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Small (o.d. = 1.8 mm, t = 0.37 mm) polystyrene shells filled with either pressurized argon gas or boron powder have been fired into DIII‐D plasmas for disruption mitigation experiments. The pellet shells were observed to burn up at ρ≈0.5, roughly consistent with ablation rate calculations. Pellet slowing from 350 m/s down to 100 m/s was observed, which is not well‐understood at present. Negligible plasma current contraction or MHD onset were seen as a result of the shell burn‐up in the plasma edge, consistent with calculations. The pellet payloads were observed to ionize rapidly in the pellet vicinity (

Resistive neoclassical tearing modes (NTMs) will be the principal limit on stability and performance in the ITER standard scenario as the resulting islands break up the magnetic surfaces that confine the plasma. Drag from rotating island‐induced eddy current in the resistive wall can also slow the plasma rotation, produce locking to the wall, and cause loss of high confinement H‐mode and disruption. The NTMs are maintained by helical perturbations to the pressure‐gradient driven “bootstrap” current. Thus, this is a high beta instability even at the modest beta for ITER. A major line of research on NTM stabilization is the use of radio frequency (rf) current drive at the island rational surface. While large, broad current drive from lower hybrid waves has been shown to be stabilizing (COMPASS‐D), most research is directed to small, narrow current drive from electron cyclotron waves (ECCD); ECCD stabilization and/or preemptive prevention is successful in ASDEX Upgrade, DIII‐D and JT‐60U, for example, with a...

Complete suppression of the m=2/n=1 neoclassical tearing mode (NTM) is reported for the first time using electron cyclotron current drive (ECCD) to noninductively generate current at the radius of the island O‐point. Experiments on the DIII‐D tokamak show that the maximum shrinkage of the m=2/n=1 island amplitude occurs when the ECCD location coincides with the q=2 surface. Estimates of the ECCD radial profile width from the island shrinkage are consistent with ray tracing calculations but may allow for a factor‐of‐1.5 broadening from electron radial transport.

A271 ELECTRON CYCLOTRON CURRENT DRIVE IN DIII-D: EXPERIMENT AND THEORY. Experiments on the DIII-D tokamak in which the measured off-axis electron cyclotron current drive has been compared systematically to theory over a broad range of parameters have shown that the Fokker-Planck code CQL3D provides an excellent model of the relevant current drive physics. This physics understanding has been critical in optimizing the application of ECCD to high performance discharges, supporting such applications as suppression of neoclassical tearing modes and control and sustainment of the current profile.

Tearing modes have been shown to limit β and confinement in conventional ELMing H-mode tokamak regimes. The tearing modes grow from a “seed” island due to the destabilizing effect of pressure flattening in the island leading to a reduction in the local bootstrap current. Recent experiments on the DIII-D tokamak have demonstrated stabilization of m=3/n=2 tearing modes in the presence of sawteeth through localized electron cyclotron current drive (ECCD). Variation of the deposition location indicates the ECCD remains localized despite the beam traversing an ELMing edge. The effect of the ECCD on the mode is consistent with predictions that the ECCD must be within the island for stabilization. The calculated EC current density (JEC) is greater than the calculated local bootstrap current density (JBS) also in accord with predictions. A closed-loop feedback scheme has been successfully operated for the first time using position control and magnetic signals as the actuator and sensor, respectively.

Neoclassical tearing modes are found to limit the achievable beta in many high performance discharges in DIII-D. Electron cyclotron current drive within the magnetic islands formed as the tearing mode grows has been proposed as a means of stabilizing these modes or reducing their amplitude, thereby increasing the beta limit by a factor around 1.5. Some experimental success has been obtained previously on Asdex-U. Here we examine the parameter range in DIII-D in which this effect can best be studied.