Confinement time of electron plasma approaching magnetic pumping transport limit in small aspect ratio C-shaped torus

Abstract

A long confinement time of electron plasma, approaching magnetic pumping transport limit, has been observed in SMARTEX-C (a small aspect ratio partial torus with Ro/a∼1.59). Investigations of the growth rate reveal that they are governed by instabilities like resistive wall destabilization, ion driven instabilities, and electron-neutral collisions. Successful confinement of electron plasmas exceeding >1×105 poloidal E→×B→ rotations lasting for nearly 2.1±0.1 s is achieved by suppressing these instabilities. The confinement time has been estimated in two ways: (a) from the frequency scaling of the linear diocotron mode launched from sections of the wall that are also used as capacitive probes and (b) by dumping the plasma onto a charge collector at different hold times.

Unprecedented confinement time of electron plasmas with a purely toroidal magnetic field in SMARTEX-C

“…They argued that the confinement time approached the limit set by MPT (for the trap major radius of 17.4 cm and assumed temperature of 1 eV), although no direct evidence of MPT was observed. Results from SMARTEX-C followed, whose reported confinement time (2.14±0.1 s) was also very close to their theoretically predicted time scales [28]. Its worth mentioning here that no such theoretical limitations are known to apply to plasmas trapped in other B field topologies like in Stellarators or, say, in a torus with levitated dipole magnetic field.…”

“…Multiple shots have been acquired at each instant to verify the shot-to-shot reproducibility and error in confinement time is obtained from goodness of fit. Earlier experiments in SMARTEX-C had reported [28] a confinement time of 2.14 ± 0.1 s at 380 Gauss. The confinement seemed to be severly limited by droop ∼ 35% in the B field.…”

“…, where Q is total stored charge, L p plasma length, R w wall radius and B is toroidal magnetic field. The frequency evolution can therefore provide us with an estimation of confinement time [28]. Note that in toroidal electron plasmas, shift in equilibrium position, if any, has to be accounted for and B field at equilibrium position has to be used.…”

“…In a typical experiment, a natural diocotron mode appears soon after injection but quickly damps in less than one-fourth of a second resulting in a quiescent plasma (See Fig 2(a)). Such a quiescent plasma in stable equilibrium has been achieved through a series of measures leading to control of various instabilities [28]. With instabilities arrested, charge loss rate reduces and lifetime increases.…”

“…In such a scenario the presence of plasma can be detected by launching the diocotron mode externally. A small electric field perturbation is applied to the equilibrium plasma using a pair of oppositely located wall probes at a toroidal location [28]. While the applied perturbation may contain a chirp of frequency within a few kHz, the linear m = 1 mode is excited with a frequency that is proportional to the charge content in the plasma at the instant of launch.…”