Transition From Collisional to Collisionless Confinement in WHAM

The Wisconsin HTS Axisymmetric Mirror (WHAM) is a new fusion experiment based on the magnetic mirror concept [1]. Currently, most WHAM plasmas are collisional (gas dynamic), but recent experiments have seen increased ion temperatures, pushing the plasma into the collisionless regime. Rognlien et al. [2] suggest that particle confinement is a linear addition of gas dynamic trapping and 90^◦ pitch angle scattering. This work investigates this transition by simulating the Fokker-Planck evolution of a decaying particle population at various ion temperatures. These simulations show a transition between collisional and collisionless confinement and are compared against analytic confinement times. Additionally, we compute mean free paths within WHAM to characterize confinement regimes. Select shots show a sharp transition between regimes, and calculated confinement times are compared with simulation data to identify transition points.

[1] D. E. et al. Physics basis for the wisconsin hts axisymmetric mirror. Journal of Plasma Physics, 89, 2023.

[2] T. D. Rognlien. Transition from pastukhov to collisional confinement in a magnetic and electrostatic well. Nuclear Fusion, 20, 1980.