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DISRUPTION MITIGATION STUDIES IN DIII-D Data on the discharge behavior, thermal loads, halo currents, and runaway electrons have been obtained in disruptions on the DIH-D tokamak [J.L. Luxon and L.G. Davis, Fusion Technology 8,2A 441 (198.31. These experiments have also evaluated techniques to mitigate the disruptions while minimizing runaway electron production. Experiments injecting cryogenic impurity "killer" pellets of neon and argon and massive amounts of helium gas have successfully reduced these disruption effects. The halo current generation, scaling, and mitigation are understood and are in good agreement with predictions of a semianalytic model. Results from "killer" pellet injection have been used to benchmark theoretical models of the pellet ablation and energy loss. Runaway electrons are often generated by the pellets and new runaway generation mechanisms, modifications of the standard Dreicer process, have been found to explain the runaways. Experiments with the massive helium gas puff have also effectively mitigated disruptions without the formation of runaway electrons that can occur with "killer" pellets.

A set of neutron counters and a pair of scintillators measure the 2.5 MeV neutron emission produced by the DIII-D tokamak. The neutron counter set provides a large dynamic range ͑ϳ7 orders of magnitude͒ while the scintillators provide the very fast resolution needed for studying transient events. The counters are absolutely calibrated in situ with a 252 Cf source and the scintillators are cross calibrated to the counters. The historic variations in the emission measured by the various detectors have been compared and are consistent within the estimated accuracy of the absolute calibration ͑15%͒. In the discharges with the highest emission levels ͑2.4ϫ1016 n/s͒, the signals from the neutron counters and the scintillators agree well. Comparisons with other diagnostics also corroborate the neutron measurements.

A new type of toroidally localized kink instability, which we named the "slinky mode," was observed in a reverse-field-pinch plasma in the OHTE device. It is found that the slinky mode is the result of the phase locking of several internal kink modes due to nonlinear coupling and is an effective way to approach the Taylor relaxed state.PACS numbers: 52.35.Py, 52.55.EzReverse-field pinches (RFP's) were successfully operated in the OHTE device with the resistive shell, and discharges were sustained over 10 ms, much longer than the resistive-shell time constant of 1.5 ms. Global plasma characteristics with the resistive shell were very similar to those obtained with the conducting shell.' Since the linear MHD theories predict that RFP plasmas are unstable with a resistive shell and that unstable modes grow on the resistive-shell time scale, ^""^ magnetic fluctuations were studied in detail. A new type of toroidally localized kink instability was observed. In this Letter we report characteristics of the instability and its role in RFP discharges.The OHTE device has a major radius of 1.24 m and a plasma radius of 0.183 m. The resistive shell is placed at a minor radius of 0.200 m. The details of the device and the operation mode are described elsewhere. ^ In order to investigate detailed MHD behavior of the plasma, an ar-300

Both external and internal magnetic probes have been used in low-current discharges and external probes in high-current discharges to study the magnetic field configuration and its fluctuations in the OHTE toroidal pinch with toroidal field reversal. The equilibrium magnetic field configuration is close to that of the Taylor state in the central half of the plasma (μ≡μ0bJ/B is constant) but differs in the outer half (μ gradually goes to zero at the plasma edge). Measurements of the magnetic fluctuations indicate that the dominant fluctuation mode observed is the resistive internal kink with m=1, n≂18. The measured relative level of magnetic field fluctuations scales as B̃/B∼S−1/2, where S is the magnetic Reynolds number.