Fast electron transport in improved-confinement RFP plasmas

Hard-x-ray bremsstrahlung, with photon energies reaching 150 keV, is detected in reversed-field pinch (RFP) plasma discharges with reduced tearing mode amplitudes, indicative of improved confinement of fast electrons compared to the standard case. Current-driven tearing modes in standard RFP plasmas create stochastic magnetic fields; fast electrons, generated by a strong electric field in the plasma core, are nearly collisionless and stream along these stochastic fields, diffusing radially out of the plasma core at a rate proportional to their velocity. In these discharges, emitted x-rays do not exceed energies of about 10 keV. By comparison, when tearing mode amplitudes are sufficiently reduced, magnetic flux surfaces are restored and fast electrons become well-confined, diffusing radially at a rate independent of their velocity. Experiments were performed on the Madison Symmetric Torus RFP using an x-ray spectroscopy diagnostic. Twelve CdZnTe photodiode detectors measure 10-150 keV hard-x-ray bremsstrahlung from fast electrons, and a silicon detector measures 2-10 keV soft-x-ray bremsstrahlung from thermal and fast electrons. The detectors are placed primarily along a radial array of beryllium x-ray windows. They are run in pulse mode, and the signals are digitized to enable the separation of pulses from noise and the resolution of pulse pileup. Pulse heights are proportional to x-ray energy and are calibrated with a known source, and pulses are binned by time, energy, and radius to obtain spectra. Hard x rays are measured from the core of discharges with pulsed parallel current drive, which modifies the current profile to reduce tearing mode amplitudes and thus magnetic stochasticity. In these discharges, the Fokker-Planck code CQL3D can be used to infer the effective ion charge Zeff and the particle diffusion coefficient Dr. Hard x rays are also detected when a large magnetic island forms in the plasma core, usually the result of quasi-single helicity, where one tearing mode grows large while the rest are suppressed. Stochasticity is reduced within the magnetic island and fast electrons are well-confined in this region.