Density Threshold for Magnetohydrodynamic Activity in Alcator C

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, n¯_c. For a given toroidal field, n¯_c is remarkably repeatable. We find n¯_c to be proportional to B² and independent of I_p.