A new sawtooth control mechanism relying on toroidally propagating ion cyclotron resonance frequency waves: Theory and Joint European Torus tokamak experimental evidencea) Phys. Plasmas 17, 056118 (2010); 10.1063/1.3363201 Sawtooth control using beam ions accelerated by fast waves in the DIII-D tokamakClosed and open loop control techniques were applied to growing m/nϭ2/1 rotating islands in wall-stabilized plasmas in the High Beta Tokamak-Extended Pulse ͑HBT-EP͒ ͓J. Fusion Energy 12, 303 ͑1993͔͒. HBT-EP combines an adjustable, segmented conducting wall ͑which slows the growth or stabilizes ideal external kinks͒ with a number of small ͑6°wide toroidally͒ driven saddle coils located between the gaps of the conducting wall. Two-phase driven magnetic island rotation control from 5 to 15 kHz has been demonstrated powered by two 10 MW linear amplifiers. The phase instability has been observed and is well modeled by the single-helicity predictions of nonlinear Rutherford island dynamics for 2/1 tearing modes including important effects of ion inertia and finite Larmor radius, which appear as a damping term in the model equations. The closed loop response of active feedback control of the 2/1 mode at moderate gain was observed to be in good agreement with the theory. Suppression of the 2/1 island growth has been demonstrated using an asynchronous frequency modulation drive which maintains the inertial flow damping of the island by application of rotating control fields with frequencies alternating above and below the natural mode frequency. This frequency modulation control technique was also able to prevent disruptions normally observed to follow giant sawtooth crashes in the plasma core.

The High Beta Tokamak-Extended Pulse ͑HBT-EP͒ experiment ͓J. Fusion Energy 12, 303 ͑1993͔͒ combines an internal, movable conducting wall with a high-power, modular saddle coil system to provide passive and active control of long wavelength magnetohydrodynamic ͑MHD͒ instabilities. Systematic adjustment of the radial position, b, of the conducting wall elements in relation to the surface of the plasma ͑minor radius a͒ resulted in the suppression of ␤-limiting disruptions for discharges in which b/aϽ1.2 and a positive plasma current ramp was maintained. Conducting wall stabilization of kink instabilities was observed in discharges with strong current ramps and in plasmas with ␤ values near the Troyon stability boundary. The frequency of slowly growing modes that persisted in wall-stabilized discharges was controlled by applying oscillating mϭ2, nϭ1 resonant magnetic perturbations. A compact, single-phase saddle coil system permitted modulation of the rotation velocity of internal m/nϭ2/1 instabilities by a factor of 2.

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High-frequency multimegawatt polyphase resonant power conditioning techniques have recently been realized as a result of key component developments, cooperative efforts, research and development funding contracts, and newly applied engineering techniques. The first generation 10-MW pulsed converter-modulators, implemented at Los Alamos National Laboratory, Los Alamos, NM, are now utilized for the Oak Ridge National Laboratory, Oak Ridge, TN, Spallation Neutron Source (SNS) accelerator klystron radio frequency amplifier power systems [1]. Three different styles of polyphase resonant converter-modulators were developed for the SNS application. The various systems operate up to 140-kV, or 11-MW pulses, or up to 1.1 MW average power, all from a direct current input of + 1.2 kV. The component improvements realized with the SNS effort coupled with new applied engineering techniques have resulted in dramatic changes in overall power conditioning topology. As an example, the 20-kHz high-voltage transformers are less than 1% the size and weight of equivalent 60-Hz versions. With resonant conversion techniques, load protective networks are not required.A shorted load de-tunes the resonance which results in limited power transfer. This provides for power conditioning systems that are inherently self-protective, with automatic fault "ride-through" capabilities. By altering and iterating the Los Alamos design, higher power and continuous wave power conditioning systems can now be realized with improved performance and flexibility. This paper will examine the SNS engineering data, briefly review the underlying theory of polyphase resonant conversion techniques, and apply this knowledge to future system topologies.