Speaker: Aaron Tran, University of Wisconsin-Madison
Abstract: WHAM's "peak-performance" beam-ion plasma may induce drift-cyclotron loss-cone (DCLC) instability: a coupled ion Bernstein / drift wave excited by the plasma’s radial density gradient and loss-cone velocity distribution. We present 3D plasma simulations, using kinetic ions and isothermal fluid electrons, of various WHAM configurations with sloshing (45 deg. pitch angle) beam-ion distributions from the collisional Fokker-Planck code CQL3D-m as an initial condition. Edge-localized electrostatic waves grow and saturate in ~1–10 μs with ω ~ 1–2× the ion cyclotron frequency. Wave properties can be explained by linear theory of DCLC in a planar slab. DCLC scattering fills the loss cone, so particle confinement is set by axial free streaming (aka "gas dynamic" confinement). Adding cool (~1 keV) ions to the plasma edge improves confinement by ~2–5×. I will also briefly comment on (i) other ways to stabilize DCLC, (ii) how DCLC fits into a broader landscape of instabilities in mirrors, and (iii) the effect of externally-driven shear flows.