29th Symposium on Fusion Technology (SOFT 2016)

P4.065 90 degrees cylindrical energy analyzer for the plasma potential fluctuations measurements on the tokamak ISTTOK

8 Sep 2016, 14:20

1h 40m

Foyer 2A (2nd floor), 3A (3rd floor) (Prague Congress Centre)

Board: 65

Poster D. Diagnostics, Data Acquisition and Remote Participation P4 Poster session

Speaker

Igor Nedzelskiy (Instituto de Plasmas e Fusao Nuclear)

Description

The heavy ion beam diagnostic of the tokamak ISTTOK is operated with a 20 keV Xe⁺+  ion beam and a multiple cell array detector to collect the secondary Xe²⁺2+ ions created along the primary beam path by ionizing collisions with plasma electrons. In this multichannel mode of operation, the use of standard Proca-Green 30^oo parallel plate energy analyzer for the plasma potential measurements is of reduced applicability to collect all the secondary ions along the plasma cross-section. The multichannel time-of-flight method has been implemented in the recent past as an alternative approach. However, the results of the measurements have indicated a lack of enough resolution for investigations of plasma potential fluctuations in the ΔE/E ~ 10^-4-4 range. In this presentation a multi-channel multi-slit 90^oo cylindrical analyzer for the plasma potential fluctuations measurements in ISTTOK is described. Preliminary estimations for the energy resolution of the ideal 90^oo cylindrical analyzer indicate 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^oo cylindrical analyzer is characterized by finite first order angular aberration. The SIMION code has been used to investigate the properties of real 90^oo 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. Almost the first order focusing in the range of ±4^oo has been obtained exploiting the effect of fringing field together with optimization of the initial conditions at the beam entrance (angle and shift relative to standard trajectory). Also, multiple cell detection at the analyzer exit is discussed as an alternative to the standard split-plate operation.