Update on the Viborg fusor 1 Operation

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Schistad
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Real name: Bernhard Schistad

Update on the Viborg fusor 1 Operation

Post by Schistad »

I think it is time for an update on the fusor, here in Viborg, Denmark.

We have built a fusor for use in our A level physics classes in order to demonstrate nuclear fusion and plasma behavior. The vacuum tank was built by our mechanical workshop, which has some very skilled stainless steel craftsmen. It uses a rather large vacuum chamber for a fusor, (Ø 27 cm). The internal grid is made of 1mm tungsten wire with a 4 cm diameter. Because of the large vacuum chamber, we have added an external grid, which is Ø 15 cm.

The vacuum system consists of a Leybold Trivac D8B foreline pump (obtained from eBay) and an Agilent Ax-65 air-cooled diffusion pump. We measure the pressure with a 536 Thermocouple gauge and XGS-600 controller.

The high voltage is delivered by a very compact Iseg 350 W, -40 kV power supply with our home made controller. To monitor the voltage we use a 100 micro amp meter with a suitable high voltage resistor chain in series. In order to stabilize the load on the power supply, we use a 100kΩ serial power resistor (actually 4 x 50 kΩ resistors).

The deuterium supply uses a Sigma-Aldrich 10L lecture bottle with regulator and two additional vacuum valves. To limit the flow, we have also included a 10μ tube section from Lenox Laser.

This is the total system:
System.jpg
The system has three large flanges:
• Left flange with vacuum output, vacuum gauge connection and Deuterium input
• Center flange with a 3.5” viewport with borosilicate glass
• Right flange with HT feedthrough
The large endplates have been cut with our plasma cutter. The flanges are just plane flanges with microgrooves. We use Nitrile rubber gaskets with a little vacuum grease. This is not 100% tight, but the leakage under vacuum is about 5 microns per minute, which is manageable with such a large vacuum chamber.
When the fore line pump is switched on, the system reaches about 15 microns in 5 minutes. When the diffusion pump is switched on, the pressure starts dropping steeply after 6 minutes:
Pressure vs time.jpg
The pressure gauge can only measure down to 0.15 microns, but the diffusion pump can give a couple of orders of magnitude more than that.
Our power supply is current limited to nine mA. This means that the voltage is highly dependent on the plasma pressure. We have measured the voltage versus pressure with deuterium:
Voltage vs Pressure.jpg
If we start by filling the system with deuterium, the voltage at 15 microns will stabilize at 4 kV. We get a beautiful deuterium plasma:
4 kV plasma.jpg
Starting the diffusion pump will cause the pressure to fall and the voltage starts rising. If no deuterium is added, the plasma will extinguish at around 2 microns. In order to keep a stable plasma at high voltage, we need to keep the system at around 4 microns by feeding deuterium or shutting off the throttle valve in the vacuum output. At 20 kV, we get a very hot plasma and we can feel the vacuum chamber walls starting to heat up.
Plasma at 20 kV:
20 kV plasma.jpg
Above 12 kV, the system starts to emit x-ray radiation. When we reach 20 kV the intensity at the viewport starts to get dangerous. We have measured radiation vs voltage with an SEI Inspector Geiger counter:
Radiation level.png
The radiation is not coming from electrons hitting the chamber or viewport glass, but from the plasma at the center of the internal grid. The highest radiation level I have measured was 1200 μSv/h (120 mrem/h) at a distance of 25 cm from the center.
The next posting will report gamma spectroscopy of the radiation and measurements of neutron output.

Cheers,
Bernhard Schistad
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Richard Hull
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Re: Update on the Viborg fusor 1 Operation

Post by Richard Hull »

I find it amazing that you had readings in the 120mrem/hr range. I have a well calibrated victoreen ion chamber with a thin aluminum window about 3 inches in diameter. At 40kv with my fusor the reading at about 20cm is about 10-20 mrem/hr through the SS chamber. However, at the same distance from the 1" viewport I get about 60mrem/hr at 40 KV.

At about 2 meters, the level is below 1mrem from the SS chamber. You noted you used 20kv? I wonder how you read more radiation at 20kv than I get at 40kv.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Schistad
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Real name: Bernhard Schistad

Re: Update on the Viborg fusor 1 Operation

Post by Schistad »

I was amazed by the high radiation levels. The Inspector has a very large pancake GM tube and it starts picking up photons at 12 kV and goes totally bananas above 20 kV.
The instrument crashes because of overload when it gets above 120 mRem/h.
I also have a GMC-320-Plus counter, which is much less sesitive to the soft x-rays. It starts ticking around 15 kV and shows about 1/3 of the Count of the Inspector below 20 kV.

Also the scintillation tube registers a very high photon count. Because it is enclosed in a 1 mm Aluminium tube, it only register photons above 8 keV.

But certainly, a fusor is a very bright x-ray source. Also, notice that at 20 kV we have a plasma at thermal equilibrium, with a photon energy spectrum peaking at the kinetic temperature of the plasma.

Cheers,
Bernhard
Bruce Meagher
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Re: Update on the Viborg fusor 1 Operation

Post by Bruce Meagher »

Hi Bernhard,

As you are probably well aware, using a geiger counter to measure dose rate is pretty inaccurate. These instruments rely on a calibration factor to convert counts per minute (CPM) to dose rate, and many use a CS-137 source (662 keV) for this factor. As Richard mentioned above, a thin windowed ion chamber is the go to instrument for this measurement. Have you done anything special to calibrate your geiger counter to accurately measure dose rate for your furor's x-ray spectrum?
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Schistad
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Real name: Bernhard Schistad

Re: Update on the Viborg fusor 1 Operation

Post by Schistad »

Hi Bruce,

No I am only using the geiger counters to monitor the Count rate when adjusting the pressure and Deuterium flow.

The spectrum was measured with i scintillation counter, which was calibrated with a Cs137 source.

Bernhard
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