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

Abstract

The ELMO Bumpy Torus (EBT) [Plasma Physics and Controlled Nuclear Fusion (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.

Summary of ELMO Bumpy Torus (EBT-S) experiments from 1982 to 1984

“…Performance limits for EBT plasmas may be obtained by examining plasma parameters both as a function of the ambient neutral pressure with differing magnetic fields and during the 'ring modifier' and 'ring killer' experiments [17]. The data are shown in Fig.…”

“…It is important that the hot electron interchange mode be stable because the interchange mode prohibited lowering the pressure to obtain higher bulk plasma temperatures. [10,17] Since the hot electrons in EBT-S did not have sufficient energy density to modify the vacuum magnetic field so as to form an MHD-stable configuration, the plasma might have been expected to be flute unstable. Surprisingly [10,17], only broadband turbulence [n e fi/n e )2 (peak to peak) « 5% with frequencies up to 500 kHz] was observed during normal operation.…”

“…[10,17] Since the hot electrons in EBT-S did not have sufficient energy density to modify the vacuum magnetic field so as to form an MHD-stable configuration, the plasma might have been expected to be flute unstable. Surprisingly [10,17], only broadband turbulence [n e fi/n e )2 (peak to peak) « 5% with frequencies up to 500 kHz] was observed during normal operation. Shallow pressure gradients in the interior of the plasma suggest that the bulk plasma was at least partially stabilized by compressibility [10], although other stabilization mechanisms, such as charge uncovering [27] or finite Larmor radius effects [28], may also have played a role.…”

“…For example, the power incident on the limiter increased as the fluctuation level increased. [7,9,10,13,14,17] Much of the crucial physics of EBT plasmas can be determined from an examination of the 2-D potential contours [31 ] displayed in Fig. 2.…”

ELMO Bumpy Torus programme

“…Typical EBT parameters are given in Table I, together with the parameters of a proposed modification into a new configuration, the ELMO Bumpy Square. The programme has provided fundamental information about relativistic electrons [5][6][7], non-Maxwellian distributions [8], ambipolar electric fields [9], and RF heating [10,11] -both in the ECH and ICH frequency ranges, and has also provided technological leadership in the production, distribution and launching of highpower, high-frequency microwaves.…”