Convective contributions to local power loss in a bumpy torus

Abstract

Power flow in the ELMO Bumpy Torus [Plasma Physics and Controlled Nuclear Fusion Research, 1974, Tokyo (IAEA, Vienna, 1975), Vol. 2, p. 141; Plasma Phys. 25, 597 (1983)] was investigated by measuring the power received by a limiter. This power was found to be a small fraction of the gyrotron output power (in one case, 13 out of 100 kW). To investigate the reason for the small fraction that appeared on the limiter, power was selectively removed from various cavities, including the cavity containing the limiter. These experiments have demonstrated that the majority of the power is lost locally. Observations of the potential structure demonstrate that asymmetric potential contours are present that can lead to enhanced plasma loss.

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

“…The first treatment suggests that, since the outward ballooning force is due to the toroidal nature of the plasma, modification of the magnetic field in the vicinity of the rings would not eliminate the need for a counterbalancing horizontal electric field, and so one might expect a poloidal asymmetry to persist, even in the EBS. [13] The loss of plasma particles by convection due to the asymmetric electric field has been mentioned previously. The symmetric and horizontal asymmetric electric fields cancel each other in two places near the electron rings: one is inside and the other is outside the major radius at the midplane.…”

“…(5) The global energy confinement time, as a measure of the average confinement for inner and outer (convective) loss regions, was found to be approximately 100 JUS and did not exceed200 fis even if the energy of the warm (non-Maxwellian) electrons was included [14]. This confinement time is comparable with the E X B drift time across the plasma [13] and with the particle confinement time measured by a diagnostic neutral beam [8] (see Sections 2.6 and 2.7).…”

“…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.…”

“…This gives rise to a horizontal electric field that induces particle convection, as discussed in the following sections. [9,13] The aforementioned potential asymmetry played a major role in plasma confinement. In the C-mode, where all of the potential contours were open, the confinement time was determined by r = <£>B/E where <£> is the average length of the path along a potential contour before encountering a wall.…”

“…(4) 2-D potential contours measured during T-mode operation showed nested contours at the plasma center with open contours outside [12]. In the outer region, plasma loss could occur convectively as plasma streamed along the open potential contours [13] (see Sections 2.4 and 2.5).…”