Measurement of sloshing ions in WHAM

The Wisconsin High-field Axisymmetric Mirror (WHAM) [1] experiment has achieved robust plasma heating using neutral beam injection (NBI) at 25 kV and 40 A [2]. Neutrals are launched axially at a 45° angle and exhibit the expected bounce behavior at approximately 2B_o. WHAM supports a wide range of central plasma densities, from 2x10¹⁷ to 2x10²¹ m^-3, with NBI shinethrough varying from 100% to near-zero. A dedicated shinethrough detector array measures the non-captured fraction of the NBI and is used to reconstruct the radial density profile.

Between the first two experimental campaigns, an extensive cleaning procedure—including vessel bakeout and glow discharge cleaning—significantly reduced edge neutral populations and charge exchange losses. As a result, sloshing ion populations have been observed and NBI-sustained discharges are now routinely maintained. Ongoing studies are exploring plasma behavior at reduced mirror ratios using central trim coil fields to further optimize confinement and performance. Diagnostic development to better characterize the fast ion population is also underway and machine characterization to further reduce the neutrals population is underway.

[1] Endrizzi D, Anderson JK, Brown M, et al. Physics basis for the Wisconsin HTS Axisymmetric Mirror (WHAM). Journal of Plasma Physics. 2023;89(5):975890501. doi:10.1017/S0022377823000806

[2] A. Sorokin et al. Characterization of 1 MW, 40 keV, 1 s neutral beam for plasma heating. Review of Scientific Instruments. 2010. doi:https://doi.org/10.1063/1.3266141