1983). [Preview Abstract] |
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CP12.00076: Installation of a Thomson scattering diagnostic on the Compact Toroidal Hybrid Experiment P.J. Traverso, D.A. Maurer, D.A. Ennis, G.J. Hartwell, M.R. Cianciosa A Thomson scattering system is being commissioned for the non-axisymmetric plasmas of the Compact Toroidal Hybrid (CTH), a five-field period current-carrying torsatron. The initial system takes a single point measurement on the magnetic axis and will be used to assess options for an expansion to a multi-point system to enable better 3D equilibrium reconstructions using the V3FIT code. A single point measurement will reduce the uncertainty in the reconstructed peak pressure by an order of magnitude for both current-carrying plasmas and future gyrotron-heated stellarator plasmas. The beam, generated by a frequency doubled Continuum 2 J, Nd:YaG laser, is passed vertically through an entrance Brewster window and a two-aperture optical baffle system to minimize stray light. The beam line is designed to propagate $\sim$ 8 m to the mid-plane of the CTH device with the beam diameter $<$ 3 mm inside the plasma volume. An Andor iStar DH740-18U-C3 image intensified CCD camera is used in conjunction with a Holospec f/1.8 spectrograph to collect the red-shifted scattered light from 532-580 nm. A single point system will initially measure plasmas with core electron temperatures of 100 to 200 eV and densities of $5\times10^{18}$ to $5\times10^{19}$ m$^{-3}$. [Preview Abstract] |
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CP12.00077: Electron cyclotron ray tracing and absorption predictions for Compact Toroidal Hybrid plasmas using TRAVIS S.F. Knowlton, G.J. Hartwell, D.A Maurer, N.B. Marushchenko, Y. Turkin, T. Bigelow Plasmas in the Compact Toroidal Hybrid (CTH), a five field period, $\ell = 2$ torsatron ($B_0 = 0.5$\,T $R_0 = 0.75$\,m, $a_p\approx0.2$\,m) will be heated by second harmonic X-mode electron cyclotron heating with power provided by a 28\,GHz gyrotron capable of producing up to 200\,kW. Ray-tracing calculations that will guide the selection of the launching position, antenna focal length, and beam-steering characteristics are performed with the TRAVIS code [1]. Non-axisymmetric vacuum and current-carrying CTH equilibria for the ray tracing are modeled with the V3FIT code [2]. The calculated absorption is highest for vertically propagating rays that traverse the region where a saddle of resonant field strength exists. However, the absorption for top-launched waves is more sensitive to variations in the magnetic equilibria than for a radial side launch where the magnetic field profile is tokamak-like.\\[4pt] [1] N.B. Marushchenko, Y. Turkin, H. Maassberg, Comp. Phys. Comm. 185 165 (2014)\\[0pt] [2] J. D. Hanson et al., Nucl. Fusion 49, 075031 (2009) [Preview Abstract] |
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CP12.00078: Island Divertor Plate Modeling for the Compact Toroidal Hybrid Experiment G.J. Hartwell, S.D. Massidda, D.A Ennis, S.F. Knowlton, D.A. Maurer, A. Bader Edge magnetic island divertors can be used as a method of plasma particle and heat exhaust in long pulse stellarator experiments. Detailed power loading on these structures and its relationship to the long connection length scrape off layer physics is a new Compact Toroidal Hybrid (CTH) research thrust. CTH is a five field period, $\ell=2$ torsatron with $R_0~=~0.75$\,m, $a_p\sim 0.2$\,m, and $|B|$~$\leq 0.7$\,T. For these studies CTH is configured as a pure stellarator using a 28\,GHz, 200\,kW gyrotron operating at 2nd harmonic for ECRH. We report the results of EMC3-EIRENE [1] modeling of divertor plates near magnetic island structures. The edge rotational transform is varied by adjusting the ratio of currents in the helical and toroidal field coils. A poloidal field coil adjusts the shear of the rotational transform profile, and width of the magnetic island, while the phase of the island is rotated with a set of five error coils producing an $n=1$ perturbation. For the studies conducted, a magnetic configuration with a large $n=1$, $m=3$ magnetic island at the edge is generated. Results from multiple potential divertor plate locations will be presented and discussed.\\[4pt] [1] Y. Feng, M. Kobayashi, T. Lunt, and D. Reiter, Plasma Phys. Control. Fusion, 53 (2011) 024009 [Preview Abstract] |
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CP12.00079: HSX Program Overview and Research Directions Simon Anderson HSX is a neoclassical-transport optimized stellarator. Research has concentrated on neoclassical transport, turbulent transport and concept optimization, and the plasma edge. For neoclassical transport, an optimized diagnostic has improved equilibrium reconstruction. Counter-streaming Pfirsch-Schluter flow measurements have been made to examine the core electron-root \textbf{E}$_{\mathrm{\mathbf{r}}}$. Turbulent transport studies have included heat transport stiffness and direct comparisons with non-linear GENE calculations. Optimization of the HSX magnetic configuration to turbulent transport has been initiated. Edge studies have concentrated on measurements of 2D edge profiles and comparison to EMC3-EIRENE. The HSX program will continue in these main areas, with extension into energetic ion confinement with DNB injection. Diagnostic upgrades will permit direct \textbf{E}$_{\mathrm{\mathbf{r}}}$ measurements through MSE, and improvements in density and temperature fluctuation measurements will improve understanding of turbulent transport and facilitate continued GENE modeling. Edge studies will be extended to measure neutral fueling and recycling, which will permit use of a single reservoir particle balance model to provide a complete particle inventory. * This work supported by US DOE Grant DE-FG02-93ER54222 [Preview Abstract] |
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CP12.00080: Progress in understanding flows and electric field in HSX S.T.A. Kumar, J. Smoniewski, T. Dobbins, J.N. Talmadge, F.S.B. Anderson, D.T. Anderson Experimental measurements of the radial electric field in HSX could not reproduce the high positive values calculated with a neoclassical model. Several approaches to improve the measurement and modeling are being undertaken to understand this discrepancy. In particular: (a) The CHERS system on HSX has recently been modified to measure the counter-streaming Pfirsch-Schl\"{u}ter (PS) parallel ion flows that could provide an improved measurement of the radial electric field in the plasma. Preliminary measurements indicate a larger radial electric field than the previous measurements, but still significantly less than the neoclassical values. (b) We are complementing the improvements in the experiment with improvements in the neoclassical modeling. We are using SFINCS and FORTEC-3D to validate the monoenergetic approximation used in DKES and PENTA codes. (c) A biased electrode is being used to observe the ion resonance with the electric field which would appear as a sudden drop in the biased electrode current and a sudden increase in the parallel flow. (d) Work is progressing on utilizing a complementary approach to measure the radial electric field due to the Motional Stark Effect (MSE). [Preview Abstract] |
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CP12.00081: Coherent Density Fluctuations in the HSX Stellarator C.B. Deng, D.L. Brower, D.T. Anderson, F.S.B. Anderson, K.M. Likin, J. Smoniewski, J.N. Talmadge A multi-channel interferometer system is used to measure equilibrium density profile and its fluctuations in the HSX stellarator. Low-frequency, coherent density fluctuations are observed in certain quasi-helically symmetric (QHS) plasma conditions and has characteristic frequency of 15kHz. The mode is observed for small displacement of the 1$^{\mathrm{st}}$ harmonic O-mode ECRH location inward from the magnetic axis. This mode is also observed on magnetic fluctuation signal, using external coils, which shows n$=$1. When HSX is operated without quasi-helical symmetry (mirror configuration), a coherent electrostatic mode at 28 kHz is observed. While the coherent mode in QHS plasmas shows ballooning effect, the coherent mode in Mirror plasma exhibits an anti-ballooning characteristic. Mode radial structure can be obtained from inversion of interferometer measurement when the m number is known. Under certain Mirror conditions, the coherent modes display strong bi-coherence on Langmuir probe signals. Detailed characterization of the observed coherent modes will be reported and their identification will be explored. \textit{*Supported by USDOE grants DE-FG03-01ER54615 and DE-FG02-93ER54222.} [Preview Abstract] |
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CP12.00082: A MSE Polarimetry diagnostic for the measurement of radial electric fields on the HSX stellarator T.J. Dobbins, S.T.A. Kumar, F.S.B. Anderson, D.T. Anderson The radial electric field in HSX has been measured using Charge Exchange Recombination Spectroscopy. These impurity ion flow measurements could not resolve a large positive radial electric field ($\sim$ 40-50 kV/m) near the core of the HSX plasma, predicted by neoclassical codes. A dual PEM (Photo Elastic Modulator) MSE polarimetry system has been designed for direct measurement of the radial electric field in the HSX plasma. The polarimetry design has been optimized to get a maximum change in polarization angle from an electric field while still providing good spatial resolution. It is expected that a radial electric field as small as 2 kV/m can be detected. A synthetic diagnostic is added to the optimization model to see the effect of spot size and beam width on the measurement of electric field. In addition, work is being done to use the optical system for a BES system on HSX. Design and implementation of the BES system is in progress. Both diagnostic designs and initial calibrations are presented. This work is supported by US DOE grant DE-FG02-93ER54222. [Preview Abstract] |
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CP12.00083: Simulation and experiment investigating neoclassical effects of impurities on bootstrap current in HSX J. Smoniewski, J. Talmadge, M. Landreman Impurity accumulation is a major concern for stellarators, and HSX provides a chance to understand impurity dynamics in a low-shear quasi-symmetric device. Calculations with SFINCS have shown that small changes in $Z_{eff}$ impact the bootstrap current in W7-X.\footnote{A. Moll\'en et al, J. Phys.: Conf. Ser., 561, 012012, 2014.} The PENTA and SFINCS codes are in use at HSX and can solve the drift-kinetic equation with multiple plasma species. PENTA is a momentum conserving extension of the DKES code, which calculates the monoenergetic transport matrix on a flux surface using a moments approach. SFINCS is a four dimensional drift-kinetic continuum code. We compare calculations with PENTA and SFINCS for HSX plasmas with varying impurities, focusing on effects on the bootstrap current. Experimental measurements of the bootstrap current using a Rogowski coil are compared to simulation results where possible. [Preview Abstract] |
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CP12.00084: Neutral source and particle balance in the HSX edge Laurie Stephey, Santhosh Kumar, Aaron Bader, Adrian Akerson, Oliver Schmitz, David Anderson, Simon A, Joseph Talmadge, Chris Hegna The ability to control the neutral particle and impurity source in fusion devices is critical to obtaining high purity, high confinement plasmas. The neutral particle source defines the edge density gradients and plasma flows. To understand the relationship between the neutral particle source, plasma density gradients and plasma edge and core transport in HSX, a single reservoir particle balance is being used to provide a complete particle inventory. Detailed spectroscopic measurements of hydrogen and helium emission have yielded neutral and plasma profiles and ionization length estimations. The plasma puff source rate has been directly measured. To determine the recycling source rate, two specially designed limiters will be inserted to intercept 99{\%} of the field lines, resulting in a well-defined LCFS and plasma interaction zone. Single limiter insertion resulted in a 50{\%} reduction in global line emission, implying a reduction in wall recycling. Future camera and probe measurements will provide a recycling source rate. HSX neutral physics is also being investigated using EMC3-EIRENE. All results are discussed along with complementary plans for the Wendelstein 7-X startup phase. [Preview Abstract] |
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CP12.00085: Measurements of Edge Plasma Properties in the HSX Stellarator with Comparison to EMC3-EIRENE A.R. Akerson, A. Bader, O. Schmitz, F.S.B. Anderson, C.C. Hegna, D.T. Anderson 2D profiles of plasma edge temperature, density and flow have been obtained in the edge of the Helical Symmetric Experiment (HSX) using a multi-pin Langmuir probe. Comparison of these profiles with a 3D edge fluid and kinetic neutral transport model (EMC3-EIRENE) show significant deviations. In particular, measurements show peaked density and potential profiles within an edge magnetic island. These features appear coincident with a bundle of closed field lines according to Biot-Savart calculations, suggesting a link between transport and topology. Measurements of the plasma response to changes in edge field-line connection length with the use of a limiter are presented. These observations are important because the presence of potential structures and corresponding ExB flows are not included in the EMC3-EIRENE modeling, necessitating further investigation to understand the origin and impact that these structures have on edge plasma properties. This work supported by US DOE Grant DE-FG02-93ER54222 [Preview Abstract] |
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CP12.00086: WAVES, SHOCKS AND INSTABILITIES |
(Author Not Attending)
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CP12.00087: Saturation of Alfven modes in tokamaks Roscoe White, Nikolai Gorelenkov, Marina Gorelenkova, Mario Podesta, Yang Chen The effect of Alfven modes on high energetic particles in tokamaks is important in general, and could be of significance for ITER. This work is a combination of analytic models and numerical simulation to find the saturation levels of unstable Alfven modes and the resulting effect on beam and alpha particle distributions. Solving the drift kinetic equation with a guiding center code in the presence of Alfven modes driven unstable by a distribution of high energy particles requires the use of a $\delta f$ formalism, wherby the initial distribution $f_0$ is assumed to be a steady state high energy particle distribution in the absense of the modes, and $f = f_0 + \delta f$ describes the particle distribution in the presence of the modes. The Hamiltonian is written as $H = H_0 + H_1$ with $H_0$ giving the unperturbed motion, conserving particle energy $E$, toroidal canonical momentum $P_\zeta$, and magnetic moment $\mu$. By writing the initial particle distribution in terms of these variables, a simple means of calculating mode-particle energy and momentum transfer results, giving a very accurate $\delta f$ formalism. [Preview Abstract] |
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CP12.00088: Two Fluid Kelvin-Helmholtz Instability in a Tokamak Plasma Omar Lopez Ortiz, Luca Guazzotto For the study of equilibrium configurations of tokamak plasmas when toroidal and poloidal flows are present, single and two fluid models are available in the literature. In the two fluid description there appears a component of the poloidal velocity perpendicular to the magnetic flux surfaces, which does not occur in a single fluid description. As an illustration of the impact the normal velocity has on the stability of a plasma, we investigate its effect on a Kelvin-Helmholtz instability driven by a radial gradient in the toroidal flow. The analysis is performed by consistently using single and two fluid equations. The model considers an approximate high beta equilibrium configuration obtained by asymptotically expanding a functional for the single fluid Grad-Shafranov Bernoulli system of equations in terms of the inverse aspect ratio [1]. The normal component of the velocity comes from two fluid theory and it represents a small correction to the single fluid poloidal velocity. The equilibrium and stability analysis is pursued with an analytic approach.\\[4pt] [1] E. Hameiri, \textit{Phys. Plasmas} \textbf{20}, 024504 (2013) [Preview Abstract] |
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CP12.00089: Simultaneous existence of Kelvin Helmholtz and Drift wave Instabilities in IMPED P.K. Chattopadhyay, Sayak Bose, J. Ghosh, Y.C. Saxena The Kelvin-Helmholtz (KH) and drift wave instabilities are observed to be existing simultaneously in the \textbf{I}nverse \textbf{M}irror \textbf{P}lasma \textbf{E}xperimental \textbf{D}evice (IMPED). Further, the generation of side-bands are also observed due to mode coupling of Kelvin-Helmholtz and drift wave instability. In IMPED, magnetized plasma is produced using a multi-filamentary source of much larger diameter located in the low magnetic field region followed by long uniform plasma column in the main chamber with uniform high magnetic field. The uniqueness of IMPED [1] enables the radial profiles of plasma density and temperature to be changed by varying the ratio of the magnetic field in the source and in the main chamber. The instabilities are identified by measuring the wavelength and radial profiles of density, temperature and plasma potential. The evolution of the asymmetry of the side-bands is studied for different radial profiles of density and temperature. Experimental results describing the co-existence of interacting KH and drift instabilities will be presented.\\[4pt] [1] Bose et al. Rev. Sci. Instrum. 86, 063501 (2015). [Preview Abstract] |
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CP12.00090: Analytical and numerical treatment of drift-tearing and resistive drift instabilities in plasma slab V.V. Mirnov, C.C. Hegna, J.P. Sauppe, C.R. Sovinec We consider modification to linear resistive MHD instability theory in a slab due to two categories of non-MHD effects: (1) electron and ion diamagnetic flows caused by equilibrium pressure gradients and (2) electron and ion decoupling on short scales associated with kinetic Alfven and whistler waves. The relationship between the expected stabilizing response due to the effects (1) and the destabilizing contribution caused by the dispersive waves (2) is investigated. An analytic solution combining the effect of diamagnetic flows and the ion-sound gyroradius contribution is derived using a perturbative approach. Linear numerical simulations using the NIMROD code are performed with cold ions and hot electrons in plasma slab with a doubly periodic box bounded by two perfectly conducting walls. Configurations with magnetic shear are unstable to current-driven drift-tearing instability. A second linearly unstable resistive drift type mode with largely electrostatic perturbations is also observed in simulations. The resistive-drift mode is suppressed by magnetic shear in unbounded domains but can remain unstable in the simulations with finite slab thickness and perfectly conducting wall. Additionally, the growth rate is sensitive to the magnetic shear length. We analyze whether these modes can be unstable in cylindrical configurations with magnetic shear typical for reversed field pinches. [Preview Abstract] |
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CP12.00091: Electromagnetic particle simulation of the linear mode conversion and the nonlinear parametric decay instability of lower hybrid waves in tokamaks Jian Bao, Zhihong Lin, Animesh Kuley, Zhixuan Wang An electromagnetic fluid-kinetic model is developed to study the lower hybrid (LH) waves in tokamaks with low numerical noise, in which electron density is pushed forward by the continuity equation, and the kinetic markers are introduced for closure. A generalized weight-based particle-in-cell scheme is also applied to the simulation for the local high resolution in phase space. This new model has been successfully implemented into the global gyro-kinetic toroidal code (GTC), and the electromagnetic particle simulations of the LH waves have been carried out with a realistic electron-to-ion mass ratio. The simulation shows that toroidal effects induce an upshift of the parallel reflective index when LH waves propagate from the tokamak edge toward the core, which modifies the radial position for the mode conversion between slow and fast LH waves. The broadening of the poloidal spectrum of the wave-packet due to the wave diffraction is also observed in the simulation of LH wave propagation, and both the toroidal upshift and broadening effects of the wave-packet spectrum modify the parallel phase velocity and thus the linear absorption of LH waves by electrons through Landau resonance. In the nonlinear simulation, the LH wave can drive a net current during the propagation when its phase velocity gets closed to the local electron thermal speed. Finally, the parametric decay instability is observed when we increase the power of LH waves, in which a LH sideband and a low frequency ion plasma waves are generated. [Preview Abstract] |
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CP12.00092: MHD Stability and Constraints Tommaso Andreussi, Philip J. Morrison, Francesco Pegoraro Following up on previous work [1-3] we present examples of MHD equilibria for which we compare/contrast Lagrangian, Eulerian, and Dynamically Accessible stability. These kinds of stability differ by the constraints employed. Such constraints will be discussed, along with explicit examples including simple convective instability and cylindrical configurations with plasma flow.\\[4pt] [1] T. Andreussi, P.J. Morrison, and F. Pegoraro, {\it Plasma Phys. Control. Fusion}, {\bf 52}, 055001 (2010);\\[0pt] [2] T. Andreussi, P.J. Morrison, and F. Pegoraro, {\it Phys. Plasmas}, {\bf 19}, 052102 (2012); \\[0pt] [3] T. Andreussi, P.J. Morrison, and F. Pegoraro, {\it Phys. Plasmas}, {\bf 20}, 092104 (2013) [Preview Abstract] |
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CP12.00093: Studies of waves and instabilities using increased beta, warm ion plasmas in LAPD Troy Carter, Seth Dorfman, Walter Gekelman, Steve Vincena, Bart Van Compernolle, Shreekrishna Tripathi, Pat Pribyl, George Morales A new plasma source based on a Lanthanum Hexaboride (LAB$_6$) emissive cathode has been developed and installed on the LArge Plasma Device (LAPD) at UCLA. The new source provides a much higher discharge current density (compared to the standard LAPD Barium Oxide source) resulting in a factor of $\sim 50$ increase in plasma density and a factor of $\sim 2-3$ increase in electron temperature. Due to the increased density the ion-electron energy exchange time is shorter in the new plasma, resulting in warm ions (measured spectroscopically to be $\sim 5-6$eV, up from $\alt 1$eV in the standard source plasma). This increased pressure combined with lowered magnetic field provides access to magnetized plasmas with $\beta$ up to order unity. Topics under investigation include the physics of Alfv\'{e}n waves in increased $\beta$ plasmas (dispersion and kinetic damping on ions), electromagnetic effects and magnetic transport in drift-Alfv\'en wave turbulence, and the excitation of ion-temperature-anisotropy driven modes such as the mirror and firehose instabilities. The capabilities of the new source will be discussed along with initial experimental resuls on electromagnetic drift-Alfv\'{e}n wave turbulence and Alfv\'{e}n wave propagation with increased plasma $\beta$. [Preview Abstract] |
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CP12.00094: High-Beta Electromagnetic Turbulence in LAPD Plasmas G. Rossi, T.A. Carter, M.J. Pueschel, F. Jenko, D. Told, P.W. Terry The introduction of a new LaB6 cathode plasma source in the Large Plasma Device has enabled the study of pressure-gradient-driven turbulence and transport variations at significantly higher plasma $\beta$. Density fluctuations are observed to decrease with increasing $\beta$ while magnetic fluctuations increase. Furthermore, the perpendicular magnetic fluctuations are seen to saturate while parallel (compressional) magnetic fluctuations increase continuously with $\beta$. These observations are compared to linear and nonlinear simulations with the GENE code. The results are consistent with the linear excitation of a Gradient-driven Drift Coupling mode (GDC) which relies on grad-B drift due to parallel magnetic fluctuations and can be driven by density or temperature gradients. [Preview Abstract] |
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CP12.00095: Laboratory Studies of Nonlinear Alfv\'{e}n Interactions and Decay Instabilities S. Dorfman, T. Carter, S. Vincena, P. Pribyl, G. Rossi, R. Sydora, Y. Lin Alfv\'{e}n waves, a fundamental mode of magnetized plasmas, are ubiquitous in lab and space. The non-linear behavior of these modes is thought to play a key role in important problems such as the heating of the solar corona, solar wind turbulence, and Alfv\'{e}n eigenmodes in tokamaks. In particular, theoretical predictions show that these Alfv\'{e}n waves may be unstable to various decay instabilities, even at low amplitudes ($\delta B/B < 10^{-3}$). The present work, conducted at UCLA's Large Plasma Device, represents the first fundamental laboratory study of the non-linear Alfv\'{e}n wave interactions responsible for Alfv\'{e}n wave decay instabilities. Experiments include the first laboratory observation of the Alfv\'{e}n-acoustic mode coupling at the heart of the Parametric Decay Instability [1]. More recently, efforts have focused on the non-linear decay of a KAW into daughter modes with frequencies and wave numbers that suggest co-propagating KAWs. The observed process is parametric, with the frequency of the daughter modes varying as a function of pump amplitude. Efforts are underway to fully characterize this set of experiments and compare with decay instabilities predicted by theory and simulations. \\[4pt] [1] S Dorfman and T Carter, Phys. Rev. Lett. 110, 195001 (2013). [Preview Abstract] |
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CP12.00096: Evidence of a New Instability in Gyrokinetic Simulations of LAPD Plasmas P.W. Terry, M.J. Pueschel, G. Rossi, F. Jenko, D. Told, T.A. Carter Recent experiments at the LArge Plasma Device (LAPD) have focused on structure formation driven by density and temperature gradients. A central difference relative to typical, tokamak-like plasmas stems from the linear geometry and absence of background magnetic shear. At sufficiently high $\beta$, strong excitation of parallel (compressional) magnetic fluctuations was observed. Here, linear and nonlinear simulations with the \textsc{Gene} code are used to demonstrate that these findings can be explained through the linear excitation of a Gradient-driven Drift Coupling mode (GDC). This recently-discovered instability, unlike other drift waves, relies on the grad-B drift due to parallel magnetic fluctuations in lieu of a parallel electron response, and can be driven by density or temperature gradients [M.J.~Pueschel et al., Phys.~Plasmas 22, 062105 (2015)]. The linear properties of the GDC for LAPD parameters are studied in detail, and the corresponding turbulence is investigated. It is found that, despite the very large collisionality in the experiment, many properties are recovered fairly well in the simulations. In addition to confirming the existence of the GDC, this opens up interesting questions regarding GDC activity in astrophysical and space plasmas. [Preview Abstract] |
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CP12.00097: Laboratory Investigation of the Electromagnetic Electron-Ion Hybrid Instability C. Lon Enloe, Erik Tejero, Bill Amatucci, Chris Crabtree, Guru Ganguli The electromagnetic to electrostatic transition of the electron-ion hybrid instability is currently being studied in the Space Physics Simulation Chamber at NRL. It has been shown by theory that strong gradients in plasma flows perpendicular to the magnetic field can drive electromagnetic waves in the whistler branch. Velocity-sheared flows of this type may naturally arise in the boundary layer between plasmas of different characteristics, such as in the plasma sheet in the Earth's magnetosphere and laser produced plasma expansions across a magnetic field. When the wave vector normalized to the electron skin depth is much larger than 1, the waves are predominantly electrostatic in character and electromagnetic otherwise. These waves are eigenmodes in the direction of the velocity shear and demonstrate a dramatic increase in width after transitioning to an electromagnetic wave. Results from recent experiments will be presented in which this transition is observed. [Preview Abstract] |
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CP12.00098: Landau-fluid closure and drift-wave dispersion relations for arbitrary collisional plasmas Wonjae Lee, M.V. Umansky, J.R. Angus, M.A. Dorf, R.H. Cohen, M.R. Dorr, S.I. Krasheninnikov The Landau fluid model has been revisited to describe drift-wave instabilities in edge plasmas where the plasma parameters can vary by an order of magnitude or more. Usually, simple fluid models without Landau-fluid closure have been used to describe edge plasma dynamics. However, the collisionality conditions for the simple fluid descriptions are only marginally satisfied in present-day tokamaks and the validity conditions for such models will not be satisfied for future devices. As a result, the simple fluid models without Landau closure cannot properly describe the electron kinetic effects (e.g. the wave-electron resonances) in weakly collisional plasmas. We compare the analytical growth rates of drift-wave instabilities from the electromagnetic Landau-fluid model and the electromagnetic drift-kinetic model by conducting linear analysis on both models in various plasma parameters. Consequently, we demonstrate that both the electromagnetic Landau-fluid model and the electromagnetic drift-kinetic model, which yield similar linear growth rates, can be used to describe drift wave turbulence in a wide range of plasma parameters. We also present comparative simulations of drift wave instability using BOUT++ and COGENT(M. Dorf, invited talk, this meeting). [Preview Abstract] |
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CP12.00099: Simulation and Theoretical Study of Spontaneous Excitation of Convective Cells by Kinetic Alfven Waves Yu Lin, Fulvio Zonca, Liu Chen It has been recently demonstrated that, generally, electrostatic (ES) and magnetostatic (MS) convective cells (CCs), or zonal flows, can be excited simultaneously by kinetic Alfven waves (KAWs) [1]. In this paper, spontaneous excitations of electrostatic as well as magnetostatic convective cells by KAWs are investigated through hybrid simulations, and the results are compared with the analytical theory based on the nonlinear gyrokinetic equations. In the hybrid simulation, ions are treated as fully kinetic particles, and electrons are treated as a massless fluid. It is found that finite ion-Larmor-radius (FILR) effects play a crucial. Furthermore, ES and MS convective cells are intrinsically coupled and must be treated on an equal footing. Excellent agreement is obtained for mode structure and generation rate of convective cells by KAW, demonstrating that ESCC and MSCC are indeed coupled, and that spontaneous CC excitation is suppressed at long wavelength, showing the crucial destabilizing role of FILR effects in the excitation via modulational instabilities.\\[4pt] [1] F. Zonca and L. Chen, ``Spontaneous excitation of convective cells by kinetic Alfven waves.'' Presented at the 2014 International Sherwood Fusion Theory Conference, San Diego, California, March, 2014. [Preview Abstract] |
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CP12.00100: Ion acoustic wave collapse via two-ion wave decay: 2D Vlasov simulation and theory Thomas Chapman, Richard Berger, Jeffrey Banks, Stephan Brunner The decay of ion acoustic waves (IAWs) via two-ion wave decay may transfer energy from the electric field of the IAWs to the particles, resulting in a significant heating of resonant particles. This process has previously been shown in numerical simulations to decrease the plasma reflectivity due to stimulated Brillouin scattering. Two-ion wave decay is a fundamental property of ion acoustic waves that occurs over most if not all of the parameter space of relevance to inertial confinement fusion experiments, and can lead to a sudden collapse of IAWs. The treatment of all species kinetically, and in particular the electrons, is required to describe the decay process correctly. We present fully kinetic 2D+2V Vlasov simulations of IAWs undergoing decay to a highly nonlinear turbulent state using the code LOKI. The scaling of the decay rate with characteristic plasma parameters and wave amplitude is shown. A new theory describing two-ion wave decay in 2D, that incorporates key kinetic properties of the electrons, is presented and used to explain quantitatively for the first time the observed decay of IAWs. [Preview Abstract] |
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CP12.00101: Breakdown of modulational approximations in multimode nonlinear interactions Felipe Rizzato, Paulo Iorra, Samuel Marini, Eduardo Peter, Renato Pakter, Abraham Chavez The present work investigates the breakdown of the modulational approximation in a multimode extension of the three wave (triplet) nonlinear interaction. The modulational approach is accurate when the nonlinear wave coupling is weak, which causes amplitudes and phases to evolve slowly in time. We examine the types of dynamics arising when the coupling rises from very small to large values. At small couplings, when the modulational approach is valid, amplitude excursions are small and energy remains confined to the most unstable triplet subset of the multimode system. Above a critical value of the coupling, amplitude excursions abruptly become much larger and energy distributes more evenly among the active modes. Estimates for the critical coupling and relaxation times can be obtained with proper analysis of the most unstable triplet.\footnote{Iorra et al, Physica A {\bf 436}, 686 (2015).} [Preview Abstract] |
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CP12.00102: KEEN and KEEPN wave simulations from 2D to 4D Michel Mehrenberger, Bedros Afeyan, David Larson, Nicolas Crouseilles, Fernando Casas, Erwan Faou, Adila Dodhy, Eric Sonnendrucker, Magdi Shoucri We show for well-driven KEEN (Kinetic Electrostatic Electron Nonlinear) waves and their analogs in pair plasmas KEEPN (Positron) waves, how the dynamics is captured in a variety of complimentary numerical approaches. Symplectic integration and quadrature node based techniques are deployed to achieve satisfactory results in the long time evolution of highly nonlinear, kinetic, non-stationary, self-organized structures in phase space. Fixed and composite velocity grid arbitrary-order interpolation approaches have advantages we highlight. Adaptivity to local phase space density morphological structures will be discussed starting within the framework of the Shape Function Kinetics (SFK) approach. Fine resolution in velocity only in the range affected by KEEN waves makes for more efficient simulations, especially in higher dimensions. We explore the parameter space of unequal electron and positron temperatures as well as the effects of a relative drift velocity in their initial conditions. Ponderomotively driven KEEPN waves have many novelties when compared to KEEN waves, such as double, staggered, vortex structures, which we highlight. [Preview Abstract] |
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CP12.00103: Nonlinear frequency shift of electrostatic waves in general collisionless plasma: unifying theory of fluid and kinetic nonlinearities Ilya Y. Dodin, Chang Liu The nonlinear frequency shift is derived in a transparent asymptotic form for intense dissipationless Langmuir waves in general collisionless plasma (arXiv:1505.03498). The formula describes both fluid and kinetic effects simultaneously. The fluid nonlinearity is expressed, for the first time, through the plasma dielectric function, and the kinetic nonlinearity accounts for both smooth distributions and trapped-particle beams. Various known limiting scalings are reproduced as special cases. The calculation avoids differential equations and can be extended straightforwardly to other nonlinear plasma waves. [Preview Abstract] |
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CP12.00104: Lagrangian geometrical optics of classical vector waves and particles with spin D.E. Ruiz, I.Y. Dodin Linear vector waves, both quantum and classical, experience polarization-driven bending of ray trajectories and polarization dynamics that can be interpreted as the precession of the ``wave spin.'' In this work, we present a universal Lagrangian theory that describes these effects by extending the geometrical-optics approximation to small but nonvanishing $\lambda/\ell$, where $\lambda$ is the wavelength, and $\ell$ is the characteristic inhomogeneity scale (arXiv:1503.07829; arXiv:1503.07819). When applied to classical waves, this theory correctly predicts, for example, the difference between the polarization-driven bending of left- and right-polarized electromagnetic wave rays in isotropic media (arXiv:1507.05863). When applied to quantum waves, the same general theory yields a Lagrangian point-particle model for the Dirac electron, i.e. the relativistic spin-1/2 particle. The model captures both the Bargmann-Michel-Telegdi spin precession theory and the Stern-Gerlach spin-orbital coupling theory. Moreover, we present, for the first time, a calculation of the fully relativistic ponderomotive Hamiltonian for a Dirac electron in a vacuum laser field. This Hamiltonian captures not only the usual relativistic mass shift but also spin effects. [Preview Abstract] |
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CP12.00105: Small scale instabilities and anomalous electron current in Hall plasmas A. Smolyakov, I. Romadanov, W. Frias, J. Carlsson, I. Kaganovich, Y. Raitses Small scale instabilities in Hall plasmas with ExB drift of magnetized electrons and non-magnetized ions are investigated analytically and in numerical simulations. Interrelation of various branches such as collisionless Simon-Hoh, low hybrid and electron cyclotron modes are investigated in local and nonlocal regimes. It is shown that long wavelength Simon-Hoh instability driven by plasma gradients smoothly connects to the low hybrid instability with a cut-off at the wavelength of the electron Larmor radius. It is shown that the electron-neutral collisions are destabilizing at small scale but reduce the growth rate at moderate values of the wavelengths. The electron cyclotron mode is compared against the low hybrid mode operating in the same range of the frequencies and wave vectors. Relative contribution of various modes into electron transport is studied in nonlinear fluid and Particle-in-Cell simulations. [Preview Abstract] |
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CP12.00106: A TWT upgrade to study wave-particle interactions in plasma Fabrice Doveil, Meirielen Caetano de Sousa, Didier Guyomarc'h, Aissa Kahli, Yves Elskens Beside industrial applications, Traveling Wave Tubes (TWT) are useful to mimic and study wave-particle interaction in plasma [1-3]. We upgraded a TWT, whose slow wave structure is a 4 m long helix (diameter 3.4 cm, pitch 1 mm) of Be-Cu wire (diameter 0.6 mm) wrapped in insulating tape. The helix is inserted in a vacuum glass tube. At one end, an electron gun produces a beam propagating along the helix, radially confined by a constant axial magnetic field. Movable probes, capacitively coupled to the helix through the glass tube, launch and monitor waves generated by an arbitrary waveform generator at a few tens of MHz. At the other end of the helix, a trochoidal analyzer allows to reconstruct the electron distribution functions of the beam after its self-consistent interaction with the waves. Linear properties of the new device will be reported. The measured coupling coefficients of each probe with the helix are used to reconstruct the growth and saturation of a launched wave as it interacts with the electron beam.\\[4pt] [1] G. Dimonte {\&} J.H. Malmberg, Phys. Fluids 21, 1188 (1978).\\[0pt] [2] S.I. Tsunoda, F. Doveil {\&} J.H. Malmberg, Phys. Rev. Lett. 58, 1112 (1987).\\[0pt] [3] F. Doveil, A. Macor {\&} A. A\"{\i}ssi, Celest. Mech. Dyn. Astr. 102, 255 (2008). [Preview Abstract] |
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CP12.00107: Interaction of plasma oscillations with a background ion density perturbation Sayak Bose, Manjit Kaur, P.K. Chattopadhyay, J. Ghosh, Y.C. Saxena In a quasineutral plasma, electrons undergo collective oscillations, known as plasma oscillations, when perturbed locally. The oscillations propagate due to finite temperature effects. However, the wave loses its coherence in an inhomogeneous plasma (phase mixing) because of the dependence of plasma oscillation frequency on plasma density [1]. Phase mixing is characterized by transfer of energy from wave to particles. For detailed experimental investigation of the above mentioned phenomena a new device, Inverse Mirror Plasma Experimental Device (IMPED) [2], has been designed and fabricated. The machine produces uniform plasma, $L_{uniform}\sim120\; cm$, with quiescence, $\delta n / n\sim 0.2\%$, in argon at filling pressure of $\sim10^{-4}\; mbar$ and axial magnetic field of $B_{main}\sim900\;G$. Plasma oscillations excited in the presence of a background ion density perturbation clearly showed that the power in the coherent plasma oscillations decreased significantly with the increase in the amplitude of the ion density perturbation. The experimental details and results are presented.\\[4pt] [1] J. Dawson Phys. Rev. 113, 383 (1959).\\[0pt] [2] Bose \textit{et al.} J. Plasma. Phys. 81, 345810203 (2015). [Preview Abstract] |
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CP12.00108: Simulations of Electron Density Perturbations in a Gas Discharge James Caplinger, Vladimir Sotnikov, Daniel Main Beginning with the idealized case of the Pierce diode, a series of particle-in-cell (PIC) simulations are conducted in order to characterize density perturbations in a laboratory gas discharge. This work is conducted to support future experimental investigations into electromagnetic scattering off of electron density perturbations excited by plasma flows. As a first step, 2D PIC simulations were conducted for the Pierce diode case, which is a simple model that exploits instabilities of a monochromatic electron beam between two grounded electrodes. These results were compared to the standard analytical solution. Departing from this idealized case we will include in the simulations electron-neutral collisions, particle creation from ionization, as well as an electric field generated by biased electrodes. A parameter study of electric field strength and collision frequency will be performed for values approaching the Pierce diode as well as extending to cases of expected laboratory parameters. If we can extract physical density spectra from simulations with parameters approaching experimental values, it may be possible to analyze electromagnetic scattering characteristics. [Preview Abstract] |
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CP12.00109: Ion thermal and dispersion effects in Farley-Buneman instabilities Sandeep Litt, Andrei Smolyakov, Ehab Hassan, Wendell Horton Farley Buneman instability is most commonly observed in the collisional part of ionospheric E-layer and solar chromosphere. Despite high collisionality, the kinetic effects associated with finite temperature are important for determination of the mode frequencies and growth rate, especially for largely unmagnetized ion component. The kinetic theory offers a comprehensive tool for studies of thermal effects but remains to be a challenge even for modern computers. Alternatively, we develop an extended ion fluid model that incorporates ion thermal and kinetic effects via the linear closures for higher order moments. The ion thermal effects on dynamics of FB type modes are investigated in the short wavelength region using the first and second order closure and the full kinetic response. It is shown that the ion thermal effects are primarily reasons for mode cutoff at shorter wavelength and FB instability is limited by the finite range of wavevectors. The proposed fluid like equations with closures could be useful alternative for the analysis of weakly driven situations, in contrast to the PIC simulations which can handle strongly driven cases but are noisy near the marginal stability boundary. Our results also indicate that the mode growth rate is a nonmonotonic function of the wave vector and also depends on the collisionality. The critical phase velocity (or threshold) for the unstable modes is shown to be modified due to the ion thermal effects. [Preview Abstract] |
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CP12.00110: Characterization of Ion-Acoustic Wave Reflection Off A Plasma Chamber Wall Jorge Berumen, Feng Chu, Ryan Hood, Sean Mattingly, Anthony Rogers, Fred Skiff We present an experimental characterization of the ion acoustic wave reflection coefficient off a plasma chamber wall. The experiment is performed in a cylindrical, magnetized, singly-ionized Argon inductively-coupled gas discharge plasma that is weakly collisional with typical conditions: n $\sim$ 10$^{10}$cm$^{-3}$ T$_{\mathrm{e}}$ $\sim$ 3 eV and B $\sim$ 1 kG. The main diagnostics are laser-induced fluorescence and Langmuir probe measurements. A survey of the ion velocity distribution function's zeroth and first order as well as density fluctuations at different wave excitation frequencies is obtained. Analysis of the reflection coefficient's dependence on the phase velocity and frequency of the wave is done through the characterization of waves utilizing Case-Van Kampen modes and the use of Morrison's G-transform [1].\\[4pt] [1] F. Skiff, H. Gunell, C.S. Ng A. Bhattacharjee, and W.A. Noonan. Electrostatic Degrees of Freedom in Non-Maxwellian Plasma. Physics of Plasmas, 9(5):1931, 2002. [Preview Abstract] |
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CP12.00111: Radiation patterns and reciprocity of whistler mode antennas J. Manuel Urrutia, Reiner Stenzel Whistler modes can be excited and received with magnetic loop antennas. The radiation pattern has been measured in a large laboratory plasma for a low frequency whistler mode ($\omega \simeq 0.3 \omega_c \ll \omega_p$). The difference in the radiation patterns for group and phase velocities is shown and discussed. Plane waves have been generated using antenna arrays. These are used to measure the antenna patterns of receiving antennas which are usually different. Examples are small loops which radiate along the resonance cone but receive all waves within the resonance cone. The reciprocity of antennas has been investigated. Directional antennas and phased array antennas are not reciprocal. A relative motion between an antenna and a plasma modifies transmitting and receiving properties. When a loop antenna moves rapidly across the dc magnetic field a continuous wave of the source excites wave packets in the form of a whistler wing in the stationary plasma. Moving receiving antennas are subject to frequency shifts by the convective derivative such as Doppler shifts. Motion violates reciprocity, e.g. radiation cannot be received from a downstream source, but transmitted to a downstream receiver. These results are of interest to space and laboratory plasmas. [Preview Abstract] |
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CP12.00112: Antenna arrays for producing plane whistler waves Reiner Stenzel, J. Manuel Urrutia In a large uniform laboratory plasma helicon modes with mode numbers $m=1-8$ have been excited. Using a circular phased array it is shown that positive and negative modes can propagate equally well. The phase fronts of helicons form Archimedian screw surfaces. The electromagnetic field carries linear momentum due to the axial propagation and angular momentum due to the azimuthal propagation. Associated with the orbital angular momentum is a transverse Doppler shift. It is demonstrated that a rapidly rotating ``receiver'' observes a different frequency than the wave. This implies that a rotating electron can undergo cyclotron resonance when moving against the field rotation. Analogous to the axial Doppler shift cyclotron damping and cyclotron instabilities are possible due to the field rotation in helicons. Since helicons exist in unbounded laboratory plasma they should also exist in space plasmas. The angular wave-particle interaction may be an alternate approach for the remedial of energetic electrons. [Preview Abstract] |
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CP12.00113: Vlasov Simulation of the Effects of Collisions on the Damping of Electron Plasma Waves Jeff Banks, Richard Berger, Thomas Chapman, Stephan Brunner, T. Tran Kinetic simulation of two dimensional plasma waves through direct discretization of the Vlasov equation may be particularly attractive for situations where minimal numerical fluctuation levels are desired, such as when measuring growth rates of plasma wave instabilities. In many cases collisional effects can be important to the evolution of plasma waves because they both set a minimum damping rate for plasma waves and can scatter particles out of resonance through pitch angle scattering. Here we present Vlasov simulations of evolving electron plasma waves (EPWs) in plasmas of varying collisionality. We consider first the effects of electron-ion pitch angle collisions on the frequency and damping, Landau and collisional, of small-amplitude EPWs for a range of collision rates. In addition, the wave phase velocities are extracted from the simulation results and compared with theory. For this study we use the Eulerian-based kinetic code LOKI that evolves the Vlasov-Poisson system in 2+2-dimensional phase space. We then discuss extensions of the collision operator to include thermalization. Discretization of these collision operators using 4th order accurate conservative finite-differencing will be discussed. [Preview Abstract] |
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CP12.00114: Density Waves in Systems of Non-Interacting Particles E.J. Kolmes, V.I. Geyko, N.J. Fisch Under certain conditions, systems of non-interacting particles can give rise to density waves. In general, these waves do not require any particular perturbations in the initial density or velocity distributions, but they do tend to be strongly dependent on the boundary conditions of the system; one of the simplest examples is a collection of non-interacting particles bouncing in a constant gravitational field. A wide variety of different potentials can produce density waves, which change in both shape and behavior as the potential changes. We examine the structure and origin of these waves numerically and analytically. We also analyze the sensitivity of these waves to changes in different parameters of the system, including the effects of interparticle interactions on these structures. Strong interparticle interactions tend to disrupt the structure that develops in the non-interacting-particle case. We discuss possible experimental consequences of these phenomena. [Preview Abstract] |
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CP12.00115: SHOCK WAVE AND DISCONTINUITY |
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CP12.00116: Hybrid Simulations and Scaling Laws for Shock Formation in the UCLA Collisionless Shock Experiment David Larson, Dan Winske, Misa Cowee, S. Eric Clark, Christoph Niemann, Stephen Brecht Two- and three-dimensional simulations are used to compare and contrast the plasma expansion, formation of a magnetic cavity, and generation of an outgoing shock wave for conditions relevant to the laser experiment at UCLA, as a function of the background ion mass. A model of the shock formation process is constructed that yields an expression for the speed of the shock, which we show is in good agreement with the simulations. In addition, the criteria for generating strongly-coupled shocks are derived and simulations are used to examine the velocity scaling obtained via momentum conservation. [Preview Abstract] |
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CP12.00117: Effects of trapped electrons on ion reflection in an oblique shock wave Mieko Toida, Junya Inagaki A magnetosonic shock wave propagating obliquely to the external magnetic field can trap some electrons and accelerate them to ultrarelativistic energies [1]. These electrons significantly influence electromagnetic fields near the shock front. In the 2D simulation, the trapped electrons excite whistler-wave instabilities. As a result of nonlinear development of the instabilities, 2D electromagnetic fluctuations along the shock front grow to large amplitudes [2]. We studied effects of trapped electrons on ion motions in an oblique shock wave [3]. We analytically derived the condition for ions to be reflected from the shock front. It was predicted that the fraction of reflected ions is enhanced by the 2D electromagnetic fluctuations excited by trapped electrons. This prediction was confirmed by 2D electromagnetic particle simulations with full ion and electron dynamics and calculation of test ions in the electromagnetic fields averaged along the shock front. A comparison between 2D and 1D electromagnetic particle simulations is also shown. \\[4pt] [1] N. Bessho and Y. Ohsawa, Phys. Plasmas, {\bf 6} (1999) 3076.\\[0pt] [2] K. Shikii and M. Toida, Phys. Plasmas, {\bf 17} (2010) 082316.\\[0pt] [3] M. Toida and J. Inagaki, Phys. Plasmas, {\bf 22} (2015) 062305. [Preview Abstract] |
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CP12.00118: Numerical calculation of ion runaway distributions Sarah Newton, Ola Embr\'eus, Adam Stahl, Eero Hirvijoki, T\"unde F\"ul\"op Ion acceleration by electric fields is of interest in many plasma scenarios. Limitations of analytic descriptions prevent their general use in following the evolution of such ``runaway ion'' populations. Therefore we have implemented an initial value solver, CODION, for the linearized ion drift kinetic equation, with a non-relativistic Fokker-Planck collision operator. A spectral-Eulerian discretization scheme is used for 2D velocity space. The background plasma is taken to be homogeneous and static, with arbitrary composition. We demonstrate the use of the numerical distribution function to study ion acceleration in solar flares and tokamak plasmas. The variation of the strength and duration of the electric field required to produce a significant fast ion population is illustrated. Low frequency magnetic activity, indicative of toroidal Alfv\'{e}n eigenmode excitation, has been observed during tokamak disruptions. Taking typical disruption parameters, we show that accelerated bulk ions are unlikely to reach a sufficient velocity to provide the resonant drive. [Preview Abstract] |
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CP12.00119: Collision of an Arched Plasma-Filled Flux Rope with a Target Cloud of Initially Neutral Gas Pakorn Wongwaitayakornkul, Paul M. Bellan The Caltech solar loop experiment apparatus [1] had been used to create an arched plasma-filled flux rope that expands to collide with a pre-injected initially-neutral gas. We investigated such a situation in two regimes: (i) plasma made by heavy gas impacting a much lighter neutral gas cloud and (ii) a light-gas plasma impacting much heavier neutral gas. The neutral gas became ionized immediately upon impact. In regime (i), multiple shock layers were formed in the target cloud; these magnetized collisionless shocks are relevant to solar physics as such shocks develop ahead of Coronal Mass Ejections and occur in Co-rotating Interaction Regions. In regime (ii), plasma expansion was inhibited. In both cases, fast camera images, magnetic probe measurements, and spectroscopy data will be reported. The analysis of plasma and shock expansion, as well as associated density and temperature changes, will be presented.\\[4pt] [1] Stenson, E. V. \& Bellan, P. M. 2012, Physical Review Letters, 109, 075001 (2012) [Preview Abstract] |
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CP12.00120: Overview and recent results of the Magnetized Shock Experiment (MSX) T.E. Weber, R.J. Smith, S.C. Hsu, Y. Omelchenko Recent machine and diagnostics upgrades to the Magnetized Shock Experiment (MSX) at LANL have enabled unprecedented access to the physical processes arising from stagnating magnetized ($\beta \approx $ 1), collisionless, highly supersonic ($M, M_{A}\approx $ 10) flows, similar in dimensionless parameters to those found in both space and astrophysical shocks. Hot (100s of eV during translation), dense (10$^{22}$ -- 10$^{23}$ m$^{-3})$ Field Reversed Configuration (FRC) plasmoids are accelerated to high velocities (100s of km/s) and subsequently impact against a static target such as a strong parallel or anti-parallel (reconnection-wise) magnetic mirror, a solid obstacle, or neutral gas cloud to recreate the physics of interest with characteristic length and time scales that are both large enough to observe yet small enough to fit within the experiment. Long-lived (\textgreater 50 $\mu $s) stagnated plasmas with density enhancement much greater than predicted by fluid theory (\textgreater 4x) are observed, accompanied by discontinuous plasma structures indicating shocks and jetting (visible emission and interferometry) and copious \textgreater 1 keV x-ray emission. An overview of the experimental program will be presented, including machine design and capabilities, diagnostics, and an examination of the physical processes that occur during stagnation against a variety of targets. [Preview Abstract] |
(Author Not Attending)
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CP12.00121: Conditions for collisionless shocks formation in magnetized plasma interaction with kinetic-scale obstacles F. Cruz, E.P. Alves, R.A. Fonseca, L.O. Silva, R.A. Bamford, R. Bingham The interaction between plasmas and kinetic-scale magnetic obstacles is a problem of interest in space and laboratory plasmas. In general, this interaction is purely three dimensional, highly nonlinear and happens over a wide range of plasma/obstacle parameters. The complexity of the problem limits the development of analytical models and requires the use of computer simulations. In this work, we model the interaction between a magnetized plasma colliding with a small-scale dipolar magnetic obstacle from first principles using multidimensional PIC simulations, with the aim of determining under which conditions a shock can be formed. We identify that the global system behavior can be determined by the flow Alfv\`{e}nic Mach number M$_{\mathrm{A}}$ and the ratio between the effective obstacle size and the ion gyroradius measured in the upstream plasma conditions, L/$\rho _{\mathrm{i}}$. We determine that a shock is critically formed for L/$\rho _{\mathrm{i}}$\textgreater 1 and show that this is a~very restrictive condition on the maximum possible shock M$_{\mathrm{A}}$ for small obstacle sizes. We describe the magnetopause and shock dynamics of different 2D planes and compare the results with full 3D simulations. We also identify the optimal parameter regimes to explore this physics in the laboratory. [Preview Abstract] |
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CP12.00122: Initial Results from 3D Electric and Magnetic Field Measurements of the Interaction of a Laser-Produced and Ambient Plasma P.V. Heuer, D.B. Schaeffer, L.R. Hofer, C.G. Constantin, A.S. Bondarenko, E.T. Everson, S.E. Clark, W. Gekelman, C. Niemann Utilizing high-repetition lasers combined with a high-repetition ambient plasma allows for detailed 3D scans of the interaction of the laser-produced and ambient plasmas. We present the first results from experiments combining a newly-commissioned high-repetition (1 Hz) laser with the 1 Hz ambient plasma of the Large Plasma Device (LAPD) at the University of California, Los Angeles. The laser (20 J, 14 ns) was focused on a cylindrical plastic target embedded in the ambient LAPD plasma, resulting in an ablated debris-plasma that expanded perpendicular to the background magnetic field. The debris-ambient plasma interaction was studied with 3-axis magnetic flux probes, mounted on a 3D motion drive for detailed, high-resolution planar scans both along and perpendicular to the background field. Measurements were also taken using filtered fast-gate (ns) imaging, emissive Langmuir probes, and emissive spectroscopy. The results show that the debris ions are de-energized inside the diamagnetic cavity, while the ambient ions are accelerated through laminar electric fields. [Preview Abstract] |
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CP12.00123: Electromagnetic Fields of a Laboratory Axial Plasma Jet Stephen Vincena, Jeffrey Bonde, Walter Gekelman Measurement are presented of the time-varying electromagnetic fields of a plasma jet directed along the background magnetic field in an ambient plasma. The jet is formed by irradiating a solid carbon target at $\sim 1 \times 10^{10}$W/cm$^{2}$ suspended in a cylindrical argon plasma ($B_{0}=750$G, $n_{e}=5\times 10^{12}$/cm$^{3}$) so that the parallel expansion velocity matches the ambient Alfven speed. The experiments are conducted in the Large Plasma Device (LAPD) which operates at a 1 Hz cadence (matching the laser-target firing) and allows the collection of ensemble datasets. Measurements are made in two orthogonal planes that intersect the diamagnetic cavity formed by the laser-produced plasma jet. Three-axis magnetic induction coils as well as a novel emissive probe reveal the total electric field: $E = -\nabla V_{p} - \partial_{t} A$. The measured symmetry in the $xy$ plane is exploited to form a cylidrically symmetric reconstruction of the dynamic, three-dimensional process. [Preview Abstract] |
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CP12.00124: Three Dimensional Hybrid Simulations of Super-Alfv\'enic Laser Ablation Experiments in the Large Plasma Device Stephen Clark, Derek Schaeffer, Anton Bondarenko, Erik Everson, Dan Winske, Carmen Constantin, Christoph Niemann We present 3D hybrid simulations of laser produced expanding debris clouds propagating though a magnetized ambient plasma in the context of magnetized collisionless shocks. New results from the 3D code are compared to previously obtained simulation results using a 2D hybrid code. The 3D code is an extension of a previously developed 2D code developed at Los Alamos National Laboratory. The new simulations are used to verify scaling relationships, such as shock onset time and coupling parameter ($R_m/\rho_d$), developed via 2D simulations. Previous 2D results focus primarily on laboratory shock formation relevant to experiments being performed on the Large Plasma Device, where the shock propagates across the magnetic field. The new 3D simulations show wave structure and dynamics oblique to the magnetic field that introduce new physics to be considered in future experiments. [Preview Abstract] |
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CP12.00125: Diagnostic Progress and Results on the Magnetized Shock Experiment R.J. Smith, T.E. Weber The Magnetized Shock Experiment (MSX) at LANL is reliably producing Field Reversed Configuration (FRC) plasmas spanning peak densities of $\sim$ 10$^{21-23}$ m$^{-3}$, combined Te$+$Ti of 10s-500eV and velocities of 100-300km/s as a means to producing a laboratory supercritical collision-less shock. Visible light images showing discontinuities indicative of shocks and jetting have been obtained on various targets: co-solenoid B field, a metal wall and counter-solenoidal B fields (FRC capture and reconnection). Two chord interferometry, external and internal magnetic probing are routinely employed and x-ray diagnostic capability has recently been added. The pulsed polarimetry technique is being deployed which can measure the local magnetic field using Lidar Thomson scattering. In addition, a fiber optic version of pulsed polarimetry using a new specialty fiber that enhances fiber backscatter with Fiber Bragg Gratings is being developed. Magnetic fields of order $\sim$ 1T have been measured, however a new modified shock chamber geometry and recent machine modifications enabling operation at increased $\theta $-coil voltage are expected to improve translation speed and hence stagnation pressures. Progress on these diagnostics and results will be presented. [Preview Abstract] |
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CP12.00126: Z-PINCH, X-PINCH, EXPLODING WIRE PLASMA AND DENSE PLASMA FOCUS/MEASUREMENTS AND DIAGNOSTIC TECHNIQUES |
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CP12.00127: A Multiple Z-Pinch Configuration for the Generation of High-Density, Magnetized Plasmas Alfonso G. Tarditi The z-pinch is arguably the most straightforward and economical approach for the generation and confinement of hot plasmas, with a long history of theoretical investigations and experimental developments. While most of the past studies were focused on countering the natural tendency of z-pinches to develop instabilities, this study attempts to take advantage of those unstable regimes to form a quasi-stable plasma, with higher density and temperature, possibly of interest for a fusion reactor concept. For this purpose, a configuration with four z-pinch discharges, with axis parallel to each other and symmetrically positioned, is considered. Electrodes for the generation of the discharges and magnetic coils are arranged to favor the formation of concave discharge patterns. The mutual attraction from the co-streaming discharge currents enhances this pattern, leading to bent plasma streams, all nearing towards the axis. This configuration is intended to excite and sustain a ``kink'' unstable mode for each z-pinch, eventually producing either plasmoid structures, detached from each discharge, or sustained kink patterns: both these cases appear to lead to plasmas merging in the central region. The feasibility of this approach in creating a higher density, hotter, meta-stable plasma regime is investigated computationally, addressing both the kink excitation phase and the dynamics of the converging plasma columns. [Preview Abstract] |
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CP12.00128: Kr gas puff implosion experiments on the Z generator David Ampleford, Christopher Jennings, Stephanie Hansen, Adam Harvey-Thompson, Gregory Rochau, Derek Lamppa, Brent Jones, Arati Dasgupta, John Giuliani, J. Ward Thornhill We discuss experiments imploding large diameter Kr gas puffs on the Z generator. Thermalization of kinetic energy leads to high pinch temperatures; the plasma conditions achieved are conducive to 13-keV K-shell emission from Kr. By tailoring the density profile and designing experiments using hydrodynamic gas flow modeling coupled to MHD modeling [C.A. Jennings \emph{et al.}, Phys. Plasmas 22, 056316 (2015)] we are able to implode these gas puffs at high velocities ($>100 cm/\mu s$) from 12-cm initial diameters to a tight ($\sim$ 1 mm diameter) uniform stagnated pinch. Data indicates that changes to the initial density profile affect the implosion stability and significantly affect the radiated output, with the most stable implosion radiating $\sim$ 8 kJ at $>$10 keV, the majority of which is radiated in the Kr He$\alpha$ line. In this poster we will compare an extensive suite of yield, spectral, imaging and pulse shape diagnostics to MHD modeling, and discuss the plasma conditions inferred from comparing data to atomic modeling. [Preview Abstract] |
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CP12.00129: Operation of Two-stage Wire Array Z-pinches on the Magpie Generator Jian Wu, Guy Burdiak, S. Lebedev, A. Harvey-Thompson, G. Hall, G. Swadling, F. Suzuki-Vidal, S. Bland, L. Suttle, E. Waisman, G. Wang, Q. Yang We describe the operation of two-stage wire array z-pinches driven by the 1.4MA, 240ns Magpie generator at Imperial College. In this setup, an inverse wire array acts as a fast current switch, delivering a 20ns, 5kA current pre-pulse into a cylindrical load array, before rapidly switching the majority of the generator current into the load after a 100ns dwell time. Measurements of load resistivity and energy deposition during the pre-pulse suggest significant bulk heating of the array mass occurs, leaving it in a mixed liquid-vapour state. Preconditioning of the load dramatically alters the ensuing implosion dynamics; the ablation phase is eliminated, together with trailing mass during the final implosion. The main current switch occurs as the inverse array explodes and plasma expands into the load region. Electrical and imaging diagnostics indicate that the main current switch may evolve as a plasma flow switch, driven by the expansion of a magnetic cavity along the length of the load array. Analysis of implosion trajectories suggests that approximately 1MA switches into the load in 100ns, corresponding to a doubling of the generator dI/dt. Attempts to measure the current profile throughout the current switch will be presented. In addition, we present results from preconditioned x-pinch experiments, and attempts to perform point projection radiography of preconditioned single wires by fielding an x-pinch in parallel with a two-stage array. [Preview Abstract] |
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CP12.00130: Deuterium Gas-Puff Z-pinch as a Source of Fast Ions Producing Intensive Pulse of Neutrons K. Rezac, J. Cikhardt, B. Cikhardtova, D. Klir, J. Kravarik, P. Kubes, O. Sila, A. Shishlov, R. Cherdizov, F. Fursov, V. Kokshenev, B. Kovalchuk, N. Kurmaev, A. Labetsky, N. Ratakhin, K. Turek A deuterium gas-puff with outer plasma shell has been examined on GIT-12 generator (on the current level of 3 MA) since 2013. Such a configuration caused more stable implosion at final stage of z-pinch. The consequence of this was a production of intensive pulses of fast ions. During last 4 campaigns in 2013-2015, fast ions were examined by several in-chamber diagnostics such as: stack detector (ion energy), pinhole camera (location of ion source), multi-pinhole camera (asymmetry and anisotropy of ion emission), and ion beam detector (dynamics of ion pulses). A CR-39 track detectors and also GAFCHROMIC HD-V2 films from these diagnostics will be presented. On the basis of obtained results, the solid sample for increasing of neutron yield up to 1e13 could be placed below the cathode mesh. Except of neutron yield, other properties such as: neutron energies (up to 33 MeV), neutron emission time (about 20 ns), and emission anisotropy of neutrons were measured. Such a short and intensive neutron pulse provides various applications. [Preview Abstract] |
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CP12.00131: Measurements of fusion neutrons from Magnetized Liner Inertial Fusion Experiments on the Z accelerator K.D. Hahn, G.A. Chandler, C.L. Ruiz, M.R. Gomez, S.A. Slutz, A.B. Sefkow, D.B. Sinars, S.B. Hansen, P.F. Knapp, P.F. Schmit, E.C. Harding, T.J. Awe, J.A. Torres, B. Jones, J.A. Bur, G.W. Cooper, J.D. Styron, V.Yu. Glebov Strong evidence of thermonuclear neutron production has been observed during Magnetized Liner Inertial Fusion (MagLIF) experiments on the Z accelerator. So far, these experiments have utilized deuterium fuel and produced primary DD fusion neutron yields up to 2e12 with electron and ion stagnation temperatures in the 2-3 keV range. We present MagLIF neutron measurements and compare to other data and implosion simulations. In addition to primary DD and secondary DT yields and ion temperatures, other complex physics regarding the degree of fuel magnetization and liner density are elucidated by the neutron measurements. Neutron diagnostic development for deuterium and future deuterium-tritium fuel experiments are also discussed. [Preview Abstract] |
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CP12.00132: Flyer-Plate-Based Current Diagnostic for Magnetized Liner Inertial Fusion Experiments Joseph Reneker, Matthew Gomez, Mark Hess, Christopher Jennings Accurate measurements of the current delivered to Magnetized Liner Inertial Fusion (MagLIF) [1] loads on the Z machine are important for understanding the dynamics of liner implosions. Difficulty acquiring a reliable load current measurement with the standard Z load B-dots [2] has spurred the development of alternative load current diagnostics. Velocimetry of an electromagnetically-accelerated flyer plate can be used to infer the drive current on a flyer surface. A load current diagnostic design is proposed using a cylindrical flyer plate in series with the MagLIF target. Aspects of the flyer plate design were optimized using magnetohydrodynamic simulations. Design and preliminary results will be presented. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.\\[4pt] [1] S.A. Slutz et al, Phys. Plasmas 17, 056303 (2010)\\[0pt] [2] D. V. Rose et. al., Phys. Rev. ST Accel. Beams, 13, 040401 (2010) [Preview Abstract] |
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CP12.00133: First experiment on LMJ facility: pointing and synchronisation qualification, sequences qualification Olivier Henry, Didier Raffestin, Dominique Bretheau, Michel Luttmann, Herve Graillot, Michel Ferri, Frederic Seguineau, Emmanuel Bar, Loic Patissou, Philippe Canal, Fran\c{c}oise Sautarel, Yves Tranquille-Marques The LMJ (Laser mega Joule) facility at the CESTA site (Aquitaine, France) is a tool designed to deliver up to 1.2 MJ at 351 nm. The experiment system will include plasma diagnostics: UV and X energy balances, imagers (Streak and stripe camera, CCD), spectrometers, and a Visar/pyrometer. The facility must be able to deliver, within the hour following the shot, all the results of the plasma diagnostics, alignment images and laser diagnostic measurements. Part of the end of 2014 was devoted to the qualification of system pointing on target and synchronization within and between beams. The shots made with one chain (divided in 2 quads -- 8 laser beams) have achieved 50$\mu$m of misalignment accuracy and a synchronization accuracy in the order of 50 ps. The performances achieved for plasma diagnostic (in the order of less 100 $\mu$m of alignment and timing accuracy less than 150 ps) comply with expectations. At the same time the first automatic sequences were tested. They allowed a shot on target every 6h:30 and in some case twice a day by reducing preparation actions, leading to a sequence of 4h:00. These shooting sequences are managed by an operating team of 7 people helped by 3 people for security aspects. [Preview Abstract] |
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CP12.00134: Plasma Profile Measurements for Laser Fusion Research with the Nike KrF Laser Jaechul Oh, J.L. Weaver, V. Serlin, S.P. Obenschain The grid image refractometer of the Nike laser facility (Nike-GIR) has demonstrated the capability of simultaneously measuring electron density ($n_e$) and temperature ($T_e$) profiles of coronal plasma.\footnote{J. Oh, et al, Rev. Sci. Instrum. 86 (2015) in press.} For laser plasma instability (LPI) research, the first Nike-GIR experiment successfully measured the plasma profiles in density regions up to $n_e \sim 4 \times 10^{21}$ cm$^{-3}$ (22\% of the critical density for 248 nm light of Nike) using an ultraviolet probe laser ($\lambda_p = 263$ nm). The probe laser has been recently replaced with a shorter wavelength laser ($\lambda_p = 213$ nm, a $5^{th}$ harmonic of the Nd:YAG laser) to diagnose a higher density region. The Nike-GIR system is being further extended to measure plasma profiles in the on-going experiment using 135$^\circ$-separated Nike beam arrays for the cross-beam energy transfer (CBET) studies.\footnote{J. Weaver, et al, Anomalous Absorption Conference, Ventura, CA, June 14-19, 2015.} We present an overview of the extended Nike-GIR arrangements and a new numerical algorithm to extract self-consistant plasma profiles with the measured quantities. [Preview Abstract] |
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CP12.00135: Measuring spatial distributions of nuclear burn in ICF implosions at OMEGA and the NIF using proton emission imaging Fredrick Seguin, H.G. Rinderknecht, A. Zylstra, H. Sio, J. Frenje, C.K. Li, R. Petrasso, M. Rosenberg, F.J. Marshall, T.C. Sangster, P. Mckenty, S. Craxton, J.R. Rygg, S. le Pape, V. Smalyuk, P.A. Amendt, S.C. Wilks, A. Mackinnon, N.M. Hoffman Fusion reactions in ICF implosions of D$^{3}$He-filled capsules produce 14.7-MeV D$^{3}$He protons and 3-MeV DD protons. Spatial distributions of the D$^{3}$He and DD reactions are studied with a penumbral imaging camera [1-2] that utilizes a CR-39-based imaging detector to detect the protons. Up to three orthogonal cameras have been used simultaneously at OMEGA to study the 3-D structure of asymmetric implosions, and two orthogonal cameras have now been used to study an exploding-pusher implosion at the NIF. Recent data from OMEGA and from the NIF will be shown. \\[4pt] [1] F. H. S\'{e}guin \textit{et al}., Rev. Sci. Instrum. \textbf{75}, 3520 (2004).\\[0pt] [2] F. H. S\'{e}guin \textit{et al}., Phys. Plasmas \textbf{13}, 082704 (2006). [Preview Abstract] |
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CP12.00136: Investigation of neutron-induced background in Magnetic-Recoil-Spectrometer CR-39 data using a DT neutron source and MCNP simulations Lucio M. Milanese, Johan Frenje, Maria Gatu Johnson, Brandon Lahmann, Hong Sio, Richard Petrasso The Magnetic Recoil neutron Spectrometers (MRS) installed on the OMEGA laser facility and the National Ignition Facility (NIF) are routinely used to measure neutron yield, areal density and ion temperatures from DT implosions. The observed background in the lower-energy part of MRS spectra is significantly higher than expected from analysis of neutron-induced background data obtained in stand-alone CR-39 experiments at OMEGA. A possible explanation relates to the scattering of neutrons in the MRS housing vessel, which is not accounted for in current modeling. To test experimentally the impact of individual vessel components on the observed background, parts of the MRS housing have been mocked up and CR-39 data have been collected employing a DT neutron source. The experimental results are contrasted to MCNP simulations to improve our understanding of the mechanism behind the enhanced neutron background. The results will be used to correct measured spectra from OMEGA and the NIF to allow detailed analysis of lower energy data. [Preview Abstract] |
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CP12.00137: Absolute x-ray and neutron calibration of CVD-diamond-based time-of-flight detectors for the National Ignition Facility N. Kabadi, H. Sio, H. Rinderknecht, A. Zylstra, M. Gatu Johnson, J. Rojas-Herrera, J.A. Frenje, R.D. Petrasso, V. Glebov The particle-time-of-flight (pTOF) detector at the National Ignition Facility routinely measure proton and neutron nuclear bang-times in inertial confinement fusion (ICF) implosions. The active detector medium in pTOF is a chemical vapor deposition (CVD) diamond biased to a high voltage. This work discusses absolute measurement of CVD diamonds sensitivity to neutrons and x-rays, for different thickness and bias voltage. Although the impulse response of the detector is regularly measured on diagnostic timing shot, absolute sensitivity of the detector's response to neutron and x-ray have not been fully established. X-ray and DT-n sources at the MIT accelerator facility provide a continuous source for testing. CVD diamond detectors are also fielded on OMEGA experiments to measure sensitivity to protons, DD neutrons, and DT neutrons. Implications for absolute neutron yield measurements at the NIF using pTOF detectors will be discussed. [Preview Abstract] |
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CP12.00138: The MIT HEDP Accelerator Facility for education and advanced diagnostics development for OMEGA, Z and the NIF R. Petrasso, M. Gatu Johnson, E. Armstrong, H.W. Han, N. Kabadi, B. Lahmann, D. Orozco, J. Rojas Herrera, H. Sio, G. Sutcliffe, J. Frenje, C.K. Li, F.H. S\'eguin, R. Leeper, C.L. Ruiz, T.C. Sangster The MIT HEDP Accelerator Facility utilizes a 135-keV linear electrostatic ion accelerator, a D-T neutron source and two x-ray sources for development and characterization of nuclear diagnostics for OMEGA, Z, and the NIF. The ion accelerator generates D-D and D-$^{3}$He fusion products through acceleration of D ions onto a $^{3}$He-doped Erbium-Deuteride target. Fusion reaction rates around 10$^{6}$ s$^{-1}$ are routinely achieved, and fluence and energy of the fusion products have been accurately characterized. The D-T neutron source generates up to 6 $\times$ 10$^{8}$ neutrons/s. The two x-ray generators produce spectra with peak energies of 35 keV and 225 keV and maximum dose rates of 0.5 Gy/min and 12 Gy/min, respectively. Diagnostics developed and calibrated at this facility include CR-39 based charged-particle spectrometers, neutron detectors, and the particle Time-Of-Flight (pTOF) and Magnetic PTOF CVD-diamond-based bang time detectors. The accelerator is also a vital tool in the education of graduate and undergraduate students at MIT. [Preview Abstract] |
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CP12.00139: The magPTOF diagnostic for shock-bang and compression-bang time measurement and charged-particles spectroscopy at the NIF H.W. Han, H. Sio, H. Rinderknecht, J. Frenje, A. Zylstra, M. Gatu Johnson, F. Seguin, C. Li, R. Petrasso, A. House, J.R. Rygg, J. Kimbrough, A. Macphee, G.W. Collins, A. Mackinnon, S. le Pape, L. Berzak Hopkins, M. Bedzyk, J. Magoon, M. Shoup, C. Sangster, J. Kilkenny, R. Olson A magnetic particle-time-of-flight (MagPTOF) diagnostic has been fielded at the National Ignition Facility (NIF) for measurements of both shock- and compression-bang times. This type of measurement, combined with the measured shock-burn-weighted $\rho $R, is used to understand shock convergence and implosion dynamics. The MagPTOF design is an upgrade to the existing particle time-of-flight (pTOF) diagnostic, which has recorded bang times in cryogenic DT implosions, DT exploding pushers and D3He implosions with accuracy better than 70 ps. The inclusion of a deflecting magnet should increase proton signal-to-background by a factor of 1000, allowing for measurements of shock bang time (using 14.7 MeV D3He protons) and compression bang time (using 2.45 MeV DD neutrons) in D3He-filled surrogate implosions. For exploding pushers with D3He, D2, T3He, or DT fuel, from which several charged fusion products escape, CR39 surrounding the CVD diamond detector can also be used for low-energy charged-particle spectroscopy. Implementation and initial data at the NIF will be discussed. [Preview Abstract] |
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CP12.00140: A compact Charged-Particle Spectrometer for OMEGA and the NIF D. Orozco, M.J. Rosenberg, F.H. Seguin, M. Gatu Johnson, H. Sio, A.B. Zylstra, H.G. Rinderknecht, J.A. Rojas, J.A. Frenje, C.K. Li, R.D. Petrasso, V. Yu, Glebov A very compact scattering pinhole diagnostic (SPD) has been implemented and used to measure the mean energy of charged particles produced in Inertial Confinement (ICF) experiments. This was done by measuring the spatial distribution of mono-energetic particles that passed through a small pinhole, scattered in a thin foil that was positioned about a centimeter in front of a CR-39 detector. To determine the mean energy from the spatial distribution of the scattered particles on the CR-39, an empirical relationship between the scattering angle and the incoming particle energy for a given foil was determined using simulations. Two methods for the energy determination are discussed in this presentation. The capabilities of this diagnostic are demonstrated with DD proton and D3He alpha data from the OMEGA laser. To check the fidelity of the SPD measurements, the results are contrasted to data obtained with other well-established techniques. [Preview Abstract] |
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CP12.00141: Simultaneous measurements of the X-ray and nuclear shock-bang times in ICF plasmas G. Sutcliffe, H. Sio, H. Rinderknecht, J. Frenje, A. Zylstra, M. Gatu Johnson, F. Seguin, C.K. Li, R. Petrasso, J.R. Rygg, A. Macphee, A. Mackinnon, S. le Pape, L. Berzak Hopkins, S.P. Regan, C. Sangster, J. Kilkenny, R. Olson Recent measurements of nuclear and x-ray shock-bang times in ICF implosions at OMEGA and the NIF provide new constraints on implosion modeling and may elucidate the underlying physics of e-i equilibration during the shock phase. As the ions are predominantly heated by the converging and rebounding shock, the ion temperature is initially much higher than the electron temperature and the difference relaxes at the e-i equilibration time scale. Nuclear and x-ray bang times are expected to differ because of different temperature dependence. At OMEGA, nuclear shock-bang time and burn history are routinely measured using streak camera diagnostics, while x-ray self-emission is observed with x-ray framing cameras. We are exploring the possibility of measuring both x-ray and nuclear shock-bang times with a single diagnostic with high relative accuracy, and will discuss the precision with which they can be made and the diagnostics necessary at OMEGA. [Preview Abstract] |
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CP12.00142: Design of a compact, low energy charged particle spectrometer for stellar nucleosynthesis experiments at OMEGA and the NIF E. Armstrong, J. Frenje, M. Gatu Johnson, C.K. Li, D. Orozco, H. Rinderknecht, M. Rosenberg, F.H. S\'eguin, H. Sio, A. Zylstra, R.D. Petrasso Simulations have been used to model an ``Orange Spectrometer'' for measuring alpha and proton energy spectra in the range $\sim$1-5 MeV for experiments at the OMEGA laser facility and the National Ignition Facility (NIF). An important application will be the study of stellar nucleosynthesis reactions, in particular 3He$+$3He$\to \alpha +$p$+$p, which is a step in the solar proton-proton chain. Experiments to study this reaction have been undertaken at OMEGA before, but no diagnostics have been able to measure the low-energy, low-yield alpha particles generated in the reaction. Feasibility studies were performed with particle trajectory calculations utilizing magnetic field models from COMSOL, and several designs have been identified for testing and development. Ability to study the alpha particles in addition to the protons is essential for understanding the nuclear physics governing the final-state interactions between pairs of particles in the three-body final state. [Preview Abstract] |
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CP12.00143: Single Crystal X-ray Spectropolarimeter for HED Plasmas Matthew Wallace, Showera Haque, Paul Neill, Alan Kastengren, Nino Pereira, Radu Presura When energetic electrons in a plasma have a preferred direction the resulting X-rays can be polarized. This makes plasma X-ray polarization spectroscopy, spectropolarimetry, a useful way to reveal information about the anisotropy of the electron velocity distribution. X-ray spectropolarimetry has been used for characterizing the anisotropy of space and laboratory plasmas environments. The spectrum's polarization, typically measured with two crystals both at a 45 degree Bragg angle or one on successive shots, can now be determined in a new way using one crystal. Crystals with hexagonal symmetry present pairs of internal planes that diffract incident X-rays in two directions that are perpendicular to each other and the incident ray. The diffracted components are linearly polarized perpendicularly to each other. The polarization splitting properties of quartz crystals were confirmed with linearly polarized X-rays from the Advanced Photon Source. A Y-cut crystal that splits polarization with [11-20] planes at 7.15 keV was among those tested. An X-cut crystal with [10-10] polarization splitting planes was then tested on Al wire array z-pinches at UNR. We will present the use and development of a single crystal X-ray polarimeter for the characterization of anisotropy in HED plasmas. [Preview Abstract] |
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CP12.00144: Helical Striation Pattern Generation and Axial Field Compression in Aluminum Liner Experiments at 1 MA Levon Atoyan, Tom Byvank, John Greenly, Bruce Kusse, Sergei Pikuz, William Potter, Tania Shelkovenko, David Hammer Awe \textit{et al.} [\textit{Phys. Plasmas 21}, 235005, 2014] found on the 20 MA Z machine that applying an externally generated axial magnetic field to an imploding liner produces a helical plasma pattern near the surface of the liner. Here we show that this phenomenon is also observed using 10 mm long cylindrical metal liners having 16 mm diameter and 3 to 6 $\mu $m wall thickness on the 1 MA, 100-200 ns COBRA pulsed power generator [T. A. Shelkovenko et al, \textit{Rev. Sci. Instrum. 77}, 10F521, 2006]. The magnetic field in these experiments is created using a 150 $\mu $s rise time Helmholtz coil, and the pattern is observed using extreme ultraviolet imaging. Moreover, using B-dot probes we show that there is a 4-8{\%} axial magnetic field compression relative to the initially applied B$_{z}$. Using a visible light framing camera, we show that this compression begins before the outside surface of the liner has become a visible light emitting plasma. [Preview Abstract] |
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CP12.00145: Spectroscopic Studies of the Soft X-Ray Radiation from Gas-Puff Z-Pinches on Cobra T.A. Shelkovenko, S.A. Pikuz, P.W.L. de Grouchy, N. Qi, L. Atoyan, B.R. Kusse, D.A. Hammer Gas-puff Z-pinch experiments have been conducted on the 0.8-1.2 MA, 100-240 ns pulse duration COBRA pulsed power generator. Triple nozzle gas-puff loads consisting of Ne, Ar and Kr gases in different combination and pressures with pre-ionization were used in the most recent experiments. Photo-conducting diodes (PCDs) and pinhole cameras with different filters were used to study the X-ray timing, intensity and spatial distribution in different energy bands. Spectrographs with spatial and temporal resolution were used to study the soft x-ray radiation from the gas-puff Z-pinches. One spectrograph with two spherically bent mica crystals was used to study radiation with 200 micron spatial resolution and high spectral resolution. An x-ray streak camera with one spherically bent quartz crystal was used to study the x-ray radiation with up to 10 ps temporal resolution. The x-ray spectra were used to estimate spatial and temporal distributions of plasma parameters and determine the intensity of the line and continuum radiation from the Z-pinches plasma. [Preview Abstract] |
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CP12.00146: Study of Explosive Electron Emission from a Pin Cathode Using High Resolution Point-Projection X-Ray Radiography S.A. Pikuz, T.A. Shelkovenko, D.A. Hammer, E.V. Parkevich, I.N. Tilikin, A.R. Mingaleev, A.V. Agafonov Most studies of Explosive Electron Emission (EEE) are based on the idea of cathode flares developing after explosion of metal whiskers (micron scale pins) on the cathode surface. The physical state of the pin material, the spatial structure of the explosion and its origin are still a matter of conjecture. In this work we used high-resolution point projection x-ray radiography to observe micron scale pin explosion in a high-current diode. Pin cathodes made from 10-25 micron Cu or Mo wires were placed in gaps in return current circuits of hybrid X-pinches on the XP and BIN pulsers. Pin lengths were varied over a range 1-4 mm and pin-anode gaps within 0.05-3 mm. The diode current and voltage were measured. In experiments with small pin-anode gap (0.1 - 1 mm) development of an expanded dense core of the pin was observed except the pin tip with length 100-200 microns indicating significant energy deposition in the wire material. In experiments with bigger gaps there was no visible wire core expansion within the spatial resolution of the experimental technique. [Preview Abstract] |
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CP12.00147: Studies of the dynamics of a 1-microsecond X-pinch Richard Appartaim, Danielle Green The 1-$\mu s$ X-pinch (0.3 kA/ns) has been shown to produce intense soft x-rays with a spatially reproducible source location and fine size $(i.e. < 10\ \mu m)$. For certain applications these x-rays are comparable in their utility to those produced on pulsed-power devices but have the advantage of a much lower component of hard x-rays. Many of the critical plasma dynamics are also similar to those observed in the fast rise-time (1 kA/ns) experiments. However, the longer rise time of the microsecond discharge can lead to important differences in wire ablation rates and transition to coronal plasma, plasma current distribution and plasma dynamics. We present recent results of these plasma dynamics using optical techniques such as shadowgraphy, schlieren and framing photography, as well as x-ray observation techniques including filtered PCD and Si diode measurements, pinhole photography and x-ray spectroscopy. We demonstrate potential applications including the relevance of the observed plasma jets to astrophysical jets. [Preview Abstract] |
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CP12.00148: Radiative Properties of Argon Gas-Puff Implosions on COBRA Nicholas Ouart, Niansheng Qi, Phil de Grouchy, Tatiana Shelkovenko, Sergei Pikuz, John Giuliani, Arati Dasgupta, John Apruzese, Robert Clark, David Hammer, Bruce Kusse Gas-puff Z-pinch experiments were performed on the 1 MA COBRA pulsed power generator at Cornell University. The gas puffs were injected into the load region from a triple nozzle. The load region had an anode-cathode gap of 2.5 cm. The standard diagnostics on COBRA include time-integrated pinhole cameras, a time-integrated axially resolved x-ray spectrometer, filtered photo-conducting detectors, and time-gated XUV cameras. We will focus mainly on results from pinhole images and x-ray spectra from argon gas puffs including some with a SO2 dopant. The x-ray time-integrated pinhole images feature a tight axially uniform plasma column with a diameter of approximately 1 mm for argon gas implosion. The x-ray spectrometer used mica crystals (2d$=$19.84 {\AA}) and captured the argon K-shell radiation from different crystal reflections. A 1-D multi-zone argon and sulfur non-LTE kinetics code with radiation transport is used to model the K-shell emission for the purpose of inferring the plasma conditions and the interaction of gas from the inner annulus with the central jet. [Preview Abstract] |
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CP12.00149: Neutron production in deuterium gas-puff z-pinch with outer plasma shell at current of 3 MA J. Cikhardt, D. Klir, K. Rezac, B. Cikhardtova, J. Kravarik, P. Kubes, O. Sila, A.V. Shishlov, R.K. Cherdizov, F.I. Frusov, V.A. Kokshenev, N.E. Kurmaev, A. Yu. Labetsky, N.A. Ratakhin, G.N. Dudkin, A.A. Garapatsky, V.N. Padalko, V.A. Varlachev, K. Turek, J. Krasa Z-pinch experiments at the current of about 3 MA were carried out on the GIT-12 generator. The outer plasma shell of deuterium gas-puff was generated by the system of 48 plasma guns. This configuration exhibits a high efficiency of the production of DD fusion neutrons with the yield of above $10^{12}$ neutrons produced in a single shot with the duration of about 20 ns. The maximum energy of the neutrons produced in this pulse exceeded 30~MeV. The neutron radiation was measured using scintillation TOF detectors, CR-39 nuclear track detectors, bubble detectors BD-PND and BDS-10000 and by several types of nuclear activation detectors. These diagnostic tools were used to measure the anisotropy of neutron fluence and neutron energy spectra. It allows us to estimate the total number of DD neutrons, the contribution of other nuclear reactions, the amount of scattered neutrons, and other parameters of neutron production. [Preview Abstract] |
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CP12.00150: Kinetic Simulations of Dense Plasma Focus Breakdown A. Schmidt, D.P. Higginson, S. Jiang, A. Link, A. Povilus, J. Sears, N. Bennett, D.V. Rose, D.R. Welch A dense plasma focus (DPF) device is a type of plasma gun that drives current through a set of coaxial electrodes to assemble gas inside the device and then implode that gas on axis to form a Z-pinch. This implosion drives hydrodynamic and kinetic instabilities that generate strong electric fields, which produces a short intense pulse of x-rays, high-energy (\textgreater 100 keV) electrons and ions, and (in deuterium gas) neutrons. A strong factor in pinch performance is the initial breakdown and ionization of the gas along the insulator surface separating the two electrodes. The smoothness and isotropy of this ionized sheath are imprinted on the current sheath that travels along the electrodes, thus making it an important portion of the DPF to both understand and optimize. Here we use kinetic simulations in the Particle-in-cell code LSP to model the breakdown. Simulations are initiated with neutral gas and the breakdown modeled self-consistently as driven by a charged capacitor system. We also investigate novel geometries for the insulator and electrodes to attempt to control the electric field profile. The initial ionization fraction of gas is explored computationally to gauge possible advantages of pre-ionization which could be created experimentally via lasers or a glow-discharge. Prepared by LLNL under Contract DE-AC52-07NA27344. [Preview Abstract] |
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CP12.00151: Design of a 100 J Dense Plasma Focus Z-pinch Device as a Portable Neutron Source Sheng Jiang, Drew Higginson, Anthony Link, Jason Liu, Andrea Schmidt The dense plasma focus (DPF) Z-pinch devices are capable of accelerating ions to high energies through MV/mm-scale electric fields. When deuterium is used as the filling gas, neutrons are generated through beam-target fusion when fast D beams collide with the bulk plasma. The neutron yield on a DPF scales favorably with current, and could be used as portable sources for active interrogation. Past DPF experiments have been optimized empirically. Here we use the particle-in-cell (PIC) code LSP [1,2] to optimize a portable DPF for high neutron yield prior to building it. In this work, we are designing a DPF device with about 100 J of energy which can generate 10$^{6}$ -- 10$^{7}$ neutrons. The simulations are run in the fluid mode for the rundown phase and are switched to kinetic to capture the anomalous resistivity and beam acceleration process during the pinch. A scan of driver parameters, anode geometries and gas pressures are studied to maximize the neutron yield. The optimized design is currently under construction. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and supported by the Laboratory Directed Research and Development Program (15-ERD-034) at LLNL.\\[4pt] [1] D. R. Welch, D. V. Rose, R. E. Clark, T. C. Genoni, and T. P. Hughes, Comput. Phys. Commun. 164, 183 (2004)\\[0pt] [2] A. Schmidt, V. Tang, D. Welch, Phys. Rev. Lett. 109, 205003 (2012) [Preview Abstract] |
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CP12.00152: Progress in Development of Dense Plasma Focus Pinch for AmBe Radiological Source Replacement Steve Falabella, Alex Povilus, Andrea Schmidt, Jennifer Ellsworth, Anthony Link, Jason Sears, Drew Higginson, Sheng Jiang A dense plasma focus (DPF) is a compact plasma gun accelerator that can produce intense, high energy ion beams (multiple MeV). These ion beams could be used to replace radiological sources for a variety of applications. Using a 2kJ DPF with a helium gas fill, alpha particles are accelerated into a beryllium target in order to generate a neutron spectrum similar to an AmBe source. We report on initial observations of neutron yields for this system and efforts to optimize and improve repeatability of pinch performance. In particular, incorporating results from newly-developed kinetic LSP simulations, we demonstrated higher neutron yields by adjusting the geometry of the anode electrode. In addition, we present preliminary measurements for energy distributions of ions accelerated by the pinch. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. This work supported by US DOE/NA-22 Office of Non-proliferation Research and Development. [Preview Abstract] |
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CP12.00153: Measuring $\sim$10~T B-Fields Using Zeeman Splitting of Sodium Emission Lines on a 500~kA Pulsed Power Machine Jacob Banasek, Joseph Engelbrecht, David Hammer, Sergei Pikuz, Tatiana Shelkovenko Following earlier work by M. Gomez \textit{et al.} [1], we have shown that Zeeman splitting of the Sodium (Na) D-lines at 5890~{\AA} and 5896~{\AA} can be used to measure the magnetic field (B-field) produced in high current pulsed power experiments. In the present experiments, we have measured the B-field next to a return current post during hybrid X-pinch experiments near peak current. These measurements were performed at 500~kA current on the XP machine at Cornell University using a 150~ns current rise time. Na was added to the system by applying a few drops of a NaCl solution onto the inner surface of one of two 4~mm radius return current posts, located about 4~cm away from the hybrid X-pinch. The Na is desorbed from the return current post by radiation from the hybrid x-pinch, enabling observation of the Na vapor's spectral lines. Measurements have shown a B-field of 10-15~T near the return current post, which is consistent with the calculated B-field. Future experiments will explore applying this approach to regions of higher B-field in higher current machines.\\[4pt] [1] M. R. Gomez, \textit{et al.} Rev. Sci. Instrum. \textbf{85}, 11E609 (2014) [Preview Abstract] |
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CP12.00154: Magneto Rayleigh-Taylor, Sausage, and Kink Instability Experiments on a MegaAmpere Linear Transformer Driver D.A. Yager-Elorriaga, A.M. Steiner, S.G. Patel, N.M. Jordan, R.M. Gilgenbach, Y.Y. Lau, M.R. Weis, P. Zhang At the Michigan Accelerator for Inductive Z-Pinch Experiments (MAIZE) facility, a 1-MA Linear Transformer Driver (LTD) is being used to deliver 500-600 kA to cylindrical liners in order to study the magneto Rayleigh-Taylor (MRT), sausage, and kink instabilities in imploding and exploding Al plasmas. The liners studied in this experiment had thicknesses of 400 nm to 30 $\mu $m, heights of 1-2 cm, and diameters of 1-6 mm. The plasmas were imaged using 4-time-frame, laser shadowgraphy and shearing-interferometry at 532 nm. For imploding liners, the measured acceleration was found to be less than predicted from the current pulse, indicating significant diffusion of the azimuthal magnetic field. A simple experimental configuration is presented for ``end-on'' laser probing in the r-$\theta $ plane in order to study the interior of the liner. Finally, the effects of axial magnetic fields are determined by modifying the return current posts and incorporating external coils. Experimental growth rates are determined and discussed. [Preview Abstract] |
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CP12.00155: The Non-Lte Stagnation Physics Of A Z Pinch: Spectroscopy Coupled With Rad-Hydro Simulations T. Mehlhorn, J. Giuliani, W. Thornhill, A. Dasgupta, Y. Maron, E. Kroupp, G. Rosenzweig, C. Coverdale, J. Apruzese, C. Deeney We present modeling of the non-LTE ionization kinetics in radiation-MHD simulations of Z pinches, focusing on the origin of the large effective ion temperatures and the energy balance during the stagnation phase. Effective ion temperatures (Ti$_{\mathrm{eff}}$), based on the widths of emission lines, have long been reported to exceed the electron temperature by more than 10X. Ne gas puff experiments at the Weizmann Institute also display this effect, and provide extensive time and space resolved measurements of the plasma during stagnation. MACH2-TCRE has been used to model this Ne. Ti$_{\mathrm{eff}}$ has been computed analogously to the experimental technique. The 2D model results are significantly larger than the ion thermal temperatures early in the K-shell pulse, in agreement with the data. This implies that the broad line widths reflect strong radially velocity gradients near the axis. Spectroscopic data from Al/Ti arrays on Z at SNL and gas puffs at the Weizmann Institute indicate that the stagnating pinch is defined by an accreting shock with the pressure behind the shock balanced by the ram pressure of the imploding material. Polarization spectroscopy indicates that the magnetic field does not penetrate deeply into the stagnating plasma. [Preview Abstract] |
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CP12.00156: A Plasma Opening Switch Based on a Gas-Puff/Axial Wire Configuration Joseph Engelbrecht, Philip de Grouchy, Nicholas Ouart, Niansheng Qi, Levon Atoyan, Jacob Banasek, William Potter, David Hammer, Bruce Kusse, John Giuliani We are investigating an idea for switching current from a gas-puff shell to an axial metal wire as a mechanism for generating inductive voltage spikes and x-rays above 10 keV. The outer annulus of a 7 cm. diameter triple-annular gas-puff nozzle is used to inject gas into the electrode gap of the COBRA 1 MA generator, with a single wire on-axis. We show that the current pulse produced by COBRA initially travels through the lower inductance pre-ionized outer shell plasma, generating an azimuthal magnetic field which drives this shell radially inwards. Rayleigh-Taylor instability growth occurs on the outer edge of this imploding plasma, which disrupts the current carrying column, inhibiting the axial flow of current through the gas-puff plasma and possibly causing the current to switch to the higher inductance wire. A disruption to the current through the gas-puff shell caused by instability growth should be measurable as a voltage spike, as the current finds a new path either through the wire or elsewhere in the imploding plasma shell. We investigate this effect as instability conditions are varied, by adjusting the density and species of the injected gas. [Preview Abstract] |
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CP12.00157: Characteristic Differences Between Wire and Foil X-pinches Gilbert Collins, Julio Valenzuela, Igor Krasheninnikov, Farhat Beg, Mingsheng Wei We conducted X-pinch experiments using laser-cut Ni and Cu foils on the 250kA GenASIS current driver at UC San Diego. General Atomics' Laser Micro-Machining (LMM) Center manufactured the X's. To characterize the foil X-pinches, we measured and compared the evolution, emission spectra, yield, and source size of these new arrays to that of comparably massed wire X-pinches on the same driver. Diagnostics included Si PN diodes and diamond PCDs, optical probing, X-ray spectroscopy, an XUV framing camera, a slit-wire camera, and current probes. We used novel structures machined into the crosspoint in an effort to better understand the effects of the initial geometry on the final pinch and to spatially confine the source location. Some designs entirely prohibited pinching. In other designs, when pinching occurred, the sources were comparable to ideal wire shots on GenASIS both in size (at or less than five microns) and X-ray flux (5-10 MW @ 1-10 keV). The data collected here also show considerable differences between successful foil and wire pinches. The X-ray spectra are not identical, and we find that the foil X's produce a single \textgreater 2.5 keV emission pulse with none of the additional later and longer-lasting hard emission pulses found in wire X-pinches. [Preview Abstract] |
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CP12.00158: Instability formation on thin liners in small-scale experiments using MACH2 Jeff Narkis, Julio Valenzuela, Hafiz Ur Rahman, Paul Ney, Frank Wessel, Farhat Beg Initial experimental studies on instability formation on thin liners have been conducted on GenASIS (200 kA, 150 ns rise time). In those experiments, thecurrent was applied to Cu and Ni liners with initial length, radius, and thickness of 7 mm, 1 mm, and 3 $\mu $m, respectively. Plasma perturbations of the minimum observable wavelength (20 $\mu $m) were seen as early as 70 ns. In both materials the most significant perturbations reach a limiting wavelength of the order of the liner radius, but this occurs faster (20 ns) for Cuthan in Ni(100 ns), suggesting the seeding mechanism must be different early in time, when the resistivities are different for the two materials. The 2-1/2 MHD code MACH2 was used to investigate this early-time instability development. The ratio of initial length to thickness (7000:3) presents computational challenges: the initial length was reduced to 2 mm, and to reduce initial density gradients an initial liner thickness of 30 $\mu $m was used. To seed instability growth, up to a 10{\%} initial density perturbation was used. Preliminary results have been inconclusive; additional work will explore sub-micron resolutions and alternative initial density configurations. [Preview Abstract] |
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CP12.00159: High Voltage Coaxial Vacuum Gap Breakdown for Pulsed Power Liners Samuel Cordaro, Simon Bott-Suzuki, Luis Sebastian Caballero Bendixsen The dynamics of Magnetized Liner Inertial Fusion (MagLIF)$^{1}$, are presently under detailed study at Sandia National Laboratories. Alongside this, a comprehensive analysis of the influence of the specific liner design geometry in the MagLIF system on liner initiation is underway in the academic community. Recent work at UC San Diego utilizes a high voltage pulsed system (25kV, 150ns) to analyze the vacuum breakdown stage of liner implosion. Such experimental analyses are geared towards determining how the azimuthal symmetry of coaxial gap breakdown affect plasma initiation within the liner. The final aim of the experimental analysis is to assess to what scale symmetry remains important at high (MV) voltages. An analysis of the above will utilize plasma self-emission \textit{via} optical MCP, current measurements, voltage measurements near the gap, exact location of breakdown via 2D b-dot probe triangulation, as well as measuring the evolution of the B-field along the length of the liner \textit{via} b-dot array. Results will be discussed along with analytical calculations of breakdown mechanisms [Preview Abstract] |
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CP12.00160: ABSTRACT WITHDRAWN |
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