EXPERIMENTAL BETA-LIMITS OF SYMMETRICAL LINEAR HELIAC CONFIGURATIONS

Helically symmetric heliac equilibria [H. P. Furth, Plasma Physics and Controlled Fusion Research (International Atomic Energy Agency, Vienn a, 1966), Vol. 1, p. 103] are formed on the High Beta Q Machine (HBQM) [C. M. Greenfield, Phys. Fluids B 2, 133 (1990)] by using a fast-risi ng central conductor (hardcore) current in conjunction with a shock-he ated l = 1 stellarator configuration. The equilibria are found to poss ess a high global beta and the plasma pressure is approximately a flux -surface quantity. Under the effects of plasma, the magnetic well is f ound to deepen and the rotational transform is greatly increased and b ecomes highly sheared, owing to plasma currents induced by the fast-ri sing hardcore current. In the second phase of the experiment, the equi librium fields of the symmetric heliac are lowered while maintaining t he same shock heating in an attempt to raise the global beta. No subst antial change in global beta is seen, indicating that the configuratio n forms at the beta limit in the shock-heated HBQM, and that the plasm a beta seen in the first phase of the experiment is the symmetric heli ac beta limit.