SiPhoRE

The project aims at demonstrating the feasibility of a nearly complete neutralization of a beam of H- or D- by photodetachment. This is to be applied to the production of energetic neutral beams, suitable for the heating of fusion plasmas, like the one of ITER and the future fusion reactors (DEMO). This would be an important simplification with respect to the current neutral beam production technique, which relies on collisional neutralization and suffers from important technical drawbacks such as very low overall injector efficiency (~20%) and the necessity to set high-current negative ion sources and all their accessories at very high voltages. Despite its conceptual simplicity, the photodetachment way presents a major difficulty, namely the necessity to illuminate the ion beam with a very high flux of photons, so as to reach the saturation regime. Fortunately, laboratoire ARTEMIS has accumulated an outstanding expertise in high power laser light injection, which makes photodetachment in a high finesse cavity a realistic solution. Using an optical cavity with a finesse of a few thousand would make it possible to reach the multi-megawatt regime suitable for saturation with only one kW of laser input. Recycling of the light is made possible - and necessary - by the very low individual absorption of a single negative ion. The first goal of the project is to make the experiment at a reduced scale, on a negative ion beam of only a few millimetres in diameter, and to increase the cavity finesse, hence the light power progressively. Having several MW of intracavity light power is a complementary objective, for which several technical issues have to be addressed, first of all the thermal effects inside the optical substrates. Association of an industrial partner to the project aims at demonstrating that the cavity mirrors can have the properties necessary for reaching the objective. During the program, the demonstration experiment at LAC will make use of magnetic coils, so as to investigate the amplification of the detachment cross-section at Landau resonances. Though this implies solving additional problems, especially to make the magnetic field homogeneous on large volumes, this could be helpful for future industrial developments by reducing the laser power to be applied.