Abstract
The Heavy Ion Beam Diagnostic of the tokamak ISTTOK is operated with a 20keV Xe+ ion beam and a multiple cell array detector to collect the secondary Xe2+ ions created along the primary beam path by ionizing collisions with plasma electrons. In this paper an improved multi-channel multi-slit 90° cylindrical analyzer for the plasma potential fluctuations measurements in ISTTOK is described. The energy resolution of the ideal 90° cylindrical analyzer indicates the value of minimal detectable change in plasma potential to approximately 5 times higher than the expected plasma potential fluctuation level. In addition, the ideal 90° cylindrical analyzer is characterized by finite first order angular aberration. The SIMION code has been used to investigate the properties of real 90° cylindrical analyzer with inclusion of fringing fields. As a result of numerical simulations, the necessary energy resolution has been obtained in novel approach of operation in combining electric field deflection and deceleration with biased exit electrode. In addition, almost the first order focusing in the range of ±2° has been obtained exploiting the lens effect of fringing field together with optimization of the initial conditions at the beam entrance. A split-plate and a multiple cell detection systems at the analyzer exit are also discussed.
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