Effect of a Helical Field on the Plasma Behavior in the Model C Stellarator

The superposition of an l = 3 helical field on the uniform confining magnetic field of the Model C Stellarator introduces a rotational transform of up to 58° at 5 cm radius. Measurements have been made of the effect on the plasma of superimposing this helical field on the confining field. If the confining field of the stellarator is made ``symmetric'' (if each line of force is closed when no helical field is applied), superimposing the helical field improves plasma equilibrium and increases the plasma temperature and particle confinement time. When the rotational transform from the Ohmic heating current is in the opposite sense to that from the helical winding, such that the net transform near the outer edge of the plasma is zero, a rapid loss of plasma occurs. The effect of a deliberately introduced lack of closure in the vacuum field is minimized by the helical field.