The ITER Neutral Beam Test Facility is a part of the International Thermonuclear Experimental Reactor (ITER) in Padova, Veneto, Italy.[1] The facility will host the full-scale prototype of the reactor's neutral beam injector, MITICA (Megavolt ITer Injector & Concept Advancement), and a smaller prototype of its ion source, SPIDER (Source for the Production of Ions of Deuterium Extracted from a Radio frequency plasma).[2] SPIDER started its operation in June 2018. SPIDER will be used to optimize the ion beam source, to optimize the use of caesium vapor, and to verify the uniformity of the extracted ion beam also during long pulses.
To deliver power to the fusion plasma in ITER, two heating neutral beam injectors will be installed. They are designed to provide the power of 17MW each, through the 23m beamlines, up to the four-meter diameter container: in order to deposit sufficient heating power in the plasma core instead of the plasma edges, the beam particle energy shall be about 1MeV, thus increasing the neutral beam system complexity to an unprecedented level. This will be the main auxiliary heating system of the reactor. Due to its low conversion efficiency, the neutral beam injector first needs to start a precursor negative ion beam of 40A, and then neutralizes it by passing it through a gas cell (with an efficiency <60%), and then by a residual ion dump (the remaining 40—20% negative, 20% positive). The neutralized beam is then dumped on a calorimeter during conditioning phases, or coupled with the plasma. Further reionization losses or interception with the mechanical components reduce its current to 17A.[3]
The role of the test facility includes research and development on the following topics:
SPIDER is the first large experimental devices to start the operation at the test facility (May 2018). The components of MITICA are currently under procurement, with its first operation expected in late 2023.
The design parameters of SPIDER are the following:
During 2018, the plasma discharge by eight ion source RF drivers were optimised. In 2019 the operation with hydrogen negative ion beam begun: for the first year, SPIDER will operate with a reduced number of beamlets (80 instead of 1280) due to limitations in the vacuum system. In 2021, the first operation with caesium was performed.
The capabilities of SPIDER and MITICA are listed in the following table in comparison with the objectives of the ITER Heating Neutral Beam and with other existing devices based on RF-driven sources. The obtained results reported in table refers to the operation at low filling pressure of 0.3 Pa; a marked improvement of performances is found for higher operating pressures, but a low pressure is requried to minimise the heat loads due to stray particles, generated by interaction of the beam ions with the background gas along the multi-grid electrostatic accelerator of MITICA and ITER HNB sources.
| Experiment | First operation | Beam energy (achieved/ target) | negative ion beam current (achieved/ target) | negative ion beam current density (achieved/ target) | Ion source type | Accelerator type | Neutraliser type | Beamline length | Neutral beam equivalent current | Target single beamlet divergence at 0.3 Pa (gaussian 1/e) | Achieved single beamlet divergence at 0.3 Pa ±10% (gaussian 1/e) |
|---|---|---|---|---|---|---|---|---|---|---|---|
| BATMAN Upgrade[4] | upgraded in 2018 | ~60 kV | ? (hydrogen) | 350 A/m2 [5] / 330 A/m2 (hydrogen) | RF-driven caesiated surface-plasma source | Multi-aperture electrostatic triode | - | ~3 m | - | - | 11 mrad (core divergence including ~75% beamlet current) |
| ELISE[6] | Feb 2013 | ~60 kV | ~27 A (hydrogen) | ~280 A/m2 [7] / 330 A/m2 (hydrogen) | RF-driven caesiated surface-plasma source | Multi-aperture electrostatic triode | - | ~5 m | - | - | - |
| SPIDER | May 2018 | 50 kV[8] / 110 kV | ~1 A/ 54 A (hydrogen) | 225 A/m2 / 330 A/m2 (hydrogen) | RF-driven caesiated surface-plasma source | Multi-aperture electrostatic triode | - | ~5 m | - | <7 mrad | 12 mrad |
| MITICA | 2025 (expected) | 880 kV (hydrogen) / 1000 kV (deuterium) | -/ 40 A (hydrogen) | -/ 330 A/m2 (hydrogen) | RF-driven caesiated surface-plasma source | Multi-grid multi-aperture concept (7 electrodes) | 4 Gas cells | ~13 m | 16.7 A | <7 mrad | - |
| ITER HNB | TBD | 880 kV (hydrogen) / 1000 kV (deuterium) | 40 A | -/ 330 A/m2 (hydrogen) | RF-driven caesiated surface-plasma source | Multi-grid multi-aperture concept (7 electrodes) | 4 Gas cells | ~22.5 m | 16.7 A | <7 mrad | - |