Abstract
To study the fast-ion transport in a well controlled background plasma, a 3-cm diameter rf ion gun launches a pulsed, ∼300eV ribbon shaped argon ion beam parallel to or at 15° to the magnetic field in the Large Plasma Device (LAPD) [W. Gekelman, H. Pfister, Z. Lucky, J. Bamber, D. Leneman, and J. Maggs, Rev. Sci. Instrum. 62, 2875 (1991)] at UCLA. The parallel energy of the beam is measured by a two-grid energy analyzer at two axial locations (z=0.32m and z=6.4m) from the ion gun in LAPD. The calculated ion beam slowing-down time is consistent to within 10% with the prediction of classical Coulomb collision theory using the LAPD plasma parameters measured by a Langmuir probe. To measure cross-field transport, the beam is launched at 15° to the magnetic field. The beam then is focused periodically by the magnetic field to avoid geometrical spreading. The radial beam profile measurements are performed at different axial locations where the ion beam is periodically focused. The measured cross-field transport is in agreement to within 15% with the analytical classical collision theory and the solution to the Fokker–Planck kinetic equation. Collisions with neutrals have a negligible effect on the beam transport measurement but do attenuate the beam current.
Highlights
Fast ions may have velocities b that are much larger than the thermal velocity of the background plasma ions i but much less than the thermal velocity of the background plasma electrons e, i.e., i Ӷ b Ӷ e
Time evolution of the LAPD plasma and the current measured by the energy analyzer. ͑a Plasma line density measured by the interferometer. ͑b Beam current signals measured by the energy analyzer when rf is on or off
The fast-ion beam energy and its deceleration process have been measured in the quiescent LAPD afterglow plasma when the ion beam source was in the direction par
Summary
Fast ions may have velocities b that are much larger than the thermal velocity of the background plasma ions i but much less than the thermal velocity of the background plasma electrons e, i.e., i Ӷ b Ӷ e. The modified classical theory predicts reduced deceleration rates and cross-field transport. The deceleration of fast ions has been measured in tokamak plasmas and agrees with the standard classical Fokker– Planck theory to within 10%.9–14. The classical theory could predict the diffusion of the test particles distribution for up to 20% of the 90°-collision time in the long-time experiments.. Cross-field diffusion is caused by pitch-angle scattering in velocity space. The result of an experiment measuring the cross-field transport of test ions with velocity f Ͻ 2i in a. This article describes experiments that measure accurately the deceleration and spatial diffusion of fast ions in a quiescent laboratory plasma, which is generated in the Large Plasma DeviceLAPD ͑Ref. 23͒ at UCLA. The measurements of the fast-ion slowing-down time and cross-field transport are discussed separatelySecs.
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