Bulletin of the American Physical Society
56th Annual Meeting of the APS Division of Plasma Physics
Volume 59, Number 15
Monday–Friday, October 27–31, 2014; New Orleans, Louisiana
Session GP8: Poster Session III: DIII-D Tokamak; Computer Simulation Methods: Shocks, Waves, Dynamo and Dipole; Low Temperature Plasmas Science and Engineering |
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Room: Preservation Hall |
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GP8.00001: DIII-D TOKAMAK |
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GP8.00002: Evolution of Radial Electric Field due to RMP-induced density pump-out R.J. Groebner, S.P. Smith, T.E. Evans, X. Chen, C. Paz-Soldan, K.H. Burrell, R. Nazikian, B.A. Grierson, R.A. Moyer, D. Orlov, C. Chrystal, G.R. McKee The time history of shear in the ExB field during application of resonant 3D magnetic perturbations (RMP) in DIII-D is studied with CER spectroscopy. Application of the RMP typically causes density pump-out and can ultimately lead to ELM suppression. Thus, understanding the origin of the density transport is an important issue for understanding ELM suppression by this technique. One hypothesis is that the RMP causes a reduction of ExB shear at the pedestal top, which then allows for an increase in density transport [1]. The ExB shear is examined in a new experiment in which the RMP was varied by a small amount around the threshold for causing density pump-out. In and on top of the pedestal, $E_{r}$ is observed to become more positive coincident with density pumpout. $E_{r}$ and its shear are examined over a range of RMP fields to determine if there is a relation between these quantities and the magnitude of density pumpout.\par \vskip6pt \noindent [1] S. Mordijck, R. A. Moyer, and G. R. McKee, Phys. of Plasmas {\bf19}, 024504 (2012) [Preview Abstract] |
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GP8.00003: Boundary and PMI Diagnostics for the DIII-D National Fusion Facility D.M. Thomas, B.D. Bray, C. Chrobak, A.W. Leonard, S.L. Allen, C.J. Lasnier, A.G. McLean, A.R. Briesemeister, J.A. Boedo, D. Elder, J.G. Watkins The Boundary and Plasma Materials Interaction Center is planning an improved set of boundary and divertor diagnostics for DIII-D in order to develop and validate robust heat flux solutions for future fusion devices on a timescale relevant to the design of FNSF. We intend to develop and test advanced divertor configurations on DIII-D using high performance plasma scenarios that are compatible with advanced tokamak operations in FNSF as well as providing a comprehensive testbed for modeling. Simultaneously, candidate PFC material solutions can be easily tested in these scenarios. Additional diagnostic capability is vital to help understand and validate these solutions. We will describe a number of desired measurements and our plans for deployment. These include better accounting of divertor radiation, including species identification and spatial distribution, divertor/SOL main ion temperature and neutral pressure, fuller 2D $T_e/n_e$ imaging, and toroidally separated 3D heat flux measurements. [Preview Abstract] |
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GP8.00004: Inferring DIII-D Edge Neutral Density from Fast-Ion D-Alpha Emission N.G. Bolte, W.W. Heidbrink, D. Pace, M. Van Zeeland Promptly-lost beam ions produce Doppler-shifted Balmer-Alpha light after charge exchanging with edge neutrals. Spectra of this edge-localized fast-ion D-alpha (FIDA) emission have been measured at DIII-D using six chords that view the edge region. A new simulation P-FIDASim has been developed that models prompt-loss radiation. P-FIDASim uses modules from the active FIDA code, FIDASIM [1] but uses fast-ion orbits from a single beam in place of FIDASIM's use of a theoretical fast-ion distribution function and considers CX with edge, not beam or halo neutrals. Initial results show good correlation between experiment and simulation in spectral shape. Intensity variations between chords show that empirical results are inconsistent with neutral density being a pure flux function. Modeling a neutral source term at the wall gives the z-dependence of the neutral density by inversion. Results will be presented of 2D (R,z) cross-sectional values of neutral density found by this method.\par \vskip6pt \noindent [1] W.W. Heidbrink, et al., Commun. Comput. Phys. {\bf10}, 716 (2011) [Preview Abstract] |
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GP8.00005: The Effects of Non-axisymmetric Fields on Divertor Conditions in the DIII-D Tokamak A.R. Briesemeister, R.C. Isler, J.-W. Ahn, E.A. Unterberg, D.L. Hillis, A.G. McLean Measurements of impurity ion density, temperature and flow velocity made using the multichord divertor spectrometer (MDS) on DIII-D are presented for plasmas both with and without externally applied resonant magnetic perturbations (RMPs). Large parallel flows, measured to be on the order of 25 km/s, are driven by the Bohm sheath criteria at the plasma/wall interface. Changes in the connection lengths of the magnetic field, which can cause changes in the ion flow in the divertor, are predicted to occur when the RMPs are applied. Measurements of both C II and C III are analyzed to assess changes in both the flow and the spatial localization of the different carbon ionization states when RMPs are applied. In high-density plasmas, where RMPs had no effect on the core plasma conditions, no measurable changes were seen in the carbon flow in the divertor. Flows measured in lower density conditions will be used to investigate the relationship between density and the effects of RMPs on divertor plasmas. [Preview Abstract] |
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GP8.00006: Validation of the SOLPS Parallel Heat Transport Model J.M. Canik, A.R. Briesemeister, C.J. Lasnier, A.G. McLean, M.A. Makowski, A.W. Leonard, J.G. Watkins Recent SOLPS 2D fluid plasma/neutrals edge transport simulations have shown a consistent under-prediction of radiated power that when accounted for allows simulations to successfully match high resolution divertor and scrape-off-layer density ($n_e$) and temperature ($T_e$) measurements near detached conditions in DIII-D. The parallel heat transport model has been evaluated in simulations with the upstream $n_e$ and $T_e$ and divertor heat flux matched to experiments. Simulations of L-mode discharges near detachment onset require either increased carbon sources or hydrogenic recombination radiation to match measured radiative losses. With this increase, the poloidal $T_e$ profile shows good agreement with 2D divertor Thomson scattering data, including an extended region with very low $T_e$, which cannot be reproduced without the additional radiative loss. Similar scaling of the radiated power also results in agreement for the $T_e$ profile measured in H-mode experiments; however, in this case the plasma data show a poloidally extended region of high ne that is not captured in simulations. [Preview Abstract] |
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GP8.00007: OEDGE Modeling of Power Balance in DIII-D Density Scan Discharges Leading to Detachment J.D. Elder, P.C. Stangeby, A.W. Leonard, B.D. Bray, N.H. Brooks, J.G. Watkins, E.A. Unterberg, A.G. McLean The OEDGE code is used to model the outer divertor plasma for discharges from a density scan experiment on DIII-D. In this experiment the plasma density was increased over a series of L-mode and H-mode discharges starting with both targets attached and ending with both targets fully detached. These discharges used large X-point sweeps to obtain 2D Thomson profiles of the divertor plasma, target Langmuir probe profiles and spectroscopic emission profiles. OEDGE is run with a plasma solution consistent with divertor Thomson measurements and target recycling fluxes. This reproduces the experimental hydrogenic emissions. Carbon sources are then modeled in OEDGE to try to match the carbon experimental spectroscopic emissions. The combination of total hydrogenic and total carbon radiated power is then compared to outer divertor bolometric measurements. This process assesses the consistency of both the model and the experimental measurements of radiative power balance and identifies whether additional power terms may be required. [Preview Abstract] |
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GP8.00008: The role of parallel and poloidal heat flux in setting the detachment threshold in DIII-D D.N. Hill, S.L. Allen, C.J. Lasnier, A.G. McLean, T.W. Petrie, A.W. Leonard, M. Groth Experimental results show that the threshold density for divertor detachment is reduced even as the parallel scrape-off-layer (SOL) heat flux ($q_{||}$) is more than doubled, contrary to expectation. The work is part of a systematic study to identify the physics basis for obtaining detached divertors in future high power burning plasma experiments, consistent with requirements for high confinement steady-state operation. Parallel heat flux [$P_{SOL}*(B_{tor}/B_{pol})/2\pi R\lambda_q$; $\lambda_q$ is the SOL width] is independent of poloidal flux expansion and is commonly used to quantify the divertor heat flux challenge. In these experiments, the parallel heat flux was varied either by changing the heating power (thereby $P_{SOL}$), plasma current (the SOL width), or toroidal field (the projection of $P_{SOL}$ onto $B_{tor}$). The data point to poloidal-field physics effects (e.g., neutral penetration field, line length, and impurity radiation volume) playing a dominant role in setting the detachment threshold. Comparison with 2D simulation will be shown. [Preview Abstract] |
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GP8.00009: Compatibility of Detached Divertor Operation with Robust Edge Pedestal Performance A.W. Leonard, T.H. Osborne, P.B. Snyder, M.A. Makowski, A.G. McLean The compatibility of radiative detached divertor operation with the maintenance of a robust H-mode pedestal is examined in DIII-D. A density scan with deuterium injection into H-mode spanned a range of divertor conditions from fully attached, $\sim$30 eV at the target, with little divertor radiation to a fully detached with $T_e <\,$ 5 eV throughout the divertor up to the \mbox{X-point}. Over this scan of pedestal density from $n/n_{GW}\,$=30\% to 60\% the pedestal $T_e$ was reduced from 800 eV to 350 eV, representing a $\sim$20\% reduction in pedestal pressure with a similar reduction in normalized energy confinement. The reduction in pedestal pressure at high density was found to be consistent with a reduced pedestal ELM MHD stability limit at high collisionality. The scaling of the pedestal top pressure with density was also consistent with the EPED model, which assumes an additional constraint on the local pressure gradient. The MHD stability limit at the highest collisionality depends on details of the ELM instability growth rate normalization. This result is encouraging for future burning plasmas where a low collisionality pedestal is expected to be maintained even for high density detached divertor operation. [Preview Abstract] |
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GP8.00010: Characterizing the transition from high recycling to partial detachment A.G. McLean, S.L. Allen, M. Fenstermacher, C. Lasnier, W.H. Meyer, G. Porter, V. Soukhanovskii, B.D. Bray, T.N. Carlstrom, A.W. Leonard, C. Liu, D. Eldon, M. Groth, P.C. Stangeby, C. Tsui Experiments at DIII-D have explored the transition from the high recycling to the partially detached divertor condition in L- and H-mode with an unprecedented level of detail. Improved divertor and core Thomson scattering diagnostics were coupled with high resolution spectroscopic studies of molecular and neutral emissions. 2-D $T_{e}$ and $n_{e}$ profiles of the outer leg reveal the earliest indications of formation of the detachment front at the target plate, reducing local $T_{e}$ at the outer strike point from 8-10 eV to 2-3 eV with a marginal ($<10\%$) increase in $ |
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GP8.00011: Dependence of the Heat Flux Width on the Connection Length in DIII-D M.A. Makowski, C.J. Lasnier, V.A. Soukhanovski, A.W. Leonard, T.H. Osborne, T.W. Petrie, P.B. Snyder The heat flux width characterizes the scale length of peak power deposition in the divertor. The total heat flux width, $\lambda_{int}\approx \lambda_q+1.74S$, has contributions from the scrape-off layer itself, characterized by the quantity $\lambda_q$, and from the private flux region, characterized by a Gaussian-like width, $S$. Most work to date has focused on the physics of $\lambda_q$, with the essential finding that it depends approximately inversely on the plasma current. Here, the emphasis is on the $S$ parameter and, in particular, its dependence on the connection length, $L_{conn}$. Data from high X-point discharges ($L_{conn}\sim\,$30 m) have been used to extend the DIII-D heat flux width database beyond discharges with a standard divertor configuration ($L_{conn}\sim\,$20 m). Snowflake divertor discharges ($L_{conn}>40\,$m) will also be analyzed to further extend the range of $L_{conn}$. Preliminary results indicate that $S$ increases with Lconn, consistent with $S$ being determined by a diffusive process. This result has important implications for advanced divertor designs as it demonstrates that long connection lengths increase the heat flux width. [Preview Abstract] |
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GP8.00012: Heat Deposition on Inner and Outer Wall During Diverted DIII-D Discharges C.J. Lasnier, W.H. Meyer, S.L. Allen, M.A. Van Zeeland In order to get more complete information on power loss, we have quantified heating of the inner and outer wall of DIII-D in diverted DIII-D discharges due to several effects, using a wide-angle tangential viewing IR camera system. These effects include prompt fast ion losses to outer wall and bumper limiters during counter-toroidal-field neutral beam injection; shine-through of neutral beam power on the inner wall accentuated during low-density operation; small amounts of heat deposited on inner and outer walls during edge localized modes, and anomalous heat deposition on the outer wall near beam ports, which may be due to re-ionization of neutral beam particles. Prompt losses of fast ion losses distributed on the outer wall with counter-injected beams have resulted in a surface temperature rise of 25$^\circ$C, but localized heating of a bumper limiter has resulted in a temperature rise of up to 245$^\circ$C. Heating of the same limiter by an ELM has been observed to increase the surface temperature by 50$^\circ$C. [Preview Abstract] |
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GP8.00013: Snowflake Divertor Configuration Studies in DIII-D Tokamak V.A. Soukhanovskii, S.L. Allen, M.E. Fenstermacher, C.J. Lasnier, M.A. Makowski, A.G. McLean, W.H. Meyer, E. Kolemen, R.J. Groebner, A.W. Hyatt, A.W. Leonard, T.H. Osborne, T.W. Petrie Recent DIII-D studies show that the snowflake (SF) divertor enables significant manipulation of divertor heat transport for power exhaust in attached and radiative divertor conditions, between and during edge localized modes (ELMs), while maintaining good \mbox{H-mode} confinement. Results include: 1) Increased scrape-off layer (SOL) width suggesting enhanced divertor heat transport; 2) Direct measurements of divertor null-region poloidal beta $\beta_p>>1$ in support of the theoretically proposed instability mechanism leading to fast convective plasma redistribution, especially efficient during ELMs, and contribution to 1); 3) Weak effect on pedestal profile and stability resulting in essentially unchanged ELM regime; 4) Reduction of Type-I ELM energy loss; 5) In radiative SF divertor regimes with D2 seeding, a significant reduction of peak heat fluxes between and during ELMs, as in standard H-modes. [Preview Abstract] |
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GP8.00014: Divertor Optimization via Control at DIII-D E. Kolemen, S.L. Allen, M.A. Makowski, V.A. Soukhanovskii, B.D. Bray, D.A. Humphreys, R. Johnson, A.W. Leonard, C. Liu, B.G. Penaflor, T.W. Petrie, D. Eldon, A.G. McLean, E.A. Unterberg DIII-D divertor performance and heat-handling capabilities are optimized using advanced control techniques. The world's first real-time snowflake divertor detection and control system was implemented on DIII-D in order to stabilize and optimize this configuration. A new control system was implemented to regulate and study detachment and radiation, since future fusion reactors will require detached or partially detached plasmas to achieve acceptable divertor target heat fluxes. The algorithm regulates the $D_{2}$ and impurity gas injection level by using the divertor temperature measurements from real-time Thomson diagnostics to compute the detachment level, and the real-time bolometer diagnostics to determine core and divertor radiation. This control allows the optimization of the detachment and radiation from the core and the divertor to achieve high core performance compatible with reduced heat-flux to the divertor. [Preview Abstract] |
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GP8.00015: Application of the Radiating Divertor Approach to Innovative Divertor Concepts T.W. Petrie, A.W. Leonard, T.C. Luce, F. Turco, S.L. Allen, M.E. Fenstermacher, C.T. Holcomb, C.J. Lasnier, V.A. Soukhanovskii, E. Kolemen, J.G. Watkins Recent experiments on DIII-D have assessed the effectiveness of 3 innovative tokamak concepts under radiating divertor (RD) conditions: (1) high performance standard double-null divertor (DND) plasmas, (2) high performance double-null ``snowflake" (SF-DN) plasmas, and (3) single-null H-mode plasmas with different isolation from their divertor targets but otherwise identically prepared. Significant reductions in both divertor heat flux and divertor electron temperature were observed in both standard DND and SF-DN plasmas under neon/deuterium-based RD conditions, while maintaining high performance metrics, such as $\beta_N\simeq 3.0$ and $H_{98(Y,2)} \simeq 1.4$. Not only is the peak heat flux reduced by extending the parallel connection length ($L_{||-XPT}$) between the X-point and divertor targets, thereby enhancing the effect of cross-field diffusion, but also the effectiveness of the RD conditions is markedly improved at larger $L_{||-XPT}$. In general, our analyses support the attractiveness of all three of the above concepts under RD conditions, both in reducing peak heat flux and in maintaining good to excellent H-mode energy confinement. [Preview Abstract] |
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GP8.00016: The boundary effects on tokamak edge instabilities Weigang Wan, Scott Parker, Yang Chen Tokamak edge instabilities are extremely difficult to model with gyrokinetic simulation because of the strong pressure gradients and the sensitivity to the magnetic equilibrium near the x-point. Our previous global simulations have assumed fixed radial boundary conditions with the simulation domain inside the last closed flux surface, i.e. the separatrix. The pressure profiles have to be smoothed near the boundary. Although it is typical that edge instabilities peak inside the separatrix, these restrictions may affect the physical results. Here, we extend the simulation domain into the scrape-off layer, still assuming closed flux surfaces, and keep the original density and temperature profiles to study any possible boundary effects in the previous model. With the newly implemented general magnetic equilibrium in GEM (rather than Miller parametrization), this model is even more realistic. Simulations are carried out using direct experimental parameters of recent DIII-D discharges. [Preview Abstract] |
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GP8.00017: ELM Triggering Dependence on Deuterium Pellet Size on DIII-D L.R. Baylor, N. Commaux, S.J. Meitner, C.J. Lasnier, M.E. Fenstermacher, S.L. Allen, A.W. Leonard, P.B. Parks, R.A. Moyer The triggering of small ELMs by pellet injection has been demonstrated as a method to prevent large ELMs that can erode plasma facing components [1]. Small deuterium pellets $<$ 1 mm in size have been shown to reliably trigger ELMs on the DIII-D tokamak in the ITER like scenario plasmas. A variation in pellet size and speed was used to determine the minimum pellet size needed to trigger ELMs as a function of edge pedestal pressure. Pellets $<$ 0.8 mm in size were found to be insufficient to trigger ELMs. These results show smaller pellets than predicted by nonlinear MHD simulations can destabilize high-n ballooning modes from a local pressure perturbation well in excess of the pedestal pressure [2]. The implications of these results for pellet ELM mitigation and the design of the pellet injection system for ITER will be discussed.\par \vskip6pt \noindent [1] L.R. Baylor et al., Phys. Rev. Lett. 245001 (2013)\par \noindent[2] S. Futatani et al., Nucl. Fusion. {\bf54}, 073008 (2014). [Preview Abstract] |
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GP8.00018: Understanding the Physics of EHO Generation in DIII-D Including the Role of Rotational Shear K.H. Burrell, A.M. Garofalo, P.B. Snyder, W.M. Solomon The key to QH-mode operation is an edge electromagnetic mode, the edge harmonic oscillation (EHO), which provides the extra transport to allow the edge plasma to reach a transport equilibrium with edge pressure gradient and current density just below the edge localized mode (ELM) limit [1]. Experimental results are consistent with the theoretical prediction that the EHO is a kink-peeling mode destabilized by edge rotational shear at edge conditions just below the ELM limit [1]. Theory suggests that the essential rotation speed is $E_r/RB_\theta$; initial analysis of experimental data is consistent with this expectation [2,3]. Recent results show that the change in shear between QH-mode and ELMing H-mode occurs in the small radius side of the edge $E_r$ well near the top of the edge pedestal. Experiments have been carried out to test the whether $E_r/RB_\theta$ is the essential shear and, if so, how that critical shear varies with $\nu^\ast$.\par \vskip6pt \noindent [1] K.H.\ Burrell, et al., Nucl.\ Fusion {\bf 49}, 085024 (2009).\par \noindent [2] A.M.\ Garofalo, et al., Nucl.\ Fusion {\bf 51}, 083018 (2011).\par \noindent [3] K.H.\ Burrell, et al., Phys.\ Plasmas {\bf 19}, 056117 (2012). [Preview Abstract] |
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GP8.00019: Separation of Particle and Energy Transport in the H- and QH-mode Pedestal D.J. Battaglia, C.S. Chang, A. Diallo, B.A. Grierson, K.H. Burrell, R.J. Groebner Net particle transport through the H-mode pedestal is dictated by anomalous transport mechanisms; however, a significant fraction of the energy transport is governed by enhanced transport of high-energy ions on collisionless orbits. The pedestal radial electric field ($E_r$) is constrained to the value that balances this ion flux with a pinch of colder main ions and impurities as demonstrated using XGC0, a self-consistent full-f multi-species neoclassical calculation that includes neutral recycling and transport. These calculations resolve how edge modes can increase the anomalous particle transport with only a small effect on energy transport, the observed scaling of the height of the density pedestal with $I_p$, and the structure of $E_r$ in the pedestal. Quantitative agreement between XGC0 and the unique features of QH-mode, such as $T_i$ anisotropy, large scrape-off layer $T_i$ and intrinsic co-$I_p$ edge rotation provide confidence that the simulation captures the kinetic effects in the pedestal that drive the neoclassical energy transport. [Preview Abstract] |
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GP8.00020: Modeling EHO Formation in QH-mode on DIII-D Xi Chen, K.H. Burrell, N.M. Ferraro, T.H. Osborne, A.M. Garofalo, R.J. Groebner, L.L. Lao, P.B. Snyder, R. Nazikian, W.M. Solomon, B.J. Tobias, G.R. McKee, Z. Yan, C.M. Muscatello The 3D MHD code M3D-C1 is being used to model the edge harmonic oscillation (EHO) in QH-mode plasmas. Preliminary simulations show unstable low-n modes in some reconstructed QH-mode equilibria with high edge density fluctuations similar to experiments. QH-mode is a stationary edge localized mode (ELM)-stable high confinement operation mode while EHO drives the additional particle transport allowing the edge plasma to reach a transport equilibrium just below the ELM limit [1]. Experiments and theory suggest that the EHO is a kink-peeling mode destabilized by edge rotational shear at edge conditions just below the ELM limit [1] and the essential rotation is the toroidal angular ExB drift frequency [2]. Detailed comparison of two-fluid M3D-C1 simulations and fluctuation measurements from multiple diagnostics on DIII-D will be presented, along with the EHO onset condition between experiment and simulation from various pedestal ExB shears.\par \vskip6pt \noindent [1] K.H.\ Burrell, et al., Nucl.\ Fusion {\bf 49}, 085024 (2009).\par \noindent [2] A.M.\ Garofalo, et al., Nucl.\ Fusion {\bf 51}, 083018 (2011). [Preview Abstract] |
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GP8.00021: Super H-mode: Prediction and Discovery of a New High Performance Regime P.B. Snyder, E.A. Belli, K.H. Burrell, A.M. Garofalo, R.J. Groebner, T.H. Osborne, W.M. Solomon, H.R. Wilson Fusion performance of tokamak plasmas increases strongly with the pressure at the top of the edge transport barrier (or ``pedestal height''). As understanding of the physics controlling the pedestal improves, this can be used not only to predict performance in existing regimes, but also to uncover new regimes of operation. The EPED model predicts pedestal height by combining calculated peeling-ballooning and kinetic ballooning mode constraints, and has been extensively tested. EPED predicts that, for strongly shaped plasmas, the pedestal bifurcates at high density into the usual H-mode solution, and a very high pressure ``Super H Mode'' solution. Prediction of Super H-Mode access via dynamic variation of the density has led to its recent discovery on DIII-D, including observations of bifurcation and very high pedestal pressure. We discuss pedestal theory, DIII-D observations, and coupling to core physics to globally optimize Super H-Mode. [Preview Abstract] |
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GP8.00022: Limit Cycle Oscillations and L/H Transitions From Mean Field Momentum Transport Equations G.M. Staebler, R.J. Groebner The momentum transport of the mean field ExB toroidal and ion parallel velocities are modeled with both collisional and turbulent contributions to the transport equations. It will be shown that there are normal one-step L/H transitions to suppressed turbulence and newly discovered limit cycle oscillations (LCO), from this two dimensional system. The suppression of tubulence by ExB velocity shear provides the drive for both types of transitions converting fluctuation intensity into parallel and ExB flow. The results of the new model will be compared with recent high-resolution measurements. The frequency of the LCO and the L/H transition timescale can be matched by the model. The phase shift between the density fluctuation amplitude of the turbulence and the ExB velocity shear is shown to depend on the evolution of the linear growth rate of the turbulence. The density dependence of the H-mode power threshold is consistent with the model with a strong increase in the H-mode threshold at low density. [Preview Abstract] |
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GP8.00023: RMP-Induced Plasma Transport Near X point J.D. Callen, C.C. Hegna The experimentally most noticeable effect of resonant magnetic perturbations (RMPs) on H-mode plasmas is ``density pump-out." This effect is most evident in the near-separatrix region where RMPs can cause the electron density gradient there to decrease by a factor of up to two. The previously developed flutter model [1] produces some RMP-induced transport in this region. However, two other electron effects need to be taken into account in the X~point region of the divertor separatrix. First, small 3D fields can cause significant (many cm) radial motion of field lines in ``homoclinic tangles" very near the X~point which have been observed experimentally in DIII-D. A flutter-type plasma transport model based on parallel electron collisional effects caused by RMP-induced ``radial" motion of field lines away from the lowest order axisymmetric magnetic flux surfaces in the X~point region is being developed. The second effect is that a small fraction of long length magnetic field lines in the near-separatrix region are ``open" ones which are directly connected to material walls in the divertor region. Electrons on such field lines could conduct significant electron heat to the divertor plates.\par \vskip6pt \noindent [1] J.D.\ Callen, et al., Nucl.\ Fusion {\bf 53}, 113015 (2013). [Preview Abstract] |
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GP8.00024: Developing Boundary/PMI Solutions for Next-Step Fusion Devices H.Y. Guo, A.W. Leonard, D.M. Thomas, S.L. Allen, D.N. Hill, Z. Unterberg The path towards next-step fusion development requires increased emphasis on the boundary/plasma-material interface. The new DIII-D Boundary/Plasma-Material Interactions (PMI) Center has been established to address these critical issues on a timescale relevant to the design of FNSF, adopting the following transformational approaches: (1) Develop and test advanced divertor configurations on DIII-D compatible with core plasma high performance operational scenarios in FNSF; (2) Validate candidate reactor PFC materials at reactor-relevant temperatures in DIII-D high-performance plasmas, in collaboration with the broad material research/development community; (3) Integrate validated boundary-materials interface with high performance plasmas to provide viable boundary/PMI solutions for next-step fusion devices. This program leverages unique DIII-D capabilities, promotes synergistic programs within the broad PMI community, including linear material research facilities. It will also enable us to build a compelling bridge for the US research on long-pulse facilities.\par [Preview Abstract] |
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GP8.00025: Formation of Counter-Flows by Magnetic Perturbations in Computer Simulations of the Plasma Boundary of Tokamaks H. Frerichs, O. Schmitz, T.E. Evans, Y. Feng, D. Reiter Simulations of the plasma boundary of an ITER similar shape H-mode plasma at DIII-D with the EMC3-EIRENE code have shown that a pattern of counter-flow channels emerges when resonant magnetic perturbations (RMPs) are applied. This pattern is found to be correlated with a flow-reversal in the perturbed scrape-off layer bounded by the perturbed separatrix. As a result of small non-axisymmetric perturbations to an axisymmetric equilibrium field, stable and unstable invariant manifolds associated with the separatrix split and intersect transversely. This so-called homoclinic tangle determines where field lines may connect from inside of the original separatrix to plasma facing components, and it introduces a checkerboard pattern of field lines with short and long connection lengths. In the present contribution we focus on the resulting plasma flows and we give a detailed analysis of the emerging flow pattern. [Preview Abstract] |
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GP8.00026: Role of plasma response in determining density pump-out with Resonant Magnetic Perturbations (RMPs) in DIII-D S. Mordijck, S.P. Smith We are studying the effect of RMPs on the density pump-out threshold in order to determine whether the transport changes are the result of change in turbulence or rotation. Applying RMPs strongly reduces the core rotation and increases the edge rotation, which reduces the ExB shear thus increasing turbulent transport. The toroidal rotation measurements made at two different toroidal location show no phase lag during rotating n=2 RMP experiment, which is an indication that there is a strong n=0 response. This n=0 response could be the result of MHD effects, or due to changes in turbulence characteristics. New low $\nu^*$ experiments at lower RMP strength allow us to test, whether this change in the toroidal rotation is the main drive behind the increase in particle transport in low collisionality H-mode plasmas on DIII-D as well as examine what is causing the n=0 response to the toroidal rotation. [Preview Abstract] |
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GP8.00027: Imaging of Boundary Plasma Displacements During RMPs in DIII-D R.A. Moyer, D.M. Orlov, T.E. Evans, N. Ferraro, J. King, T. Strait, C. Paz-Soldan, A. Wingen, R. Nazikian, B. Grierson, L. Zeng Visible imaging is used to measure the boundary displacement due to n = 2 and n = 3 RMPs in H-mode plasmas in DIII-D. Displacements $\approx$2 cm on the outer midplane are measured in LSN H-modes using \break active imaging of Doppler shifted deuterium beam emission with n = 2 RMPs rotating in the co-current direction [1] where the kink response is expected to be maximized. In contrast, displacements due to static n = 3 RMPs are $\approx$4 mm in similar LSN H-modes, with no measurable change when the n = 3 RMP phase is ``flipped'' by 60$^{\circ}$ toroidally. Plasma shape is also found to have a strong effect on the plasma response: n = 3 RMPs in Double Null Divertor plasmas are $\approx$2 mm, 10x smaller than the displacements in similar LSN plasmas, consistent with magnetics measurements. We will compare boundary displacements measured with active beam emission and passive C III imaging to separatrix manifold displacements and kink response in plasma response models.\par \vskip6pt \noindent [1] R.A. Moyer, et al., Nucl. Fusion {\bf52} 123019 (2012) [Preview Abstract] |
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GP8.00028: Effect of Perturbation Spectrum on RMP ELM Suppression in DIII-D D.M. Orlov, R.A. Moyer, C. Holland, T.E. Evans, N.M. Ferraro, C. Paz-Soldan, P.B. Snyder, J.S. deGrassie, C.M. Greenfield, R. Maingi, R. Nazikian, J.-K. Park, N. Logan, M.E. Fenstermacher Recent experiments in DIII-D have demonstrated that while the edge localized modes (ELMs) in a tokamak can be controlled with only 5 of 12 magnetic perturbation coils with only a small increase in the coil current, the width of the $q_{95}$ resonant window for ELM suppression is smaller with a reduced coil set due to an overall reduction of the resonant n=3 field in the plasma. In each I-coil configuration tested, a strong similarity of the plasma parameters was observed during ELM suppression phase in the core and pedestal regions. Vacuum and M3D-C1 plasma response modeling elucidate the role of the toroidal sidebands for resonant magnetic perturbation (RMP) ELM suppression. TGLF and TGYRO simulations are performed to understand the effect of the perturbation spectrum on the core transport.\par [Preview Abstract] |
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GP8.00029: ELM Suppression in DIII-D ITER-like Plasmas Using n=2 Magnetic Perturbations R. Nazikian, B.A. Grierson, M. Okabayashi, B.J. Tobias, D. Eldon, T.E. Evans, N.M. Ferraro, R.J. Groebner, C. Paz-Soldan, E.J. Strait, S.R. Haskey, J.D. King, G.R. McKee, R.A. Moyer, D.M. Orlov, M.W. Shafer A robust window of edge localized mode (ELM) suppression was observed at elevated magnetic safety factor ($q_{95}\approx$ 4.1) in ITER-like plasmas with even parity n=2 resonant magnetic perturbation (RMP) using the internal I-coils. Variation of the upper and lower I-coil phasing was used to explore the importance of pitch alignment vs kink alignment for ELM suppression. Both the pedestal density and ELM suppression were strongly dependent on I-coil phasing and a large variation in the plasma response amplitude was measured on multiple diagnostics. Surprisingly, toroidal rotation of the even parity n=2 RMP led to the loss of ELM suppression, indicating that components of the residual error field orthogonal to the kink mode may be important near the threshold for ELM suppression.\par [Preview Abstract] |
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GP8.00030: Measurements of Islands and Screening with Resonant Magnetic Perturbations on DIII-D M.W. Shafer, E.A. Unterberg, A. Wingen, J.M. Canik, J.D. Lore, J.H. Harris, D.L. Hillis, S.P. Hirshman, T.E. Evans, N.M. Ferraro, M.E. Austin Recent experiments on DIII-D have advanced the understanding of plasma response to resonant magnetic perturbations (RMPs) in low magnetic shear, inner wall limited, L-mode plasmas. Fine torque scans using mixtures of co- and counter-current neutral beam injection reveal that large RMP-induced n=1 islands are present at multiple mode-rational surfaces (m=2,3,4) at low rotation, but are completely screened at higher rotation. There is an observed nonlinear threshold for this torque, where small torque increments lead to a completely screened plasma response. Initial analysis indicates that near-zero $\omega_{e,\perp}$ is found not to be a sufficient condition for island formation, as it is observed to be approximately zero in cases where islands are completely screened. Comparisons are underway with two-fluid MHD modeling via the M3D-C1 code, 3D fluid transport with the EMC3-EIRENE code and nonlinear resistive MHD with the SIESTA code.\par [Preview Abstract] |
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GP8.00031: Edge Radial Electric Field and Ion Orbit Loss in DIII-D J.S. deGrassie, R.J. Groebner, J.A. Boedo, B.A. Grierson The edge radial electric field, $E_r$, may be largely determined by the value necessary to supply the neoclassical return current to balance the loss current due to ion orbit loss. This is the indication from a phenomenological model, motivated by recent Mach Probe measurements of the edge co-$I_p$ flow layer in DIII-D [1,2], based on a simple empty loss cone orbit loss model [3,4]. Probe and charge exchange recombination measurements also show a relatively large positive edge $E_r$ just inside the LCFS in Ohmic conditions, $\sim10$ kV/m, which is explained in this model by the propensity of the flow layer to drive return current. The Er level is also dependent on $Z_{eff}$ in the edge lower $Z_{eff}$ promotes greater negative $E_r$ for current balance. The model will be compared with measurements in Ohmic, L- and H-mode conditions.\par \vskip6pt \noindent [1] J.A. Boedo et al., Phys. Plasmas 18, 035510 (2011).\par \noindent[2] S.H. M\"uller et al., Phys. Rev. Lett. 106, 115001 (2011).\par \noindent[3] J.S. deGrassie et al., Nucl. Fusion 49, 085020 (2009).\par \noindent[4] J.S. deGrassie et al., Nucl. Fusion 52, 013010 (2012). [Preview Abstract] |
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GP8.00032: Ion Orbit Loss Effect on Structure of Radial Electric Field T.M. Wilks, W.M. Stacey, T.E. Evans The radial electric field is an important factor in the L-H transition, the suppression of ELMs, etc. Therefore, the modeling of the causes and the dynamics of the radial electric field in the edge and scrape off layer regions are of interest. We are investigating the interdependence of ion orbit loss, the compensating ion return current, rotation, and magnetic flux surface geometry on the radial electric field in the edge region and the coupling to the electric fields in the scrape off layer. Both thermalized plasma ions and fast beam ion losses are modeled in realistic geometry. The importance of toroidal and poloidal rotation in relating ion orbit loss to the radial electric field is examined. The ion orbit loss is modeled with conservation equations and supplemented by Lorentz orbit tracking calculations of the fraction of ions that recross the separatrix back into the edge plasma. [Preview Abstract] |
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GP8.00033: Inter-ELM Edge Transport Evolution in DIII-D H-mode Plasmas J.P. Floyd, W.M. Stacey, S.C. Mellard, R.J. Groebner This study examines the time evolution and pedestal recovery dynamics of ion transport in the edge pedestal between ELMs on DIII-D. Three plasmas from a DIII D H-mode current scan are analyzed: discharges ($I_p$ = 0.5 MA), ($I_p$ = 1.0 MA) and \break ($I_p$ = 1.5 MA). The profile evolution during these shots is interpreted to infer thermal diffusivities, ion diffusion coefficients, ion pinch velocities, and other important transport properties constructed using the momentum balance framework of Ref. [1]. The evolution of these computed properties is examined alongside the evolution of measured quantities, such as the densities, temperatures, rotation velocities, and radial electric field strength, in order to gain insight about the mechanisms of edge transport and pedestal recovery after Type-I ELMs. Both diffusive and non-diffusive (e.g. ion orbit loss) edge transport processes are quantified using the aforementioned framework and the GTEDGE code developed for DIII-D data interpretation. The analysis is focused on maximizing the time resolution of the profile evolutions.\par \vskip6pt \noindent [1] W.M. Stacey and R.J. Groebner, Nucl. Fusion {\bf51}, 063024 (2011). [Preview Abstract] |
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GP8.00034: The characteristics of the micro-turbulence in the pedestal region of DIII-D Tokamak Jingfei Ma, Xueqiao Xu, Rich Groebner Two types of the electromagnetic micro-instabilities have been identified in the pedestal region of DIII-D H-mode ELM-free plasmas (shot number 132016) numerically, using a six-field landau-fluid model under BOUT$++$ framework. One is the Alfvenic ion temperature gradient (AITG) mode, localized at the outer mid plane, and the other is the drift Alfven instability, localized at the top and bottom of the Tokamak. The AITG mode is driven by the ion temperature gradient and finite $\beta$, which is affected by the kinetic effects, such as Finite Larmor Radius (FLR) and Landau resonance. Typically, the FLR destabilizes the modes while the Landau resonance stabilizes them. Besides, the global simulation shows that the pedestal height and width have an evident impact on the growth rate and mode structure of the AITG instability. In order to identify the AITG instability, a set of the global self-consistent equilibria with different pedestal height (``Varyped'') are generated, which later are also used to explore the strong impact of $\beta $ on the AITG. The drift Alfven instability, however, has a very weak dependence on $\beta $. Moreover, the drift Alfven instability is the dominant mode, while the AITG is subdominant mode in the steep gradient region of the pedestal. [Preview Abstract] |
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GP8.00035: Effect of Collisionality and Effective Charge on the \mbox{H-mode} Pedestal Structure in DIII-D and JET M.J. Leyland, K.J. Gibson, T.H. Osborne, R.J. Groebner, P.B. Snyder, M.N.A. Beurskens, C. Giroud, S. Saarelma, X. Chen, R. Nazikian After the installation of the ITER-like-wall, the energy confinement of high triangularity D$_2$ fueled JET baseline plasmas was degraded by up to 40\% due to a reduction in pedestal performance. This could be partially recovered by changing the collisionality ($\nu^\ast$) and/or effective charge ($Z_{eff}$) when seeding N$_2$. Pedestal measurements revealed a widening of the pedestal and a variation in gradient. Comparison to EPED pedestal-model predictions highlights the potential importance of a low-Z, carbon-like, impurity at the plasma edge. We report on a dedicated DIII-D experiment that studied the role of $\nu^\ast$ and $Z_{eff}$ on the pedestal structure through means of D$_2$-fueling, N$_2$-seeding and Li-dropping. Initial analysis shows with increasing D$_2$ fueling the pedestal does not widen and the ELM frequency increases in contrast to equivalent JET plasmas. [Preview Abstract] |
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GP8.00036: Bifurcation to Expanded H-mode Pedestal Width and Height with Lithium Dust Injection in DIII-D R. Maingi, D.K. Mansfield, D.J. Battaglia, B. Grierson, R. Nazikian, A.L. Roquemore, G.L. Jackson, T.H. Osborne, C. Chrobak, J.S. deGrassie, R.J. Groebner, P.B. Snyder, Z. Yan, G.R. McKee, A.G. McLean Lithium (Li) aerosol injection into the SOL of the DIII-D tokamak has facilitated a rapid $\sim100\%$ expansion of the H-mode pedestal width in a class of ELMy discharges. ELM-free H-modes with $\tau_E$ increasing by $<60\%$ are observed; the radiated power held steady during ELM-free periods. The pedestal $T_e$ and $P_e$ doubled, while the $T_i$ increased by $\sim20\%$. Substantial $L_i$ density was observed in the core, reaching up to $15\%$ at the top of the pedestal. The onset of a continuous pedestal-localized instability measured on beam emission spectroscopy correlated with the pedestal expansion, which can occur on a $<$10 ms timescale. These enhanced pedestals are limited by onset of giant ELMs, which appear to be consistent with ideal stability calculations. [Preview Abstract] |
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GP8.00037: Impact of Lithium Injection on the H-mode Pedestal in DIII-D T.H. Osborne, G.L. Jackson, C. Chrobak, J.S. deGrassie, R.J. Groebner, P.B. Snyder, R. Maingi, D.K. Mansfield, D.J. Battaglia, B.A. Grierson, R. Nazikian, A.L. Roquemore, Z. Yan, G.R. McKee, A.G. McLean Lithium injection into ELMy H-mode discharges triggered unusual, up to 350 ms, ELM-free periods (EFPs) during which the pedestal width, $w_{PED}$, increased on a short time scale $\approx$10 ms reaching 2$\times$ the width seen in the ELMy phase. The electron pedestal pressure in EFPs with Li was 2$\times$ that of the ELMy phase and 1.5$\times$ that of similar $e$ was reduced by similar factors in EFPs with $L_i$. Rapid $w_{PED}$ expansion and enhanced particle transport was associate with pedestal localized density fluctuations seen on BES. $w_{PED}$ during EFPs with $L_i$ was $40\%$ larger than predicted by $E_{PED}$1.0 scaling, while $w_{PED}$ in EFPs without $L_i$ agreed with this scaling. EFPs terminated in a large ELM when the peeling-ballooning mode stability limit was reached. Sustainment of large $w_{PED}$, $P_{PED}$ could open a regime of improved energy confinement and high $\beta$ stability. [Preview Abstract] |
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GP8.00038: Onset of a ``Broadband Bursty'' with Lithium Aerosol Injection in DIII-D Z. Yan, G. McKee, R. Maingi, D.K. Mansfield, D.J. Battaglia, B.A. Grierson, R. Nazikian, A.L. Roquemore, G.L. Jackson, T.H. Osborne, C.P. Chrobak, J.S. deGrassie, R.J. Groebner, P.B. Snyder, A.G. McLean A long wavelength density fluctuation with moderately broad spectral structure (40 kHz-150 kHz) was observed with 2D beam emission spectroscopy (BES) in DIII-D plasmas when lithium aerosol was injected into the scrape-off layer region. The onset of such modes happens on a very fast $<$10 ms time scale and seems to correlate with the pedestal expansion, increase in pedestal height and increase on energy confinement time that occur at higher lithium injection levels. The mode fluctuations seem very bursty in time. It is localized to the region 0.90$<$r/a$<$0.95 with poloidal wavenumber about 0.46 rad/cm. The normalized density fluctuation amplitude peaks $\sim$8\% at r/a$\sim$0.92. This mode may drive additional particle transport that could alter gradients and allow for pedestal growth by changing pedestal stability parameters. [Preview Abstract] |
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GP8.00039: Evolution of High-Frequency Turbulence During Limit-Cycle Oscillations on DIII-D J.C. Rost, A. Marinoni, E.M. Davis, M. Porkolab, K.H. Burrell Limit-cycle oscillations (LCO) can provide insight into the interplay between shear and turbulence in triggering the H-mode transition. The Phase Contrast Imaging (PCI) diagnostic on DIII-D is particularly sensitive to density fluctuations in the highly sheared flow in the H-mode/LCO edge due to sensitivity to finite radial wave number ($k_r\sim k_\theta$) and large bandwidth (10 kHz $<$ f $<$ 2 MHz). Each roughly 1 ms oscillation in the LCO (10s of ms) exhibits a period of highly Doppler shifted, highly sheared turbulence which terminates at a burst of low-f turbulence. As the Doppler backscattering (DBS) diagnostic records a gradual increase in fluctuation amplitude rather than a burst [1], the PCI signal can be explained by a sudden decrease in radial correlation length caused by a burst in zonal flows. Both diagnostics are consistent with results of 1D models [2]. Comparison of LCOs of different durations reveals a threshold-like behavior in mean flow.\par \vskip6pt \noindent [1] L. Schmitz et al., Phy. Rev. Lett. {\bf108}, 155002 (2012).\par \noindent[2] K. Miki et al, Phys. Plasmas {\bf19}, 092306 (2012) [Preview Abstract] |
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GP8.00040: A Novel Technique for Estimating Edge Pedestal Density Gradients Using Reflectometer Time Delay Data L. Zeng, E.J. Doyle, W.A. Peebles, T.L. Rhodes, G. Wang A new technique for fast pedestal density gradient estimation has been developed, using profile reflectometer time delay data without a direct profile inversion. Such a fast profile gradient estimator is a potentially key to providing new ``real-time'' analysis of profile reflectometer data, suitable for use in plasma control systems. The new approach utilizes a simple edge plasma model to provide an analytic estimate for the measured differential time delay between two adjacent reflectometer frequencies. The model predicts that the measured differential time delays should be inversely proportional to the local pedestal density gradient. Using existing DIII-D profile reflectometer data, it has been demonstrated that the estimated gradient using this new technique is in good agreement with the actual density gradient for a number of DIII-D discharges. Further tests of this technique in a variety of DIII-D plasma conditions will be presented. [Preview Abstract] |
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GP8.00041: Bayesian Inference of Physics Parameters in the DIII-D Charge-Exchange Recombination Spectroscopy System C. Bowman, K.J. Gibson, R.J. La Haye, R.J. Groebner, N.Z. Taylor, B.A. Grierson A Bayesian inference framework has been developed for the DIII-D charge-exchange recombination (CER) system, capable of computing probability distribution functions (PDFs) for desired parameters. CER is a key diagnostic system at DIII-D, measuring important physics parameters such as plasma rotation and impurity ion temperature. This work is motivated by a case in which the CER system was used to probe the plasma rotation radial profile around an m/n=2/1 tearing mode island rotating at $\sim1$ kHz. Due to limited resolution in the tearing mode phase and short integration time, it has proven challenging to observe the structure of the rotation profile across the island. We seek to solve this problem by using the Bayesian framework to improve the estimation accuracy of the plasma rotation, helping to reveal details of how it is perturbed in the magnetic island vicinity. Examples of the PDFs obtained through the Bayesian framework will be presented, and compared with results from a conventional least-squares analysis of the CER data.\par [Preview Abstract] |
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GP8.00042: Improved Beam Diagnostic Spatial Calibration Using In-Situ Measurements of Beam Emission C. Chrystal, K.H. Burrell, D.C. Pace, B.A. Grierson, N.A. Pablant A new technique has been developed for determining the measurement geometry of the charge exchange recombination spectroscopy diagnostic (CER) on DIII-D. This technique removes uncertainty in the measurement geometry related to the position of the neutral beams when they are injecting power. This has been accomplished by combining standard measurements that use in-vessel calibration targets with spectroscopic measurements of Doppler shifted and Stark split beam emission to fully describe the neutral beam positions and CER views. A least squares fitting routine determines the measurement geometry consistent with all the calibration data. The use of beam emission measurements allows the position of the neutral beams to be determined in-situ by the same views that makeup the CER diagnostic. Results indicate that changes in the measurement geometry are required to create a consistent set of calibration measurements. However, changes in quantities derived from the geometry, e.g. ion temperature gradient and poloidal rotation, are small.\par [Preview Abstract] |
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GP8.00043: Synthetic Image Correction of Microwave Imaging Reflectometer Signals C.M. Muscatello, N.C. Luhmann, Jr., G.J. Kramer, R. Nazikian, B.J. Tobias A microwave imaging reflectometer (MIR), capable of simultaneously measuring the poloidal and radial structure of density fluctuations, is operational on DIII-D. MIR probes the plasma at 4 simultaneous frequencies, generating a poloidal array of fluctuation measurements at 4 different radial locations. When probing the pedestal in the steep gradient region, the active cutoffs are closely spaced and MIR performs well as an imaging system. However, in the lower gradient regions of the core, the active cutoffs are often spaced farther apart than the optical depth-of-field, thereby inhibiting simultaneous imaging over multiple radial locations. A numerical procedure is implemented that propagates the electric field at the cutoff layer to the location of the optical focus. The procedure relies on symmetric detection of sidebands of the scattered field. This synthetic correction is applied to MIR data during Alfv\'en eigenmode activity to demonstrate its ability to resolve images at multiple core-localized locations as well as to demonstrate its limitations. [Preview Abstract] |
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GP8.00044: Upgraded High Spatial Resolution ECE System for DIII D Z. Yang, M.E. Austin, D.D. Truong An upgrade to the DIII D high resolution electron cyclotron emission (HRECE) diagnostic is being implemented by replacing the eight fixed frequency filters with tunable Yttrium-Iron-Garnet (YIG) filters. The YIG filters are adjustable between 3.75 and 18 GHz and have a varying bandwidth that increases with increasing center frequency, from 110-290 GHz. The exceptionally wide tuning range permits high spatial resolution measurements over a broad range of DIII-D radii without adjusting $B_T$. Also, the ability to vary the center frequencies independently offers the means to optimize radial coverage to view plasma structures of different sizes. Correlation ECE techniques have been evaluated to take advantage of the new channels' ability to have overlaping filter spans. Additionally, a B-field ramp technique has been developed to do relative calibration between the HRECE channels. [Preview Abstract] |
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GP8.00045: ABSTRACT WITHDRAWN |
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GP8.00046: COMPUTER SIMULATION METHODS |
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GP8.00047: Overview of Recent NIMROD-Based Computational Work at the Univ. of Wisconsin-Madison C.R. Sovinec, A.L. Becerra, T.A. Bechtel, K.J. Bunkers, T.A. Cote, E.C. Howell, J.B. O'Bryan, J.P. Sauppe, P. Zhu The NIMROD code (https://nimrodteam.org) is a versatile and well tested computational tool for modeling macroscopic dynamics in magnetized plasma. The Center for Plasma Theory and Computation at the Univ. of Wisconsin-Madison presently applies NIMROD in studies of magnetic relaxation in reversed-field pinches and spheromaks, non-inductive current drive in spherical tokamaks, vertical-displacement instability, resistive-wall mode, edge localized modes and resonant magnetic perturbation in tokamaks, and magnetic topology evolution in a stellarator-tokamak hybrid. Recent contributions to NIMROD development include spectral-element stabilization, nodal trigonometric basis functions, resistive-wall options, and magnetization effects in viscosity. A summary of this application and development work is presented, along with a perspective on NIMROD modeling. [Preview Abstract] |
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GP8.00048: High-beta extended MHD simulations of toroidal stellarators T.A. Bechtel, C.C. Hegna, M.G. Schlutt, J.D. Hebert The nonlinear, extended MHD code NIMROD is used to study high-beta, 3D magnetic topology evolution of a toroidal stellarator. The configurations under investigation derive from the geometry of the Compact Toroidal Hybrid (CTH) experiment. However, the vacuum rotational transform profile is artificially raised in an effort to examine the sensitivity of low order rational surfaces and/or magnetic islands. Finite beta plasmas are created using a heating source and anisotropic heat conduction in a manner similar to previous calculations of CTH where the effects of Ohmic current drive were simulated.\footnote{M. G. Schlutt et al, Nucl. Fusion 52, 103023 (2012).} The onset of MHD instabilities and nonlinear consequences are monitored as a function of beta as well as the fragility of the magnetic surfaces. [Preview Abstract] |
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GP8.00049: Fully Parallel MHD Stability Analysis Tool Vladimir Svidzinski, Sergei Galkin, Jin-Soo Kim, Yueqiang Liu Progress on full parallelization of the plasma stability code MARS will be reported. MARS calculates eigenmodes in 2D axisymmetric toroidal equilibria in MHD-kinetic plasma models. It is a powerful tool for studying MHD and MHD-kinetic instabilities and it is widely used by fusion community. Parallel version of MARS is intended for simulations on local parallel clusters. It will be an efficient tool for simulation of MHD instabilities with low, intermediate and high toroidal mode numbers within both fluid and kinetic plasma models, already implemented in MARS. Parallelization of the code includes parallelization of the construction of the matrix for the eigenvalue problem and parallelization of the inverse iterations algorithm, implemented in MARS for the solution of the formulated eigenvalue problem. Construction of the matrix is parallelized by distributing the load among processors assigned to different magnetic surfaces. Parallelization of the solution of the eigenvalue problem is made by repeating steps of the present MARS algorithm using parallel libraries and procedures. Initial results of the code parallelization will be reported. [Preview Abstract] |
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GP8.00050: A Fast Multipole Method based Grad-Shafranov solver Antoine Cerfon, Travis Askham, Zydrunas Gimbutas, Jungpyo Lee, Leslie Greengard We present a fast, high order accurate, adapative Grad-Shafranov solver for complex plasma geometries with or without X-points. The solver uses two main ingredients: 1) the reformulation of the Grad-Shafranov equation as a nonlinear Poisson problem; 2) a fast Poisson solver based on integral equation methods. To be more specific regarding the second ingredient, the solution of Poisson's equation is written as the sum of a volume potential and a double layer potential. The volume potential is calculated in optimal time with the Fast Multipole Method (FMM), and the layer potential is computed using high order quadrature techniques. Beside its speed, this new solver has two properties that make it a desirable option for transport, heating, or stability codes that require coupling with an equilibrium solver. First, the solver automatically refines the mesh in regions of steep gradient, such as the edge pedestal. Second, the integral equation formulation does not only lead to high order accuracy for the solution of the Grad-Shafranov equation, but also for its derivatives. This means that the safety factor and the magnetic shear, among other quantities, can be computed with very good accuracy. [Preview Abstract] |
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GP8.00051: Linear hybrid kinetic-MHD model of rotating plasmas via the interface of MINERVA stability and VENUS-LEVIS delta-f PIC codes David Pfefferle, Nobuyuki Aiba, Jonathan P. Graves, Wilfred A. Cooper In the framework of hybrid kinetic-MHD with plasma rotation, this project focuses on computing, via a delta-f PIC scheme, the non-adiabatic contribution to the MHD pressure tensor from supra-thermal populations. The orbit code VENUS-LEVIS is employed to evolve an ensemble of weighted markers in the rotating magnetic equilibria produced by the MHD stability code MINERVA. The linearly perturbed Vlasov equation is solved by evolving the marker weights in the presence of MINERVA's most unstable MHD modes. Moments of the perturbed distribution are sequenced to yield the hot ion kinetic response. The Laplace transform of the perturbed parallel and perpendicular pressure is calculated at the resonance as a function of the radial position and the poloidal and toroidal mode number. The resulting profiles are fed back into MINERVA as an additional source term in the MHD force balance equation. The mode structure, the frequency and the growth rate of the perturbations are modified due to resonances with the hot particles' bounce/transit motion and their toroidal precession drift. The effect of toroidal plasma rotation on the mode stability is assessed. [Preview Abstract] |
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GP8.00052: Variational Algorithms for Drift and Collisional Guiding Center Dynamics C. Leland Ellison, John M. Finn, Hong Qin, William M. Tang The simulation of guiding center test particle dynamics in the upcoming generation of magnetic confinement devices requires novel numerical methods to obtain the necessary long-term numerical fidelity. Geometric algorithms, which retain conserved quantities in the numerical time advances, are well-known to exhibit excellent long simulation time behavior. Due to the non-canonical Hamiltonian structure of the guiding center equations of motion, it is only recently that geometric algorithms have been developed for guiding center dynamics. This poster will discuss and compare several families of variational algorithms for application to 3-D guiding center test particle studies, while benchmarking the methods against standard Runge-Kutta techniques. Time-to-solution improvements using GPGPU hardware will be presented. Additionally, collisional dynamics will be incorporated into the structure-preserving guiding center algorithms for the first time. Non-Hamiltonian effects, such as polarization drag and simplified stochastic operators, can be incorporated using a Lagrange-d'Alembert variational principle. The long-time behavior of variational algorithms which include dissipative dynamics will be compared against standard techniques. [Preview Abstract] |
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GP8.00053: Trinity Multiscale Transport Code Development for Experimental Comparison E. Highcock, M. Barnes, G. Colyer, J. Citrin, D. Dickinson, N. Mandel, F. Van Wyk, C. Roach, A. Schekochihin, W. Dorland The Trinity multiscale transport code has been extensively upgraded to further its use in experimental comparison. The upgrades to Trinity have extended its capability to work with experimental data, allowed it to evolve the magnetic equilibrium self-consistently (at fixed current) and significantly enhanced the range and performance of its turbulent transport modeling options. To enhance its capability to reproduce experiment, Trinity is now able to take output from the CRONOS integrated modelling suite, which is able to provide high quality reconstructions of experimental equilibria of, for example, JET. Trinity has also been integrated with the CHEASE Grad-Shafranov code. This allows the magnetic equilibrium to be re-computed self consistently as the pressure gradient evolves. Trinity has been given new options for modeling turbulent transport. These include the well-known TGLF framework, and the newly developed GPU-based nonlinear code GRYFX. These will allow rapid initial scans with Trinity before more detailed gyrokinetic modeling. Trinity's performance will benefit from an extensive programme to upgrade one of its primary gyrokinetic turbulence modeling options, GS2. We present a summary of these improvements and preliminary results. [Preview Abstract] |
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GP8.00054: Guiding-centre and full-Lorentz orbit solving in 3D magnetic coordinates for fast particle simulations Wilfred A. Cooper, David Pfefferle, Jonathan P. Graves Designed to accurately solve the motion of energetic particles in the presence of 3D magnetic fields, the VENUS-LEVIS code leans on a non-canonical general coordinate Lagrangian formulation of the equations of motion. It switches between full-orbit particle following and guiding-centre tracing by verifying the perpendicular variation of magnetic vector field, not only including gradients and curvature terms but also the shearing of field-lines. The criteria is particularly relevant for the study of fast ion redistribution in the kinked core of hybrid plasmas, where the compression of flux-surfaces against the axisymmetric outer mantle creates strongly varying magnetic field-lines and large parallel currents. Slowing-down simulations of NBI fast ions show that co-passing particles helically align in the opposite side of the plasma deformation whereas counter-passing particles are barely affected by the kinked structure. Results are compared with experimental neutron camera traces and FIDA measurements during long-lived modes (LLM). [Preview Abstract] |
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GP8.00055: Implicit $\delta f$ Lorentz Ion Sub-Cycling and Orbit Averaging Scott Parker, Benjamin Sturdevant, Yang Chen, Benjamin Hause A second order, implicit Lorentz ion drift-kinetic electron model has been developed to study low-frequency, quasi-neutral plasmas [1,2]. This model is useful, for example, as an alternative to gyrokinetics in the tokamak edge region where gradient scale lengths are short. In the presence of a strong guide field, however, the applicability of the model is limited due to the time step size required to fully resolve the ion gyromotion. The aim of this research is to develop GPU accelerated sub-cycling and orbit averaging methods to be used with the Lorentz ion model making its utilization more viable. Sub-cycling pushes computational particles independently over several micro time steps for each macro time step interval over which the fields are advanced. Orbit averaging uses the deposition data from the sub-cycled particles to obtain time averaged source terms used in the field solving stage. This provides a filtering effect, allowing for clean simulations at low frequencies. Simulation results and analysis for an ion acoustic model are presented along with performance results for GPUs.\\[4pt] [1] Y. Chen, S.E. Parker, Phys. Plasmas 16 (2009).\\[0pt] [2] J. Cheng, S.E. Parker, Y. Chen, D. Uzdensky, J. Comput. Phys. 245 (2013), 364 [Preview Abstract] |
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GP8.00056: 3D field solver in toroidal geometry for the long wavelength E\&M modes Salomon Janhunen, Bei Wang, Jan Hesthaven, Mark Adams, Seung-Hoe Ku, Choong-Seock Chang Gyrokinetic simulations -- such as those performed by the XGC code -- provide a self-consistent framework to investigate a wide range of physics in strongly magnetized high temperature laboratory plasmas, global modes usually considered to be in the realm of MHD simulations. However, the present simulation models generally concentrate on short wavelength electro-magnetic modes mostly to convenience the field solver performance. To incorporate more global fluid-like modes, also non-zonal long wavelength physics needs to be retained. In this work we present development of a fully 3D mixed FEM/FDM electro-magnetic field solver for the gyrokinetic code XGC1. We present optimization for use on massively parallel computational platforms, investigation of numerical accuracy characteristics using the method of manufactured solutions and evaluate the regime of validity for the current physics model. [Preview Abstract] |
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GP8.00057: On the Numerical Dispersion of the Electromagnetic Particle-In-Cell Code: Finite Grid Instability M.D. Meyers, C.-K. Huang, Y. Zeng, S. Yi, B.J. Albright The widely used Particle-In-Cell (PIC) method in relativistic particle beam and laser plasma modeling is subject to numerical instabilities that can render unphysical simulation results or even destroy the simulation. For electromagnetic relativistic beam and plasma modeling, the most relevant numerical instabilities are the finite grid instability and the numerical Cherenkov instability. We rigorously derive the faithful 3D PIC numerical dispersion relation, and specialize to the Yee FDTD scheme. The manner in which the PIC algorithm updates and samples the fields and distribution function, along with any temporal and spatial phase factors, is accounted for. Numerical solutions to the 1D dispersion relation are obtained for parameters of interest. We investigate how the finite grid instability arises from the interaction of the numerical modes admitted in the system and their aliases. The most significant interaction is due critically to the correct placement of the operators in the dispersion relation. We obtain a simple analytic expression for the peak growth rates due to these interactions. [Preview Abstract] |
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GP8.00058: Gyrokinetic simulation studies on the energetic-particle-induced geodesic acoustic mode Kazuhiro Miki, Yasuhiro Idomura Understanding of the energetic particles physics is of great interest in the future burning plasmas. Particularly, particle loss in the presence of EGAM may be critical for ITER. We thus need to know how EGAM is excited and interacts with turbulence. We here introduce energetic particles in a full-f gyrokinetic code (GT5D). (i) We find linear dynamics of the EGAM driven by bump-on-tail particle distributions. We examine flat-q, homogeneous, axisymmetric, electrostatic gyrokinetic simulations. Above a certain level of the beam intensity, an oscillatory mode grows with about a half of the standard GAM. The observed frequencies are consistent with the eigenmode analyses derived from the perturbed gyrokinetic equations. The theoretical analyses also indicate a bifurcation of the excited modes depending on q-value. Estimation of the finite-orbit-width effects can provide a size dependency of the EGAM growth rate. (ii) We find linear and nonlinear dynamics of the EGAM driven by slowing-down distributions. We examine the axisymmetric gyrokinetic simulations with DIII-D-like parameters. The observed growth rates and frequencies are consistent with results of other hybrid code. Furthermore, we will focus on nonlinear phase space dynamics, namely chirping mode. This work is supported by HPCI Strategic Program Field No.4: Next-Generation Industrial Innovations, funded by the MEXT, Japan. [Preview Abstract] |
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GP8.00059: Computational Diagnostics for Extreme Scale Toroidal Gyrokinetic Particle Simulations Yuan Shi, Bei Wang, Bruce Scott, William Tang The capability of monitoring processes in extreme scale simulation is crucial for extracting information about global and non-linear dynamics, as well as checking the integrity of the simulation. A set of post-processing computational diagnostics for toroidal gyrokinetic particle simulations is developed and optimized for efficient performance on multi- and many-core modern computational platforms. These diagnostics track the time evolution of parallel mode structure, radial profile, toroidal/poloidal spectra, and nonlinear energy transfer spectra. To demonstrate the performance of this diagnostic tool set, diagnosis results for drift wave turbulence with numerical dissipation are presented. [Preview Abstract] |
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GP8.00060: Benchmarking of the Gyrokinetic Microstability Codes GENE, GS2, and GYRO over a Range of Plasma Parameters Ronald Bravenec, Tobias Goerler, Daniel Told, Frank Jenko, M.J. Pueschel, George McKee, Jeff Candy, Andrea Garofalo, Sterling Smith, Gary Staebler, Michael Barnes, Chris Holland, Siye Ding, Terry Rhodes Comparing results from different gyrokinetic codes as a means of verification (benchmarking) is only convincing if the codes agree over a wide range of plasma conditions. Otherwise, agreement may simply be fortuitous. We present here linear and nonlinear comparisons of the Eulerian codes GENE, GS2, and GYRO for a variety of discharges and radii. These include the outer regions of DIII-D discharges with localized electron-cyclotron heating applied at different locations, the steep-gradient region of an EAST H-mode pedestal, a high poloidal beta DIII-D discharge with reversed magnetic shear in the core, and well-matched DIII-D discharges with varying degrees of toroidal rotation. [Preview Abstract] |
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GP8.00061: Gyrokinetic simulation of the tearing mode instability Edward Startsev, Weixing Wang, Wei-li Lee A recently developed split-weight perturbative particle simulation scheme for finite-$\beta$ plasmas in the presence of background inhomogeneities which analytically separates the additional adiabatic response of the particles associated with the quasi-static bending of the magnetic field lines [1] has been generalized to the sheared magnetic field geometry. The new scheme has been implemented in a 2D particle-in-cell code in slab geometry with drift-kinetic electrons and gyrokinetic ions. The electrons pitch-scattering collision operator has also been implemented to study collisionless as well as collisional tearing, and drift-tearing instabilities. In this paper the results of linear simulations of tearing and drift-tearing modes for realistic mass ratio $m_i/m_e=1837$ and different values of plasma $\beta$, electron-ion collision frequency, density and temperature gradients are presented and compared to the solution of the eigenvalue equation [2]. We will also present preliminary results of collisionless tearing mode simulations in cylindrical geometry using tokamak turbulence code GTS.\\[4pt] [1] E. A. Startsev and W. W. Lee, Phys. Plasmas 21, 022505 (2014).\\[0pt] [2] J. F. Drake and Y. C. Lee, Phys. Fluids 20, 1341 (1977). [Preview Abstract] |
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GP8.00062: SHOCKS, WAVES, DYNAMO AND DIPOLE |
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GP8.00063: First results of transcritical magnetized collisionless shock studies on MSX T.E. Weber, R.J. Smith, T.M. Hutchinson, S.F. Taylor, S.C. Hsu Magnetized collisionless shocks exhibit transitional length and time scales much shorter than can be created through collisional processes. They are common throughout the cosmos, but have historically proven difficult to create in the laboratory. The Magnetized Shock Experiment (MSX) at LANL produces super-Alfv\'enic shocks through the acceleration and subsequent stagnation of Field Reversed Configuration (FRC) plasmoids against a strong magnetic mirror and flux-conserving vacuum boundary. Plasma flows have been produced with sonic and Alfv\'en Mach numbers up to $\sim$10 over a wide range of plasma beta with embedded perpendicular, oblique, and parallel magnetic field. Macroscopic ion skin-depth and long ion-gyroperiod enable diagnostic access to relevant shock physics using common methods. Variable plasmoid velocity, density, temperature, and magnetic field provide access to a wide range of shock conditions, and a campaign to study the physics of transcritical and supercritical shocks within the FRC plasmoid is currently underway. An overview of the experimental design, diagnostics suite, physics objectives, and recent results will be presented. [Preview Abstract] |
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GP8.00064: Pulsed Polarimetry and magnetic sensing on the Magnetized Shock Experiment (MSX) R.J. Smith, T.M. Hutchinson, T.E. Weber, S.F. Taylor, S.C. Hsu MSX is uniquely positioned to generate the conditions for collision-less magnetized supercritical shocks with Alvenic Mach numbers (M$_{A})$ of the order 10 and higher. Significant operational strides have been made in forming plasmas over wide parameter ranges: (T$_{e}+$T$_{i})$ of 10-200 eV, average n$_{e}$of 5-60x10$^{+21}$ m$^{-3}$, speeds up to 150 km/s and fields up to 1T with a highest plasma flow M$_{A}$ of 5 to date. The MSX plasma is unique in regards to large plasma size of 10 cm and average $\beta $ higher than 0.8 making the FRC and the magnetized shock structure candidates for the application of Pulsed Polarimetry, a polarization sensitive Lidar technique. The shock dynamics are presently being investigated using internal probes, interferometry and imaging. Internal probe results and an assessment of the shock parameters will dictate the use of the UW pulsed polarimeter system in which internal n$_{e}$, T$_{e}$and B are to be measured. Recent results will be presented. [Preview Abstract] |
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GP8.00065: Three Dimensional Hybrid Simulations of Super-Alfv\'enic Laser Ablation Experiments in the Large Plasma Device Stephen Clark, Dan Winske, Derek Schaeffer, Erik Everson, Anton Bondarenko, Carmen Constantin, Christoph Niemann We present 3D hybrid simulations of laser produced expanding debris clouds propagating though a magnetized ambient plasma in the context of magnetized collisionless shocks. New results from the 3D code are compared to previously obtained simulation results using a 2D hybrid code. The 3D code is an extension of a previously developed 2D code developed at Los Alamos National Laboratory. It has been parallelized and ported to execute on a cluster environment. The new simulations are used to verify scaling relationships, such as shock onset time and coupling parameter ($R_m/\rho_d$), developed via 2D simulations. Previous 2D results focus primarily on laboratory shock formation relevant to experiments being performed on the Large Plasma Device, where the shock propagates across the magnetic field. The new 3D simulations show wave structure and dynamics oblique to the magnetic field that introduce new physics to be considered in future experiments. [Preview Abstract] |
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GP8.00066: Characterization of Laser-Produced Plasmas Relevant to Magnetized Collisionless Shocks D.B. Schaeffer, A.S. Bondarenko, E.T. Everson, S.E. Clark, C.G. Constantin, C. Niemann Recent experiments performed at the University of California, Los Angeles (UCLA) have generated magnetized collisionless shocks driven by a laser-mediated magnetic piston. The effectiveness of the piston at coupling energy between a laser-plasma and an ambient plasma depends highly on the nature of the laser-plasma, which has been hitherto poorly characterized. We present experiments that provide new details on the composition and evolution of laser-produced plasmas relevant to a magnetic piston. Thomson scattering was used to probe the electron temperature and density up to several cm from the target and several microseconds after ablation. Ionization states and blow-off velocities of the ablated plasma were further measured with emission spectroscopy and fast-gate filtered photography. The data compares well to analytic models describing the spatial and temporal temperature and density evolution of the plasma. 1D HELIOS simulations of the laser-target interaction also agree favorably with data that shows fast ions dominated by a single charge state primarily drive the magnetic piston. [Preview Abstract] |
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GP8.00067: Polarization studies of radiation spectra of relativistic collisionless shocks Ujjwal Sinha, Joana Martins, Jorge Vieira, Ricardo Fonseca, Luis Silva Collisionless electromagnetic shocks generated by counterpropagating plasma flows mediated by the Weibel or Current Filamentation Instability (WI or CFI) may be at the origin of Gamma Ray Bursts (GRBs) and cosmic ray acceleration in astrophysics. These instabilities can also strongly amplify initial seed magnetic fields, leading to synchrotron and other radiation processes originating from particle scattering in self-generated WI or CFI magnetic fields. In this work we present OSIRIS particle-in-cell simulations of shocks generated by colliding relativistic electron positron plasmas. Using multidimensional simulations we examine the dynamics of plasma particles in the magnetic filaments generated at the shock front and explore the corresponding polarization signatures. We find that plasma particles can get trapped in these filaments leading to radiation bursts as a result of their jitter motion. Further, we analyse the polarization spectra of the radiation emitted from such particles and determine the fraction of radiated energy attributed to different polarizations. Such analysis is of special significance towards deeper understanding of bright afterglows produced when relativistic jets emanating from accreting black holes collide with ambient medium. [Preview Abstract] |
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GP8.00068: Non-axisymmetric Perturbations of the LDX Laboratory Magnetosphere by Lithium Pellet Injection D. Garnier, M. Mauel, J. Kesner In most toroidal magnetic plasma confinement systems, transport within helical flux surfaces serve to symmetrize the plasma temperature and density. In contrast, a plasma torus confined by a dipole field lacks a rotational transform and therefore the confined plasma is not necessarily axisymmetric. The plasma, however, self organizes into a time-averaged symmetric state through particle drifts and turbulent transport. Recent experiments in the LDX laboratory magnetosphere have been conducted to study large non axisymmetric perturbations of the dipole confined plasma. A high speed gas gun was used to inject lithium pellets tangentially through the peak of the plasma density profile. High speed video shows the pellet ablating as it traverses the bulk plasma. As the pellets approach the mid plane they encounter the deeply trapped energetic electron ring (formed during ECH) and absorb energy deeply into pellet. This causes a rapid ablation fracturing of the pellet into multiple droplets; the exploding pellets will vaporize and then ionize leading to a tripling of the line integrated density. Similar processes occur when objects enter the Van Allen belts. The high density plasma presents an improved target for ICRF heating. We will present recent experimental results. [Preview Abstract] |
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GP8.00069: Dipole Transport: a New Confinement Paradigm J. Kesner, D. Garnier, M. Mauel In a tokamak-like device turbulence will grow up to a level determined by non-linear processes. The associated transport, in combination with particle and energy sources then determines the density and temperature profiles of the plasma. This paradigm is fundamentally different for a plasma that is confined in a dipole field. In a dipole, levitated to avoid losses to the supports, the plasma will assume a stationary profile determined only by the specific volume, $V(\psi)$, (which is determined by the magnetic geometry). Independent of the source and sink profiles for particles and energy, turbulence will grow up to a sufficient level so that diffusion and pinch dynamics will establish stationary profiles characterized by $n_e\propto1/V$ and $p\propto 1/V^{5/3}$. This process is observed in magnetospheric plasmas and we have observed it in the laboratory in LDX. For example, with edge fueling in LDX we observed that the stationary (peaked) density profile ($n\propto 1/V$) was established by a turbulence-driven density pinch [1] whereas in recent experiments with core (pellet) fueling turbulence was observed to relax the density back to the stationary profile on a similar timescale. \\[4pt] [1] A.C. Boxer, et al., Nature-Physics 6, 207 (2010) [Preview Abstract] |
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GP8.00070: Linear and Quasilinear Model for Pressure-Driven Interchange and Entropy Modes in a Warm Electron Dipole Plasma M. Mauel, D. Garnier, T. Roberts, J. Kesner The measured structures of electrostatic interchange modes in dipole-confined plasma cause global mixing when driven by energetic trapped electrons, sonic plasma, or warm electron pressure. Global circulation also appears in planetary magnetospheres driven by solar wind, but differences exist in underlying physics. Breaking azimuthal symmtry in magnetospheres caused currents to flow through the ionosphere, which regulate interchange motion.\footnote{Lyon, \textit{Science}, \textbf{288}, 1987 (2000).} In the laboratory, there are no field-aligned currents and perturbations induce ion-inertial currents, which determine the global linear model structure. In this poster, the linear description of global interchange and entropy modes are presented for the CTX and LDX laboratory magnetospheres computed from the flux-tube averaged gyrofluid equations.\footnote{Ricci, \emph{et al.}, \textit{Phys Plasmas}, \textbf{13} 062102 (2006).} Additionally, the quasilinear particle and heat flux are calculated and show turbulent self-organization that drives profiles to become centrally-peaked.\footnote{Kesner, \emph{et al.}, \textit{Phys Plasmas},\textbf{18}, 050703 (2011).} [Preview Abstract] |
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GP8.00071: Bounce-Averaged Gyrokinetic Simulation of Current-Collection Feedback in a Laboratory Magnetosphere T.M. Roberts, D. Garnier, J. Kesner, M.E. Mauel A self-consistent, nonlinear simulation of interchange dynamics including the bounce-averaged gyro-kinetics of trapped electrons was previously used to understand frequency sweeping\footnote{B.~Levitt, \textit{Phys Plasmas}, \textbf{9}, 2507 (2002).} and the turbulent cascades\footnote{B.~Grierson, \textit{Phys Plasmas}, \textbf{16}, 055902 (2009). } observed in dipole-confined plasmas. Through adjustment of the particle and heat sources this code reproduces dynamics that resemble the turbulence measured experimentally, both in spectral power-law trends and in the onset of a steepened density profile. Time stepping is performed in an explicit leap-frog manner and a flux-corrected transport algorithm is implemented. In this presentation, we discuss the physics and numerical methods of the simulations as well as plans for including the effects of a biasing electrode which can collect or inject electrons. By varying this source/sink of electrons at the electrode location based on the potential fluctuations occurring elsewhere, we study the effects of current-collection feedback to compare to recent experiments observed to regulate interchange turbulence. [Preview Abstract] |
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GP8.00072: Gyrokinetic theory of auroral arc formation and electron acceleration in the magnetosphere-ionosphere coupling Tomo-Hiko Watanabe We have constructed a unified theoretical model of auroral arc growth and electron acceleration by means of the gyrokinetic and two-fluid equations for the magnetosphere-ionosphere (M-I) coupling system. The new theory describes destabilization of kinetic (or dispersive) Alfven waves (KAWs) in the M-I coupling, where development of upward and downward field aligned currents carried by the KAWs leads to ionospheric density enhancement and depletion, respectively. The feedback M-I coupling through KAWs elucidates growth of auroral arcs, excitation of KAWs, and field-aligned acceleration of electrons self-consistently. The unified theoretical model of M-I coupling provides a possible explanation to the Alfvenic auroras observed by the FAST spacecraft, and is also appreciated as a successful application of gyrokinetics to auroral physics. [Preview Abstract] |
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GP8.00073: Initial Results from the Magnetorotational Instability Experiment (MRI) Upgrade Erik Gilson, Eric Edlund, Jeremy Goodman, Hantao Ji, Aveek Kapat, Ethan Schartman, Peter Sloboda, Xing Wei The Magnetorotational Instability experiment (MRI) has been upgraded to enable reliable operation at the high rotation rates required to obtain the MRI by implementing mechanical improvements, and to increase the expected saturation amplitude of the MRI by installing electrically conductive end caps. Initial experimental results are presented that determine the extent to which the optimized baseline configuration matches the ideal Taylor-Couette rotation profile in the absence of a magnetic field. Radial velocity measurements with an improved ultrasound Doppler velocimetry configuration are compared to numerical simulations of the Ekman configuration. High-speed runs extend the results of Roach [1] to speeds between 60\% and 100\% of the design limit in order to investigate whether the observed simple scaling of the normalized azimuthal velocity with the normalized magnetic field can serve as a suitable baseline against which to identify the MRI. Plans are discussed for an experimental campaign to identify the MRI by measuring radial velocities and magnetic fields, in addition to changes to the azimuthal velocity, as predicted by simulations. \\[4pt] [1] Austin Roach, Ph. D. Thesis, Princeton University (2013). [Preview Abstract] |
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GP8.00074: Upgrade of the Magnetorotational Instability Experiment Apparatus E. Schartman, E.P. Gilson, E. Edlund, J. Goodman, H. Ji, P. Sloboda, X. Wei The Princeton MagnetoRotational Instability (MRI) Experiment was designed to investigate the MRI in a liquid gallium alloy Taylor-Couette flow generated between concentric spinning cylinders. To achieve magnetic Reynolds numbers sufficiently large to excite the MRI, flow velocities of order 20m/s are required. Experimental operation at such velocities has been hampered by mechanical limitations of the apparatus. Dynamic pressures generated by the alloy cause distortion and binding, which is laborious to correct. High surface speeds lead to excessive seal wear. Modifications to the apparatus were implemented to enable extended operation at full design speed. The inner cylinder was also modified to carry diagnostics such as Doppler ultrasound, torque and magnetic field sensors. Details of the modifications will be presented. This work is supported by U.S. DOE, NASA and NSF. [Preview Abstract] |
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GP8.00075: Theoretical study of plasma confinement by magnetic multicusp field Ivan Khalzov, Cary Forest Plasma confinement in a magnetic multicusp field is studied numerically using both collisional particle-in-cell and isothermal two-fluid MHD codes and tested against the empirical model. The simulation domain is two-dimensional, periodic in one direction and bounded by absorbing boundaries with multicups field in other direction. First, we study the dependence of plasma loss width on plasma parameters and field strength and compare the results with the well-known empirical formula $w=2\sqrt{\rho_e\rho_i}$ (two hybrid gyro-radius). Our results show that the loss width has the same scaling with magnetic field $w\propto1/B$, but dependence on other plasma parameters does not agree with this formula. Second, we study the plasma flow drive in the cusp region due to electric field applied by discrete electrodes. The electrode positions are optimized for achieving the highest plasma flow. Comparison with available experimental data from Madison Plasma Dynamo Experiment (MPDX) is made. [Preview Abstract] |
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GP8.00076: Theoretical foundations of MHD spectroscopy in Madison Plasma Dynamo Experiment Robert Siller, Ivan Khalzov, Cary Forest We develop a theoretical basis of active MHD spectroscopy for Madison Plasma Dynamo Experiment (MPDX). This new diagnostic is based on an analysis of incompressible shear Alfven modes and compressible acoustic modes for spherical plasmas, and the influence of plasma flow on the corresponding eigen-spectrum. Alfven modes in plasma are assumed to be excited in the presence of an external axial magnetic field. The mode frequencies depend on the distribution of plasma parameters. Inverting this dependence for a given (experimentally measured) set of modes, we are able to infer the spatial structure of plasma characteristics. We demonstrate this inversion technique by determining the rigid plasma rotation from the splitting of the low-frequency resonance Alfven modes driven by a localized antenna. Compressible acoustic waves in plasma are assumed to be excited with and without the presence of an external magnetic field. For the acoustic waves we determine the velocity dependence of the normal mode spectrum and magnetic field structure. We consider the feasibility of using the developed diagnostic in MPDX. [Preview Abstract] |
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GP8.00077: Helioseismology in the Lab Ethan Peterson, Matthew Brookhart, Mike Clark, Chris Cooper, Jan Egedal, John Wallace, David Weisberg, Cary Forest A novel diagnostic technique for measuring plasma flows in the Madison Plasma Dynamo Experiment (MPDX) has been designed and implemented. The technique, inspired by helioseismology, launches ion acoustic waves from the boundary of a spherical (1.5m radius), unmagnetized, spinning plasma and measures the doppler shifted wave at two longitudinal locations of the same latitude. These two measurements yield a line integrated velocity measurement from the source to the receivers. The ion acoustic waves are produced via the mode conversion of a magnetosonic wave excited by a current loop antenna located in the confining cusp field of MPDX. Probe measurements of the electric field in the plasma and the magnetic field fluctuations (Bdot) in the cusp are used to observe the wave and deduce velocities along two chords. This technique is used to measure 10km/s flows and to validate mach probe measurements near the edge of the plasma. The Bdot measurements in the cusp provide the proof-of-concept for a surface array of probes to measure global velocities. [Preview Abstract] |
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GP8.00078: Taylor-Dean flow on the Plasma Couette Experiment (PCX) K. Flanagan, M. Clark, C. Collins, C. Cooper, I. Khalzov, J. Wallace, C. Forest A Taylor-Dean flow profile is implemented on the upgraded Plasma Couette Experiment (PCX) aimed at exciting the magnetorotational instability (MRI). Taylor-Dean flow profiles are set up by injecting torque via a radial current crossing an axial magnetic field. A ``virtual cathode" is set up in the center of PCX by applying a bias from a cathode located at the top center of the vessel to an anode at the bottom. This circuit is biased with respect to anodes at the outer wall to drive a radial current. A small vertical magnetic field is then applied via external Helmholtz coils in order to induce a ${\bf J}\times{\bf B}$ torque. Theoretical investigations have shown that the ion-neutral drag in PCX plasmas with low ionization fractions negatively affect the MRI threshold and growth rate. In order to increase the ionization fraction in PCX, upgrades to the multicusp magnet system and microwave power are underway. New SmCo magnets, like those used on the Madison Plasma Dynamo Experiment (MPDX), will provide better confinement and tolerate higher plasma temperatures than the previous ceramic ones. A MRI stability analysis of Taylor-Dean flows under relevant PCX parameters as well as early flow data will be presented. [Preview Abstract] |
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GP8.00079: Development of an ECH System on the Madison Plasma Dynamo Experiment Jason Milhone, Alf Koehn, Paul Nonn, Cary Forest The Madison plasma dynamo experiment (MPDX) is a 3 meter diameter spherical vessel lined with 3000 SmCo permanent magnets (B \textgreater 3 kG) that create an axisymmetric multi-cusp ring for confining the plasma. The MPDX is designed to study flow driven MHD instabilities and dynamo action in the regime of high magnetic Reynolds number Rm$=$vL/$\eta $. This will be achieved through electron cyclotron heating of the electrons leading to good electrical conduction and large ionization fraction. The system consists of five 20 kW, CW magnetrons operating at 2.45 GHz. The system will be described in detail, including the power supplies, RF vacuum feedthroughs, and modulator/regulator circuit used to control the magnetrons. The power will be injected at various latitudes and is resonant at the fundamental cyclotron frequency in the multi-cusp edge. Prototype experiments in a smaller version of the device routinely operate in a density regime that is overdense. Experiments and numerical modeling will be described that determine how the power is absorbed in this mode. [Preview Abstract] |
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GP8.00080: Experimental Optimization of High Magnetic Reynolds Number, Two-Vortex Flows on the Madison Plasma Dynamo Experiment David Weisberg, Christopher Cooper, Mike Clark, Ken Flanagan, Ivan Khalzov, Jason Milhone, Mark Nornberg, John Wallace, Cary Forest Laminar counter-rotating two-vortex flows, predicted to excite a large-scale dynamo, are generated and optimized in the Madison plasma dynamo experiment (MPDX). Numerical simulations show that the topology of these simply-connected flows may be optimal for creating a plasma dynamo in the lab and predict a critical threshold of $Rm_{crit}=\mu_0\sigma LV=250$ for optimal flows. MPDX plasmas are shown to exceed this critical $Rm$, generating large ($L=1.4$\,m), hot ($T_e>10$\,eV) plasmas where $Rm=600$. Plasma flow is driven using eight thermally emissive LaB$_6$ cathodes which generate a $J\times B$ torque at the magnetized edge of spherical He plasmas. Mach probe measurements show counter-rotating flows at speeds of $V>10$\,km/s; the driven flow at the plasma edge viscously couples inward to the unmagnetized core via ion-ion collisions, and flow measurements along radial chords compare favorably to hydrodynamic calculations using Braginskii viscosity. To optimize flow for dynamo generation, cathode bias and positioning are varied along with plasma viscosity ($\nu\sim T_i^{5/2}/n_i$) and the frictional neutral-ion drag force ($\mu=L^2/(\nu\tau_{in})$). Prospects for observing a dynamo, hydrodynamic transitions to turbulence, and eventual large Rm fast dynamos will be presented. [Preview Abstract] |
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GP8.00081: Optical Emission Spectroscopy in an Unmagnetized Hot Plasma Blair Seidlitz, Chris Cooper, Dave Weisberg, Mark Nornberg, John Wallace, Cary Forest Recently, a new technique has been developed to create in the laboratory, a large (1.5m), weakly magnetized, fast flowing ($0.1< V/C_s <1$), and hot (10-20eV) plasma. These unique conditions make it possible to study a wide variety of phenomena in plasma astrophysics which is the goal of the Madison plasma dynamo experiment. Accurate measurements of plasma properties such as density and temperature can be challenging with Langmuir probes due to contamination, their perturbative nature, plasma flow, and probe overheating due to large T$_e$. To achieve a non-invasive measurement of relevant parameters, optical emission spectroscopy techniques have been implemented using a 1.5nm resolution fiber-coupled broad wavelength spectrograph. Measurements of HeI and HeII emission are utilized to predict relative density ratio using the coronal model. This has yielded HeII ground state measurements which is particularly important in MPDX's plasma regime containing HeI, HeII, and HeIII. Radially resolved measurements are also made with a periscope like device. Finally, full collisional radiative modeling is being explored to characterize T$_e$ via HeI emission. [Preview Abstract] |
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GP8.00082: Establishing high magnetic Reynolds numbers (Rm) in MPDX for dynamo studies Christopher Cooper, Michael Clark, Ken Flanagan, Ivan Khalzov, Jason Milhone, Ethan Peterson, Blair Seidlitz, John Wallace, David Weisberg, Cary Forest The Madison plasma dynamo experiment (MPDX) is a basic plasma research device designed to investigate flow driven MHD instabilities, such as the dynamo, in parameter regimes relevant to astrophysical systems and numerical simulations. A 3 m diameter vacuum vessel lined with an axisymmetric magnetic multipole cusp confines a nearly magnetic-field-free plasma. Thermally emissive lanthanum hexaboride cathodes biased $<$ 400 V create the plasma and generate toroidal {\bf $J \times B$ } torques in the cusp region. Plasma viscosity propagates this momentum throughout the unmagnetized core, driving poloidal and toroidal flows for studying the interactions between flows and magnetic fields in high conductivity regimes. In plasma, the viscosity and conductivity scale dramatically with $T_e$, $n_e$, and $Z_{eff}$ which are uniquely determined by particle and energy confinement. Varying the MPDX input power and fill pressure with the achieved 10 km/s driven flows creates a scaling of the fluid Reynolds number $10 |
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GP8.00083: Status of the Wisconsin Plasma Astrophysics Laboratory John Wallace, Matthew Brookhart, Mike Clark, Chris Cooper, Ken Flanagan, Ivan Khalzov, Jason Milhone, Ethan Peterson, Joseph Olson, Aaron Stemo, Dave Weisberg, Jan Egedal, Cary Forest The Wisconsin Plasma Astrophysics Laboratory (WiPAL) is a facility that now encompasses a collection of novel plasma astrophysics experimental configurations. In the MPDX configuration large, un-magnetized, fast flowing, hot plasma is being used to investigate a variety of flow driven MHD instabilities. The experiment is 3 meters in diameter and utilizes a permanent magnet multicusp plasma confinement. Five 20KW, 2.45 GHz, CW magnetrons produce electron cyclotron heating for plasma generation. Ten lanthanum hexaboride (LaB6) stirring rods and molybdenum anodes are inserted into the vessel to produce JxB flows. The chamber has a variety of multiuse ports, and is able to split open to allow experimental apparatus to be inserted. This poster will describe future experimental configurations including reconnection (TREX), jet and plasma wind experiments. [Preview Abstract] |
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GP8.00084: Complex Dynamics of Line-tied Flux Ropes and Screw Pinches Matthew Brookhart, Aaron Stemo, Amanda Zuberbier, Cary Forest It has been suggested that flux ropes -- self-contained plasma structures with axial current and magnetic field -- may be the basic building blocks of many astrophysical plasmas. Many of these plasmas -- including coronal loops, the solar wind, and astrophysical jets -- also show ``line-tying'' where the ends of flux ropes are magnetically fixed. The Line-tied Reconnection Experiment is a basic plasma research facility designed to study the behavior, instability, and self-organization of multiple line-tied flux ropes in a variety of geometries and plasma conditions. The recent construction of a 300 coil magnetic probe array has allowed for direct, time and space resolved observations of flux rope dynamics in a wide range of conditions. Complex dynamics of 2 and 3 flux ropes are observed both with and without background plasma. These plasmas exhibit complex instabilities and interactions. Observations show that larger numbers of flux ropes merge into azimuthally-symmetric screw pinch equilibria. High safety factor kink-like instabilities are seen in both hollow and reversed current profile plasmas, akin to tokamak current holes and certain models of solar flares. The equilibria, instability criteria, and dynamics of these plasmas are explored. [Preview Abstract] |
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GP8.00085: Construction of 4 meter diameter Helmholz coils using square cross section hollow conductor on the Madison Plasma Dynamo Experiment (MPDX) M.M. Clark, A. Alt, J. Egedal, D. Endrizzi, S. Greess, A. Jaeger, E. Jewell, J. Olson, J.P. Wallace, C.B. Forest A pair of Helmholz coils has been constructed in situ for the Madison Plasma Dynamo Experiment (MPDX). The MPDX vessel itself is a 3 meter diameter spherical vacuum chamber. Each Helmholz coil consists of 88 turns of 13mm square conductor with a 9mm diameter round channel centered in the cross section. The coils will produce 277Gauss of magnetic field at 800Amps DC. A steel roller frame and aluminum wheel to support the conductor was built in the MPDX lab. The conductor was then wound onto the wheel making 16 pancakes per coil. The ends of the conductor needed finishing so that series electrical connections would result as well as parallel water cooling. The resistance of each coil was measured to be 187mOhm. The design and construction process will be shown. [Preview Abstract] |
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GP8.00086: Exciting Alfven Waves using Modulated Electron Heating by High Power Microwaves Yuhou Wang, Walter Gekelman, Patrick Pribyl, Bart Van Compernolle, Konstantinos Papadopoulos Experiments exploring the physics of ionospheric modification with intense perpendicular propagating waves ($\vec{k}\perp\vec{B_{0}}$) on the Large Plasma Device (LaPD) at UCLA have been upgraded with the addition of a high power rapidly pulsed microwave source. The plasma is irradiated with ten pulses (250 kW X-band) near the upper-hybrid frequency. The pulses are modulated at a frequency of a fraction (0.1-1.0) of $f_{ci}$ (ion cyclotron frequency). Based on a previous single-pulse experiment [1], the modulated electron heating may drive a large amplitude shear Alfv\'{e}n wave ($f < f_{ci}$), making the plasma a virtual antenna. This wave driving mechanism may have important application in terrestrial radio communications by low frequency waves, which are difficult to launch directly due to their enormous wavelengths. Various heating methods involving X-mode, O-mode, and electron Bernstein mode are investigated in plasmas with controllable parameters ($n_{e}=10^{8}\sim10^{12}cm^{-3},T_{e}=0.1\sim6 eV ,T_{i}\ll T_{e},B_{0}=100\sim3000 G,\nabla n_{e}/n_{e}=0\sim1cm^{-1}$). Mode-conversion between these waves and the subsequent structural changes of the plasma near the conversion region are also under investigation.\\[4pt] [1] B. Van Compernolle, et al, Geophys. Res. Lett. 32.8 (2005) [Preview Abstract] |
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GP8.00087: Solar-relevant plasma loop expansion in strapping field Bao Ha, Paul Bellan Tokamak-like forces may explain fundamental behaviors of solar plasma arches. The hoop force causes arched, current-carrying plasma loops to expand. This expansion was slowed and even inhibited by a magnetic ``strapping'' field in previous solar loop experiments at Caltech [1] but no attempt was made to control the field's spatial profile. Kliem and Torok [2] predicted an explosive-like transition from slow expansion to fast eruption if the spatial decay rate of the strapping field exceeds a threshold. Smaller, independently-powered auxiliary coils placed inside the vacuum chamber produce strapping fields with above-threshold decay rate and strong enough to act on the plasma. The plasma is observed, however, to bypass regions of stronger strapping field and expand into regions of weaker field. Added external inductance slows plasma expansion allowing the strapping coils to hold down the plasma. Different interactions between arched plasma loops and strapping magnetic fields will be presented. \\[4pt] [1] J. F. Hansen and P. M. Bellan, Astrophys. J. Lett. \textbf{563}, L183 (2001)\\[0pt] [2] B. Kliem and T. Torok, Phys. Rev. Lett. \textbf{96}, 255002 (2006) [Preview Abstract] |
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GP8.00088: Gendrin mode vortices and helicons in unbounded plasmas J. Manuel Urrutia, Reiner Stenzel Magnetic loop antennas are used to excite cw whistler modes in a large laboratory plasma for parameters $\omega \simeq 0.3 \omega_{ce} \ll \omega_{pe}$. The wave topology and propagation is measured in 3D space and time with magnetic probes. When the antenna dipole field is aligned with the uniform background field $\mathbf{B}_0$, the spatial wave packets have conical phase fronts and linked toroidal and poloidal fields. These whistler ``vortices'' resemble $m=0$ helicons in bounded plasmas. The topology resembles that of $m=1$ helicon modes when the antenna dipole field is perpendicular to $\mathbf B_0$, except the phase fronts are inclined at the Gendrin angle. The 3D field lines form two nested and opposing helices along $\mathbf{B}_0$. The wave field is force free. Using linear superposition, the fields from two phased loops, spaced axially apart by $\lambda/4$, are superimposed, resulting in directional radiation. It is more efficient than rotating field antennas. Whistler standing waves have been generated with oppositely propagating helicons. These waves produce no perfect nodes and have wave polarizations varying spatially between linear and circular. The results are of interest to space and laboratory plasmas. [Preview Abstract] |
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GP8.00089: Antenna arrays for producing plane whistler waves Reiner Stenzel, J. Manuel Urrutia Linear whistler modes with $\omega \simeq 0.3 \omega_{ce} \ll \omega_{pe}$ are excited in a large laboratory plasma with magnetic loop antennas. A single antenna always produces a spatially bounded wave packet whose propagation cannot be directly compared to plane wave theories. By superimposing the fields from spatially separated antennas, the wavenumber along the antenna array can be nearly eliminated. 2D arrays nearly produce plane waves. The angle $\theta$ of wave propagation has been varied by a phase shift along the array. The refractive index surface $n (\theta)$ has been measured. The parallel phase and group velocities for Gendrin modes has been demonstrated. The interference between two oblique plane whistlers creates a whistler ``waveguide'' mode, i.e. standing waves for $\mathbf{k} \perp \mathbf{B}_0$ and propagation for $\mathbf{k} \parallel \mathbf{B}_0$. It also describes the reflection of oblique whistlers from a sharp discontinuity in the refractive index or conductivity. Radial reflections are also a dominant factor in small plasma columns of helicon devices. These results are of interest to space and laboratory plasmas. [Preview Abstract] |
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GP8.00090: Measurements of the Linear Kinetic Plasma Response to Alfv\'en Waves J.W.R. Schroeder, F. Skiff, G.G. Howes, C.A. Kletzing, T.A. Carter, S. Dorfman Alfv\'en waves likely account for a significant fraction of auroral electron acceleration. However, a direct test of electron acceleration by Alfv\'en waves has never been accomplished. Complex trajectories and limited resolutions have prevented \emph{in situ} observations from completing thorough tests of existing theory. Until now, laboratory diagnostics have not been sensitive to the predicted small fluctuations in the tail of the electron distribution function $f_e$. A novel diagnostic developed at the University of Iowa uses the absorption of a small-amplitude whistler wave to measure $f_e$ up to $1$ keV with $0.1$\% accuracy. Inertial Alfv\'en waves ($v_{te}/v_A \sim 0.2$) with $\delta B/B \sim 10^{-5}$ are launched in an overdense plasma at the Large Plasma Device (LaPD) with $B_0=1800$G. Under these conditions, only the whistler mode propagates parallel to the background magnetic field at frequencies just below the electron cyclotron frequency. Results show fluctuations in the tail of the distribution function at the frequency of the Alfv\'en wave. An analytic solution from the Boltzmann equation is used to describe experimental results. Further analysis of measurements is presented and is compared to theoretical predictions. [Preview Abstract] |
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GP8.00091: Generation of shear Alfv\'{e}n waves due to resonant interactions with a spiraling ion beam on the Large Plasma Device Shreekrishna Tripathi, Bart Van Compernolle, Walter Gekelman, Patrick Pribyl, William Heidbrink The role of Landau and Doppler-shifted ion-cyclotron resonances (DICR) in extracting the free-energy from an ion-beam and destabilizing Alfv\'{e}n waves was explored. The experiment was conducted on the Large Plasma Device (LAPD) in a dual-species magnetized plasma ($n \approx 10^{10}$--10$^{12}$ cm$^{-3}$, T$_e \approx$ 5.0 eV, B = 1.0--1.8 kG, 92$\%$ He$^+$ and 8$\%$ H$^+$ ions, 19 m long, 0.6 m diam). A hydrogen ion beam (15 kV, 10 A) was obliquely injected into the plasma. The interaction of the beam with the plasma was diagnosed using a retarding-field energy analyzer, three-axis magnetic-loop, and Langmuir probes. Measurements of the beam profiles at multiple axial locations evinced a spiraling ion-beam (J $\approx$ 50-140 mA/cm$^2$, pitch-angle $\approx$ 53$^{\circ}$) that traveled at Alfv\'{e}nic speed (beam-speed/Alfv\'{e}n-speed = 0.2--1.2). Although, a variety of waves were generated by the beam, this presentation will focus on shear Alfv\'{e}n waves. Parameters of the ion-beam and ambient plasma were varied to examine the resonance conditions under a variety of scenarios. The experimental results demonstrate that the DICR process is particularly effective in exciting left-handed polarized shear Alfv\'{e}n waves that propagate in the direction opposite to the ion beam. [Preview Abstract] |
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GP8.00092: Spatial and temporal measurements of electrostatic fields of a field-aligned, magnetized laser-produced plasma expansion Jeffrey Bonde, Stephen Vincena, Walter Gekelman Laser-produced plasmas (LPPs) in laboratory environments form supersonic and
super-Alf\'{e}nic flows that, when properly scaled, can model naturally
occurring phenomena such as shock structures in supernovae and astrophysical
jets. Our interest lies in understanding the evolution of these flows and
how they interact with ambient, magnetized plasma. An LPP was generated
using a $10^{11}W/cm^{2}$ laser pulse on a solid target. It expanded into a
pre-formed, magnetized plasma and was directed along the background magnetic
field ($c_{s} < |
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GP8.00093: Observation of Rayleigh-Taylor instability growth and evolution toward longer wavelengths at a decelerating magnetized-plasma interface Colin Adams, Auna Moser, Scott Hsu, John Dunn, Mark Gilmore The interaction of a high-Mach-number plasma jet propagating into a background magnetic field is studied experimentally on the Plasma Liner Experiment [1]. The jets, generated by plasma railguns, have densities and temperatures of order $10^{14}$~cm$^{-3}$ and 1~eV, respectively, at the time of interaction with the magnetic field ($\sim$ few hundred G). Due to ringing railgun current, the jet is comprised of a series of ``blobs'' traveling at $\sim 40$--70~km/s, arriving at the region with the applied field at $\sim 20$--30~$\mu$s intervals. When a trailing jet arrives and interacts with the remnants of the leading jet and compressed magnetic field, growing fingers are observed with a multi-frame camera at the front of the trailing jet. The fingers evolve toward longer mode wavelength ($\sim$ few cm) as the incoming jet penetrates into the magnetized region. Spectrometer and interferometer data show deceleration of the incoming jets against the lower-density magnetized background at approximately $10^{10}$~m/s$^2$. We compare experimental results to theoretical and computational predictions, showing consistency of the observations with Rayleigh-Taylor instabilities with magnetic and/or viscous stabilization.\\[4pt] [1] S. C. Hsu et al., Phys. Plasmas 19, 123514 (2012). [Preview Abstract] |
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GP8.00094: Vlasov simulations of negative mass instability of Langmuir waves Kentaro Hara, Thomas Chapman, Jeffrey Banks, Richard Berger, Ilon Joseph, Stephan Brunner, Iain Boyd An unambiguous signal of the negative mass instability (NMI) of large amplitude Langmuir waves has been observed for the first time using a 1D-1V Vlasov simulation code. During the NMI, recently proposed by Dodin (PRL \textbf{110}, 215006 (2013)), particles trapped in the potential well move to different trapped orbits with different bounce frequencies due to mutual Coulomb repulsion and potentially undergo phase bunching. The NMI in Langmuir waves has been studied using the Vlasov simulation with initial conditions conducive to comparison with theoretical estimates of the growth rate. In order to investigate the instability, Fourier analysis of the trapped particle distribution has been performed in action-angle coordinates. Theoretical and numerical growth rates of the NMI are in good agreement when the trapped particle population is initialized as a delta-like function in energy. The mechanism of nonlinear saturation of the NMI is also discussed. \it{This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and supported by the U.S. Department of Energy Office of Science, Fusion Energy Sciences Program, Grant DESC0001939.} [Preview Abstract] |
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GP8.00095: Study of Coupling between a Plasma Source and Plasma Fluctuations Jorge Berumen, Feng Chu, Ryan Hood, Sean Mattingly, Anthony Rogers, Fred Skiff An experimental study on the coupling between a plasma source and plasma fluctuations in a cylindrical, magnetized, singly-ionized Argon inductively-coupled gas discharge plasma that is weakly collisional is presented. Typical plasma conditions are n $\sim$ 10$^{10}$cm$^{-3}$ T$_{\mathrm{e}}$ $\sim$ 3 eV and B $\sim$ 1 kG. Amplitude Modulation (AM) of the inductively-coupled RF plasma source is produced near the fundamental-mode ion-acoustic wave frequency ($\sim$ 1 kHz) to study the effects of the source-wave interaction and plasma production. Density fluctuation measurements are implemented using Laser-Induced Fluorescence techniques and Langmuir probes. We apply coherent detection with respect to the wave frequency to obtain the perturbed ion distribution function associated with the waves. Measurements of fluctuating I-V traces from a Langmuir probe array and antenna current load are also used to show the effects of the interaction. [Preview Abstract] |
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GP8.00096: Simulation of mode conversion of lower hybrid waves Guozhang Jia, Nong Xiang, Xueyi Wang, Yu Lin Conversion between slow- and fast-waves in the lower hybrid range of frequencies in an inhomogeneous plasma is investigated using the particle-in-cell simulation code based on the gyrokinetic electron and fully kinetic ion (GeFi) model [Yu Lin, Xueyi Wang, Zhihong Lin and Liu Chen, Plasma Phys. Control. Fusion 47, 2005, 657]. For a low input power, it is found that the occurrence of the mode conversion sensitively depends on the value of the parallel wave refractive index as shown by the linear theory, and good agreement with the linear theory is obtained. With the input power increasing, it is shown that the mode conversion process is significantly affected by nonlinear plasma-wave interactions. [Preview Abstract] |
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GP8.00097: Electrostatic instabilities induced by counter streaming ions in supersonic wake Chuteng Zhou, I.H. Hutchinson, Christian Bernt Haakonsen The wake behind an object moving at supersonic speed in a plasma contains a region of depleted density into which the plasma expands. This replenishing mechanism results in counter streaming ion beams accelerated by ambipolar electric fields, which can be inherently unstable. It is widely believed, for example, that the intense electrostatic noise in the central lunar wake arises from such instabilities. To understand better this and related phenomena, a code has been developed to calculate the linear wave growth rates in the Vlasov-Poisson system for arbitrary ion distribution functions. The research aims to give a comprehensive description of the electrostatic instabilities and their parametric dependences. A contour plot of maximum growth rates in parameter space will be presented in cases where the ion distribution function can be represented by a sum of Gaussians. Our calculations consider unequal beams and oblique modes in contrast to previous published results, which mostly treat equal beams and parallel propagation. We further apply our methods to investigate the regions of electrostatic instabilities in the wake of high Mach number plasma flow. [Preview Abstract] |
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GP8.00098: Excitation and control of autoresonant ion acoustic waves Lazar Friedland Controlled excitation of nonlinear waves is one of the important goals of both basic and applied research. We study the excitation of large amplitude ion acoustic waves in plasmas from a trivial equilibrium by the adiabatic nonlinear phase-locking (autoresonance) with a chirped frequency drive. In this case, under certain conditions, the nonlinearity and variation of parameters work in tandem to preserve the phase-locking in the system via excursion of the ion acoustic wave in its parameter space, yielding controlled growth of the wave amplitude. We analyze formation of autoresonant ion acoustic waves via both the fluid and kinetic Vlasov-Poisson models. A weakly nonlinear, long wavelength limit of the fluid approximation for the ion acoustic waves is frequently associated with the KdV equation. We go beyond the driven KdV problem and study the formation of driven, strongly nonlinear, fluid-type ion acoustic waves. The Whitham's averaged vriational principle is applied in analyzing these autoresonant excitations. At larger ion temperatures, our simulations show the possibility of formation of a phase-locked void (hole) in the ion distribution, yielding a particular type of autoresonant ion Bernstein, Green, and Kruskal (BGK) mode. [Preview Abstract] |
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GP8.00099: Two-stream instability with time-dependent drift velocity Hong Qin, Ronald Davidson The classical two-stream instability driven by a constant relative drift velocity between two plasma components is extended to the case with time-dependent drift velocity. For periodic oscillating relative velocity, the linear dynamics can be characterized by a one-period map from which the growth rate of the perturbation can be rigorously defined and calculated. Using this tool, we are able to obtain a comprehensive picture of the linear two-stream stability driven by a general time-dependent drift velocity. Stability diagrams for the oscillating two-stream instability are presented over a large region of parameter space. It is shown that the maximum growth rate for the classical two-stream instability can be significantly reduced by adding an oscillatory component to the relative drift velocity. [Preview Abstract] |
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GP8.00100: The Birth of Kinetic Electrostatic Electron Nonlinear (KEEN) Waves Weakly Driven by the Ponderomotive Force of Crossing Laser Beams in High Energy Density Plasmas Compared to Strongly Driven KEEN Waves Michel Mehrenberger, Bedros Afeyan, Adila Dodhy, Eric Sonnendr\"uker We vary the amplitude and duration of the ponderomotive force driving KEEN waves in Vlasov-Poisson simulations. We use variable-resolution velocity-grids, so as to maintain accuracy, no matter how small the driven waves get at weak drive. We further accelerate the long time simulations by the use of a large time step which is allowed by a sixth-order time-splitting symplectic scheme. How KEEN waves are born, the fragmentation into vorticlets and their subsequent (sometimes successful and sometimes unsuccessful) merger in the case of weak and/or short duration drives is compared to longer duration and to larger amplitude drives. Our aim is to extract the scaling laws dictating how multiple harmonic, phase locked electric field structures which are the essential feature of KEEN waves are related to the ponderomotive drive properties and how they vary at different locations in the Brillouin diagram. New diagnostics revealing partitioning of phase space, particle orbit statistics and partial mode reconstructions are used to demonstrate the nonstationary and intricate yet robust physics of KEEN wave self-organization in phase space. [Preview Abstract] |
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GP8.00101: Nonlinear growth and damping rates of a plasma wave Didier Benisti We provide, within the same theoretical framework, a full description of the nonlinear stage of the beam-plasma instability and the derivation of the nonlinear Landau damping rate of a plasma wave. The latter issue is addressed whether the duration of the plasma pulse is long or comparable to the plasma period. Therefore, the present work generalizes previous derivations of the nonlinear Landau damping rate of an electron plasma wave [1], that were only for slowly varying waves, as those generated by Raman scattering in a laser fusion device. Such a generalization is needed to address backward Raman amplification, and to discuss how nonlinear effects may enlarge the parameter window for amplification of short wave lengths [2].\\[4pt] [1] D. B\'{e}nisti, D. Strozzi, L. Gremillet and O. Morice, Phys. Rev. Lett. \textbf{103}, 155002 (2009).\\[0pt] [2] V.M. Malkin and N.J. Fisch, Phys. Plasmas, \textbf{17}, 073109 (2010). [Preview Abstract] |
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GP8.00102: LOW TEMPERATURE PLASMA SCIENCE AND ENGINEERING |
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GP8.00103: Numerical Confirmation of the Dramatically Reduced Secondary Electron Emission Yield of Velvet-like Surfaces C. Swanson, I.D. Kaganovich, Y. Raitses Recent experimentation with Hall Thrusters [1] has shown that the effective secondary electron emission yield of Hall Thruster walls is dramatically reduced by application of velvet-like fibers to the walls. This secondary electron emission suppression is presumably due to re-collision of secondary electrons with the fibers before emitted electrons can return to plasma. A numerical evolution of the resulting electron velocity distribution function of emitted electrons returning to the plasma was performed for this surface geometry; and the results were benchmarked against analytic calculations and experimental findings. \\[4pt] [1] Y. Raitses et. al., IEEE Trans. Plasma Sci. \textbf{39}, 995 (2011). [Preview Abstract] |
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GP8.00104: Gradient Drift Instabilities in Two Dimensional Hybrid Hall Thruster Simulations Jacob Aley, Caleb Dowdy, Eduardo Fernandez Instabilities triggered by a variety of mechanisms have been theoretically predicted for Hall thruster plasmas. Experimentally, fluctuations spanning a wide range of frequencies and wave numbers have been observed. Perhaps more importantly, fluctuations have been postulated to play a role in regulating cross-field electron transport in Hall thrusters. However, a clear understanding of what instabilities are responsible for such transport is presently lacking. In this work we focus on analysis of long wavelength gradient drift instability in the Hall thruster via two dimensional hybrid fluid-PIC simulations that resolve azimuthal dynamics. Recent theoretical analysis by Frias\footnote{Winston Frias, Andrei I. Smolyakov, Igor D. Kaganovich, and Yevgeny Raitses, ``Long wavelength gradient drift instability in Hall plasma devices. I Fluid Theory,'' Physics of Plasmas 20, 072112 (2012).} et al. shows that previous stability criteria for drift instabilities are modified due to compressibility of the electron flow. In our simulations, we test this improved criterion by examining the transient waves that emerge in the simulation from a smooth initial condition. The simulations give good agreement with the theory, both in the frequency/growth rate characteristics of the waves as well as the region of the thruster where such disturbances are predicted to emerge. These results suggest that gradient drift instabilities play a significant role in Hall thruster plasmas. [Preview Abstract] |
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GP8.00105: Saturated Fluctuations and Transport in Axial, Azimuthal Hybrid Hall Thruster Simulations Caleb Dowdy, Jacob Aley, Eduardo Fernandez Simulation studies of Hall thrusters aimed at describing the global domain typically employ hybrid schemes instead of more expensive kinetic approaches. Such simulations are generally in the radial and axial coordinates, assuming axisymmetry in order to circumvent azimuthal dynamics. Cross-field electron transport is enhanced (in an ad-hoc manner) in order to sustain the plasma and produce simulation profiles in semi-quantitative agreement with experimental measurements. In this work we present results from an axial/azimuthal hybrid fluid-PIC model of Hall thrusters that treats the azimuthal dynamics self-consistently, without employing ad-hoc transport parameters. Unlike previous simulation efforts with this model, the current work has succeeded at obtaining fully saturated states at high voltage, resolving the longest (breathing mode) timescales in the system. Equilibrium profiles and fluctuations predicted by the simulation will be presented. The latter are analyzed in terms of their frequency and propagation characteristics, as well as their contribution to transport. Linear stability theory is used to comment on the possible origin of the disturbances. Finally, the role of EXB flow shear on the potential regulation of fluctuation-induced electron transport is discussed. [Preview Abstract] |
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GP8.00106: Study of Breathing Oscillations in a Hall Thruster Scott Keller, Yevgeny Raitses, Ahmed Diallo Breathing oscillations are the most powerful low frequency (10-30 kHz) oscillations that are typically observed in different types of Hall thrusters [1,2]. We report on investigations of the effects of both natural and artificially driven breathing oscillations on the discharge and plasma properties of a cylindrical Hall thruster. In order to produce artificially coherent oscillations, a sinusoidal modulation up to 30 V$_{\mathrm{AC}}$ of the anode potential in the range of 5-30 kHz is applied to the thruster. These driven modes are studied in operating regimes with and without naturally occurring oscillations. The imposed periodicity allows for measurement of the time-dependent ion velocity distribution through a novel heterodyne approach to laser-induced fluorescence (LIF) using phase-sensitive detection. Further comparison between natural and driven modes is performed through the analysis of the discharge and ion currents, as well as high-speed imaging data. Results serve both to validate the LIF technique and to improve understanding of breathing oscillations. In particular, we show oscillations of the ion velocity distribution function due to breathing oscillations and explain their correlation with oscillations of the discharge and ion currents. \\[4pt] [1] J. P. Boeuf and L. Garrigues, J. Appl. Phys. 84, 3541 (1998).\\[0pt] [2] Y. Raitses \textit{et al}, Phys. Plasmas 16, 057106 (2009). [Preview Abstract] |
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GP8.00107: Time-Averaged and Oscillatory Characterization of a Hall Plasma Discharge Chris V. Young, Andrea Lucca Fabris, Nicolas Gascon, Mark A. Cappelli We characterize a 70 mm diameter stationary plasma thruster operating on xenon at 200-500 W using nonintrusive laser measurements. This study resolves both time-averaged properties and oscillatory phenomena in the plasma discharge. Specifically, we explore how the plume ion velocity field evolves in time with respect to periodic discharge current oscillations using time-synchronized laser induced fluorescence (LIF) techniques. In this LIF scheme, a triggered signal acquisition gate is locked at a given phase of the current oscillation period, allowing for drift in the oscillation. The laser is modulated at a characteristic frequency and the induced fluorescence signal is extracted out of the bright background emission using homodyne detection with a lock-in amplifier. [Preview Abstract] |
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GP8.00108: Incorporating a Turbulence Transport Model into 2-D Hybrid Hall Thruster Simulations Eunsun Cha, Mark A. Cappelli, Eduardo Fernandez 2-D hybrid simulations of Hall plasma thrusters that do not resolve cross-field transport-generating fluctuations require a model to capture how electrons migrate across the magnetic field. We describe the results of integrating a turbulent electron transport model into simulations of plasma behavior in a plane spanned by the E and B field vectors. The simulations treat the electrons as a fluid and the heavy species (ions/neutrals) as discrete particles. The transport model assumes that the turbulent eddy cascade in the electron fluid to smaller scales is the primary means of electron energy dissipation. Using this model, we compare simulations to experimental measurements made on a laboratory Hall discharge over a range of discharge voltage. Both the current-voltage trends as well as the plasma properties such as plasma temperature, electron density, and ion velocities seem agree favorably with experiments, where a simple Bohm transport model tends to perform poorly in capturing much of the discharge behavior. [Preview Abstract] |
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GP8.00109: Experimental investigation of drift instabilities in ExB discharges Nicolas Gascon, Chris V. Young, Andrea Lucca Fabris, Tsuyohito Ito, Mark A. Cappelli Drift plasma instabilities are characterized in three ExB discharges operating on noble gases: two Hall-type plasma thrusters with insulating channel walls (70mm outer diameter, 20 mm long, and 90mm outer diameter, 80mm long), and a small magnetron discharge (5mm diameter). Plasma instabilities in the ExB discharges are investigated using arrays of electrostatic probes. The signals from the probes arrays are processed with wavelet filtering, and frequency-wavelength dispersion analysis tools. Results are compared to hybrid PIC-fluid axial azimuthal simulations and analyzed in light of recent theories of gradient-driven drift instabilities, in an effort to better understand the relation between drift instabilities and anomalous electron transport in these discharges. [Preview Abstract] |
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GP8.00110: Low-temperature plasma simulations with the LSP PIC code Johan Carlsson, Alex Khrabrov, Igor Kaganovich, David Keating, Svetlana Selezneva, Timothy Sommerer The LSP (Large-Scale Plasma) PIC-MCC code has been used to simulate several low-temperature plasma configurations, including a gas switch for high-power AC/DC conversion, a glow discharge and a Hall thruster. Simulation results will be presented with an emphasis on code comparison and validation against experiment. High-voltage, direct-current (HVDC) power transmission is becoming more common as it can reduce construction costs and power losses. Solid-state power-electronics devices are presently used, but it has been proposed that gas switches could become a compact, less costly, alternative. A gas-switch conversion device would be based on a glow discharge, with a magnetically insulated cold cathode. Its operation is similar to that of a sputtering magnetron, but with much higher pressure (0.1 to 0.3 Torr) in order to achieve high current density. We have performed 1D (axial) and 2D (axial/radial) simulations of such a gas switch using LSP. The 1D results were compared with results from the EDIPIC code. To test and compare the collision models used by the LSP and EDIPIC codes in more detail, a validation exercise was performed for the cathode fall of a glow discharge. We will also present some 2D (radial/azimuthal) LSP simulations of a Hall thruster. [Preview Abstract] |
(Author Not Attending)
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GP8.00111: Influence of power on the surface loss reaction of F radicals in a low pressure CF$_{4}$:O$_{2}$ ICP discharge Mahsa Setareh, Morteza Farnia, Ali Maghari A zero dimensional modeling code Global\textunderscore kin, developed by Kushner is applied to model the CF$_{4}$/O$_{2}$ radio frequency inductively coupled plasma at applied powers of 80-300W, pressure of 25mTorr and temperature of 400K. The calculated results indicated that the Fluorine (F) is the dominant radical produced in CF$_{4}$:O$_{2}$ discharge which is lost mostly at the walls rather than in formation of F$_{2}$ molecules. We calculated the time integrated rate of F loss at the wall together with the relative contribution of wall reactions on the total loss of F corresponding to the sticking probabilities. The model predicts that although the absolute time integrated loss rates at the walls increase with power, but the relative contribution of the wall loss process decreases slightly upon higher powers. Furthermore, at lower O$_{2}$ contents (or high CF$_{4}$ contents), the relative contribution of the wall loss process is much lower because F radicals can also get lost in reactions with other plasma species such as CF$_{3}$ to form again CF$_{4}$. At equal contents of O$_{2}$ and CF$_{4}$, 35-45{\%} of the F radicals are lost at the walls, depending on the power. The numerical modeling results for CF$_{4}$ decomposition into new products are validated based on experimental data from literature. [Preview Abstract] |
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GP8.00112: Modification of polypropylene foils by low pressure oxygen plasma and its influence on the formation of titanium dioxide films Rafal Sadowski, Wojciech Macyk Commercially available polypropylene foils were pre-treated with low pressure, room temperature radio frequency (RF) oxygen plasma at constant power and pressure. Various durations of pre-treatment process were applied. Afterwards the samples were covered with titanium dioxide thin film by dip-coating technique and photosensitized by titanium(IV) surface complexes formed upon impregnation with catechol-like ligands. Optical emission spectroscopy (OES) measurements were used for determining plasma species. The surface properties before and after plasma treatment were characterized by contact angle measurements, FTIR-ATR, UV-Vis, and X-ray photoelectron spectroscopy (XPS). Titanium dioxide thin films were characterized by scanning electron microscopy (SEM) and UV-Vis spectroscopy. The photoactivity of TiO2 films was tested by photocurrent measurements. It was shown that plasma pre-treatment is essential for oxygen groups formation which contribute to titanium dioxide binding to polymer surface. [Preview Abstract] |
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GP8.00113: A high voltage nanosecond pulser with independently adjustable output voltage, pulse width, and pulse repetition frequency James Prager, Timothy Ziemba, Kenneth Miller, John Carscadden, Ilia Slobodov Eagle Harbor Technologies (EHT) is developing a high voltage nanosecond pulser capable of generating microwaves and non-equilibrium plasmas for plasma medicine, material science, enhanced combustion, drag reduction, and other research applications. The EHT nanosecond pulser technology is capable of producing high voltage (up to 60 kV) pulses (width 20 -- 500 ns) with fast rise times (\textless 10 ns) at high pulse repetition frequency (adjustable up to 100 kHz) for CW operation. The pulser does not require the use of saturable core magnetics, which allows for the output voltage, pulse width, and pulse repetition frequency to be fully adjustable, enabling researchers to explore non-equilibrium plasmas over a wide range of parameters. A magnetic compression stage can be added to improve the rise time and drive lower impedance loads without sacrificing high pulse repetition frequency operation. [Preview Abstract] |
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GP8.00114: Solutions of Boltzmann Equation for Simulation of Particle Distributions in Plasmas Jason Hammond We are investigating the time evolution of the electron and excited state distribution functions. To accomplish this, we solve the time dependent Boltzmann equation to overcome some typical limitations of modeling high pressure plasmas using Monte Carlo methods. Here we focus on the numerical approach to solving the time dependent Boltzmann equation using a multi-term approximation of the electron distribution function. We also compare Boltzmann results for electron distribution evolution against multiple plasma simulations using experimental collisional cross-section data. [Preview Abstract] |
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GP8.00115: Development of a laser diagnostic using Raman and Thomson scattering in atmospheric microplasma sources Bradley Sommers, Steven Adams A laser scattering system utilizing a triple grating spectrometer and a 266 nm ultraviolet laser has been developed in order to investigate Rayleigh, Raman, and Thomson scattering within atmospheric plasma sources. Such laser scattering interactions offer a non-invasive technique for investigating atmospheric microplasma sources, which have potential applications in remote optical sensing, materials processing, and environmental decontamination. In this work, laser scattering measurements were performed on atmospheric discharges composed of nitrogen and air. The laser signal was calibrated using a heated nitrogen vacuum cell held at atmospheric pressure. Preliminary temperature measurements were performed on a D.C. microdischarge operating in normal glow mode. This provides a non-thermal plasma in which the translational, rotational, vibrational and electron temperatures are not in equilibrium. All gas temperatures were calculated by fitting simulated scattering spectra to the experimental data obtained using the triple grating spectrometer. Measured temperatures were also compared with those obtained using standard optical emission spectroscopy methods. [Preview Abstract] |
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GP8.00116: Laser-induced incandescence diagnostic for \textit{in situ} monitoring of nanoparticle synthesis in an atmospheric plasma James Mitrani, Shane Patel, Mikhail Shneider, Brent Stratton, Yevgeny Raitses A DC arc discharge with a consumed graphite electrode is commonly used for synthesis of carbon nanoaparticles in a low temperature (0.1 -- 1 eV), atmospheric pressure plasma. The formation of nanoparticles in this plasma is poorly understood; it is not clear where nanoparticles nucleate and grow in the arc discharge. Therefore, a laser-induced incandescence (LII) diagnostic for \textit{in situ} monitoring of the nanoparticles' spatial distribution in the plasma is currently being constructed. The LII diagnostic involves heating the particles with a short-pulsed laser, and measuring the induced spatiotemporal incandescence profiles on longer timescales. By appropriately modeling the induced spatiotemporal incandescence profiles, one can measure particle diameters and volume fraction. LII diagnostics have been extensively used to study soot particles in flames. However, they have never been applied in a strongly coupled plasma background. Even though the spatial dimensions for soot and nanoparticles are similar, great care is needed in developing an LII diagnostic for monitoring nanoparticles in a plasma background. Therefore, we will calibrate our LII diagnostic by measuring spatiotemporal incandescence profiles of known, research grade soot and nanoparticles. [Preview Abstract] |
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GP8.00117: Self-Organization of Plasma and Material Processes in the Carbon Arc Discharge Yevgeny Raitses, Jonathan Ng The atmospheric pressure carbon arc in helium is an important method for the production of nanomaterials [1]. Typical arcs operate in a dc mode between a graphite anode, which is consumed, and a cathode which may be a lower melting point material, which remains undamaged [2,3]. During the arc operation, a carbon deposit is formed on the cathode surface and plays a crucial role in conducting the arc current. Temperature measurements demonstrate that a sufficiently large area of the cathode deposit is hot enough for thermionic emission to be the source of most of the arc current [4,5]. Structural evaluation of the carbon deposit and an analysis of the energy balance at the anode and the arc-cathode interface suggest that the evaporation of the graphite anode and formation of the carbon deposit on the cathode are self-organized to maintain the current conduction in the arc and can probably be generalized for other arc synthesis methods with consumed anodes [5].\\[4pt] [1] Journet et. al. Nature 388:756 (1997);\\[0pt] [2] Keidar, Beilis, J. Appl. Phys 106, 103304 (2009);\\[0pt] [3] Fetterman, Raitses, Keidar, Carbon 46, 1322 (2008);\\[0pt] [4] Hantzsche, Beitr. Plasmaphys., 22, 325(1981);\\[0pt] [5] Ng and Raitses, in press, Carbon (2014). [Preview Abstract] |
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GP8.00118: Investigation of Sterilization Effect by various Gas Plasmas and Electron Microscopic Observation of Bacteria Yota Sasaki, Toshihiro Takamatsu, Kodai Uehara, Takaya Oshita, Hidekazu Miyahara, Akitoshi Okino, Keiko Ikeda, Yuriko Matsumura, Atsuo Iwasawa, Masahiro Kohno Atmospheric non-thermal plasmas have attracted attention as a new sterilization method. It is considered that factor of plasma sterilization are mainly reactive oxygen species (ROS). However, the sterilization mechanism hasn't been investigated in detail because conventional plasma sources have a limitation in usable gas species and lack variety of ROS. So we developed multi-gas plasma jet which can generate various gas plasmas. In this study, investigation of sterilization effect by various gas plasmas and electron microscopic observation of bacteria were performed. Oxygen, nitrogen, carbon dioxide, argon and air were used as plasma gas. To investigate gas-species dependence of sterilization effect, \textit{S.aureus} was treated. As a result, nitrogen plasma and carbon dioxide plasma were effective for sterilization. To investigate sterilization mechanism, the surface of \textit{S.aureus} was observed by scanning electron microscope. As a result, dimples were observed on the surface after irradiation of nitrogen plasma, but no change observed in the case of carbon dioxide plasma. These results suggest that bactericidal mechanism of nitrogen and carbon dioxide plasma should be different. In the presentation, Measurement result of ROS will be reported. [Preview Abstract] |
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GP8.00119: ABSTRACT WITHDRAWN |
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GP8.00120: Comparison of short-lived medical isotopes activation by laser thin target induced protons and conventional cyclotron proton beams Joseph Murray, Galina Dudnikova, Tung-Chang Liu, Dennis Papadopoulos, Roald Sagdeev, J.J. Su Production diagnostic or therapeutic nuclear medicines are either by nuclear reactors or by ion accelerators. In general, diagnostic nuclear radioisotopes have a very short half-life varying from tens of minutes for PET tracers and few hours for SPECT tracers. Thus supplies of PET and SPECT radiotracers are limited by regional production facilities. For example 18F-fluorodeoxyglucose (FDG) is the most desired tracer for positron emission tomography because its 110 minutes half-life is sufficient long for transport from production facilities to nearby users. From nuclear activation to completing image taking must be done within 4 hours. Decentralized production of diagnostic radioisotopes will be idea to make high specific activity radiotracers available to researches and clinicians. 11C, 13N, 15O and 18F can be produced in the energy range from 10-20 MeV by protons. Protons of energies up to tens of MeV generated by intense laser interacting with hydrogen containing targets have been demonstrated by many groups in the past decade. We use 2D PIC code for proton acceleration, Geant4 Monte Carlo code for nuclei activation to~compare the yields and specific activities of short-lived isotopes produced by cyclotron proton beams and laser driven protons. [Preview Abstract] |
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