Photodetector Module Helps Monitor Magnetic Fusion

Michael D. Wheeler

For years, research scientists had had trouble navigating the cramped electronics hall at San Diego's DIII-D magnetic fusion facility. Packed with electronic and data-acquisition equipment, the hall had very little room for the large electronics racks that house the photomultipliers, preamplifiers and high-voltage supplies. The situation became critical when it was decided to triple the number of views inside the fusion chamber -- hence tripling the number of photomultipliers and accessories required.
Oak Ridge National Laboratory researcher Chris Klepper found a remedy. After seeing an advertisement in 1994 for Hamamatsu Corp.'s newly developed photomultiplier tubes (PMTs) (see also "Photomultiplier Tubes See the Light" by Earl Hargert and Craig Walling, Photonics Spectra, December 1996), he set out to design a compact module appropriate for the fusion facility.
Klepper bundled five H5783 modules -- each including the TO-8 PMTs and a high-voltage supply -- into a 7/8 3 7/8 3 2-in. package that was inserted into a standard, double-width nuclear imaging module plug-in. The plug-in incorporates a notch filter, small focusing optic and a circuit that supplies the necessary voltage and preamplification. Klepper also included an innovative safety circuit to protect the photosensors from overexposure to room light.
The new modules replaced the old setup that had occupied an entire electronics rack.

An eye inside
Klepper's photodetector modules perform an essential task: They provide scientists with a means of monitoring magnetic fusion, a potentially viable energy source in years to come. Inside the doughnut-shaped fusion chamber, developed by Russian scientists in the 1950s, gas is heated to temperatures in excess of 100 million °C. As the plasma -- really a soup of ions and electrons -- becomes heated, it emits light at different wavelengths. These wavelengths are characteristic of the various types of elements and the degree to which the atoms are stripped of their electrons. Researchers monitor the activity via optical fibers that carry the light from a separate view in the chamber. Klepper's design splits light into three channels -- each with a different notch filter -- to view different ions or neutrons.
Apart from magnetic fusion applications, Klepper said he is considering potential applications in the semiconductor industry. One clear advantage, he said, is the relatively low cost of $4000 for one plug-in and $500 to $800 for the photosensor alone.