Review of the Los Alamos FRX-C Experiment

Abstract

The FRX-C device is a large field-reversed theta pinch experiment with linear dimensions twice those of its FRX-A and FRX-B predecessors. It is used to form field-reversed configurations (FRCs), which are high-beta, highly prolate compact toroids. The FRX-C has demonstrated an R2 scaling for particle confinement in FRCs, indicating particles are lost by diffusive processes. Particle losses were also observed to dominate the energy balance. When weak quadrupole fields were applied to stabilize the n = 2 rotational mode, FRC lifetimes >300 µs were observed. Detailed studies of the FRC equilibrium were performed using multichord and holographic interferometry. Measurements of electron temperature by Thomson scattering showed a flat profile and substantial losses through the electron channel. The loss rate of the internal poloidal flux of the FRC was observed to be anomalous and to scale less strongly with temperature than predicted from classical resistivity.Following a modification to the device, FRCs were translated from the theta-pinch coil into a direct current (dc) solenoid and metallic vacuum chamber. The translation process was observed to be in reasonable agreement with adiabatic theory. The FRCs were translated and trapped in a dc solenoid without active auxiliary coils. Trapping was aided by the inelastic reflection of FRCs off a magnetic mirror. Measurements of the radiated power from translating FRCs indicated that radiation is a small component in the power balance; thus it appears that electron thermal conduction is more important.The particle confinement of an FRC is expected to improve as s, which measures the number of local ion gyroradii between the field null and the separatrix, increases. A regime of increased susceptibility to magnetohydrodynamic modes, notably the internal tilt, however, has recently been predicted to have a threshold in s of 3 to 4. To address these issues, as well as the issues of electron energy loss and poloidal flux loss, a three-stage experiment has been proposed that is predicted to reach s of ˜7. The FRX-D device will consist of separate formation, heating, and confinement regions. Plasma translation permits separation of these functions in a manner thought to be desirable for a fusion reactor.