Magnetic surface mapping with an emissive filament technique on the Auburn torsatron

Abstract

Torsatron and stellarator plasma confinement devices rely on magnetic surface mapping to determine the critical vacuum magnetic field structure. A recently developed method employing an emissive filament offers some advantages over the traditional technique of mapping with a directed electron beam. On the Auburn torsatron a comparative study between the emissive filament and directed electron beam techniques has been conducted. The parameters varied in the comparative study are filament geometry, emission current, bias voltage, background gas pressure, and magnetic field strength. This comparative study indicates that the emissive filament technique is reliable over a broad and easily accessible range of parameters. We have also measured the spatial distribution of electrons on a given magnetic surface. As an application of the emissive filament technique, the optimization of the magnetic surfaces on the Auburn torsatron is shown.

Toroidal stellarator diode: discharge features and electron transport

“…At a fixed external applied field E r , the dependence of the electron confinement time τ e on the magnetic field B in the TSD can be determined correctly in an experiment, without any restrictions, from the expression τ e ∝ 1/I em [15] by measuring I em (B) (at steady-state conditions, I em = j ⊥ S, j ⊥ = enV ⊥ , V ⊥ = r/τ e → τ e = (en r S)/I em ). To determine the conditions for applying the "stellarator diode" method to map the vacuum magnetic surfaces, the dependence of the minimum emission current I em min on B (near the magnetic axis) was measured on the Auburn Torsatron (the multipolarity of helical magnetic field is l = 2, and B = 0.02 − 0.1 T) [3,4]. The procedure of a least mean-squares fitting to the measured data gave the scaling I em min ∝ B −0.87 .…”

“…electron distributions measured on the CMS [1][2][3][4][5]). The maximum electron density is measured in a layer of the toroidal magnetic surfaces related to the injection radius r inj (see figure 1).…”

“…Two of these [1][2][3][4] predict the dependence of the emission current I em on the magnetic field B as I em ∝ B −2 and the linear relation between I em and the emitter-anode potential difference U a , these predictions being significantly different from the experimental results [1-4, 12, 15]. For the straight helicalsymmetry magnetic field, Dikij et al [13] proposed a scheme for the construction of a stellarator diode theory based on the solution of the kinetic equation for the case, where collisions of electrons with neutral atoms play the main role in electron transport.…”

“…The density distribution in the electron cloud depends on both the radial position of the emitter and the diffusion process determined by the structure of the confining magnetic field. Studies of electron radial distributions in torsatrons [1][2][3][4][5] have shown that electrons are injected in a rather narrow layer of toroidal drift surfaces around the small emitter. Hence, an electron-filled layer of the toroidal drift surfaces enclosing the thermionic emitter serves as an electron source in the TSD.…”

“…In the process, in order to determine the parameter space of exact magnetic surfaces, only a few features of the TSD discharge were studied [1][2][3][4]. The degree of suppression of the thermionic cathode emission in the TSD system characterizes the electron confinement on the magnetic surfaces.…”