Trapped-Particle-Mediated Damping and Transport

Abstract

Abstract. Weak axial variations in B(z) or φ (z) in Penning-Malmberg traps cause some particles to be trapped locally. This causes a velocity-space separatrix between trapped and passing populations, and collisional separatrix diffusion then causes mode damping and asymmetry-induced transport. This separatrix dissipation scales with collisionality as ν 1/2 , so it dominates in low collisionallity plasmas. The confinement lifetime in the "CamV" apparatus was dominated by a weak magnetic ripple with δ B/B ∼ 10 −3 , and it appears likely that the ubiquitous (L/B) −2 lifetime scalings and other applied asymmetry scalings represent similar TPM effects. TPM transport will limit the containment of large numbers of positrons orps, since TPM loss rates generally scale as total charge Q 2 , independent of length.

Trapped-Particle-Mediated Collisional Damping of Non-Axisymmetric Plasma Waves

“…For V sq = 0, a (more accurate) damping rate is obtained over longer times, and this is interpreted as γ M due to the inherent magnetic ripple. Prior experiments on crossfield transport and diocotron mode damping clearly isolated the magnetic ripple effects [2,3,4]; here, the requirement of 2 sectored cylinders precludes removal of the ripple.…”

“…In typical pure electron plasma columns, the magnetic field is axially "invariant" to within 1 part in 10 3 ; but the slow electrons trapped in the weak ripples are predicted to have near-discontinuous velocity distributions, and this apparently causes dominant damping and transport effects. These trapped-particle-mediated (TPM) effects arise from both magnetic and electric ripples, and have been observed to cause new modes [1], damping of drift modes [2], cross-field tranport of particles [2,3,4], as well as the damping of electron plasma waves discussed here. Theory suggests that TPM effects dominate when collision rates ν are small compared to wave frequencies ω, since TPM effects are predicted to scale as (ν/ω) 1/2 rather than as (ν/ω) 1 .…”