High School Fusor: One Year Milestone, New Equipment, and Significant Changes for the Near Future
When I started this project about a year ago I had just completed my sophomore year and now I’m about to be a senior in high school (yay!). A lot has changed since then including nearly every component of my fusor system (I think only the NST system is still in use). My fusor prototype has evolved from a piece of borosilicate glass stuck between two hand-milled plates to a much larger lab-grade steel chamber with its own dedicated high vacuum system. Likewise the fusor project has grown from a summer project to an all-encompassing (perhaps too much so) venture with its own specialized equipment and (home) lab space.¹
At the time of my last post, I had obligations to other activities and school (AP exams are awful), so I was unable to put much work into the fusor for a few months. However now that I have had marginally more time, I have been able to make another significant step towards fusion: buying a turbopump. Alongside that, I have bought some radiation detection equipment that I am learning how to use. This will become very necessary after I set up my 30kV x-ray transformer. Lastly, I designed a simple relay control board that I will continue upgrading and adding functionality to as the fusor system increases in complexity.
I recently returned from spending the month in a residential summer research program at UNC Charlotte where I was able to further my knowledge of fusion phenomenon at a molecular level and get professional feedback to my project. Returning in August, I have begun redesigning Fusor 1 to accommodate my new turbopump acquisition. This entails constructing a scaffold to hold the fusor chamber, pumps, and vacuum plumbing and will greatly increase the modular upgradability of the system.
At that point, the only remaining steps before fusion will be building a new 30kV power supply, designing a gas handling and injection system, and building a neutron detector of some type. While a large part of the project remains to-be-completed, I believe that once I complete the turbopump addition I will be more than halfway to my goal of fusion with an entire year to go.
Control Board V1 (and future expansion)
The first iteration of Fusor 1’s control board is meant solely to replace the separate switches controlling power to the roughing pump and the variac with a centralized relay based system. While this alone is very simple, as Fusor 1 gets increasingly complex (with the impending additions of a turbopump and its requisite cooling system) any control board must be easily upgradeable and “forwards compatible” with later fusor evolutions. Opposing this design goal, I did not want work on the control board to eclipse work on the fusor itself as overly complex designs would distract from its nature as strictly secondary to the fusor itself.
For these reasons, I settled on a design centered around an Arduino Uno R3 that flips a latching relay when given binary input from a switch.
I built the circuit on a breadboard that I plan to mount on a DIN rail in a small electrical cabinet. This would be significant overkill for such a simple design, but as I plan to expand this system to handle all power input and data output from the system, I decided that such complexity was warranted from the start.
The most substantial planned upgrade to this system will be transitioning metering activities from isolated multimeters and controllers to a centralized unit. For pressure readings, that will just entail reading analog output from the Varian 804–a thermocouple controller. Reading voltage and amperage can be accomplished using the Arduino’s analog pins. I have not yet designed the proper filtering circuits for that, but I do not doubt that it is possible. Unfortunately, my Geiger counter is from a Ukrainian manufacturer and has very little English documentation so although it can output to a computer, I am unsure of how I can manage that connection with an Arduino. More exploration will be required.
Regardless of the capabilities that are eventually added to the system, its main benefit is safety. Especially as I am planning to finally set up my 30kV transformer, having a breaker or fuse control the transformer will become essential. Additionally, I plan to implement various indicators that high voltage systems are active to increase Fusor 1’s safety of operation.
New Radiation Metering
Much like the new metering setup, while a Geiger counter is not necessary currently (as no radiation is currently emitted from the system), it will become increasingly useful as I continue to upgrade Fusor 1.
The primary radiation risk associated with a non-fusing plasma system comes from x-rays. An x-ray is a high energy photon that is produced when an electron impacts a suitably dense surface.² In a fusor, this occurs when electrons from the grid’s electron jet hit the wall of the chamber.³ The energies of these photons approximate a Maxwellian distribution, meaning that for a 30kV grid input, a few photons will attain 30keV while most may only attain 10-15kV.
With the current NST and vacuum pump, the highest attainable x-ray energy (penetrating power) is about 6keV which is easily attenuated by even the unleaded 1.5mm thick borosilicate viewport. However, at the 30kV supplied by the x-ray transformer, x-rays will be able to easily penetrate an unshielded viewport and may even be able to “leak” at low volumes through gaps in the steel chamber walls.
At this point, I will most certainly need a ways to measure this radiation to quantify and subsequently control my exposure. To accomplish this, I bought a Geiger counter system and a pen dosimeter set, both surplus devices from Ukraine, on eBay (I believe their heritage to be unrelated to current events as both listings had been posted last year). That situation did, however, push me to buy the Geiger counter very quickly.
The Geiger counter system includes a control unit, a mica windowed Alpha/Beta/Gamma detector tube, and a Beta/Gamma tube. It came very well packaged with a certificate of calibration as well. The only issue was that the only language available is Ukrainian and there is very limited English documentation online. Relying on European friends and the fusor.net community,⁶ I was able to decipher the menus enough to use it.
Along with the Geiger counter, I also bought a set of vintage Soviet dosimeters. Unlike a Geiger counter which measures in radiation dose per unit of time (μSv/hr), a dosimeter measures the total amount of radiation that it has absorbed (rad⁷). I bought a set of ten pen-style dosimeters that can be worn on my clothes or placed in different areas around the lab to record the level of radiation exposure that I am subjected to.⁸
When combined, these two metering systems will allow me to quantify the level of ionizing radiation that Fusor 1 generates once I set up the 30kV transformer. This will help me design safety precautions and shielding devices to reduce my exposure as much as possible.
New Turbomolecular Pump
While the other upgrades and additions listed in this post are certainly important, acquiring a secondary vacuum pump is the latest major milestone to fusion that I have reached.
Within a fusor system, voltage, current, and pressure are implicitly related.⁹ ¹⁰ ¹¹
- Fusion becomes more likely to occur at higher voltages as the greater potential difference between the negative grid and grounded chamber walls increases the speed of ions to a certain point where the strong nuclear force can initiate fusion.¹²
- Current increases as a direct function of voltage and also increases the likelihood of fusion by increasing the number of ions in the plasma. However, more ions flying through the chamber and colliding with the grid as well as more amperage running through the grid both heat it (potentially) to the point of melting.
- Higher deuterium pressure increases the likelihood of fusion, however it also increases the current required to ionize the increased gaseous density. Lowering deuterium pressure reduces heating issues, however pressure can easily be reduced so low that the voltage required to create any fusion exceeds what the power supply is able to provide.
This delicate balancing act of three variables is what makes fusor design and operation so difficult. In my system, this evens out to around 30kV at 10mA minimum current at 5–20 microns of deuterium pressure.
At this point, a secondary pump becomes necessary to attain such pressures. My roughing pump (a Precision D-25) has been able to pump down to a minimum chamber pressure of 16 microns when properly conditioned and run very carefully. While this is below the maximum of 20 microns as stated above, were I to add deuterium to the chamber to meet this maximum, the mixture would still be 80% atmospheric gasses.¹³ To attain as near to a pure deuterium environment as possible, the chamber must be pumped down as far below 1 micron as possible and then back-filled with deuterium until pressure increases to 5 microns of nearly pure deuterium. As even the best roughing pumps bottom out around 10 microns, a secondary high vacuum pump becomes necessary.
For this application, two main types of secondary pumps can be used, an oil diffusion pump or a turbo molecular (turbo) pump. Each pump is designed for a different use case with various pros and cons and many professionals on fusor.net give conflicting recommendations.
I ended up deciding to use a turbopump for a number of reasons, however the most influential by far was this specific unit’s availability. Alongside this, using a turbopump entails other more objective benefits.
An oil diffusion pump requires moderate quantities of specialized oil to operate. While that oil does not need frequent changing, it adds another layer of complexity as well as the risk of oil backflow out of the pump should the chamber re-pressurize unexpectedly. A diffusion pump is also physically large and must be oriented properly. Compared to this, a turbopump is an entirely dry system with a much simpler startup and shutdown sequence. It can also be oriented in nearly any configuration and can reach deeper levels of vacuum faster. As far as my fusor project is concerned, a turbopump is the superior choice from both an ease of use as well as a sophistication standpoint.
It was also much more readily available than a diffusion pump for a very reasonable price.
The basic principle of operation behind a turbo molecular pump relies upon the fact that once the gaseous pressure within an enclosed space drops below a certain point, the gas ceases to act as a fluid and instead behaves as a collection of individual molecules. At that point, traditional fluid pumps such as the Precision D-25 that I am currently using become ineffective. Instead of pumping out molecules, turbopumps function instead by trapping molecules within a stacked assembly of fast moving blades and eventually ejecting them into a higher pressure exhaust where they can behave as fluids again (other high vacuum pumps employ other methods to the same effect).
The TPU — 170 pump that I was able to get is massive overkill for my application, however its usage has the benefit of increased pumpdown speed. The unit is supplied power through a sophisticated control system that ramps pump speed as pressure decreases.
With the addition of a secondary pump into the system, the fusor capabilities will be drastically expanded. This expansion is drastic enough to call it another iteration on the Fusor 1 system, Fusor 1.3. This upgrade also pushes Fusor 1 definitively out of the solely demo fusor territory and (I believe) places it in the proto-fusor classification for the near future.
I plan to begin assembly of the new turbopump system (Fusor 1.3) as soon as possible. This will involve disassembling large parts of the system to add the required components. Because of this, I have decided to disassemble the entire system for cleaning and maintenance as well as to replace old vacuum gaskets that I believe are beginning to develop leaks.
Once the turbopump is properly installed and the system reassembled, I will be able to begin the process of comprehensively testing and quantifying the behavior of Fusor 1.3. At that point, I will be ready to start seriously working on the 30kV power supply that I have been slowly preparing to work on.
A secondary vacuum pump is one of the last major investments necessary for fusion. While I still must acquire gas injection and neutron detection equipment, with the purchase of a turbopump system I can confidently say that I am nearing fusion.
(hopefully my next update will take weeks rather than months)
Notes & Citations:
¹ The project’s associated cost has also, unfortunately, ballooned from Nintendo Switch territory to performance desktop computer territory.
⁷ In this case, the outdated unit, not the outdated expression.
¹³ Not to mention the fact that chamber pressure should start at the minimum of operation range and then be slowly increased.