Electron confinement in a bumpy torus without the influence of hot‐electron rings—‘‘Ring killer’’ experiment

The ELMO Bumpy Torus (EBT) [P l a s m a P h y s i c s a n d C o n t r o l l e d N u c l e a r F u s i o n (IAEA, Vienna, 1975), Vol. II, p. 141] normally has an energetic electron ring in each of its 24 mirror sectors. The original intention of using this hot‐electron population was to provide an average local minimum in the magnetic field (through its diamagnetism) to stabilize the simple interchange and flute modes, which otherwise are theoretically inherent in a closed‐field‐line bumpy torus. To study the confinement properties of a bumpy torus without the influence of hot‐electron rings, a water‐cooled stainless steel limiter in each mirror sector was extended into the plasma to the ring location; this eliminated the hot‐electron ring population. These limiters were aptly named ‘‘ring killers.’’ Electron temperature, density, space potential, and plasma fluctuations have been measured during the ring killer experiment and are compared to standard EBT operation. The results of these experiments indicate that the hot‐electron rings in EBT do enhance the core plasma properties of EBT and do, in fact, reduce plasma fluctuations; however, these improvements are not large in magnitude. These measurements and recent theoretical models suggest that simple interchange/flute modes are stabilized, or fluctuation levels reduced, well before that condition is obtained for average minimum‐B stabilization. Several possible mechanisms for this stabilization are discussed.