It has been a while since I have done any real work on my fusor. In fact, it is a actually dissassembled and in a variety of boxes that I have only begun to unpack. I started college and have been going through my first year whilst also moving, and working two jobs. I havent been able to scrounge up the time to work on a new project until now.
With some space set aside and available, I am read to begin the next version of my fusor. Whilst I was able to be more successful operating in a high pressure (35-50 mTorr range) at -50KV and 5ma, I was not able to get above an average yield of 200K n/s. Whilst good for the power input and the small size of the fusor, it was nowehere near the neutron numbers I was hoping for. A matter of fact, this very power supply had been used by another user on this forum some time ago giving them a 500K n/s output. I had a few suspicions as to why this was, and now that I have working experience with a very high performance fusion device and have had the input and advice of many of the people there, the conclusions as to why my yields were lower than expected are as follows:
Contamination (I): I only used a roughing pump, leaving many impurities in the chamber. My TC gauge read me down to 1mTorr, and while I believe the pump is (in a perfect world), capable of these numbers, I know for a fact that my vacuum practice was extremely poor, especially for a fusion system, and that the system’s most likely ultimate vacuum before a gas fill was about 10mTorr. This means that setting aside any pump-oil contaminates, that almost 25% of the pressure was not usable fuel! Fusion plasmas tend to love to dump their heat into anything but fusion, and my chamber was full of heavy ions that would love to soak up heat from Deuterium!
Contamination(II): Beam/Wall Interactions. Hear the beams from the fusor (as well as fast neutrals) are hitting the walls heating it. Not only does this cause outgassing of normally embedded materials that contaminate the fusion plasma, but the beams and uv radiation can ablate material from the walls. Specifically Carbon (found in stainless steel), loves soak up heat and be a bad actor in a fusion plasma. Heavier Ions such as Iron and the grid materials will contaminate the plasma too, but some of these far heavier ions are harder to deposit energy into than carbon making their effect leastened.
Contamination(III): Vacuum Leaks. The system was poor and leaky. Valves were strung together with a shoestring budget and I used tubing and cables that were not vacuum rated giving way to air contamination both during pump down times and normal operation.
Thermionic Runaway: This has been discussed in depth on this forum before. I wont go in depth on it here. Andrew Seltzmann has done a lot of work with water cooled grid to reduce this effect.
Pressure Swings: Manual operation in the gas feed/vacuum control system lead to operator error and experience being a huge factor in the operation of the device. My poor gas feed system and clunky vacuum valve lead me to over/underpressure the system many times during operation, and I struggled to hold constant pressure without massive swings.
So what do I hope to address in the future? First, I need a working system. More specifically a working system with better vacuum. I have purchase a turbopump and controller. With this combined with the previous vacuum pump (with the addition of an inline oil trap) I hope to achieve an ultimate vacuum of 7.0e-7 Torr. This is far beyond what is required for an amaeture fusor, but it is my personal goal. That way I can eliminate the idea of the initial vacuum system leading to contaminate. Even contamination from pressures in the low 1.0e-5 ranges can have an effect on a fusion plasma (from an academic/industrial system standpoint).
I also plan on creating a better gas feedthrough system using mass flow controllers, and I want to be able to switch between a Deuterium and Helium working gas for runs with/without neutrons (testing characteristics of the discharge). I also want to address the issue of the beams/plasma heating the chamber and contaminating the plasma with heavy ions by inserting a protective liner into the system (similiar to the first walls in tokamaks). Ideally this is lithium or beryllium. Not due to lithiums favorable neutronics which have been discussed before (not to mention the unfavorable scattering Beryllium would cause effecting the fast neutron flux), but rather the fact they won’t effect the plasma as much due to their low atomic mass (and any molecules kicked off would ionized very quickly). Almost certainly I will have to make due with copper, which can be electropolished or cleaned in some other fine manner to ensure a favorable outer surface. Copper is much harder to kick off into plasma than carbon in steel is.
A water cooled grid and shell is, of course in the future. While I am hoping the above methods will get me into the 1e6 n/s mode of operation , I would be happy with 500K n/s. With a water cooled grid and shell, using the same power supply I should be over the 1e6 n/s range. With talk of increasing the yield, I also have a few ideas for the far future. This includes adding a Helicon Ion Source (Higher density than a typical IC-ion source). I also want to attempt to backfill the chamber with D2 and operate with a steady fill of gas rather than a flow system, this way the pressure is more stable. I very clean liner is almost certainly required hear to rpevent outgassing from being an issue due to heating (long bakeouts and plasma cleaning will be required on top of this). There are also odd ideas such as adding targets in the center of the grid (Heavy Water ice, Liquid Deuterium, or Deuterated Targets), but these are in the very far future and would require advanced cryogenic systems to operate. These are just ideas right now, but ones I have in the back of my head looking forward for this system.
What am I going to use? A while back I purchase Andrew Seltzmann’s 6-way vacuum cube, and I plan to use it, with the equipment on it to operate this device. The small size will allow me to operate in the high pressure regime, but there are enough ports on it (as well as proper and very well made pressure gauges), for the system to remain versatile. Of course, one of the more interesting things I hope to do with this fusor, is investigate high-yield operation of a small fusor operating in the high-pressure regime. I have the number in my head of achieving 1e7 n/s as my ultimate goal of this device, but only time will tell. For now, it’s only optimism. I will use this forum post to continue to post updates and I will eventually add pictures of my of my new purchases in the next few days as well (they currently on my computer for me to access).
For posts specifically relating to fusor design, construction, and operation.
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