B. McGeehan

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

A new field-reversed configuration (FRC) formation technique is described, where a spheromak transitions to a FRC with inductive current drive. The transition is accomplished only in argon and krypton plasmas, where low-n kink modes are suppressed; spheromaks with a lighter majority species, such as neon and helium, either display a terminal tilt-mode, or an n=2 kink instability, both resulting in discharge termination. The stability of argon and krypton plasmas through the transition is attributed to the rapid magnetic diffusion of the currents that drive the kink-instability. The decay of helicity during the transition is consistent with that expected from resistivity. This observation indicates a new scheme to form a FRC plasma, provided stability to low-n modes is maintained, as well as a unique situation where the FRC is a preferred state.

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

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.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.

New method for inductively forming an oblate field reversed configuration from a spheromak

A new method for inductively forming a field reversed configuration is demonstrated, based on the inductively driven transformation of a spheromak. The driven transition can be achieved in argon and krypton plasmas, in which MHD modes are suppressed; simulations indicate that stability through the transition is explained by magnetic diffusion. Spheromaks with lighter working gas, such as neon and helium, either display a tilt mode or an n = 2 kink instability, both resulting in discharge termination.

Electromagnetic Perturbations in the Reconnecting Current Sheet in MRX

Magnetic reconnection is a fundamental plasma process in which magnetic field lines break and reconnect, converting magnetic field energy into particle kinetic energy. Electromagnetic fluctuations, which may play a role in fast reconnection, are studied from both an experimental and theoretical standpoint. The waves, which are in the lower hybrid range of frequencies, may be produced by a plasma instability known as the oblique lower hybrid drift instability. When the electron drift velocity is large, the theory predicts coupling between whistler and acoustic waves in the ion frame that may lead to an instability in the vicinity of the current sheet. On the experimental side, an antenna placed in the Magnetic Reconnection Experiment (MRX) at the Princeton Plasma Physics Laboratory is used to apply perturbations, and their propagation characteristics are measured. Results from a 2mm diameter antenna indicate that any induced fluctuations are confined to the current sheet and are preferentially excited in the direction of electron flow within the layer. Preliminary data from a 2cm diameter antenna shows a wave propagating in the electron flow direction at the local electron drift velocity. Thus electron drift appears to play a crucial role in the appearance of fluctuations.