Laboratory experiment for the development of a laser neutralizer in view of DEMO NNBI

Abstract

Laser neutralization of an accelerated negative ion beam is a promising alternative to the gas neutralizer of current NNBI systems. It has the prospect to boost the overall wall-plug efficiency of the NNBI system from currently about 28 % at ITER to about 60 % for the next step after ITER, a DEMO tokamak. Laser neutralization requires resonant coupling of a cw laser to an amplification cavity built around the ion beam. In the final stage, the amplified laser radiation of several hundreds of MW needs to intersect an ion beam cross section of ∼ m 2 at energies up to 1 MeV. Such a sophisticated concept needs to be tested in small scale at laboratory experiments. Challenges include achieving coverage of the entire ion beam cross section, thermal lensing of the cavity mirrors due to the huge optical power loads as well as coupling to a cavity under vibrations due to vacuum pumps and possible interferences during ion beam extraction. Using a medium power laser (8 W), investigations regarding the influence of vibrations are performed on an optical table, while in a second step the application to an ion beam with reduced cross section (∼ mm 2 ) and beam energy (∼ keV) will be studied.