Overview on electrical issues faced during the SPIDER experimental campaigns

Maistrello, A.; Agostini, M.; Bigi, M.; Brombin, M.; Dan, M.; Casagrande, Riccardo; Nardi, Marco De; Ferro, A.; Gaio, E.; Jain, Palak; Lunardon, Francesco; Marconato, N.; Marcuzzi, D.; Recchia, M.; Patton, T.; Pavei, M.; Santoro, Francesco; Toigo, Vanni; Zanotto, L.; Barbisan, M.; Baseggio, L.; Bernardi, M.; Berton, G.; Boldrin, M.; Bello, S. Dal; Fasolo, D.; Franchin, L.; Ghiraldelli, R.; Grando, L.; Milazzo, Ruggero; Pimazzoni, A.; Rigoni, A.; Sartori, E.; Serianni, G.; Shepherd, A.; Ugoletti, M.; Zaniol, B.; Zella, Daniele; Zerbetto, Enrico; Decamps, Hans; Rotti, C.; Veltri, P.

doi:10.1016/j.fusengdes.2023.113510

Cited by 8 publications

(6 citation statements)

References 19 publications

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“…and the operation of the 4 RF oscillators, each one feeding 2 drivers connected in series. The design power level of 100 kW/driver could not be reached, and the achieved power was limited to about 70kW/driver when operating one generator alone and to about 50kW/driver when operating more than two drivers, because of interference of various resonant circuits [12]. Such finding led to the choice of substituting the oscillators with solid state amplifiers for the ITER HNB.…”

Section: The First Operations In Spidermentioning

confidence: 99%

Optimizing the ITER NBI ion source by dedicated RF driver test stand

Mario,

Pimazzoni,

Sartori

et al. 2024

J. Phys.: Conf. Ser.

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The experimental fusion reactor ITER will feature two (or three) heating neutral beam injectors (NBI) capable of delivering 33(or 50) MW of power into the plasma. A NBI consists of a plasma source for production of negative ions (extracted negative ion current up to 329 A/m2 in H and 285 A/m2 in D) then accelerated up to 1 MeV for one hour. The negative ion beam is neutralized, and the residual ions are electrostatically removed before injection. The beamline was designed for a beam divergence between 3 and 7 mrad. The ion source in ITER NBIs relies on RF-driven, Inductively-Coupled Plasmas (ICP), based on the prototypes developed at IPP Garching; RF-driven negative-ion beam sources have never been employed in fusion devices up to now. The recent results of SPIDER, the full size ITER NBI ion source operating at NBTF in Consorzio RFX, Padova, measure a beamlet divergence minimum of 12mrad and highlighted beam spatial non-uniformity. SPIDER results confirmed the experimental divergence found in smaller prototype sources, which is larger compared to filament-arc ion sources. Although prototype experiments have shown that the extracted current requirement can be achieved with minor design improvements, the beamlet divergence is expected to marginally achieve the design value of 7 mrad, which in multi-grid long accelerators results in unexpected heat loads over the accelerator grids. A contributor to the beam divergence is the energy/temperature of the extracted negative ions, so it is believed that plasma differences between the two source types play a role. Research is focused on the plasma parameters in the ion source. One RF driver, identical to the ones used in SPIDER, installed in a relatively small-scale experimental set-up, inherently more flexible than large devices, is starting operations devoted to the investigation of the properties of RF-generated plasmas, so as to contribute to the assessment of negative ion precursors, and of their relationship with the plasma parameters, particularly when enhancing plasma confinement. The scientific questions, that have arisen from the preliminary results of SPIDER, guided the design of the test stand, which are described in this contribution, together with the diagnostic systems and related simulation tools. The test stand, which shares with the larger experiment all the geometrical features and constraints, will allow technological developments and optimized engineering solutions related to the ICP design for the ITER NBIs.

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Section: The First Operations In Spidermentioning

confidence: 99%

Optimizing the ITER NBI ion source by dedicated RF driver test stand

Mario,

Pimazzoni,

Sartori

et al. 2024

J. Phys.: Conf. Ser.

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“…10, and a new less cumbersome bias busbar on the back of RF source [16]. -RF circuit layout improvement to reduce the mutual coupling between different driver pairs, which was found responsible for the modulation of the plasma light when two or more driver pairs were supplied [17] -Extraction Grid (EG) modifications: EG segments will be modified to be electrically insulated among themselves, in order to discriminate the electrical current collected by each segment from extracted beam, hence to identify excessive non-uniformities potentially dangerous for the segment integrity; moreover, the EG apertures will be tapered in order to increase the margin preventing the beamlets from colliding with the aperture' rims. See Fig.…”

Section: Source Modificationsmentioning

confidence: 99%

“…The original configuration of RF systems includes 4 generators based on free-running tetrode oscillators in the push-pull self-excited Colpitts configuration [17,18] supplying 4 driver pairs on board the ion source, used to ionize the gas and excite plasma. Four Normal L-type matching networks are mounted on the backside of the ion source, to tune the driver pair impedance to that of the RF generators [19].…”

Section: Rf Power Supply Generatorsmentioning

confidence: 99%

“…The above-mentioned evidences led to the decision to replace the RF generators in SPIDER and MITICA and change the current ITER baseline from self-excited free running tetrode oscillator technology to solid-state amplifier technology [17,23,24]. The solid-state amplifier technology is believed to be strongly attractive for ion source operation, since no frequency flip phenomena are expected and there are no tetrodes requiring high voltage.…”

Section: Rf Power Supply Generatorsmentioning

confidence: 99%

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Lessons learned after three years of SPIDER operation and the first MITICA integrated tests

Marcuzzi¹,

Toigo²,

Boldrin³

et al. 2023

Fusion Engineering and Design

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“…The replacement of the four RF generators based on self-oscillators with solid-state RF generators [28][29][30] is expected to provide a large improvement, as the latter ones do not rely on internal resonance to set the frequency. In principle, this will avoid frequency instabilities (i.e.…”

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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Overview on electrical issues faced during the SPIDER experimental campaigns

Cited by 8 publications

References 19 publications

Optimizing the ITER NBI ion source by dedicated RF driver test stand

Optimizing the ITER NBI ion source by dedicated RF driver test stand

Lessons learned after three years of SPIDER operation and the first MITICA integrated tests

Highlights of recent SPIDER results and improvements

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