Radial transport reduction in tandem mirrors using end-wall boundary conditions

Abstract

Results from the Tandem Mirror Experiment-Upgrade (TMX-U) demonstrate that radial transport in the plasma volume can be affected by the boundary conditions at the end wall. Measurements in which end-wall plates, mapping to the plasma core, are switched from floating to grounded during plugging operation demonstrate that this floating increases the build-up rate of the central cell plasma (by increasing the radial confinement time), steepens the core density profile, and affects the plasma throughout the entire cross section.

Plasma-surface interactions in large tandem mirror devices—MFTF-B

“…In practice, this bias must be a function of radius, so segmentea plates are used. This technique has been demonstrated qualitatively in current experiments[24].Another improvement is the use of drift pumping to remove ions in the thermal barrier regions. Neutral beam-;--which remove ions by charge exchange-injected nearly parallel to the axis of the machine are the more common method.…”

A new simpler way to obtain high fusion power gain in tandem mirrors

“…Here (σv) e ≈ 10 −14 m 3 s −1 is the ionization rate for 10 eV electrons (atom or molecule [43]), and v o = 3 × 10 4 m s −1 for 10 eV deuterium ions. Numbers above give N o = 0.001 N amb with n p Δ edge = 10 19 giving Δ edge = 0.2 m and P ECH = 1.5 MW for N amb = 3 × 10 16 (1 µTorr), or 5 MW for 3µTorr as in MFTF [40]. Recycling of neutrals from the wall is negligible if f * (σ x /(σ x + σ i )) < 1 where f * is the number of cold neutrals returning when a hot neutral created by charge exchange strikes the wall [55]; hence not a problem in steady state reactor End Plugs where σ x /σ i = 10 −3 for 1 MeV ions, nor in the proposed experiment with f * 1 by gettering.…”

“…The other unique feature is access from the ends to apply radial electric fields across the pedestal, shown to affect radial transport in the Macrator Electric Tokamak at UCLA [16]. The open ends in tandem mirrors allow the introduction of segmented end plates at reactor scale, with an array of voltages externally controllable.…”

The ambipolar trap

“…The TMX-U device has been used to study transverse plasma transfer and its effect on the disruption of ambipolar confinement. Radial sectioning of the end plasma receivers made it possible, by using the shapes of the calculated particle drift surfaces [16], to measure the nonambipolar ion losses as a function of the plasma radius and to analyze the dependence of these losses on a number of experimental parameters (see Panov's review article [17]). In most plasma confinement modes, transverse ion transfer proved to be much larger than neoclassical resonance diffusion.…”