Self-stabilization of ideal modes in a heliac

Fusion devices operating in the second stability regime are very attractive because there is no limit to the amount of plasma pressure contained in the device. Second stability regime has been predicted theoretically for tokamaks and stellarators; moreover self-stabilization has been observed experimentally in the first operation of the ATF device. In this paper we try to understand the self-stabilization behavior to Mercier modes of some TJ-II configurations whose equilibria were obtained using the 3D VMEC code. The Flexible Heliac TJ-II, under construction in Spain is characterized by a high degree of flexibility: changing the ratio of currents in its two central coils, different magnetic configurations can be achieved with a wide range of parameters (0.22≤ι/4≤0.628, -1%≤ magnetic well ≤6%). We studied the behavior of three configurations to Mercier modes belonging to two different rotational transform 'families'. (author) 5 refs., 4 figs., 1 tab