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ABSTRACT: Stabilization of the Resistive Wall Mode (RWM) in the NSTX tokamak is important to achieve high-beta plasmas. This paper numerically investigates state-space control algorithms for improved performance of RWM control using the existing six external control coils with off-midplane poloidal magnetic field sensors in NSTX. Experimentally Ã<sub>N</sub> = 5.6 was achieved with the present proportional gain controller. The proposed LQG controller is capable of reaching Ã<sub>N</sub> = 6.7 for slowly rotating plasma modes and the ideal wall limit Ã<sub>N</sub> = 7:06 for plasma modes with higher natural rotation speed.
Decision and Control, 2009 held jointly with the 2009 28th Chinese Control Conference. CDC/CCC 2009. Proceedings of the 48th IEEE Conference on; 01/2010
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S A Sabbagh,
J W Berkery,
R E Bell, J M Bialek,
S P Gerhardt,
J E Menard,
R Betti,
D A Gates,
B Hu,
O N Katsuro-Hopkins,
B P Leblanc,
F M Levinton,
J Manickam,
K Tritz,
H Yuh
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ABSTRACT: Stabilizing modes that limit plasma beta and reduce their deleterious effect on plasma rotation are key goals for the efficient operation of a fusion reactor. Passive stabilization and active control of global kink/ballooning modes and resistive wall modes (RWMs) have been demonstrated on NSTX and research is now advancing towards understanding the stabilization physics and reliably maintaining the high beta plasma for confident extrapolation to ITER and a fusion component test facility based on the spherical torus. Active n = 1 control experiments with an expanded sensor set, combined with low levels of n = 3 field phased to reduce error fields, reduced resonant field amplification and maintained plasma rotation, exceeded normalized beta = 6 and produced record discharge durations limited by magnet system constraints. Details of the observed RWM dynamics during active control show the mode being converted to a rotating kink that stabilizes or saturates and may lead to tearing modes. Discharges with rotation reduced by n = 3 magnetic braking suffer beta collapse at normalized beta = 4.2 approaching the no-wall limit, while normalized beta greater than 5.5 has been reached in these plasmas with n = 1 active control, in agreement with the single-mode RWM theory. Advanced state-space control algorithms proposed for RWM control in ITER theoretically yield significant stabilization improvements. Values of relative phase between the measured n = 1 mode and the applied correction field that experimentally produce stability/instability agree with RWM control modelling. Experimental mode destabilization occurs over a large range of plasma rotation, challenging the notion of a simple scalar critical rotation speed defining marginal stability. Stability calculations including kinetic modifications to the ideal MHD theory are applied to marginally stable experimental equilibria. Plasma rotation and collisionality variations are examined in the calculations. Intermediate rotation levels are less stable, consistent with experimental observations. Trapped ion resonances play a key role in this result. Recent experiments have demonstrated magnetic braking by non-resonant n = 2 fields. The observed rotation damping profile is broader than found for n = 3 fields. Increased ion temperature in the region of maximum braking torque increases the observed rate of rotation damping, consistent with the theory of neoclassical toroidal viscosity at low collisionality.
Nucl. Fusion. 01/2010; 503030(52).
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ABSTRACT: The resistive-wall mode is actively stabilized in the National Spherical Torus Experiment in high-beta plasmas rotating significantly below the critical rotation speed for passive stability and in the range predicted for the International Thermonuclear Experimental Reactor. Variation of feedback stabilization parameters shows mode excitation or suppression. Stabilization of toroidal mode number unity did not lead to instability of toroidal mode number two. The mode can become unstable by deforming poloidally, an important consideration for stabilization system design.
Physical Review Letters 08/2006; 97(4):045004. · 7.37 Impact Factor
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W Zhu,
S A Sabbagh,
R E Bell, J M Bialek,
M G Bell,
B P LeBlanc,
S M Kaye,
F M Levinton,
J E Menard,
K C Shaing,
A C Sontag,
H Yuh
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ABSTRACT: Dissipation of plasma toroidal angular momentum is observed in the National Spherical Torus Experiment due to applied nonaxisymmetric magnetic fields and their plasma-induced increase by resonant field amplification and resistive wall mode destabilization. The measured decrease of the plasma toroidal angular momentum profile is compared to calculations of nonresonant drag torque based on the theory of neoclassical toroidal viscosity. Quantitative agreement between experiment and theory is found when the effect of toroidally trapped particles is included.
Physical Review Letters 07/2006; 96(22):225002. · 7.37 Impact Factor
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S A Sabbagh,
J W Berkery, J M Bialek,
R E Bell,
S P Gerhardt,
O N Katsuro-Hopkins,
J E Menard,
H Reimerdes,
R Betti,
L Delgado-Aparicio,
D A Gates,
B Hu,
B P Leblanc,
J Manickam,
D Mastrovito,
J.-K Park,
Y.-S Park,
K Tritz
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ABSTRACT: Maintaining steady fusion power output at high plasma beta is an important goal for future burning plasmas such as in ITER advanced scenario operation and a fusion nuclear science facility. Research on the National Spherical Torus Experiment (NSTX) is investigating stability and control physics to maintain steady high plasma normalized beta with minimal fluctuation. Resistive wall mode (RWM) instability is observed at relatively high rotation levels. Analysis including kinetic effects using the MISK code shows a region of reduced stability for marginally stable experimental plasmas caused by the rotation profile falling between stabilizing ion precession drift and bounce/transit resonances. Energetic particle (EP) effects are stabilizing but weaker than in tokamaks due to a reduced EP population in the outer plasma. Calculations for ITER show that alpha particles are required to stabilize the RWM at anticipated rotation levels for normalized beta of 3. Combined RWM and new beta feedback control capability were used to generate high pulse-averaged normalized beta with low fluctuation. Non-resonant braking by applied 3-D fields was used to alter plasma rotation compatibly with beta feedback. A newly implemented RWM state space controller produced long pulse, high normalized beta plasmas at low internal inductance. Neoclassical toroidal viscosity (NTV) torque by applied 3-D fields could be used to actuate rotation control and avoid rotation profiles unfavorable for RWM stability. As the ExB frequency is reduced, the NTV torque is expected to increase as collisionality decreases, and maximize when it falls below the ∇B drift frequency (superbanana plateau regime). Increased non-resonant braking was observed at constant applied field and normalized beta in experiments when rotation and ExB frequency were reduced to low values. The RWM multi-mode spectrum is computed in high beta plasmas using the multi-mode VALEN code. The computed RWM growth rate for instabilities and natural mode rotation for stabilized modes agrees with experiment. The computed multi-mode RWM spectrum shows significant amplitude in low-order ideal eigenfunctions other than the least-stable eigenfunction of single-mode analysis.
