Retarding field energy analyzers for the ion temperature measurements in the SOL plasmas of the tokamak ISTTOK and the TJ-II stellarator
Measurements of the ion temperature in the plasma boundary of fusion devices typically employs the Retarding Field Analyzer (RFA) that is based on selective rejecting or retarding the plasma ions by a varying braking electric field. Normally, the RFA comprises an input slit and collector electrode with two or more separately biased fine grids placed in front of it. With an assumption of Maxwellian ion distribution, the exponential decay of the RFA collector currentvoltage characteristic is determined by the ion temperature. This contribution describes 1- and 3channels RFAs for the ion temperature measurements in the scrape-off-layer of the tokamak ISTTOK and the TJ-II stellarator. Selected results of the ion temperature measurements on the ISTTOK and the TJ-II are presented. In addition, limitations related to the RFA operation (including the alignment along magnetic field direction and plasma loading) are considered.
On the reliability of scrape-off layer ion temperature measurements by retarding field analyzers.
The retarding field analyzer (RFA) is one of the only widely accepted diagnostics for measurements of ion temperature Ti in the tokamak scrape-off layer. In this paper we analyze some instrumental effects of the RFA and their influence on Ti measurements. It is shown that selective ion transmission through the RFA slit is responsible for an overestimation of Ti by less than 14%, even for a relatively thick slit plate. Therefore, thicker slit plates are preferable, since they reduce, e.g., the risk of melting during off-normal events, and the effect of positive space charge inside the cavity. The influence of the electron repelling grid, as well as misalignment of the slit with respect to the magnetic field on Ti measurements are negligible.
A TIME-OF-FLIGHT ELECTRON VELOCITY SPECTROMETER
Abstract : A velocity selector is described which is designed for measurement of the total collision cross section of electrons in the energy region below one volt. The system determines the distributions of times of flight of single electrons over a measured path at selected pressures of a specimen gas. The electrons are generated by repetitive illumination of a photo-emitter by a spark gap, pass over a shielded drift path, enter an accelerating field, and impinge on an electron multiplier. Electronic equipment and data reduction for analysis of time-of-flight and amplitude distributions of electron counts are described; data illustrative of performance are presented, including typical transmission measurements in helium and argon which extend to 100 millivolts. Applications and extensions of the technique are discussed.