SPIDER, the Negative Ion Source Prototype for ITER: Overview of Operations and Cesium Injection

An overview of the recent operations and the main results of cesium injection in the Source for the Production of Ions of Deuterium Extracted from Rf plasma (SPIDER) negative ion source are described in this contribution. In experiments without cesium injection, all SPIDER plants were tested to verify the basic expectations on the operational parameters (e.g., electron cooling effectiveness of magnetic filter field) and to determine its operational region. For beam properties, it was shown that the current density varies across the beam in the vertical direction. In preliminary cesium experiments, the expected increase of negative ion current and simultaneous decrease of co-extracted electrons were found, along with the influence of the control parameters (polarization of the plasma electrodes, magnetic filter field) on the SPIDER beam uniformity in the horizontal and vertical directions. It was shown that non-Gaussian tails can be identified in the angular distribution on the plane perpendicular to the beam propagation direction. Stray particles, nonhomogeneous beam and large divergence might result in unexpected heat and particle loads over ITER neutral beam injector (NBI) accelerator grids; it is the goal of SPIDER to assess and possibly to identify suitable methods for controlling these beam features. A major shutdown, planned for late 2021, to solve the issues identified during the operation and to carry out scheduled modifications, is outlined. Such improvements are expected to allow SPIDER to pursue the ITER requirements in terms of negative ion current, electron-to-ion ratio, and beam duration.