THE STOR-1M TOKAMAK: EXPERIMENTS ON CURRENT REVERSAL AND FAST CURRENT RAMPING
Wolfe, Sean William
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Experiments on the STaR-1M tokamak have addressed the following problems in magnetic confinement fusion research: 1) the attainment of quasi-continuous operation in a fusion reactor by inductively driving a current which alternates polarity; 2) plasma heating by inducing turbulence to enhance the Ohmic dissipation; and, 3) the stability of tokamak plasmas carrying large currents. STOR-1M plasmas have major and minor radii of 22 cm and 3.5 cm, respectively, and are produced with hydrogen at a pressure of 0.9 mTorr. Typical discharges carry a current of 5 kA with a toroidal field of 1 T. Around the current peak, electrons at a density of 1 x 1013 cm-3are heated to a temperature of 80 eV. The ions, with an effective charge number of 2, reach a temperature of 30 eV. Input energy is confined for almost 0.1 ms at the current peak, and the total discharge length is usually 4.5 ms. To simulate the current reversal phase in an ac tokamak reactor, a sinusoidal plasma current has been sustained for one cycle. Peak currents of 8 kA and electron densities of 1.8 x 1013 cm-3have been attained. The electron density at the reversal is always at least 2 x 1012 cm-3. The unexpected equilibrium when the toroidal current goes through zero may be due to vertical plasma currents closing through the limiter or chamber walls. To induce turbulence for plasma heating, an electric field pulse of amplitude up to 360 Vim and width 20 µs drives up to 10 kA of current. on top of a normal discharge. After the pulse, electron temperatures of 300 eV and ion temperatures of 200 eV have been recorded. About 200 µs after the pulse, the electron density and temperature reach a peak, implying that containment of energy is enhanced. The safety factor at the plasma surface during the pulse can be as low as 1.5. Disruptive behaviour, in the form of current interruption and loop voltage spikes, is observed when the safety factor is between 1.85 and 2.1. Outside this range, the plasma is grossly stable.