Design of a sawtooth-free plasma in an inductively-operated pulsed tokamak reactor

An inductively-operated pulsed tokamak reactor, where a current profile is automatically determined with the combination of Ohmic current and bootstrap current, has been designed for various aspect ratio plasmas with a sawtooth-free constraint; i.e., a safety factor at the plasma center q(0) is larger than unity. In low aspect ratio tokamak (A=3), q(0) value decreases deeply below unity (typically q(0)-0.5) when the safety factor at the plasma surface, qΨ(a), is as low as 3-4. The fusion power of 10-30% will fluctuate only by the modification of plasma parameters due to sawtooth crash, and it would be an important issue if the sawtooth is not completely stabilized by some mechanism. To eliminate q=1 surface, qΨ(a) value should be raised to 5-6. This sawtooth-free condition with q(0)>1 results in larger machine size (R=7m->8.5m, typically) for a plasma current sufficient for the ignition (Ip-20MA). On the other hand, for A=5 plasmas, the fraction of the bootstrap current, which has a relatively broad current profile, is about 50% of the total plasma current, because of its high βp value. Reactor with R-9.5m can be, therefore, designed in satisfying both conditions of q(0)≥1 and qΨ(a)≥3, and the plasma current necessary for the ignition condition is also reduced to ∼12MA. The A=5 plasma has a smaller plasma volume than the A=3 plasma, although the major radius is slightly large. While, in A=7 plasma the current profile becomes very broad because of its enormous increase of the bootstrap current, resulting in the weak magnetic shear configuration. If the condition that qΨ(a)≥3 is imposed on the plasma design, the q(0) value raises up to 1.8 or more. (author)