DOE OSTI2015
The test facility ELISE, equipped with a large radio frequency (RF) driven ion source (1×0.9 m2) of half the size of the ion source for the ITER neutral beam injection (NBI) system, is operational since beginning of 2013. The first experimental campaign was dedicated to a thorough qualification of the test facility and its diagnostic tools at low RF power (80 kW in total, i.e. 20 kW per driver) in volume operation, i.e. operation without cesium, where the negative hydrogen ion production is done in the plasma volume only. This paper reports on the main results of the second and third experimental campaigns, where Cs was inserted in the ion source for an enhancement of the negative ion production by the surface process. The second experimental campaign was done still with low RF power, both for hydrogen and deuterium, with pulse lengths of up to 500 s. The results of this campaign are rather encouraging, especially in hydrogen, where large current densities with respect to the low RF power could be achieved at a ratio of co-extracted electrons to extracted ions of 0.5-0.6 at the relevant source pressure of 0.3 Pa. Similar large extracted ion currents could be achieved also in deuterium, but with larger amounts of co-extracted electrons. The required ratio of co-extracted electrons to extracted ions of one could be achieved only in short pulses. The third experimental campaign aimed then for approaching the required ITER NBI parameters with respect to the ion and electron extracted currents, both for hydrogen and deuterium, by increasing the RF power with short pulses, i.e. beam-on times of up to 10 s and RF-on time up to 20 s. Current densities near the ITER NBI requirements could be achieved in hydrogen at a ratio of co-extracted electrons to extracted ions of 0.5-0.6 at the relevant source pressure of 0.3 Pa. As it was the case for the low RF operation, the required filter field was significantly lower than expected from the experience with the small prototype RF source. Similar large extracted ion currents could be achieved also in deuterium, but with larger amounts of co-extracted electrons; the main problem in deuterium operation are the non-stable currents of the co-extracted electrons, most probably caused by the high dynamic of the Cs redistribution in the system. The reasons for this larger instability are still under investigation.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗