Tokamak de Varenne’s diagnostic neutral beam injector

Abstract

A fully automated and remote controlled neutral beam injector has been designed, built, and characterized on a test stand for use as a diagnostic aid on Tokamak de Varennes (TdeV). The injector has a duoPIGatron ion source with a LaB6 coaxial cathode. A cusped magnetic field surrounding the plasma chamber is used to enhance the plasma confinement. The beam current is modulated by switching the arc current, and it has a 70 μs rise time. Two, in-house-built, titanium sublimation pumps provide up to 16 000 l s−1 pumping capacity for differential pumping between the source and the tokamak. The injector operates with its source at ground potential, and its neutralizer canal at a negative potential |Vb|⩽40 kV. An electrostatic trap biased to −6 kV with respect to the neutralizer canal retains the electrons in the neutralizer. The residual ions are collected on an ion collector that is biased to −5 kV. The design of the extraction system and the electrostatic trap was optimized with a numerical code. The injector has been operated with hydrogen and helium beams. A multi-aperture three-grid extraction system provides up to 2.4 A of proton or 1.2 A of He+ current. The maximum average proton current density extracted from the 19 extraction holes was 650 mA cm−2 at 35 kV extraction voltage, which was obtained with 60 A of arc current.