Low-aspect-ratio tokamak start-up and operational current limits in CDX-U

Abstract

A new, small, low-aspect-ratio tokamak (LART), named the Current Drive Experiment-Upgrade (CDX-U), has been designed and built for the investigation and development of the LART configuration, as well as for the investigation of novel current drive methods. Recently, an inductive ohmic heating and current drive (OH) system, including a compact high-field OH transformer coil, was designed and installed in CDX-U, enabling the study of LART plasmas with higher plasma currents and temperatures.

Electron cyclotron resonance heating (ECH) was used in CDX-U to assist plasma breakdown, allowing breakdown with low initial induced voltage. Plasma start-up was achieved with transmitted ECH power of approximately l% of the maximum coupled OH power, at loop voltages as low as 1 Volt, and in toroidal magnetic fields ranging by a factor of 2.5 in strength. The reduction in loop voltage necessary for start-up minimized large, induced eddy currents in the toroidally continuous vessel walls common to LARTs. Plasma start-up and control in the presence of these significant vessel eddy currents was demonstrated, an important achievement for LART operation. Calculated ohmic efficiency, in terms of the Ejima coefficient. $C\sb{E}$, compared favorably with that found in other tokamaks, yielding $C\sb{E} \geq$ 0.3-0.4.

An operational current limit was found during extensive CDX-U ohmic operation, corresponding to an MHD safety factor, q(a), of approximately 3.5, a new low demonstrated q-limit for an aspect ratio, A, of 1.6. Studies of magnetic fluctuations in a range of plasma current from 15 kA to 40 kA revealed a coherent, saturating, 10-15 kHz frequency mode, with a toroidal mode number of n = l and a poloidol mode number ranging from m = 1 to m = 3. Numerical stability analysis of a magnetic reconstruction of a typical discharge exhibiting this mode indicated ideal stability. Previous studies of this mode at the lower plasma currents showed the amplitude increasing dramatically as the safety factor approached the operational current limit of q(a) = 3.5, and a radial mode structure consistent with magnetic island formation. These n = 1, low m, resistive modes are a good candidate for an MHD instability causing the observed operational current limit.