Improvement of SPIDER diagnostic systems

Pasqualotto, R.; Sartori, E.; Agnello, R.; Brombin, M.; Candeloro, V.; Fadone, M.; Mario, I.; Patton, T.; Poggi, C.; Segalini, B.; Serianni, G.

doi:10.1016/j.fusengdes.2023.113889

Cited by 2 publications

(2 citation statements)

References 37 publications

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“…Despite all the instruments already installed on SPIDER (described in [17]), the spatial resolution of the existing diagnostic systems in the SPIDER source is still not enough to investigate some inhomogeneities in beamlet profiles that were outlined during the experimental activities [14]. In fact, in both Cs-free operations and during caesium injection, the global imbalance of current density in the vertical direction corresponded to the non-uniform distribution of positive ions in the extraction region.…”

Section: Integrating the Diagnostic Systemmentioning

confidence: 99%

Study and development of diagnostic systems to characterise the extraction region in SPIDER

Segalini,

Candeloro,

Poggi

et al. 2024

J. Inst.

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SPIDER, an RF-driven negative ion source in the Neutral Beam Test Facility (NBTF), serves as the prototype for ITER's neutral beam injector (NBI). It is composed of 8 drivers powered by 4 RF generators, aiming to accelerate 50 A of negative hydrogen ions to 100 KeV with a beam uniformity target of 10%. The experiment, launched in 2018, tested negative ion production using caesium. Results match those of similar facilities, but SPIDER faces challenges due to its size, multiple drivers, and non-uniform plasma expansion. These issues impact beam uniformity, preventing the machine from reaching expected performance. To address this, SPIDER initiated a significant shutdown at the end of 2021 for improvements. One the most important aspects studied during the first experimental campaign is source uniformity, addressed both in terms of plasma and of caesium distribution. The latter is particularly relevant since its quality is directly related to the beam uniformity and divergence. To have more insight about these issues, monitoring the plasma properties in the extraction region is crucial, hence in the present contribution, the design and development of two new diagnostic systems are described: a movable Langmuir probe and a Retarding Field Energy Analyser (RFEA). The first can provide a vertical scan of the main plasma parameters close to the plasma grid. The spatial resolution would improve with respect to the already installed set of fixed Langmuir probes embedded in the grid system, and the newly installed diagnostic could interact with other sensors to produce complementary measurements (namely, electron photo-detachment). The latter, instead, allows the monitoring of the positive ion energy distribution: positive ions, in fact, can be precursors of the negative ones produced at the caesiated surface, but also influence the energy of negative ion and their extraction probability and thus collecting information about their energy distribution allows inferring details about the extracted negative ion beam. The two diagnostics are designed focusing on the experimental constraint of integrating the diagnostics in a harsh and complex environment such as SPIDER plasma: a preliminary study of the placement inside the source is carried out, then the electrode of the movable probe and the RFEA sensor are sized according to the spatial and energy resolution requested by the system.

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Section: Integrating the Diagnostic Systemmentioning

confidence: 99%

Study and development of diagnostic systems to characterise the extraction region in SPIDER

Segalini,

Candeloro,

Poggi

et al. 2024

J. Inst.

Self Cite

View full text Add to dashboard Cite

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“…Particle-in-cell simulations were carried out to calculate the loss fraction in a cusp field when the plasma potential is different than the floating potential [34] (in SPIDER, the plasma potential at the lateral walls is of the order of 30-50 V while the electron temperature is reduced between 5 and 1.5 eV), and also characterization of the deposits on the lateral wall surface was conducted [19], to clarify the extent of the plasma wall interaction. In order to allow a better experimental characterization of the discussed non uniformities, new diagnostics were designed to either characterize the homogeneity of the plasma properties among the drivers, or to measure the profiles of plasma properties in front of the extraction apertures [35,36], integrating with the set of already existing diagnostics [37].…”

Section: Vertical and Horizontal Homogeneity Among Rf Driversmentioning

confidence: 99%

Highlights of recent SPIDER results and improvements

Sartori,

Agnello,

Agostini

et al. 2023

J. Inst.

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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.

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Improvement of SPIDER diagnostic systems

Cited by 2 publications

References 37 publications

Study and development of diagnostic systems to characterise the extraction region in SPIDER

Study and development of diagnostic systems to characterise the extraction region in SPIDER

Highlights of recent SPIDER results and improvements

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