Farnsworth Fusor


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!

Neutron Detector Details

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.

Power Balance

Under the optimal operating conditions mentioned above, our fusor consumes about 102 watts of power from the HV supply. With this input it stimulates enough D+D fusion to create about 104 neutrons/sec. Each of the neutron-forming D+D reactions releases 3.27 MeV. Presumably there are also about an equal number of undetectable D+D reactions occurring that yield 3H, a proton, and 4.03 MeV. Thus the total energy yield per emitted neutron is about 7 MeV. At 104/sec, that’s a total fusion power output of 10-8 watts!!!….10-10 of the input power.

Counting the heat power generated by this experiment, the observed ratio of Pout/Pin is therefore about 1.0000000001. Yes, EarthTech has finally observed the excess heat phenomenon! Now we just need to make it bigger…a lot bigger!

Thanks to Richard Hull for generous technical support of our efforts!

Search for Recoil Proton Tracks in a Diffusion-Type Cloud Chamber – 22MAR99

A diffusion-type cloud chamber is a continuously active chamber developed by Langsdorf in 1939. A heated vapor source is located at the top of the chamber and the floor of the chamber is chilled significantly. A vertical temperature gradient will result and there will be a certain zone in which the vapor is sufficiently supersaturated that condensation will take place on ions.

This photo shows a diffusion-type cloud chamber mounted beside our fusor. The cloud chamber consists of a clear plastic container inverted over a black-painted Al plate, which is cooled from below by a slab of dry ice (pressed up against the plate with a piece of spring steel). A layer of felt has been glued into the bottom of the plastic container (which is on top now) and this felt is saturated with isopropyl alcohol to serve as the vapor source. It is heated simply by the ambient.

About 15-20 minutes after closing up the chamber and installing the dry ice, the chamber becomes active. For some reason, the active layer in this chamber is quite shallow, about 1/2″ deep right at the bottom.

This chamber is sensitive to natural radiation. During a 10 minute period, I counted 7 tracks that were sufficiently long and distinct that I might be tempted to consider them recoil proton tracks had the fusor been running.

During another 10 minute period with the fusor running nearly continuously, I counted 45 such tracks!

Proton recoils from 2.5 MeV neutrons can have any energy from 0 to 2.5 MeV and the average should be about 1.25 MeV. A 1.25 Mev proton has a range of about 3 cm in air so we should expect tracks in the cloud chamber about that long.

This photo shows the bottom of the cloud chamber, under illumination from the side, with a millimeter ruler lying in it for scale. The major numbered divisions are centimeters.

Following are several photos of tracks that I think are made by recoil protons. By the way, the source of the protons is hydrogen atoms in either the black paint or the clear plastic container. In this photo the track is very dense and bright. It is about 2-3 cm long.

The next two photos (below) show longer, more diffuse tracks. If the proton gets the full 2.5 MeV, it can make a 10 cm track. Also, if the proton actually flies thru the chamber above the active layer, the ions it produces can drift downward into the active layer and create a diffuse-looking track.

This last photo (below) shows a thready little track (lower center) about 3 cm long. Apparently it originated in the black paint on the right and rose at a very slight angle as it progressed to the left. At the extreme left end of the track you can see the reflection of the track in the bottom of the chamber, which is quite reflective as it is covered with a layer of liquid alcohol. Other lines, which look a little like tracks in this photo, are just scratches on the plastic.