Uniformity of the Large Beam of ELISE during Cs Conditioning.

The ELISE test facility is based on a modular concept design which has been replicated in the ITER Negative Beam Ion (NBI) source with twice the size in the vertical direction. The target of ELISE is to achieve simultaneously the ITER NBI parameters in terms of negative ion extracted current (in H and D), pulse duration and ratio of co-extracted electron to ion below 1 at a source filling pressure of 0.3 Pa. While the source plasma can be operated for 1 h, only 9.5 s of beam extraction every ≈ 150 s can be applied, with a maximum total voltage of 60 kV due to limitations of the HV power supply; only divergences larger than 1 deg have been measured so far. The beam in ELISE is investigated by various diagnostics: the currents in the grid system are electrically measured and provide the total extracted negative ion current as well as information on the beam losses separately measured in the top and bottom grid segments of the extraction and grounded grids; the Beam Emission Spectroscopy (BES) gives information on the beam intensity and divergence (vertical and horizontal profiles); Infra-red (IR) analysis of the diagnostic calorimeter provides a 2D map of the beam power density and, consequently, the accelerated current density distribution as well as the total accelerated current. The spatial resolution of the beam diagnostics at ELISE (BES; diagnostic calorimeter) allows to resolve and describe in intensity and divergence the two beam segments associated to the two top and bottom grid segments. Following the evolution of the beam segments during the Cs conditioning process (i.e. Cs evaporation in the source to move from volume production of negative ions to surface production) allows to optimize the Cs conditioning by using the beam as an indirect measurement of the negative ion production. At high source performance and stable conditioning, the vertical beam uniformity in terms of accelerated current can be easily achieved: the difference between top and bottom beam segments is almost always well within the 10%. [ABSTRACT FROM AUTHOR]