Resistivity scaling of rotating magnetic field current drive in FRCs

摘要

Rotating magnetic fields (RMFs) have been used to both form and sustain low density, prolate FRCs in the translation confinement and sustainment (TCS) facility. The two most important factors governing performance are the plasma resistivity, which sets the maximum density for which toroidal current can be maintained, and the energy loss rate, which sets the plasma temperature. The plasma resistivity has been determined by carefully measuring the amount of RMF power absorbed by the RFC. When the ratio of RMF magnitude, B_ω, to external poloidal confinement field, B_e, is high, this resistivity is very adversely affected by the RMF drive process. However, when B_ω/B_e falls below about 0.3, the resistivity returns to values typical of non-driven FRCs. The observed scaling leads to a density dependence of n_e ～ B_ω/r_sω~(1/2) where r_s is the RFC separatrix radius and ω is the RMF frequency. Since the FRC contains little or no toroidal field, B_e is proportional to (n_eT_t)~(1/2) where T_t = T_e + T_i is the sum of the electron and ion temperatures. In the present experiments, except for the initial start-up phase where T_t can exceed 100 eV, the plasma temperature is limited to about 40 eV by high oxygen impurity levels. Thus, low B_ω/B_e, low resistivity operation was only realized by operating at low values of B_ω. The RMF drive sustains particles as well as flux, and resistive input powers can be in the MW, range at higher values of B_ω, so that high temperature, steady-state operation should be possible once impurity levels are reduced. Changes are being made to the present 'O-ring' sealed, quartz chambered TCS to provide bakable metal walls and wall conditioning as in other quasi-steady fusion facilities.