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 a 12×18 cm aperture, and a 10‐m ‘‘module’’ focus. TFTR modules are unfocused, with a 120‐kV gap and 12×43‐cm mask. The first CLPS was tested in the TFTR configuration, at 120 kV, 2s. Optimum current was 73 A, or 1.76 μpervs (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 (1/e half‐angle). The combination of an axial cusp magnetic bucket and slot accelerator apertures gives the common long pulse source about twice the beam power per unit cross section of other long pulse sources, plus lower divergence in the direction parallel to the slots.