The Columbia Nonneutral Torus: A New Experiment to Confine Nonneutral and Positron-Electron Plasmas in a Stellarator

Sarasola

et al. 2010

Contrib. Plasma Phys.

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.

“…1 [21]. Steady state pure electron plasmas are created by thermionic emission from a heated and biased tungsten filament held near the magnetic axis by an insulating ceramic rod.…”

Section: The Cnt Experimentsmentioning

confidence: 99%

Confinement Studies in the Columbia Non‐Neutral Torus

Brenner

Sarasola

et al. 2010

Contrib. Plasma Phys.

“…1. Details of the CNT design and construction can be found in References [1,2]. CNT was assembled rather easily primarily by graduate students, an advantage of the error field resilience in the design.…”

Section: Introductionmentioning

confidence: 99%

Dynamics of Electron-Rich Plasmas in the CNT Stellarator

Plasma and Fusion Research

Berkery

Boozer

et al. 2008

Self Cite

Pure electron plasmas and electron plasmas with a finite ion fraction have been studied in the Columbia Nonneutral Torus (CNT) since the end of 2004. Results from the first three years of operation are summarized. Stable, small Debye length pure electron plasmas are routinely created, and have confinement times up to 20 msec. The confinement is limited by radial transport caused by internal rods, as well as electron-neutral collisions. The neutral driven transport rate is indicative of poor particle orbits in CNT, despite the strong radial electric field. Numerical simulations shed light on this issue, demonstrating the detrimental effects of variations in the electrostatic potential on a magnetic surface. With the installation of a magnetic surface conforming electrostatic boundary and the transition to external diagnostics, significantly longer confinement times should be possible. Also presented are observations of sudden confinement jumps that have a hysteretic behavior, and observations of an ion driven instability.

“…This single mode was previously discovered and studied in CNT's standard configuration (64 • tilt angle) and has been identified as an ion-driven instability [21]. In the standard configuration, the rotational transform of the magnetic surfaces varied from ι = 0.12 on the magnetic axis to 0.23 at the plasma edge [12]. The tilt angle between the CNT's interlocking coils has since been switched to 78 • .…”

Section: Temporal Behaviourmentioning

confidence: 91%

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

Sarasola

Plasma Phys. Control. Fusion

2012

Abstract. The first detailed experimental characterization of the physics of plasmas of arbitrary neutrality is reported. It is shown that the degree of neutralization of the plasma in the Columbia Non-Neutral Torus stellarator can be varied continuously from quasi-neutral to pure electron plasma, and that the plasma can be sustained indefinitely at any degree of neutralization. The physical phenomena change significantly as the degree of neutralization is varied. Quasi-neutral plasmas exhibit a single mode low frequency flute instability. This mode aligns almost perfectly with the magnetic field lines, presenting a resonant m = 3 poloidal structure. For weakly magnetized ions, and moderate degrees of neutralization, multi-mode behaviour is observed, in some cases fully turbulent. At the other extreme of the neutralization scale, electron-rich plasmas with a small but non-negligible amount of ions exhibit single mode fluctuations. Essentially identical phenomena were previously reported by Marksteiner [Q. Marksteiner et al., 2008, Phys. Rev. Lett., 100, 065002]. In addition to the already known poloidal mode number m = 1 of these fluctuations, the toroidal mode number is now known: n = 0. This not only confirms the previous observations that the electron fluid force balance appears to be broken, but the frozen-in flux condition is also violated.