This photo shows our implementation of the Farnsworth Fusor. It is housed in a 6″ Conflat cross. The outer grid is about 3.9″ OD, made of 0.062″ diameter 308 stainless wire (welding rod) spot welded. A portion of the outer grid can be seen in the photo as three dark lines crossing the viewport. The inner grid is 1.25″ OD, made of 0.020″ diameter Ta wire spot welded. This grid is glowing visibly in the photo.
The inner grid is supported by a 0.094″ diameter stainless rod which is part of the 30 kV feedthrough visible on top of the chamber. The stainless rod is insulated with a 99.8% alumina ceramic tube that tends to glow alarmingly red during operation!
By carefully adjusting the D2 pressure to around 10-15 millitorr (measured with a capacitance manometer) and applying about 20,000 volts across the grids, a thin glow discharge can be established. The current is typically around 6 milliamps. Under these conditions, the system produces D+D fusion in the center due to head-on collisions between ~20 keV deuterons.
Evidence of this fusion reaction is the emission of ~104 neutrons/sec. Neutrons are detected with a Bicron BC-720 fast neutron scintillator which consists of ZnS(Ag) phosphor embedded in a clear hydrogenous plastic. It is 2″ in diameter. We have coupled it to a 2″ PM tube (bi-alkali photocathode) and enclosed it in the cardboard tube visible on the left. The detector electronics are visible in the lower left corner. That box is the all-in-one (HV supply, amplifier, single-channel analyzer, and scaler) Texas Nuclear 9200 system that was originally sold as portable XRF system in the 1960’s and 1970’s.
The total neutron emission rate was calculated from the observed count rate of up to 1.5 count/sec (background is 0.2 count/sec), taking into account the 1/36 geometry factor and 0.6% detector efficiency for 2.5 MeV neutrons.
In front of the chamber is an 8mm thick sheet of yellowish leaded glass that does an excellent job of stopping the torrent of soft x-rays that pours out of the glass viewport during operation.
At the very top of the photo you can see the Plastic Capacitor’s HV power supply and a neon sign transformer. The secondary of this transformer is wired in series with the chamber to serve as a filter. It’s inductance measures 190 henries!
One of the advantages of the BC-720 is gamma rejection. The following waveforms, collected from the amplifier output of the TN9200, show how this is accomplished:
The left waveform shows a typical gamma pulse. This waveform was generated by exposing the detector to a large piece of 232Th, which emits multiple gamma energies up to 2.6 MeV. The waveform on the right was collected during operation of the fusor. A single-channel analyzer is used to discriminate against small the pulses, making the detection system essentially insensitive to gammas.
We also looked at some background pulses. As our system is presently adjusted, background pulses occur approximately once every 5 seconds. Some of these pulses look just like the neutron pulse shown on the right above. Occassionally, there are some odd ones:
The background pulse on the left is very narrow compared to either the gamma or neutron pulses. The background pulse on the right is huge! Presumably these are different forms of cosmic radiation.