Second stability in the ATF torsatron

Access to the MHD second stability regime has been achieved in the ATF torsatron. Experimental β values (${\beta }_0$≤3%, with fast ions contributing ≊(1/3 of the pressure at high β) are well above the theoretical transition value (${\beta }_c$≊1.3% for ideal modes) required to reach this regime. The relatively low ${\beta }_c$ results from operation with peaked pressure profiles. The measured magnetic fluctuations decrease with increasing β, and the pressure profile broadens. This behavior is consistent with theoretical predictions for β self-stabilization of resistive interchange modes.