G.C. Harper
Tests of the RF discharge source for use in the tandem terminal were completed this year, and two experiments were run using the source.
Bench tests last year1 with a 1 mm diameter canal produced
32 µamps
of unanalyzed beam using 4He as the gas with a measured gas load
of 2.0 mTorr-l/sec and a focused FWHM of 1.3 mm. The emittance
of the 4He+ beam from the 1 mm canal was estimated to be about 1.1
mm mrad
.
The source was then installed in the terminal of the
Van de Graaff. An unanalyzed beam of
22 µamps
using 3He gas was measured
before the magnet. An analyzed beam of
3 µamps
of 3He+ was measured on
a flap just after the magnet. The size and shape of the beam
spot on this flap indicated some problem with the magnet. The
source and magnet were removed and tested in the 24 inch chamber.
The strange beam spot repeated itself on a faraday cup inside the 24 inch chamber. Several possible causes were investigated. Ultimately, a more careful measurement of the dipole magnetic field indicated that the vertical component of the fringe field was extremely irregular. It was determined that the 8C permanent magnet material has a very low relative permeability and is unable to produce uniform fields in the fringe regions. Pole faces of soft steel with high permeability were attached to the 8C poles. The edge of the steel was beveled by 45° to avoid saturation in that region. This produced a beam profile, intensity, and diameter as originally expected. Electrostatic deflectors were also built and tested and these proved to be a usable alternative to the dipole magnet if the need ever arose for applications involving heavier beams.
The source and magnet were again installed in the terminal. The source ran uninterrupted with a steady output of 32 µamps for 15 days in an experiment that ran from 15 April to 30 April. The source produced 32 µamps of analyzed beam with a measured gas load of 0.76 mTorr-l/sec. At the 5.5 MV terminal voltage required by the experiment, 90% of the beam was transported through the spiral inclined field tubes. In the terminal voltage range of 1.8 MV to 7.5 MV over 50% of the beam was transported. The source was not disassembled or cleaned after this experiment.
A second experiment was scheduled from 1 December to 21 December. After the installation just prior to this experiment an electrolytic capacitor in the RF power supply collapsed under tank pressure. A new capacitor was purchased and both capacitors for the RF supply were potted in thin-wall stainless steel tubes with epoxy. During the experiment the source sparked on a regular basis but ran with satisfactory beam output. The source was disassembled after this experiment and the pyrex bottle was found to be coated with a metallic film. The canal was partially sputtered away. The coating was believed to be the cause of the sparking.