Confinement Studies in the Columbia Non‐Neutral Torus

Abstract

We present results from a systematic study of confinement of pure electron plasmas in the Columbia Nonneutral Torus (CNT) stellarator. Recent results show more than an order of magnitude improvement in confinement, with the new best confinement time measured to be 0.32 seconds. This large improvement is the combined result of lower neutral pressure (nearly a factor of five lower), higher magnetic field (a factor of two), and the installation of a conducting boundary designed to control electrostatic boundary conditions. New external diagnostics are being developed in preparation for electron-positron plasma operation. Confinement times measured with an edge potential probe are in good agreement with confinement times calculated based on the steady state emission current of static discharges. Plasmas created from an emitter that is extracted in 0.02 seconds do not survive after extraction even in cases where the confinement time is initially much longer than the rod retraction time.

First experimental studies of the physics of plasmas of arbitrary degree of neutrality

“…The anisotropic tensor would lead to force terms, some along B, usually neglected both in the electron fluid equation and the ideal MHD Ohm's law. Numerical simulations show that > 50% of the electrons are on magnetically trapped orbits, and that there are complicated loss structures in phase space in CNT [31], also consistent with experimental findings [17,32]. The dominant collisional process in these plasmas is electron-neutral collisions, and they are quite rare [17] (ν en ≈ 500 s −1 for η = 0.3).…”

Pure electron plasmas confined for 90 ms in a stellarator without electron sources or internal objects

“…Pure electron plasmas are created in CNT by thermionic emission from heated, negatively biased filaments. A small Debye length is routinely achieved, initially down to 1.5 cm [10,11], and recently 1.0 cm [12], with typical temperatures of T e ≈ 4 eV, densities of n e ≈ 2 × 10 12…”

“…Confinement was seen to increase with increasing magnetic field (linearly for rod driven transport, stronger than linearly for neutral collision driven transport) and decrease with neutral pressure (linear relationship between transport rate and neutral pressure) [14]. More recently, τ = 0.32 sec was achieved through reduction of the neutral pressure, maximization of the magnetic field strength, and the installation of a flux surface conforming electrostatic boundary [12]. A moveable emitter was installed, capable of full retraction from the magnetic axis to the edge of the magnetic surface in 20 milliseconds [16], ie.…”

“…This, however, turned out to be more complicated than expected. Initial results from retraction experiments were disappointing: There appeared to be no plasma left after retraction, even during operational conditions that favor long confinement times (B-field up to 0.2 T and low neutral pressures p n ≈ 5 × 10 −9 Torr) [12].…”