Heavy-ion beam probe diagnostic systems (invited)

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Abstract
Heavy-ion beam probing generally consists of passing a beam of 1+ ions through a plasma imbedded in a magnetic field. Secondary ions with higher ionization levels are produced by ionizing collisions with the plasma electrons. Detection of the secondary ions with a small-aperture electrostatic energy analyzer allows continuous fluctuation measurements of the plasma density and space potential with both spatial and temporal resolution. Spatial resolution is the order of 0.1 cm3 and temporal resolution is presently electronics limited to ∼1 μs. The energy of the probing beam is determined primarily by the requirement that the secondary ion must escape from the plasma. Typical beam energies extend from 10 to 500 keV. The range of plasma densities that have been investigated is 1012 cm−3<ne<1014 cm−3. At the higher densities, beam attenuation becomes a serious problem. Higher beam energies provide better penetration of the magnetic field, and reduced beam attenuation. Heavy-ion beam probes were first used to measure a coherent density fluctuation on a hollow cathode arc in 1969, and soon afterward to measure the space potential. Since then beam probes have been used to measure the space potential and fluctuations in both density and space potential for plasmas with varying magnetic geometries. There is continuing development work to study the feasibility of using beam probes to measure magnetic fluctuations and magnetic field structure. Sensitivity for measuring density and potential fluctuations is best demonstrated by what is the most sophisticated beam probe to date: the 500-keV system on TEXT. For broadband measurements (50–250 kHz), the TEXT beam probe has demonstrated a sensitivity to space potential fluctuations of 2 V (rms), and resolution for ñe/ne of 10−3. Recent measurements on both ISX-B and TEXT have demonstrated the capability of obtaining simultaneous ñ and φ̃ measurements at three separate locations in the plasma. For some locations the sample volumes are poloidally separated and S(kθ, w) can be estimated for both ñ and φ̃. This permits evaluation of the net electrostatic-fluctuation-induced particle flux. Some of the problems still being encountered with present beam probes are the nonideal behavior of the energy analyzer, cross talk between ñ and φ̃ for high-wave-number fluctuations, the effect of finite sample volume and sample volume spacing on the evaluation of k spectra, simultaneous measurement of two components of the k vector, probing of the complex 3-D magnetic fields, and extension of the measurements to higher-energy beam probe systems.