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.

Source for the production of ions of deuterium extracted from a radio frequency plasma (SPIDER) is the full-scale prototype for the ITER heating neutral beam (HNB) source. Beam operation in SPIDER, with the eventual aim of achieving 100-keV, 46-A hydrogen negative ion beam for $1$ h, has mainly been performed in volume operation without cesium. Recently, with the introduction of cesium, SPIDER has moved to the surface operational regime, with a noted increase in beam current. Electrical and calorimetric measurements of the beam current have previously shown discrepancies between the two, possibly due to background gas interactions in the accelerator and upstream of the STRIKE calorimeter. Taking advantage of the reduced number of open apertures due to the plasma grid (PG) mask, a series of fluxgate current transducers have been installed in SPIDER to measure the current of individual beamlets. These beamlet current monitors (BCMs), positioned downstream of the grounded grid (GG) around individual grid apertures, have provided direct measurement of the current of up to five individual beamlets during the recent SPIDER campaigns with and without cesium. This contribution describes the installation of the BCM dc sensors, analysis of the dc signal, and initial characterization of the beamlet direct current component in volume and surface operational regimes.

This article presents the basic physical and numerical principles of a fluid model implemented into the numerical code fluid solver for SPIDER in 2D (FSFS2D) used for simulations of the source for the production of ions of deuterium extracted from RF plasma (SPIDER) negative ion source. It gives self-consistent 2-D description of the source, including the neutral gas flow, plasma chemistry, radio frequency (RF) coupling in the source driver, and plasma transport through the magnetic filter (MF). In order to capture the neutral depletion and its impact on the plasma dynamics the continuity equation for molecules and the corresponding surface chemistry have been included in the model. An important element in the development of the numerical framework is the validation of the code results against the experimental data. For this aim, a series of numerical simulations have been performed and compared with the experimental data from the SPIDER experimental campaigns. This article presents critical assessment of the validation results and outlines the necessary code/model enhancements required to improve the predictive capability of the FSFS2D code.

For negative ion beam sources there are several methods of measuring the accelerated beam current, most commonly electrical measurements at the power supply and calorimetric measurements. On SPIDER, the ITER Heating Neutral Beam full-scale beam source prototype, electrical measurements at the acceleration grid power supply (AGPS) are complemented by polarizing the diagnostic calorimeter STRIKE to provide an additional electrical measurement of the accelerated current. This is in addition to the calorimetric measurements provided by STRIKE. These diagnostics give differing measurements of the beam current. Exploiting the reduced number of open apertures on SPIDER a new beam diagnostic has been installed to measure the individual beamlet currents directly. The so called Beamlet Current Monitor (BCM) has been used to measure the current of five beamlets during the most recent SPIDER campaign. This work compares the BCM current to the electrical measurements at the AGPS and STRIKE. The average BCM current agrees well with the STRIKE electrical measurements, indicating that the AGPS overestimates the beam current. The individual beamlets are compared to the STRIKE calorimetric measurements, showing similar current trends with the source parameters.

SPIDER is the full-sized ion source prototype for the ITER Heating Neutral Beam (HNB). Hosted by Consorzio RFX in Padova, Italy, it forms part of the ITER Neutral Beam Test Facility (NBTF). SPIDER has, for the first time, operated with fully open beam segments: up to 1 4 of the designed beam extraction area. By increasing in the maximum achieved RF power and acceleration voltage, this has enabled SPIDER to take a substantial step towards the ITER HNB requirements of 66 A extracted current at 100 keV. As the first negative-ion beam source fully enclosed in a single vacuum vessel, SPIDER has presented some unique challenges for high-power operation. RF-induced breakdowns at the rear of the source drove the initial decision to reduce the number of open beam apertures, in order to reduce the pressure at the back of the source while keeping the same filling pressure in the plasma chamber. The recent increase in the number of open apertures from 28 to 300 has resulted in an increase in the beam-generated plasma downstream of the accelerator. Due to the enclosed nature of the vacuum and the electromagnetic field generated by the source, the high voltage power supplies collect some of these additional charges, complicating the measurement of the beam current. This contribution describes the assessment of the SPIDER beam current and its performance over the recent experimental campaigns.