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Positrons : practical plasma probe? Ziemelis, Ugis Oskars

Abstract

The feasibility of using positrons to probe plasmas is investigated. Analysis of the y rays resulting from the annihilation of positrons with plasma electrons may yield information about the momentum distribution, temperature and number density of these electrons. A number of 'positron probe1 success criteria are introduced and a wide range of positron-plasma systems are evaluated in light of these criteria. Of special importance are positron annihilation time (Ta) and thermalization time (Tt) calculations, which indicate that the most important success criterion, Tt < Ta, is satisfied by the class of plasmas characterized by kTe > 10 eV. Two potential fusion plasmas, namely Tokamak and laser compression plasmas, belong to this class (where some positron diagnostic techniques may be practical). Detailed fractional thermalization calculations for positrons from a ²²Na source in hypothetical, fully ionized H₂ plasmas of these types indicate that most positrons will thermalize and annihilate while the plasma environment is in existence. Positron sources, including pair creation, which is predicted to occur in both Tokamak and laser compression plasmas, and y ray detectors are discussed. Annihilation y ray counting rates are estimated under a variety of circumstances for both NaI(Tl) and Ge(Li) detectors to prepare the way for the assessment of specific positron diagnostic techniques. Four such techniques are evaluated with reference to both Tokamak and laser compression plasmas. It is concluded that diagnostic techniques involving measurements of shift and broadening of the annihilation line appear most promising in both cases. The measurement of positron lifetimes may be possible in some Tokamak plasmas, but is not feasible in laser compression plasmas. 'Slow' positron probing of plasmas would be ideal, but slow positron sources of sufficient intensity do not yet exist. Angular correlation measurements do not appear feasible at this time.

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