Bulletin of the American Physical Society
50th Annual Meeting of the Division of Plasma Physics
Volume 53, Number 14
Monday–Friday, November 17–21, 2008; Dallas, Texas
Session VI2: Current Drive |
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Chair: Nathaniel Fisch, Princeton Plasma Physics Laboratory Room: Landmark B |
Thursday, November 20, 2008 3:00PM - 3:30PM |
VI2.00001: Non-solenoidal Plasma Startup in the Pegasus Toroidal Experiment Invited Speaker: Non-solenoidal (NS) startup will simplify the design of future tokamaks by eliminating need for a central solenoid and is required for an ST based CTF. In Pegasus, washer-stack current sources (plasma guns) are used to initiate NS discharges via point-source DC helicity injection. Current injected parallel to the helical vacuum field can relax into a tokamak-like configuration with toroidally-averaged closed flux and tokamak-like confinement. This requires no modification of the vacuum vessel and is scalable to fusion grade systems with proper geometry. Guns in the divertor region create discharges with $I_{p}$ up to 50 kA, 3 times the vacuum windup. Nonlinear 3D simulation with NIMROD shows excitation of a line-tied kink, producing poloidal flux amplification. Evidence of flux amplification includes: reversal of edge poloidal magnetic flux; $I_{p}$ increase over vacuum geometric windup; plasma position subject to radial force balance; and persistence of I$_{p}$ after gun shut-off. Equilibria show high edge current ($l_{i}$ = 0.2) and elevated $q$ ($q_{min} \quad >$ 6), allowing access to high $I_{N}$ ($I_{N} \quad >$ 12). Guns at the outboard midplane produce $I_{p}$ up to 7 times the vacuum windup with large $n$=1 activity when edge $q$ passes through rational surfaces. Line averaged density up to 2x10$^{19}$ m$^{-3}$ after relaxation shows an increase in particle confinement over non-relaxed cases. Maximum $I_{p}$ is determined by helicity and radial force balance, tokamak stability, and Taylor relaxation. Coupling midplane gun discharges to other CD is straightforward due to $I_{p}$ decay times $>$3 ms. Poloidal field induction has been used to create NS discharges up to 80 kA and gun plasmas with $I_{p}$ of 60 kA have been ramped to over 100 kA by including OH drive. Present research is aimed at understanding the physics of this technique in order to form NS targets in excess of 200 kA and design NS startup systems for larger devices. [Preview Abstract] |
Thursday, November 20, 2008 3:30PM - 4:00PM |
VI2.00002: Validation of On- and Off-axis Neutral Beam Current Drive Against Experiment in DIII-D Invited Speaker: Neutral beam current drive (NBCD) experiments in DIII-D using vertically shifted plasmas to move the current drive away from the axis have clearly demonstrated off-axis NBCD. Time-dependent measurements of magnetic pitch angles by the motional Stark effect diagnostic are used to obtain the evolution of the poloidal magnetic flux, which indicates a broad off-axis NBCD profile with a peak at about half the plasma radius. The measured off-axis NBCD profile is consistent with calculations using an orbit-following Monte-Carlo code for the beam ion slowing down including finite-orbit effects, provided there are no large-scale MHD activities such as AE modes or sawteeth. Agreement is found between the measured pitch angles and those from simulations using transport-equilibrium codes. The fast-ion density profile is inferred from neutron and fast-ion $D_\alpha$ diagnostics. As expected, prompt losses are larger (smaller) for off-axis (on-axis), perpendicular (tangential), or counter-current (co-current) injection. Some evidence of non-classical transport is observed. Off-axis NBCD is planned for ITER, so detailed comparison of theoretical models with experimental measurements is needed for accurate projections. Steady-state, high $\beta_N$ tokamak scenarios require current drive that is maximum off-axis. The magnitude of off-axis NBCD is sensitive to the alignment of the beam injection relative to the helical pitch of the magnetic field lines. If the signs of $B$ and $I$ yield the proper helicity, both measurement and calculation indicate that the efficiency is good, even better than for on-axis NBCD because the increased fraction of trapped electrons reduces the electron shielding of the injected ion current, in contrast with electron current drive schemes where the trapped electrons degrade the efficiency. [Preview Abstract] |
Thursday, November 20, 2008 4:00PM - 4:30PM |
VI2.00003: An assessment of full-wave effects on the propagation and absorption of lower hybrid waves Invited Speaker: Lower hybrid (LH) waves have the attractive property of damping strongly via electron Landau resonance on relatively fast tail electrons. Consequently these waves are well-suited to driving current in the plasma periphery where the electron temperature is lower, making LH current drive (LHCD) a promising technique for off--axis (r/a$\sim $0.60) current profile control in reactor grade plasmas. Established modeling techniques use WKB expansions with non-Maxwellian self-consistent distributions. Higher order WKB expansions have shown some effects on the parallel wavenumber evolution and consequently on the damping due to diffraction [1]. A massively parallel version of the TORIC full-wave electromagnetic field solver valid in the LH range of frequencies has been developed [2] and applied to scenarios at the density and magnetic field characteristic of devices such as Alcator C-Mod and ITER [B0$\approx $5 T, ne$\approx 1\times10^{20}$ m$^{-3}$]. We find that retaining full wave effects due to diffraction and focusing has a strong effect on the location of wave absorption. Diffraction occurs at caustic surfaces and in resonance cones resulting in a large upshift of the parallel wavenumber and localized power deposition. For some values of density and magnetic field when the waves are fully accessible to the center of the plasma, the full wave description predicts all power being damped at larger radii (r/a $\sim $ 0.7) in contrast to ray tracing which shows more central power absorption. By incorporating a Fokker-Planck code for self-consistent treatment of the electron distribution and using an synthetic hard X-ray diagnostic we compare the code predictions by both full wave and ray tracing methods with recent Alcator C-Mod experiments. We will compare full-wave and ray tracing for low and high single pass damping regimes. \\[0pt] [1] G. Pereverzev, Nucl. Fusion 32 1091 (1991). \\[0pt] [2] J. C. Wright, E. J. Valeo, C. K. Phillips and P. T. Bonoli, Comm. in Comput. Physics 4 545 (2008). [Preview Abstract] |
Thursday, November 20, 2008 4:30PM - 5:00PM |
VI2.00004: Advanced Techniques for Neoclassical Tearing Mode Control by Electron Cyclotron Current Drive in DIII-D Invited Speaker: Novel techniques have been developed in DIII-D for (1) control of rapidly rotating neoclassical tearing modes (NTMs) and (2) control of NTMs that have locked to a residual error field or the resistive wall. Electron cyclotron current drive (ECCD) has been successful at suppression of NTMs in present tokamaks, but will face new challenges in ITER where NTMs are expected to be more prone to locking. In order to avoid locking, rotating islands must be controlled at small widths that are expected to be narrower than the ECCD deposition. Under these conditions, modulated ECCD is predicted to stabilize more efficiently than continuous current drive. (1) A new technique developed at DIII-D detects the island using oblique electron cyclotron emission with a line of sight equivalent to that of the ECCD. This removes much of the uncertainty in mapping the island structure from the detector to the current drive location. This method was used both to measure the radial alignment between ECCD and the island, and to synchronize the modulation in phase with the island O-point, successfully stabilizing an NTM with mode numbers $m/n=3/2$. (2) If islands do grow large enough to lock, locked mode control will be necessary for recovery or avoiding disruption in ITER. A potential difficulty associated with locking is that the mode can lock in a position not necessarily accessible to ECCD. To obviate this problem, magnetic perturbations were used for the first time to unlock and reposition a locked $m/n=2/1$ mode in order to bring it in view of the gyrotron beam, leading to a significant reduction in island size. Once unlocked, magnetic perturbations were also used to sustain and control the mode rotation, which has the potential for easier ECCD modulation [Preview Abstract] |
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