Development of an ion beam detector for the atomic beam probe diagnostic

Abstract

The atomic beam probe diagnostic concept aims at measuring the edge magnetic field and through that edge current distribution in fusion plasmas by observing trajectories of an ion beam stemming from a diagnostic neutral beam. The diagnostic potentially has microsecond scale time resolution and can thus prove to be a powerful option to study fast changes in the edge plasma. A test detector has been installed on the COMPASS tokamak as an extension of the existing lithium beam diagnostic system. It employs a relatively simple concept of an array of conductive detection plates measuring the incident ion current, which is then amplified and converted to a voltage signal. The aim of the test detector is to experimentally examine the idea of the diagnostic and provide background data for design and installation of a final detector. Also, a numerical code based on the CUDA parallel computing platform has been developed for modeling lithium ion trajectories in the given COMPASS plasma discharges. We present the developments of the detector design and test measurements of the diagnostic performed both in a laboratory beam system and on the COMPASS tokamak.

Micro-Faraday cup matrix detector for ion beam measurements in fusion plasmas

“…Previous measurements 20 showed that electrons and ions as well as UV radiation reaching any metallic surface induce secondary electrons which make the measurement hard to interpret. The detector plate has to be masked so that the ions can only reach the detector at the Fara- day cups.…”

“…(30 keV, Li, 0.4 T homogenous field, 2 cm path in the field, r L =16 cm, deflection: 2.5 mm, 7 • ) The major difference of the setup described in Ref. 20 is that the magnetic field was less homogenous at the detector position which is indicated with a blue line in Figure 7. Figure 8 shows the Computer-Aided Design (CAD) model with one magnet and the covering mask removed (a), the full setup with the vacuum flange (b) and the detector head with the magnet holder zoomed (c).…”

“…According to computer simulation of the COMPASS measurement setup, the ion beam toroidal shift on the detector in response to an edge current perturbation is 0.1-1 mm/kA. In earlier experiments a Lithium beam reduced to 5 mm still gave good signal 20 , therefore we considered this beam diameter. We intend to detect at least 1 mm beam movement, thus the 5 mm beam needs to be resolved to a few measurement channels.…”

“…For detection of the ion beam distribution a collection plate matrix was proposed and tested in the COMPASS tokamak 20 . Results showed that several improvements are necessary to be able to extend the operation space towards standard H-mode scenarios.…”

“…The detector position is indicated with a red line. The detector position for the Ref 20. setup with less homogenous field is also indicated with a blue line amplifier input to protect them from overcurrent from the plasma or beam.…”