Saturated Drift-Cyclotron Loss-Cone (DCLC) Instability in 3D Kinetic-Ion Simulations of WHAM*

The Wisconsin High-Temperature Superconductor Axisymmetric Mirror (WHAM) will form a plasma column of radius ~10 cm, spanning ~2–5 ion Larmor radii. Its small size 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 associated with both the magnetic mirror and a self-consistent ambipolar hole. We present 3D plasma simulations, using kinetic (particle-in-cell) ions and isothermal fluid electrons in a hybrid approximation, of various WHAM operating 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 ~ 1–3x the ion cyclotron frequency. Wave properties agree with linear theory of DCLC in a planar slab. DCLC scatters ions into the loss cone at a rate balanced by finite outflow time. We will further discuss (i) how DCLC loss rate scales with device parameters, and (ii) ways to mitigate DCLC in WHAM and next-step mirror devices, including the efficacy of trapped warm ions.