A new high performance field reversed configuration operating regime in the C-2 device

Abstract

Large field reversed configurations (FRCs) are produced in the C-2 device by combining dynamic formation and merging processes. The good confinement of these FRCs must be further improved to achieve sustainment with neutral beam (NB) injection and pellet fuelling. A plasma gun is installed at one end of the C-2 device to attempt electric field control of the FRC edge layer. The gun inward radial electric field counters the usual FRC spin-up and mitigates the n = 2 rotational instability without applying quadrupole magnetic fields. Better plasma centering is also obtained, presumably from line-tying to the gun electrodes. The combined effects of the plasma gun and of neutral beam injection lead to the high performance FRC operating regime, with FRC lifetimes up to 3 ms and with FRC confinement times improved by factors 2 to 4.

Achieving a long-lived high-beta plasma state by energetic beam injection

“…The stability of the l ¼ 2 rotational mode has been achieved by applying various control techniques, for example, external static fields 4 , rotating magnetic fields 34 and the plasma gun 12,13 . However, active stabilization of the most dangerous l ¼ 1 tilt mode has not been demonstrated before this work.…”

“…In addition, we have demonstrated an active means of improving the stability and confinement property of the merged FRC as a suitable target plasma for NBI by applying line-tying and boundary-plasma biasing with plasma guns at the end divertors 12,13 . The former provides a restoring force that facilitates the centring of the plasma and reduces the off-centre 'wobble', while the latter produces an inward electric field E r with the E r  B z drift opposing the ion diamagnetic drift B z  rp, thus reducing bulk plasma rotation.…”

“…Spectacular progress has been made on both physics and technology fronts of FRC development in the C-2 CT-merging FRC experiment (Fig. 1) [10][11][12][13] , an effort entirely funded by private resources. It has recently achieved a long-lived FRC regime, with a marked improvement in confinement over one order of magnitude, departing from the predictions based on the scaling law in the previous experiments 14 .…”

Hybrid MHD model for a driven, ion-current FRC

“…[2] FRCs formed by a reversedfield θ-pinch, [6] also report toroidal spin-up, e.g., in the C-2 device, formed by merging two θ-pinch plasmas. [7] The origin of the spin-up has not been resolved, but has been suggested to be due to end-shorting of the radial-electric field, found on open, magnetic-field lines, and particle-loss jetting from the FRC, inside the separatrix. [6] This paper employs the use of a hybrid, two-fluid approach, to study the formation and equilibration of the FRC, driven by an external, inductive-electric field.…”

Formation of a compact toroid for enhanced efficiency

“…10,11 FRCs have been successfully formed in a field-reversed theta pinch, 12,13 by spheromaks merging, 14,15 with rotating magnetic field 16,17 and self-generated toroidal field, 18 annual coaxial discharge system, 19,20 and combined processes. 21,22 There are some disadvantages to these methods, such as low energy input into plasma or high complexity. All these plasma configurations have low coefficient of captured magnetic flux and no internal mechanism, leading to self-heating of the plasma.…”