Measurements of classical transport of fast ions

Abstract

To study the fast-ion transport in a well controlled background plasma, a 3-cm diameter rf ion gun launches a pulsed, ϳ300 eV ribbon shaped argon ion beam parallel to or at 15°to the magnetic field in the Large Plasma Device ͑LAPD͒ ͓W. Gekelman, H. Pfister, Z. Lucky, J. Bamber, D. Leneman, and J. Maggs, Rev. Sci. Instrum. 62, 2875 ͑1991͔͒ at UCLA. The parallel energy of the beam is measured by a two-grid energy analyzer at two axial locations ͑z = 0.32 m and z = 6.4 m͒ 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.

Dependence of fast-ion transport on the nature of the turbulence in the Large Plasma Device

“…The classical diffusion of fast ion beams in LAPD is well understood 17 and a Monte-Carlo (MC) code simulates the beam broadening due to classical effects. Another indicator of the classical diffusion effect is the beam FWHMs in early afterglow plasmas (<10 ms after the discharge ends).…”

“…14,15 The fast ion campaign at the upgraded Large Plasma Device 16 (LAPD) at the University of California, Los Angeles has been focused on understanding fast ion transport mechanisms in various background waves in a basic linear device. Several experiments has been conducted and reported, including study of fast ion classical transport 17 and study of Doppler-shifted resonance of fast ions with linearly 18 or circularly 19 polarized shear Alfvén waves (SAW). LAPD provides a probe-accessible plasma with direct and accurate diagnostic tools.…”

Measurements of interactions between waves and energetic ions in basic plasma experiments

“…The standard detector at the LAPD [8] utilizes both biased grids and collimation ( figure 3(a)) to suppress interference from the background plasma. Early LAPD experiments [13,14] used only biased grids in the relatively tenuous 'afterglow' LAPD plasma but this collector was overwhelmed by electron currents in the denser, active-phase portion of the discharge. The standard collimated/gridded detector is unable to measure the energy of the beam (probably due to charge accumulation on insulators), so a new collector was designed and successfully tested [15].…”

Lithium ion sources for investigations of fast ion transport in magnetized plasmas

“…Then the profile data are extracted along radial lines and fitted to Gaussian distributions with the goodness of fitting ͑ 2 ͒ recorded as statistical weights. 3 An example of a radial line is shown in Fig. 9͑b͒ connecting the cross hair and the orbit center.…”

“…2͒ and used to investigate classical transport 3 and preliminary turbulent transport measurements. 4 Although fairly reliable in operation, it could not be operated at pitch angles larger than 25°a nd only at magnetic fields of ϳ1 kG or less.…”