Confinement of pure electron plasmas in the Columbia Non-neutral Torus

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.

Section: Understanding and Improving Confinementsupporting

confidence: 74%

Dynamics of Electron-Rich Plasmas in the CNT Stellarator

Plasma and Fusion Research

Berkery

Boozer

et al. 2008

Self Cite

“…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).…”

Section: Discussionsupporting

confidence: 76%

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.

“…The magnitude and approximate 1/p n scaling for the confinement time for helium dominated experiments is consistent with measurements of the transport in steady state experiments with helium. The 1/p n scaling is well understood as being electron-neutral collision driven transport and was also observed for nitrogen dominated steady state experiments [10,14].…”

Section: Ion Accumulation Cause Of Fast Collapsementioning

confidence: 60%

“…Thus the Debye length is much smaller than the 15 cm minor radius. It was established early on that the presence of internal structures (ceramic rods) caused significant transport [10,14], but it was also found that these rods prevented continuous ion accumulation, and that the ion content would therefore reach a steady state value. At low neutral pressures (p n < 5 × 10 −9 Torr) the ion content would be less than 1 % [15].…”

Section: The Cnt Experimentsmentioning

confidence: 99%

“…During the first year of operation, steady state confinement times up to 20 msec were measured. 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].…”

Section: The Cnt Experimentsmentioning

confidence: 99%

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

Brenner

2012

Physics of Plasmas

We report on the creation and up to 90 millisecond sustainment of pure electron plasmas confined in a stellarator without internal objects. Injection of positrons into such plasmas are expected to lead to the creation of the first electron-positron plasma experiments. These newly created plasmas will also allow a study of pure electron plasmas without the perturbing presence of internal objects.The plasmas were created by thermionic emission of electrons from a heated, biased filament that was retracted in 20 milliseconds. The confinement of these transient plasmas is different from that of steady state plasmas with internal objects and emissive filaments, and is generally shorter, limited by ion buildup. The decay time is increased by lowering the neutral pressure, lowering the electron plasma temperature, or operating with neutrals with high ionization energies (helium). These findings are all consistent with ion accumulation being the cause for the shorter than expected confinement times. The magnetic field strength also moderately increases the decay times. The deleterious effect of ions is not expected to imply a similar deleterious effect when introducing positrons, but it implies that ion accumulation must be avoided also in an electronpositron experiment.