Design and Operation of the Electrical Noise Suppression System for CHI on NSTX and NSTX-U

Authors: Z. Gao, R. Ramakrishnan, C. Neumeyer, D. Mueller

Abstract: Coaxial Helicity Injection (CHI) is a novel system for solenoid-free plasma current initiation on Tokamaks and Spherical Tokamaks (ST). The method uses divertor coils to produce magnetic flux that connects the lower divertor plates on NSTX/NSTX-U, which are electrically insulated from each other. A voltage is applied across the divertor plates using a 50 mF, 2 kV capacitor bank and D2 gas in injected in the lower gap, which drives a current along the helical field lines connecting the plates. Electromagnetic forces cause the discharge to expand into the vessel and a combination of magnetic reconnection and inductively driven current form a plasma with closed flux surfaces inside the vessel.  On NSTX-U it is planned to increase the voltage limits to up to 3 kV.

As the injected flux closes on itself to generate a tokamak-like equilibrium, rapid changes to the plasma circuit inductance can generate high voltage spikes. To suppress these spikes, two voltage suppression systems are employed. The first is a capacitor based snubber system, load balanced between the inner and outer vessel components of NSTX/NSTX-U.  The second is a Metal Oxide Varistor (MOV) system. The MOV system is deployed at three toroidal locations around the device. Design calculations are able to closely match the observed behavior on NSTX. Design details for both these systems, including improvements to the MOV system for NSTX-U, will be discussed in conjunction with experimental measurements from NSTX.

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