Confinement of Injected Plasmas in Stellarators

Abstract

Plasmas produced by a washer plasma gun have been injected into B‐3, which is a twisted figure‐eight stellarator, and Etude, which is a racetrack stellarator. Both have $\text{l\hspace{0.17em}=\hspace{0.17em}3}$ helical windings. In the injected plasmas n_e ≤ 5 < 10⁹, T_e ≈ 5‐8 eV, and T_i is probably greater than T_e. The mean free paths for electron collisions were in the range 10‐100 machine lengths. The observed confinement times varied with confining field as B^m with $\text{0.6 < m < 1.2}$ for $\text{B ≤ 20}\mathrm{kG}$ in B‐3. In Etude, the confinement times varied from the Bohm value at zero rotational transform ι and increased by a factor 20 when $\text{ι\hspace{0.17em}=\hspace{0.17em}5.43}$ rad. In B‐3 the confinement times varied from about twice the Bohm value when only the geometric transform was present and increased by a factor 30 when ι was increased to 19 rad. It was not possible to decide if the increase of confinement time was due to effects of shear or rotational transform.