Development of the Cs-seeded RF negative ion beam source in Korea
The development of a Cs-seeded RF negative ion beam source has been started as a collaborative research project between two institutes KAERI and NFRI, towards ITER neutral beam injector (NBI) technology. The final performance objective of the source is to produce 200 keV, 0.5 A deuterium ion beams. The preliminary experiments for the performance evaluation of the RF plasma source was conducted. The RF-compensated Langmuir probe was used to measure the electron energy probability functions (EEPFs), and the electron temperatures and the electron densities were calculated from the obtained EEPFs. The obtained plasma parameters in this way were used to calculate various RF source characteristic parameters. In particular, the negative ion density were estimate through the balance model calculations, in which the negative ions are assumed to be produced mainly by the volume production mechanism, that is, the dissociative attachments of low energy electrons to vibrationally excited hydrogen molecules. The extraction and the acceleration grids for the beam energy up to 30 keV were installed and the beam extraction experiments were carried out to check the actual performance of the source. Then, the evaporated Cs was seeded into the expansion chamber controlled by the dispenser current. In order to find the optimum operating conditions for high-performance ion beams, various beam extraction experiments were accomplished with several external parameters such as the Cs-evaporation rate, the temperature of the plasma grid (PG), etc. Detailed system description and the experimental results are presented.