Bulletin of the American Physical Society
51st Annual Meeting of the APS Division of Plasma Physics
Volume 54, Number 15
Monday–Friday, November 2–6, 2009; Atlanta, Georgia
Session TP8: Poster Session VII: DIII-D Tokamak III; MST, RFP, and Stellarator; Magnetic Reconnection; Magneto-Inertial-Fusion |
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Room: Grand Hall East |
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TP8.00001: DIII-D TOKAMAK III |
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TP8.00002: Effect of Self-consistent Poloidal Electric Field on Neoclassical Angular Momentum Transport S.K. Wong, V.S. Chan Existing calculations of toroidal angular momentum flux in the banana regime of neoclassical transport theory have neglected the self-consistent electric field, or the poloidal variation of the electric potential, that arises from quasi-neutrality. Recently, it has been shown that this variation cannot be neglected in the Pfirsch-Schluter regime, where it in fact gives the dominant contribution for large aspect ratio flux surfaces. We show that the same conclusion applies in the banana regime. We have revisited the calculation of this variation in the banana regime, and found an analytic expression that does not have the divergence at the inner most point of the flux surface noted in an earlier work, although it is still discontinuous there. Using this expression, and an analytic solution of the linearized drift kinetic equation based on a model collision operator, we have obtained a closed form for the angular momentum flux to leading order of an expansion in inverse aspect ratio. The result is larger than the existing one by the order of square root of inverse aspect ratio. [Preview Abstract] |
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TP8.00003: Toroidal Rotation With Near-Balanced NBI in \hbox{DIII-D} H-Mode Discharges J.S. deGrassie, K.H. Burrell, R.J. Groebner, W.M. Solomon Intrinsic rotation exists in the tokamak with no applied auxiliary torque and is important to understand for projection to burning plasmas and ITER. Rice's scaling, that $V \sim W/I_p$ [1], exists in other tokamaks, including DIII-D, where $V$ is toroidal velocity, and $W$ the stored energy. A dimensionless casting of this scaling is being sought [1]. Heating by rf waves is the primary tool to investigate intrinsic rotation in H-mode conditions. It is difficult to attain ITER-relevant values of $\beta_N \sim 2$ simply due to the typical rf power capabilities installed, compared with neutral beam injection (NBI). In DIII-D we are using the balanced beam capability to investigate the intrinsic rotation scaling at these $\beta_N$ values. An issue is the localized remnant torque density that exists to some extent with mirrored beam injection, because of the opposite radial drift of co- and counter-injected fast ions. We will show how these higher $\beta_N$ conditions compare with the Rice scaling and account for remnant torque. \vskip6pt \noindent [1] J.E. Rice, Nucl. Fusion {\bf 47}, 1618 (2007). [Preview Abstract] |
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TP8.00004: H-mode Power Threshold, ELM Characteristics and Control, and Plasma Startup in Helium Plasmas in DIII-D P. Gohil Determining the physics of helium plasmas and comparing with hydrogen and deuterium plasmas is important for the validation of key issues in plasma physics. This is also important for the first operational phase of ITER, which will use helium or hydrogen plasmas. Physcs issues for helium plasmas include: (a) the H-mode power threshold and the H-mode pedestal and ELM characteristics; (b) ELM suppression by resonant magnetic perturbations; (c) requirements for ITER plasma start-up and ramp-down. There is also the question of whether recent results from deuterium and hydrogen plasmas hold for helium plasmas. For example, recent studies of the H-mode power threshold in hydrogen and deuterium indicate that the power required to induce the L-H transition is dependent on the applied beam torque, and the H-mode power threshold using ECH (or a combination of ECH+NBI) is 20\%-40\% lower than that for discharges using NBI alone. Results from experiments to investigate issues (a) to (c) above in helium plasmas will be presented. [Preview Abstract] |
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TP8.00005: Core Turbulence and Flow Dynamics Across L-H Transition on DIII-D G. Wang, W.A. Peebles, T.L. Rhodes, J.C. Hillesheim, E.J. Doyle, L. Schmitz, L. Zeng, A.E. White, G.R. McKee, C.C. Petty, K.H. Burrell, W.M. Solomon First measurements of core low and intermediate-k correlation lengths as well as the dynamic turbulence amplitude behavior across near-balanced NBI-heated L- to H-mode transitions have been obtained on DIII-D. In these discharges, poloidal turbulence flow increases with little change in its shear as the L-H transition is approached. Leading up to the transition, fluctuation levels $(\tilde n/n)$ of low-k ($<$3 cm$^{-1}$) show little variation, while that of intermediate- (3$-$6 cm$^{-1}$) and high-k ($\sim$ 35 cm$^{-1}$) increase. At the same time, core radial correlation lengths of both low and intermediate-k decrease. In contrast, electron temperature fluctuation levels $(\tilde T_e/T_e)$ first increase then drop. With these data a multi-scale and multi-field picture of the L to H transition dynamics is being developed allowing detailed comparison to theory and simulation (e.g. linear gyrokinetic stability simulations, TGLF). [Preview Abstract] |
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TP8.00006: Turbulence and Transport Dependence on Plasma Shape in DIII-D T.L. Rhodes, W.A. Peebles, L. Schmitz, J.C. Hillesheim, G. Wang, L. Zeng, E.J. Doyle, C. Holland, G.R. McKee, A.E. White, J.C. DeBoo, K.H. Burrell, C.C. Petty Using a unique array of diagnostics the dependence of turbulence and transport on plasma shape (elongation, elongation shear, and triangularity) is studied. The diagnostic set includes FIR scattering, Doppler backscattering, correlation ECE, BES, reflectometry, and high-$k$ backscattering. This covers a broad range in wavenumber $(0 \leq k\rho_s\leq 10)$ and fields (density, temperature, flows). The measurements concentrated primarily on L-mode plasmas, although, H-mode was obtained in one shape. Data were obtained using two different heating methods, neutral beam and EC heated. Initial results show strong decreases in density and temperature fluctuation levels as the plasma elongation is increased at fixed $q_{95}$. Thermal transport is reduced as well. This data set was acquired in order to make detailed comparisons to turbulence and transport models such as GYRO and TGLF. [Preview Abstract] |
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TP8.00007: 2D Core Turbulence Properties on DIII-D M.W. Shafer, G.R. McKee, R.J. Fonck, D.J. Schlossberg, Z. Yan, C. Holland, A.E. White Quantitative measurements of the inherently 2D turbulence characteristics in magnetized plasmas are compared with nonlinear simulation. This comparison substantiates key aspects of the $E\times B$ shear model of turbulence suppression that explains enhanced confinement. The critical dynamics underlying turbulent transport occur in the plane perpendicular to the magnetic field $(k_\| \ll k_\perp)$. These localized long-wavelength $(k_\perp \rho_i < 1)$ density turbulence measurements are obtained in the core $(0.3 < r/a < 0.9)$ of DIII-D L-mode plasmas with a 2D rectangular array of Beam Emission Spectroscopy channels. Radial and poloidal correlation lengths are found to scale with the ion gyroradius and demonstrate a poloidally elongated eddy structure. $S(k_r,k_\theta )$ spectra are compared with GYRO simulations: key features (wavenumber peak, correlation lengths) compare well, however the simulations indicate a sheared eddy structure at outer radii that is not observed. Measured local decorrelation and shearing rates are also compared. [Preview Abstract] |
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TP8.00008: Density Fluctuation Enhancement During RMP ELM-Suppressed Discharges G.R. McKee, R.J. Fonck, D.J. Schlossberg, M.W. Shafer, Z. Yan, T.E. Evans, R.A. Moyer Fluctuation behavior changes dramatically over the radial range $0.6 < r/a < 1.0$ during ELM-suppressed phases of H-mode discharges with externally applied resonant magnetic perturbations (RMPs) on DIII-D. The 2D spatiotemporal characteristics of density fluctuations during ELM suppression are characterized with beam emission spectroscopy and exhibit a broadband structure from 80$-$400 kHz. The fluctuation power increases by up to one order of magnitude in the mid-core region $(0.6 < r/a < 0.8)$, with more modest increases in the pedestal region. The increase in turbulence coincides with the ELM-suppressed phases, which typically start a few 100 ms after the application of a radial magnetic field. The radial and poloidal correlation lengths of the increased fluctuations are a few centimeters, typical of long-wavelength turbulence. The increased fluctuations appear to be related to observed profile changes, reduction of the line integrated density, and increased radial particle transport. [Preview Abstract] |
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TP8.00009: Transport of Energetic Ions Due to Interaction With Microturbulence in DIII-D D.C. Pace, W.W. Heidbrink, Z. Lin, W. Zhang, G.R. McKee, M. Murakami, J.M. Park, C.C. Petty, M.A. Van Zeeland Cross-field diffusion of energetic ions due to microturbulence is observed during neutral beam injection in the DIII-D tokamak. The energetic ion energy spectrum and radial density profile are measured with fast-ion $D_\alpha$, neutron detection, and motional Stark effect systems. DIII-D discharges study the range $5 < E/T_e < 20$, where E is the energy of the energetic ions injected by neutral beams and $T_e$ is the electron temperature, in order to test theoretical results [1] that show energetic ion diffusion increasing as this ratio decreases. The parameter space is relevant to energetic ions in present tokamaks and helium ash in ITER. Density fluctuations related to the microturbulence are measured with beam emission spectroscopy and are consistent with ion-temperature gradient and trapped electron modes. Progress on simulating this phenomenon with the gyrokinetic codes GYRO and GTC will be presented. \vskip6pt \noindent [1] W. Zhang, et al., Phys. Rev. Lett. {\bf 101} (2009) 095001. [Preview Abstract] |
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TP8.00010: Modeling the H-mode Barrier with TGLF G.M. Staebler, J.E. Kinsey, R.E. Waltz The trapped gyro-Landau fluid (TGLF) transport model was designed to be valid in the near separatrix region of tokamaks and spherical tori. Since the last published version [1], a number of improvements have been made to TGLF: general numerical equilibrium magnetic geometry, improved electron-ion collision model, improved saturation rule. Most importantly, momentum transport has been added. Both the parallel and $E\times B$ velocity shear and the coriolis drift are now included in the eigenmode calculation so that the linear wavefunction and viscous stress matrix can be computed quasi-linearly. The $E\times B$ velocity shear is included using a generalization of the successful quench rule that includes a model for the radial wavenumber induced by the $E\times B$ velocity shear. With these improvements, the TGLF model is ready to push out to the edge of the closed flux surface region. The first TGLF results of linear stability calculations and particle, energy and momentum transport simulations of H-mode and L-mode near edge regions of the DIII-D tokamak will be reported. \vskip6pt \noindent [1] J.E. Kinsey, et al., Phys. Plasmas {\bf 15}, 055909 (2008). [Preview Abstract] |
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TP8.00011: Predictions of the Confinement in DIII-D Hybrids Using the TGLF Transport Model J.E. Kinsey, G.M. Staebler, R.E. Waltz, J. Candy In previous work, the TGLF gyro-Landau-fluid transport model [1,2] was validated against a profile database of 96 L- and H-mode discharges. This work focuses on modeling hybrid discharges using an upgraded collision model in TGLF. Recent comparisons between TGLF and GYRO [3] nonlinear simulations of long wavelength driftwave turbulence with collisions motivated improving the collision model in TGLF. Using a newly developed collision model results in the TGLF diffusivities more accurately fitting a database of 35 nonlinear GYRO runs performed with collisions and Miller geometry. TGLF transport simulations of 32 DIII-D hybrid discharges show good agreement with both the ion and electron experimental temperature profiles. The transport simulations show the ion transport tends to be close to the neoclassical level while the electron transport tends to be dominated by short wavelength ETG modes. \vskip6pt \noindent [1] J.E. Kinsey, et al., Phys. Plasmas {\bf 15}, 055908 (2008). \vskip2pt \noindent [2] G.M. Staebler, et al., Phys. Plasmas {\bf 12}, 102508 (2005). \vskip2pt\noindent [3] J. Candy, R.E. Waltz, Phys. Rev. Lett. {\bf 91}, 45001 (2003). [Preview Abstract] |
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TP8.00012: Experimental Tests of Elongation Scaling in Gyrokinetic Turbulence Simulations C. Holland, G.R. Tynan, T.L. Rhodes, W.A. Peebles, L. Schmitz, J.C. Hillesheim, G. Wang, L. Zeng, E.J. Doyle, G.R. McKee, Z. Yan, M.W. Shafer, A.E. White, J. Candy, R.E. Waltz, J.E. Kinsey, G.M. Staebler, J.C. DeBoo, R. Prater, K.H. Burrell, C.C. Petty, M.A. Makowski Comparisons of nonlinear GYRO simulations of DIII-D discharges with low and high elongation against experimental measurements are presented. Comparisons of measured low-$k$ density spectra (from beam emission spectroscopy), intermediate-$k$ density spectra (from Doppler backscattering), and low-$k$ electron temperature spectra (from correlation electron cyclotron emission radiometry) to synthetic spectra predictions at multiple flux-surfaces are shown. Results using both direct fits to profile measurements and flux-matching profiles predicted by the new TGYRO code [1] are presented, with agreement between model and experiment assessed via a simple set of newly developed validation metrics. \vskip4pt \noindent [1] J. Candy, et al., General Atomics Report GA-A26380 (2009). [Preview Abstract] |
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TP8.00013: Study of the Variation of Turbulence with Poloidal Angle on DIII-D Using Phase Contrast Imaging J.C. Rost, M. Porkolab, J.R. Dorris, K.H. Burrell The Phase Contrast Imaging (PCI) diagnostic on DIII-D, which measures density fluctuations with $k < 30$ cm$^{-1}$, has operated with three different beam paths; vertical through the last closed flux surface at the outboard midplane (edge), 11 deg from vertical through the outer plasma reaching $r/a = 0.75$ (Phase I), and now vertical through $r/a = 0.4$ (Phase II). PCI measures modes propagating perpendicular to the beam path, i.e. $k_\theta < 0.1\ k_r$ with the edge path and $k_\theta \sim k_r$ at the edge in Phase I and II. The beam paths also sample the turbulence at different poloidal angles. Results from the three beam paths combined provide a more complete description of the turbulence. Results from the edge and Phase I paths show that the turbulence phase velocity varies more with $k_\theta /k_r$ than is accounted for by Doppler shifts. Also, the spectra $S(k)$ show a complex variation in shape with $\theta$ and $k_\theta /k_r$. Initial results from Phase II will be presented to help disambiguate the various possible dependencies. [Preview Abstract] |
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TP8.00014: Measurements of the Wavenumber Spectrum of Multi-scale Turbulence in the Core of DIII-D H-mode Plasmas L. Schmitz, G. Wang, J.C. Hillesheim, W.A. Peebles, T.L. Rhodes, E.J. Doyle, L. Zeng, C. Holland, A.E. White, G.R. McKee, J.C. DeBoo, J.S. deGrassie, K.H. Burrell, C.C. Petty The wavenumber spectrum and spectral index of multi-scale turbulence $(0.5 \leq k_\theta \rho_s \leq 6)$ in the core of L- and H-mode DIII-D plasmas have been determined by Doppler Backscattering (DBS). A comparison is made to quasi-linear spectra from the trapped gyro-Landau fluid code (TGLF) to assess the contributions of ITG and TEM/ETG turbulence to transport fluxes $(0.4 \leq r/a \leq 0.8)$. Initial results of comparisons to spectra calculated via nonlinear gyrokinetic (GYRO) simulations are also presented. In high temperature, low density plasmas, ITG-scale and intermediate-scale core turbulence is found reduced by at least an order of magnitude across the L- to H-mode transition $(T_i/T_e \geq 2)$. This reduction is attributed to the combined effects of reduced turbulence drive and increased core $E\times B$ flow shear in H-mode. [Preview Abstract] |
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TP8.00015: High Resolution Density Profile Measurement for Electron Transport Studies in DIII-D L. Zeng, E.J. Doyle, T.L. Rhodes, W.A. Peebles, C.C. Petty, J.C. DeBoo, W.M. Solomon, T. Tala High temporal (to 10 $\mu$s) and spatial ($\sim$0.5 cm) resolution measurements of density profile evolution via profile reflectometry have been performed in a variety of plasma conditions in DIII-D. These measurements are currently applied to study particle transport. As one example, in a D$_2$ gas puffing modulation experiment, it is observed that the $n_e$ modulation can propagate from the edge inward to $\rho\sim 0.4$. The dependence of the modulations on $q$ and collisionality is under investigation. In an experiment with EC power alternately deposited at two close positions, it has been seen that the $n_e$ profile is significantly modulated by the EC modulations when the EC power at two locations is unbalanced. The $n_e$ modifications become smaller when the EC power is balanced. For these $n_e$ modulation datasets, an FFT transport analysis technique will be applied to investigate transport coefficients and particle pinch velocity. In addition, the associated $n_e$ fluctuations over a broad range of $k$ will be presented. [Preview Abstract] |
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TP8.00016: Analysis of Particle Transport in DIII-D H-mode Plasma with a Generalized Pinch-Diffusion Model L.W. Owen, W.M. Stacey, R.J. Groebner, J.D. Callen, X. Bonnin Interpretative analyses of particle transport in the pedestal region of H-mode plasmas typically yield diffusion coefficients that are very small ($<$0.1 m$^2$/s) in the steep gradient region when a purely diffusive particle flux is fitted to the experimental density gradients. Previous evaluation of the particle and momentum balance equations using the experimental data indicated that the pedestal profiles are consistent with transport described by a pinch-diffusion particle flux relation [1]. This type of model is used to calculate the diffusion coefficient and pinch velocity in the core for an inter-ELM H-mode plasma in the DIII-D discharge 98889. Full-plasma SOPLS simulations using neutral beam particle and energy sources from ONETWO calculations and the model transport coefficients show good agreement with the measured density pedestal profile. \vskip6pt \noindent [1] W.M. Stacey and R.J. Groebner, Phys. Plasmas {\bf 12}, 042504 (2005). [Preview Abstract] |
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TP8.00017: Analysis of Pedestal Transport J.D. Callen, R.J. Groebner, T.H. Osborne, J.M. Canik, L.W. Owen, A. Pankin, T. Rafig, T.D. Rognlien, W.M. Stacey To clarify plasma transport properties in the pedestal, we have undertaken an H-mode Edge Pedestal (HEP) Benchmarking Exercise (BE) for a single DIII-D pedestal. Codes used include 1.5D interpretive (ONETWO, WMS), 1.5D predictive (ASTRA) and 2D (SOLPS, UEDGE) codes. The particular DIII-D discharge considered is 98889, which has a typical low density H-mode pedestal. Transport properties are analyzed in near transport equilibrium between Type I ELMs. Both 2D and 1.5D transport properties are obtained and compared. Inferred radial diffusivities are smallest near the midpoint of the pedestal, about $0.1 - 0.3$ m$^2$/s for electron and ion heat but 10 times smaller for net particle transport. The small effective particle diffusivity could be the result of an inward particle pinch nearly balancing a diffusive outward radial particle flux. These and other pedestal plasma transport properties will be discussed. [Preview Abstract] |
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TP8.00018: Further Development of a Predictive Pedestal Model P.B. Snyder, R.J. Groebner, A.W. Leonard, T.H. Osborne, H.R. Wilson The pressure at the top of the edge transport barrier (or ``pedestal height'') strongly impacts tokamak fusion performance, and first principles prediction of the pedestal height remains an important challenge. A recently developed model, EPED1, combines a calculated peeling-ballooning stability constraint with a simple equation describing kinetic ballooning mode (KBM) onset to yield a predictive model of the pedestal height and width. The model has been successfully tested under a wide range of conditions on several tokamaks, included in a dedicated experiment where predictions were made before the experiment was conducted. Here we discuss continuing development of the model, including a more comprehensive KBM model based directly on gyrokinetic calculations, and improved treatment of diamagnetic stabilization. [Preview Abstract] |
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TP8.00019: Test of a Model for Limits to Pedestal Pressure Gradient in DIII-D R.J. Groebner, P.B. Snyder, T.H. Osborne, A.W. Leonard, T.L. Rhodes, L. Zeng, Z. Yan, G.R. McKee The EPED1 pedestal model predicts that kinetic ballooning modes (KBM) limit the magnitude of the H-mode pedestal pressure gradient, prior to the onset of the ELM instability. Time-resolved measurements of the total pedestal pressure profile are used to test this hypothesis in Type I edge localized mode (ELM) discharges in DIII-D. These measurements show that there is a significant spatial and temporal variation of the pedestal pressure gradient as the pedestal builds up between ELMs. In some regions of the pedestal, particularly in the outer half of the pedestal, the pressure gradient becomes nearly time-stationary long before an ELM crash. When this happens, increases of pressure gradient often are observed in other regions of the pedestal. Thus, the entire pedestal pressure gradient does not usually saturate at one time. The observed pressure gradients will be examined to see if they scale with predictions of linear theory for ballooning modes. [Preview Abstract] |
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TP8.00020: Quiescent H-mode Plasmas in DIII-D with Strong, Co-Current Edge Rotation K.H. Burrell, T.H. Osborne, P.B. Snyder, R.J. Groebner, P. Gohil, M.E. Fenstermacher, W.M. Solomon For the first time in any tokamak, quiescent H-mode (QH-mode) plasmas have been created with neutral beam injection in the direction of the plasma current (co-injection) and with edge rotation in the co-current direction. Previous QH-mode plasmas have always exhibited counter-current rotation of the plasma edge. The existence of QH-mode with strong edge co-rotation is a confirmation of the theoretical prediction that QH-mode should exist with either sign of the edge rotation provided the magnitude of the shear in the edge rotation is sufficiently large. In addition, detailed comparison of the edge plasma conditions with peeling-ballooning mode stability theory shows good agreement with that theory. This extends the previous detailed confirmation of the theory to co-rotating QH-mode plasmas. Furthermore, the existence of QH-mode with edge co-rotation demonstrates that counter NBI and counter edge rotation are not essential conditions for QH-mode. [Preview Abstract] |
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TP8.00021: H-Mode Pedestal Structure and Transport in Hybrid Plasmas During Magnetic Perturbation in the DIII-D Tokamak B. Hudson, A.W. Leonard, T.E. Evans, T.H. Osborne, C.C. Petty, P.B. Snyder The effect of resonant magnetic perturbation (RMP) on the H-mode pedestal structure is studied in hybrid discharges in the DIII-D tokamak. The empirical window for complete edge localized mode (ELM) suppression appears to be the same as in standard H-mode plasmas, which is $3.5 < q_{95} < 3.9$. A reduction in the pedestal bootstrap current during RMP is inferred through a decrease in $q_{95}$, at fixed $I_p/aB_T$, consistent with the measured reduction in the edge pressure gradient. Statistical analysis indicates that magnetic and MSE measurements are consistent with an edge parallel current profile given by the Sauter bootstrap current model. Small amplitude ELMs are observed to return, when the rotation frequency becomes small or when $q_{95}$ is outside the resonance window for ELM suppression, associated with an increase in pedestal electron temperature and a decrease in the calculated magnetic field line diffusion. The EPED1 model and ELITE code are used to predict the pedestal height and determine ELM stability. [Preview Abstract] |
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TP8.00022: Non-Ideal ELM Stability and Non-Axisymmetric Field Penetration Calculations with M3D-C1 N.M. Ferraro, M.S. Chu, P.B. Snyder, S.C. Jardin, X. Luo Numerical studies of ELM stability and non-axisymmetric field penetration in diverted DIII-D and NSTX equilibria are presented, with resistive and finite Larmor radius effects included. These results are obtained with the nonlinear two-fluid code M3D-C1, which has recently been extended to allow linear non-axisymmetric calculations. Benchmarks of M3D-C1 with ideal codes ELITE and GATO show good agreement for the linear stability of peeling-ballooning modes in the ideal limit. New calculations of the resistive stability of ideally stable DIII-D equilibria are presented. M3D-C1 has also been used to calculate the linear response to non-axisymmetric external fields; these calculations are benchmarked with Surfmn and MARS-F. New numerical methods implemented in M3D-C1 are presented, including the treatment of boundary conditions with C$^1$ elements in a non-rectangular mesh. [Preview Abstract] |
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TP8.00023: 3D Equilibria for DIII-D ELM Suppression Experiments E.A. Lazarus 3D equilibria have been successfully calculated for DIII-D ELM-suppression experiments in the n=3 configuration using the VMEC equilibrium code. These include the usual poloidal coils as well as the I-coils that produce the perturbation suppressing the ELMs, the error correction coils (C-coils) and the 24 discrete B-coils that produce the TF ripple. No stellarator symmetry is assumed. Relatively minor changes to VMEC enabled such solutions to be obtained, Details of the solutions will be discussed. Solutions take of order a cpu-day per case. Thus far we have used $N_{fp}=$ 3, and obtained solutions with toroidal mode numbers $-24\le n\cdot N_{fp} \le 24$, poloidal mode numbers $0\le m\le 12$, and 193 radial zones. We expect to increase the radial resolution and poloidal mode upper bound. Such solutions allow investigation of the possibility that the interaction of the I-coils with the TF ripple plays a role in the observed phenomenology. Results will be presented. [Preview Abstract] |
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TP8.00024: Simulations of Nonlinear ELM Instabilities in Tokamaks T.E. Evans, A. Wingen, K.H. Spatschek, C.J. Lasnier A model describing the 3D nonlinear evolution of ELMs in DIII-D has been developed. The model invokes field-aligned currents flowing through short flux tubes embedded in the pedestal plasma that connect the outer to the inner target plate. These flux tubes are initially formed by field-errors and a field-error correction coil. Magnetic perturbations due to these currents cause the separatrix topology to evolve which increases the area of the flux tubes and the magnitude of the current. This creates a positive feedback loop. Simulation results are compared to fast IR camera images of ELM heat flux in the divertor. Good agreement between the calculated magnetic structures on the divertor components and camera images during an ELM cycle is found. The simulation predicts a bifurcation from an $n=1$ homoclinic tangle and an $n=2$ heteroclinic tangle as the separatrix topology evolves during the growth of an ELM. Implications for suppressing ELMs with external magnetic perturbations are also discussed. [Preview Abstract] |
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TP8.00025: Initial Results from Radiating Divertor Experiments with RMP ELM Suppression T.W. Petrie, N.H. Brooks, T.E. Evans, J.R. Ferron, A.W. Hyatt, T.C. Luce, P.A. Politzer, C.C. Petty, M.J. Schaffer, M.E. Fenstermacher, C.J. Lasnier, G.D. Porter, J.G. Watkins, S. Mordijck The``radiating divertor'' has been posited as an important way of controlling heat flux at the divertor targets. Significant theoretical and experimental progress has been made during previous campaigns at DIII-D at identifying conditions leading to optimal radiating divertor operation [1]. During this same period, several studies at DIII-D clearly demonstrated that eliminating edge localized modes (ELMs) from H-mode plasmas using the resonant magnetic perturbation (RMP) approach could be an attractive possibility for solving the ``ELM-issue'' in ITER. Whether this ELM suppression approach is compatible with radiating divertor scenarios, however, has been an open question. In this presentation, we present results from the first attempts at combining active ELM by RMP suppression with radiating divertor scenarios. \vskip6pt \noindent [1] T.W. Petrie, et al., Nucl. Fusion {\bf 49} (2009) 065013. [Preview Abstract] |
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TP8.00026: Particle Exhaust and Scrape-off Layer Conditions During RMPs in Deuterium and Helium Discharges on DIII-D E.A. Unterberg, T.E. Evans, J.M. Canik, O. Schmitz, R. Maingi, N.H. Brooks The complete suppression of ELMs in a tokamak using the resonant component of a 3D magnetic perturbing field (RMP) has been demonstrated on DIII-D at ITER similar pedestal-$\nu_e^*$ and cross-sectional shapes. Recent analysis using global particle balance and measurements of the $D_\alpha$ poloidal distribution show that the wall inventory can be strongly affected by changing the average triangularity of the plasma. Further investigations using vacuum field-line tracing identified a bifurcation in edge plasma conditions and divertor pumping due to a difference in the perturbed separatrix in the two configurations and an apparent increase in the scrape-off layer neutral density. Comparisons with helium discharges will also be made. These results support a goal of understanding the role of particle sources and sinks during the RMP and demonstrate ELM suppression without significant wall pumping, a feature that is essential in long-pulse reactors with saturated walls. [Preview Abstract] |
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TP8.00027: Scaling of Type-I ELM Divertor Energy, Heat Flux, and Profile Width in DIII-D C.J. Lasnier, M.A. Makowski, J.A. Boedo, N.H. Brooks, A.W. Leonard, W.P. West, J.G. Watkins In this paper, we show scaling of divertor plate energy per ELM, peak heat flux and heat flux profile width of Type-I ELMs in DIII-D. We obtained data using an IR camera that viewed the divertor during discharges with large low-frequency ELMs. We independently varied the input power $P_{in}$, line-averaged density $\bar n_e$, and toroidal field $B_T$ at constant $q_{95}$, with other parameters (nearly) fixed. We also scanned $I_p$ while allowing the natural $\bar n_e$ (i.e., no gas puffing during the H-mode), with other quantities fixed. Previous scaling efforts have examined the pedestal energy loss during ELMs. We compare the deposited divertor energy with those analyses. We examine the fraction of total ELM energy reaching the target, the variation of ELM heat flux rise time, and the fraction of ELM energy deposited before the surface temperature peak. \vskip6pt \noindent [1] A.W. Leonard et al., Plasma Phys. Control. Fusion {\bf 44}, 945 (2002). \noindent [2] A. Loarte et al., J. Nucl. Mater. {\bf 313-316}, 962 (2003). [Preview Abstract] |
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TP8.00028: Characterization and Scaling of Heat Flux Transport in the DIII-D SOL M.A. Makowski, C.J. Lasnier, A.W. Leonard, J.A. Boedo In this study, we compare the divertor heat flux profile (width) to the upstream $T_e$ profile in DIII-D. It is generally thought that the parallel heat flow in the scrape off layer (SOL), in competition with cross-field transport, governs the heat flux to the divertor. The relative importance of conduction versus convection within the SOL is not yet completely quantified for many operating regimes. In this study, we focus on an operational regime that generates Type I edge localized modes (ELMs). Two diagnostics, a fast framing IRTV and a Thomson scattering system, allow us to examine the relative role of conduction and convection of the $T_e$ channel by comparing the upstream $T_e$ width with the heat flux width in the divertor. We will present data scalings for a number of parameters scans including variations of plasma current, plasma density, and toroidal field at constant safety factor and input power. [Preview Abstract] |
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TP8.00029: Characteristics of the Secondary Divertor on DIII-D J.G. Watkins, C.J. Lasnier, A.W. Leonard, T.E. Evans, R. Pitts, P.C. Stangeby, J.A. Boedo, R.A. Moyer, D.L. Rudakov In order to address a concern that the ITER secondary divertor strike plates may be insufficiently robust to handle the incident pulses of particles and energy from ELMs, we performed dedicated studies of the secondary divertor plasma and scrape-off layer (SOL). Detailed measurements of the ELM energy and particle deposition footprint on the secondary divertor target plates were made with a fast IR camera and Langmuir probes and SOL profile and transport measurements were made with reciprocating probes. The secondary divertor and SOL conditions depended on changes in the magnetic balance and the core plasma density. Larger density resulted in smaller ELMs and the magnetic balance affected how many ELM particles coupled to the secondary SOL and divertor. Particularly striking are the images from a new fast IR camera that resolve ELM heat pulses and show spiral patterns with multiple peaks during ELMs in the secondary divertor. [Preview Abstract] |
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TP8.00030: OEDGE Modeling of Deuterium Recycling in \hbox{DIII-D} J.D. Elder, P.C. Stangeby, S. Lisgo, A.W. Leonard, B.D. Bray, N.H. Brooks, M.E. Fenstermacher, M. Groth, J.A. Boedo, D.L. Rudakov, J.G. Watkins, E.A. Unterberg The OEDGE code is used to examine the role of deuterium recycling on core and pedestal fueling. In a previously modeled DIII-D discharge, 119925, with a partially detached inner divertor, attached outer divertor and significant plasma wall interaction, it is found that the core fueling is dominated by target and volume recombination sources in the inner divertor where 53\% of the neutral deuterium source accounted for 61\% of the core influx. In addition, main chamber recycling accounts for 24\% of the core influx but only 11\% of the source. Sensitivity studies assess the effect of physically plausible variations of the ``plasma background.'' The spatial distribution of the core neutral deuterium influx is reported for representative plasma solutions for attached, partially detached and fully detached divertors. [Preview Abstract] |
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TP8.00031: Some Divertor Scaling Considerations P.C. Stangeby A case is advanced for ``divertor non-scaling'', viz that absolute values of divertor density $n_d \sim 10^{21}$ m$^{-3}$ and temperature $T_d \sim$ 5 eV need to be achieved for optimal demo/reactor-relevant studies. For $T_d >$ 10 eV sputtering is very strong; for $T_d <$ 2 eV there is risk of detachment and density limit. High $n_d$ is required for high power, high duty cycle devices so that net erosion $\ll$ gross erosion via prompt local re-deposition of sputtered material. This occurs when impurity neutral ionization mean free path $\ll$ fuel ion gyro-radius (magnetic pre-sheath thickness); for $B\sim$ 5~T this requires $n_d\stackrel {>} {\sim} 10^{21}$ m$^{-3}$. Thus peak parallel power flux density $\sim 0.1 - 0.5$ GW/m$^2$. Modified two-point modeling then gives that: (a) ``upstream'' (e.g. outside midplane, separatrix), conditions, $n_{eu}$, $T_u$, are almost fixed, independent of R (device size) and P$_{\rm SOL}$ (power entering the SOL), and (b) the required P$_{\rm SOL}\sim$ R$^1$, R$^{1.5}$ or R$^2$, depending on assumptions about target power width; the latter are discussed. A test device with these absolute $n_d$, $T_d$ values will reproduce the most critical edge aspects of demo/reactors including power handling and material erosion/migration. [Preview Abstract] |
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TP8.00032: Neutral Fueling From Recycling and Contributions to Pedestal Density Profiles in DIII-D A.W. Leonard, R.J. Groebner, G.D. Porter Pedestal density widths in DIII-D are compared with the ionization profiles due to recycled neutrals. The neutral ionization profile is determined by neutral Monte-Carlo calculations based on ion flux measurements to the divertor and a plasma background reconstruction constrained by measurements. Previous studies have indicated that pedestal ionization is dominated by the inboard divertor neutral sources with smaller contributions from the main chamber in DIII-D. A density scan in H-mode is examined where the pedestal density width remains constant. The pedestal ionization rate increases at high density and inboard divertor detachment due to a longer mean free path for neutrals and an increased recombination source for neutrals. The possibility of an inward plasma pinch is explored as a possible explanation for the nearly constant pedestal density width under changing conditions of neutral sources and divertor plasma parameters. [Preview Abstract] |
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TP8.00033: Analysis of Neutral Recycling Fueling of the DIII-D Edge Pedestal Z.W. Friis, W.M. Stacey, A.W. Leonard, M.R. Rensink A detailed analysis of neutral atom recycling and pedestal fueling in a DIII-D high-confinement mode discharge is presented. Experimental data and 2D edge plasma fluid code (UEDGE) calculations are employed to provide ion wall recycling and recombination neutral sources and background edge plasma parameters for a 2D edge neutral code (GTNEUT) calculation of detailed neutral density, ionization and charge-exchange distributions throughout the edge pedestal, scrape-off layer and surrounding halo region, divertor, and private flux regions. The relative effectiveness of the different neutral sources recycling from the divertor and baffles for fueling the confined plasma is evaluated. [Preview Abstract] |
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TP8.00034: Generalized Diffusion Theory Calculations of the Edge Pedestal Density Profile J.-P. Floyd, W.M. Stacey It has been shown [1] that momentum balance requires a pinch-diffusion form of the radial particle flux in the edge plasma. Combined with the particle conservation requirement, this implies that the particle density satisfies a generalized diffusion equation with a convective term. This equation will be solved numerically and compared with the measured density profile in a DIII-D H-mode shot. \vskip6pt \noindent [1] W.M. Stacey, Contrib. Plasma Phys. {\bf 48}, 94 (2008). [Preview Abstract] |
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TP8.00035: Evaluation of Particle Pinch and Diffusion Coefficients in the Edge Pedestal of DIII-D H-mode Discharges W.M. Stacey, R.J. Groebner Momentum balance requires that the radial particle flux satisfy a pinch-diffusion relationship. The pinch can be evaluated in terms of measurable quantities (rotation velocities, $E_r$, etc.) by the use of momentum and particle balance [1,2], the radial particle flux can be determined by momentum balance, and then the diffusion coefficient can be evaluated from the pinch diffusion relation using the measured density gradient. Applications to several DIII-D H-mode plasmas are presented. \vskip6pt \noindent [1] W.M. Stacey, Contr. Plasma Phys. {\bf 48}, 94 (2008). \break \noindent [2] W.M. Stacey and R.J. Groebner, Phys. Plasmas {\bf 15}, 012503 (2008). [Preview Abstract] |
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TP8.00036: Intrinsic Rotation of Pellet Ablation Clouds P.B. Parks, T. Lu, R. Samulyak The finite resistivity code Frontier-MHD [1] is used to simulate the ablation rate of refueling pellets, including the effect of electrostatically induced $E\times B$ rotation of the ablation cloud about its symmetry axis parallel to the magnetic field [2]. The radial electric field $E$ is set up as a consequence of charge and current neutralization at the end sheaths of the ablation column. The key finding is that the centrifugal force of cloud rotation pushes the cloud density radially outwards, creating a more ``transparent" ablation channel. With reduced shielding, the {\it steady-state} ablation rate of a deuterium pellet significantly increases by $~35$\% to 100\%, depending on the $B$-field strength. This effect brings the ablation rate into better accord with a known theoretical scaling law, which agrees with most current experiments. However, the {\it transient} ablation rate is shown under the rather extreme situation of a fast pellet traveling across the steep-gradient pedestal region of ITER, to be a factor of 2 lower than predictions from the quasi-steady models. \vskip6pt\noindent [1] R. Samulyak et al., Nucl. Fusion {\bf 47}, 103 (2007). \break\noindent [2] P.B. Parks et al., Phys Plasmas {\bf 16}, 060705 (2009) [Preview Abstract] |
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TP8.00037: Shell Pellet Experiments on DIII-D E.M. Hollmann, A.N. James, J.H. Yu, N. Commaux, T.C. Jernigan, T.E. Evans, D.A. Humphreys, G.L. Jackson, P.B. Parks, E.J. Strait, W. Wu Injection of hollow shell pellets filled with a dispersive payload is a possible method for rapid shutdown of future large tokamaks to avoid wall damage during disruptions. Preliminary shell pellet experiments have been performed in the DIII-D tokamak by firing small (OD $\sim 2$ mm) polystyrene shells filled with either pressurized (10 atm) argon gas or with boron powder into quiescent discharges. Pellet slowing from 350 m/s down to 100 m/s was observed, which is not well-understood at present. Using the measured pellet velocity, the observed pellet burn up at $r/a \sim 0.5$, appears consistent with ablation rate calculations. Successful delivery and rapid ($<15~{\rm ms}$) dispersal of the pellet payloads into the plasma core was observed. Negligible plasma current contraction or MHD onset were seen as a result of the shell burn up in the plasma edge, consistent with calculations. Planned experiments with large (OD $\sim 1$ cm) shell pellets will also be discussed. [Preview Abstract] |
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TP8.00038: Development of a Robust Current Profile Feedback Controller in DIII-D Y. Ou, E. Schuster, J.R. Ferron, T.C. Luce, M.L. Walker, D.A. Humphreys, T.A. Casper, W.H. Meyer Setting up a suitable current profile has been demonstrated to be a key condition for one candidate advanced tokamak scenario with improved confinement and steady-state operation. A control-oriented model of the current profile evolution in DIII-D was recently developed for the plasma current ramp-up and early-flattop phases [1] and used to synthesize open-loop control schemes tested both in simulations and experiments [2]. A robust closed-loop controller is proposed to regulate the current profile under the presence of model uncertainties in the plasma resistivity. Using the POD/Galerkin technique, the control-oriented PDE model is reformulated into a low-dimensional ODE model that preserves the dominant dynamics, and an $H_\infty$ controller is designed to minimize the regulation/tracking error. A Corsica-based simulation assessment is presented. \vskip6pt\noindent [1] Y. Ou, et al., Fusion Engin. Design {\bf 82} (2007) 1153. \par\noindent [2] Y. Ou, et al., Plasma Phys. Control. Fusion {\bf 50} (2008) 115001. [Preview Abstract] |
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TP8.00039: Model-based Adaptive Control of Resistive Wall Modes in DIII-D F. Xie, E. Schuster, D.A. Humphreys, M.L. Walker One of the major non-axisymmetric instabilities under study in the DIII-D tokamak is the resistive wall mode (RWM), a form of plasma kink instability whose growth rate is moderated by the influence of a resistive wall. The General Atomics/FARTECH DIII-D/RWM dynamic model represents the plasma surface as a toroidal current sheet and the wall using an eigenmode approach. We report first on the experimental validation and reconciliation of the proposed dynamic model, which is a required step previous to the potential implementation in the Plasma Control System (PCS) of any model-based controller. The dynamic model is then used to synthesize an adaptive control law for the stabilization of the RWM under time-varying $\beta$ conditions. Simulation results are presented comparing the performance of the model-based adaptive controller and present non-model-based PD controllers. [Preview Abstract] |
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TP8.00040: MST, RFP, AND STELLARATOR |
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TP8.00041: Overview of MST Results and Plans J.A. Goetz MST progress in basic and fusion plasma physics investigations and the development of the RFP fusion configuration is reported. Pellet injection into high current, improved confinement plasmas allows the empirical density limit to be surpassed. The reduction in magnetic fluctuations and enhancement in energy confinement is not quite as large as for low-density plasmas. A heavy ion beam probe is providing measurements in the plasma interior to assess electrostatic turbulence when magnetic fluctuation-induced transport is reduced. A multi-pulse Thomson scattering diagnostic allows electron temperature dynamics to be investigated at up to 25 kHz. This new capability has been used for measurement of electron thermal transport through the sawtooth cycle. In addition, temperature fluctuations correlated with magnetic islands have been identified. The ion mass dependence and anisotropy (parallel vs. perpendicular) in non-collisional ion heating have been measured and are useful in testing proposed heating mechanisms. A new 1 MW, 20 ms, 25 keV neutral beam injector will be used to investigate energy and momentum deposition, MHD stability and the beta limit, and super-Alfvenic ion effects on tearing and kinetic instabilities. Progress on lower hybrid and electron Bernstein wave injection for current drive and heating, and on oscillating field current drive for current sustainment will be described. Work supported by USDoE and NSF. [Preview Abstract] |
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TP8.00042: Advances in pellet-fueled, improved confinement MST plasmas K.J. Caspary, B.E. Chapman, A.F. Almagri, J.K. Anderson, D.J. Clayton, D.J. Den Hartog, F. Ebrahimi, A.F. Falkowski, G. Fiksel, J.A. Goetz, S. Kumar, S.T. Limbach, R.M. Magee, M.B. McGarry, S.P. Oliva, E. Parke, J.A. Reusch, J.S. Sarff, H.D. Stephens, P. Franz, W.F. Bergerson, D.L. Brower, W.X. Ding, L. Lin, T. Yates, S.K. Combs, C.F. Faust Pellet injection fueling of improved confinement MST plasmas has led to a seven-fold increase in line-averaged density with a core density now exceeding 9x10$^{19}$ m$^{-3}$. This has been achieved by more than doubling the size of the injected pellets, relative to previous work on MST. The improved confinement is achieved using inductive current profile control. As was previously observed at low toroidal current (0.2 MA) in MST, pellet fueling has now allowed the Greenwald density to be surpassed at high current (0.5 MA). Utilizing new Thomson scattering capability for measurement with a 2 kHz repetition rate, the central electron temperature is observed to rise from 0.4 keV to a maximum 1 keV in only a few ms after pellet ablation. Some plasmas exhibit an increased core temperature lasting longer than the nominal duration of the auxiliary inductive current drive. Work supported by DOE. [Preview Abstract] |
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TP8.00043: Electron density and toroidal current density profile evolution in high-performance pellet-fueled MST plasmas L. Lin, D.L. Brower, W.X. Ding, W.F. Bergerson, T.F. Yates, K.J. Caspary, B.E. Chapman, J.S. Sarff High-density, high-$\beta $ plasmas have been achieved in MST using pellet injection in conjunction with inductive current profile modification. Here, we present measurements during such plasmas of the electron density (n$_{e})$ and toroidal current density (j$_{tor})$ profiles obtained using a three-wave far-infrared (FIR) laser polarimetry-interferometry diagnostic. Electron density profiles are obtained by inverting the line-integrated interferometry measurements. However, simultaneous measurements of the electron density gradient from a differential interferometry configuration are used to assist in removal of interferometric fringe jumps during the pellet ablation and improve the overall accuracy of the inverted density profiles. Toroidal current density profiles are measured by handling the polarimetry measurements with a parameterized functional fitting method. The evolution of n$_{e}$ and j$_{tor}$ during pellet ablation is resolved, and profile dynamics will be presented. [Preview Abstract] |
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TP8.00044: Quasi-Single-Helicity Structures in the MST Reversed-Field Pinch W.F. Bergerson, W.X. Ding, D.L. Brower, B.E. Chapman, J.S. Sarff, J.K. Anderson, D.J. Den Hartog, J.A. Reusch, H.D. Stephens Tearing modes produce a stochastic magnetic field, which degrades particle confinement in the reversed-field pinch (RFP), and characterize the multiple helicity (MH) state. MHD simulations predict an alternate to the MH state where the tearing modes condense into a single helical mode (SH). The term quasi-single helicity (QSH) describes plasmas with a dominant mode and smaller secondary modes. Studies have been carried out in the MST on both MH and QSH plasmas to investigate the fluctuation and confinement properties. For the first time the internal magnetic topology as well as magnetic and density fluctuations associated with these modes are measured directly in the plasma core using a high-speed, laser-based, polarimetry-interferometry diagnostic. The density and radial magnetic field fluctuations have been correlated in order to evaluate the magnetic-fluctuation-induced particle flux in the high-temperature core region. The electron temperature profile is 50 eV hotter in the region 0$<$ r/a $<$0.75 in the poloidal plane of the island O-point in QSH plasmas, as opposed to the X-point. Global particle confinement measurements are underway to identify properties of the QSH and MH states. This work is supported by the DOE. [Preview Abstract] |
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TP8.00045: Role of Electron Density Fluctuations in Momentum Transport in a Stochastic Magnetic Field W.X. Ding, D.L. Brower, W.F. Bergerson, L. Lin, T.F. Yates, A. Almagri, G. Fiksel, D.J. Den Hartog, S.C. Prager, J.A. Reusch, J.S. Sarff Plasma density fluctuations associated with magnetic fluctuations are not simply the result of passive advection. They can feed back on the plasma flow through fluctuating pressure, causing parallel momentum transport. The pressure fluctuations resulting from density fluctuations have been measured by using a high-speed polarimetry-interferometry (for both radial magnetic field and density fluctuations), Thomson scattering (for mean electron temperature), Rutherford scattering (for mean ion temperature). Measurements show that the density-fluctuation-induced momentum flux is comparable to the total momentum flux during a sawtooth crash. The temperature fluctuation-induced momentum flux is yet to be measured. Work supported by US DOE and NSF. [Preview Abstract] |
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TP8.00046: Measuring Density Fluctuations And Particle Transport On MST Using Laser Polarimetry-Interferometry Travis Yates, Weixing Ding, Troy Carter, David Brower Simultaneous interferometry-polarimetry measurements with a bandwidth $\sim $500 kHz and 8 cm chord spacing have been utilized to determine the core density and magnetic field fluctuations in MST. Density fluctuations arising from magnetic fluctuations contribute to electron particle transport. An outward radial flux of particles is caused by a non-zero correlation between velocity and density fluctuations. It is also possible to measure the energy balance, proportional to $\tilde {n}^2$, of density fluctuations during a sawtooth event where the stochasticity of the magnetic field is largest. Measurements show that density fluctuations cannot be balanced by the energy coming from the gradient in the mean plasma density profile. Initial results indicate that, for core modes, density fluctuations are unstable and likely damped by nonlinear coupling, proportional to the gradient of the fluctuating plasma density. Measurements will focus on nonlinear interactions between the dominant, core-resonant modes, (m,n): (1,7)-(1,6)=(0,1). At the sawtooth, a burst of (0,1) mode activity acts to enhance the 3-wave interaction. [Preview Abstract] |
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TP8.00047: Characterization of density and potential fluctuations with a Heavy Ion Beam Probe in MST improved confinement RFP plasmas X. Chen, P.M. Schoch, D.R. Demers, P.J. Fimognari, B.E. Chapman In improved confinement (pulsed poloidal current driven) plasmas in MST, magnetic fluctuations are suppressed, and electrostatic fluctuations may be the dominant transport mechanism. We have acquired measurements in these plasmas of the fluctuating electron density ($\tilde {n}_e /n_e )$ and potential ($\tilde {\phi })$ at multiple locations in the plasma interior with a heavy ion beam probe (HIBP). This allows us to evaluate the frequency spectra of the fluctuations up to 500kHz, assess wavenumbers, and estimate the electrostatic-fluctuation induced particle flux. Our analysis shows that the spectra of both $\tilde {n}_e /n_e $ and $\tilde {\phi }$ are broadband, and decrease with frequency, with most power below 100kHz. The features of the computed coherence and phase of $\tilde {n}_e /n_e $ at two spatial points are different than that of simultaneously acquired $\tilde {\phi }$. Fluctuation data acquired during improved confinement discharges, results of this analysis, and the role played by electrostatic fluctuations will be presented. [Preview Abstract] |
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TP8.00048: Heavy Ion Beam Probe Measurements of Equilibrium Potential in the Interior of the MST RFP D.R. Demers, X. Chen, P.M. Schoch, P.J. Fimognari The 200keV HIBP in operation on MST is being used to measure the equilibrium plasma potential in the interior of standard and improved confinement plasmas with 40-45keV K$^{+}$ beams. The injected beam angle is varied by an electrostatic sweep system which allows us to alter the sample volume location during a single shot. Simultaneous measurements at two sample locations within the plasma are obtained using two apertures to the high voltage electrostatic analyzer; high levels of UV radiation from the RFP plasma require use of the third detector set for noise subtraction. Toroidal and poloidal displacement of the sample volumes allows for inference of the electric field. Improved beam operation is enabling us to obtain increased ion current, and measurements as a function of time throughout the improved confinement period. Measurements in the interior of 380kA standard and improved confinement discharges reveal a positive electric potential. Variations in the potential as a function of discharge condition and individual characteristics will be presented. [Preview Abstract] |
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TP8.00049: Characterization of the Heavy Ion Beam Probe ion trajectory and sample volume in the MST RFP P.J. Fimognari, J.K. Anderson, J.A. Reusch, D.R. Demers, X. Chen Operation of an HIBP on MST is complicated by temporal and spatial variations of the beam trajectory and sample volume. The foremost contributors are the uncertainty and temporal changes of the equilibrium magnetic field, throughout a sawtooth cycle in standard discharges, or with edge current drive during improved confinement discharges. Other contributors include edge magnetic field errors, plasma suppression structure magnetic fields, and magnetic mode activity. Accurate modeling of each feature is crucial to sample volume shape and localization calculations used for the analysis of equilibrium plasma potential and fluctuations in electron density and potential. A finite element model of the primary and secondary beamline electrostatic sweep systems has also been developed. This simulation also enables studies of finite-cross-section beams and associated scrape-off effects. An overview of the HIBP system on MST and in-depth spatial and temporal characterization of the trajectory and sample volumes will be presented. [Preview Abstract] |
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TP8.00050: Simulated and Measured Electron Thermal Transport with Varying Stochasticity in the MST RFP J.A. Reusch, J.K. Anderson, D.J. Den Hartog, F. Ebrahimi, C.B. Forest, R. O'Connell, D.D. Schnack, H.D. Stephens Recent results show that the MST RFP exhibits Rechester-Rosenbluth thermal transport only at the sawtooth crash, when the MHD activity peaks. This is in contrast with previous results [Biewer, 2003], which suggested stochastic transport between sawteeth. Results from a new set of first principles simulations using the resistive MHD code DEBS will be presented. The simulations are designed to match experimental conditions as closely as possible. Both Spitzer and neoclassical resistivity models were tried for the case of a fixed, experimentally measured resistivity at a central Lundquist number of $3.8\times10^6$. The measured electron thermal diffusion, $\chi_e$, obtained through power balance is compared to the Rechester-Rosenbluth electron thermal diffusion obtained from DEBS, $\chi_{_{RR}} = v_{_{T_e}} \pi L_{_{eff}} \tilde{b}^2/B^2$, and the expected thermal diffusion from the field line tracing code MAgnetic Lines (MAL), $\chi_{_{MAL}}=v_{_{T_e}}D_{mag}$ where $D_{mag}=\left(\Delta r\right)^2/\left(2\Delta l\right)$, where the traced magnetic field is also from DEBS. $\chi_{_{RR}}$ is generally found to be an order of magnitude larger than $\chi_e$ except at the sawtooth crash, where the two are in agreement, suggesting that while magnetic fluctuations are always the dominant transport mechanism, the magnetic field is only fully stochastic at the sawtooth crash. This work is supported by the U.S. DOE. [Preview Abstract] |
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TP8.00051: Initial results from 2 kHz Thomson scattering measurements on MST Y.M. Yang, B.E. Chapman, D.J. Den Hartog, E. Parke, J.A. Reusch, H.D. Stephens The laser system for the Thomson scattering diagnostic on the MST RFP has been upgraded to a pulse-burst system enabling operation at a 2 kHz repetition rate. Single-shot evolution of the electron temperature (Te) profile can now be recorded. This capability is especially useful for analysis of plasmas with shot-to-shot variation, such as those with Enhanced Confinement (EC) or Pulsed Poloidal Current Drive (PPCD). In discharges with PPCD at toroidal plasma current (0.5 MA), the central Te increases by $\sim $1500 eV in 10 ms, reaching a maximum $>$ 2000 eV. Simultaneously, Te closer to the plasma boundary remains at a few hundred eV. For reasons not yet understood, the time evolution of the Te profile and maximum central Te can vary substantially shot to shot. A global transport analysis for 0.4 MA standard, non-reversed, EC, and PPCD plasmas will also be presented. The energy confinement time during EC and PPCD plasmas is calculated for specific shots and shows a factor 2-3 improvement during EC and $\sim $6 during PPCD, relative to standard discharges. [Preview Abstract] |
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TP8.00052: Pulse-Burst Laser Systems for Thomson Scattering on MST D.J. Den Hartog, M.T. Borchardt, W.S. Harris, J.A. Reusch, Y.M. Yang A new purpose-built ``pulse-burst'' laser system is being constructed for the Thomson scattering diagnostic on the MST reversed-field pinch. This new laser will produce a burst of 1--2 J $Q$-switched pulses at repetition rates 5--250 kHz. It will operate at 1064 nm and is a master oscillator, power amplifier (MOPA) system. Variable pulse-width drive (0.15--20 ms) of the flashlamps in this laser will be accomplished by IGBT switching of large electrolytic capacitor banks. A subset of these power supplies has already been constructed and is currently being used to drive the flashlamps in the two existing commercial Nd:YAG lasers used for Thomson scattering on MST. Each of these upgraded lasers now produces a burst of up to fifteen 2 J $Q$-switched pulses (1064 nm) at repetition rates 1--12.5 kHz. Direct control of the laser Pockels cell drive enables optimal pulse energy extraction, and up to four 2 J laser pulses during one flashlamp pulse. These lasers are currently being used to study the dynamic evolution of electron temperature in MST. The new purpose-built ``pulse-burst'' laser system will further expand this capability. [Preview Abstract] |
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TP8.00053: Two-Color SXR Tomography on MST M.B. McGarry, J.A. Goetz, D.J. Den Hartog, P. Franz Recent improvements in the analysis technique used to calculate plasma electron temperature using a soft-x-ray (SXR) tomography diagnostic on MST are presented. The diagnostic is comprised of four 20-channel cameras at the same toroidal angle but different poloidal angles. The two-color configuration uses two pairs of Be filters in the four cameras, giving measurements of two distinct energy ranges dominated by bremsstrahlung radiation. The ratio of emissivity in these two energy ranges gives a poloidal map of electron temperature. Improvements to the original inversion technique have reduced numerical oscillations. The model used to simulate SXR emission from the plasma has also been improved to incorporate measured density and temperature profiles. The model can also simulate line and recombination radiation. Furthermore, the model includes a radially dependent enhancement factor that can be used to assess other sources of radiation, as well as possible radial variations in Z$_{eff}$. Persistent artifacts in the temperature calculation are being investigated, and the improved technique will be applied to recent experimental results. Work supported by U.S.D.O.E. [Preview Abstract] |
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TP8.00054: Tomographic SXR emissivity measurements on MST P. Franz, F. Bonomo, M.B. McGarry, B.E. Chapman, J.A. Goetz, D.J. Den Hartog We present a survey of soft x-ray (SXR) measurements in standard and improved confinement plasmas in MST. Data are collected by a SXR tomographic diagnostic made of four 20-channel probes installed on a cross-section of the MST torus. The plasma has been analyzed using different thicknesses of Be foil filters, with values ranging between 15 and 821 microns, in order to select various energy ranges in the SXR spectrum. Correlation of the reconstructed SXR emissivity distributions with magnetic mode measurements enables investigation of MHD core dynamics in MST plasmas. For example during QSH (when one $m=1$ tearing mode dominates the MHD magnetic spectrum and all remaining modes have reduced amplitudes), SXR tomographic reconstruction shows a more emissive structure on top of a symmetric profile. Data have been taken in the \textit{one-color} configuration (same Be filter in all probes), \textit{two-color} (two pairs of Be foils in the four cameras) and \textit{multicolor} (four different Be filters). The \textit{one-color} configuration measures only SXR emissivity, while the \textit{two-color} configuration with thicker filters allows a reconstruction of the electron temperature from measurement of bremsstrahlung. [Preview Abstract] |
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TP8.00055: Similarities in QSH thermal measures in RFX-mod and MST experiments in high current regimes Alberto Alfier, Alessandro Fassina, Paolo Franz, Joshua A. Reusch, Hillary Stephens, Meghan McGarry , Daniel J. Den Hartog, Jay K. Anderson In the two RFP devices, Madison Symmetric Torus (MST) and RFX-mod, experiments have been recently performed at the aim of characterizing and comparing quasi single helicity (QSH) thermal properties. They have been dedicated to study the appearance of helical structures in the core region and to its transient evolution. Shots have been run at an electron density and current in the two devices such that the I/N parameter ($I$ plasma current, $N$ line averaged density) was in the range 3-9*10-14Am. Both spontaneous and induced QSH have been considered. Hot temperature structure, in some cases occupying most of the plasma core, is measured on both devices, corresponding to a more emissive plasma detected through the X-ray tomography. Their occurrence is found to be statistically related to the I/N values. Heat transport analysis in similar cases provides similar heat diffusivity profiles. [Preview Abstract] |
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TP8.00056: Measurement of $Z_{eff}$ and Particle Diffusion via X-ray Spectroscopy in MST D.J. Clayton, A.F. Almagri, J.K. Anderson, D.R. Burke, B.E. Chapman, C.B. Forest, R. O'Connell, R.W. Harvey Measured x-ray spectra constrain Fokker-Planck modeling of MST discharges and provide a means to determine the effective ionic charge $Z_{eff}$ and the radial particle diffusion coefficient $D_{r}$. A new radial array of Si photodiode detectors measures 2-10 keV x rays from thermal and runaway electrons in most plasma conditions. An array of CdZnTe detectors measures 10-150 keV x rays from high energy runaway electrons present only in plasmas with reduced stochasticity and improved particle confinement. The Fokker-Planck code CQL3D models the electron distribution function and predicts the resulting bremsstrahlung emission expected along each detector's line of sight. The code is run iteratively to find the $Z_{eff}(r)$ and $D_{r}(r)$ that produce the best fit to the data. Tests of this measurement technique with various plasma conditions will be presented, including largely stochastic plasmas with locally improved confinement within a magnetic island, and plasmas with reduced tearing modes and globally improved confinement. In the core of a typical high-temperture, low-density, improved-confinement plasma, $Z_{eff}$ = 4-6 and $D_{r}$ = 1 m$^{2}$/s. Work supported by the USDOE. [Preview Abstract] |
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TP8.00057: Parallel and perpendicular ion heating in the MST RFP Richard Magee, Daniel Den Hartog, Gennady Fiksel, Santhosh Kumar, Darren Craig The anomalous energization of plasma ions during magnetic reconnection has long been observed in numerous laboratory and astrophysical plasmas. In the MST reversed-field pinch, the reconnection impulsively heats the ions, more than doubling their temperature in $\sim $100 $\mu $sec. A critical outstanding question has been whether or not this heating is isotropic with respect to the magnetic field, which we attempt to address here with charge exchange recombination spectroscopy measurements. A new toroidal view allows localized measurements of the impurity ion (C$^{+6})$ temperature both perpendicular and parallel to the mean magnetic field. We observe that in the core of low-density discharges, the parallel temperature rise is comparable to the perpendicular rise, and a large increase in neutron flux occurs at the time of reconnection. In high-density discharges, the perpendicular temperature increase is the same as in low, but there is almost no heating seen in the parallel direction, and no increase in neutron flux is observed. Possible mechanisms will be discussed. [Preview Abstract] |
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TP8.00058: C$^{+6}$ impurity profiles in MST plasmas Santhosh T.A. Kumar, D.J. Den Hartog, R.M. Magee, G. Fiksel, D. Craig Temporally-resolved radial profiles of Doppler broadened C$^{+6}$ emission (343.4 nm) have been measured on the MST reversed-field pinch using Charge-Exchange Recombination Spectroscopy. Experimental observations indicate a hollow C$^{+6}$ density profile for both standard and improved confinement plasmas, throughout the duration of the diagnostic neutral beam pulse, even though the C$^{+6}$ ion temperature profile peaks at the center. Measurements are made on deuterium plasmas with density $\sim 1\times 10^{19} ~ m^{-3}$ and plasma current $\sim$ 400 kA. Experimental results are presented. [Preview Abstract] |
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TP8.00059: High Power Neutral Beam Injection on the MST Gennady Fiksel, J.K. Anderson, B.E. Chapman, S. Limbach, M. Nornberg, S.P. Oliva, J.S. Sarff, J. Waksman, G.F. Abdrashitov, V.P. Belov, V.I. Davydenko, P.P. Deichuli, A.A. Ivanov, V.A. Kapitonov, V.V. Kolmogorov, A.N. Selivanov, A.V. Sorokin, N.V. Stupishin, R.V. Voskoboinikov A high power neutral beam injection system has been installed on the MST RFP. The hydrogen beam is designed for a power of 1 MW, an energy of 25 keV, and a duration of 20 ms. The goals for the system include investigation of the beam energy and momentum deposition into the plasma, studying the dependence of MHD stability on the plasma pressure, and investigation of fast particles effects on tearing and kinetic instabilities in the MST. First tests of this system on MST are expected shortly. [Preview Abstract] |
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TP8.00060: Computational Simulation Of Neutral Beam Injection On MST Jeff Waksman, Jay Anderson, Gennady Fiksel MST has recently installed a 1 MW, 20 ms tangential neutral beam injection system. Understanding of initial measurements and guidance of future diagnostics relies heavily on accurate calculations of the beam characteristics. The transport code TRANSP was used to simulate the beam's effect on the plasma, such as shine-through, power and momentum deposition and current drive. While the code was originally developed for tokamaks, its Monte Carlo Beam Deposition and Slowing-Down Model is of particular interest for MST. Initial studies used real Grad-Shafranov equilibrium reconstructions of MST discharges without field reversal, satisfying a TRANSP requirement that the toroidal flux be a monotonic function of radius. Subsequent computations were performed on RFP equilibria modified in one of two different ways: either the plasma boundary was defined to be inside of the reversal surface, or a small artificial toroidal field was added near the boundary. Results from the simulations will be compared with the initial data from NBI operation on MST. Work supported by the U.S.D.O.E. [Preview Abstract] |
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TP8.00061: Oscillating Field Current Drive Experiments on MST K.J. McCollam, J.K. Anderson, D.J. Den Hartog, F. Ebrahimi, J.A. Reusch, J.S. Sarff, D.R. Stone, H.D. Stephens, D.L. Brower, W.X. Ding Oscillating field current drive (OFCD) is a proposed method of efficient, steady-state RFP sustainment in which applied AC poloidal and toroidal loop voltages interact with magnetic relaxation to produce a DC plasma current. OFCD is added to a standard RFP in the MST device, increasing the net plasma current by up to about 10\%, with Ohmic efficiency and without a net decrease in energy confinement or beta ($\beta\propto p/B^2$). The total current can be increased or decreased according to the phase between the two AC voltages. Time series of equilibria are reconstructed using internal measurements for the cases of OFCD with different phases and of OFCD off. Of these the OFCD case with the most added current is calculated to have, on cycle average, the highest confinement time and beta, the lowest fluctuation-induced dynamo electric field, and the lowest magnetic helicity decay rate. To date, increases in input power have not led to further increases in the OFCD-added current, perhaps due to discrete magnetic fluctuation events often observed at increased input power. This work is supported by the US DOE. [Preview Abstract] |
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TP8.00062: Magnetic Relaxation with Oscillating Field Current Drive on MST D.R. Stone, A.F. Almagri, G. Fiksel, K.J. McCollam, M.C. Miller, R.M. Magee, S.T.A. Kumar, J.S. Sarff, D.C. Brower, W.X. Ding, W.F. Bergerson In oscillating field current drive (OFCD), poloidal and toroidal frequency-matched ac magnetic fields are inductively applied to the plasma to drive dc plasma current through magnetic relaxation. Measurements of the MHD [$<\tilde {v}\times \tilde {B}>=<\tilde {E}_\bot \times \tilde {B}_\bot >$] and Hall [$\frac{<\tilde {j}\times \tilde {B}>}{ne}$] dynamo mechanisms associated with magnetic relaxation are conducted during OFCD both to better understand the relaxation dynamics and to aid in optimizing OFCD performance. Initial measurements of the MHD and Hall dynamo in the core and edge are reported. Charge exchange recombination spectroscopy and far-infrared interferometry-polarimetry are used in the core. Insertable probes are used in the edge, including a secondary emission capacitive probe developed to measure the electric fields. The fluctuation-induced magnetic helicity flux [$<\tilde {\phi }\tilde {B}_r >$] associated with magnetic relaxation is also measured in the edge. This flux is enhanced during OFCD by $\sim $100{\%} relative to standard RFP operation. This work is supported by the US DOE. [Preview Abstract] |
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TP8.00063: Lower Hybrid Antenna Coupling on MST M.C. Kaufman, D.R. Burke, J.A. Goetz, C.B. Forest The particular constraints of MST lead to the use of a novel interdigital-line structure rather than the traditional waveguide grill antenna for launching lower hybrid waves. The antenna has been designed to launch the slow wave at 800 MHz and an $n_{||}$ of 7.5 with maximum absorption at r/a $\sim$ 0.8 for current drive scenarios. While there are several drawbacks to this type of antenna including the lack of fine phasing control, the launched spectrum displays fairly good directivity. Loading studies indicate that the antenna operates well in a variety of plasma conditions, and agrees well with theory. With LH wave injection, toroidally localized hard x-rays in standard plasmas with energies up to 50 keV have been observed. Additional x-ray measurements at the antenna indicate that the progenitor fast electrons are lost more quickly than the ohmic field can accelerate them to the inferred velocities. Monte Carlo modeling shows that the emission is likely the result of edge interaction of electron gyro-orbits with gradients in the antenna near field. [Preview Abstract] |
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TP8.00064: Lower hybrid wave absorption on the MST D.R. Burke, A.F. Almagri, D.J. Clayton, C.B. Forest, J.A. Goetz, M.C. Kaufman Rf waves near the lower hybrid frequency have long been used to accelerate electrons in tokamaks. Lower hybrid wave injection experiments are underway on the MST RFP targeting electrons at r/a $\sim$ 0.8 (where $\nabla J_{\parallel}$ is steep) with the goal of stabilizing tearing fluctuations. Fast electrons are observed through bremsstrahlung emission using an Amptek Si-PIN photodiode to measure x rays from 3-10 keV. Increased x ray flux is observed during rf experiments, toroidally localized near the antenna. This is believed to be caused by the high field winding ($q \simeq 0$) and the high radial diffusion in the RFP edge. Bremsstrahlung emission is enhanced by inserting a probe with a molybdenum tip into the edge, which provides a higher Z target than the plasma, making edge emission more prominent relative to core emission. This probe has been refined, providing a plasma-free line of sight, and an interchangeable target. These probe studies reveal more about the local electron distribution function. Computational studies of wave propagation and absorption have also been undertaken for various plasma conditions, using a Grad-Shafranov fitter coupled with GENRAY (a ray-tracing code) and CQL3D, a Fokker-Planck equation solver. Results from target and computational studies are presented. [Preview Abstract] |
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TP8.00065: Development of a MW-Level EBW Heating and Current Drive Experiment on the MST Jay Anderson, Cary Forest, Andrew Seltzman The electron Bernstein wave experiment on MST is motivated by its potential to deposit heat and drive off-axis current in the RFP plasma. Positive results of emission, coupling and 100kW-level injection experiments compel the development of a new megawatt-level heating and current drive experiment. A 1.2 MW klystron which operates at 5.5 GHz has been selected as a transmitter. Construction is underway on an 80kV 40A IGBT-based modulated power supply to drive the tube, following the design and utilizing spare parts from the Los Alamos National Laboratory on a similar pulsed supply. A half-wavelength quartz window is designed and has been bench-tested, and a cylindrical molybdenum antenna will be used as the initial launching structure. This experiment is at higher frequency (shorter wavelength) than the previous MST injection experiments (3.6 GHz) and therefore the characteristic size of the antenna is reduced. This is favorable from the standpoint of small ports in the vacuum vessel and extension to multiple tubes and multiple antennas in the future. Initial coupling data from the molybdenum launcher are presented. [Preview Abstract] |
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TP8.00066: Electron Bernstein Wave Injection and Heating in MST Andrew Seltzman, Jay Anderson, Cary Forest The electron Bernstein wave (EBW) has been suggested as a potential means of further improving RFP confinement with sustained, off-axis current drive to stabilize the resistive tearing modes which govern thermal transport. An S-band EBW heating system was tested on MST resulting in measurement of localized soft-x-ray emission concurrent with rf injection. In order to further investigate coupling, an insertable rf probe was constructed. This allowed measurement of the transverse electric field of the x-mode wave as it crossed the right-hand cutoff layer and showed evanescent decay of the electric field amplitude. This motivated the fabrication of an improved RF probe with a dual axis pair of dipole antennas and pair of loops to measure the electric and magnetic components, respectively, of the injected wave. Upcoming analysis will enable verification of conversion of the injected x-mode wave to the electron Bernstein wave at the upper hybrid resonance layer. Further, the rf probe may measure the parametric decay instability, providing further evidence of coupling to the EBW mode. [Preview Abstract] |
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TP8.00067: Modeling of EBW Propagation and Damping in MST S.J. Diem, J.K. Anderson, C. Forest, A. Seltzman, R.W. Harvey Edge current profile modification is being explored to improve particle and energy transport in the Madison Symmetric Torus (MST) reversed field pinch (RFP). To this end, we report on modeling results of an electron Bernstein wave experiment under development in MST. Numerical modeling of EBW propagation and damping has been explored using the GENRAY ray-tracing and CQL3D Fokker-Planck codes in support of this upgraded 5.55 GHz EBW heating and current drive system. Calculations were performed for EBWs launched with a 4.5 cm poloidal extent, -19 degrees below the midplane, to investigate off-axis current drive. Current was driven at $\rho$/a $>$ 0.65 with current drive efficiency $>$ 10 kA/MW via the Ohkawa current drive method. The effect of high RFP diffusion on the current drive efficiency was investigated by varying the radial transport coefficient included in CQL3D. Additionally, CQL3D has been used to model the soft x-ray flux resulting from the EBW distortion of the electron distribution function, for comparison with experimental data. [Preview Abstract] |
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TP8.00068: Flow Dynamics and Transport in the Edge of MST M.C. Miller, A.F. Almagri, D. Craig, D.A. Ennis, G. Fiksel, S. Gangadhara, A. Kuritsyn, S.C. Prager, D. Stone, T.D. Tharp Understanding plasma flow dynamics is important to the study of reconnection, momentum transport, and particle transport. In MST, abrupt changes in flow patterns occur during quasi-periodic magnetic reconnection events (sawteeth). During these events, fluctuation levels are observed to increase in many measurable quantities and can lead to the transport of particles, momentum, and energy. Probes have been used in the edge to measure the three components of velocity and magnetic field, as well as density and temperature. This poster presents first time measurements of plasma flows associated with tearing reconnection. In momentum transport studies, the fluctuation-induced Maxwell and Reynolds stresses were unexpectedly found to be much larger than the rate of change in plasma momentum but approximately in balance with each other. Fluctuation-induced particle transport, measured directly as $\langle\widetilde{n} \widetilde{v_{r}}\rangle$, increases dramatically during a reconnection event, reaching several times its initial value. From these measurements we can begin to understand the plasma dynamics that lead to transport. Work is supported by the US DOE and the NSF. [Preview Abstract] |
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TP8.00069: Experimental study of anisotropic magnetic turbulence in a reversed field pinch Y. Ren, A.F. Almagri, G. Fiksel, S.C. Prager, J.S. Sarff, P.W. Terry In many space and laboratory plasmas with a background magnetic field, magnetic turbulence is predicted to be anisotropic, \textit{i.e.} the turbulence structure has a large perpendicular wavenumber ($k_{\perp}$) and much smaller parallel wavenumber ($k_{||}$). The observed small scale turbulence is often thought to cascade from large scale perturbations, forming inertial and dissipation ranges in the wavenumber power spectrum characterized with power and exponential laws, respectively. We carried out our experimental study of magnetic turbulence in MST, a medium-sized reversed- field pinch. MST plasmas exhibit a broad spectrum of magnetic fluctuations, ranging from low frequency coherent tearing modes (10-20 kHz) to high frequency fully developed turbulence (hundreds of kHz), and are ideal for studying magnetic turbulence. Radial profile measurements using a magnetic probe array show that the observed magnetic turbulence has a locally resonant feature, $\vec{k}\cdot\vec{B}$, and exhibits strong anisotropy with $k_{\perp}\gg k_{||}$ and $\delta B_{\perp} \gg\delta B_{||}$. We identified power and exponential law dependencies in the $k_{\perp}$ and $k_{||}$ spectra, which indicates a cascade picture for the origin of the small scale magnetic fluctuations, and the turbulence is shown to have a radial standing wave structure. We will discuss the observed magnetic fluctuation polarization and possible types of high frequency fluctuations. This work is supported by NSF and DoE. [Preview Abstract] |
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TP8.00070: Measurements of Hall Reconnection in MST T.D. Tharp, A.F. Almagri, M.C. Miller, V.V. Mirnov, S.C. Prager, J.S. Sarff Previous measurements in MST have established that two-fluid Hall effects produce a dynamo EMF during magnetic relaxation events (sawteeth), and therefore two-fluid dynamics are important to the macroscopic effects of reconnection. This Hall dynamo was established by measuring the nonlinear Hall term $(\tilde{J} \times \tilde{B})$ in the axisymmetric (flux-surface-averaged) Ohm's Law. Here, we report measurements of terms in the \em non\em-axisymmetric Ohm's Law, including the reconnection electric field and terms that balance this field. This analysis is applied to tearing modes with poloidal mode number $m=0$ and toroidal mode number $n\ge 1$. Measurements are performed with probes in the vicinity of the resonant surface, and clearly indicate that single fluid physics is not sufficient to explain this reconnection. In particular, nonlinear three-wave interaction Hall terms are a strong contributor to the dynamics of this reconnection and the associated island formation. [Preview Abstract] |
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TP8.00071: Numerical Studies of Two-Fluid Tearing and Dynamo in a Cylindrical Pinch Jacob King, Carl Sovinec, Vladimir Mirnov The nonlinear evolution of two-fluid tearing modes in a cylindrical pinch is investigated with the NIMROD code. Nonlinear single helicity evolution is examined by constraining the number of unstable modes with a small aspect ratio. Without ion gyroviscosity, the saturated island size and the combined nonlinear Hall and MHD dynamo profile are relatively independent of the ion sound gyroradius, $\rho _{s}$, and the Hall dynamo is broad relative to a linear prediction. However, at large $\rho _{s}$, the electron flow is modified into a more global structure, with the characteristic m=1 shift across the axis. This effect is not observed at small $\rho _{s}$. With ion gyroviscosity, the saturated island width decreases as $\rho _{s }$ is increased, indicating that the gyroviscous force opposes the driving forces. The saturation mechanism in the simulations is analyzed, and the amplitude and structure of the dynamo are compared with MST experimental results. Mode coupling and modal energy exchange are examined in a multihelicity case with an experimentally realistic aspect ratio. [Preview Abstract] |
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TP8.00072: Extended MHD Simulations of Tearing Instabilities and the Dynamo Effect in the Reversed-Field Pinch K. Germaschewski, J. Dearborn, A. Bhattacharjee Observations on MST indicate the importance of the Hall current in sawtooth crashes and the dynamo effect in a RFP. We employ our Magnetic Reconnection Code (MRC) to perform fully 3D extended MHD simulations in the RFP, including the Hall current and electron pressure gradient in a generalized Ohm's law. The MRC is an MPI-parallelized finite-volume based simulation code that integrates the extended MHD equations. It supports arbitrary curvilinear coordinate mappings, allowing it to be adapted to cylindrical and toroidal geometries. In order to overcome restrictive time-step limits, it uses implicit time integration. We have benchmarked the code for linear tearing instabilities, and performed fully nonlinear simulations. Due to the presence of the Hall current, novel vortical flows are seen in the vicinity of rational surfaces, akin to those seen in recent sawtooth studies in tokamaks, when the peak of the current density separates from the stagnation point of the flow. We calculate the dynamo field by averaging, and compare simulations with observations. [Preview Abstract] |
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TP8.00073: Relaxed states and finite pressure effects in the reversed field pinch Ivan Khalzov, Fatima Ebrahimi, Dalton Schnack , Vladimir Mirnov Two-fluid relaxation and finite pressure effects in the reversed field pinch (RFP) are studied. First, we analytically find the relaxed states of a cylindrical plasma column in standard (single-fluid) and Hall (two-fluid) MHD minimizing the energy with constraints imposed by invariants inherent in corresponding models. The relaxed state in Hall MHD is the force-free magnetic field with uniform axial flow and/or rigid azimuthal rotation. The relaxed states in standard MHD are more complex due to the coupling between velocity and magnetic field. $F-\Theta$ diagrams (reversal parameter vs. pinch) are obtained for different values of cross helicity and total angular momentum. Analytical results are also compared with numerical simulations performed with the extended MHD code NIMROD (nimrodteam.org). Second, we examine the behavior of resistive interchange instability at high beta RFP plasma. Equilibrium profiles from MSTFIT for high beta plasma are loaded into the Grad-Shafranov solver NIMEQ. Two-fluid and MHD stability analysis of the high beta MST plasma is then performed using NIMROD. The growth rate of high-$n$ localized interchange modes is found to be significantly reduced by finite Larmor radius effect. The work is supported by NSF and DOE. [Preview Abstract] |
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TP8.00074: The role of Nonlinear Ion Temperature Gradient Driven Drift Modes in a Reversed Field Pinch Plasma Varun Tangri, Paul Terry, R.E. Waltz The Ion Temperature gradient (ITG) mode has been rarely investigated in Reversed Field pinch (RFP) plasmas, although its role tokamak turbulence has been studied extensively. In this work, we investigate if it is plausible that ITG may play a role in particle and heat confinement in such devices. The linear stability and nonlinear saturation of ITG is investigated in the RFP geometry by modifying the gyrokinetic code GYRO\footnote{J. Candy and R.E. Waltz, J. Comp. Phys. 186, 545 (2003).} in a low beta, collisionless limit with and without non-adiabatic electrons. A simple toroidal equilibrium has been devised that is specified by just two parameters: the pinch parameter and the radial position. The level of transport is shown to be sensitive to temperature and density gradients and the threshold is found. To determine the nature of the instability, we study parametric scaling and also compare results with the well-known CYCLONE base case for tokamak simulations. We also estimate mixing level transport for MST parameters using linear simulations to determine if the instability is relevant to the small-scale turbulence observed in MST. [Preview Abstract] |
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TP8.00075: Gyrokinetic simulations of ITG turbulence in the Reversed Field Pinch I. Predebon, C. Angioni, S.C. Guo The Reversed Field Pinch geometry has been implemented in the gyrokinetic code GS2\footnote{M.~Kotschenreuther {\em et al.}, Comput. Phys. Commun. {\bf 88}, 128 (1995)}$^,$\footnote{W. Dorland {\em et al.}, Phys. Rev. Lett., {\bf 85}, 5579 (2000)}, to investigate the instability of ion temperature gradient (ITG) modes in typical plasmas of the RFX-mod device. The numerical results are found in agreement with previous analytical\footnote{S.~C.~Guo, Phys. Plasmas {\bf 15}, 122510 (2008)} and numerical\footnote{F.~Sattin {\em et al.}, Easter Plasma Meeting 2009, Torino, Italy} estimates, in the appropriate limits. Consistently, ITG modes are found to be only marginally unstable, in correspondence to the steep temperature gradients at the boundary of the helical structure in the quasi-single helicity states. We discuss the dependence of the ITG instability threshold on the relevant parameters, like density gradient and magnetic shear. The excitation of trapped electron mode (TEM) turbulence is also considered. [Preview Abstract] |
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TP8.00076: Overview of RFX-mod Results Piero Martin, Maria Ester Puiatti RFX-mod (R=2m, a=0.457m) is proceeding in its mission to explore RFP confinement in the plasma current range $>$1MA, to extend the knowledge base on MHD stability active control and to contribute to fusion science, in particular in the low-magnetic field regime. The main recent results is the discovery, at high plasma current (I$>$1 MA), of plasma self-organization in a 3d single helical axis equilibrium, with m=1,n=7 helicity [Nature Phys 2009]. The helical state has almost conserved magnetic flux surfaces, with strong core electron transport barriers. Electron temperature Te reaches 1.3keV @1.7MA. Ti is $\sim $(0.5-0.7)Te, consistent with collisional ion heating. Magnetic surfaces quality improves with current. Together with a global overview of the recent results, advances on active feedback control of tearing modes and RWM will be presented (RFX has a system of 192 independent feedback coils), as well as, in particular, the progress in understanding density limit and plasma edge physics. At the plasma edge temperature barriers are also observed and current density fluctuations are measured and interpreted as Drift-Alfven vortices, similarly to what happens in Earth magnetosphere. First wall lithization is under development. The flexibility of the device allows for low-current tokamak operation, for both edge turbulence and active MHD stability control studies. [Preview Abstract] |
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TP8.00077: Initial Exploitation of RFX-mod at 1.8 MA Plasma Current R. Cavazzana, S. Dal Bello, P. Franz, R. Lorenzini, G. Marchiori, G. Spizzo, M.E. Puiatti, M. Spolaore, D. Terranova, N. Vianello, L. Zanotto The recent remarkable results [1] of RFX-mod in the 1.5 MA range push forward the exploration of the confinement properties of Reversed Field Pinch (RFP) magnetic configuration at even higher current. During the first exploration campaigns aimed at increasing the plasma current, discharges beyond 1.8 MA have been obtained, highlighting the key issues needed to bring RFX-mod in the 2 MA current range: the most effective start-up and current ramping scheme; the conditioning of the graphite first wall in order to limit the hydrogen influx and the impurity concentration; the optimization strategy needed for tuning the active MHD mode control system (192 actuators, full coverage), aiming at the mitigation of plasma wall interaction still preserving the Helical States and their good confinement properties. \\[4pt] [1] Lorenzini R. et al. {\em Self-organized helical equilibria as a new paradigm for ohmically heated fusion plasmas } Nat.Phys. 2009 - doi:10.1038/nphys1308. [Preview Abstract] |
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TP8.00078: Existence of single helicity ohmic states in the RFP Dominique Escande, D. Bonfiglio, S. Cappello, P. Zanca The existence of ohmic single helicity (SH) stationary states for the reversed field pinch (RFP), a longstanding issue [1,2,3,4], is analytically proved in the frame of resistive MHD in cylindrical geometry by using perturbation theory for a paramagnetic pinch with a low edge axial magnetic field. A necessary criterion for this existence involves the radial profile of the logarithmic derivative of the Newcomb eigenfunction of the pinch. It suggests that imposing the radial magnetic field to vanish at the plasma edge is not optimal. Numerical simulations with the SpeCyl code check the dependence of the SH RFP states on the boundary radial magnetic field. The existence criterion is checked against the data of RFX-mod discharges in quasi SH states. \\[4pt] [1] V. D. Pustovitov, Pis'ma Zh. Eksp. Teor. Fiz. 35, 3 (1982) [JETP Lett. 35,1 (1982)] \\[0pt] [2] P. N. Vabishchevich et al., Fiz. Plazmy 9, 484 (1983) [Sov. J. Plasma Phys. 9, 280 (1983)] \\[0pt] [3] J. M. Finn et al., Phys. Fluids B4, 1262 (1992) \\[0pt] [4] S. Cappello et al., 26$^{th}$ EPS conference on Controlled Fusion and Plasma Physics, Maastricht 1999, p. 981. [Preview Abstract] |
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TP8.00079: Numerical reconstruction of spontaneous helical equilibria in RFX-mod M. Gobbin, A.H. Boozer, S.P. Hirschman, L. Marrelli, N. Pomphrey, D. Terranova The Reversed Field Pinch (RFP) plasmas can spontaneously access regimes in which a single mode dominates the magnetic perturbation spectrum (Quasi Single Helicity, QSH) [1]. For sufficiently large values of the dominant mode the magnetic island separatrix is expelled and a helical magnetic axis appears. This regime is called SHAx (Single Helical Axis) and is characterized by a helically symmetric plasma, with the periodicity of the dominant mode [2]. In order to describe the SHAx regime, the VMEC code has been adapted to the RFP configuration by using as flux coordinate the poloidal flux instead of the toroidal flux. In this work we present the first results obtained with the VMEC code reproducing the magnetic topology of a SHAx plasma in a fixed boundary approach by providing as input the safety factor profile, the shape of the last closed flux surface and taking into account also the effect of pressure. \\[4pt] [1] P.Martin et al., Plasma Phys. Control. Fusion 49 (2007) A177-A193\\[0pt] [2] R. Lorenzini et al., to be published on Nature Physics, DOI 10.1038/NPHYS1308 [Preview Abstract] |
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TP8.00080: Reversed Field Pinch helical self-organization studies with the volume preserving field line tracing code NEMATO D. Bonfiglio, M. Veranda, S. Cappello, L. Chacon, D.F. Escande, P. Piovesan The existence of a Reversed Field Pinch (RFP) dynamo as a (laminar) helical self-organization was anticipated by MHD numerical studies [1]. High current operation in RFX-mod experiment shows such a helical self-organization: strong internal electron transport barriers (ITB) appear and magnetic chaos healing is diagnosed when Single Helical Axis (SHAx) regimes are achieved [2]. We present results of the field line tracing code NEMATO [3] applied to study the magnetic topology resulting from 3D MHD simulations, with the aim of clarifying the conditions for chaos healing in SHAx states. First tests confirm the basic picture: the magnetic chaos due to island overlap is significantly reduced after the expulsion of the dominant mode separatrix. The possible synergy with the presence of magnetic and/or flow shear at the SHAx ITB will also be discussed [4].\\[4pt] [1] S. Cappello, Plasma Phys. Control. Fusion (2004) {\&} references therein \\[0pt] [2] R. Lorenzini et al., Nature Phys. (2009) \\[0pt] [3] J. M. Finn and L. Chacon, Phys. Plasmas (2005) \\[0pt] [4] M.E. Puiatti et al invited presentation EPS 2009 conference, submitted to Plasma Phys. Control. Fusion [Preview Abstract] |
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TP8.00081: Neoclassical transport in the helical Reversed-field pinch G. Spizzo, M. Gobbin, L. Marrelli, R.B. White In this work we study particle transport in the Reversed-Field Pinch magnetic field topology. The guiding centre test particle code \textsc{Orbit} is used. We find that passing particles, which in the chaotic multiple helicity (MH) state experience long-correlation L\'{e}vy flights, in the low-chaos, Single Helical Axis (SHAx) regime~\footnote{R.Lorenzini, E.Martines, P.Piovesan \emph{et al}, Nature Physics in press} remain well confined within magnetic surfaces. Residual diffusion is given by trapped particles and the associated neoclassical diffusion coefficient $D$ is explored as a function of collisionality. Helically trapped (superbanana) particles appear to provide negligible contribution~\footnote{M. E. Puiatti, In {\em Proc. of the 36$^{\mathrm th}$ EPS Conference on Controlled Fusion and Plasma Physics, Sofia (Bulgaria), 29 June- 3 July, 2009}}. As a result, we recover the classical Tokamak and Stellarator transition from the banana to the plateau and Pfirsch-Schl\"uter regimes, but without the $1/\nu$ dependence at low collisionality which is typical of the un-optimized stellarator. [Preview Abstract] |
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TP8.00082: Progresses in resonant modes control in RFX-mod Lionello Marrelli, Federica Bonomo, Paolo Piovesan, Lidia Piron, Gianluca Spizzo, Paolo Zanca, Barbara Zaniol The RFX-mod device is equipped with a sophisticated active feedback system which can produce non axisimmetric magnetic field aimed at controlling both MHD modes and reducing error fields. Due to its flexibility, the system can deal simultaneously with multiple harmonics. A proper tuning of the feedback laws on resonant $m=1$ modes allowed to put them into rotation and to reduce edge values of the radial fields, allowing to explore the high current regime ($I_p \leq 1.8 $MA). A recently developed model of the non-linear dynamics of resonant modes (also named Tearing Modes) has been used to try to further improve the feedback law. The simulated dependence of the edge radial field associated to Tearing harmonics has been compared to the results of a PID parameter scan campaign at $0.8 \div 1.1$ MA, confirming the presence of a minimum value for the edge radial field for optimal PID parameters. Statistical analysis of two important inputs of the code, namely $m=0$ modes phase relations and plasma flow velocity, as measured by passive spectroscopy of intrinsic impurities, will also be presented. [Preview Abstract] |
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TP8.00083: Pseudo-decoupler approach to error field correction in RFX-mod Paolo Piovesan, Marco Gobbin, Luca Grando, Giuseppe Marchiori, Lionello Marrelli, Lidia Piron, Anton Soppelsa, David Terranova An optimal design of feedback schemes for error field and mode control in fusion plasmas has to take into account the electromagnetic effects of real walls. The 3D wall structure introduces significant couplings among the plasma modes, affecting their stability and the feedback performance. A dynamic decoupling scheme, based on the experimentally identified mutual inductances among active coils and sensors, has been developed in the RFX-mod reversed-field pinch. RFX-mod is equipped with an advanced feedback system made of 192 saddle coils and 192 sensors fully covering the torus. A large subset of the 192$\times $192 mutual inductances was identified from vacuum shot measurements and a decoupling scheme based on a pseudo-inverse of this matrix was developed. Control schemes based on this approach were tested in the experiment. Error fields due to the non-uniform penetration of the vertical magnetic field through the shell gaps have been reduced. A similar strategy is being developed for tearing mode control, which may allow reducing their edge radial magnetic field component. [Preview Abstract] |
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TP8.00084: Characterization of magnetic field profiles at RFX-mod by Faraday rotation measurements Fulvio Auriemma, Matteo Brombin, Alessandra Canton, Leonardo Giudicotti, Paolo Innocente, Enrico Zilli A multichannel far-infrared (FIR, $\lambda $=118.8 $\mu $m) polarimeter has been recently upgraded and re-installed on RFX-mod to measure the Faraday rotation angle along five vertical chords. Polarimetric data, associated with electron density profile, allow the reconstruction of the poloidal magnetic field profile. In this work the setup of the diagnostic is presented and the first Faraday rotation measurements are analyzed. The measurements have been performed at plasma current above 1.2 MA and electron density between 2 and 6x10$^{19}$ m$^{-3}$. The actual S/N ratio is slightly lower than the expected one, due to electromagnetic coupling of the detectors with the saddle coils close to the polarimeter position. Due to this limit, only average information in the flat-top phase of the discharge could be so far obtained. The experimental data have been compared with the result of the $\mu ${\&}p equilibrium model [1], showing a good agreement between experiment and model, whereas the main differences are in the external region of the plasma. A different parameterization of the $\mu =\mu _{0}$ J$\cdot $B/B$^{2}$ profile has been proposed to enhance the agreement between model and experiment. \\[0pt] [1] Ortolani and Snack, World Scientific (1993) Singapore [Preview Abstract] |
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TP8.00085: Experimental background emissivity at the transitions useful for the active CXRS in RFX-mod F. Bonomo, M. Valisa, L. Carraro, M.E. Puiatti, B. Zaniol We present the investigation of the background emissivity of the transitions useful for active charge exchange (CX) spectroscopy. In particular, we focused on the Carbon and Oxygen emissivities at those transitions which could be attractive for the neutral beam diagnostic installed in RFX-mod. These transitions are: the n=8$\rightarrow$7 (529.0nm), n=7$\rightarrow$6 (343.4nm), and n=6$\rightarrow$5 (207.0nm) for C VI; the n=10$\rightarrow$9 (606.8nm) for O VIII. Their experimental background emissivities have been studied and compared to those predicted by an active CX simulation code (M.von Hellermann), in order to highlight which transition features the best signal to background ratio and to set up the most effective experimental arrangement for a charge exchange diagnostic on RFX-mod. [Preview Abstract] |
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TP8.00086: Transport and linear stability studies for PPCD optimization in RFPS Gian Luca Delzanno, John M. Finn, John Sarff We have combined 1D transport simulations of pulsed poloidal current drive (PPCD) together with linear stability studies for a wide spectrum of m=1 and m=0 modes. The present model assumes zero beta and a specified form of resistivity increasing near the edge. The model includes a ``dynamo'' term in Ohm's law which is gradually decreased to zero during the early part of the PPCD cycle, simulating the decrease in tearing mode activity as PPCD progresses. We present several initial studies with ad-hoc waveforms in time for the wall electric field, and measure the fraction of time over which m=1, m=0 stability is achieved. We have also developed a more systematic scheme by which we decrease the toroidal electric field at the wall and determine the poloidal electric field there by requiring the parallel electric field to be zero there. (This ensures zero helicity dissipation at the edge.) This programming is designed to match at a specific time a self-similar rampdown (SSRD) state [1]. After this time the wall electric field components are then programmed to decay exponentially. We have mapped out the full parameter space of these SSRD states and their linear stability to m=1 and m=0 modes. A PPCD scenario is considered optimal if it has a slow decay rate and develops into a SSRD state well inside the stable SSRD regime. \\[4pt] [1] R. Nebel, D. Schnack, and T. Gianakon, Phys. Plasmas 9, 4968 (2002). [Preview Abstract] |
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TP8.00087: High frequency MHD activity due to resistive interchange modes in a reversed field pinch plasma Matteo Zuin, Silvia Spagnolo, Roberto Paccagnella, Emilio Martines, Roberto Cavazzana, Gianluigi Serianni, Monica Spolaore, Nicola Vianello The results of an experimental activity aimed at investigating the origin of high frequency, high toroidal mode number (n$>$20) magnetic fluctuations in the RFX-mod reversed field pinch device are presented. The space-time properties of the fluctuations are characterized by means of 2 arrays of in-vessel toroidal and poloidal pick-up coils covering the whole toroidal torus, and by means of an insertable edge probe, housing two radial arrays of triaxial pick-up coils, closely spaced in the toroidal direction. High levels of magnetic fluctuation at frequencies above 30 kHz are observed when the reversal of the magnetic field at the edge is driven to deep values. These fluctuations are due to rotating coherent modes whose resonant radii are located between the plasma edge and the reversal surface. A comparison of the experimental spectral properties to the theoretical predictions by a linear MHD stability analysis for a RFP plasma allows to interpret the observed instabilities as resistive interchange modes. [Preview Abstract] |
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TP8.00088: Resistive g-modes and RFP confinement Jan Scheffel, Ahmed Mirza The role of pressure driven resistive modes in the reversed-field pinch remains unclear. It was early shown that unstable resistive $g$-modes would always exist for an inwardly directed pressure gradient. It now appears that pressure profile smoothing, due to inclusion of heat conductivity terms in the energy equation, enables completely stable RFP states at moderate plasma beta. These calculations, apart from being restricted to linearized perturbations, suffer from the use of rather forced scalings, thus their accuracy can be questioned. Also, they have so far only been applied to conventional RFP states, where confinement-limiting tearing fluctuations maintain the reversed axial magnetic field. In the advanced RFP, current profile control has largely eliminated current driven tearing modes. Fully nonlinear, numerical studies have shown that energy confinement and poloidal beta increase substantially, but that weak residual modes usually remain. The nature of these residual modes, which limit energy confinement, is studied using a novel semi-analytical, spectral scheme for solving the resistive MHD equations; the generalized weighted residual method (GWRM). Results from the analysis as well as comparisons with the competing linear resistive $g$-mode theories will be presented. [Preview Abstract] |
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TP8.00089: Progress in Low-Aspect-Ratio RFP Research in RELAX S. Masamune, A. Sanpei, R. Ikezoe, T. Onchi, K. Oki, M. Sugihara, Y. Konishi, S. Fujita, M. Nakamura, A. Fujii, H. Motoi, H. Himura, R. Paccagnella, A. Ejiri, D. DenHartog RELAX is a low-aspect-ratio RFP experiment with R=0.51m/a=0.25m (A=2). Initial results have shown the advantages of low-A RFP configuration with I$_{p}$ of up to 100kA and V$_{l}$ of down to $\sim$30V; the configuration tends to relax to a quasi-single helicity (QSH) state. A high-speed camera diagnostic has revealed simple helix structure. In the extreme case, rotating Helical Ohmic equilibrium state has been realized. The pressure driven bootstrap current fraction is shown to be less than 5\% of the total current in the present RELAX plasmas. Recent progress includes a test of field error compensation; the control system will also be used in feedback control of RWM. The electron density measurements with a 104 GHz microwave interferometer show that the density lies in the range from $\sim$5 $\times$10$^{18}$m$^{-3}$ to $\sim$10$^{19}$m$^{-3}$. Design study of Thomson scattering system is also in progress. [Preview Abstract] |
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TP8.00090: Imaging Diagnostics for MHD Studies in Low-Aspect-Ratio Machine RELAX T. Onchi, R. Ikezoe, K. Oki, Y. Konishi, M. Sugihara, S. Fujita, A. Sanpei, H. Himura, S. Masamune, N. Nishino Visible-light and soft-X ray imaging diagnostics have been applied to a low-aspect-ratio RFP machine RELAX for the study of MHD dynamics. The visible-light images with a high-speed (80,000fps) camera from a tangential port of RELAX have revealed time evolution of the simple helical structure for the first time in the RFP. The visible-light helical structure agrees well with magnetic structure of the helically deformed RFP (Helical Ohmic RFP), rotating toroidally in the same direction as the magnetic fluctuation. The simple helical structure is one of the results of low-A nature of RELAX. A soft-X ray (SXR) imaging diagnostic technique has also been developed for MHD studies in RELAX. In this diagnostic, a 3-D magnetic structure in the low-A RFP plasma is reconstructed from simultaneously measured tangential and vertical SXR images. Initial results from tangential SXR imaging will be reported, in comparison with the visible-light images of the simple helical structure. [Preview Abstract] |
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TP8.00091: Rotating Helical Ohmic Configuration in a Low-Aspect-Ratio Reversed Field Pinch Kensuke Oki, Ryuya Ikezoe, Takumi Onchi, Shin-Ichi Fujita, Akio Sanpei, Haruhiko Himura, Sadao Masamune, Roberto Paccagnella Recent theoretical studies have shown that a low-aspect-ratio Reversed Field Pinch (RFP) may have several advantages. In a low- aspect-ratio RFP machine ``RELAX'', inner magnetic field profiles have been measured by a radial array of magnetic probes inserted to the magnetic axis. In a type of plasma, the profiles largely oscillate and are good agreement with rotating ``helical Ohmic equilibrium'' (helical RFP configuration with a helical magnetic axis). The helical structure indicated by edge magnetic fields at various places corresponds to a helical instability mode having a singular surface in the core region. Thus, the helical deformation may be caused by growth of the single helical mode at the singular surface. It appears that this growth is enhanced by increase in separation of major singular surfaces due to low- aspect-ratio. [Preview Abstract] |
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TP8.00092: Toroidal rotation of localized dynamo event in a low-aspect-ratio RFP plasma R. Ikezoe, T. Onchi, K. Oki, Y. Konishi, M. Sugihara, S. Fujita, A. Sanpei, H. Himura, S. Masamune A burst of the edge magnetic fields has been observed in low density discharges in the low-aspect-($A)$ ratio RFP machine, RELAX. The burst occurs at a toroidally localized position, then rotating in the opposite direction to the plasma current at 10-20 km/s. The evolutions of the edge toroidal field and the toroidal flux in the toroidal section where the burst occurs show that the toroidal flux increases at the burst and the toroidal field profile is modified accordingly; the burst will be referred to as a localized dynamo event. The toroidal flux keeps rising between the bursts at much slower rate than at the burst. This indicates that the discrete dynamo event may be superimposed on the mild continuous dynamo in RELAX. The time evolution of soft-x-ray and $H_{\alpha }$ emissions, has indicated that the localized dynamo is closely related to particle and energy transport. We present the detailed characteristics of this burst phenomenon with emphasis on the effect of low aspect ratio. [Preview Abstract] |
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TP8.00093: Simultaneous measurement of electron temperature and density by a line pair method in the RFP plasma Masayuki Watanabe, S. Shimizu, H. Ogawa, T. Shinohara A line-pair-method has been applied for a simultaneous measurement of the electron temperature and density in ATRAS RFP plasma. Three helium spectrum lines (668nm, 706nm, 728nm) were measured during the discharge at the same time and the electron temperature and density is estimated by using a Collision-Radiation model. To get the signal of the helium impunity line from the RFP discharge, the RFP plasma in the hydrogen gas with a few mixed helium gas was formed. In the typical ATRAS RFP discharge of the plasma current of 60kA, the electron temperature was approximately 50-150 eV and the electron density is the order of 10$^{18}$ m$^{-3}$. During the discharge, the change of the temperature and density are mutually related and this correlation was the almost reverse phase. The periodically change of the temperature and density were also observed. This change synchronizes with a periodically increase of the averaged toroidal magnetic field, which is caused by the toroidal rotation of the increase of the toroidal magnetic field. This rotation, which is deeply related with dynamo effect, makes the plasma energy lose and particles also diffuse toward the plasma edge. As a result, the recycling of the particle and energy are occurred at the same time. [Preview Abstract] |
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TP8.00094: Effect of a strong equilibrium electric field on the zonal flow residual in quasisymmetric stellarators Matt Landreman, Peter Catto It was recently shown [1] that several neoclassical formulae for tokamaks can be significantly modified if the radial electric field is as large as that observed in a pedestal -- specifically, if $d\Phi/dr \sim T/(e \rho_{\mbox{pol}})$, where $\rho_{\mbox{pol}}$ is the poloidal ion gyroradius. In the present work, we consider the same modification of neoclassical quantities in quasisymmetric stellarators, beginning with the zonal flow residual [2]. The tokamak analysis in [1] exploited the conservation of canonical angular momentum. The analysis therefore requires modification in stellarators, in which canonical angular momentum is not conserved. In its place, one can use the ``helical momentum'' which is known to be conserved if the magnetic field is quasisymmetric [3].\\[4pt] [1] Kagan and Catto, \emph{Phys. Plasmas} {\bf 16}, 056105 (2009).\\[0pt] [2] Rosenbluth and Hinton, \emph{Phys. Rev. Lett.} {\bf 80}, 724 (1998).\\[0pt] [3] Boozer, \emph{Phys. Fluids} {\bf 26}, 496 (1983). [Preview Abstract] |
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TP8.00095: Transport in a stellarator with imperfect flux surfaces and quasisymmetry Harold Weitzner With the use of techniques developed in Refs. 1 it is possible to study low shear stellarator steady states. A steady state near a magnetic field with flux surfaces and approximate quasisymmetry is modified by the addition of small magnetic fields restricted only by the condition that the perturbation preserve the underlying stellarator symmetry. The perturbations constrain the profile functions of the electrons and ions. Equations for the profile functions are given and a simple model of a linearized collision operator is used to obtain further results. \\[4pt] [1] Phys. Plasmas 1, 3942 (1994); 4, 575 (1997); and 5, 417 (1998). [Preview Abstract] |
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TP8.00096: Rotation roots and neoclassical viscosity in quasi-symmetry A.J. Cole, C.C. Hegna, J.D. Callen In a quasi-symmetric device, there exists a symmetry angle $\alpha_h = \theta -N\zeta/M$, such that $|B| = B_0 \left(1 - \epsilon_h \cos{M \alpha_h} \right)$ along a field-line, with several much smaller helical `sidebands.' Provided the departure from symmetry is small, i.e. $\delta B_{\rm eff}/B_0 \ll \epsilon_h$ where $\delta B_{\rm eff}/B_0$ is the effective helical sideband strength, flow damping and thus flow evolution along and `cross' the direction of symmetry in a flux surface decouple [1,2], and can be determined successively. In the context of a fluid-moment approach [3], the momentum equation in the symmetry direction is equivalent to the ambipolarity condition. Steady state rotation solutions of this equation are equivalent to ambipolar radial electric field `roots' in conventional stellarator theory and will be presented for various banana-drift neoclassical flow damping regimes [2].\\[4pt] [1] J.~D.~Callen, A.~J.~Cole, and C.~C.~Hegna, Tech.~Rep.~UW-CPTC 08-7, Univ.~of Wisconsin, http://www.cptc.wisc.edu (2009).\\[0pt] [2] A.~J.~Cole, C.~C.~Hegna, and J.~D.~Callen, Tech.~Rep.~UW-CPTC 08-8, Univ.~of Wisconsin, http://www.cptc.wisc.edu (2009).\\[0pt] [3] K.~C.~Shaing and J.~D.~Callen, Phys.~Fluids 26, 3315 (1983). [Preview Abstract] |
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TP8.00097: Characterization of magnetic configurations of LHD in terms of boundary shape and the direction of their improvements Shoichi Okamura LHD in Japan is the largest stellarator / heliotron device with superconducting magnetic coils (major radius is 3.9 m with 3 T magnetic field). It has been demonstrating good confinement properties comparable to major tokamaks. For the magnetic configuration control, the vertical field control is very effective with which the confinement characteristics such as the magnetic field curvature and trapped particle trajectories are largely changed. For high temperature discharge experiments or high density discharge experiments, two distinct configurations have been used respectively: inward shifted configuration and outward shifted configuration which are produced by selecting different vertical field. In this paper, these two configurations are analyzed in terms of Fourier components of boundary shape for the purpose of understanding them with general physical quantities rather than specific device parameters. The direction of improving magnetic configurations for better confinement properties will be discussed. [Preview Abstract] |
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TP8.00098: New QP/QI optimized stellarator configurations D.A. Spong, J.H. Harris A unique characteristic of the quasi-poloidal/isodynamic transport optimization strategy is that it can lead to stellarators that deviate from the usual ``doughnut'' shape; i.e., they can have extended relatively straight cylindrical sections of plasma (connected by corner regions). This offers a number of potential design advantages, including simplified coil geometries, novel divertor approaches, low bootstrap current (less potential for ELMs and disruptions), more acceptable wall heat fluxes, and demountable blankets for reactors. The STELLOPT approach has been used to develop optimized configurations of this type for two, three and four field periods; the primary optimization targets used so far have been: effective ripple, J* closure and aspect ratio. From a top view, the two field period device is an extended racetrack with $\rlap{--} {i}=0.2-0.25$, $\varepsilon _{eff}^{3/2} =8\times 10^{-4}-3\times 10^{-3}$ and outboard $R_{max}/R_{min}$\textit{ $\sim $3}; $\left\langle R \right\rangle /\left\langle a \right\rangle 's$ in the range of 9 to 20 have been examined so far. The physics characteristics and remaining optimization targets that are under consideration for such devices will be discussed. [Preview Abstract] |
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TP8.00099: Neoclassical and anomalous flows in stellarators A.S. Ware, T. Marine, D.A. Spong The impact of magnetic geometry and plasma profiles on flows and viscosities in stellarators is investigated. This work examines both neoclassical and anomalous flows for a number of configurations including a particular focus on the Helically Symmetric Experiment (HSX) and other quasi-symmetric configurations. Neoclassical flows and viscosities are calculated using the PENTA code [1]. For anomalous flows, the neoclassical viscosities from PENTA are used in a transport code that includes Reynolds stress flow generation [2]. This is done for the standard quasi-helically symmetric configuration of HSX, a symmetry-breaking mirror configuration and a hill configuration. The impact of these changes in the magnetic geometry on neoclassical viscosities and flows in HSX are discussed. Due to variations in neoclassical viscosities, HSX can have strong neoclassical flows in the core region. In turn, these neoclassical flows can provide a seed for anomalous flow generation. These effects are shown to vary as the ratio of electron to ion temperature varies. In particular, as the ion temperature increases relative to the electron flow shear is shown to increase. \\ { } [1] D. A. Spong, Phys. Plasmas 12, 056114 (2005). \\ { } [2] D. E. Newman, et al., Phys. Plasmas 5, 938 (1998). [Preview Abstract] |
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TP8.00100: Linear and nonlinear gyrokinetic studies of turbulence in stellarator geometry with the GS2 code J.A. Baumgaertel, G.W. Hammett, D.R. Mikkelsen, W. Dorland, E.A. Belli The GS2 gyrokinetic code is being used to study microinstabilities and turbulence in flux-tubes in non-axisymmetric geometries, including stellarators. Stellarators have a number of interesting features, such as natural negative magnetic shear and a large number of shaping parameters, which offer possibilities for reducing microturbulence and improving performance. GS2 traditionally uses numerical equilibria for these studies, but recently an analytical stellarator equilibrium model\footnote{SUGAMA, WATANABE, and FERRANDO-MARGALET, Plasma and Fusion Research, Vol. 3, p.041 (2008)} has been implemented for benchmarking with the GKV and GENE gyrokinetic codes. In addition to studying ITG modes, we will present results on kinetic ballooning mode growth rates and instability thresholds, using both the analytical model and numerical stellarator equilibria from recent design studies. Nonlinear results will also be discussed. [Preview Abstract] |
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TP8.00101: Use of High Temperature Superconducting Monoliths for Field Shaping in Stellarators and Tokamaks Thomas Brown, Leslie Bromberg, Allen Boozer, Phil Heitzeroeder, Michael Zarnstorff , Joseph Minervini Monolithic High Temperature Superconductors (HTS) can be used for field shaping. Design issues relevant to stellarator magnets using single crystal or highly textured YBCO monoliths will be discussed. The excellent properties of YBCO operating at elevated temperatures ($>$ 10 K) will be summarized. High field, cryo-stable, highly complex magnet field topologies can be generated using the techniques discussed in this paper. Engineering constraints, such as stresses in the superconducting monoliths, support, quench protection, superconducting stability of the monoliths and required external support structure will be described. The limitations imposed by different fusion environments on the performance and lifetime of the HTS monoliths will be reviewed, both for near term experiments as well as long term stellarator fusion reactors. Since the HTS monoliths require no insulation or copper for stability/quench protection, some of the irradiation limits on these components are eliminated. [Preview Abstract] |
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TP8.00102: Overview of HSX Results and Future Directions D.T. Anderson HSX is a quasihelically symmetric (QHS) stellarator with minimal toroidal curvature and high effective transform. We present recent results at 1 Tesla operation which highlight the unique features of the bootstrap and Pfirsch-Schlueter currents. Large parallel flows, usually neglected in stellarator calculations, have been observed by CXRS in qualitative agreement with results first predicted by the PENTA code [1]. Electron temperatures in the core during ECRH are up to 2.5 keV with 100 kW input power and drop to 1.5 keV when the symmetry is intentionally degraded. The steep temperature gradient in the core is indicative of a core electron root confinement (CERC) mode. PENTA calculations support the conclusion that even with small symmetry-breaking, it is possible to achieve a neoclassical ITB based on the proximity of an electron root near an ion root. A Weiland ITG/TEM tokamak model for anomalous transport supports the conclusion that E x B suppression of turbulence is responsible for the improved confinement in the plasma core. At a lower field of 0.5 Tesla, an instability due to fast electrons is observed, which disappears when the symmetry is broken. Experimental measurements indicate that the mode is acoustic. We will also briefly summarize topics of current research and future plans. [Preview Abstract] |
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TP8.00103: Modeling and Measurement of Toroidal Currents in the HSX Stellarator J.C. Schmitt, J.N. Talmadge, J. Lore A set of magnetic diagnostics are installed to measure the magnetic field at several locations around HSX. Rogowski coils and diamagnetic loops measure the toroidal current and toroidal flux, and two poloidal belts, separated by $\sim $1/3 of a field period measure the magnetic field vector at 16 poloidal locations at each toroidal angle. A self-consistent calculation of the Pfirsch-Schl\"{u}ter (PS) and bootstrap currents is found with VMEC and BOOTSJ, and the 3D equilibrium reconstruction code, V3FIT calculates the expected signal of the magnetic diagnostics. The lack of toroidal curvature in HSX results in a dipole PS current that has a helical rotation and nearly reverses at the two toroidal locations. Also, the bootstrap current reduces the rotational transform ($\sim $1) but increases the effective transform ($\sim $3). Compared to a tokamak, the magnitude of each current is reduced by $\sim $3. Improvements in the modeling include: 1) Comparison of the measured bootstrap current to the PENTA code which includes the effects of parallel momentum conservation and finite electric field. 2) Comparison of the evolution of the net toroidal current to a 3D model involving the susceptance matrix. 3) Analysis of the sensitivity of the diagnostic set and the ability of the V3FIT code to reconstruct the plasma pressure and current profiles based on the magnetic signals. [Preview Abstract] |
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TP8.00104: Determining the Radial Electric Field in HSX Using Impurity Ion Velocity Measurements from CHERS A. Briesemeister, K. Zhai, D.T. Anderson, F.S.B. Anderson, J. Lore, J.N. Talmadge A Charge Exchange Recombination Spectroscopy (CHERS) system is used to measure the velocity, density, and temperature of fully stripped carbon impurities in the Helically Symmetric Experiment (HSX), a quasi-helically symmetric stellarator. Velocity measurements are obtained by reversing the magnetic field between sets of shots in order to reverse the plasma flow velocity. This doubles the distance the measured photons are Doppler shifted and eliminates the need to accurately determine the value of the unshifted emission line which can depend on plasma conditions. Since two viewing angles (one approximately poloidal and the other approximately toroidal) are used at each radial location, the velocity magnitude and direction can be determined. The measured velocity is used to determine the radial electric field from force balance. A comparison of the measured values to those calculated using a momentum-conserving neoclassical transport model will be presented. [Preview Abstract] |
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TP8.00105: Disruptions in current-driven discharges in the Compact Toroidal Hybrid experiment S. Knowlton, G. Hartwell, J. Hanson, B. Stevenson, J. Eckberg, K. Kamdin Disruption avoidance in stellarators is relevant to helical configurations with tokamak-like levels of bootstrap current, e.g. quasi-axisymmetric devices, and stellarator-tokamak hybrids. Disruptions are investigated in the Compact Toridal Hybrid (CTH) experiment (R$_{0}$ = 0.75 m, a $\sim $ 0.2 m, B$_{0} \quad \le $ 0.7 T, $\bar {n}_e $ = 0.2 -- 1.5 x 10$^{19}$ m$^{-3})$, a flexible heliotron operating with significant ohmic current. At B$_{0 }$= 0.5 T, the edge vacuum rotational transform is variable from $\iota _{VAC}$(a) = 0.05 to 0.5${\rm g}$nd plasma currents up to 40 kA are driven in plasmas generated by electron-cyclotron resonant heating at 14 GHz. At the lowest vacuum transform $\iota _{VAC}$(a) = 0.05, current-driven disruptions leading to a complete loss of plasma can be induced for a total rotational transform $\iota _{TOT}$(a) $>$ 0.3 at plasma densities $\bar {n}_e \quad \ge $ 0.8 x 10$^{19}$ m$^{-3}$. Disruptions are preceded by m = 2 tearing activity. Complete disruptions have not yet been observed in CTH with h $\iota _{VAC}$(a) $\ge $ 0.2, although partial current collapses can take place. Efforts are underway to study the transitional behavior of the disruptions as the vacuum transform is continuously raised. [Preview Abstract] |
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TP8.00106: Soft X-Ray Diagnostics on the Compact Toroidal Hybrid Experiment G.J. Hartwell, J. Eckberg, J.D. Hanson, S.F. Knowlton, K. Kamdin Soft X-Ray (SXR) Diagnostics are used on the Compact Toroidal Hybrid (CTH) torsatron experiment (R = 0.75 m, a $\sim $ 0.2 m, B $\le $ 0.7 T, n$_{e}$ $\le $ 10$^{19}$ m$^{-3}$, T$_{e} \quad \le $ 250 eV) for tomographic reconstruction of the emissivity profile, electron temperature measurement, and as input to a 3D reconstruction code. SXR tomography is performed with three cameras with up to 60 chords viewing a poloidal cross-section. Each camera consists of a 20-channel AXUV-20EL photo-diode array filtered with 500nm Al foil. Electron temperatures are being measured with an Amptek X123-SDD spectrometer. The spectrometer views the Bremsstrahlung emission along a single chord through the plasma in the energy range from 1-10 keV. Signals from the 60 channel tomographic camera system and signals from similar cameras located at other toroidal locations are being incorporated into the V3FIT reconstruction code[1]. [1]. J. D Hanson et. al., Nucl. Fusion \textbf{49}, 075031 (2009) [Preview Abstract] |
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TP8.00107: Reconstruction of current-driven equilibria in the Compact Toroidal Hybrid using magnetic diagnostics B.A. Stevenson, M. Cianciosa, J. Hanson, G. Hartwell, J. Hebert, S. Knowlton There is a growing need for rapid reconstruction capability of fully three-dimensional equilibria in toroidal confinement experiments. Test and validation of the new V3FIT 3D magnetic equilibrium reconstruction code are underway on the Compact Toroidal Hybrid (CTH). The CTH is a heliotron-type device in which the magnetic configuration can be strongly modified by plasma current. The present suite of magnetic diagnostics includes internal and external 8-part and full Rogowski coils, four flux loops, and a diamagnetic loop. The measured signals from these diagnostics include contributions from the plasma current, externally applied currents, vacuum vessel current, and various sources of pickup and drift. The induced vacuum vessel current ($<$15kA) also contributes to the magnetic diagnostic signals. In order to include this contribution in the reconstruction process, the VALEN code was used to model the time varying vacuum vessel current distribution. For reconstructions, the plasma contribution is extracted from the total signal to provide the experimental input to V3FIT which utilizes least-squares fitting and the VMEC equilibrium code to reconstruct 3D plasma equilibria. [Preview Abstract] |
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TP8.00108: Plasma Equilibrium in a Magnetic Field with Stochastic Regions A. Reiman, J. Krommes, D. Monticello, M. Zarnstorff The nature of plasma equilibrium in a magnetic field with stochastic regions is examined, with particular application to the reconstruction of equilibria for the W7AS stellarator. It is shown that the magnetic differential equation that determines the equilibrium Pfirsch-Schl\"{u}ter currents can be cast in a form similar to various nonlinear equations for a turbulent plasma, allowing application of the mathematical methods of statistical turbulence theory. An analytically tractable model, previously studied in the context of resonance- broadening theory, is applied with particular attention paid to the periodicity constraints required in toroidal configurations. It is shown that even a very weak radial diffusion of the magnetic field lines can have a significant effect on the equilibrium in the neighborhood of the rational surfaces, strongly modifying the near-resonant Pfirsch-Schl\"{u} ter currents. Implications for the numerical calculation of 3D equilibria are discussed, with specific application to the calculation of equilibria for the W7AS stellarator. [Preview Abstract] |
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TP8.00109: MAGNETIC RECONNECTION |
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TP8.00110: Laboratory Study of Magnetic Reconnection with Variable Collisionality and its Application to Space Astro-physics M. Yamada, D. Uzdensky, R. Kulsrud, H. Ji, S. Dorfman, E. Oz, J. Yoo The fundamental physics of magnetic reconnection derived from the recent MRX experiment [1] is presented focusing on the different dynamics of electrons and ions during reconnection. Both local and global physics issues for reconnection and magnetic field dissipation are discussed. In the MRX scaling [1], the reconnection rate increases rapidly when the ion skin depth becomes larger than the Sweet-Parker width; the rate depends linearly on $\lambda_{mfp}/L$, a ratio of the electron mean free path to the scale length. This scaling can guide comparisons between laboratory results and astrophysical plasmas [2]. The recent experimental study of the global dynamics of line-tied solar flux ropes in a half-toroid plasma arc is also presented. \\[4pt] [1] M. Yamada et al, Phys. Plasmas, 13, 052119 (2006)\\[0pt] [2] D. A. Uzdensky, Phys. Rev. Lett., v.99, 261101 (2007). [Preview Abstract] |
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TP8.00111: A Next-Generation Experiment To Study Magnetic Reconnection and Related Explosive Phenomena in Large and Collisionless Plasmas H. Ji, M. Yamada, S. Prager, W. Daughton, V. Roytershteyn Magnetic reconnection, a topological change in magnetic field in plasmas, often occurs explosively leading to rapid conversion of magnetic energy to plasma particle energy in space, astrophysical and laboratory fusion plasmas. The Magnetic Reconnection Experiment (MRX, http://mrx.pppl.gov) is a primary dedicated experiment to study reconnection in a controlled environment. However, further critical understanding and contributions to space and astrophysical plasmas are limited by the parameters achievable in MRX and other dedicated experiments. The MRX plasmas are relatively collisional (Lundquist numbers $S \sim 10^3$) and effectively small (plasma size normalized by ion skin depth or ion sound radius $\sim 10$). In this paper, we discuss plans for a next-generation reconnection experiment based on MRX. By a combination of larger physical size, stronger magnetic field, and higher heating power, we aim to increase $S$ by a factor of 100 and effective size by a factor of 10, representing a very large jump in the laboratory capabilities. Kinetic simulations in realistic boundaries will be used to guide the experimental design. Research topics include: (1) transition of collisional to collisionless reconnection and its scaling with collisionality and size, (2) interacting multiple reconnections as a possible cause of explosive phenomena, (3) particle energization by reconnection, (4) relation between local reconnection and global magnetic self-organization in 3D realistic geometry and boundary. [Preview Abstract] |
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TP8.00112: Local and Global 3-D Effects in the Magnetic Reconnection Experiment (MRX) S. Dorfman, H. Ji, M. Yamada, E. Oz, J. Yoo, W. Daughton, V. Roytershteyn One of the key open questions in Magnetic Reconnection is the nature of the mechanism that governs the reconnection rate in real astrophysical and laboratory systems. Comparisons between fully kinetic 2-D simulations of the Magnetic Reconnection Experiment (MRX) and experimental data show that the 2-D, collisionless expression for the electric field due to particle dynamics [1] does not match MRX data; related to this is a factor of 3-5 discrepancy in the layer width [2,3]. Adding collisions to the simulation leads to a broadening of the layer, but the level of collisionality present in MRX may not be high enough to resolve the discrepancy. Ongoing research on MRX explores the role of fluctuations and 3-D effects in the force balance. Significant toroidal asymmetries have been found, manifested by regions of high inductive electric field moving in the electron flow direction within the layer. Electromagnetic fluctuations in the lower hybrid frequency range [4] tend to occur in discharges with high local currents and a rapid local reconnection rate. The precise relation of these phenomena to fast reconnection is actively being investigated. [1] M. Hesse, et al., Phys. Plasmas, 6:1781 (1999). [2] Y. Ren, et al., Phys. Plasmas 15, 082113 (2008). [3] S. Dorfman, et al., Phys. Plasmas 15, 102107 (2008). [4] H. Ji, et al., Phys.Rev.Lett. 92 (2004) 115001. Supported by NDSEG, DOE, NASA, and NSF. [Preview Abstract] |
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TP8.00113: Measurement of an Electrostatic Potential Well during Magnetic Reconnection in MRX Jongsoo Yoo, Masaaki Yamada, Hantao Ji, Seth Dorfman, Erdem Oz Numerical simulations and space observations have demonstrated that an electrostatic potential well develops during magnetic reconnection, along the direction normal to the current sheet . The existence of an electrostatic potential well is evidence of two-fluid effects in the diffusion region. Electrons and ions have different heating mechanisms, due to the difference in their fundamental length scales. Unmagnetized ions are considered to obtain energy from the electrostatic potential as they approach the X line. Thus, it is important to measure the magnitude of the potential well to study how ions are accelerated and heated during reconnection. With the help of a 10-tip floating potential probe and Langmuir probes, electrostatic potential wells are observed during `Pull' reconnection in the Magnetic Reconnection eXperiment (MRX) . The measured potential drop across the current sheet is on the order of 10 V. The location and width of the current sheet are measured by a nearby magnetic probe. The relationship between the magnitude of the potential drop and ion temperature is under investigation. [Preview Abstract] |
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TP8.00114: Study of Ion heating and acceleration in magnetic reconnection and laboratory flux loop experiments at MRX Erdem Oz, Masaaki Yamada, Seth Dorfman, Hantao Ji, Jongsoo Yoo Although it has been studied for many decades there is no comprehensive theoretical model that describes particle heating and acceleration commonly observed in space and astrophysical plasmas. Magnetic reconnection is considered to be the main process that causes ion heating and acceleration; however, the fundamentals of reconnection heating mechanism is not yet well understood. In the Magnetic Reconnection Experiment, we recently built a new type of ion Doppler spectroscopy probe in order to measure local ion temperature and flow velocity. The light from the plasma is collected by an optical fiber bundle and transported to a high resolution spectrometer where spectral images of 4 slits are recorded using an intensified gated camera capable of recording 2 images in a single plasma shot. The new light collection system gives an order of magnitude improvement in signal intensity compared to previous studies on MRX [1]. Initial results from this probe in two specific experimental campaigns, the ``pull'' reconnection experiments, and the laboratory flux loop experiment [2], will be reported. \\[4pt] [1] S. C. Hsu, et al, Phys. Plasmas 8, 1916 (2001)\\[0pt] [2] Abstract APS 2008 DPP GP6.19 [Preview Abstract] |
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TP8.00115: Fully kinetic 3D simulations of magnetic reconnection in MRX-relevant geometry including Coulomb collisions V. Roytershteyn, W. Daughton, L. Yin, B.J. Albright, K.J. Bowers, S. Dorfman, H. Ji, M. Yamada, H. Karimabadi Considerable progress has been made in understanding of quasi-stationary reconnection in 2D systems. In contrast, 3D reconnection and, in particular, the role of current-aligned instabilities is less understood. This work investigates the effects of these instabilities on reconnection using fully kinetic simulations with a Monte-Carlo model for Coulomb collisions and boundary conditions mimicking Magnetic Reconnection Experiment (MRX)~[1]. The motivation comes from a) experimental observations of both electrostatic and electromagnetic fluctuations in the current sheet made in MRX~[2,3] and b) our previous 2D simulations demonstrating a discrepancy in the thickness of the current sheet between both collisionless [1] and weakly collisional simulations and the experimental observations. We describe 3D simulations performed at MRX-relevant levels of collisionality that demonstrate the existence of both short-wavelength Low-Hybrid Drift Instability with $k \rho_e \sim 1$ localized at the edge of the current layer and long-wavelength electromagnetic modes localized at the center. The role of these instabilities in global reconnection dynamics is discussed. [1] S. Dorfman, et al., Phys. of Plasmas {\bf 15}, 102107 [2] T. Carter, et al., Phys. Plasmas, {\bf 9}, 3272 [3] H. Ji, et al., Phys. Rev. Letters {\bf 92}, 115001 [Preview Abstract] |
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TP8.00116: Fast Reconnection in High-Lundquist-Number Plasmas Due to Secondary Tearing Instabilities Yi-Min Huang, Amitava Bhattacharjee, Hongang Yang, Barrett Rogers Thin current sheets in systems of large size that exceed a critical value of the Lundquist number are unstable to a super-Alfv\'enic tearing instability. The scaling of the growth rate of the fastest growing instability with respect to the Lundquist number is shown to follow from the classical dispersion relation for tearing modes. As a result of this instability, the system realizes a nonlinear reconnection rate that appears to be weakly dependent on the Lundquist number, and larger than the Sweet-Parker rate by an order of magnitude (for the range of Lundquist numbers considered). This regime of fast reconnection appears to be realizable in a dynamic and highly unstable thin current sheet, without requiring the current sheet to be turbulent. [Preview Abstract] |
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TP8.00117: Two-dimensional Studies of Super-Alfvenic Instabilities of Thin Current Sheets L. Ni, K. Germaschewski, A. Bhattacharjee, Yi-Min Huang, H. Yang In large systems such as the solar corona or the magnetotail, extended thin current sheets are formed in situations involving free or forced reconnection. These current sheets are characterized by large values of the tearing instability parameter delta- prime, which is strongly destabilizing, and shear in the outflow velocity along the current sheet, which can be stabilizing. We have carried out a systematic analytical and computational study of this problem in resistive MHD, including the effect of finite plasma compression. When the shear in the outflow velocity is weak, we obtain an entire class of super-Alfvenic instabilities in the high-S regime, predicted by Lourerio et al., and generalized recently by Bhattacharjee et al. In the presence of strong velocity shear along the current sheet, the system tends to be more stable, and the wave number of the fastest growing instability itself evolves as a function of time. In the regime of small growth rates, it is possible to define a critical length below which the system is stable. The parametric dependence of this critical length is determined by analysis and simulations. [Preview Abstract] |
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TP8.00118: Current sheet extension and reconnection scaling in collisionless, hyperresistive, Hall MHD B.P. Sullivan, A. Bhattacharjee, Y.M. Huang We present Sweet-Parker type scaling arguments in the context of collisionless, hyper-resistive, Hall magnetohyrdodynamics (MHD). The predicted steady state scalings are consistent with those found by Chac\'on et al. [PRL 99, 235001 (2007)], and Uzdensky, [PoP 16, 040702 (2009)], though our methods differ slightly. As with those studies, no prediction of electron dissipation region length is made. Numerical experiments confirm that both cusp like \& extended geometries are realizable. Importantly, the length of the electron dissipation region (taken as a parameter by several recent studies) is found to depend on the level of hyper-resistivity. Although hyper-resistivity can produce modestly extended dissipation regions, the dissipation regions observed here are much shorter than those seen in many kinetic studies. The thickness of the dissipation region scales in a similar way as the length,so that the reconnection rate is not strongly sensitive to the level of hyperresistivity. The length of the electron dissipation region depends on electron inertia as well.The limitations of scaling theories that do not predict the length of the electron dissipation region are emphasized. [Preview Abstract] |
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TP8.00119: Fast magnetic reconnection and particle acceleration in pair plasmas Naoki Bessho, A. Bhattacharjee Magnetic reconnection without a guide field in both non-relativistic and relativistic regimes has been studied in pair plasmas by 2D PIC simulations. We have found that in both regimes, particle acceleration enhances reconnection rates by an interesting feedback effect. Reconnection rates in the impulsive phase become of the order of 1 when the background density in a Harris sheet equilibrium is of the order of 0.01 of the density in the current sheet. Fast reconnection becomes possible not only because of increase of the time derivative part of the inertial term in the generalized Ohm's law, but also by a positive feedback on the pressure tensor term, bootstrapped by particle acceleration that produces a reduction in the particle density in the diffusion region. In this impulsive phase, the extension of the diffusion region along the outflow region is accompanied by a broadening of the width of the current sheet in the inflow region, keeping the aspect ratio small and sustaining a large reconnection rate. The collisionless resistivity originating from non-ideal terms in the generalized Ohm's law becomes large and nearly spatially uniform along the diffusion region even after the extension of the diffusion region, and can account quantitatively for the fast reconnection rate. We will compare and contrast this with reconnection and particle acceleration in hydrogen plasmas. [Preview Abstract] |
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TP8.00120: Plasmoid generation in resistive-MHD reconnection Ravi Samtaney, Nuno Loureiro, Dmitri Uzdensky, Alexander Schekochihin, Stephen Cowley A detailed numerical study of resistive-MHD magnetic reconnection for very large, previously inaccessible, Lundquist numbers ($10^4\le S\le 10^8$) is reported. Large-aspect-ratio Sweet-Parker current sheets are shown to be unstable to super-Alfv\'enically fast formation of plasmoid (magnetic-island) chains. The plasmoid number scales as $S^{3/8}$ and the linear growth rate of the instability as~$S^{1/4}$, in agreement with the theory by Loureiro~et~al.\ [Phys.\ Plasmas~{\bf 14}, 100703 (2007)]. In the nonlinear regime, plasmoids continue to grow faster than they are ejected and completely disrupt the reconnection layer. These results suggest that high-Lundquist-number reconnection is inherently time-dependent and thus call for a substantial revision of the standard Sweet-Parker quasi-stationary picture for~$S>10^4$. [Preview Abstract] |
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TP8.00121: Analytical model for fast reconnection in large guide field plasma configurations A.N. Simakov, L. Chac\'on, D. Grasso, D. Borgogno, A. Zocco Significant progress in understanding magnetic reconnection {\it without a guide field} was made recently by deriving quantitatively accurate analytical models for reconnection in electron [1] and Hall [2] MHD. However, no such analytical model is available for reconnection {\it with a guide field}. Here, we derive such an analytical model for the large-guide-field, low-$\beta$, cold-ion fluid model [3] with electron inertia, ion viscosity $\mu$, and resistivity $\eta$. We find that the reconnection is Sweet-Parker-like when the Sweet-Parker layer thickness $\delta_{SP} > (\rho_s^4 + d_e^4)^{1/4}$, with $\rho_s$ and $d_e$ the sound Larmor radius and electron inertial length. However, reconnection changes character otherwise, resulting in reconnection rates $E_z/B_x^2 \approx \sqrt{2 \eta/\mu} (\rho_s^2 + d_e^2)/(\rho_s w)$ with $B_x$ the upstream magnetic field and $w$ the diffusion region length. Unlike the zero-guide-field case, $\mu$ plays crucial role in manifesting fast reconnection rates. If it represents the perpendicular viscosity [3], $\sqrt{\eta/\mu} \sim \beta^{-1} \sqrt{(m_e/m_i)(T_i/T_e)}$ and $E_z$ becomes dissipation independent and therefore potentially fast.\\[0pt] [1] L. Chac\'{o}n, A. N. Simakov, and A. Zocco, {\it PRL} {\bf 99}, 235001 (2007).\\[0pt] [2] A. N. Simakov and L. Chac\'on, {\it PRL} {\bf 101}, 105003 (2008).\\[0pt] [3] D. Biskamp, {\it Magnetic reconnection in plasmas}, Cambridge University Press, 2000. [Preview Abstract] |
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TP8.00122: Influence of Current Aligned Instabilities on Magnetic Reconnection in Neutral Sheet Geometry W. Daughton, V. Roytershteyn, L. Yin, B.J. Albright, K.J. Bowers, H. Karimabadi The influence of current aligned instabilities on collisionless reconnection is studied within neutral sheet geometry using petascale 3D kinetic simulations, which permit ion to electron mass ratios in the range $m_i/m_e =200 - 400$. Open boundary conditions are employed to avoid artificial recirculation effects and better mimic large-scale systems in nature. During the onset and initial evolution, intense lower-hybrid drift activity is observed immediately upstream of the electron diffusion region and along the separatrices, with characteristic wavelength on the electron gyroscale $k_y \rho_e \sim 1$. These fluctuations do not penetrate into the central electron layer, and are gradually weakened as the upstream density gradients relax and a highly elongated electron-scale current layer is formed. At later times, an electromagnetic instability is observed within the elongated electron layer with wavelength $k_y (\rho_i \rho_e)^{1/2} \sim 1$ roughly consistent with previous predictions from Vlasov theory\footnote{Daughton, Phys. Plasmas, {\bf 10}, 3103, 2003}. This instability gives rise to a pronounced kinking or undulation of the electron layer in a manner qualitatively similar to large-scale electron-positron plasmas\footnote{Yin et al, PRL {\bf 101}, 125001, 2008}. The possible influence of these instabilities on the dissipation rate and energy partition is discussed. [Preview Abstract] |
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TP8.00123: Bi-directional Beams by Electron Trapping during Anti-parallel Reconnection Jan Egedal, A. Le, N. Katz, L.-J. Chen, B. Lefebvre, W. Daughthon So-called bi-directional electron beams have been observed by a number of spacecraft missions in the inflow regions of anti-parallel reconnection. Here we analyze electron distribution functions measured by the four Cluster spacecraft and we show that the beam features can be accounted for by electron trapping mainly by parallel electric fields. In turn, the parallel electric fields can be described by a parallel acceleration potential $\Phi_{\parallel}$ (defined in Ref [1]). In the analysis we determine the profiles of $\Phi_{\parallel}$ along the paths of the Cluster spacecraft during their encounter with a reconnection region. $\Phi_{\parallel}$ is typically in excess of 1kV and therefore all thermal electrons are trapped. This is important for the internal structure of the Hall current system because extended trapping significantly alters the pressure tensor of the electron fluid [2].\\[1ex] [1] J.~Egedal, W.~Daughton, J.~Drake, N.~Katz, and A.~Le, {\sl Physics of Plasmas}, {\bf 16}, 050701 (2009).\\[0ex] [2] A.~Le, J.~Egedal, W.~Daughton, W.~Fox, and N.~Katz, {\sl Phys.~Rev.~Lett.}, {\bf 102}, 085001 (2009). [Preview Abstract] |
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TP8.00124: Observation of electron holes and lower-hybrid turbulence during magnetic reconnection experiments on VTF W. Fox, M. Porkolab, J. Egedal, N. Katz, A. Le We report a detailed study of electrostatic turbulence observed during magnetic reconnection experiments on the Versatile Toroidal Facility (VTF) [1], including identification of modes, exploration of instability mechanisms, and studies of correlation with the reconnection events and electron energization. Electrostatic fluctuations are observed by arrays of small, high-bandwidth, impedance-matched Langmuir probes. Broadband fluctuations are found, including lower-hybrid (LH) waves and higher-frequency Trivelpiece-Gould (TG) waves. Strong nonlinear turbulence, consisting of large-amplitude, positive-potential spikes identified as electron phase-space holes, is also observed [2]. We believe that the LH modes are driven unstable by steep electron temperature gradients, while TG and electron holes arise from bump-on-tail instability of high energy electrons. In both cases, it is believed that the modes arise as a consequence of electron energization by the reconnection events.\\{} [1] J. Egedal, \textit{et al.}, \textit{PRL} \textbf{98}, 015003 (2007). \\{} [2] W. Fox, \textit{et al.}, \textit{PRL} \textbf{101}, 255003 (2008). [Preview Abstract] |
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TP8.00125: Rogowski Coil Array for Collisionless Magnetic Reconnection Experiments at VTF A. Le, J. Egedal, W. Fox, N. Katz, A. Vrublevskis, M. Porkolab Recent experiments on magnetic reconnection at the Versatile Toroidal Facility (VTF) indicate that the electron current may become filamented during spontaneous reconnection events [1]. A new array of Rogowski coils, which has centimeter-scale resolution, will measure the parallel currents driven by fast reconnection in the presence of a guide magnetic field. The diagnostic should confirm the filamentary structure of the current profile, determine the distribution of filaments around the X-line, and reveal when the filaments are generated with respect to the onset of reconnection. In addition, multi-shot scans performed by moving the Rogowski coil array across a poloidal section and to several different toroidal angles will yield high-resolution measurements of the reproducible features of the current profile that develops during three-dimensional reconnection at VTF. Preliminary data will be presented if available.\\[1ex] [1] J. Egedal et al., Phys. Rev. Lett., {\bf 98}, 015003 (2007) [Preview Abstract] |
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TP8.00126: Washer-Gun Plasma Source for Magnetic Reconnection Experiments on VTF A. Vrublevskis, J. Egedal, W. Fox, N. Katz, A. Le, M. Porkolab We present a recently built electrostatic washer-gun plasma source for the Versatile Toroidal Facility (VTF). The source produces plasmas with estimated densities of $\sim10^{19}$ m$^{- 3}$ and electron temperatures of $\sim 5-20$ eV. The present plasma source for VTF is microwave-induced electron cyclotron resonant breakdown and requires a strong toroidal magnetic field, which acts as a guide field in reconnection experiments. The gun will allow reconnection experiments with no guide field. The source is based on the design developed by Sterling Scientific [1, 2]. To operate, gas is injected into a channel formed by a stack of alternating molybdenum and boron nitride washers with a molybdenum electrode washer at each end. A capacitor bank is discharged through these electrodes and the gas. The resulting plasma escapes the channel into the main chamber of the experiment. If available, we will present data on argon plasma produced by the gun inside the VTF. \\[1ex] [1] G.~Fiksel, et al., Plasma Sources Sci. Technol., {\bf 5}, 78 (1996)\\[0ex] [2] D.~Hartog et al., Plasma Sources Sci. Technol., {\bf 6}, 492 (1997) [Preview Abstract] |
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TP8.00127: Reconnection properties in Hall MHD Plasmas Zhi-Wei Ma, Xing-Qiang Lu, Ya-Wei Hou Dynamics and properties of magnetic reconnection with an asymmetry initial condition are studied by using Hall MHD simulation. In the early phase of the magnetic reconnection, the geometry of the magnetic field in the near reconnection region tends to form a Y-type structure under an asymmetry initial condition instead of to exhibit an X-type structure in the symmetry initial condition. The growth rate of the current density in the early stage is proportional to the square of the ion inertial length. In the late phase, the thin elongated current sheet associated with the Y-type geometry of the magnetic field breaks-up to form a magnetic island due to the secondary tearing instability. It is found that the onset time of the secondary tearing instability is inversely dependent on the square root of the ion inertia length. The generated Hall electric field and net charge are larger in the region inside than outside the magnetic island. The strengths of the Hall electric field and net charge density linearly increase with the increase of the ion inertia length. [Preview Abstract] |
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TP8.00128: Suppression of Double-Tearing Modes by Poloidal Shear Flows Xiaogang Wang, Wenbin Xu, Zhengxiong Wang, Yue Liu Suppression effects of shear flows on double tearing modes are investigated in a reduced resistive magnetohydrodynamics model. It is found that as poloidal flow shear increased, the growth of the magnetic island generated by the double tearing mode can be greatly suppressed. [Preview Abstract] |
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TP8.00129: The origin of the abrupt nonlinear growth in Double Tearing Mode Miho Janvier, Yasuaki Kishimoto, Jiquan Li Reversed shear configuration represents an effective way to achieve a better confinement of the plasma inside tokamaks via the formation of Internal Transport Barriers (ITB). However, this configuration leads to MHD instabilities such as the Double Tearing Mode, where the dynamics are enhanced due to the presence of two rational surfaces. Kink type or tearing type evolutions of the DTM have been investigated and recently, an evolution in three regimes (linear, slowdown and fast growth) has attracted much attention [1][2]. However, there is still no clear explanation on the triggering mechanism leading to the nonlinear growth of the DTM and fast reconnection. Here, we investigate the physical process that leads to the fast growth regime and we try to understand the trigger mechanism as a result of secondary instability evolving at different time scales. We perform numerical simulations for the DTM in slab geometry by solving the 2-field reduced MHD equations and investigate the stability of the developed magnetic island with flow in the slowdown regime as a quasi-steady equilibrium with slow time scale. The high order magnetic structure is therefore found to play an important role in triggering the abrupt growth of the DTM flows. [1] Y.Ishii, \textit{et al,} Phys. Rev. Lett., Vol.89,\textbf{20 }(2002) [2] Z.X.Wang, \textit{et al, }Phys. Plasmas, \textbf{15} 082109 (2008) [Preview Abstract] |
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TP8.00130: Turbulence driven magnetic reconnections causing magnetic islands with long wavelengths Akihiro Ishizawa, Noriyoshi Nakajima Numerical simulations of reduced two-fluid equations show that micro-turbulence produces magnetic reconnection and magnetic islands with long wavelengths even if the equilibrium is stable against tearing modes. Thus, the turbulence modifies the threshold of magnetic island formation against stability parameter of tearing mode. The length of magnetic islands is the same order as the system size and the width is several times of ion Larmor radius. This suggests that micro-turbulence can cause seed magnetic islands of neoclassical tearing modes which are nonlinear instability and are considered to give the main limitation to plasma pressure in magnetically confined plasmas. [Preview Abstract] |
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TP8.00131: Ion heating during magnetic reconnection with a guide field: a threshold for pickup behavior J.F. Drake, P.A. Cassak, M.A. Shay, M. Swisdak, E. Quataert The acceleration of ions during magnetic reconnetion with a guide field is explored with simulations and analytic analysis. Ions crossing into Alfv\'enic reconnection outflows can behave like pickup particles and gain an effective thermal velocity equal to the Alfv\'en speed. However, with a sufficiently strong ambient out-of-plane magnetic field, which is the relevant configuration for most laboratory experiments and astrophysical systems, the ions can become adiabatic and their heating is then dramatically reduced. The threshold for non-adiabatic behavior, where ions are strongly heated, becomes a condition on the ion mass-to-charge-ratio, $m_i/m_pZ_i>10\sqrt{\beta_{0x}/2}/\pi$, where $m_i$ and $Z_i$ are the ion mass and charge state, $m_p$ is the proton mass and $\beta_{0x}=8\pi nT/B_{0x}^2$ is the ratio of the plasma pressure to that of the reconnecting magnetic field $B_{0x}$. Thus, during reconnection high mass-to-charge particles gain energy more easily than protons. A simple model reveals that in the case of flares the abundances of high mass-to-charge ions are enhanced, which is consistent with observations. Implications for understanding ion heating during sawtooth events in the MST reversed field pinch are also discussed. [Preview Abstract] |
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TP8.00132: Lower Hybrid Instability and Coherent Formation of Electron Holes Haihong Che, James Drake, Marc Swisdak, Peter H. Yoon Using particle-in-cell simulations and kinetic theory, we discover that lower hybrid instability can form not only oblique localized structures but also much stronger and more stable bipolar structures parallel to magnetic field. This is a direct result of nonlinear coupling between waves and particles which allows the perpendicular phase speed of lower hybrid waves to be in pace with the undergoing cross-field $\mathbf{E} \times \mathbf{B}$ drift of its trapped electrons. The coherent increasing of phase and group velocity strengthen the stability of the electron holes. The increasing phase speed of lower hybrid waves transfer the momentum from high velocity electrons to ions. [Preview Abstract] |
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TP8.00133: Effects of Plasma Beta on Multiple Island Ion Acceleration Kevin Schoeffler, James Drake, Marc Swisdak Magnetic reconnection has been found to be associated with energetic ions and electrons. Gamma ray emission from the solar photosphere, and energetic particles found by Wind in the magnetosphere confirm this association. One explanation for the acceleration of these particles is by 1st order Fermi acceleration of particles bouncing in contracting magnetic islands. The contracting islands heat up particles in the direction of the magnetic fields, causing a temperature anisotropy with $T_\parallel > T_\perp$. When the anisotropy reaches the marginal fire hose condition, this mechanism should end. Larger initial values of $\beta$ might cause the fire hose condition to be reached more quickly. Using a particle-in-cell code, we investigate the effects of $\beta$ by simulating island growth with initial equilibria with multiple Harris current sheets. [Preview Abstract] |
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TP8.00134: Kinetic effects in gyrokinetic tearing instability Ryusuke Numata, William Dorland, Nuno Loureiro, Barrett Rogers, Alexander Schekochihin, Tomoya Tatsuno We present numerical results of tearing instability simulations in a strong guide magnetic field limit using the \texttt{AstroGK} astrophysical gyrokinetics code. The tearing growth rates from the gyrokinetic simulations show quantitative discrepancy with those of the fluid model based on the compressible two-fluid MHD model [1], which may be ascribed to the treatment of pressure. In the kinetic model, the pressure tensor cannot be described one single relation to the density. Moreover, the behavior of the pressure can be very different in the inner current layer and the outer ideal MHD region. In this presentation, we show results of pressure tensor diagnostics, and discuss the effect of the pressure tensor on the linear tearing instability. Preliminary results from nonlinear simulations will also be presented. \\[4pt] [1] E. Ahedo and J.J. Ramos, Plasma Phys. Control. Fusion 51, 055018 (2009). [Preview Abstract] |
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TP8.00135: Progress in kinetic MHD simulation of magnetic reconnection in Harris sheet equilibrium Jianhua Cheng, Yang Chen, Scott Parker We have developed a Lorentz force ion, fluid electron kinetic MHD hybrid model [D. Barnes, \emph{et al}, Phys. Plasmas {\bf{15}}, 055702 (2008)]. Here we focus on the implementation of an isothermal fluid electron model in the GEM turbulence code. A second-order accurate implicit scheme that generalizes the previous implicit scheme for Vlasov ions and drift kinetic electrons [Yang Chen and Scott E. Parker, Phys. Plasmas {\bf{16}}, 052305 (2009)] has been implemented. The generalized Ohm's law is solved for the Harris sheet equilibrium configuration by Fourier decomposing the electric field along the equilibrium field and solving for each Fourier component in the direction perpendicular to the current sheet using direct matrix inversion. This presentation focuses on the linear instabilities for Harris sheet equilibrium. We will also provide some preliminary reconnection results and their direct comparison with MHD simulations. [Preview Abstract] |
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TP8.00136: 3D Ion Temperature Reconstruction Hiroshi Tanabe, Setthivoine You, Alexander Balandin, Michiaki Inomoto, Yasushi Ono The TS-4 experiment at the University of Tokyo collides two spheromaks to form a single high-beta compact toroid. Magnetic reconnection during the merging process heats and accelerates the plasma in toroidal and poloidal directions. The reconnection region has a complex 3D topology determined by the pitch of the spheromak magnetic fields at the merging plane. A pair of multichord passive spectroscopic diagnostics have been established to measure the ion temperature and velocity in the reconnection volume. One setup measures spectral lines across a poloidal plane, retrieving velocity and temperature from Abel inversion. The other, novel setup records spectral lines across another section of the plasma and reconstructs velocity and temperature from 3D vector and 2D scalar tomography techniques. The magnetic field linking both measurement planes is determined from in situ magnetic probe arrays. The ion temperature is then estimated within the volume between the two measurement planes and at the reconnection region. The measurement is followed over several repeatable discharges to follow the heating and acceleration process during the merging reconnection. [Preview Abstract] |
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TP8.00137: Local and Global Transient Effect of Magnetic Reconnection Yoshinori Hayashi, Toru Ii, Michiaki Inomoto, Yasushi Ono Transient effect of magnetic reconnection has been investigated in the TS-4 torus plasma merging device. We found axial mergings of two and three loops, which were caused by local and global effect, respectively. The two loop merging with pull reconnection converts one common flux to two private fluxes. Under strongly driven inflow, the plasma and magnetic flux inflow exceeded the outflow ones, causing flux and density piled-up in the current sheet. This pile-up effect was found to increase the inflow speed without anomalous resistivity effect. Under strong guiding field, a plasmoid grew in the current sheet during plasma pile-up. When flux pile-up reached critical value, a plasmoid was ejected from reconnection region and reconnection speed transiently increased. The plasmoid ejection made the reconnection rate maximum when its acceleration was maximized. In the case of three loop merging, two parallel loops stays for a while in a stable state because of another anti-parallel loop. When global magnetic configuration change, two loop reconnected into one loop impulsively forming another equilibrium state. We fond both of these local and global effect cause fast but unsteady magnetic reconnection. [Preview Abstract] |
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TP8.00138: Experimental Study of Three-Dimensional Localized Magnetic Reconnection by Use of Merging Torus Plasmas Toru Ii, Yoshinori Hayashi, Michiaki Inomoto, Yasushi Ono Three-dimensional (3-D) localized magnetic reconnection was studied experimentally in the University of Tokyo Spherical Torus (TS-4) plasma merging device using controlled external compression force and guide field. We found two unsteady effects: 3-D deformation and ejection of current sheet cause fast magnetic reconnection. When strong compression force $I_{\rm Acc}\sim 60$ kA compressed two compact toroids with low guide field $B_t\sim B_\parallel$ ($B_\parallel$ is reconnecting field component), toroidal modes $n=1$--$3$ of current sheet was observed to grow only during their reconnection. A new finding is that 3-D deformation of current sheet promotes mass ejection from current sheet and causes the reconnection rate as well as the reconnection outflow to increase. When weak compression force $I_{\rm Acc}\sim 0$ kA compressed two compact toroids with high guide field $B_t\sim 7B_\parallel$, their reconnection rate was maintained small. These phenomena indicate that 3-D localized reconnection is one of fast reconnection mechanisms. [Preview Abstract] |
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TP8.00139: Pulsed High-Power Heating of Magnetic Reconnection for Spherical Torus Plasmas Yasushi Ono, Michiaki Inomoto, Takuma Yamada The merging/ reconnection heating of spherical torus plasmas (ST, Spheromak, RFP, FRC) has been developed in the TS-3 TS-4 and UTST experiments, leading us to its new extension to their pulsed high-power heating. Two spherical torus plasmas were produced inductively by two or four PF coil currents without using any center solenoid (CS) and they were merged together for high-power reconnection heating. The reconnection outflow speed was observed to be equal to the Alfven speed under no guiding field condition. The outflow energy is converted mostly into ion thermal energy through ion viscosity and/or fast shock. This fact indicates that the ion temperature increment (and thermal energy) scales with squares of reconnecting magnetic field (Alfven speed). This unique method enables us to utilize the highest heating power MW-GW among all CS-less startups and the heating time much shorter than the energy confinement time and the electron-ion collision time. These facts indicate that the merging of two spherical torus plasmas possibly provides a direct path to the burning plasma formation. The TS-3 and TS-4 scaling data suggest that two merging spherical torus with B=1-3T, n=10$^{20}$m$^{-3}$ will increase its ion temperature into the ITER regime plasma of T$\approx $20keV [Preview Abstract] |
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TP8.00140: Simulated and experimental compression of a compact toroid Jeffrey Johnson We present simulation results and experimental data for the compression of a compact toroid by a conducting nozzle without a center electrode. In both simulation and experiment, the plasma flow is obstructed by even modest magnetic fields. A simple mechanism for this obstruction is suggested by our simulations. The configuration of the plasmoid's magnetic field plays a significant role in its compression. We analyze two types of plasma configurations under compression and demonstrate that the results from the simulations match those from the experiments, and that the mechanism predicts the different behaviors observed in the two cases. [Preview Abstract] |
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TP8.00141: Mass dependent ion heating in the SSX reconnection device M. Brown, T. Gray, J. Santner, M. Korein, D. Weinhold Ion heating due to magnetic reconnection is measured in the SSX plasma merging device for a variety of ion masses and charge states with a high resolution ion Doppler spectrometer. The SSX IDS instrument measures the width and Doppler shift of the nascent $C_{III}$ impurity $229.7~nm$ line, a doped $He_{II}$ impurity $468.6~nm$ line, or a doped $Ar_{II}$ impurity line to determine the temperature and line-averaged flow velocity. The velocity resolution of the instrument is $\le 5~km/s$. There is enough signal to resolve the full line within an MHD dynamical time (about $1~\mu s$ in SSX). Peak ion temperatures of $80~eV$ have been recorded during reconnection events as well as flows up to $40~km/s$. Spheromak merging in a new slightly prolate flux conserver ($R=0.2~m, L=0.4~m$) often results in excitation of several unstable MHD modes. After reconnection and instability, we measure a period of reconnection driven ion heating with peak temperatures for carbon $T_C \cong 50~eV$ and for helium $T_{He} \cong 70~eV$ (averaged over many shots). During the decay phase, we observe rapid ion cooling likely due to energetic ion loss. Results from a new ion energy analyzer and Mach probe will be presented. [Preview Abstract] |
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TP8.00142: MAGNETO-INERTIAL-FUSION |
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TP8.00143: Field Reversed Configuration (FRC) formation and compression using liners J.H. Degnan, P. Adamson, D.J. Amdahl, R. Delaney, M. Domonkos, F.M. Lehr, E.L. Ruden, W. Tucker, W. White, C. Grabowski, D. Gale, M. Kostora, J. Parker, W. Sommars, M.H. Frese, S.D. Frese, J.F. Camacho, S.K. Coffey, V. Makhin, T.F. Intrator, G.A. Wurden, P.J. Turchi, R.E. Siemon, A.G. Lynn, N.F. Roderick FRC capture, field and compression experiments all use 2 T guide and mirror fields established inside the liner. A 12 MA, 4.5 MJ discharge drive the liner implosion. The capture experiments use 3 capacitor discharges into a segmented theta coil surrounding the formation region to form a bias field, pre-ionize the deuterium gas, and provide the reverse field theta discharge which forms the FRC. Two cusp field discharges aid this. The guide and mirror fields enable translation and capture of the FRC. Diagnostics include field exclusion, interferometry, radiography, and radiation detection. Design and parameters are guided by MHD simulations. Supported by DOE-OFES. [Preview Abstract] |
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TP8.00144: Progress on Field Reversed Configuration target for Magnetized Target Fusion T. Intrator, G.A. Wurden, P.E. Sieck, W.J. Waganaar, R. Oberto, T.D. Olson, D. Sutherland, J.H. Degnan, E.L. Ruden, M. Domonkos, P. Adamson, C. Grabowski, D.G. Gale, W. Sommars, M. Kostora, M.H. Frese, S.D. Frese, J.F. Camacho, S.K. Coffey, N.F. Roderick, D.J. Amdahl, P. Parks, R.E. Siemon, T. Awe, A.G. Lynn We overview the experimental high density Field Reversed Configuration (FRC) approach for a LANL AFRL collaborative physics demonstration of Magnetized Target Fusion (MTF). We show some initial translation data from the Los Alamos FRC experiment FRXL that characterize the MTF translated target plasma. The conical theta coil is expected to generate toroidal magnetic field, helicity, and good curvature field lines, and should increase stability and lifetime. The suitability of the present FRXL data for MTF implosion at AFRL Kirtland will be discussed, along with the hardware, diagnostics, and pre-compression plasma formation and trapping experiments. [Preview Abstract] |
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TP8.00145: Translation and Capture of High-Density Field Reversed Configurations for Magnetized Target Fusion P.E. Sieck, T.P. Intrator, G.A. Wurden, W.J. Waganaar, R.J. Cortez, R.J. Oberto A physics demonstration of Magnetized Target Fusion (MTF) is being pursued by a collaborative team from Los Alamos National Laboratory and Air Force Research Laboratory. The LANL facility, known as the Field Reversed eXperiment --- Liner (FRX-L), focuses on the physics of producing high-density Field Reversed Configurations (FRCs), translating them, and capturing them in a static flux conserver. Observations of FRCs in translation and capture will be presented. The data suggest FRCs are formed at density above 10$^{22}$/m$^3$, translate over the one meter chamber at 97~km/s, and a captured portion having radius 4~cm lives for 10$\mu$s. The repeatability of FRC capture will be discussed in context of that necessary for MTF. This work is supported by the Office of Fusion Energy Sciences, and DOE/LANL contract DE-AC52-06NA25396. [Preview Abstract] |
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TP8.00146: Proposed profile measurements of internal B, ne and Te on the Magnetized Target Fusion (MTF) plasmas using Pulsed Polarimetry R.J. Smith, G.A. Wurden The novel pulsed polarimetry technique combines both the Faraday effect and Thomson scattering into a comprehensive Lidar-like diagnostic that measures the sightline parameters, B(s), ne(s) and Te(s) remotely and non-perturbatively. The target FRC: FRX-L at LANL (development of formation and translation hardware and scenario for MTF) and FRCHX at AFRL, Kirtland AFB (formation + translation + liner implosion) have sufficiently large optical activity to allow polarimetric measurements in the near infrared. The proposed diagnostic is based on an ultra-short pulsed NdYag laser with streak camera detection. Magnetic field spatial resolutions of several \textit{cms} are possible on FRX-L and sub-cm on FRCHX. The pulsed polarimeter system and the relevance of the internal measurements to the MTF program will be described. [Preview Abstract] |
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TP8.00147: Visibility Measurements on a Fiber-Optic Probe Interferometer System J.F. Camacho, A.G. Lynn, E.L. Ruden A HeNe laser interferometer operating at 632.8 nm with several single-mode optical fiber probe beams has been assembled to measure the electron density of a field-reversed configuration for a magnetized target fusion experiment. Our system features probe beams whose path lengths are many times longer than the reference beam paths. We have generalized the condition for maximum interference visibility to the case of a heterodyned interferometer in which portions of the probe and reference beams propagate through different lengths of single-mode fiber. This analytic development also takes into account the effect of the line width of the individual modes of our multimode laser on mode coherence, which imposes an upper bound on the fiber length for which a good interference signal can be obtained. When this maximum fiber length is exceeded, an interference signal can still be observed, but the signal is amplitude modulated at frequencies much lower than the 80-MHz carrier signal. This modulation is likely due to ``mode pulling'' that occurs because of fluctuations in laser cavity length arising from acoustic modes of the laser tube. Visibility measurements establishing the viability of our design will be presented. [Preview Abstract] |
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TP8.00148: Time resolved visible spectroscopy of surface plasma radiation Tasha Goodrich, Thomas Awe, Stephan Fuelling, Richard Siemon Experiments at UNR demonstrate that interesting multiply ionized aluminum plasma (T=1-30 eV, n $\sim $ 10$^{18}$ cm$^{-3}$, Z $\sim $ 3) can be created on the surface of mm-diameter aluminum rods by MA pulses of current. Surface temperature has been estimated by measuring the surface brightness in the green part of the visible spectrum (Awe et al., this conference). To investigate the spectral distribution throughout the visible region, which should vary as one over wavelength to the fourth power for temperatures above about 2 eV, a Jobin-Yvon f/2 spectrometer has been coupled with a linear array of silicon photodiodes. This instrument has spectral resolution of 30 nm on each of ten array elements covering the spectrum between 400 -- 700 nm and uses a Nikon lens/fiber optic collection system to enable observation of temperatures ranging from 1-30 eV. Experimental data with comparisons to the blackbody spectrum will be presented. [Preview Abstract] |
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TP8.00149: Overview of the Plasma Liner Experiment (PLX) S.C. Hsu, F.D. Witherspoon, M.A. Gilmore, J.T. Cassibry The Plasma Liner Experiment (PLX), to be built at LANL, will explore and demonstrate the feasibility of forming imploding spherical ``plasma liners'' that can reach peak pressures $\sim 0.1$~Mbar upon stagnation. The liners will be formed via merging of 30 dense, high Mach number plasma jets ($n\sim 10^{17}$~cm$^{-3}$, $M\sim 10$--35, $v\sim 50$--70~km/s, $r_{jet}\sim 5$~cm) in spherically convergent geometry. This is a staged, exploratory project where scientific issues will be studied first at modest stored energies ($\sim 300$~kJ) before attempting to reach HED-relevant pressures (requiring $\sim 1.5$~MJ)\@. We have arrived at these numbers via extensive 3D hydrodynamic simulations. The primary scientific goals are to identify/resolve physics issues and to develop a predictive understanding of plasma liner formation, liner ram pressure amplification during liner convergence, conversion of liner kinetic energy to thermal/radiation energy of the stagnated system, and confinement time of this energy. We are aiming for two scaled-up follow-on applications for this work if it is successful: (1)~assembling repetitive, macroscopic (cm and $\mu$s scale) plasmas suitable for fundamental HEDP studies and (2)~a standoff driver solution for magneto-inertial fusion. This poster provides an overview of the project and the research plan. Supported by the DOE Joint Program in HEDLP. [Preview Abstract] |
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TP8.00150: Plasma Guns for the Plasma Liner Experiment (PLX) F.D. Witherspoon, R. Bomgardner, A. Case, S.J. Messer, S. Brockington, L. Wu, R. Elton, S.C. Hsu, J.T. Cassibry, M.A. Gilmore A spherical array of minirailgun plasma accelerators is planned for the Plasma Liner Experiment (PLX) to be located at LANL. The plasma liner would be formed via merging of 30 dense, high Mach number plasma jets ($n\sim 10^{16-17}$~cm$^{-3}$, $M\sim $10--35, $v\sim $50--70~km/s, $r_{\rm jet}\sim $5~cm) in a spherically convergent geometry. Small parallel-plate railguns are being developed for this purpose due to their reduced system complexity and cost, with each gun planned to operate at $\sim$300~kA peak current, and launching up to $\sim$8000~$\mu$g of high-Z plasma using a $\sim$50~kJ pfn. We describe experimental development of the minirailguns and their current and projected performance. Fast operating repetitive gas valves have recently been added to allow injection of high density gases including helium, argon, and (eventually) xenon. We will present the latest test results with the high-Z gases, and discuss future plans for augmenting the rails, optimizing the nozzle configuration, preionizing the injected gas, and configuring the pulse forming networks with the capacitors available to the program. [Preview Abstract] |
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TP8.00151: Hydrodynamic Modeling of the Plasma Liner Experiment (PLX) Jason Cassibry, Scott Hsu, Doug Witherspoon, Marc Gilmore Implosions of plasma liners in cylindrically or spherically convergent geometries can produce high pressures and temperatures with a confinement or dwell time of the order of the rarefaction timescale of the liner. The Plasma Liner Experiment (PLX), to be built at LANL, will explore and demonstrate the feasibility of forming imploding plasma liners with the spherical convergence of hypersonic plasma jets. Modeling will be performed using SPHC and MACH2. According to preliminary 3D SPHC results, high Z plasma liners imploding on vacuum with $\sim $1.5MJ of initial stored energy will reach $\sim $100kbar, which is a main objective of the experimental program. Among the objectives of the theoretical PLX effort are to assist in the diagnostic analysis of the PLX, identify possible deleterious effects due to instabilities or asymmetries, identify departures from ideal behavior due to thermal and radiative transport, and help determine scaling laws for possible follow-on applications of $\sim $1~Mbar HEDP plasmas and magneto-inertial fusion. An overview of the plan to accomplish these objectives will be presented, and preliminary results will be summarized. [Preview Abstract] |
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TP8.00152: Diagnostics for the Plasma Liner Experiment (PLX) Elizabeth Merritt, Mark Gilmore, Alan Lynn, Jason Cassibry, F. Douglas Witherspoon, Scott Hsu High pressure imploding plasma liners are possible sources for assembling plasmas suitable for scientific studies of HEDP and a potential stand-off driver solution for magneto-inertial fusion (MIF). The Plasma Liner Experiment (PLX) will merge 30 dense, high Mach number plasma jets to study plasma liner formation and convergence dynamics. Measurements of the ion and electron densities, temperatures, and velocities will be crucial for characterizing the thermodynamic state of the liner from formation through stagnation. Hydrodynamic simulations predict widely varying density and temperature ranges in the liner over its evolution, from 10$^{15}$ - 10$^{19}$ cm$^{-3}$ and a few to hundreds of eV. To examine the plasma over this wide range of parameters, we will employ laser interferometry, Schlieren imaging, visible imaging, visible and VUV spectroscopy, pressure probes, x-ray photodiodes and bolometry. Simulation results for the proposed interferometers and Schlieren imaging are presented as well as an overview of the entire diagnostic suite. [Preview Abstract] |
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TP8.00153: Optical Measurements of Dense Hypervelocity Plasmoids from Coaxial and Railgun Plasma Accelerators Andrew Case, Sarah Messer, Richard Bomgardner, Samuel Brockington, Douglas Witherspoon, Ray Elton High velocity dense plasma jets are under continued experimental development for fusion applications including refueling, disruption mitigation, momentum injection/rotation drive, and magnetized target fusion. We present measurements taken on the plasmoids produced by a half-scale coaxial plasmoid accelerator, a full scale coaxial plasmoid accelerator, and a novel minirailgun accelerator. The data presented includes spectroscopic measurements of velocity and density, two point interferometric measurements of line integrated density and velocity, and fast framing camera imaging. Results from these measurements are in agreement with each other and with time of flight measurements taken using photodiodes, as well as total plasmoid momentum measurements taken using a ballistic pendulum technique. Plasma density is greater than $5 \times 10^{15}$ cm$^{-3}$, and velocities range up to 100 km/s, with a small component in some cases exceeding 120 km/s. [Preview Abstract] |
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TP8.00154: The HyperV Full-Scale Contoured-Gap Coaxial Plasma Railgun Samuel Brockington, Andrew Case, Sarah Messer, Richard Bomgardner, Raymond Elton, Linchun Wu, F. Douglas Witherspoon HyperV has been developing pulsed plasma injected coaxial railguns with a contoured gap profile designed to mitigate the blowby instability. Previous work using half-scale guns has been successful in launching 150~$\mu$g plasmas at 90~km/s [1]. In order to meet the original goal of 200~$\mu$g at 200~km/s the full-scale coaxial plasma gun has been constructed, and initial testing is beginning. This new plasma gun consists of two machined aluminum electrodes and a UHMW polyethylene breech insulator. The gun is breech fed by 64 ablative polyethylene capillary discharge units identical to the half-scale gun units. Maximum accelerator energy storage has also been increased 50\%. Refractory coatings may be necessary to allow full current ($\sim$800~kA) operation. The outer electrode includes 24 small diagnostic ports for optical and magnetic probe access to the plasma inside the gun to allow direct measurement of the plasma armature dynamics. Initial test data from the full-scale coax gun will be presented along with plans for future testing. Work supported by the U.S. DOE Office of Fusion Energy Sciences.\\[4pt] [1] F. D. Witherspoon, A. Case, S. Messer, R. Bomgardner, M. Phillips, S. Brockington, R. Elton, ``Contoured Gap Coaxial Plasma Gun with Injected Plasma Armature'' Rev. Sci. Instr. submitted (2009) [Preview Abstract] |
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TP8.00155: One-Dimensional Burn Dynamics of Plasma-Jet Magneto-Inertial Fusion John Santarius This poster will discuss several issues related to using plasma jets to implode a Magneto-Inertial Fusion (MIF) liner onto a magnetized plasmoid and compress it to fusion-relevant temperatures [1]. The problem of pure plasma jet convergence and compression without a target present will be investigated. Cases with a target present will explore how well the liner's inertia provides transient plasma stability and confinement. The investigation uses UW's 1-D Lagrangian radiation-hydrodynamics code, BUCKY, which solves single-fluid equations of motion with ion-electron interactions, PdV work, table-lookup equations of state, fast-ion energy deposition, and pressure contributions from all species. Extensions to the code include magnetic field evolution as the plasmoid compresses plus dependence of the thermal conductivity and fusion product energy deposition on the magnetic field.\\[4pt] [1] Y.C. F. Thio, et al.,``Magnetized Target Fusion in a Spheroidal Geometry with Standoff Drivers,'' in Current Trends in International Fusion Research, E. Panarella, ed. (National Research Council of Canada, Ottawa, Canada, 1999), p. 113. [Preview Abstract] |
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TP8.00156: Laser-driven preheat simulations of magnetized fusion fuel for magnetically-driven liner implosions on pulsed power accelerators A.B. Sefkow, M.C. Herrmann, S.A. Slutz, R.A. Vesey Magnetically-driven implosions of cylindrical metal liners containing magnetized and preheated fuel are being studied to determine whether significant inertial confinement fusion yields can be economically obtained on pulsed power accelerators. Preliminary radiation-MHD simulations of dense (1-5 mg/cc), axially-magnetized (3-30 T), and preheated (200-500 eV) deuterium-tritium (DT) fuel driven by a pulsed power accelerator similar to Z (25-60 MA) indicate fast implosions (100-300 ns) and high yields (100s kJ - 10s MJ) may be feasible. Efficient alpha particle trapping for self-heating and reduced heat conduction losses in the radial direction can be provided by compression of the axial magnetic field (to $\sim $ 10 kT), which substantially reduces the traditional threshold value of the fuel $\rho $R at ignition. Preheating the fuel before compressing it is advantageous because ignition temperatures can thereby be accessed without requiring extraordinarily large convergence ratios or implosion velocities. A few kJ of 2$\omega $ laser light delivered over a few ns by Z Beamlet (ZBL) may sufficiently preheat the fuel, and Z and ZBL experiments are planned to test the various components of this concept. [Preview Abstract] |
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TP8.00157: Simulation of laser-driven magnetic flux compression in inertial confinement fusion Chuandong Zhou, Ammar Hakim, John Loverich, Peter Stoltz Laser-driven magnetic-flux compression is an innovative approach to achieve magneto-inertial fusion (MIF). A cylindrical target with initial seed magnetic field is compressed by energetic laser beams. The magnetic field that is ``frozen-in'' plasma gets compressed with the target. The resulting high magnetic field reduces electron thermal conductivity and improves alpha particle confinement, thus providing an additional thermal insulation of the fuel forming the hot spot. Computer simulation is a main tool in this area, but an easy-to-use and easy-to-access, parallel, 3D code is not available to this community. New features, including laser energy deposition in under-dense plasma, are incorporated into the Tech-X fluid framework, TxFluids. We discuss our approach to determine the best algorithms for properly modeling laser-driven shock implosions with magnetic fields in conditions relevant to cylindrical MIF and benchmark magnetic compression with one-dimensional simulation. [Preview Abstract] |
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TP8.00158: Progress in plasma liner modeling for MIF John Loverich, Ammar Hakim, Sean Zhou Magnetic confinement fusion and inertial confinement fusion represent the two extremes in terms of density and confinement time in fusion energy research. Both approaches have been studied extensively through the decades pushing technology to the limits. An alternative fusion approach exists between these regimes called magnetized target fusion. In magnetized target fusion longer confinement times are achieved than in ICF through the use of strong magnetic fields, the long confinement time reduces the required plasma density to reach ignition--the approach has advantages over MFE in that it is much more compact and higher density. This work explores computationally a form of magnetized target implosion using a plasma liner. This concept is to be compared with solid liner implosion approach which may not be commercially viable as a reactor due to the ``standoff'' problem, portions of the device are destroyed with each target implosion. We present simulation results of plasma liner formation, jet merging, and plasma jet magnetized target interaction using a fluid plasma code (TxFluids) developed at Tech-X corporation. [Preview Abstract] |
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TP8.00159: Hybrid-PIC Algorithms for Simulation of Large-Scale Plasma Jet Accelerators Carsten Thoma, Dale Welch Merging coaxial plasma jets are envisioned for use in magneto-inertial fusion schemes as the source of an imploding plasma liner. An experimental program at HyperV is considering the generation of large plasma jets (length scales on the order of centimeters) at high densities (10$^{16}$-10$^{17}$ cm$^{-3}$) in long coaxial accelerators. We describe the Hybrid particle-in-cell (PIC) methods implemented in the code LSP for this parameter regime and present simulation results of the HyperV accelerator. A radiation transport algorithm has also been implemented into LSP so that the effect of radiation cooling on the jet mach number can be included self-consistently into the Hybrid PIC formalism. [Preview Abstract] |
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TP8.00160: Plasma Formation and Evolution from Thick Metal Pulsed with Megagauss Magnetic Field Thomas Awe, Bruno Bauer, Stephan Fuelling, Richard Siemon, Jaspreet Billing, Tasha Goodrich The threshold for plasma formation on the surface of thick metal, in response to a pulsed multi-megagauss magnetic field, is experimentally measured. Thick Aluminum rods with initial diameters ranging from 0.5 to 2.0 mm are pulsed with the 1.0 MA, 100-ns Zebra generator. Surface magnetic field rise rates vary from 30 to 80 MG per microsecond, with corresponding peak fields of 1.5 to 4 MG. The onset of thermal plasma is observed through an abrupt increase in the rate of surface heating when the surface temperature reaches about 0.7 eV. Plasma forms when the surface magnetic field exceeds 2 MG, independent of the rise rate of the applied field. High resolution (30 micron, 2 ns) images detail surface dynamics during this transition from warm-dense-aluminum to plasma. Measurements of magnetic field, brightness temperature, spectrum of emitted radiation, time of plasma formation, expansion velocity, and growth of instabilities are presented. [Preview Abstract] |
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TP8.00161: Modeling of an aluminum wall-confined plasma created by MG magnetic fields Richard Siemon, Irvin Lindemuth Experiments at UNR demonstrate that interesting fully ionized aluminum plasma (T=1-30 eV, n $\sim $ 10$^{18}$ cm$^{-3}$, Z $\sim $ 3) can be created on the surface of mm-size aluminum rods by MA pulses of current. The transport properties of such plasma and nearby wall material at higher density and lower temperature are not well established, but numerical modeling using SESAME tables or Russian material model gives surprisingly good agreement with experimental observations. During the experimental time of current rise, the observed plasma temperature typically increases from 1-eV to 15 eV or more. In order for that to happen the plasma must be sufficiently dense and in a layer sufficiently thick for Ohmic heating to be larger than conduction cooling. That requirement is equivalent to saying that the electron omega-tau must be less than a small number that depends weakly upon plasma Z. This demonstration that a fully ionized plasma can be held by magnetic pressure against a solid surface in what is known as a ``wall confined'' geometry supports one of the basic assumptions often made in the context of magneto-inertial fusion. [Preview Abstract] |
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TP8.00162: Simulation of Formation and Implosion of Plasma Liners for Magnetized Target Fusion Roman Samulyak, Lingling Wu, Paul Parks Spherically symmetric simulations of the implosion of plasma liners and compression of plasma targets in the concept of Magnetized Target Fusion have been performed using the method of front tracking. The cases of single deuterium and xenon liners and double liners containing both deuterium and xenon layers have been investigated. Simulations have been compared with recent theoretical works. In most favorable setups, the state of ignition and large fusion burn up fraction and energy gains have been achieved. The main conclusion of the study is that the efficiency of the method significantly increases if a large initial target (up to 30 cm in radius) is compressed by a high Mach number liner containing a heavy gas layer (xenon). Full 3D simulations of the propagation and merger of high Mach number deuterium jets and the formation of liner have also been performed. It has been shown that the jet merger reduces the Mach number. After the 5 m propagation and full merger of 144 jets with initial Mach number 60, the average Mach number of the liner was approximately 10. [Preview Abstract] |
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