Direct measurement of the plasma loss width in an optimized, high ionization fraction, magnetic multi-dipole ring cusp

Abstract

The loss width of plasma in the WiPAL multi-dipole magnetic ring cusp [Cooper et al., Phys. Plasmas 21, 13505 (2014); Forest et al., J. Plasma Phys. 81, 345810501 (2015)] has been directly measured using a novel array of probes embedded in the insulating plasma limiters. The large plasma volume (∼10 m3), small loss area associated with strong rare earth permanent magnets (Bo∼2.23 kG at face), and large heating power (≤200 kW) produces a broad range of electron temperatures (2<Te<15 eV), ion temperatures (0.03<Ti<2 eV), plasma densities (3×1010<ne<2×1012 cm−3), and ionization fractions (0.05<ne/(ne+nn)<1), in both argon and helium, all of which were accurately measured. This plasma regime, accessible with high magnetic fields, differs from previous devices: the cusp loss width is much larger than the Debye length and electron gyroradius and comparable to the collision length. Plasma parameters measured at the surface of ceramic limiter tiles covering the magnets and along radial chords in the cusp magnetic field indicate that electron density and temperature are nearly constant on magnetic field lines and that the mirror forces play little role in confining the plasma other than to constrict the loss area. Particle balance modeling is used to determine the cross field diffusion coefficient base on the measured losses to the limiters. The experimentally determined cross field diffusion coefficient (which determines the cusp loss width) is consistent with ambipolar diffusion across five orders of magnitude. The ambipolar diffusion across a given field line is set primarily by the electron-neutral collisions in the region where the magnetic field is the weakest, even though these plasmas can have ionization fractions near 1.