Development testing of the U.S. common long pulse source at 120 kV

The U.S. magnetic fusion energy program has developed a single design long pulse neutral beam source for TFTR, MFTF-B, and DIII-D. The arc is a very compact axial magnetic line cusp. The accelerator is an actively cooled tetrode with water cooled grid tubes of shaped molybdenum forming 'slot' beamlets. DIII-D and MFTF-B configurations have an 80 kV accelerator gap, with 12 x 48 cm aperture, and a 10 meter 'module' focus. TFTR modules are unfocused, with a 120 kV gap and 12 x 43 cm mask. The first CLPS was tested in the TFTR configuration, at 120 kV, 2 seconds. Optimum current was 73 Amperes, or 1.76 ppervs (deuterium), with 80% - 85% atomic fraction. Optimum divergence of ions plus neutrals was 0.4' parallel to the slots, and 0.7' perpendicular to the slots ( l / e half angle). The combination of an axial cusp magnetic bucket and slot accelerator apertures gives the CLPS about twice the beam power per unit cross section of other long pulse sources, plus lower divergence in the direction parallel to the slots.