April 2018
Spotlight Summary by Alain Simonin
Telescope-based cavity for negative ion beam neutralization in future fusion reactors
Beyond the ITER machine, the European fusion program aims at demonstrating the technological and commercial viability of a nuclear fusion reactor to produce clean electrical power safely (with high safety and environmental performance). The production and control of the magnetically confined deuterium-tritium (D-T) plasma at the required fusion temperature in the reactor chamber involves the injection into the plasma core of high-power and high-energy deuterium atom beams, e.g., ~100 MW of D° at 1 MeV. Such beams are produced by accelerating D- ions from a plasma source to 1 MeV and subsequently neutralizing them. The photo-detachment process is the only way to obtain more than 90% neutralization efficiency. However, the low photo-detachment cross-section necessitates a MW range photon flux to illuminate the energetic D- beam. To provide this flux, a folded optical Fabry-Perot cavity with a finesse of a few thousand is proposed. The cavity is filled by a kW range highly stabilized external laser. Moreover, to fully overlap the ion beam of 1 cm width, the intra-cavity photon beam diameter in the interaction region has to be close to 1.5 cm FWHM leading to a cavity optical length of 100 m.
Integrating such a long optical cavity into the harsh nuclear environment of the fusion reactor constitutes a major challenge. Stringent nuclear requirements lead to multiple levels of protections and barriers (bioshield) to confine the radioactive inventories within the nuclear island of the reactor. To ease the cavity integration and fulfill the safety requirements, the cavity optical length must be shortened: the optical tank containing the mirrors has to be located within the safety controlled zone ("bioshield"), at a reasonable distance away from the injector tank. The authors of this paper propose making use of a telescope, to reduce the cavity optical length from 100 m to 30 m, allowing implementation of the optical tanks, only meters away from the injector (still within the bioshield), without affecting cavity stability. This concept is inspired from the proven and mature technology of the gravitational waves detectors, but was not yet tested for the parameters described above. Therefore, it needs major research and development work until its implementation in a neutral beam injector connected to a fusion reactor.
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Integrating such a long optical cavity into the harsh nuclear environment of the fusion reactor constitutes a major challenge. Stringent nuclear requirements lead to multiple levels of protections and barriers (bioshield) to confine the radioactive inventories within the nuclear island of the reactor. To ease the cavity integration and fulfill the safety requirements, the cavity optical length must be shortened: the optical tank containing the mirrors has to be located within the safety controlled zone ("bioshield"), at a reasonable distance away from the injector tank. The authors of this paper propose making use of a telescope, to reduce the cavity optical length from 100 m to 30 m, allowing implementation of the optical tanks, only meters away from the injector (still within the bioshield), without affecting cavity stability. This concept is inspired from the proven and mature technology of the gravitational waves detectors, but was not yet tested for the parameters described above. Therefore, it needs major research and development work until its implementation in a neutral beam injector connected to a fusion reactor.
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Article Information
Telescope-based cavity for negative ion beam neutralization in future fusion reactors
Donatella Fiorucci, Ali Hreibi, and Walid Chaibi
Appl. Opt. 57(7) B122-B134 (2018) View: Abstract | HTML | PDF