Status of SPIDER beam source after the first 3.5 years of operation

Self Cite

Three years of experiments on SPIDER allowed characterization of the main features of the source plasma and of the negative ion beam, in the original design configuration. For the large dimensions of the source chamber, and of the extraction area, the investigation of the single-beamlet currents and of the source plasma uniformity had to be carried out to extend the knowledge gained in smaller prototype sources. The configuration of the multiple RF drivers and filter field topologies were found to cause a peculiar behavior in the plasma confinement in the drivers, creating left-right asymmetries which were also visible in the extracted negative ion currents, even after the early implementation of a new scheme of plasma-grid current send and return busbars that greatly improved performance at high filter fields. The plasma properties in the driver and expansion region as well as the positive ion energy at the extraction region were studied in different experimental conditions, and interpreted also with the support of numerical models, suggesting that an improved plasma confinement could contribute to the increase of the plasma density, and to a certain extent to a lowering of the plasma potential profile; both effects shall contribute to increase the presence of cold negative ions for the formation of low-divergence beamlets. Early results related to unwanted RF discharges on the back of the plasma source and the gas conductance of the beam source suggested the reduction of the vessel pressure as mitigation, leading to the definition of a new pumping system. The difficulties related to the simultaneous operation, stable control and high-power operation of multiple RF self-oscillating vacuum tube based RF generators were an unambiguous obstruction to the experimentation, calling for the implementation of RF solid-state amplifiers. The initial tests related to caesium management, the non-uniform plasma properties at different locations across the plasma grid, and the challenges in the measurement of the current and divergence of the accelerated beamlet, unambiguously resulted in the need of new diagnostic systems to investigate with better resolution the spatial uniformities. This contribution summarises how the main experimental findings in the previous experimental campaigns are driving modifications to the SPIDER experiment, during the present shut down, in view of future operations.

Section: Control Of Caesium Evaporation and Improvement Of The Emissi...supporting

confidence: 71%

Section: Control Of Caesium Evaporation and Improvement Of The Emissi...mentioning

confidence: 99%

Highlights of recent SPIDER results and improvements

Sartori,

Agnello,

Agostini

et al. 2023

Self Cite

“…It highlights that the metallic components in correspondence to the fourth segments are shiny, i.e. Cs coverage is poor, compared to the other segments, where the presence of Cs is recognizable from the matted effect of the PG and bias plate surfaces [19]. The reason behind the strong removal of Cs and/or poor deposition on the bottom segment is not known.…”

Section: Jinst 18 C09006mentioning

confidence: 97%

Plasma emission monitored via optical emission spectroscopy during the Cs conditioning at SPIDER

Mario,

Zaniol,

Serianni

et al. 2023

Self Cite

The SPIDER test facility is the full-size ITER neutral beam injector (NBI) ion source, required to provide 355 A/m2 extracted negative ion current density in hydrogen (285 A/m2 in deuterium) with an electron-to-ion ratio lower than 0.5 (one in deuterium). The negative ion source is attached to a three-grids extraction and acceleration system. The operational conditions for the cases presented in this work involve short pulses (up to about 30 s length) repeated every five to six minutes. The duty cycle can be adjusted. In order to fulfil the requirement on the extracted negative ion current with reduced amount of co-extracted electrons, the evaporation of caesium into the ion source through Cs ovens (three in SPIDER) and the optimisation of Cs conditioning techniques are mandatory. At SPIDER, the plasma is monitored via optical emission spectroscopy techniques measuring the plasma emission in a line-of-sight (LOS) integrated manner in several positions inside the ion source. In particular, close to the extraction region, two sets (centred at 5 mm and at 35 mm distance from the plasma grid) of four horizontal LOSs are used to retrieve the vertical profile of the plasma emission. During the Cs conditioning campaign performed at SPIDER, although the extraction capabilities were reduced due to technical problems, the RF power coupled to the plasma reached 400 kW with all four RF generators working simultaneously. The negative ions produced by surface emission affects the plasma radiation, and the ion source performances in terms of extracted negative ions and co-extracted electrons. The aim of this work is to study the evolution of the plasma emission over the initial phase of the Cs conditioning campaign. The effect of Cs conditioning on the extracted negative ions and electrons is also presented and discussed.

“…Here we present the ratio between Cs and Mo concentration, even though similar results are obtained considering the sole Cs concentrations. The whole process is described in detail in [20]. Due to the closeness of the permanent magnets, it was possible to collect only one sample at the position of each magnet and one sample between each magnet.…”

Section: Cs On the Source Case Lateral Wall (Sclw)mentioning

confidence: 99%

Summary of caesium evaporation and deposition during SPIDER first campaign

Fadone,

Pouradier Duteil,

Sartori

et al. 2024

Self Cite

SPIDER (Source for the Production of Ions of Deuterium Extracted from a Radio frequency plasma) is currently in a major shutdown period dedicated to the upgrade of several components in order to enhance its performances and guarantee their agreement with the ITER requirements. During this phase, an in-depth inspection of the beam source components is fundamental to understand SPIDER behaviour during the previous experimental campaigns, and in particular caesium evaporation inside the source by means of Cs ovens. Caesium evaporation and its deposition on the plasma grid (PG) is fundamental to minimize the work function of PG surface causing an increased generation of negative ions and a reduction of the electron density in front of the PG, which means less co-extracted electrons. This work presents the analysis of the caesium ovens performances and of their status after removal. Inductively Coupled Plasma Mass Spectrometry (ICP-MS) analyses were carried out on samples of plasma source walls, and the experimental data was compared to the results obtained from other investigations: numerical models for the caesium evaporation and deposition, chemical surface analyses (performed in strategical locations of the source walls and extraction grid), and Laser Absorption Spectroscopy (LAS) measurements. The non-uniformity of Cs distribution has been taken into account since it has repercussions on negative ion extraction efficiency and uniformity.