Low Vacuum Plasma

Here is a picture from a very low vacuum run (just under 2000 microns). We found that the atmospheric plasma would only ignite up to 2000 microns, and that the low vacuum plasma was much redder in color. Also, this plasma would ignite at a much lower voltage (around -400V) as compared to the higher vacuum plasma at -1500V.

Interesting Effects...

This video is a clip from a very low vacuum run (around 200 microns). Notice how the plasma is not diffused throughout the chamber but forms a "halo" around the wires of the grid.

It Works!

Today, February 26, 2010 I turned the completed reactor on for the first time and it performed wonderfully! At a vacuum pressure of 20 microns and a potential of -1500V on the inner grid, a bright purple-colored plasma was observed. On the very first run, I was able to adjust the variac all the way to full power (-10kV on the inner grid) but the high voltage managed to arc from the container to the metal guide wire in the vacuum hose, grounding the whole system and extinguishing the plasma. After that we were not able to get the voltage past -5kV, but we still managed to get plasma, and played around with varying the voltage and pressure in the chamber. Below is a video of one of the runs:

As you can hear in the video, the plasma starts to appear when the variac is at 20 volts (corresponding to roughly -1500V from the power supply). From what I have read so far, the shape of the plasma is due in part to the geometry of the inner grid, and the jet, or "bugle" coming out of the left side has to do with how the grid is shaped.

While this plasma looks really cool, this test was just the first in a series of steps to achieving nuclear fusion. This test run was just done on atmospheric gas, which is much too heavy to fuse with the voltages that I am running at. However, the next step is to build an inlet to let deuterium gas into the chamber, which should fuse when the reactor is turned on.

2 More Days Until the First Test Run!

This Friday, I hope to run the fusor for the first time. Today, Anatol and I put the system together and mounted the chamber on some vertical supports that Anatol made. We connected the chamber in series with Dr. Bulman's so we can measure the pressure inside the chamber. After everything was tightened down, I turned on the pump and started evacuating the chamber. Hopefully by Friday the chamber will be down to around 35 microns or so.

The first test of the chamber will not actually produce fusion reactions, but should ionize the atmospheric gas that is left in the chamber. If we can manage to ionize this gas, then the fusor proof-of-concept is good and we can proceed to add a gas inlet to the chamber to let deuterium in.

Power Supply

In order to create the high voltages required to energize the inner grid, a special power supply was needed. As commercial kV power supplies are very expensive, I decided to build my own. This supply is comprised of a variac tied to the main transformer which is output through a reverse full-wave bridge rectifier and then through a filter capacitor. Because the voltages were too high to measure with a regular multimeter, I made a voltage divider circuit to cut the voltage down enough to measure it. Unfortunately, the first time I measured the voltage, I failed to take into account the fact that the internal impedance of the meter was the same as the resistor I was measuring over, so my calculated results were wrong. However, once I factored in the internal impedance of the meter, I was able to (correctly) measure a maximum output voltage of about 20kV. I also measured a no-load maximum current of 30 mA. Finally, I measured the output using an oscilloscope. There was an appreciable ripple, so I might build more smoothing circuitry for future generations of the power supply.

Inner Grid and Feedthrough

The basic operating principle of this fusor is that there will be two "grids" in a vacuum. The outer grid will be a positive ground reference and the inner grid will be a highly negative voltage. When the grids are energized, the free particles in the chamber will ionize (i.e. the electrons will be ripped from the nucleus). The free electrons will be attracted to the positive ground (the metal chamber in my case) and the positively charged nuclei will be attracted to the negative inner grid. As you can see from the picture on the left, the inner grid should be as transparent as possible to allow the nuclei to fly through and collide in the center. These collisions are what should produce fusion reactions once deuterium is introduced into the chamber.


In order to get the high voltage into the vacuum chamber, it is necessary to have a vacuum sealed inlet for the high voltage. In the picture below, my feedthrough is shown in the vice. For the feedthrough I decided to use a spark plug (credit to Andrew Seltzman of RTF technologies for the idea). I chipped away the insulation on the inside of the plug and used a crimp connector to attach a copper wire to the center post of the spark plug. Then, Anatol and I J-B welded a high-alumina ceramic jacket over the wire to stop it from arcing to the casing of the spark plug. We cut and tapped a piece of aluminum to J-B weld to the side of chamber so that the spark plug could simply be screwed in (we used teflon tape around the threads to ensure a vacuum seal).

Vacuum Test 3

For the third vacuum test, I tested the system with the addition of the high-voltage port. For the high voltage feedthrough, I will be using a Bosch spark plug threaded through a 1/2'' thick aluminum plate, J-B welded to the side of the chamber. The schematic on the right is of the aluminum plate that was epoxied to the side of the chamber. In order to connect the spark plug, Anatol and I wrapped the threads in Teflon tape and used a wrench to screw the plug into the threaded hole in the aluminum plate.

In addition to making sure that the system could still hold a vacuum after the feedthrough had been attached, we found another (newer) vacuum gauge that fit the plug for the sensor in Dr. Bulman's chamber, and compared the readings of both gauges.

It appears that the third test was a success, as no audible leaks were detected when the pump was first turned on, and the chamber got down to a pressure of about 200 microns on the old gauge. However, when we swapped the old gauge for the new one, we recorded a pressure of 34 microns! As we have no way of calibrating the gauges at this time, it is impossible to decisively say which gauge is the "correct" one, or even if either of the gauges are correct. However, at this point, Anatol and I agreed that the newer gauge was probably more accurate just on the account of it being new, so we will use that one for the time being.

Vacuum Setup

Here is a picture of the vacuum setup that was described in the post for the first vacuum test (although this picture is of the second test). My chamber is the one on the ground, and is connected to the orange and black roughing pump by the clear line. Dr. Bulman's chamber is the black cylinder on the back of the table, and the pressure gauge is in front of it. The gauge is connected to a thermocouple sensor which is mounted inside of the chamber. In the future, I hope to buy a thermocouple sensor and build a feedthrough directly into my fusor, as the roughing pump has to work twice as hard in this configuration to pump out both chambers.

Vacuum Test 2

After the viewport had been attached, we needed to make sure that the seal was vacuum tight, so we assembled the system and hooked it up to Dr. Bulman's rig again. After pumping for a few days, the system got down to about 200 microns again, so it appears that the container is still vacuum tight.

View Through the Viewport

Here is a view through the viewport with the vacuum container assembled. The hole in the other side is the 3/4'' fitting which is attached to the vacuum pump. When the fusor is completed, the inner grid should be visible through the viewport.

Assembled Container with Viewport

Here is the assembled container with the viewport attached.

Lathe

Here is the lathe that I have been using to machine the metal parts. In this picture, the aluminum round used for the viewport base is being prepared for machining of the hole and the counter-sink.

Viewport Design

In order to be able to see into the chamber, Anatol and I designed a viewport to be attached to the side opposite the vacuum fitting. We cut a slice off of the 5'' aluminum round to be the base and then I used the lathe to put a 2'' diameter hole in the center of the round. In order to have a place to put the viewport, I cut an additional 2 3/4'' diameter hole partially into the round to act as a counter-sink for the port itself. The port was made by cutting a 2'' round piece from a 1/2'' thick slab of acrylic. As per usual, we used J-B Weld to attach the base to the bowl and then the acrylic to the base.

A Short Introduction to Pressure Terminology

The units of pressure were a bit confusing to me before I started this project, so to clarify things for the reader in advance, here is a quick tutorial about pressure and its units.

In order for the fusor to produce fusion reactions, the inside of the chamber must be at a very low pressure, or put another way, at a very high vacuum. Out in space, there is an almost perfect vacuum. On the earth, the atmosphere is filled with gas, which exerts pressure on everything. You are used to this pressure and do not notice it in your everyday life, but if you drive up a mountain or fly on an airplane, your ears will pop, as the atmosphere is thinning and the pressure is dropping.

The standard pressure at sea level is defined as 1 atm (atmosphere), which is roughly 15 pounds per square inch. The pressure of the earth's atmosphere at sea level will apply a force of 15 lbs on every square inch exposed to the atmosphere. Easy, right? Well, here is where things start to get confusing.

There are many different systems of measuring pressure. The SI unit of pressure is the pascal (Pa), but I have not encountered this notation very often in my online fusor research. More common is the Torr, which is roughly the fluid pressure exerted by a millimeter of mercury. One atm is equal to 760 Torr. However, vacuum pressure is much, much less than atmospheric pressure, so normally only a tiny fraction of a Torr is measured. For example, light bulbs are normally evacuated to 0.1 to 0.01 Torr. The vacuum pressure on the moon is roughly 10^-11 Torr.

From my online research so far, it seems that a vacuum of about 20-60 mTorr (m stands for 10^-3) is needed. Because one Torr is roughly equivalent to a millimeter of mercury, another symbol for a Torr is mmHg. Thus, one mTorr will be equal to one micrometer (10^-6 meters) of Hg, and so the term "micron" is interchangeable with "mTorr." Whew! Long story short, my fusor will need to obtain pressures of about a tenth of the vacuum pressure in a light bulb in order to operate.

There are many other measurements of pressure, and if you are really interested, wikipedia has good articles on pressure (http://en.wikipedia.org/wiki/Pressure) and vacuum (http://en.wikipedia.org/wiki/Vacuum) that I obtained most of the information above from. Hope this helps!

First Vacuum Test

After letting the J-B Weld set over the weekend, the system was ready to be tested for the first time. Dr. Bulman had a chamber set up with a thermocouple pressure gauge, so we attached my chamber in series with his, with both chambers being connected to my Alcatel roughing pump. We also included a shutoff valve between the pump and the two chambers in order to test the chamber's ability to hold a vacuum once the pump had been shut off. A sketch of the system is shown below. The middle chamber is Dr. Bulman's (with the gauge) and the one on the right is mine:


When we turned the pump on for the first time, Dr. Bulman said it sounded fine so there weren't any large leaks. It made a constant gurgling sound and didn't sputter. We left the pump running for a few days, and the pressure went down to 60 microns or so, but then increased to 200 microns and stayed there. Anatol then closed off the valve (as seen in the top picture) and the system did not drop pressure immediately, which was a good sign. It took about a day for the pressure to rise close to standard atmospheric pressure.

The first test was a success! Not only did we manage to get the pressure down to a very low level, but when the pump was turned off, the chamber was able to hold the vacuum for a while. Also, when we disassembled the chamber later, the gasket was still in good condition.

Vacuum Container: First Stage

After the custom flange was machined on the lathe, Anatol and I used J-B Weld to epoxy the flange to one of the stainless steel bowls. We used a drill press and lots of cutting fluid to punch a 3/4'' hole through the top of the bowl before epoxying the fitting.

I measured the outer and inner diameter of the lip on the stainless steel bowls and cut a gasket out of a sheet of medium-grade rubber. Then, I cut some particle-board pieces (seen behind the bowl) to fit around the outside of the bowls. A hole was drilled in each corner of the particle board so that the boards could be fit around the bowls and tightened down with bolts to secure the bowls together.

Here, both bowls are fit together, without the particle-board jig. We used vacuum grease around the gasket to prevent any small leaks into the container.