An E × B analyzer to study the particle fluxes, energy, and charge state in the scrape-off layer of tokamaks

Abstract

We present a new E × B analyzer for the measurement of charged particle fluxes and of the energy distribution and the charge state of the ions. The device is based on the fact that (1) the drift velocity v ̄ D = E ̄ × B ̄ /B 2 is independent of the kind of particle and its parallel velocity v ∥ and (2) that E × B fields have the property of stigmatic focusing (trochotron mass spectrometer). The particles (electrons and ions) passing through an entrance slit penetrate an E × B region. The gross effect is the deviation of the trajectories by an angle tanα = v D v ∥ which depends on the particle energy. An array of charged particle collectors is placed behind the E × B field region. The position of the impact on the array is a measure of v ∥, i.e. of the parallel energy. For impurities, the distribution of implanted atoms on the collector shows density peaks located at equally spaced nodes occurring at positions corresponding to time-of-flights being multiples of the gyration period. The distance between nodes is a measure of the charge state of the ions. The analyzer is used to study the scrape-off layer of PDX and hydrogen fluxes at different energies are presented.