Inductive sustainment of oblate field-reversed configurations with the assistance of magnetic diffusion, shaping, and finite-Larmor radius stabilization

Abstract

Oblate field-reversed configurations (FRCs) have been sustained for >300μs, or >15 magnetic diffusion times, through the use of an inductive solenoid. These argon FRCs can have their poloidal flux sustained or increased, depending on the timing and strength of the induction. An inward pinch is observed during sustainment, leading to a peaking of the pressure profile and maintenance of the FRC equilibrium. The good stability observed in argon (and krypton) does not transfer to lighter gases, which develop terminal co-interchange instabilities. The stability in argon and krypton is attributed to a combination of external field shaping, magnetic diffusion, and finite-Larmor radius effects.

Formation and sustainment of field reversed configuration (FRC) plasmas by spheromak merging and neutral beam injection

“…This improved stability was understood from a rigid-body model, which indicates that the specific external field configuration and plasma shape in MRX are responsible for this improvement in stability [11]. These experiments demonstrated for the first time the role of both passive stabilization and plasma shape in improving oblate FRC stability in the MHD regime, are the first experimental identification of the co-interchange mode [26].…”

“…Oblate FRCs have been sustained for longer period than 15 magnetic diffusion times by use of an inductive solenoid. It was again found on MRX the spheromak merging experiments [26,27] that a significant amount of magnetic energy is converted to ion thermal energy during plasma merging and that the energy conversion rate is much larger than the value expected from classical dissipation mechanisms. The reconnection speed was factor of 10 faster than the Sweet-Parker time.…”

“…7] [24][25][26]. An inward pinch is observed during sustainment, leading to a peaking of the pressure profile and maintenance of the FRC equilibrium.…”

“…They could be removed if necessary. Ability to form oblate FRC by spheromak merging and to modify the FRC shape in a controlled manner through the use of equilibrium field shaping, have been demonstrated; the n=1 tilt and shift modes stabilization by central conductor and strong shaping fields; and unstable n>1 co-interchange modes studied experimentally, and these modes were shown to be reduced in very oblate plasmas [24][25][26]. To generate large ion radius which induce more FLR stabilization, Argon gas was used.…”

Field-reversed configuration formation scheme utilizing a spheromak and solenoid induction

“…8͑c͒. Also shown is the Spitzer parallel resistivity 71 ͑ ʈ = 0.5ϫ 10 −4 T e −3/2 Z eff ln ⌳͒, assuming Z eff = 1.5 ͑note that many collisional plasmas in MRX have resistivities near the collisional value 72,73,67 ͒. The value of T e in this calculation comes from a triple Langmuir probe, which was located at R = 0.35 m during these experiments.…”