EBT-II conceptual design study

Abstract

The proposed EBT-II will use 48 superconducting coils to provide the required steady-state, cyclotron resonant, toroidal magnetic field of 4.3 T for electron cyclotron heating (ECH) with microwaves at 120 GHz. The peak design fields are 7.8 T in the coil, 6.0 T in the coil throat, and 3.0 T in the cavity midplane. Bulk ECH will be provided by eight 120-GHz, 200-kW cw gyrotrons, and provisions have been made for an additional 800 kW of cw profile heating in the range of 70 to 90 GHz. The torus major radius and mean coil radius are 520 cm and 28 cm respectively, yielding a mechanical aspect ratio of approximately 20. The basic device parameters of microwave frequency and power, magnetic field, and size should allow experiments that penetrate well into the collisionless regime and thus provide a definitive test of neoclassical scaling. This scaling test will be pushed further by using a built-in provision which permits changing the effective aspect ratio by varying the current through 96 copper toroidal aspect ratio enhancement (ARE) coils (2 per microwave cavity). Phase 1 of the experiment will be the study of transport and aspect ratio scaling of basic device parameters for comparison with results from EBT-I and EBT-Scale (EBT-S). Under optimum conditions, we project n tau approximately 10/sup 13/ sec/cm/sup 3/ with T/sub i/ and T/sub e/ approximately 3 to 8 keV. Phase 2 will center on the addition of 1 MW of neutral beam heating for direct heating of ions in conjunction with microwave heating of electrons so as to permit control of the ambipolar potential. With this added heating power, it appears possible to attain near-breakeven conditions in a hydrogen plasma at ..beta.. approximately equal to 10%.