Second stability in the ATF torsatron

Abstract

Access to the MHD second stability regime has been achieved in the ATF torsatron. Experimental \ensuremath{\beta} values (${\ensuremath{\beta}}_{0}$\ensuremath{\le}3%, with fast ions contributing \ensuremath{\approxeq}(1/3 of the pressure at high \ensuremath{\beta}) are well above the theoretical transition value (${\ensuremath{\beta}}_{c}$\ensuremath{\approxeq}1.3% for ideal modes) required to reach this regime. The relatively low ${\ensuremath{\beta}}_{c}$ results from operation with peaked pressure profiles. The measured magnetic fluctuations decrease with increasing \ensuremath{\beta}, and the pressure profile broadens. This behavior is consistent with theoretical predictions for \ensuremath{\beta} self-stabilization of resistive interchange modes.