Speaker
Dr
Cyril Drag
(Laboratoire Aimé-Cotton)
Description
Neutral beam injection, for plasma heating, will supposedly be achieved, in ITER, by collisional detachment of a pre-accelerated D${^-}$ beam. Collisional detachment however makes the presence of a D2-filled neutralisation chamber necessary, on the beam line. This has severe drawbacks, due to collisions with the injected gas up- and downstream from the collision chamber and the impossibility of setting the chamber at high voltages, which makes it necessary to set the D${^-}$ ion source itself at -1 MV.
Photodetachment, in comparison, has many advantages as a neutralisation method: there is no further need of gas injection, the photodetachment zone can be set at a high (positive) voltage, which makes the ion source operation much easier. Photodetachment efficiencies can get infinitely close to 100%. The only difficulty of the photodetachment scheme, as has been known for decades, is that it requires a high laser flux. Fortunately continuous-wave lasers are now available commercially with higher and higher output powers and a monochromaticity good enough to make efficient injection into a high-finesse optical cavity possible.
The present work has consisted in implementing such an optical cavity, with a finesse greater than 1000, on a 1.2 keV H${^-}$ beam (which is only 20 times slower than the D${^-}$ ion beams prepared for ITER). About 26 kW of continuous wave illumination could be obtained with 24 W of laser input, at the wavelength 1064 nm. Photodetachment efficiency has reached a more-than-50% level in a preliminary version of the experiment, which demonstrates that development of real-scale photodetachment-based neutral beam injectors has become realistic.
This experiment also provides the opportunity of a measurement of the photodetachment cross-section, which was seldom determined in actual experiments [1].
[1] Vandevraye, M., Babilotte, P., Drag, C., & Blondel, C., "Laser measurement of the photodetachment cross section of H${^-}$ at the wavelength 1064 nm", *Phys. Rev. A* **90**, 013411 (2014)
Primary author
Dr
David BRESTEAU
(CEA)
Co-authors
Dr
Christophe BLONDEL
(Laboratoire Aimé-Cotton)
Dr
Cyril Drag
(Laboratoire Aimé-Cotton)
