Design and optimization of the electrostatic input module for the ISTTOK Tokamak HIBD cylindrical energy analyzer

Abstract

The Heavy Ion Beam Diagnostic of the ISTTOK Tokamak is based on the injection of a single ionized primary beam (Xe+, Cs+) of energy 20–25 keV and on the collection of all doubly ionized ions emerging from the primary beam due to the impact ionization with the plasma electrons. In the present configuration three local plasma parameters can be retrieved: plasma density (ne), electron temperature (Te), and poloidal magnetic field (Bp). A new detection system is being design in order to determine the plasma potential (Vp) profile via the secondary ions energy measurements. To that end an improved 90̂ cylindrical electrostatic analyzer (CEA) has been designed and optimized to measure the energy of the secondary ions. This paper reports on the design optimization of the input stage of the new detection arrangement to guide and shape 4 secondary beams into the entrance of the CEA. It is composed of a set of four electrostatic cylindrical plates followed by four pairs of steering plates and a multiple aperture Einzel-like electrostatic lens, consisting of four planar vertical electrodes containing four passing slits. Individual secondary beams with dimension of 8 mm × 2.5 mm were obtained without overlapping between the four beams from different slits. By changing the incident beam energy from 20 keV to 20.2 keV (in order to simulate a local plasma potential value of +200 V), without adjusting bias voltage of the input system, results in a vertical beam shift of 1 mm which can be recovered by changing the voltage on the parallel plate arrangement by 40 V. The flexibility and tolerance of the optic system was tested successfully for the expected angular uncertainty of the beam by +/−0.5̂ at the input module entrance and with the expected energy spread of the beam corresponding to different plasma positions inside the tokamak.