The setup of a neutron generating system is shown in Fig. 1. This photograph was taken just after the system was first completed. The vacuum chamber, neutron detector, vacuum pump, and deuterium tank sit on the large table. To the right in the photograph are the electronics. The gray box between the two tables is a 12 KV 1.5 kVA potential transformer. This transformer feeds a voltage doubler which sits on top of the transformer. The maximum unloaded voltage at the output of the doubler is about 40 KV when 140 V is fed into the primary of the potential transformer.
| Fig. 1. View of entire system as it looked when first built. |
The electronics required to operate the system in glow-discharge mode are shown in Fig. 2. An 18" high, 19" wide benchtop rack enclosure holds the electronics. The enclosure features extra unused panels for future expansion. At the top left is a MKS PDR-C-1C pressure readout that controls the chamber capacitance manometer. To the left is a MKS PDR-C-2C pressure readout that controls the backing line capacitance manometer. At the bottom right is the vacuum control box which controls the rotary and turbo pumps. To the left of the vacuum control box is the power supply control. The box contains a variac and electronics to display the output voltage and current of the high voltage supply; this box connects to an external transformer and filter assembly.
A neutron detector is also required for operation. The radiation analyzer used with this system, not shown in the photo, is a Model 5000 Radiation Analyzer made by The Nucleus. This all-in-one unit powers the neutron detector, amplified the detector output, then runs it through a single channel analyzer and on to a counter and rate meter.
| Fig. 2. Electronics for operation of the system in glow discharge mode. |
Two photographs of the system in operation are shown in Fig. 3 and Fig. 4. These photographs were taken while observing the discharge in deuterium at relatively low voltages. At voltages above 15 KV x-ray production begins to become significant, as does neutron production. In the photos, beams can be seen radiating out of the centers of the grid openings. This is often termed "star mode" operation and caused by electron beams exiting the center grid and exciting neutral deuterium atoms in the chamber. The glow results from recombination of the deuterium ions, which occurs a short time later.
In Fig. 4 the inner grid is running red hot. This is the normal mode of operation where significant neutron outputs are obtained. A stainless steel grid limits system output since it easily becomes white hot. The problem with such a hot grid is that thermionic emission begins when the grid reaches orange or yellow heat. These electrons increase the current through the device and a runaway condition results. If the voltage to the system is not reduced, the inner grid will melt within a few seconds.
| Fig. 3. View of system operating at about 15 KV and 10 mA. |
| Fig. 4. Another shot of the system in operation. Grid is running red hot due to ion bombardment. |
Easily measurable fusion rates occur in the system at voltages above 13 - 15 KV. The system is normally operated at voltages of about 20 - 45 KV and currents of 5 - 20 mA. Deuterium pressure in the chamber ranges from about 8 - 20 mTorr. Maximum output obtained so far in glow-discharge mode is about 675,000 neutrons/sec. This occurred at 42 KV and about 18 mA current with a deuterium pressure of 11.5 mTorr. Fig. 5 shows how neutron output scales with increasing voltage. Current was held at 5 mA for all data points. The output rapidly increases in this area since the cross section of the deuterium-deuterium reaction increases quickly through this voltage range. The increased voltage and reduced gas pressure also help to increase the output at the higher end of the graph.
| Fig. 5. Plot of neutron output vs. acceleration voltage. Current was held at 5 mA. |