SPIDER, the negative ion source prototype for ITER: first operations with caesium

To reach fusion conditions and control the plasma configuration in ITER, the next step in tokamak fusion research, two neutral beam injectors (NBIs) will supply 17MW each, by neutralizing accelerated negative hydrogen or deuterium ions. The requirements of ITER NBIs (40A/1MeV D- ions for <=1h, 46A/870keV H- ions for <=1000s) have never been simultaneously attained. So in the Neutral Beam Test Facility (NBTF, Consorzio RFX, Italy) the operation of the full-scale ITER NBI prototype (MITICA) will be tested and optimised up to full performances, focussing on accelerator (including voltage holding), beam optics, neutralisation, residual ion removal. The NBTF includes also the full-scale prototype of the ITER NBI source with 100keV particle energy (SPIDER), for early investigation of: negative ion production and extraction, source uniformity, negative ion current density and beam optics. After three years of experimentation, mainly devoted to verifying the operation of the various plants and to identifying a suitable operational range, SPIDER has recently entered the next operational phase, in which the amount of negative ions available for extraction and acceleration is enhanced by employing the caesium-catalysed conversion at the plasma electrode. This contribution will describe the main results of the first campaign with caesium in SPIDER, devoted to characterizing plasma and beam parameters in these conditions. In preliminary experiments, the expected increase of the negative ion current and simultaneous decrease of the amount of co-extracted electrons were found. The caesiation procedure adopted in SPIDER will be described (effect of: duty cycle, caesium injection rate, RF power, source gas pressure) along with the influence of the control parameters (polarisation of the plasma electrode, magnetic filter field) on the SPIDER beam uniformity. A major shutdown, planned for late 2021, to solve the issues identified during the operation and to carry out scheduled modifications, will be outlined. These improvements, are expected to allow SPIDER to aim at the ITER requirements in terms of negative ion current, electron-to-ion ratio, beam duration.