Bulletin of the American Physical Society
51st Annual Meeting of the APS Division of Plasma Physics
Volume 54, Number 15
Monday–Friday, November 2–6, 2009; Atlanta, Georgia
Session PP8: Poster Session VI: C-Mod; NSTX and General Spherical Torus; Simulation and Modeling of Basic Plasma Phenomena |
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Room: Grand Hall East |
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PP8.00001: C-MOD |
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PP8.00002: Lower Hybrid Wave Induced Rotation on Alcator C-Mod Ron Parker, Yuri Podpaly, John Rice, Andrea Schmidt Injection of RF power in the vicinity of the lower hybrid frequency has been observed to cause strong counter current rotation in Alcator C-Mod plasmas [1,2]. The spin-up rate is consistent with the rate at which momentum is injected by the LH waves, and also the rate at which fast electron momentum is transferred to the ions. A momentum diffusivity of $\sim $ 0.1 m$^2$/s is sufficient to account for the observed steady-state rotation. This value is also comparable with that derived from an analysis of rotation induced by RF mode conversion [3]. Radial force balance requires a radial electric field, suggesting a buildup of negative charge in the plasma core. This may be the result of an inward pinch of the LH produced fast electrons, as would be expected for resonant trapped particles. Analysis of the fast-electron-produced bremsstrahlung during LH power modulation experiments yields an inward pinch velocity of $\sim $ 1 m/s, consistent with the estimated trapped particle pinch velocity. \\[4pt] [1] A. Ince-Cushman, et.al., Phys. Rev. Lett., \textbf{102}, 035002 (2009)\\[0pt] [2] J. E. Rice, et. al., Nucl. Fusion \textbf{49}, 025004 (2009)\\[0pt] [3] Y. Lin, et.al., this meeting [Preview Abstract] |
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PP8.00003: Integration of Fokker Planck calculation in full wave FEM simulation of LH waves Orso Meneghini, Syun'ichi Shiraiwa, Ron Parker A full wave simulation code based on Finite Element Method (FEM) was developed to solve LH waves in tokamaks [1]. The FEM approach allows a seamless handling of the antenna, first wall, SOL, divertor and core regions. In the region of plasma inside of the separatrix, electron landau damping is modeled by means of an iterative procedure. The code has been recently coupled to a bounce averaged Fokker Planck (FP) solver, which self consistently calculates the electron 2D distribution function resulting from the balance between collisions and RF quasilinear diffusion (DQL) in a toroidal geometry. The evaluation of the DQL term from the full wave fields has been done by integration of the momentum equation of test particles. Results comparing Maxwellian, 1D FP and 2D FP plasmas will be presented for the Alcator C-Mod tokamak.\\[4pt] [1] S. Shiraiwa, this conference. [Preview Abstract] |
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PP8.00004: Investigation of velocity diffusion in the presence of a broad-band Lower Hybrid wave spectrum Jungpyo Lee, Paul Bonoli, John Wright Lower Hybrid (LH) waves are attractive for driving current in Tokamak because of high efficiency. The quasi-linear diffusion coefficient\footnote{C.F. Kennel and F. Engelmann, Physics of Fluids, 9(1966),2377-2388} has been used in the past to estimate the current density by electron landau damping. We numerically evaluate the diffusion coefficient taking into account the broad band spectrum of LH wave. The motion of a single electron is simulated by a one dimensional non-linear momentum equation along the magnetic field, and the value of velocity diffusion averaged over initial positions is computed. The electron is accelerated by an electric field whose wavenumber spectrum is broad and discrete due to toroidal geometry. The broad band spectrum induces phase mixing so that it allows only a limited number of kicks every revolution of the electron. We compare the numerically computed diffusion coefficient with the quasi-linear diffusion coefficient$^{1}$ as a function of the magnitude of the electric field with Alcator C-Mod LH wave parameters. We also present diffusion coefficient profiles in velocity space overlapped by many resonance modes. [Preview Abstract] |
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PP8.00005: Full-Wave Analysis of Lower Hybrid Wave Propagation in the Edge Plasma of a Tokamak P.T. Bonoli, G.M. Wallace, J.C. Wright, O. Meneghini, R. Parker, M. Porkolab, S. Shiraiwa, R.W. Harvey, C.K. Phillips, E.J. Valeo, J.R. Wilson Recent lower hybrid current drive\textbf{ (}LHCD) experiments on Alcator C-Mod have revealed a transition density above which LHRF generated fast electrons are no longer detected [1]. This critical density is also well below the threshold density for parametric decay of the LH pump wave. Analysis of these plasmas using a ray tracing-Fokker Planck model shows that poorer wave penetration at higher density, accompanied by increased collisional damping in the Scrape off Layer (SOL) may possibly explain this transition [1]. We have used an electromagnetic field solver TORLH [2] to begin to assess the possible role of full-wave effects in this density limit. Results will be presented, including three-dimensional field reconstructions for varying edge plasma parameters such as SOL density, temperature, and density gradient scale length. \\[4pt] [1] G. Wallace \textit{et al}, 18$^{th}$ Topical Conf. on RF Power in Plasmas (Gent, Belgium, June, 2009) Paper B60. \\[0pt] [2] J. C. Wright \textit{et al}, Physics of Plasmas \textbf{16}, July (2009). [Preview Abstract] |
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PP8.00006: Full wave/Fokker-Planck analysis of driven current and hard X-ray emission profiles during lower hybrid experiments on Alcator C-Mod John Wright, Ernest Valeo, Cynthia Phillips, Paul Bonoli Analysis of lower hybrid (LH) wave propagation in the past has been done using ray tracing and the WKB approximation. Advances in algorithms and parallel computer architectures has permitted the solving of the Maxwell-Vlasov system for wave propagation directly [Wright et al., Phys. Plasmas (2009), 16, July]. Self-consistent non-Maxwellian distribution functions are included by coupling the TORIC-LH full wave code with the CQL3D Fokker-Planck code via the export of the quasilinear diffusion coefficent from TORICLH and the use of the electron distribution function from CQL3D to evaluate the dielectric response in TORICLH following the technique implemented by Valeo [Valeo et al.,18th RF Power in Plasmas Proceedings (2007)]. Using the synthetic HXR diagnostic in CQL3D we will compare self-consistent LH simulations with LH experiments on Alcator C-Mod at several different waveguide phasings. Discussion will include comparisons with ray tracing the relative importance of full wave effects, relativistic effects, and reflections. [Preview Abstract] |
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PP8.00007: Analysis and scenario modeling of LHCD on Alcator C-Mod J.R. Wilson, C. Kessel, S. Scott, E. Valeo, R.R. Parker, P.T. Bonoli, A.E. Hubbard, J.W. Hughes, J.S. Ko, O. Meneghini, M. Porkolab, J.E. Rice, A.E. Schmidt, S. Shiraiwa, G.M. Wallace, J.C. Wright Lower Hybrid Current Drive\textbf{ (}LHCD) on Alcator C-Mod is being used in plasmas with parameters similar to those expected on ITER for the purpose of tailoring the plasma current profile with the ultimate goal of obtaining Advanced Tokamak (AT) operation. New simulation codes have been developed which allow more detailed analysis of the existing data and extrapolation to the final target AT discharges. These codes allow the ability to include the scrape-off plasma, non-Maxwellian electron distribution function and relativistic effects. Scans in the parallel wave-number will be presented and compared to experimental data that have indicated radial broadening of the hard x-ray emission under certain plasma conditions. In addition, scans of the plasma density and temperature will be performed to confirm the optimal approach to achieving AT plasmas. [Preview Abstract] |
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PP8.00008: Nonthermal ECE as a diagnostic of LH driven fast electrons on Alcator C-Mod S.G. Baek, R.R. Parker, A. Hubbard, R.W. Harvey Downshifted 2$^{nd}$ harmonic ECE from the outboard horizontal midplane has been measured during Lower Hybrid Current Drive (LHCD) on C-Mod and is a direct indication of the generation of nonthermal electrons. However optical depth and radial magnetic field variation make quantitative analysis difficult. Potential single and multi chord vertical high harmonic ECE measurement systems are investigated with a view to obtaining quantitative information about nonthermal electrons in momentum or physical space. A simple distribution function model is tested to evaluate dependence of emissivity on parallel and perpendicular Maxwellian temperature. A more detailed simulation is done with GENRAY-CQL3D, using its synthetic diagnostic code for nonthermal ECE. Initial results indicate radial emissivity profile and perpendicular temperature can be inferred, but experimental issues originating from optically thin higher harmonic radiation may hinder the realization of vertical ECE diagnostics for Alcator C-Mod. [Preview Abstract] |
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PP8.00009: Detection of Lower Hybrid Waves on Alcator C-Mod with Phase Contrast Imaging Using Electro-Optic Modulators K. Arai, M. Porkolab, E. Edlund, P. Koert, R. Parker, N. Tsujii, P. Woskov, S. Wukitch Detection of 4.6 GHz lower hybrid waves with Phase Contrast Imaging (PCI) presents a particular challenge due to the limited frequency response of the photoconductive detectors used for our 10.6 $\mu$m CO$_2$ laser. Acousto-optic modulators have been used to detect ICRF waves in the 80 MHz range through optical heterodyning in Alcator C-Mod [1]. However, the scattering efficiency of the acousto-optic modulators limits modulation frequencies up to 100 MHz. Here we present the design of a waveguide loaded with cadmium telluride (CdTe) to serve as an electro-optic modulator for CO$_2$ laser amplitude modulation near 4.6 GHz. This amplitude modulator will be installed in the near future on our PCI system to detect lower hybrid waves. The expected signal quality will be discussed. \\[4pt] [1] M. Porkolab, et al, IEEE Trans. of Plasma Sci. 34, 229 (2006). [Preview Abstract] |
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PP8.00010: Multi-species particle transport in GYRO simulations of low-collisionality, peaked-density H-mode plasmas in C-Mod D.R. Mikkelsen, M. Greenwald, R. Waltz, J. Candy Experimental results from Alcator C-Mod have confirmed earlier AUG and JET findings that spontaneous peaking of the density profile in H-mode plasmas depends on collisionality. Previously reported nonlinear, 'full-radius' GYRO simulations [1] of low-collisionality, peaked-density H-mode plasmas in C-Mod generated a particle pinch that was produced exclusively by higher-k modes. Nonlinear simulations of AUG have a similar character [2], and recent detailed linear analyses [2,3] suggest that density peaking may be common in low collisionality plasmas. Here we increase the number of ion species in the simulations to determine whether impurity pinches are also expected, and whether the degree of density peaking is predicted to differ for the three hydrogen isotopes. These simulations include experimentally relevant levels of several impurities, and a range of H/D and D/T mixes.\\[4pt] [1] D.R. Mikkelsen, et al., Bull. Am. Phys. Soc. 52, (2007) No. 16, 221, NP8.71 \\[0pt] [2] C. Angioni, et al., Phys. Plasmas 16 (2009) 060702 \\[0pt] [3] M. Maslov, et al., Nucl. Fusion 49 (2009) 075037 [Preview Abstract] |
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PP8.00011: Impurity Transport Studies Using the Multi-Pulse Laser Blow-Off System on Alcator C-Mod Nathan Howard, Martin Greenwald, Jim Terry, John Rice A new laser blow-off system for use in impurity transport studies on Alcator C-Mod was installed for the 2009 run campaign. Its capabilities include: multiple impurity injections during a single plasma pulse, remote manipulation of the ablated spot size, and a laser pulse capable of ablating a wide range in target Z. The use of a 650 mJ, ND:YAG laser operating at up to 10 Hz coupled with fast beam steering via a 2-D piezoelectric mirror mount able to move spot locations in 100 ms, and a remote controllable optical train allow spot sizes to vary from approximately 1 to 5 mm. Alcator C-Mod's extensive diagnostic capabilities (soft X-ray, Vacuum Ultraviolet (VUV), charge exchange spectroscopy, etc.) together with the improvements over standard laser blow-off systems allow for detailed studies of the impurity transport dependencies and mechanisms. An overview of system and results from the 2009 campaign are presented. [Preview Abstract] |
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PP8.00012: Calculating the Source Terms in the Momentum Diffusion Equation Yuri Podpaly, John Rice, Matt Reinke, Yijun Lin, David Mikkelsen, Ian Hutchinson There have been numerous attempts explain the anomalous transport of toroidal momentum in plasmas. Current methods involve the application of diffusive, proportional to gradient v, and convective, proportional to v, terms in the simple momentum conservation equation. Including these terms, however, does not provide a proper representation of the profile shape of the plasma rotation, so other terms must be included, which could be source or off-diagonal residual stress contributions in the momentum conservation equation. Using the spatially resolving soft x-ray spectrometer at Alcator C-Mod which provides temporally and spatially resolved rotation profiles, and solving the momentum conservation equation, it is possible to quantify the effects of diffusion and convection terms and reveal the spatial and temporal shape of a source required to explain the rotation velocity profiles. This poster focuses on running this analysis on discharges with L-H mode transitions, L-mode rotation inversions, and fast SSEP sweeps, which provide strong changes in the rotation profile. [Preview Abstract] |
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PP8.00013: Simulation of turbulent impurity transport in Alcator C-Mod W.L. Rowan, I.O. Bespamyatnov, K.W. Gentle, K.T. Liao, W. Horton, X. Fu, C.L. Fiore, S. Benkadda, S. Futatani, X. Garbet Theory/experiment comparisons are presented for impurity transport in H-mode and ITB discharges in Alcator C-Mod. The impurity profiles are hollow in H-mode and peaked for ITB. This range of profiles is a stringent test for theory and represents cases with significant impact on tokamak performance and particular interest for ITER. The data was selected to optimize the comparison: only one impurity charge state must be analyzed and simulated. H-mode is modeled with a new, generalized Hasegawa-Wakatani model that describes on equal footing the fueling ion, the impurity, and the electrons. The ITB is simulated using a 3-D global fluid code which includes two ion species and electrons which are introduced into the plasma with a fixed source. Transport is due to coupled ITG and TEM modes. [Preview Abstract] |
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PP8.00014: CXRS Impurity Density Measurement Techniques in C-Mod Igor Bespamyatnov, William Rowan, Kenneth Liao, Kenneth Gentle, Catherine Fiore, Robert Granetz, Dan Thomas The wide-view CXRS system installed on C-Mod provides measurements of the time-dependent boron density, temperature, and flow velocity profiles. Conventional technique for CXRS measurement of the absolute impurity density requires accurate characterization of the local neutral beam parameters such as energy, density and excitation states. Currently this data is routinely provided by the set of beam extraction, propagation and penetration codes. A new unifying beam code (ALCBEAM) is being developed to provide the local beam parameters at any point in the plasma. As an alternative to the conventional CXRS system, a new integrated CXRS/BES system is designed and will be installed on the Alcator C-Mod tokamak. In this approach, the BES system serves as an alternative, more accurate, means for measuring of the local beam densities. Advantages and disadvantages of the both approaches will be discussed. [Preview Abstract] |
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PP8.00015: Electron thermal transport studies with improved n$_{e}$/T$_{e}$ profiles measured by Core Thomson scattering diagnostic on Alcator C-Mod Yunxing Ma, Jerry Hughes, Darin Ernst, Martin Greenwald, John Rice, Yuri Podpaly The Core Thomson scattering (TS) diagnostic on Alcator C-Mod has been upgraded to provide n$_{e}$ and T$_{e}$ measurements with improved radial spatial resolution up to 1cm (r/a$\sim $0.05) in the range of r/a$<$0.6. We present the results from experimental studies of electron thermal transport with the improved n$_{e}$ and T$_{e}$ profiles measured by TS and T$_{i}$ profiles from High Resolution X-ray Spectrometer (HIREX) in Ohmic and RF heated plasma discharges. We also compare the calculated thermal diffusivities from profiles fitted using different combinations of measurements. The purpose of this study is to investigate how the transport coefficients are sensitive to the input profiles. Preliminary results from gyrokinetic and gyrofluid analysis are also presented. Continued profile sensitivity studies and detailed gyrokinetic analysis will be performed in the future. [Preview Abstract] |
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PP8.00016: Thomson Scattering Diagnostic for the Scrape Off Layer of Alcator C-Mod Jiexi Zhang, J.W. Hughes, Yunxing Ma In order to study heat and particle transport in the edge plasma of Alcator C-Mod, a new Thomson Scattering (TS) diagnostic for the Scrape-Off-Layer (SOL) has been designed. It will complement the existing Edge TS system, which measures a minimum electron temperature ($T_{e})$ of approximately 50 eV. The SOL TS system is designed to resolve the plasma profile of $T_{e}$ of 10$\sim $200 eV and electron density ($n_{e})$ as low as 1$\sim $2x10$^{19}$ m$^{-3}$ with a radial spatial resolution, d$R$, of about 2 mm (r/a $\sim $ 0.01). With 1064 nm Nd:YAG laser light as the laser source, we used plasma data from the C-Mod Core TS system as a guide in the design of a new spectral configuration for the 4-channel polychromators. The response of the Avalanche Photodiode (ADP) detectors was modeled to estimate the expected signals and signal to noise ratios (SNR). Simulations including the filters show that the system in the new configuration could produce meaningful measurements with SNR$>$3 when $T_{e}$ is 10 eV and $n_{e}$ is 3x10$^{19}$ m$^{-3}$ or less. The spectral location and bandwidth of each filter are optimized to allow calibration using both Raman and Rayleigh scattering in H$_{2}$ and D$_{2}$. [Preview Abstract] |
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PP8.00017: Thermal issues and relevant upgrades of the MSE diagnostic on Alcator C-Mod Jinseok Ko, Steve Scott, Syun'ichi Shiraiwa, Bob Mumgaard, Mark Smith The cause of shot-to-shot drifts in polarization angle measured by the Motional Stark Effect (MSE) diagnostic in Alcator C-Mod has been identified as thermal stress birefringence in the in-vessel optics. Based on a series of bench experiments that characterized the thermal response of the system, a single-layer heat shield with gold plating and a new lens holder which reduces the thermal conduction path to the lens have been designed, fabricated, and installed. These modifications are expected to reduce the temperature variation across the in-vessel lens to less than 0.5 Celsius degrees during C-Mod discharges with high power. The expected reduction in the spurious change in polarization angle is more than a factor of 10. New dielectric mirrors with reduced retardation are expected to reduce the drift by an additional factor of 2 to 4. [Preview Abstract] |
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PP8.00018: Upgraded thermal management of the MSE diagnostic on Alcator C-Mod R. Mumgaard, S. Scott, J. Ko, S. Shiraiwa, M. Smith A major upgrade to the Motional Stark Effect (MSE) diagnostic was completed in June 2009 to reduce spurious errors in polarization angle measurements arising from thermal stress birefringence in the in-vessel optics [Thermal issues and relevant upgrades of the MSE diagnostic on Alcator C-Mod, Ko]. The upgrade comprises a gold-plated thermal shield, a novel thermally-insulating mount mechanism for the in-vessel MSE lenses, and improved dielectric mirrors with low retardance. The thermal and polarimetric performance of these systems will be evaluated over the course of the FY09 Alcator C-Mod experimental campaign. We compare the actual effectiveness of the thermal shield as determined from 20 thermocouple measurements to predicted performance. Reductions in the spurious variation in polarization angle will be evaluated by measuring the pitch angle in identical plasma discharges widely separated in time. Additionally, the effect of retardation in the mirrors and photoelastic modulators on calibration errors will be discussed. Work supported by US DOE Contracts DE-AC02-09CH11466 and DE-FC02-99ER54512. [Preview Abstract] |
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PP8.00019: The Alcator C-Mod FIR Polarimeter P. Xu, J.H. Irby, J. Bosco, A. Kanojia, R. Leccacorvi, E.S. Marmar, P. Michael, R. Murray, Y. Rokhman, R. Vieira, D.L. Brower, W.X. Ding, D.K. Mansfield A multi-chord FIR polarimetry diagnostic is being developed for the Alcator C-Mod Tokamak to be used to determine the q-profile and to study density and magnetic field fluctuations.~This poloidally viewing system using retro-reflectors on the inner wall will have geometry and fields similar to those planned for ITER. The full optical layout will be discussed, as well as simulations of the expected Faraday and Cotton-Mouton signal levels.~Bench test results from a single chord system including all optical components will be presented, and preliminary experimental results from C-Mod will be compared with simulated Faraday rotation angle calculated using Thomson Scattering density profiles and EFIT reconstructions of actual C-Mod plasmas. [Preview Abstract] |
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PP8.00020: Initial results from divertor heat-flux instrumentation on Alcator C-Mod B. LaBombard, D. Brunner, J. Payne, M. Reinke, J.L. Terry, J.W. Hughes, B. Lipschultz, D. Whyte Physics-based plasma transport models that can accurately simulate the heat-flux power widths observed in the tokamak boundary are lacking at the present time. Yet this quantity is of fundamental importance for ITER and most critically important for DEMO, a reactor similar to ITER but with $\sim $4 times the power exhaust. In order to improve our understanding, C-Mod, DIII-D and NSTX will aim experiments in FY10 towards characterizing the divertor ``footprint'' and its connection to conditions ``upstream'' in the boundary and core plasmas [2]. Standard IR-based heat-flux measurements are particularly difficult in C-Mod, due to its vertical-oriented divertor targets. To overcome this, a suite of embedded heat-flux sensor probes (tile thermocouples, calorimeters, surface thermocouples) combined with IR thermography was installed during the FY09 opening, along with a new divertor bolometer system. This paper will report on initial experiments aimed at unfolding the heat-flux dependencies on plasma operating conditions. [2] a proposed US DoE Joint Facilities Milestone. [Preview Abstract] |
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PP8.00021: Plasma sheath heat flux transmission in the Alcator C-Mod divertor D. Brunner, B. LaBombard, J. Payne A new array of embedded sensors has been installed in the Alcator C-Mod divertors to measure plasma heat flux deposition profiles. These include tile thermocouples, calorimeters [2] and a unique set of surface temperature thermocouple probes [3]. The latter sensors have the ability to measure the surface temperature evolution with $\sim $ 10 ms time response during a plasma discharge. From a simple 1-D heat transport model, surface heat fluxes can be computed as a function of time. In addition, embedded Langmuir probes simultaneously record poloidal profiles of plasma density and electron temperature at the divertor surface. Taken together, these data allow sheath heat-flux transmission factors to be inferred -- a fundamental quantity of plasma-sheath physics. This paper will report initial observations and estimates of heat flux and heat deposition profiles on C-Mod's outer divertor and the implied plasma-sheath transmission factors. [2] J. Payne, \textit{et al.}, this meeting. [3] S. Gangadhara, \textit{et al.}, Bull. Am. Phys. Soc. \textbf{41} (1996), 1550. [Preview Abstract] |
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PP8.00022: Design and analysis of divertor calorimetry in Alcator C-Mod J. Payne, D. Brunner, B. LaBombard, J.L. Terry An array of 14 calorimeter sensors was recently installed in a set of specially-design ``ramped'' tiles in C-Mod's outer divertor. Each sensor consists of a 5 mm diameter molybdenum body, thermally isolated from its host tile, with an embedded type-K thermocouple positioned 5 mm from the plasma-facing end. Optimization of the design was performed by mechanical modeling with SOLID EDGE combined with thermal analysis using the ALGOR package. The plasma-facing surface of the tiles and embedded calorimeters are ramped at 2 degrees in the toroidal direction to eliminate plasma shadowing effects and to improve signal-to-noise by enhancing the local heat flux. Temperature readings taken before and after a plasma discharge allow energy deposition profiles to be inferred. The entire array is viewed by an IR imaging system (ElectroPhysics Titanium 550M Camera), allowing a cross-check between these two diagnostics. This paper will report on the diagnostic design and thermal analysis with data from C-Mod discharges. [Preview Abstract] |
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PP8.00023: Interpretation and implementation of an ion sensitive probe (ISP) as a plasma potential diagnostic. Roman Ochoukov, Dennis Whyte, Bruce Lipschultz, Brian LaBombard An ISP is being developed as a robust diagnostic capable of measuring space potentials ($\Phi _{P})$ in the boundary plasma of tokamaks. The ISP relies on the large difference between the ion and electron gyroradii ($\rho _{i}$/$\rho _{e} \quad \sim $60) to reduce electron collection at a collector which is oriented parallel to the magnetic field, and recessed behind the ISP shield a distance of $\sim \rho _{i}$. The shield is independently biased. By sweeping the collector voltage, while maintaining a constant voltage difference of a few volts between the shield and the collector (with V$_{WALL} \quad <$ V$_{COLLECTOR})$, we obtain only ion current. The ISP was studied in a magnetized plasma chamber (DIONISOS, B = 0.04 T, n$_{e}$ = 10$^{16}$-10$^{18}$ m$^{-3}$, $\rho _{e}$ = 0.2 mm) at MIT. A model of the ISP potential shows that the collected ion current should reach zero for V$_{WALL}$ \underline {$>$} $\Phi _{P}$ and that space-charge effects are critical to the probe operation. The plasma potentials obtained from the ISP match well the plasma potentials measured with an emissive probe over a wide range of plasma conditions in DIONISOS, yet with a more robust physical design than the emissive probe. [Preview Abstract] |
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PP8.00024: Development of a novel in-situ accelerator-based surface diagnostic for Alcator C-Mod Zach Hartwig, Dennis Whyte Boundary science in magnetic fusion devices is severely hindered by a dearth of in-situ diagnosis of the surfaces of Plasma Facing Components (PFC). The customary laboratory surface diagnostic, nuclear scattering using MeV-range ions, is being adapted to the Alcator C-Mod tokamak. The design features toroidally and poloidally resolved measurements of PFC surface element and isotope compositions on a shot-to-shot basis with ~1 cm spatial resolution and sub-micron depth resolution. Several novel design features are described. An RF quadrupole accelerator supplies a high-current 0.93 MeV deuteron ion beam for injection into C-Mod between discharges. The ion beam is steered to a large fraction of the PFC surfaces by applying tokamak toroidal and poloidal fields. The deuterons have high Q, large nuclear-reaction cross-sections with most low-Z isotopes; the resulting high-energy gammas and neutrons are then detected for quantitative analysis of the surface compositions. Numerical modeling of these processes towards PFC measurements of interest are described, including boron film depth, deuterium fuel retention, isotope tracing for transport studies and PFC net erosion. [Preview Abstract] |
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PP8.00025: A Study of Tungsten Migration in the Alcator C-mod Divertor using Particle Induced X-Ray Emission Analysis Harold Barnard, Dennis Whyte, Bruce Lipschultz, Florent Sainct An ion beam analysis study of campaign integrated, poloidal Tungsten (W) migration patterns in the Alcator C-mod tokamak is presented. Eroded W from a toroidally continuous row of W tiles near the outer divertor strike point is used as a tracer to map W erosion and redeposition onto a set of Mo and W tiles that covered the poloidal extent of the C-Mod lower divertor which were removed following the 2008 experimental campaign. These tiles were examined for W using external (in air) particle induced X-ray emission (X-PIXE) analysis; a highly W sensitive ion beam analysis (IBA) technique in which a characteristic x-ray emission is induced from a material surface as it is exposed to an external proton beam, produced by a electrostatic tandem accelerator. With a set of systematic high spacial resolution measurements ($\sim 3 mm$ resolution), complete poloidal profiles of W redeposition have been constructed. These profiles indicate W transport and redeposition of up to $1.5\times 10^{21} atoms/m^2$ ($\sim 14 nm$ of equivalent W thickness) in several regions including the outer divertor, the inner divertor, and inside the private flux region. A comprehensive description and explanation of these results is presented. [Preview Abstract] |
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PP8.00026: Effect of plasma elongation on disruption runaway electrons Robert Granetz, Dennis Whyte, Valerie Izzo Studies of runaway electron (RE) populations during disruptions on a number of different tokamaks have shown two distinctly different types of behavior: (a) some machines tend to observe RE's during a significant number of current quenches, and (b) some machines rarely observe RE's during the disruption current quench. Of those that do see runaways, a general trait is that they run circular or low elongation and/or limited discharges (FTU, Tore-Supra, TEXTOR, JT-60U), and conversely, those that don't see runaways tend to run elongated, diverted discharges (C-Mod, DIII-D, ASDEX-U). This suggests that elongation and/or vertical stability might play a role in RE confinement during disruptions. An experiment to test this hypothesis on Alcator C-Mod uses lower hybrid current drive to generate a strong RE population, and gas jet injection to trigger reproducible disruptions. Behavior of runaways during disruptions in both low elongation and high elongation equilibria are compared. Experimental findings will be presented and compared to NIMROD modeling predictions, and implications for ITER will be discussed. [Preview Abstract] |
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PP8.00027: Excitation of unstable TAEs and stable n=0 modes in Alcator C-Mod J. Sears, A. Bader, R.R. Parker, G.J. Kramer Toroidicity-induced Alfv\'{e}n Eigenmodes (TAEs) are weakly damped MHD modes in tokamak plasmas. The modes occur at discrete frequencies near $\omega_{TAE}=v_A/2qR$, $\left( v_A=B/\sqrt{\mu_0\rho} \right)$ in a gap of the continuous spectrum of Alfv\'{e}n waves. Unstable TAEs are excited by ICRF heating of at least $3.5$ MW in Alcator C-Mod L-mode plasmas. These unstable modes have toroidal mode numbers in the range of $n=-6$ to $n=6$. In contrast, stable resonant modes that are observed in these plasmas at similar and lower ICRF powers by the Active MHD diagnostic in the TAE frequency range commonly have toroidal mode numbers of $n=0$, which precludes a TAE or EAE identity. The origin of these modes is explored with the NOVA-K code, and the destabilizing role of the energetic hydrogen tail as measured by the Neutral Particle Analyzer is presented. [Preview Abstract] |
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PP8.00028: The emission spectrum of fast neutrals in Alcator C Mod and its use as a fast ion diagnostic K.T. Liao, W.L. Rowan, I.O. Bespamyatnov, A. Bader, R.S. Granetz The emission spectrum of fast neutrals will be used to investigate the spatial distribution and temporal dynamics of fast ions in Alcator C-Mod. Fast hydrogen ions are formed by wave particle interaction between externally applied radio frequency (RF) fields and the plasma. These charge exchange with the neutrals in a diagnostic beam to form a population of fast neutrals. The newly formed fast neutrals emit a Doppler-shifted H$_{\alpha}$ spectrum which stands out against the wing of the broad ambient background spectrum of plasma D$_{\alpha}$. Competing spectral features are emission from the cold D and H at the edge, the Doppler-shifted, beam emission spectrum, and various continuum emission processes. The unique contribution of this diagnostic is that it measures fast ions inside the plasma. The information derived from this and other fast ion diagnostics will be used to validate the physical models for RF deposition via minority ions. [Preview Abstract] |
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PP8.00029: Measurements of Fast Ion Distribution in ICRF Heated Plasmas A. Bader, J. Sears, P. Bonoli, R. Granetz, R. Parker, S. Wukitch Alcator C-Mod uses ICRF for the bulk auxiliary heating and relies primarily on hydrogen minority heating scenarios. Measuring the resulting hydrogen ion distribution provides an opportunity to validate upgraded ICRF simulation capability that includes non-Maxwellian ions. The Compact Neutral Particle Analyzer (CNPA) is a diagnostic employed on Alcator C-Mod to measure this fast ion distribution function. The diagnostic can measure the energy distribution of the fast ion tail, serving as a benchmark for simulation results and allowing for an assessment of the simulation algorithm and physics kernel. In this poster, we will present results from the detector in the most recent campaigns, including expanded capabilities in the 2009 campaign. We will discuss the calculation of the fast ion distribution from the measured CNPA distribution and the effective temperature. [Preview Abstract] |
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PP8.00030: Experimental Studies of ICRF Mode Conversion with Phase Contrast Imaging and Comparison with Full-wave Simulations N. Tsujii, M. Porkolab, E.M. Edlund, L. Lin, Y. Lin, S.J. Wukitch, P.T. Bonli, J.C. Wright, E.F. Jaeger Mode conversion of fast wave to ion Bernstein wave and ion cyclotron wave has been observed with the phase contrast imaging diagnostic (PCI) in D(-$^3$He) plasmas in Alcator C-Mod [1, 2]. The wave electric fields are simulated for the experimentally measured equilibrium profiles using the full- wave codes TORIC [3] and AORSA [4]. Although the predictions of the two codes agree, the simulated PCI signals are larger than the measurements by at least a factor of 10. This could be partially explained by edge losses, which are not accounted for in the simulations, but is not fully understood. Predictions of direct rf flow drive for this scenario will also be presented. \\[4pt] [1] E. Nelson-Melby, et al., Phys. Rev. Lett. 90, 155004 (2003).\\[0pt] [2] Y. Lin, et al., Phys. Rev. Lett. 101, 235002 (2008).\\[0pt] [3] M. Brambilla, Plasma Phys. Control, Fusion 41, 1 (1999).\\[0pt] [4] E. F. Jaeger, et al., Phys. Plasmas 8, 1573 (2001). [Preview Abstract] |
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PP8.00031: SOL reflectometer Cornwall Lau, Greg Hanson, John Wilgen, Yijun Lin, Steve Wukitch Antenna-plasma interactions during RF heating and current drive are greatly influenced by the SOL density profile. A swept-frequency X-mode reflectometer is being built for Alcator C-Mod to measure the SOL density profiles at top, middle and bottom locations in front of both the new Lower Hybrid Launcher and the new ICRF antenna [1]. The system is planned to operate between 100 and 146 GHz at sweep rates from 10 $\mu $s to 1 ms per sweep and will cover a density range of approximately 10$^{16}$ to 10$^{20}$ m$^{-3}$ at 5-5.4 T. Design, test data, and calibration results for the electronics and waveguide runs for both the ICRF and LH antenna will be shown. Preliminary plans for using the reflectometer density profile results to study antenna-plasma interactions such as RF loading and RF sheaths will be discussed. \\[4pt] [1] Rev. Sci. Instrum. \textbf{79}, 10F114~(2008) This work is supported by U.S. DoE under awards DE-AC05-00OR22725 and DE-FC02-99ER54512. [Preview Abstract] |
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PP8.00032: ICRF Antenna Design Studies for Alcator C-Mod M.L. Garrett, S.J. Wukitch, S. Shiraiwa, O. Meneghini, Y. Lin, P. Koert, W. Beck Two major challenges for ICRF utilization are reducing impurity generation and increasing reliability at high voltages. In Alcator C-Mod, we are designing a new 4-strap antenna where we seek to lower E-parallel and improve voltage handling. Simulations were conducted to analyze the effect of antenna orientation on the parallel electric field, which is thought to influence impurity production. ICRF antenna coupling was also simulated using both cold plasma and dielectric models. Previous experiments suggest that improved voltage handling may be obtained through the use of refractory metals compared with copper. A test stand has been designed and built to characterize ICRF relevant voltage breakdown as a function of: antenna material, neutral pressure, and magnetic field orientation. Results from simulations and experiments will be presented along with details of the new design. [Preview Abstract] |
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PP8.00033: NSTX AND GENERAL SPHERICAL TORUS |
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PP8.00034: NSTX Facility Highlight and Upgrade Plan Masaayuki Ono The new NSTX facility additions included dual lithium (li) dropper to inject li powder directly into the plasma discharge complementing the dual li evaporators for between-shots li coverage of divertor region. Li evaporator together with EF/RWM feedback control enabled the record pulse length, stored energy, and confinement in NSTX. An edge sample probe allowing in-situ surface thermal desorption spectroscopy was introduced. The high harmonic fast wave heating system upgrade was implemented to double the power handling capability to support the current ramp-up and sustainment research. The Coaxial Helicity Injection (CHI) absorber coil was also introduced to form the field null to reduce the absorber arc. The CHI was able to increase the start-up plasma current by 100 kA above the ohmic induction. For the 2010 run, new upgrades are planned including a liquid-li divertor target to achieve lower collisionality and a Beam Emission Spectroscopy diagnostic to extend the localized ion gyro-radius scale turbulence measurements to complement the electron gyro-radius scale high-k scattering system. For the longer term, a new center stack and second NBI system are planned to access lower collisionality, full non-inductive operation, and current profile control. [Preview Abstract] |
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PP8.00035: Physics design of the NSTX Upgrade Jonathan Menard Access to low collisionality is important to more fully understand transport, stability, and non-inductive start-up and sustainment in the ST. For example, NSTX and MAST observe a strong (nearly inverse) scaling of normalized confinement with collisionality, and if this trend holds at low collisionality, high fusion neutron fluences could be achievable in very compact ST devices. Such considerations motivate the proposed upgrade of NSTX to higher field, current, and heating power. To enable engineering design of the upgrade, systematic free-boundary equilibrium calculations have been performed to determine the upgrade poloidal field requirements as a function of plasma shape, magnetic balance, internal inductance, and beta. Additional poloidal field coils in the divertor region are proposed to provide very high flux expansion for reduction of high predicted divertor heat flux. TRANSP simulations indicate that more tangential neutral beam injection (NBI) can increase NBI current drive efficiency by up to a factor of two, enable control of the core q profile, and ramp-up the plasma current to near mega-ampere levels. These and other physics design activities will be discussed. [Preview Abstract] |
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PP8.00036: Continuation of Lithium Aerosol Injection Experiments on NSTX D.K. Mansfield, A.L. Roquemore, H. Kugel, R. Maingi, J. Irby, Z. Wang During the 2008 run campaign, a Li powder dropper was installed on NSTX that successfully injected up to 35 mg/s of Li aerosol into the SOL. Initial improvements in the plasma performance from these initial experiments warranted the installation of a second Li dropper for the 2009 campaign. Design improvements in the dropper have resulted in accurate control of the flux of Li powder injected. The improved duel-dropper system has injected lithium fluxes of from 30 - 140 mg/s. At the highest flux, plasmas of 950 kA with 6 MW off NBI auxiliary heating have been successfully operated. This flux corresponds to 2.5 x106 - 5.8 x106 aerosol particles/s and is stoichiometrically equivalent 80 - 187 Torr L/s of D2. Operation of the Li dropper and the effects of the Li aerosol on the plasma performance will be discussed. [Preview Abstract] |
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PP8.00037: Modifications in divertor and scrape-off layer conditions with lithium coatings in NSTX V.A. Soukhanovskii, H.W. Kugel, R. Kaita, R.E. Bell, D.A. Gates, J.E. Menard, D. Mueller, B.P. Leblanc, S.F. Paul, A.L. Roquemore, D.P. Stotler, R. Maingi, R. Raman, A. Yu. Pigarov, R. Smirnov Two lithium evaporators were used in NSTX to deposit lithium coatings on graphite plasma-facing tiles in the lower divertor, enabling neutral deuterium pumping and resulting in reduced recycling. This was evident from the reduction by 50\% -- 80\% in D$_{\alpha}$ poloidal and divertor brightnesses, as well as the ion outfluxes, edge neutral pressure, core and divertor $n_e$. The two point and multi-fluid UEDGE code modeling suggested a shift of the outer divertor operating space toward the sheath-limited transport regime with lithium. Particle balance calculation showed that the deuteron inventory was well controlled and remained nearly constant in the long (1.2--1.4 s) ELM-free H-mode discharges with lithium coatings. However, $n_e$ rose due to the increasing carbon inventory. The carbon source, estimated from C~II and C~III intensities and atomic S/XB factors, was reduced, suggesting that impurity accumulation was due to the improved particle confinement. Supported by the U.S. DOE under Contracts DE-AC52- 07NA27344, DE-AC02-09CH11466, DE-AC05-00OR22725, DE-FG02-08ER54989, and W-7405-ENG-36. [Preview Abstract] |
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PP8.00038: Dependence of impurity accumulation on Ip and the outer gap in the presence of lithium deposition in NSTX Stephen Paul, M.G. Bell, S.P. Gerhardt, H.W. Kugel, J. Kallman, R. Kaita, J.A. Robinson Shortly after commencing evaporation of a lithium coating onto the carbon surfaces of the lower divertor and other plasma-facing components in NSTX, a reduction in the frequency of ELMs was observed, eventually resulting in complete suppression for periods up to about 1s as deposition continued. Co-incident with ELM suppression, the effective ion charge increased as a result of a buildup in carbon, though lithium itself remained at a low level in the core. Radiated power steadily increased as medium-Z metallic impurities, notably iron, accumulated in the core of the plasma. This phenomenon may occur in these NBI-heated, deuterium H-mode plasmas because the lithium coating modifies the recycling of hydrogenic species, affecting the plasma's edge and scrape-off layer. Another possibility, the role of sputtering from metal surfaces by fast beam ions introduced by NBI heating, has been investigated by changing the amount of fast beam ion loss by varying the plasma current, neutral beam tangency radius and the gaps between the plasma boundary and surrounding components. Results from bolometry and XUV spectroscopy show that the plasma current affects accumulation of metals much more strongly than the gap between the plasma and the outer limiter. [Preview Abstract] |
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PP8.00039: Identification and time evolution of impurities in NSTX plasmas J.K. Lepson, P. Beiersdorfer, M. Bitter, L. Roquemore, G. Zimmer, S. Gerhardt, J. Kallman, R. Kaita Operation of the NSTX tokamak with lithium-coated plasma facing components has shown many beneficial changes, including MHD quiescence and higher electron temperatures near the edge. Impurity accumulation, however, is of concern, especially during ELM suppression. Spectroscopic diagnostics are important in determining the composition and concentration of plasma impurities, which can vary dramatically between shots and for different run conditions. We present data from the Livermore XEUS and LoWEUS spectrometers, which observe the extreme ultraviolet and soft x-ray regions on NSTX. XEUS and LoWEUS are complementary, and have been set up to cover the 10-250 A range during the most recent run period. Because of their higher spectral resolution than provided by SPRED, the instruments can discern lines from different elements and charge states. We have also implemented time resolved data acquisition, providing the first data on when the particular metal impurities appear in the plasma. The time resolution has been 130 ms. This value is expected to improve in the near future. We present spectra from NSTX exhibiting a wide range of impurities, including lithium, boron, carbon, oxygen, neon, titanium, iron, nickel, and copper, which serve as reference spectra for tracking the different impurities. [Preview Abstract] |
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PP8.00040: Time dependent chemical interactions of lithium, deuterium, and oxygen on lithium-coated graphite surfaces C.N. Taylor, J.P. Allain, B. Heim, C.H. Skinner, H.W. Kugel, R. Kaita, A.L. Roquemore Lithium conditioning of plasma facing components has been used for particle control in fusion devices such as TFTR, CDX-U, FTU, T-11M, TJ-II and NSTX and has yielded improved plasma performance. A PMI probe has been installed on NSTX to provide an in-situ diagnostic for surface chemistry and deuterium retention measurements. Recent controlled laboratory experiments at Purdue University are investigating the chemical functionalities in lithiated graphite and the mechanism by which D is retained. XPS results show that Li reacts readily with residual oxygen in ATJ graphite, and immediately begins to intercalate into the substrate. Additionally, it has been found that Li-O and Li-C react to D proportional to the lithium thickness, suggesting a D saturation threshold. This work investigates the transient nature of the lithium and oxygen functionalities, their response to time varying D flux, and the implications to NSTX. [Preview Abstract] |
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PP8.00041: Modeling of low-recycling divertor with lithium coating in NSTX R.D. Smirnov, A.Yu. Pigarov, S.I. Krasheninnikov, C.H. Skinner, D.P. Stotler, R.E. Bell, H.W. Kugel, V.A. Soukhanovskii, T.D. Rognlien, M.E. Rensink, R. Maingi We have modeled D-Li-C edge-plasma transport with UEDGE code in support of lithium coating experiments on NSTX. The transition from the usual high-recycling regime (HRR) to low-recycling regimes (LRR) was simulated by varying hydrogen pumping by the Li coated divertor plates. The LRR modeling demonstrates the onset of sheath-limited divertor plasma conditions, flat parallel profile of electron temperature in SOL, and plate thermal loading dominated by parallel heat conduction. The results also show that Li impurities (mainly originating from the coated divertor plates) are well retained in the divertor volume. The modeled core contamination level for Li+3 ions is 0.1{\%} that is consistent with recent experimental NSTX data. At the same time, the simulated and experimental levels of wall-originated C+6 (modeled with anomalous inward pinch in core) can be as high as $\sim $10{\%} (maximal Zeff$\sim $4). High surface temperatures ($>$800K) are predicted to result in a divertor transition from LRR to HRR due to excessive evaporation and recycling of Li. The work is supported by DoE Grant DE-FG02-08ER54989 (UCSD) and Contracts DE-AC02-09CH11466 (PPPL), DE-AC52-07NA27344 (LLNL), DE-AC05-00OR22725 (ORNL). [Preview Abstract] |
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PP8.00042: Modeling of Balmer series deuterium spectra with the Cretin code for diagnosing the inner divertor re-attachment threshold in NSTX discharges with lithium coatings F. Scotti, V.A. Soukhanovskii, H.A. Scott Application of evaporated lithium coatings on graphite divertor tiles in NSTX led to a reduction of divertor recycling. The inner divertor electron density and recombination rate were also drastically reduced, suggesting that the normally detached inner divertor re-attached in many lithium-assisted ELM-free H-mode discharges. This observation was based on the divertor brightness profiles of Stark-broadened ultraviolet spectral lines from the Balmer series $n=2-7...12$ transitions. To understand the divertor transport regimes with reduced recycling and the density thresholds for both the inner divertor detachment and X-point MARFE formation, we are developing a simulation of the NSTX divertor spectra in a realistic viewing geometry using the Cretin code. The non-local thermodynamic equilibrium radiation transport code Cretin uses a 1-D plasma model with neutral diffusion and line shape calculations based on the quasi-static ion microfield approximation and a binary electron impact collision model. [Preview Abstract] |
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PP8.00043: Study of Thermo-electrically Driven Scrape-Off-Layer Current (SOLC) in NSTX Tokamak Hiro Takahashi, Eric Fredrickson, Josh Kallman, Stefan Gerhardt, Robert Kaita, Dennis Mansfield, Stuart Zweben, Rajesh Maingi SOLC flow in ELMing discharges in NSTX has been found qualitatively similar to that in DIII-D [1], with steady small amplitude during a quiescent period leading to a rapid change to much larger values, abruptly beginning just before ELM onset (as detected by D-alpha signal). Current density (spatially averaged normal to divertor plate) in NSTX is comparable to that in DIII-D during the main ELM event, but is much smaller during the quiescent period. The possible thermo-electric origin of the current in the quiescent period will be investigated using flush-mounted Langmuir probes and halo current sensors to see whether observed SOLC distributions can be quantitatively related to sheath parameters at divertor plates. Effect on SOLC of Lithium coating of plasma facing surfaces will also be examined. \\[4pt] [1] H. Takahashi, et al., Paper TP6.00033, 50$^{th}$ Ann. Mtg. of APS-DPP, Dallas, TX, Nov., `08. [Preview Abstract] |
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PP8.00044: Status of National Spherical Torus Experiment Liquid Lithium Divertor H.W. Kugel, M. Viola, R. Ellis, M. Bell, S. Gerhardt, R. Kaita, J. Kallman, R. Majeski, D. Mansfield, A.L. Roquemore, H. Schneider, J. Timberlake, L. Zakharov, R.E. Nygren, J.P. Allain, R. Maingi, V. Soukhanovskii Recent NSTX high power divertor experiments have shown significant and recurring benefits of solid lithium coatings on plasma facing components to the performance of divertor plasmas in both L- and H- mode confinement regimes heated by high-power neutral beams. The next step in this work is the 2009 installation of a Liquid Lithium Divertor (LLD). The 20 cm wide LLD located on the lower outer divertor, consists of four, 80 degree sections; each section is separated by a row of graphite diagnostic tiles. The temperature controlled LLD structure consists of a 0.01cm layer of vacuum flame-sprayed, 50 percent porous molybdenum, on top of 0.02 cm, 316-SS brazed to a 1.9 cm Cu base. The physics design of the LLD encompasses the desired plasma requirements, the experimental capabilities and conditions, power handling, radial location, pumping capability, operating temperature, lithium filling, MHD forces, and diagnostics for control and characterization. [Preview Abstract] |
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PP8.00045: Development of Operational Scenarios and Edge Diagnostics for the NSTX Liquid Lithium Divertor J. Kallman, R. Kaita, H. Kugel, E. Kolemen, M.A. Jaworski, J.W. Ahn, D. Gates, S. Gerhardt, R. Maingi, J. Robinson, A.L. Roquemore, V. Soukhanovskii During the 2010 run campaign, NSTX will continue to investigate the effects of lithium-coated plasma facing components utilizing a Liquid Lithium Divertor (LLD), to be installed at the end of 2009 run operations. In order to balance the pumping capabilities of the LLD (located on the outboard divertor) with the performance benefits of high triangularity discharges (typically with the strike points on the inner divertor), a new intermediate triangularity discharge was developed. Operational LLD-ready scenarios utilizing this shape and new strike point control algorithms were successfully developed. Neutral beam power and current scans were conducted to characterize density and temperature profiles as well as radiated power due to impurities. In addition, new diagnostics will be necessary to better assess the effect of the LLD on edge plasma parameters, key among these is a high density Langmuir probe array for measurements of plasma density and temperature in the vicinity of the LLD. [Preview Abstract] |
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PP8.00046: Diagnostics for Evaluating Performance of NSTX Liquid Lihium Divertor R. Kaita, H. Kugel, J. Kallman, B. LeBlanc, S. Paul, A.L. Roquemore, C. Skinner, V. Soukhanovskii, R. Maingi, J.-W. Ahn, J. Wilgen, J.-P. Allain, C. Taylor A Liquid Lithium Divertor (LLD) is being installed on NSTX to investigate particle control and power handling with liquid lithium as plasma-facing component (PFC). The LLD is expected to provide a low-recycling plasma-facing component (PFC). To study the effects of such a PFC on plasma performance, a variety of edge measurements are required. Since its surface is highly reflective at visible wavelengths, a Lyman-alpha detector array will be used to monitor the recycling. To understand changes in edge transport, electron temperature and density measurements will be made with Langmuir probes mounted in PFC's near the LLD, and the edge sightlines of a multipoint Thomson scattering system. A frequency-scanning reflectometer will also provide scrapeoff layer electron density profiles. The LLD response to heat loads will be examined with infrared cameras and thermocouples. Diagnostics are also needed to measure the erosion and codeposition of lithium. They include quartz deposition monitors and a retractable probe for exposing samples to the plasma. [Preview Abstract] |
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PP8.00047: Characteristics of energy transport of Li- and non-Li-conditioned plasmas in NSTX Siye Ding, Stanley Kaye, Ron Bell, Robert Kaita, Henry Kugel, Benoit LeBlanc, Stephen Paul, Baonian Wan The transport properties of NSTX plasmas obtained during the 2008 experimental campaign have been studied and are reported here. Transport trends and dependences have been isolated, and it is found that both electron and ion energy transport coefficients have strong dependences on local values of n*grad T, which in turn is strongly dependent on local current density profile. Without identifying this dependence, it is difficult to identify others, such as the dependence of transport coefficients on poloidal magnetic field (or q), plasma current and heating power. In addition, a comparison between discharges with and without Lithium wall conditioning has been made. While the trends in the two sets of data are similar, the thermal transport loss, especially in the electron channel, is found to strongly depend on the amount of Lithium deposited, decreasing by up to 50\% of its no-Lithium value. [Preview Abstract] |
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PP8.00048: Investigation of Power Balance and Excess Ion Heating in the National Spherical Torus Experiment P.W. Ross, R.E. Bell, D.A. Gates, S.M. Kaye, B.P. LeBlanc, S.S. Medley, R.B. White Neutral Beam modulations were used to create a transient heat pulse to investigate power balance of thermal ions in NSTX discharges. The primary effect of the beam modulation is a change in the ion temperature, with very little electron response. Classical neutral beam heating is expected to preferentially heat electrons with only $\sim $1/3 of the power going to the ions. Grad-Shafranov reconstructions of the plasma were performed using Thomson scattering, CHERS and MSE to measure the plasma temperature profiles and constrain the current profile. The ion power balance was calculated using the TRANSP time dependent transport calculation code. Assuming ion neoclassical transport, TRANSP calculates excess ion heating for some plasma conditions. This heating is strongest in the presence of high frequency MHD modes. The predicted peak of these modes correlates well with the location of strongest heating. Full orbit calculations of thermal ions in the presence of these modes show that stochastic heating can be sufficient to explain the excess heating. This work was supported by DoE contract No. DE-AC02-09CH11466. [Preview Abstract] |
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PP8.00049: Gyrokinetic studies of neoclassical poloidal rotation with finite orbits. R.A. Kolesnikov, W.X. Wang, F.L. Hinton, R.E. Bell, S.M. Kaye, W.M. Tang, W.W. Lee A significant discrepancy of impurity poloidal rotation from local theoretical predictions is found in tokamak experiments. The amplitude of carbon poloidal velocity is observed to be smaller in NSTX [1] and larger in DIIID [2] and JET [3] devices compared to appropriate neoclassical estimates. We performed global neoclassical simulations of toroidal plasmas using GTC- NEO particle-in-cell delta-f code [4] with a new multiple ion- species capability. In case of large-aspect ratio tokamak with sharp toroidal flow profile, the difference between simulated carbon poloidal flow and neoclassical estimate is found to be due to presence of finite ion orbits. The dominant mechanisms underlying this nonlocal effect are identified to be due to ion orbit shearing and orbit squeezing. Such finite orbit mechanisms result in generation of additional parallel ion flow due to shear in the toroidal rotation and shear in the radial electric field accordingly. We also compare simulations as well as theoretical estimates based on identified nonlocal effects with some experimental data from NSTX as well as other magnetic configurations to clarify the role the nonlocal effects might have on observed poloidal velocity. [1] R.E. Bell, this meeting. [2] W. Solomon, Physics of Plasmas 13, 056116 (2006). [3] K. Crombe, EPS, Sopfia (2009). [4] W.X. Wang, Physics of Plasmas 13, 082501 (2006). [Preview Abstract] |
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PP8.00050: Sensitivity of Tearing Mode Beta Limits to Rotation and Current Profile R.J. Buttery, C.D. Challis, R.J. La Haye, S.P. Gerhardt Unless carefully controlled, 2/1 tearing modes are likely to limit the $\beta$ of tokamak plasmas with $q=2$ surfaces. Mode onset may depend on changes to intrinsic tearing stability ($\Delta^\prime$) and external triggers. Studies have probed this underlying physics by perturbing plasma rotation and current profiles to determine how these mechanisms apply. On NSTX, $n=1$ and $n=3$ braking have been deployed to change rotation profiles at mode onset, and gauge whether action is through rotation shear and so intrinsic island stability, or differential rotation with respect to external triggers. This also provided key measurements of error field tolerability. On \mbox{DIII-D} saturated mode sizes also show a response to rotation changes. On JET the role of intrinsic stability was directly tested in hybrid plasmas, with changes to q profile leading to significant differences in 2/1 $\beta_N$ limit, identifying optimal conditions for stability. Results to be reported show a significant role of intrinsic tearing stability governing mode onset. [Preview Abstract] |
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PP8.00051: Study of Error Field Physics in Tokamaks and Implications for ITER Jong-Kyu Park, Jonathan Menard, Stefan Gerhardt, Allen Boozer, Steve Sabbagh, Michael Schaffer, Stephen Wolfe Tokamaks are sensitive to a small nonaxisymmetric field. Nonaxisymmetric error fields in tokamaks can cause a disruption by plasma locking, or degrade confinement by Neoclassical Toroidal Viscosity (NTV). Hence, the correction of error field is important to improve plasma performance. Recent NSTX experiments showed that the critical n=1 error field to cause a locking is much smaller at high beta than expected from the standard scaling at low beta, and also that the correction of n=3 field is essential to sustain high toroidal rotation. Both results indicate the importance of error field correction, not only for resonant n=1 field at low beta, but also at high beta and for nonresonant fields. This enlarged parametric space in error field physics requires a deeper understanding of the plasma response. Many useful insights on plasma response effects can be obtained using the Ideal Perturbed Equilibrium Code (IPEC) coupled with NTV theory. IPEC and NTV calculations for the recent NSTX results will be combined with the multi-machine locking scaling at low beta, in order to improve the predictability of error field thresholds and correction in ITER. This work was supported by US DOE Contract {\#} DE-AC02-09CH11466. [Preview Abstract] |
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PP8.00052: ABSTRACT WITHDRAWN |
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PP8.00053: SXR measurements of Resistive Wall Mode behavior in NSTX Luis F. Delgado-Aparicio, K. Tritz, M. Finkenthal, D. Stutman, S.A. Sabbagh, J. Berkery, S. Gerhardt, R. Bell, B. LeBlanc, J. Manickam, J. Menard, L. Roquemore A multi-energy soft X-ray (ME-SXR) array has been used for the determination of time and space-resolved emissivity profiles during stabilized resistive wall mode (RWM) experiments in NSTX. The main purpose of this study is to distinguish whether the changes on the plasma response ($T_{e}$, $n_{e}$, $n_{Z}$) correspond to the observation of a stabilized RWM or to the effect of the Resonant Field Amplification (RFA). Kinetic measurements of the RFA due to stable RWMs were tested using a single frequency $n=1$ traveling waveform in which the peak-to-peak amplitude was changed by an order of magnitude; neon injection was used to increase the signal-to-noise ratio of the ME-SXR diagnostic. The effect of the RFA on the kinetic profiles was observed and correlated with a suite of diagnostics including that of a toroidally displaced SXR array for $n=1$ RWM identification. This work was supported by the US DoE grant No. DE-FG02-99ER5452 at JHU and DoE-PPPL Contract No. DE-AC02-09CH11466. [Preview Abstract] |
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PP8.00054: Prototype high resolution multi-energy soft X-ray array for NSTX K. Tritz, D. Stutman, L. Delgado-Aparicio, M. Finkenthal, R. Kaita, L. Roquemore Our previous work has shown the utility of multi-energy filtered SXR data for the fast measurement of electron temperature and impurity profiles in NSTX. A novel diagnostic design with an improved spatial resolution of $<$ 1cm and an increased set of multi-energy SXR profiles allows continuous, fast (1-10kHz) measurements of both T$_{e}$ and impurity profiles ranging from the plasma core into the steep gradient region of the pedestal. With this, 4-6 filtered SXR profiles as well as an unfiltered bolometric profile will extend the temperature measurement to lower T$_{e}$. A single energy prototype system has been implemented on NSTX which is comprised of a filtered X-ray pinhole camera which converts the X-rays to visible light using a CsI:Tl phosphor. The phosphor is mounted on an image intensifier coupled to a high-speed CMOS camera using a fiber optic image bundle. Low noise SXR profiles have been measured in high beta H-mode plasmas at frame rates up to 10kHz. Comparisons to the existing, toroidally displaced tangential multi-energy SXR array will be also presented, aimed at measurements of non-axisymmetric perturbations. [Preview Abstract] |
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PP8.00055: Fast Ion Loss and Search for Current Redistribution due to Energetic Particle Mode Bursts in NSTX Douglass Darrow, E. Fredrickson, N. Crocker, N. Gorelenkov, M. Podesta, J. Kim, S. Kubota, K.C. Lee, S. Medley, K. Tritz, H. Yuh Bursts of MHD modes driven by neutral beam ions, termed Energetic Particle Modes (EPMs) are frequently observed in NSTX plasmas. These bursts typically comprise multiple n number modes in the 20-100 kHz frequency range and are believed to be beta induced Alfv\'{e}n acoustic eigenmodes (BAAEs). The bursts produce sizable drops in the neutron rate, up to 36{\%} in a 100 $\mu$s interval in one instance. A simultaneous loss of beam ions over a wide range of pitch angles is often seen. Details of beam ion diagnostic results will be presented. An effort has also been made to determine whether these modes alter the current profile of the discharge. Initial results indicate some changes in MSE pitch angle profiles concurrent with the bursts. However, these are comparable to changes seen with other types of MHD activity and with the general level of fluctuations seen in virtually every NSTX discharge. Consequently, it is inferred that any effect these bursts have on the current profile is no greater than that created by other frequently occurring phenomena. [Preview Abstract] |
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PP8.00056: Fishbone-induced fast-ion transport in NSTX N.A. Crocker, D. Liu, E.D. Fredrickson, E. Ruskov, S.S. Medley, W.W. Heidbrink, G.J. Kramer, N.N. Gorelenkov, R.B. White, B.P. LeBlanc, R.E. Bell, F.M. Levinton, S. Kubota Fast-ions from neutral beams excite bursts of large amplitude (\textit{$\delta $n}/$n \quad \sim $ 1{\%}) fishbones -- low $f$ ($\sim $ 25 kHz) global modes -- in NSTX. The bursts correlate with significant fast-ion loss, indicated by large ( $>$ $\sim $ 10{\%} ) drops in neutron flux. The role of fishbones in the loss is probed via theoretical tools, measurement and modeling. The radial structure of the fishbone density perturbation -- measured with 5 fixed-frequency reflectometers -- has been compared to the modeled reflectometer response to eigenmodes found by the NOVA-K ideal stability code. Ideally unstable internal kinks, expected to have structure similar to fishbones, were considered. The best match will be used with the ORBIT guiding-center code to estimate fast-ion redistribution and loss using an intial distribution from the TRANSP Monte Carlo beam deposition code. The perturbation to the population will be compared with measured changes in the flux and spectrum of energetic neutrals via modeling. [Preview Abstract] |
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PP8.00057: Experimental studies of TAE dynamics and induced fast ion losses on NSTX M. Podesta, W.W. Heidbrink, D. Liu, E. Ruskov, R.E. Bell, D.S. Darrow, E.D. Fredrickson, N.N. Gorelenkov, B.P. LeBlanc, N.A. Crocker, S. Kubota The dynamics of toroidicity-induced Alfv\'{e}n eigenmodes (TAEs) is studied in neutral beam heated NSTX plasmas. The results from similar discharges conducted in helium and deuterium plasmas are compared. Emphasis is put on investigating the transition of the modes from a quasi-stationary behavior into a phase characterized by frequency chirps and amplitude bursts as the injected neutral beam power is increased. The fast ion transport associated with bursting TAE activity is measured through Fast Ion D-Alpha spectroscopic diagnostics, neutral particle analyzers, neutron rate measurements and a fast ion loss probe. In particular, drops of the fast ion profile and neutron rate on time scales of $\sim $1ms are observed during so called \textit{TAE avalanches,} i.e. large bursting events accompanied by a frequency down-chirp which involve multiple TAEs. [Preview Abstract] |
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PP8.00058: Simulation of Ion loss during TAE avalanches in NSTX E.D. Fredrickson, D. Darrow, G. Kramer, N.N. Gorelenkov, S.S. Medley, B. LeBlanc, R.E. Bell, R.B. White, M. Podesta, N.A. Crocker, S. Kubota, F.M. Levinton, H. Yuh Non-linear interactions of multiple Toroidal Alfv\'{e}n Eigenmodes (TAE) can result in explosive mode growth and enhanced losses of fast ions in a repetitive cycle of TAE bursts called avalanches. Fast ion losses have been documented with NPA diagnostics and fast neutron measurements during strongly bursting TAE on NSTX. The mode structure and mode amplitudes are measured with arrays of reflectometers and Mirnov coils. These experimental data are used to identify and scale the amplitude and frequency evolution of Toroidal Alfv\'{e}n Eigenmodes simulated with the NOVA code. These scaled eigenmodes are then used in the ORBIT code to simulate the fast ion redistribution during a strong, 1ms, TAE burst. Fast ion redistribution is seen for the energies $>$ 30 keV, consistent with experimental observations. The simulated level of fast ion losses are in good agreement with the observed losses. [Preview Abstract] |
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PP8.00059: Observation of ``Anomalous'' Energetic Ion Spectra by the E$\vert \vert $B Neutral Particle Analyzer on NSTX S.S. Medley, R.E. Bell, D.S. Darrow, E.D. Fredrickson, N.N. Gorelenkov, B.P. LeBlanc, A.L. Roquemore, M. Podesta An ``anomalous'' increase in E$\vert \vert $B NPA charge exchange neutral flux ($\sim $4x) localized at the neutral beam (NB) injection full energy, E$_{b}$ = 90 keV, is observed in NSTX. This so-called ``High-Energy Feature (HEF)'' appears in discharges only when kink-type modes (f $<$ 10 kHz) are absent, TAE activity ( f $\sim $ 10-150 kHz) is weak ($\delta $B$_{rms} <$ 75 mGauss) and CAE activity (f $\sim $ 400 -- 1200 kHz) is robust. The HEF exhibits a growth time of $\sim $ 20-80 ms and develops a slowing down distribution that evolves over 100-400 ms, a time scale long compared with the $\sim$ 50 ms equilibration time of the NB injected particles. Increases of $\sim$ 10-30{\%} in the measured neutron yield and total stored energy are observed to coincide with the HEF along with broadening of the CHERS T$_{i}$(r) profile. The HEF is observed only in H-mode (not L-mode) discharges with injected NB power above 4 MW and is suppressed by vessel conditioning using lithium deposition at rates $\sim$ 100 mg/shot sufficient to suppress ELM activity. Though a definitive mechanism has yet to be develop, the HEF appears to be driven by a form of CAE resonance. [Preview Abstract] |
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PP8.00060: Comparison of Fast-ion D-alpha Measurements with Theory in MHD-quiescent Plasmas E. Ruskov, W.W. Heidbrink, M. Podesta The NSTX fast-ion D-alpha (FIDA) diagnostic measures the Doppler-shifted charge-exchange recombination light from energetic ions that neutralize in a heating beam. The present instrument views the beam nearly vertically. In a spherical tokamak (ST), the large fast-ion gyroradius is expected to cause an asymmetry between the intensity of blue-shifted and red-shifted light. In addition, for near-tangential injection, the large field-line pitch associated with the large poloidal field in an ST should also produce spectral asymmetries. In a dedicated experiment designed to test these predictions, toroidal field and plasma current scans were performed in low-power neutral-beam heated discharges that had little detectable fast-ion driven instabilities or MHD. The TRANSP NUBEAM module calculates the fast-ion distribution function $f$, which is then used in a FIDA simulation code to predict the spectra. In preliminary analysis, large discrepancies between predicted and experimental spectra are observed. [Preview Abstract] |
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PP8.00061: Anomalous electron transport induced by multiple beam ion driven global Alfven instabilities Nikolai Gorelenkov, Allen Boozer, Dan Stutman, Eric Fredrickson, Kevin Tritz, Roscoe White We investigate theoretically the electron thermal transport induced by multiple Global Alfv\'en Eigenmodes (GAE) in a toroidal plasma recently reported in NSTX when strong $P_{NBI}=6MW$ NBI heating was applied. A strong GAE activity correlates with such transport. The stochastization of electron longitudinal motion in the presence of the multiple oscillations is responsible for the electron transport. Application of the guiding center code, ORBIT, shows that the required level of the electron heat conductivity, $\chi_{e}\ge 10m^{2}/sec$ can be achieved in simulations with $\sim 20$ intermittently unstable GAEs with the local density perturbation amplitudes on the order of $\delta n_{e}/n_{e}=10^{-3}$. Our simulations address the requirements for GAE structure, amplitudes and the number of modes to achieve the electron heat diffusivity inferred from experiments. Results from the comparisons of the theory and recent NSTX experiments will be presented. The above anomalous electron transport potentially can have significant implications for fusion devices. [Preview Abstract] |
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PP8.00062: High-k scattering of low frequency MHD activities in NSTX K.L. Wong, E. Mazzucato, D.R. Smith, K. Tritz, S. Kaye Low frequency MHD modes in tokamaks usually have long wavelengths outside the detection range of high-k scattering diagnostics. However, data from the NSTX high-k scattering system exhibit many narrow low frequency (10-100 kHz) peaks associated with various low frequency MHD activities, from fishbones to Alfv\'{e}n eigenmodes. These MHD modes are basically shear Alfv\'{e}n waves, they can convert into short wavelength kinetic Alfv\'{e}n waves near Alfv\'{e}n resonance and thus become detectable by high-k scattering. Such observation was first reported in TFTR.\footnote{K.L. Wong et al., Phys. Lett. A \textbf{224} (1996) 99.} Since the NSTX scattering system mainly detects the radial component of \textbf{k}, it can see more detailed features of these mode-converted kinetic Alfv\'{e}n waves. Corroborating data from soft X-ray camera, Mirnov coils and detailed analysis of these data will be presented. [Preview Abstract] |
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PP8.00063: Modeling polarization of propagating millimeter-waves in NSTX J. Zhang, T. Carter, N.A. Crocker, W.A. Peebles, S. Kubota Magnetized plasma has an anisotropic index of refraction. For propagation perpendicular to the magnetic field, this leads to polarization elliptization via the Cotton-Mouton effect. In contrast, for propagation parallel to the field, the axis of the polarization ellipse rotates: this is known as Faraday rotation. In fusion plasmas millimeter-waves typically experience a combination of these two effects. To date, little attention has been given to the evolution of polarization for radial propagation in a spherical tokamak where a much greater variation of magnetic pitch angle and field strength exists in comparison to conventional tokamaks. This work investigates the polarization modification of millimeter-waves propagating radially in the National Spherical Torus eXperiment. Typical NSTX density and magnetic field profiles are utilized. The calculations provide the basis for optimization of the performance of a planned radial chord polarimeter. Future analysis will assess the sensitivity of polarization modifications to magnetic perturbations such as Alfven eigenmodes and tearing modes. [Preview Abstract] |
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PP8.00064: Full-Wave Codes Applied to Reflectometry Measurements of Core-Edge Turbulence in NSTX S. Kubota, W.A. Peebles, N.A. Crocker, B.C. Rose, S.J. Zweben, T.S. Hahm The interpretation of turbulence properties from reflectometry measurements is often not straightforward and requires full-wave simulations using modeled turbulence and a detailed knowledge of the equilibrium profiles. On NSTX, the unique combination of reflectometry hardware (FM-CW, fixed- frequency, and correlation reflectometers) is well-suited to turbulence measurements in both core and edge plasmas. Here we report on the simulation work required to generate quantitative estimates of turbulence properties (such as turbulence levels, wavenumber spectra, decorrelation times, correlation lengths, flow velocities) from these measurements. We focus on the core-edge region near the L-H transition, where the evolution of the turbulence and density profile characteristics are related to the edge transport barrier formation. Simulations will use the UCLA 1-D and 2-D FDTD full-wave codes. Recently these codes were upgraded to utilize the parallel processing capabilities of the NVIDIA C870 GPU card. [Preview Abstract] |
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PP8.00065: Global Gyrokinetic Electron Temperature Gradient Turbulence and Transport in NSTX Plasmas. S. Ethier, W.X. Wang, F. Poli, T.S. Hahm, S.M. Kaye, W.W. Lee, E. Mazzucato, G. Rewoldt, W.M. Tang Global, nonlinear simulations of electron temperature gradient (ETG) turbulence for experimental discharges have been carried out for the first time for direct validation against high-k scattering measurements of electron gyroradius scale fluctuations in NSTX. Qualitative agreement in the density fluctuation spectra and overall density fluctuation levels between the experiment and the simulation is obtained. The nonlinear generation of zonal flows with fine radial scale is observed during ETG turbulence development although they are too weak to break up the radially-elongated streamers during the nonlinear phase of ETG turbulence, leading to anisotropic fluctuations in the perpendicular wave number space. Comparison of density fluctuation amplitudes based on synthetic diagnosis between simulations and measurements are discussed. Also reported are sensitivity studies of simulated ETG-driven electron thermal transport with respect to the local profiles of electron temperature gradient, effective charge number Zeff, and safety factor, which are subject to significant experimental errors. Work supported by U.S. DOE Contract DE-AC02-09CH11466 and the SciDAC GPS-TTBP project. F. Poli is funded by UK EPSRC. [Preview Abstract] |
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PP8.00066: Nonlinear Gyrokinetic Turbulence Simulations of the NSTX Spherical Torus J.L. Peterson, G.W. Hammett, D. Mikkelsen, S. Kaye, J. Candy, R.E. Waltz Recent progress in the numerical simulation of plasma turbulence has led to a greater understanding of the mechanisms behind anomalous heat and particle losses in tokamaks. However, the source of turbulent transport in machines with smaller aspect ratios, such as the National Spherical Torus Experiment (NSTX), remains elusive. Leading contenders for explaining transport in spherical tori include turbulence driven by the Electron Temperature Gradient (ETG) mode and microtearing modes. We present here nonlinear GYRO\footnote{J. Candy, R. E. Waltz et al., J. Phys. Conf. Ser. {\bf 78}, 012008 (2007).} simulations of microturbulence in a variety of NSTX discharges and make comparisons between numerically simulated and experimentally measured levels of electron-scale turbulence. [Preview Abstract] |
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PP8.00067: Study of boundary diffusion via density fluctuation measurement using the FIReTIP system on NSTX K.C. Lee, C.W. Domier, M. Johnson, W.-C. Tsai, N.C. Luhmann, Jr., H. Park, B.P. LeBlanc, R.E. Bell, R. Kaita, S.A. Sabbagh The Far Infra Red Tangential Interferometry/Polarimetry (FIReTIP) system on the National Spherical Torus Experiment (NSTX) measured boundary density fluctuations with upgraded time resolution up to 3.3 MHz in 2009. The density fluctuation level compared with the energy confinement from EFIT showed agreement with the turbulence induced diffusion coefficient which was recently introduced [1]. According to the gyrocenter shift theory, the plasma diffusion at the boundary is dependent on the density fluctuation level and the density fluctuation level is dependent on the Reynolds number of the poloidal ion gyrocenter drift arising from collisions with boundary neutrals. FIReTIP density fluctuation data are also compared with various plasma parameters which determine the Reynolds number such as the radial electric field and plasma temperature in the vicinity of separatrix for the study of the L$\backslash $H transition mechanism. [1] K. C. Lee, Plasma Phys. Control. Fusion, Vol. 51, 065023 (2009) [Preview Abstract] |
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PP8.00068: Preliminary study of HHFW driven plasma fluctuation measurement using FIReTIP in NSTX J. Kim, K.C. Lee, C.K. Phillips, R. Kaita, C.W. Domier, N.C. Luhmann, Jr., H. Park, M. Cho, W. Namkung NSTX is equipped with an FIR interferometry (FIReTIP) and high k scattering system, which can monitor high frequency density fluctuations driven by RF waves. High Harmonic Fast Wave (HHFW) fields drive localized density fluctuations at 30 MHz that may be detectable by this system. The electronics for 30 MHz signal observations were designed and are being upgraded for FIReTIP. The location of the observed fluctuations will be compared to predictions from the HHFW version of the TORIC code, which solves the Maxwell equations in toroidal geometry, using a quasilocal approximation that retains large k$_{perp}\rho _{i}$ effects but assumes that only the HHFW fields are excited. The RF induced density perturbation in NSTX discharges heated by the 30 MHz HHFWs will be localized within the field distribution predicted by the TORIC code. In this presentation, the conceptual design including experimental development and the preliminary results of simulation will be presented. [Preview Abstract] |
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PP8.00069: Turbulent filaments on the divertor target plates of NSTX R.J. Maqueda, S.J. Zweben, J. Kallman, C.E. Bush Fine structured filaments are seen on the lower divertor target plates of NSTX during H-mode discharges. These filaments, not associated with edge localized modes, correspond to the footprints of the turbulent blobs seen near the midplane of the device with multiple diagnostics. The fluctuation level of the neutral lithium light observed and the skewness and kurtosis of its probability distribution function have similar characteristics than midplane blobs: increasing with increasing radii outside the outer strike point (separatrix). In addition, their toroidal and radial movement agrees with the typical movement of blobs at the midplane. Furthermore, with the appropriate magnetic topology, i.e. mapping between the portion of the target plates being observed into the field of view of the midplane gas puff imaging diagnostic, very good correlation is observed between the midplane blobs and these divertor filaments. The existence of ``magnetic shear disconnection'' due to the lower X-point, as proposed by Cohen and Ryutov [Nucl. Fusion 37, p. 621, 1997], is analyzed from the measurements obtained. [Preview Abstract] |
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PP8.00070: GPI Measurements of Edge and SOL Turbulence Across the L-H transition in NSTX A.L. Roquemore, S.J. Zweben, R.J. Maqueda, S. Kubota, T. Munsat, S. Kaye, R. Maingi, B.D. Scott, T.S. Hahm Edge turbulence across the LH transition has been measured using the gas puff imaging (GPI) diagnostic at the outer midplane edge and scrape-off-layer (SOL) of NSTX. The cross-correlation data from the GPI diagnostic has been analyzed to determine the time- and space-dependent radial and poloidal correlation lengths Lr and Lp and flow speeds Vr and Vp across the transition. The local dimensionless poloidal flow shear S=(dVp/dr)(Lr/Lp)t was estimated from these measurements by averaging the turbulence flow speed over a suitable space and time range (t=autocorrelation time). In the SOL this shear ranges between $\pm$2 (with a wide scatter), but preliminary results do not show a clear correlation between the magnitude of S and the local fluctuation level across the transition. A comparison will be made between the turbulence seen by GPI and by the UCLA edge reflectometry diagnostic to determine whether the transition begins inside or outside the separatrix, and how the turbulence changes propagate during the transition. [Preview Abstract] |
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PP8.00071: Turbulent energy transport and parallel heat flux in the SOL D.A. Russell, J.R. Myra, D.A. D'Ippolito, J-W. Ahn, R. Maqueda, D.P. Lundberg, D.P. Stotler, S.J. Zweben We model the evolution of vorticity, density, temperature and zonal fluid momentum in the outboard midplane of a tokamak with our SOLT code. Our edge region supports the electron drift wave and interchange instabilities, while sheath losses are isolated in the SOL. Curvature- and grad-B-driven charge separation are included everywhere, enabling blob transport of strong fluctuations ($\delta $n/n$\sim $1) from the edge into the SOL. Gas-puff imaging (GPI) diagnostics on NSTX are compared with the corresponding \textit{synthetic} GPI diagnostics from the SOLT simulations. Here we study cross-field energy transport and the resulting profile of parallel heat flux to the divertor, in the outboard midplane, and make comparisons with recent NSTX experiments in which variations of the SOL width were studied. [Preview Abstract] |
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PP8.00072: Utilization of Two-Point Model to Estimate Separatrix Temperature in NSTX John Rhoads, Rajesh Maingi A method of estimating the separatrix temperature was developed using a simple two-point model of the scrape-off layer (SOL). Given an input power, neutral density, and connection length, the two point model determines the target temperature, target density, and upstream temperature. Source and sink terms in the model include radiation and volume recombination in the region near the divertor, collisions with neutrals and viscous forces in the SOL, hydrogen recycling, and parallel heat convection effects. Although the connection length at the separatrix is not well defined, this technique uses a finite connection length determined from the magnetic reconstruction of the discharge at a fixed normalized flux, $\hat{\phi}$, slightly outside of the last closed flux surface. Characteristic connection lengths from the midplane to the outboard lower divertor vary from approximately 8 meters at $\hat{\phi} = 1.01$ to 12 meters at $\hat{\phi} = 1.002$. The upstream temperature from the two-point model is then mapped onto the Thomson scattering profile and the separatrix temperature is deduced from the offset in the normalized flux. Typical separatrix temperatures fall within the range 40-100 eV. [Preview Abstract] |
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PP8.00073: Investigation of ELM magnetic precursors in NSTX discharges with and without lithium wall coatings Frederick Kelly, Eric Fredrickson, Stefan Gerhardt, Rajesh Maingi, Jon Menard, Steve Sabbagh, Hiro Takahashi The evolution of ELM magnetic precursors in a series of NSTX discharges without lithium and with increasing lithium deposition [1, 2] are examined. Data from the high-n Mirnov array was used to estimate the toroidal mode number (n) of the precursors. ELMs were observed to have n=1 and/or n=2 magnetic precursors with some delayed modes in the range from n=3 to n=6, which persist as the lithium coating is increased and ELMs become partially suppressed. The D-alpha signal of a few ELMs is preceded by a slow growing plateau period which appear to be dominated by n= 3 to n=6 modes, however, n= 1 and/or n=2 modes appear as precursors to the main ELM peak. The observed n=1 precursors may be evidence of SOL currents in NSTX, similar to those observed in DIII-D [3]. [1] R. Maingi, et al., 36th Eur. Phys. Soc. Conf. on Plasma Physics (Sofia, Bulgaria), P2.175. [2] R. Maingi, et al., Phys. Rev. Lett. (2009) at press. [3] H. Takahashi, et al., Nucl. Fusion 44 (2004) 1075. [Preview Abstract] |
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PP8.00074: Improvements to CHI Plasma Start-up and Ramp-up in NSTX T.R. Jarboe, R. Raman, B.A. Nelson, D. Mueller, M.G. Bell, L. Roquemore, H.W. Kugel, V. Soukhanovskii Experiments in NSTX have now demonstrated the savings of central solenoid inductive flux after coupling of toroidal plasmas produced by the technique of Coaxial Helicity Injection (CHI) to inductive sustainment and ramp-up of the toroidal plasma current. In these discharges, the central solenoid with zero pre-charge was used to apply an inductive loop voltage to the decaying CHI started discharges. The coupled discharges ramped up to 800kA without the benefit of auxiliary heating. Inductive flux savings was realized as a result of an effort to reduce the influx of low-Z impurities during the plasma start-up phase. This was achieved through the use of 400ms long CHI discharges produced using a DC rectifier power supply to ablate low-Z surface impurities from the lower divertor electrodes, followed by the use of Lithium evaporative coatings and an effort to reduce spurious arcs in the upper divertor region by controlling the extent of CHI plasma growth in the vessel. Previous work on NSTX has shown that CHI started discharges after coupling to neutral beam heated discharges can transition to an H-mode. These important new results from NSTX demonstrate that CHI is a viable solenoid-free plasma startup method for future STs and Tokamaks. This work supported by U.S. DOE Contracts {\#} DE-AC02-09CH11466 and DE-FG02-99ER54519 AM08. [Preview Abstract] |
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PP8.00075: High Harmonic Fast Wave Heating Studies for L and H Mode NSTX Plasmas J.C. Hosea, R.E. Bell, B.P. LeBlanc, C.K. Phillips, L. Roquemore, G. Taylor, J.R. Wilson, R. Maingi, P.M. Ryan, J. Wilgen, K. Tritz Fast wave research on NSTX is directed toward understanding the coupling of some RF power to edge loss processes. These losses are driven in the vicinity of the antenna as opposed to resulting from multi-pass edge damping. PDI edge losses through ion-electron collisions and direct energetic ion losses appear to be significant, the latter possibly causing clamping of the edge rotation. Deuterium H-mode heating studies reveal that core heating is degraded at lower k$_{\phi }$ (- 8 m$^{-1}$ relative to 13 m$^{-1})$ as for the L-mode case at elevated edge density, consistent with edge wave damping depending on the location of the onset density (n$_{onset}\propto $ B*k$_{\vert \vert }^{2}$/$\omega )$ relative to the position of the antenna. Fast visible camera images clearly indicate that a major fast wave edge loss process is occurring from the plasma scrape off layer (SOL) in the vicinity of the antenna and along the magnetic field lines to the lower outer divertor plate. Large type I ELMs, which are observed at both k$_{\phi }$ values, appear after antenna arcs caused by precursor blobs, low level ELMs, or possibly dust. For large ELMs without arcs, the source reflection coefficients rise on a 0.1 ms time scale, suggesting that this rise time might be used to discriminate between ELMs and arcs. Work supported by USDOE Contract No. DE-AC02-09CH11466. [Preview Abstract] |
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PP8.00076: Comparison of measurements and simulations of fast ion profiles during high harmonic fast wave heating in NSTX D. Liu, W.W. Heidbrink, M. Podesta, S.S. Medley, R.W. Harvey, M. Choi, D. Green Combined neutral beam injection (NBI) and high harmonic fast wave (HHFW) heating at cyclotron harmonics accelerate deuterium fast ions in the National Spherical Torus Experiment (NSTX). Acceleration of fast ions above the beam injection energy is evident in the data from neutron, E$\vert \vert $B type Neutral Particle Analyzer (NPA), Solid State Neutral Particle Analyzer (SSNPA) array and Fast-Ion D-Alpha (FIDA) diagnostics. The fast-ion spatial profiles measured by the FIDA diagnostic show that the acceleration is at four harmonics (7-10) simultaneously and it is much broader than in DIII-D. This is because of the multiple resonance layers and large orbits in NSTX. The measured spatial profile of accelerated fast ions is farther from the magnetic axis and broader than predicted by the CQL3D Fokker-Planck code, for which we conjecture that finite Larmor radius and banana-width can have significant effects on the fast ions in NSTX. To test this hypothesis, simulations with ORBIT-RF code coupled with full wave code AORSA are in progress. [Preview Abstract] |
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PP8.00077: Numerical Modeling of HHFW Heating on NSTX C.K. Phillips, R.E. Bell, J.C. Hosea, B.P. LeBlanc, G. Taylor, E.J. Valeo, J.R. Wilson, L.A. Berry, E.F. Jaeger, P.M. Ryan, P.T. Bonoli, J.C. Wright, R.W. Harvey, T. Brecht Recent HHFW heating experiments on NSTX indicate that the core heating efficiency depends strongly on the antenna phasing and plasma conditions, and that it improves when the onset for wave propagation is moved away from the immediate vicinity of the launcher. Since the waves are nearly totally damped in a single transit through the plasma core, this degradation is likely due to power losses from collisions, rf sheaths, and sputtering associated with initial interactions of the waves with the antenna and vessel structures. Full wave and ray tracing codes have been used to study the wave propagation and damping characteristics in L-mode and H-mode discharges, and in lower density start-up plasmas. The extent to which significant wave propagation in the edge regions is correlated with the measured core heating efficiency will be presented for a range of discharge conditions. [Preview Abstract] |
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PP8.00078: Rotational Control of Plasma in NSTX Kunihiko Taira, Egemen Kolemen, David Gates, Clarence Rowley, Jeremy Kasdin In an effort to assist the continuous extraction of fusion energy, a plasma model and a controller are developed for a magnetic fusion devise (NSTX). The model is aimed to capture the rotational (toroidal) momentum transport inside the tokamak. The neutral beam injection and the neoclassical toroidal viscosity are considered in the model for their uses as actuator forces. Based on the proposed model, a feedback controller is designed to sustain the toroidal momentum of the plasma in a stable fashion and to achieve desirable plasma geometry. The model reduction and design approaches will be presented along with validation studies. [Preview Abstract] |
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PP8.00079: Design of a Linear-Quadratic-Gaussian controller for resistive wall mode stabilization in NSTX Oksana Katsuro-Hopkins, S.A. Sabbagh, J.M. Bialek, S.P. Gerhardt Stabilization of the resistive wall mode (RWM) in the NSTX is important to achieve and sustain high-beta plasmas. State-space control algorithms for improved RWM control performance using the existing external control coils and off-midplane poloidal magnetic field sensors in NSTX are investigated numerically. At reduced plasma rotation, normalized beta of 5.6 was achieved experimentally with the present proportional gain controller. The proposed linear quadratic Gaussian (LQG) controller is capable of reaching normalized beta of 6.7 for modes with low natural rotation speed and the ideal wall limit normalized beta of 7.1 for plasma modes with higher natural rotation speed. A Kalman filter, an integral part of the LQG controller, is tested using NSTX experimental data off-line and the RWM mode phase and amplitude are compared to results from the present RWM control system using proportional gain. The LQG algorithm is developed for use in real-time NSTX experiments using current control and voltage control methods. [Preview Abstract] |
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PP8.00080: NSTX Strike Point Position Control Egemen Kolemen, David Gates, Clarence W. Rowley, N. Jeremy Kasdin This paper presents the new control algorithm for the inner and outer strike point position for NSTX and the performance analysis of the controller. A liquid lithium diverter (LLD) will be installed on NSTX which provides better pumping than lithium coatings on carbon PFCs. The shape of the plasma dictates the pumping rate of the lithium by channeling the plasma to LLD, where strike point location is the most important shape parameter. Simulations show that the density reduction depends on the proximity of strike point to LLD. Experiments were performed to study the dynamics of the strike point, design a new controller to change the location of the strike point to desired location and stabilize it. The most effective PF coil in changing the outer strike point is PF2L. Thus, we use this coil as the sole controller for the outer strike point. In the same way, PF1AL is used for inner strike point control. The PF coil inputs were changed in a step fashion between various set points and the step response of the strike point position was obtained. From the analysis of the step response, PID controllers for the strike points were obtained and the controller was tuned experimentally. [Preview Abstract] |
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PP8.00081: Study of strike-point and flux-expansion control in diverted NSTX plasmas Hua Wang, Jonathan Menard Tokamaks commonly use a poloidal divertor to tailor the trajectory and length of open magnetic field lines outside the main plasma volume. Optimization of the magnetic field configuration at the divertor is desirable to control the heat and particle flux to the material surfaces. Such optimization is important in NSTX, in the design of new divertor configuration for the proposed upgrade of NSTX, and for future Spherical Torus (ST) devices. In particular, simultaneous control of the strike point location and the flux expansion at the strike point is highly desirable to control the location and magnitude of peak heat and particle flux at the divertor target. The free-boundary equilibrium code ISOLVER is utilized and modified to assess the boundary shape implications of controlling the strike point location. The viability of simultaneous strike point and flux expansion control is also assessed. [Preview Abstract] |
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PP8.00082: Details and Results from the FIReTIP Electronics Upgrade on NSTX C.W. Domier, W.-C. Tsai, K.C. Lee, N.C. Luhmann, Jr., R. Kaita The IF electronics system of the Multichannel Far Infrared Tangential Interferometer/Polarimeter (FIReTIP) system on the National Spherical Torus Experiment (NSTX) was upgraded in July 2009 to greatly extend its ability to monitor high frequency density fluctuations. The measurements are essential in understanding transport physics issues in NSTX as well as for future devices such as ITER in which fundamental understanding of microturbulence MHD issues is essential. The electronics, which were previously limited to $\sim $250 kHz, have now been upgraded to $>$ 3 MHz when operating as an interferometry-only configuration, and to $\sim $400 kHz when operated as a simultaneous interferometer/polarimeter system. New electronics are also being tested to monitor 30 MHz density fluctuations induced by high harmonic fast wave heating. Experimental details and test results of the new electronics will be presented. [Preview Abstract] |
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PP8.00083: Design and status of the NSTX BES diagnostic for fluctuation measurements on the ion gyro-scale D.R. Smith, R.J. Fonck, G.R. McKee, I. Uzun-Kaymak, G. Winz, H. Feder, R. Feder, G. Labik, B.C. Stratton A beam emission spectroscopy (BES) diagnostic has been installed on NSTX to investigate density fluctuations on the ion gyro-scale. The BES diagnostic measures the Doppler-shifted \textit{D}$_\alpha$ emission at 660~nm from neutral deuterium beams. The measurements provide radial and poloidal correlation functions, wavenumber spectra, decorrelation rates, and turbulent flows for investigating core and edge turbulence and energetic particle modes on NSTX\@. The initial deployment includes 16 detection channels, and the viewing optics are aligned to the local magnetic field to optimize spatial resolution. Spot sizes on the neutral beam are in the range 2--3~cm. Radial arrays can measure fluctuations from $r/a\approx0.1$ to beyond the last-closed flux surface, and poloidal arrays are positioned at $r/a\approx0.4\mbox{, }0.6\mbox{, }0.85\mbox{, and }1.1$. PIN photodiodes and low noise preamplifiers measure fluctuations with high sensitivity, and filtered data is obtained at 2~MS/s\@. The BES diagnostic will collect initial data during the 2010 run campaign. *Supported by the U.S. Department of Energy under Contract No. DE-AC02- 09CH11466. [Preview Abstract] |
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PP8.00084: The Use of a 28 GHz Gyrotron for EBW Startup Experiments on MAST J.B. Caughman, T.S. Bigelow, S.J. Diem, Y.K.M. Peng, D.A. Rasmussen, V. Shevchenko, J. Hawes, B. Lloyd The use of electron Bernstein waves for non-inductive plasma current startup in MAST has recently been demonstrated [1]. The injection of 100 kW at 28 GHz generated plasma currents of up to 33 kA without the use of solenoid flux, and limited solenoid assist resulted in up to 55 kA of plasma current. A higher power 28 GHz gyrotron, with power levels of up to 300 kW for 0.5 seconds, is currently being commissioned. It is being used to investigate the scaling of startup current with microwave power and power profile as a function of time. Power modulation experiments are also being explored. Gyrotron performance and experimental results will be presented. \\[4pt] [1] V. Shevchenko, et al., Proceedings of the 15$^{th}$ Joint Workshop on ECE and ECRH, Yosimite, USA, p. 68 (2009) [Preview Abstract] |
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PP8.00085: The {\sc Pegasus} Toroidal Experiment Program R.J. Fonck, M.W. Bongard, E.T. Hinson, B.A. Kujak-Ford, B.T. Lewicki, A.J. Redd, D.J. Schlossberg, F. Volpe, G.R. Winz The {\sc Pegasus} program is developing non-solenoidal startup and growth techniques for tokamaks, and exploring plasma stability at near-unity aspect ratio. Helicity injection from localized current sources in the plasma periphery produced $I_p \geq 0.1$ MA, consistent with a simple theory invoking helicity balance and Taylor relaxation constraints. Future efforts will concentrate on accessing $I_p = 0.3$ MA to test theory to the point where parallel conduction losses dominate the helicity loss rate. Plasma growth following startup will be pursued via Higher Harmonic Fast Wave heating (0.8 MW available) and/or Electron Bernstein Wave heating and current drive (0.5-1.0 MW EBW proposed). Proposed upgrades in support of these programs include a new centerstack for increased TF, divertors for separatrix-limited operation, and long-pulse capabilities. Plasma stability is dominated by peeling-like modes at large $\left \langle j_{edge}\bigl/B \right \rangle $ and large-scale low-$m/n=1$ core activity. The new capabilities will provide core and edge $j(r)$ manipulation to stably access the unique high $I_N$, high-$\beta_t$ regime at $A \approx 1$. [Preview Abstract] |
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PP8.00086: Non-solenoidal startup and low-$\beta$ operations in {\sc Pegasus} D.J. Schlossberg, D.J. Battaglia, M.W. Bongard, R.J. Fonck, A.J. Redd Non-solenoidal startup using point-source DC helicity injectors (plasma guns) has been achieved in the \textsc{Pegasus} Toroidal Experiment for plasmas with $I_{p}$ in excess of 100 kA using $I_{inj}<4\, kA$. The maximum achieved $I_{p}$ tentatively scales as $\sqrt{I_{TF}I_{inj}/w}$, where \textit{\normalsize $w$} is the radial thickness of the gun-driven edge. The $I_{p}$ limits appear to conform to a simple stationary model involving helicity conservation and Taylor relaxation. However, observed MHD activity reveals the additional dynamics of the relaxation process, evidenced by intermittent bursts of n=1 activity correlated with rapid redistribution of the current channel. Recent upgrades to the gun system provide higher helicity injection rates, smaller $w$, a more constrained gun current path, and more precise diagnostics. Experimental goals include extending parametric scaling studies, determining the conditions where parallel conduction losses dominate the helicity dissipation, and building the physics understanding of helicity injection to confidently design gun systems for larger, future tokamaks. [Preview Abstract] |
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PP8.00087: Edge current and pressure measurements of plasma equilibrium and stability properties in {\sc Pegasus} E.T. Hinson, M.W. Bongard, R.J. Fonck, B.A. Kujak-Ford, B.T. Lewicki, A.J. Redd, G.R. Winz The plasma edge region in the {\sc Pegasus} ST exhibits a range of fluctuation phenomena, from peeling-like modes at high $\dot{I}_p$, to standard broadband electrostatic turbulence. Measurements of the edge region will support studies of its equilibrium and stability properties, and provide information for RF coupling (HHFW and EBW) for future heating and current drive studies. Langmuir probes are used to measure the edge pressure gradient with high temporal resolution. Together with measurements of the edge $B_z$ profile from a radial array of Hall probes, the derived $p(r)$ and $j_{\parallel}(r)$ profiles will support detailed edge stability analyses. Peeling-like modes manifest as edge filamentary structures commonly observed in Ohmic discharges. Scanning Mirnov probes indicate broadband magnetic turbulence when this filamentation occurs. Peeling-mode theory predicts instability for large $ \langle j_\parallel \bigl/B \rangle$, a condition that {\sc Pegasus} may satisfy ($B_t \sim$ 0.1 T, $j_{\parallel} \sim$ 0.1 MA/m$^2$). These measurements will enable determination of $ \langle j_\parallel \bigl/B \rangle$ and $p(r)$ at $r \approx a$, and offer unique tests of peeling-ballooning theory. Future studies will employ divertors to increase magnetic shear and influence edge turbulence. [Preview Abstract] |
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PP8.00088: The Lithium Tokamak eXperiment (LTX) -- Status and Plans Richard Majeski, L. Berzak, D. Boyle, S. Gershman, E. Granstedt, C.M. Jacobson, A.D. Jones, R. Kaita, T. Kozub, B. LeBlanc, D.P. Lundberg, K. Snieckus, T. Strickler, J. Timberlake, L. Zakharov, G.V. Pereverzev, V. Soukhanovskii, C.E. Thomas LTX is a modest spherical tokamak with R=0.4 m, a=0.26 m, =1.5, B$_{toroidal }$= 3.4 kG, I$_{P} \quad <$ 400 kA, and $\tau _{flattop} >$ 50 msec. The LTX research objective is to investigate modifications to equilibria and transport when global recycling is reduced to very low values ($<$50{\%}), by means of a conformal stainless steel clad copper wall coated with liquid lithium. The resistively heated wall is capable of operating at temperatures exceeding 500C, but the nominal operating temperature range will be room temperature to 400C. Initial lithium operation will employ evaporative coatings applied to the heated wall. Pulsed fueling with gas jets and an H$_{2}$ cluster injector will be employed to transiently eliminate edge gas. Diagnostic plans are extensive, and are discussed in companion posters by L. Berzak, D. Lundberg, S. Gershman, E. Granstedt, C. M. Jacobson, and C. E. Thomas. [Preview Abstract] |
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PP8.00089: Operation of Magnetic Diagnostics on the Lithium Tokamak eXperiment L. Berzak, R. Kaita, T. Kozub, R. Majeski, J. Menard, L. Zakharov The Lithium Tokamak eXperiment (LTX) is designed to investigate the novel, low-recycling lithium wall operating regime for magnetically confined plasmas. LTX reaches this regime through a heated shell coated with liquid lithium internal to the vacuum vessel. This shell provides an area of 5 m$^{2}$ of liquid lithium, more than 90{\%} coverage of plasma facing components. An extensive array of unique magnetic diagnostics has been developed to yield detailed magnetic information in the presence of the shell and allow highly-constrained reconstructions of the plasma equilibrium. CDX-U, with a 2000 cm$^{2}$ liquid lithium limiter, observed a 5-10 fold increase in energy confinement time from pre-Li discharges with Ohmic loop voltages of only 0.4-0.5 V. The expanded magnetic diagnostics set is designed to permit more accurate evaluation of the global energy confinement time and surface loop voltage for comparison with the results of CDX-U and to perform scaling experiments in order to elucidate the variation in~confinement with toroidal field, plasma current, and plasma density. [Preview Abstract] |
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PP8.00090: Turbulent Thermal Insulation of Spheromak Fusion Reactors Robert Jones Spheromak fusion plasmas are likely to suffer from stochastic magnetic fields (Phys. Rev. Letts. vol. 91, \#4, 045004, 2003) and have confinement: t=(B/dB)(B/dB)(r/v)(r/v)f. This degradation in confinement can be minimized if the collision frequency f is intentionally enhanced by turbulence (see Jones, Plasma Phys. vol. 22, pg 753, 1980) f=(plasma frequency)* (W/nT), up to a level W/nT limited by plasma overheating: T/t = f W/n. (R. Jones, Current Science, vol. 57, \#18, pg. 991, 1988) [Preview Abstract] |
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PP8.00091: The Lithium Tokamak Experiment Thomson Scattering Diagnostic C.M. Jacobson, R. Kaita, B.P. LeBlanc, R. Majeski, T. Strickler The Lithium Tokamak Experiment (LTX) is a spherical tokamak designed to study the low-recycling regime though the use of a liquid-lithium coated shell conformal to the last closed flux surface. A low recycling rate is expected to flatten core electron temperature profiles, raise edge temperatures, and strongly affect electron density profiles. A Thomson scattering diagnostic is used to measure radial $T_e$ and $n_e$ profiles. The system uses a 15 J, 30 ns pulsed ruby laser and measures profiles of up to 16 radial points on the horizontal midplane at a single temporal point for each discharge. Scattered light is imaged though a spectrometer into an intensified CCD. Two transmission gratings are available for the spectrometer to increase temperature range. In order to protect optical windows from becoming coated with lithium, the system uses automated shutters and gate valves. [Preview Abstract] |
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PP8.00092: Molecular Cluster Injection for Fueling and Particle Transport Studies on the Lithium Tokamak eXperiment (LTX)* D.P. Lundberg, R. Kaita, T. Kozub, R. Majeski, V. Soukhanovskii LTX is designed to reduce global recycling with a liquid lithium wall. Gas-based fueling systems, such as wall-mounted gas puffers or supersonic gas injectors, are perturbative to low-recycling plasmas, as they source a significant amount of gas into the edge. Following experiments on the HL-2A tokomak by Yao, et al. (Nucl. Fusion 47(2007) 1399), a molecular cluster injector was designed to supply an increased fraction of core fueling on LTX. A fast solenoid valve is cooled with liquid nitrogen, lowering the temperature of the gas in the valve body. As the gas expands into vacuum, additional cooling yields the formation of molecular clusters. This concentrates the hydrogen into a directed, high-density jet, improving penetration into the plasma. This system can be used for fueling and particle transport studies. Details of the design, construction, and initial operation of this system will be presented, and its application to LTX discussed. *Supported by US DOE contracts DE-AC02-09CH11466 and DE-AC52-07NA27344 [Preview Abstract] |
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PP8.00093: Density Fluctuation and Transport Studies for the Lithium Tokamak eXperiment E.M. Granstedt, R. Majeski, R. Kaita, G.W. Hammett, C.E. Thomas Tokamak fusion energy confinement is generally limited by turbulent transport due to temperature-gradient-driven instabilities; however, a low-recycling plasma boundary may permit high edge temperatures to be realized, resulting in a flattened temperature profile and eliminating the drive mechanism for ITG and ETG turbulence. LTX is uniquely suited to study this regime, where the remaining transport is expected to be neoclassical or driven by density gradients. To extrapolate performance to larger devices, the contributions of both mechanisms to the transport must be understood. Characterization of the transport in the low-recycling regime requires measurement of density fluctuation spectra. The statistical nature of density fluctuations in LTX will be investigated with a digital holography diagnostic currently under development. This diagnostic uses a 9.1~$\mu$m CO$_2$ laser and an IR quantum well FPA camera to measure line-integrated plasma density fluctuations with up to 0.3~mm resolution perpendicular to the line-of-sight, exposures $\ga$1~$\mu$s, and frame rates $\la$48~kHz. Measurement predictions for this diagnostic through linear and nonlinear gyrokinetic simulations using the GYRO code are also presented. [Preview Abstract] |
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PP8.00094: High Speed Digital Holography for Density and Fluctuation Measurements on LTX C.E. (Tommy) Thomas Jr., L.R. Baylor, S.K. Combs, S.J. Meitner, D.A. Rasmussen, E.M. Granstedt, R. Majeski Digital holography has been demonstrated as a viable technique for precision measurement and for measuring the development of high-speed gas flow. A digital holography demonstration system has been used on an ORNL disruption mitigation test stand to make extremely high spatial resolution (less than one mm) 2-D density measurements of gas flow. A second generation system with a high-speed infrared camera (320 x 256 pixels) and higher power 9.1 micron CO$_2$ laser ($\sim$20W) is being developed for electron density and fluctuation measurements on LTX. Exposures as short as one microsecond are exspected and frame rates (at reduced pixel window size) of 10,000 frames per second and higher are planned. Elements of the design, components, and expected performance will be presented. [Preview Abstract] |
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PP8.00095: Design and Assembly of a Langmuir Probe for the Lithium Tokamak Experiment (LTX) Sophia Gershman, Laura Berzak, Dennis Boyle, Erik Granstedt, Craig Jacobson, Andrew Jones, Robert Kaita, Tom Kozub, Benoit P. LeBlanc, Daniel Lundberg, Richard Majeski, Kurt Snieckus, T. Strickler, John Timberlake, Leonid Zakharov, G.V. Pereverzev, V. Soukhanovskii, C.E. Thomas The presence of lithium coatings on plasma facing surfaces has been~shown to reduce recycling, increase the electron temperature, and~significantly improve the energy confinement in a tokamak. LTX is designed for detailed investigation~of the transport phenomena and plasma properties in this new low recycling regime. Edge diagnostics are an important part of this study. A swept probe has been designed to determine the plasma temperature and density in the scrape-off layer. The probe is~designed to accommodate continued lithium evaporation and the expected low edge densities and high temperatures characteristic to this regime. Data from the probe will support the study of the transport properties and provide input parameters for the codes modeling the edge plasma in lithium boundary tokamaks. [Preview Abstract] |
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PP8.00096: RF Experiments on TST-2 Yuichi Takase, Akira Ejiri, Yoshihiko Nagashima, Osamu Watanabe, Bung Il An, Kentaro Hanashima, Hiroyuki Hayashi, Junichi Hiratsuka, Hidetoshi Kakuda, Hiroaki Kobayashi, Hiroki Kurashina, Hazuki Matsuzawa, Takuya Oosako, Takuya Sakamoto, Takuma Wakatsuki, Kotaro Yamada, Takashi Yamaguchi, Paul Bonoli, John Wright RF experiments were performed on the TST-2 spherical tokamak (R= 0.38 m, a = 0.25 m, B$_{t}$ = 0.3 T, I$_{p}$ = 0.1 MA) with up to 5 kW of power at 2.45 GHz and up to 400 kW of power at 21 MHz. In HHFW electron heating of inductively produced plasmas, parametric decay instability was often observed. The spatial distribution measured by RF magnetic probes located at different toroidal locations indicated wave propagation effects as well as nonlinear effects. Spontaneous formation of the ST configuration was studied. Sustainment of the ST configuration was accomplished by low frequency (21 MHz) RF power alone for the first time. LHCD and I$_{p}$ ramp-up experiments are in preparation using up to 400 kW of power at 200 MHz. This experiment will be carried out at B$_{t}$ = 0.3 T, and the density of the EC produced plasma will be kept low during I$_{p}$ ramp-up by LHCD. Status of preparation and results of calculations will be presented. [Preview Abstract] |
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PP8.00097: Observation of non-locality of parametric decay during rf injection experiments on TST-2 Yoshihiko Nagashima, Takuya Oosako, Yuichi Takase, Akira Ejiri, Osamu Watanabe, Hiroaki Kobayashi, Takashi Yamaguchi, Byung Il An, Hiroki Kurashina, Kotaro Yamada, Hiroyuki Hayashi, Haduki Matsuzawa, Kentaro Hanashima, Junichi Hiratsuka, Hidetoshi Kakuda, Takuya Sakamoto, Takuma Wakatsuki Investigation of linear/nonlinear wave energy transfer in spectral and real spaces of plasma is important for understanding complex plasma structural formation. We present an experimental observation of non-locality of parametric decay instability (PDI) during rf injection experiments in the TST-2 spherical tokamak. The PDI is considered to be mainly associated with pump wave (high harmonic fast wave, HHFW), lower-sideband (HHFW or ion Bernstein wave), and ion cyclotron quasi-mode (ICQM). PDI is a nonlinear and local phenomenon. However, the PDI is widely observed in various locations inside the vessel, and the same frequency matching condition is satisfied beyond difference of local ion cyclotron frequencies at observation locations. We discuss origin of the non-locality in term of wave propagation and coupling phase. [Preview Abstract] |
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PP8.00098: Spherical tokamak start-up and sustainment experiments on UTST T. Yamada, R. Imazawa, S. Kamio, R. Hihara, K. Abe, M. Sakumura, Q.H. Cao, T. Oosako, H. Kobayashi, T. Wakatsuki, B.I. An, Y. Nagashima, M. Inomoto, A. Ejiri, Y. Takase, Y. Ono, H. Sakakita, H. Koguchi, S. Kiyama, Y. Hirano The University of Tokyo Spherical Tokamak (UTST) device was constructed for the purpose of formation and sustainment of ultra-high beta spherical tokamak (ST) plasma using double null plasma merging and radio-frequency (RF) / neutral beam injection (NBI) heatings. The main feature of UTST is that the poloidal field coils are located outside the vacuum vessel in order to demonstrate the start-up in a more reactor relevant situation. Initial operations were carried out using partially completed power supplies to investigate the appropriate conditions for plasma merging. Plasma current of the merged ST was 50 kA, and reached 170 kA by using the central solenoid coil for assistance of plasma formation. Merging of two ST plasmas through magnetic reconnection was successfully observed by using two-dimensional pickup coil arrays, which directly measure the toroidal and axial magnetic fields inside the UTST vacuum vessel. There are two methods, which are now in progress, to sustain the ultra-high beta ST created on UTST; radio-frequency heating (400 kW and 21 MHz) and neutral beam injection (500 kW and 15 ms). [Preview Abstract] |
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PP8.00099: Experimental Demonstration of Double Null Merging Start-up on UTST Ryota Imazawa, Shuji Kamio, Ryuma Hihara, Keita Abe, Morio Sakumura, Qinghong Cao, Takuma Yamada, Michiaki Inomoto, Yuichi Takase, Yashushi Ono We made double null merging(DNM) experiments using outer poloidal field(PF) coils on UTST. We successfully generated two STs at two null points by using a washer gun, and succeeded in merging them. The DNM scheme is started by two magnetic null points generated at the upper and lower regions inside the vacuum vessel. Then poloidal flux swing generates two STs at two null points formed by two pairs of outer PF coils. Finally, the coil currents push two STs toward the mid-plane, and merge them into single ST. Since magnetic reconnection during merging transforms a magnetic energy into a thermal energy, this merged ST plasma is expected to have ultra high beta. It must be noted that DNM scheme can make ST without center solenoid coils. The DNM scheme was demonstrated on TS-3/4(JAPAN) and MAST(UKAEA). However these devices have all PF coils inside the vacuum vessel, and initial plasmas were generated around PF coils, not null points. Since the internal coils are not feasible in the fusion reactor, it is important to demonstrate that the outer PF coils induce electric fields for the breakdown and the formation of two STs. [Preview Abstract] |
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PP8.00100: Measurements of HHFW profile in UTST Takuma Wakatsuki, Takuya Oosako, Hiroaki Kobayashi, Takuma Yamada, Byung Il An, Ryota Imazawa, Keita Abe, Akira Ejiri, Kentaro Hanashima, Hiroyuki Hayashi, Ryuma Hihara, Junichi Hiratsuka, Michiaki Inomoto, Hidetoshi Kakuda, Shuji Kamio, Hiroki Kurashina, Yoshihiko Nagashima, Yasushi Ono, Takuya Sakamoto, Morio Sakumura, Qinghong Cao, Yuichi Takase, Osamu Watanabe, Takashi Yamaguchi, Kotaro Yamada HHFW (high-harmonic fast wave, f = 21 MHz) experiments were performed on the UTST spherical tokamak (R= 0.38 m, a = 0.25 m, B$_{t}$ = 0.1 T, I$_{p}$ = 0.1 MA) [T. Yamada, this session]. The HHFW antenna consists of two poloidal current straps separated in the toroidal direction. The spatial profile of the HHFW field was measured by a 2-dimensional array of magnetic probes inserted into the plasma 45\r{ } away from the antenna toroidally. 40 channels of RF data can be acquired simultaneously by fast digitizers (1 GHz sampling frequency) out of 81 probes. A comparison of wave fields measured with out-of-phase two-strap excitation (high parallel wavenumbers) and with single strap excitation (low parallel wavenumbers) indicate higher RF fields inside the plasma for the same input power. The experimental data will be compared with results of wave field calculation. [Preview Abstract] |
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PP8.00101: Numerical Simulations of Current Channel Relaxation For Non-Inductive Startup T.M. Bird, C.R. Sovinec, D.J. Battaglia, J.B. O'Bryan Nonlinear resistive MHD computation with anisotropic thermal conduction is used to investigate the relaxation of helical current filaments into tokamak-like plasmas for non-inductive startup of spherical tokamaks. A localized, volumetric current source has been added to the NIMROD code (nimrodteam.org) to model miniature washer-gun current sources in the lower divertor region of the Pegasus Toroidal Experiment at the Univ. of Wisconsin. When the induced magnetic field is smaller than the vacuum field, a helical filamentary current channel forms on open field lines, comparable to experimental results in similar conditions. Relaxation into tokamak-like plasmas has been demonstrated in experiment with induced fields that exceed the vacuum field [N. W. Eidietis, et al., J. Fusion Energy 26, 43 (2007)]. Simulation results show that current channels begin to collect near the injector when reversing the poloidal flux between the injector and inboard wall. Both zero-beta simulations and temperature dependent simulations with anisotropic thermal conduction are presented along with comparison to experiment. [Preview Abstract] |
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PP8.00102: Development of Continuous Plasma Merging Scenario for Start-up and Formation of Spherical Tokamak Plasmas Choongki Sung, Y.S. Park, Hyunyeong Lee, J. Kang, Y.S. Hwang A new device which can start-up and sustain ST plasmas by means of continuous tokamak plasma injection has been designed. The device with two separated solenoids at both ends has been designed to generate small plasmas inductively, and then the plasmas are merged to the middle region, where main ST plasmas are formed. If the ST plasma could be sustained while two separated solenoids are recharged subsequent injections of inductively formed plasmas from partial solenoids can maintain or increase plasma currents. To make a feasible operation scenario of the device, a circuit model combined with density and power balance equations is developed. In the model, plasma is assumed to be a single filament in a circuit, and evolution of circuit plasma parameters can be evaluated from balance equations. By using the model, an operational scenario utilizing continuous plasma injections from AC operations of partial solenoids is developed to overcome the ST's weakness. Experimental verification of the operation will be demonstrated in near future. [Preview Abstract] |
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PP8.00103: SIMULATION AND MODELING OF BASIC PLASMA PHENOMENA |
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PP8.00104: Plasma Torque and Non-Ambipolar Transport Allen Boozer Poloidal symmetry breaking in toroidal plasmas causes a damping of poloidal rotation and toroidal symmetry breaking a damping of toroidal rotation. These torques are transmitted by the magnetic field to the outside world. An upper limit exists on the torque that can be transmitted by a magnetic perturbation. This limit is enforced by shielding asymmetries from the plasma, which can be an important effect for toroidal asymmetries. The torque interaction of plasmas with magnetic fields can be either through an anisotropic pressure or by the drive for magnetic islands. The physics of both types of interactions are considered and paradoxical effects are clarified. [Preview Abstract] |
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PP8.00105: Modeling high-voltage dielectric insulator breakdown John Verboncoeur, Soren Taverniers Modeling angled dielectric insulator breakdown has been notoriusly difficult due to a number of numerical challenges, including inaccuracies in the surfaces and impact angles caused by stair-stepped surfaces, and the small but surprisingly significant effects of rounding end points of structures off to grid locations. In this work, we describe techniques to overcome these challenges using a two-dimensional particle-in- cell model. Recent theoretical work has suggested that electron emission near the triple point due to an enhanced electric field is responsible for initiating flashover in high voltage insulation systems and RF breakdown in high power microwave windows [1]. In this work, 2D particle simulation is employed to calculate the breakdown voltage versus insulator angle for two parallel conductors. The model includes secondary electron emission insulator surface, and space charge effects in the volume and at the insulator surface. The model is verified for analytical cases, and is compared to the theory [1]. \vspace{0.1 in} \newline [1] N. M. Jordan, Y. Y. Lau, D. M. French, R. M. Gilgenbach, and P. Pengavich, J. Appl. Phys. 102, 033301 (2007). [Preview Abstract] |
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PP8.00106: PIC simulations of space charge limited flow Martin E. Griswold, Nathaniel J. Fisch, Jonathan S. Wurtele Achieving the maximum possible current density continues to be an important goal with far ranging applications. The space charge limit to current in diodes can be affected by a number of parameters, such as finite emitter size [1] and short pulse length [2]. Here, using a particle-in-cell code, we examine a number of interesting phenomena associated with space charge limited flow, including virtual cathode formation for time-dependent situations. \\[4pt] [1] Luginsland, Lau, et. al., Physics of Plasmas 9 (2002) 2371\\[0pt] [2] Valfells et. al., Physics of Plasmas 12 (2002) 2377 [Preview Abstract] |
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PP8.00107: Nonlinear effects due to Langmuir waves in densifying plasmas Nat Fisch, Ilya Dodin Electron Langmuir waves in plasma undergoing slow compression or rarefaction respond adiabatically to the modification of the plasma density and the pressure tensor, such that the total number of plasmons is conserved. However, the change in density modifies the ratio of the field energy to the plasma kinetic energy in ways that depend on how the density changes. The changes in field energy affect the ponderomotive forces, which can in turn govern a variety of plasma processes. [Preview Abstract] |
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PP8.00108: Action principle derivation of one-fluid models from two-fluid actions Alexander Wurm, P.J. Morrison Ideal MHD possesses a well-known action principle formulation when the theory is expressed in terms of Lagrangian (or material) variables.[1] Here we start with a general electromagnetic two-fluid action functional in Lagrangian variables and derive action principles for one-fluid models, including ideal MHD. \\[4pt] [1] W.A. Newcomb, Nuclear Fusion: 1962 Suppl. Part 2, p. 451 [Preview Abstract] |
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PP8.00109: Magnetization force in plasma physics Robert Johnson The magnetization force is well documented in materials science, thus it needs to be addressed in the context of plasma physics. Generalization of the macroscopic force densities by Lorentz-Kelvin and Korteweg-Helmholtz yields an expression $\mathbf{J} \times \mathbf{B} + \nabla \mathbf{M} \cdot \mathbf{B}$ appropriate for magnetized plasma. Application to the stationary equilibrium equation indicates its presence is necessary to enforce the reduction to the standard form in the limit of vanishing magnetization. A numerical investigation of an axially symmetric configuration confirms this behavior. Application to toroidal geometry indicates that no stationary solution exists for a curved plasma column. [Preview Abstract] |
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PP8.00110: The random path of a charged particle in a white noise magnetic field: analysis, simulation, and a physical realization Trevor Lipscombe, Don Lemons, Blake Johnson We derive a set of stochastic differential equations, parameterized with a single diffusion constant, that describes an isotropic, one-dimensional random path embedded in three dimensions. The path variously describes the trajectory of a charged particle in a white noise magnetic field, a photon undergoing weak scattering, and the random orientation of a long chain molecule. The mean and variance of the squared distance between the ends of the path depend only upon the diffusion constant and the path length. We illustrate this dependence with numerical simulations and test it by gathering data on a physical realization consisting of segments of thread suspended in glycerol. [Preview Abstract] |
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PP8.00111: A Hamiltonian electromagnetic gyrofluid model F.L. Waelbroeck, R.D. Hazeltine, P.J. Morrison An isothermal truncation of the electromagnetic gyrofluid model of Snyder and Hammett [Phys.\ Plasmas 8, 3199 (2001)] is shown to be Hamiltonian. The corresponding noncanonical Lie-Poisson bracket and its Casimir invariants are presented. The model describes the evolution of the density, the electrostatic potential, and the component of the vector potential along a strong background field. This makes it suitable for describing such phenomena as the propagation of kinetic-Alfv\'{e}n modons, the nonlinear saturation of drift-tearing modes, and the diamagnetic stabilization of the internal kink. The invariants are used to obtain a set of coupled Grad-Shafranov equations describing equilibria and propagating coherent structures. They also lead to a Lagrangian formulation of the equations of motion that is well suited to solution with the PIC method. [Preview Abstract] |
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PP8.00112: Grid-Free Electromagnetic Particle Simulations Benjamin Ong, Andrew Christlieb, Robert Krasny In this poster, we demonstrate how the existing boundary integral treecode (BIT) framework can be extended to electrodynamic problems. The key idea is a method of lines transpose methodology, where we choose to discretize in time and directly solve the resulting Helmholtz equation using an integral formulation. Using this formulation, the resulting implicit time integrator can be solved efficiently using treecode algorithms, and is able to take time steps much larger than the imposed CFL restrictions for a standard explicit integrator. Also, this method potentially recovers the magneto-static limit. [Preview Abstract] |
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PP8.00113: Fully EM algorithms for the quasi-neutral, zero-electron-inertia model Dennis Hewett An algorithm has been developed to compute the electromagnetic signature of impulsive currents in quasi-neutral, collisionless plasmas. Such plasmas are common in laser target chambers, space physics, and EMP generating events. Traditional models[1] used in this regime make the Darwin assumption that neglects purely EM waves. Computing electro-magnetic pulse behavior (EMP) requires the solution of the EM wave equations. Other algorithms designed to treat this problem [2] are susceptible to round-off problems in our applications. The current algorithm, based on the same physics model, is to be much more forgiving in regions of low density or low magnetic field. Results and initial V{\&}V analysis will be presented.\\[4pt] [1] ``Low-frequency EM (Darwin) applications in plasma simulations'', Hewett, CPC, 84, pg 243, May 1994\\[0pt] [2] ``A fully electromagnetic model for high density plasma simulations'' Jones, Thomas, Mason, and Winske, 13$^{th}$ Conference on the Numerical Simulations of Plasmas, 1983. [Preview Abstract] |
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PP8.00114: Relativistic Electron Scattering by Electromagnetic Ion Cyclotron Fluctuations: Test Particle Simulations Kaijun Liu, Don S. Lemons, Dan Winske, S. Peter Gary Relativistic electron scattering by electromagnetic ion cyclotron (EMIC) fluctuations is studied using a test particle simulation code. The EMIC fluctuation input comes from a one dimensional, self-consistent hybrid simulation model and is due to the growth of the ion cyclotron instability driven by the ion temperature anisotropy, T$_{i\bot } \quad >$ T$_{i\parallel }$, in a magnetized, homogeneous, collisionless plasma with a single ion species. The test particle code follows the motion of relativistic test electron particles in the input EMIC fluctuations. The time evolution of the mean-square pitch angle change is calculated and used to estimate the pitch angle diffusion coefficient. Finally the results are compared with quasi-linear diffusion theory. The present study has applications to relativistic electron dynamics in the terrestrial magnetosphere. [Preview Abstract] |
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PP8.00115: Stochastic Analysis and Numerical Simulation of Velocity Space Diffusion of Charged Particles by Circularly Polarized Magnetic Waves Transverse to a Uniform Magnetic Field Don Lemons, Kaijun Liu, Dan Winske, Peter Gary We describe the velocity space diffusion of charged particles in a static magnetic field composed of a uniform field and a sum of transverse circularly polarized magnetic waves. When the wave sum has many terms the auto-correlation time over which particle orbits become effectively randomized is small compared to the time required for the particle distribution to change significantly. In this regime the deterministic equations of motion can be transformed into stochastic differential equations of motion that are valid over the longer time scale. The resulting velocity space diffusion function is consistent with that produced by quasi-linear theory. Numerical solutions of the deterministic equations of motion agree with the theory for all pitch angles, for different wave spectra, and for normalized wave energy densities less one. The theory is applied to the scattering of magnetospheric relativistic electrons from Alfven-cyclotron waves. [Preview Abstract] |
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PP8.00116: Implicit Plasma Kinetic Simulation Using The Jacobian-Free Newton-Krylov Method William Taitano, Dana Knoll, Luis Chacon The use of fully implicit time integration methods in kinetic simulation is still area of algorithmic research. A brute-force approach to simultaneously including the field equations and the particle distribution function would result in an intractable linear algebra problem. A number of algorithms have been put forward which rely on an extrapolation in time. They can be thought of as linearly implicit methods or one-step Newton methods. However, issues related to time accuracy of these methods still remain. We are pursuing a route to implicit plasma kinetic simulation which eliminates extrapolation, eliminates phase-space from the linear algebra problem, and converges the entire nonlinear system within a time step. We accomplish all this using the Jacobian-Free Newton-Krylov algorithm. The original research along these lines considered particle methods to advance the distribution function [1]. In the current research we are advancing the Vlasov equations on a grid. Results will be presented which highlight algorithmic details for single species electrostatic problems and coupled ion-electron electrostatic problems. \\[4pt] [1] H. J. Kim, L. Chac\'on, G. Lapenta, ``Fully implicit particle in cell algorithm,'' 47th Annual Meeting of the Division of Plasma Physics, Oct. 24-28, 2005, Denver, CO [Preview Abstract] |
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PP8.00117: The FACETS project: integrated core-edge-wall modeling with concurrent execution J.R. Cary, S. Balay, J. Candy, J.A. Carlsson, R.H. Cohen, T. Epperly, D.J. Estep, M.R. Fahey, R.J. Groebner, A.H. Hakim, G.W. Hammett, K. Indireshkumar, S.E. Kruger, A.D. Maloney, D.C. McCune, L. McInnes, A. Morris, A. Pankin, A. Pletzer, A. Pigarov, T.D. Rognlien, S. Shasharina, S. Shende, S. Vadlamani, H. Zhang The multi-institutional FACETS project has the physics goals of using computation to understand of how a consistent, coupled core-edge-wall plasma evolves, including energy flow, particle recycling, and the variation of power density on divertor plates with plasma under different conditions. FACETS is being developed to take advantage of Leadership Class Facilities (LCFs), while still being able to run on laptops with reduced fidelity models. This presentation will provide a high-level overview of the project, discussing the issues of componentization, solvers, performance monitoring, testing, visualization and first physics results for core-edge coupling. [Preview Abstract] |
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PP8.00118: FACETS -- Infrastructure for Integrated Fusion Modeling Svetlana Shasharina, John Cary, Johan Carlsson, Ammar Hakim, Scott Kruger, Mahmood Miah, Alexander Pletzer, Srinath Vadlamani, David Wade-Stein, Satish Balay, Lois McInnes, Hong Zhang, Jeff Candy, Mark Fahey, Ron Cohen, Tom Epperly, Tom Rognlien, Don Estep, Alexei Pankin, Allen Malony, Alan Morris, Sameer Shende, Keshavamurthy Indireshkumar, Douglas McCune, Alexander Pigarov It is desirable that an infrastructure for integrated fusion modeling has support for: legacy and new components used interchangeably; consistent management of components lifecycle; allocating parallel resources consistent with the nature of participating components and the problem scope; components written in multiple programming languages; composition of sequentially and concurrently executing components respecting dependencies; tight and loose coupling of components; testing and validation of separate and integrated components; and use of multiple platforms from desktops to LCFs. In this poster we will describe the status of the FACETS with respect to these features. [Preview Abstract] |
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PP8.00119: Development of an Arbitrary Curvilinear-Coordinate PIC Code Chris Fichtl, John Finn, Gian Luca Delzanno We are developing an innovative new PIC code that couples a grid generation strategy to the standard PIC algorithm. This approach is very beneficial for studies of small objects immersed in plasmas. Standard PIC codes model curved surfaces with a stair-stepping technique, which highly distorts the shapes of small objects. This can become quite problematic when calculating forces on the surface of the object. Our grid generation strategy conforms to objects of arbitrary shape, eliminating such problems as stair-stepping along curved boundaries, and thus allows us to simulate complicated physical structures very accurately. Furthermore, we have developed a semi-implicit particle mover for based on a generalization of the leapfrog method to arbitrary (including nonorthogonal) grids. We show our mover to be symplectic and thus it preserves phase-space area exactly. Extensive testing has shown that it also conserves energy to high accuracy. Our new code is validated using OML charging theory for spherical and cylindrical dust grains. Further tests include the charging of an electron-emitting dust grain in a uniform plasma. The final goal is to study the forces between two emitting grains of like charge. [Preview Abstract] |
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PP8.00120: Modeling a short cold cathode DC discharge device with controllable plasma parameters Anatoly Kudryavtsev, Steven Adams, Vladimir Demidov, Yevgeny Bogdanov A short (without positive column) DC gas-discharge device with a cold cathode has been modeled. The device consists of the plane disk-shaped cathode and anode while the inter-electrode gap is bounded by a cylindrical wall. The cathode and anode are each 2.5 cm in diameter, and the inter-electrode gap is 12 mm. The wall is made of conducting parts divided by an insulator. The modeling has been performed for argon plasma at 1 Torr pressure. It is demonstrated in the model that spatial distributions of electron density and temperature and argon metastable atom density depend on the DC voltage applied to different conducting parts of the wall. Applied voltage can trap within the device volume energetic electrons arising from atomic and molecular processes in the plasma. This leads to a modification in the heating of slow electrons by energetic electrons and as a result modifies the controlling plasma parameters. [Preview Abstract] |
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PP8.00121: Efficient hybrid algorithms for collisional plasma simulation A.M. Dimits, B.I. Cohen, R.E. Caflisch, C.M. Wang, Y. Huang We report on the development of efficient hybrid simulation algorithms for plasma systems that span a wide range of collisionality. Investigations of their performance, using ion-sheath and electron-transport test problems, are presented. In these schemes the distribution function is decomposed of into kinetic and fluid components. The fluid component is treated via Eulerian fluid simulation methods. In one class of algorithms, the kinetic component is treated using a combination of fixed-weight particle-in-cell (PIC) and binary Monte-Carlo collision methods. Particles are created by sampling from the fluid component, and paired for collisions with the kinetic particles. In the other class of algorithms, the kinetic component is treated using evolving-weight delta-f-PIC schemes and collision-field algorithms. The performance these algorithms depends strongly on the particular sets of criteria for (a) exchange between the particle and fluid components and (b) creation, destruction, and retention of the simulation particles. [Preview Abstract] |
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PP8.00122: A Novel Approach to Solving the Vlasov-Fokker-Planck Equations through Particle-In-Cell Hybridization Paul Cummings, Alec Thomas A new computational method for modeling ultra-fast, ultra-intense laser-solid and laser-plasma interactions is proposed. The method consists of a hybridization of the particle-in-cell technique and a direct solution to the collisional Vlasov equation with the Krook operator, wherein the Vlasov equation is expanded in velocity space using Cartesian tensors, and the higher-order terms serve as a source of particles and ``holes'' to be tracked by PIC methods. The mathematics underlying the solution method for a simple implementation are presented and analyzed; this implementation features one spatial dimension and truncates the expansion to first-order. A computational scheme for this implementation of the solution method is presented. More advanced implementations of the solution method, including retaining second- order expansion terms and replacing the Krook model with the Fokker-Planck model, are discussed. [Preview Abstract] |
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PP8.00123: ABSTRACT WITHDRAWN |
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PP8.00124: Wavelet-based density estimation for noise reduction in plasma simulations using particles R. Nguyen van yen, D. del-Castillo-Negrete, K. Schneider, M. Farge, G. Chen A limitation of particle methods is the inherent noise caused by limited statistical sampling with finite number of particles. Thus, a key issue for the success of these methods is the development of noise reduction techniques in the reconstruction of the particle distribution function from discrete particle data. Here we propose and study a method based on wavelets, previously introduced in the statistical literature to estimate probability densities given a finite number of independent measurements. Its application to plasma simulations can be viewed as a natural extension of the finite size particles (FSP) approach, with the advantage of estimating more accurately distribution functions that have localized sharp features. Furthermore, the moments of the particle distribution function can be preserved with a good accuracy, and there is no constraint on the dimensionality of the system. It is shown that the computational cost of the denoising stage is of the same order as one time step of a FSP simulation. The wavelet method is compared with the recently introduced proper orthogonal decomposition approach in Ref.~[D. del-Castillo-Negrete, et al., Phys. Plasma, {\bf 15} 092308 (2008)]. [Preview Abstract] |
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PP8.00125: Collisionless stochastic electron heating in a plasma slab Alexey Arefiev, Vladimir Khudik We find the rate and the saturation energy for stochastic collisionless electron heating by an AC field in a plasma slab. The work is motivated by ``two-sided'' vacuum heating of radially moving electrons in laser-irradiated microclusters [1]. We consider an ideally conducting slab that freely emits electrons in an AC electric field. The interaction of an emitted electron with the field is analogous to a 1D random walk in velocity space, with the time between the walk steps proportional to the travel time through the slab ($t_s \sim 1/|v|$). The resulting increase of the average electron energy is slower than previously expected ($t^2$) [1] because of long delays between the interactions for slower electrons. The heating saturates when a considerable number of electrons reach a velocity limit where the Chirikov resonance-overlap criterion is not satisfied. The velocity saturation might alter the resonant heating mechanism proposed in [2], since the travel-time $t_s$ at the limiting velocity can significantly exceed the period of the AC field. [1] Phys. Plasmas 12, 056706 (2005); [2] Phys. Plasmas 12, 056703 (2005). [Preview Abstract] |
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