HomePage for the Engineering Physics 2004 Ball Model
 | Feb 8th Update. Well so we didn't *officially* win the ball model contest but we sure got the popular vote. I'd like to thank thank everyone who contributed over the past 3 weeks, your efforts helped make this project happen. The Fusor display was a huge attraction in the SUB during e-week, and also during e-ball itself. I've put some new pictures up of the fusor showing the highly symmetric grid arrangements we built just before the show. | |
Background
Ball Models are built by the various engineering clubs and demonstrated at the end of Engineering Week in February. Each club tries to make the neatest contraption that showcases their particular engineering skills. This time around Engineering Physics decided to build an electrostatic nuclear fusion reactor for the 2004 competition. It's a project that uniquely brings out the physics in fizz.
The Fusor
A brief description of the Fusor is provided here but anyone interested in more of the details should check out the links at the end of the page. The idea behind this device is the inertial confinement of ions into a tiny region of space.
This confinement is achieved with two spherical grids one inside another and maintained at a potential some tens of kilovolts apart. Ions that are formed get accelerated through the radially symmetric electric field from the outer grid to the inner grid. In the process they reach to some reasonable fraction of the speed of light. But the inner grid is just a thin wire mesh, so most of the ions will travel right through it, all aiming for the center point.
All these positive ions flying towards one location in space produce what is known as a poissor, or miniature plasma star. The result is always a brilliant visual display. Even more interesting, with the right gas ions the plasma star has enough energy to produce detectable amounts of nuclear fusion.
Components
There are a few major parts that were brought together in order to make a Fusor. A vacuum pump, vacuum chamber, HV power supply, and electrode assembly to be precise. We managed to intercept a lot of the HV equipment on it's way to a dumpster thanks to fortunate timing. Other bits were borrowed but all the interesting stuff we built ourselves thanks to the talented bunch in Engineering Physics. Total costs came to $23.86.
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Vacuum Pump
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Vacuum Chamber
High Voltage Power Supply
High Voltage Feedthrough
- Wire Grids
- Vacuum Grease, Flanges, and Connectors
- Trolley Cart
Chris here is showing off our elaborate looking vacuum system, on loan from the Structured Surface Laboratory. Due to safety regulations and complexities with the setup we didn't actually use the mercury diffusion pump that's attached to this unit. Instead all that was bypassed and the roughing pump used directly.
This was the pump we used throughout the display, it took about 10 minutes to bring the chamber pressure down to 50mTorr. We also got a free roughing pump from David Jone's lab that is now property of Fizz. This new pump is more portable and evacuates at 2 or 3 times the rate, reaching a final pressure of 23 mTorr.
This welded stainless steel vacuum chamber is borrowed from the storage rooms of TRIUMF. It looks like something from deep sea exploration and is a joy to carry around. There are many thick plexiglass viewports which make it ideal for public display. There are also more than a few holes and ports to the chamber, most of which were sealed off but others served to hold the pressure gauge and HV feedthrough.
A lot of interesting stuff was about to be thrown out of David Jone's new Lab on Jan 20th. We got there just in time to salvage box after box of high voltage equipment, including three 30kV 5mA DC transformers, lots of HV shielded wire, more than a few 40kV capacitors, and enough 1" thick plexiglass to build a greenhouse.
The main power supply was built by wiring two of the transformers in parallel. Their inputs came off of common variac, and the outputs were connected to panel meters with appropriate series/shunt resistors for voltage and current display. The entire assembly was mounted in a steel enclosure salvaged from some laser pulse circuit.
With this setup we were able to get up to about 40mA of current for brief periods and 20-25mA continuous, at ~2kV. With the vacuum system reaching an ablsolute minumum of 30mTorr after a lot of outgassing there was still too much current draw to raise the grid voltage beyond ~3kV.
To bring the large voltage through the vacuum chamber and to the inner grid requires a HV insulated and vacuum tight feedthrough. In the end we made our own custom piece for this job instead of adapting a car spark plug as initially planned
The feedthrough was made to fit in an existing 3/4" port through the top of the vacuum chamber. This was made from a mushroom shaped piece of plexiglass with a 1/4" brass bolt travelling through it. Both were made vacuum-tight with an o-ring and liberal amounts of grease. The high voltage travelled through the bolt, and then to a fine brass rod surrounded by a glass tube for electrical insulation all the way to the inner grid.
The inner and outer grids were an interesting challenge in symmetry. The outer grid was made from 6 loops of stainless steel welding rod. The layout was initially pretty random but after pondering a 12-sided dice for a long time we realized how to line the rings up in a symetric and uniform manner
Above is a sketch of the truncated dodecahedron geometry which the loops finally took. Imagine the dark loops in the foreground and the grey loops in the background. The trick to make this shape was to mark off each ring in 10 evenly spaced segments. Then the wires were looped and interwoven so that each mark point lined up with another. The first 3 rings made a simple triangle opening, and after that each additional ring completed the shape of a truncated dodecahedron.
The inner grid was even more of an adventure to make. We wanted something as fine as possible to reduce the number of ions colliding with the grid and wasting energy. But we also wanted something that could withstand the high plasma temperatures for more than a few minutes since this piece has to be on display for hours on end. A trip to the professional machine shop netted us with (we were told) several feet of 5 mil tungsten wire! Ideal for the task. Only later did the magnet test confirm that this was nothing but ordinary hardened steel wire.
Matt and Aviv spent the better part of a wednesday evening trying to figure out how to make the apparently simple three inner loops and secure them to the brass feedthrough rod. A technique was devised that involved sawing narrow notches in the brass, pinching the two vertical loops with washers, and tying tiny knots at each interesction to support the middle equatorial band
By about 9:30 pm they were just tightening the last knot, when the pliers slipped and ripped right through the delicate gridwork. That's reason enough to be cheerful and pose for a picture. The ruined inner grid is so fine you can barely see it in the photo, but it's there if you look carefully
Eventally we decided that the thin wire approach was too finnicky to be a reliable solution, we tried machining flat metal disks that would do a better job of keeping a rigid spherical geometry. The first approach was done with 3 copper rings machined out of a copper tube and notched every 90 degrees to fit together like a puzzle into a perfect sphere. This worked OK but got white hot and eventually melted through.
After that we moved to a set of steel rings that were larger in diameter. The steel is better at resisting high temperatures and by increasing the diameter we would keep the grid further from the hot plasma.
This grid was made by cutting off four thin rings of a 2" steel tube in the lathe and notching them every 60 degrees. The diagram above should serve to help visualize the geometry.
We didn't have permission to cut new holes in the vacuum chamber for attaching a flange, so Matt machined this custom vacuum hose adapter and plate out of 1" thick plexi-glass.
We found a 4-wheeled cart in the dumptser in front of Hebb theater that the high voltage supply and fusor could both sit on
Results
We succesfully ran the fusor for the first time on Saturday Afternoon, Jan 31st, using a thing wire inner grid. The ball started to glow at about 1kV and several milliamps. Beyond that the inner grid got almost white hot and under the heat it deformed considerably from spherical. The result was a large opening through which a plasma jet emereged. You can see this in one of the pictures. Fascinating to watch, but it means we'll want something more robust and symetric to get a real fusor action.
The vacauum pressure rose to ~100mTorr while the fusor was in operation, compared to ~40mTorr with the fusor off. Presumabely the plasma is causing outgassing somewhere in the system. With 100mTorr running pressure the voltage couldn't get up much beyond 2.5kV before the current limit on the power supplies kicked in. We let it run at 25mV for at least 1/2 an hour and nothing catastrophinc happened. There were interesting sparks jumping through the glass insulator tube on the feedthough though, so we may want to find a thicker material for this.
After that we worked with a redesigned inner and outer grid system that was considerably more symmetrical. It was interesting to see how the fusor behaved over time. If the fusor was turned on while the pressure was still fairly high (~500mTorr), we got a glow discharge surrounding the entire inner grid with no resemblance of a poissar in the middle.
As the vacuum got better, around 200mTorr, a more distinct plasma ball was formed in the middle. At this point the outlines of a plasma star can just be seen and the ions seem to be emerging uniformly from each of the open quadrants
At around 100mTorr, the jet tended to settle on just one opening at a time. For 5 or 10 minutes it would actually hop between the various exists in an unpredictable way.
Time Line
| Jan 2003 | ||||||
| Ball Model Schedule | ||||||
| Sun | Mon | Tue | Wed | Thu | Fri | Sat |
| 18 |
19 |
20 |
21 |
22 |
23 |
24 |
| 25 12:00pm Machine Flanges and Feedthroughs for Vacuum System |
26 -- |
27 -- |
28 -- |
29 -- |
30 -- |
31 -- |
| 1 -- |
2 -- |
3 First E-Week Display, 9:00-4:00 in SUB |
4 2nd day of Display, 9:00-4:00 in SUB |
5 -- |
6 n-- |
7 --planned |
Getting Involved
Everyone in Fizz is encouraged to help out. We need people to machine components in the Student Shop, we need people to design posters for behind the display, and we will need volunteers to handle the booth at the SUB when our Fusor on display during e-week, Feb 3rd and 4th. If you'd like to be involved in any of these ways send an e-mail to:
justinle@interchange.ubc.caLinks
There are a number of highly informative sites there, most can be found through the central fusor.net website
http://www.fusor.nethttp://www.kronjaeger.com/hv/fusor/fusorIII/index.html
http://www.kronjaeger.com/hv/fusor/construction/index.html
http://www.belljar.net/634fusor.pdf