Density Threshold for Magnetohydrodynamic Activity in AlcatorC

Abstract

Experimental measurements of magnetic fluctuations arising from resistive magnetohydrodynamic tearing modes reveal unexpected behavior in Alcator $C$. After the plasma current has risen to its steady-state level, no significant magnetohydrodynamic activity is observed unless the electron density is above a threshold value, ${\overline{n}}_c$. For a given toroidal field, ${\overline{n}}_c$ is remarkably repeatable. We find ${\overline{n}}_c$ to be proportional to $B^2$ and independent of $I_p$.