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

Abstract

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.

Axisymmetric Magnetic Mirror Fusion-Fission Hybrid

“…Using the measured power of 400-700 W to each grid rail [Vella 1988], and assuming the ions (from ionization of intra-electrode gas) accelerate to energies of the order of 10-20 keV, we obtain an erosion rate of 25-40 µm/day for Mo or 20-35 µm/day for W. These rates are high enough that we would not expect to achieve much more than a few weeks of steady-state operation. These values are quite uncertain, however, and could be lower if much of the power comes from electron bombardment of the electrodes, or our estimate of ion energy to the electrodes is wrong.…”

“…Long-pulse neutral beam accelerators on TFTR, and DIIID [Vella 1988, Kim 1987, Stevenson 1992] have demonstrated reliable operation at durations of a few seconds; these durations exceed ten thermal time constants of the accelerator electrodes, so they are effectively steady-state cooled. Ion source cathodes are estimated to last at least 12 days , and accelerator electrode erosion by sputtering could lead to similar lifetimes [Simonen 2010], as discussed below.…”

“…The first method to control erosion of accelerator electrodes is to shape the negative suppressor electrode so that electrons emitted from it will not strike other accelerator electrodes but will be dumped on the back wall of the ion source [Vella 1988]. Efforts can be made to expel as many cold ions (formed from ionization of gas by energetic ion impact) as possible from the accelerator, so that they don't hit electrodes and cause sputtering, but the ion-optics designer has little freedom here, as gas is ionized between nearly-planar electrode arrays.…”

“…The ion density at the accelerator grids must be very uniform to produce multiple beamlets that are each well focused. Uniformity was achieved in the LBNL Long-pulse source by using a magnetic-field-free ion source [Vella 1988, Chan 1983, in which ions can flow freely to fill the volume uniformly. However, similarly large ion sources for plasma processing have axial magnetic fields, which can be uniformly filled by careful ion source design [Hopwood 1992, Wu 2000, Blackwell 1997].…”

“…A reduction of roughly two orders of magnitude should achieve lifetimes exceeding a year; this can be done first by reducing the gas pressure within the ion source, and downstream neutralizer, and secondly by separating these elements from the accelerator to allow space for gas pumping. For this to be possible, we must abandon the long-pulse magnetic-field-free ion source [Vella 1988], which is currently closely coupled to the accelerator. Then to avoid losing most of the ions between the ion source and accelerator, we much apply an axial magnetic field that will allow the ions to flow to the accelerator with little radial loss.…”