Introduction: Build a Fusion Reactor

Picture of Build a Fusion Reactor

Yes, you can build your very own nuclear fusion reactor in your house! But first, a few warnings:

-This project includes lethal voltage levels. Make sure you know your high voltage safety or have a qualified electrical advisor.
-Potentially hazardous levels of x-rays will be produced. Lead shielding of viewports is a must!
-Deuterium, an explosive gas, will be used. Make sure to check for fuel leaks.
-All the other inherent dangers of a home engineering project of this degree (a wide gamut of potential injuries, damage to the checking account, and the loss of general sanity)

Here are the minimum required materials:

-A vacuum chamber, preferably in a spherical shape
-A roughing vacuum pump capable of reaching at least 75 microns vacuum
-A secondary high vacuum pump, either a turbo pump or oil diffusion pump
-A high voltage supply, preferably capable of at least 40kv 10ma - Must be negative polarity
-A high voltage divider probe for use with a digital multimeter
-A thermocouple or baratron (of appropriate scale) vacuum gauge
-A neutron radiation detector, either a proportional He-3 or BF3 tube with counting instrumentation, or a bubble dosimeter
-A Geiger counter, preferably a scintillator type, for x-ray detection and safety
-Deuterium gas (can be purchased as a gas or extracted from D2O through electrolysis - it is much easier and more effective to use compressed gas)
-A large ballast resistor in the range of 50-100k and at least a foot long
-A camera and TV display for viewing the inside of the reactor
-Lead to shield the camera viewport
-General engineering tools, a machine shop if at all possible (although 90% of mine was built with nothing but a dremel and cordless drill, the only thing you really can't build without a shop is scratch building the vacuum chamber)

Step 1: Assemble the Vacuum Chamber

Picture of Assemble the Vacuum Chamber

A quality high vacuum chamber is required for the fusor to operate. Sometimes an appropriate chamber can be found on eBay, but generally it is best to make one. Parts can be scrounged for several hundred dollars, or purchased new for $500+.

Get two stainless steel hemispheres, purchase two corresponding conflat-flanges (8" flanges in my case), bore out holes for accessory flanges, and then TIG weld it all together. Flanges are typically either of the KF or the conflat style. Conflat can be seen in the image below as the flanges with bolts, and KF (kwik-flange) are seen as those with only clamps holding an o-ring on the mating surface. Only weld on the inside, never on the outside (since virtual leaks can be formed if both inside and outside are welded). If you've never TIG welded before, it would be wise to have someone with experience do it as the welds must be flawless with no pin-sized holes or porous areas to hold a vacuum.

After machining, thoroughly clean the chamber and avoid getting fingerprints in it since these will outgas, which means at vacuum pressure molecules in the oil of finger prints or machining oil will become vapor and make it hard to maintain plasma stability or reach a good ultimate vacuum level.

Step 2: Prepare the High Vacuum Pump

Picture of Prepare the High Vacuum Pump

Install the oil diffusion pump (or turbo pump if you have a bit of luck scrounging or a higher budget). Fill the pump with quality diffusion pump oil to whatever fill level the pump documentation suggests, attach the inlet to a valve which then connects to the chamber (see diagram), and attach the outlet to a mechanical backing pump capable of reaching at least around 75 microns (any higher and the diffusion pump will not operate properly or the oil will oxidize quickly).

Make sure the pump is sufficiently cooled, many oil diffusion pumps require water cooling, smaller ones such as the one pictured can get by with a decent air flow.

Once this is assembled, turn on the mechanical pump and wait for the vacuum to reach at least 75 microns. Next you can test the high vacuum pump by turning on the boiler on the diffusion pump. After it warms up (could take a while), the vacuum should rapidly drop below the single micron range.

Step 3: Build Inner Grid

Picture of Build Inner Grid

The inner grid (where the high voltage is applied) must now be built and attached to a high voltage feedthrough.

It is best to use a metal such as tungsten for the grid wires since it has a very high melting point, and the grid will get extremely hot during high power runs.

This can be built however you wish, as long as it resembles a spherical shape of roughly 1-1.5 inches in diameter (for a 6-8" chamber), it should work fine.

The grid should be internally attached to an electrical feedthrough such as the one pictured in the second image. This feedthrough needs to be rated for the cathode voltage that will be used, typically 40kv is a good target voltage.

Step 4: Assemble the Deuterium System

Picture of Assemble the Deuterium System

Deuterium gas is used as the fuel for this fusion reactor. You will need to purchase a tank of this gas (unless you wish to do electrolysis on heavy water, this process will not be documented here but nothing more than a small Hoffman Apparatus is required - higher purity gas can be gotten from a compressed tank).

Attach a high pressure regulator directly to the tank, add an extremely fine-metering needle valve after this (or a laser drilled orifice in the range of 5 microns), then attach this to the chamber. A ball valve can also be installed between the regulator and the needle valve since needle valves are not shutoff valves.

See the attached diagram now updated with the deuterium handling system.

Step 5: High Voltage

Picture of High Voltage

If you can purchase a power supply (occasionally but not commonly found surplus) appropriate for fusion use, the high voltage becomes very simple. Simply take the output of the 40kv negative supply and attach it to the chamber with a physically large high voltage 50-100k ohm ballast resistor in series (large enough that its length will not flash-over if 40kv is applied to it in a plasma run-away or arc discharge).

The difficulty is that it is often difficult if not impossible to find an appropriate fully assembled DC supply of this voltage level that is affordable to the amateur scientist.

Pictured is my high frequency ferrite transformer pair, with a 4-stage multiplier seen behind it.

If a fully assembled power supply (typically manufactured by either Glassman or Spellman), there are a few options:
-Find an x-ray transformer, and if necessary either reverse the rectifiers for negative polarity or add rectifiers if it has none (an x-ray transformer core won't have rectifiers, it probably will if it is in its oil tank)
-Build a switching high frequency ferrite power supply. This is what I did, however it requires a bit of EE experience since several aspects must be resonant and if it is ever taken out of tune, the transistors will burn out. Probably not the best option for people with little electrical background.

Step 6: Setup Neutron Detection

Picture of Setup Neutron Detection

The proof of fusion (and a quantitative analysis of how much fusion) is obtained through detecting neutron radiation, the byproduct of a D-D fusion reaction. There are three options which will be described. They are in order of descending ease of setup.

-A Neutron Bubble Dosimeter
A bubble dosimeter is a small unit with a gel in it that forms bubbles when ionized by neutron radiation. This is the easiest form of neutron detection available since all you have to do is unscrew the top and set it next to the fusor. Some of the drawbacks are that it is an integrative detector which means all you get is a total neutron emission number over the time that it was used, rather than an instantaneous neutron rate. Additionally, they are somewhat hard to get since the only company to make them is Bubbletech in Canada, which has a minimum order of 3 with steep shipping and handling (expect to spend $700+ if ordering directly from them not in a group buy). Additionally, they tend to be fairly worn out after a year of shelf life (although I've kept mine in a refrigerated storage container at 50*F and it seems to be like new after I think more than a year). The advantage is that calibration data is provided with purchase and of course it is easy.

-Silver Activation
When silver is placed near the reactor (with a moderator [paraffin wax, water, HDPE, etc] between it and the neutron source, since only thermal neutrons will activate the material) it becomes slightly radioactive with decent neutron fluxes. It has a short half life of only a few minutes, but if you quickly put a geiger counter next to the silver, counts can be detected. In my best runs, I have gotten a piece of silver to about 250CPM over background on a CDV-700 geiger-counter. The disadvantages of this are that it requires a decent neutron flux (at least about 100,000 neutrons/s) which is above the average "beginner's first run" neutron rate. Also, it is somewhat difficult to calibrate, and the counts can't be taken until after the fusor has been shut off.

-A Proportional Tube
Tubes can be purchased which are filled with either BF3 or Helium-3 (some very old tubes are Boron-10 lined inert gas tubes). These tubes, similar to a geiger counter, can be used with a counting device to detect electrical pulses when neutrons pass through the tube. Either an all-in-one counter can be used, often made by a company called Ludlum, or a modular counting system can be made using NIM modules. The tube is surrounded by about 2 inches of moderating material such as wax or water. This is by far the most accurate and useful form of neutron detection, however the cost of a new tube is prohibitive to most people, and they are extremely rare on the surplus market. Also, counting equipment can become quite costly.

NIM Configuration: If you chose to make a NIM setup as I have, the typical layout is a charge sensitive pre-amplifier at the head of the proportional tube, which is plugged into both a high voltage power supply generating positive polarity voltage appropriate for the tube (in the range of 800V-2kV generally). The amplifier also hooks into a shaping amplifier, which is followed by a Single Channel Analyzer (for setting the detection discrimination level), followed by a pulse counter and/or rate meter.

Shown in the first picture is my NIM setup, the second picture is the pre-amplifier attached to a moderated helium-3 tube, the third picture is a bubble detector after being exposed to neutrons.

Step 7: Fire It Up (and Cross Your Fingers)

Picture of Fire It Up (and Cross Your Fingers)

Time to turn it on (don't forget to cover any viewports/cameras with lead! Also x-rays can pour out of ceramic feedthroughs so point them away from people. It is a good idea to be monitoring for x-rays where any people are present). The basic procedure is:

-Turn on the roughing pump and wait for sufficient backing pressure, turn on the diffusion or turbo pump and wait for it to fully warm up or achieve running speed
-Throttle the chamber back (with the valve between the diffusion/turbo pump and the chamber)
-Ever so slightly open the needle valve to the deuterium tank
-Turn up the high voltage until either plasma establishes on the camera, or you've reached 40kv and nothing has happened (don't forget, you only get one chance in your life to screw up with voltages of this degree)
-If nothing has happened, keep admitting more gas and the pressure should keep going up. Plasma should form around 40kv at about 10-15 microns of deuterium.

If all goes well, you should see on your camera the image below, and you should be detecting neutrons at this point.

Operation is quite a balancing act, since the voltage is controlled by both the power supply, but also by Paschen's Curve and Ohm's Law relating to the pressure in the chamber. Great patience is required to "Get the hang of it", but after doing so it becomes quite simple to run. Operation can be aided by an ion-gun which will not be discussed in this article.


waltvino (author)2017-11-14

If you are finished experimenting, would you consider selling this?

frog2343 (author)2017-10-23

how much did it cost to asemble this?

АрсенийП1 (author)2017-10-20

what is the thickness of the walls in the vacuum chamber

1/8" on this chamber. I'm sure there is literature online discussing how thick a chamber of a given size needs to be. Not only does it have to hold up vacuum, but it also just has to be structurally sound so you can hang things off flanges, mount the chamber itself, etc.

the16thDoctor (author)2017-10-16

wtf i thought fusion reactors werent a thing!

Fusion reactors that make useful amounts of power for a useful length of time are not a thing. Fusion reactors in general have been a thing for a very long time.

NukeGuy (author)2017-08-25

Does a AA battery work?

dknauss (author)2017-08-17

So for the power supply I'm assuming 30-40kv DC at ~10mA (300-400 Watts) should be sufficient. If the setup is: variac --> transformer --> Cockcroft-Walton multiplier would that work? Or would I need resistors to get the proper amperage?

guertlerj21 (author)2017-07-23

Could I use a high voltage diode instead of the 50-100k ballast resistors to prevent a voltage flashback?

guertlerj21 (author)2017-07-19

What would happen if you turned up the power immediately to 5 kV DC and 10 mA instead of using a variac? I ask this because I have a power supply that produces 5 kV to 30 kV DC and does not go down to 0 volts.

Fusioneer16 (author)2017-07-08

What measures did you take to insure your safety? Im finding it extremely tough to find methods to block the neutrons.

FelipeF72 (author)2017-07-08

i dont have the tools neither the time to do it, can i find all the parts on e-bay? ( or other on line shop?)

guertlerj21 (author)2017-07-04

Did the creator of this device use a cold trap or baffle for their diffusion pump? If so, what was the coolant they used?

christensent (author)guertlerj212017-07-04

I used a baffle right above the diff pump. A cold trap would be better but is totally overkill and unnecessary for an amateur DIY fusion effort.

guertlerj21 (author)christensent2017-07-04

Was your baffle water, air, or liquid nitrogen cooled?

guertlerj21 (author)2017-06-30

What was the valve you used between the mechanical pump and the diffusion pump?

christensent (author)guertlerj212017-07-02

Nothing special, just a general purpose ball valve. This valve is always wide open when running the system in typical conditions so you don't need anything special.

guertlerj21 (author)christensent2017-07-03

Were there any other valves or parts used in the vacuum system other than those shown in the schematics?

christensent (author)guertlerj212017-07-03

No, the schematic includes all valves I used

guertlerj21 (author)christensent2017-07-03

One final thought, what type of tubing did you use for the vacuum system? What was its diameter?

christensent (author)guertlerj212017-07-04

Something like 1" ID tube for mechanical pump to diffusion backing (but this should be sized to meet the requirements of whatever pump you use, for most small diffusion pumps this would be an appropriate size), then a variety of KF and conflat direct connections to the chamber (mostly 2.75" conflat size or the approximate KF equivalents)

guertlerj21 (author)christensent2017-07-04

What type of hose is utilized? I saw the tube in the picture, but what is it called?

guertlerj21 (author)2017-07-04

Could I use this device as a power source?:

guertlerj21 (author)2017-06-30

Could I use the 30 kV DC power supply in this video to power the fusor?:

christensent (author)guertlerj212017-07-02

If you actually want to achieve fusion, I seriously doubt that would have useful amounts of power. For a demo fusor (not actually doing fusion), you could get some plasma out of it.

guertlerj21 (author)christensent2017-07-03

How so? I have seen others use 30 kV DC of power as the input and 1 mA is the minimum current input into the fusor. If I put 24 volts DC and 12.5 amps into the hyperlinked device, I should get around 30 kV DC and at least 5 mA out of it.

christensent (author)guertlerj212017-07-03

It's unlikely the device could handle that kind of power. Also flyback systems usually have a terrible output impedance curve, such that when you actually draw 5mA from a design like that you only have a few kilovolts left.

It is possible to make a ferrite based power supply that operates a fusor, that's what any professional lab-grade HV supply is. I just really doubt that particular design is going to do it for you.

christensent (author)guertlerj212017-07-02

Actually as a follow up from watching through that video, there is zero chance you'll get measurable fusion, and even for a demo fusor you'd get virtually no plasma. Just not enough power coming out of that supply.

meredithgw (author)2017-05-15

can you use it to light up a lightbulb?

christensent (author)meredithgw2017-05-15

No, the reactor puts out something like a millionth of a watt, and there's no practical system to capture that energy so you get nothing. Even if you could capture it, this power is so low that for example, it'd take well over an entire human lifetime to charge a cellphone once.

guertlerj21 (author)2017-05-15

How did you keep the high-voltage electricity from getting into the other metallic components? The problem that I have heard that is mentioned is that since the outer core (the metal chamber) is picking up the electrons from the outer grid, some of those electrons may be conducted by the metal components that lead to other electronics such as vacuum pumps, which may lead to a few issues.

guertlerj21 (author)2017-04-30

Could I use a two-stage vacuum pump with an ultimate vacuum of 6 microns instead of buying the mechanical pump and the oil diffusion pump?

christensent (author)guertlerj212017-04-30

Sort of, it'd be highly sub optimal because fusion pressure is around 5-10 microns. Usually these pumps take a very long time to reach that 6 micron pressure you mention, so to get the atmosphere in there mostly deuterium it'd be difficult but if the chamber is really leak free probably possible. I don't recommend this method if actually achieving good fusion results is the goal.

guertlerj21 (author)christensent2017-05-01

Do you have a link or description of the oil diffusion pump that you think would work best and would also be relatively inexpensive? I am trying to build a fusor, but this part has shown itself to be quite illusive and expensive.

guertlerj21 (author)2017-04-21

What is the minimum voltage to achieve fusion? I currently have a design that works with 6 kV DC and 30 mA, but I am not certain if it will work.

christensent (author)guertlerj212017-04-21

You will not get any measurable fusion. You need 15-20kV to get any fusion with a fairly high end detection system, and 25-30kV to detect any fusion with a typical low end neutron detection system (small BF3/He3 tube, silver activation, or BTI bubble detector would be the typical options available to someone starting out).

MatthewM416 (author)2017-03-22

How hot does the tungsten grid wire get? Ball park?

It looks like it consumes a little over 400watts an hour, do you have any idea (total generated regardles of capturing) how much energy is realesed in an hour?

Last questionhow long can it contiuously run if you could capture that heat energy inside? In theory forever? Based on deuterium feedstock is there.

Great project for sure. Just trying to think of ways to capture that heat. Zirconia tubes to thermally decompose over a sodium catalyst sounds intriguing.

christensent (author)MatthewM4162017-03-23

I don't really know how hot it gets, my only basis for saying is that steel grids melt at full power, so somewhere between 1500C and 3400C.

Total fusion output is around a microwatt or less. So it's nearly a billion times inefficient.

Yes, in pure theory, if you captured ALL thermal loss plus the neutrons, you could run it forever (put in 400 watts, get 400.000001 watts out). In reality, there is currently and never will be any technology for thermal energy capture efficient enough to make this practical (you'd need a 99.999999%+ efficient thermal waste collector). So it truly is "in theory". In practice, a reactor like this will absolutely never be practical for energy generation or self-powered operation.

MatthewM416 (author)christensent2017-03-23

Is the tungsten heated by the reaction or energy input? It looks like it heats rapidly. sorry, Im a nuclear noob but infatuated with the concept. A CNO cycle would teraform mars, if ever produce efficiently.

It almost seems as if the gravity field is the true generator

christensent (author)MatthewM4162017-03-25

The tungsten is heated by wasted energy input. It appears to heat fast because it has very little thermal mass so it takes little time to start glowing.

The vast majority of the generated energy (in the form of neutron radiation) passes through the stainless steel chamber and is eventually absorbed by surrounding materials outside the reactor.

MatthewM416 (author)MatthewM4162017-03-22

Thermally decompose water over a sodium catalyst to produce co2less hydrogen.

SHOE0007 (author)2015-12-08

Is this even legal since you are maxing radiation which requires a permit.

RikHoutman (author)SHOE00072016-05-03

depens on the country you live in. In the US you need permession but that's just that no 12 year old's are going to try this

JacksonO8 (author)RikHoutman2017-03-24

Actually, I'm 12 and I'm about 1 month from completing my reactor.

offgridsid (author)RikHoutman2017-02-07

A late reply, I know and I'm trying to be Nice ... but Sorry, you are incorrect ... WE are Americans and we don't ever ask permission (especially from the Government) for ANYTHING (including building Nuclear Reactors) that we might wish to do. Also, one of the better designs WAS designed and built by a 12 year old ... American.

lol, I know. I am not even old enough to drive and am looking into building a fission generator for my science project since I really want to win and last years hydrogen battery only got me third place. Age is not even relevant.

im tying to buy this

im 12

my friend is 12

we are teaching our self's nuclear physics and hoping we can get authorization to get the plant running again.

also i have 1 year to beat the record for youngest.

Well, that's quite an ambitious goal for a 12 year old, but certainly not unattainable. Me and my friends didn't really start getting into nuclear physics until we were around 13, and its not something they really teach you in school at that age, so you are going to have to teach yourself practicaly all of it. The biggest problem with the Shoreham nuclear power plant was public opposition since most people don't really have any idea how a nuclear power plant works, and their brains are just kind of wired to "Nuclear, big explosion, radiation bad." If you plan to reopen that, you are going to have to find a way to get New York State officials on board with your plan, which is not going to be easy. If I remember right the plant was also fully decommissioned, and it is a older model as well, so it is not going to be nearly as efficient and safe as the designs we have now. You will also need a license to do any work on that reactor as well, start here:, but I warn you, nuclear licences are never cheap, but as a small entity rather than a large corporation it will be significantly easier to get such such a licence.

I would also like to add that the plant in question is a fission power plant, and as far as I know, no individual has ever build one by themselves. Jamie Edwards currently holds the title for being the youngest person to construct a fusion reactor at age 13, but you got plenty of time if you want to build a fission reactor, since you would be the first to ever build one as an individual.

(Taylor Wilson has designed a small scale Fission reactor, but as far I know his design was never built. Contact me if you need any help!)

SHOE0007 (author)2017-02-09

I like what are doing making a fusion reactor maybe it will replace a nuclear fisison reactor one day. As long as you play safe with high voltage and radiaition I have no problems. Good luck.

Blakemccoyx11 (author)2017-01-23

Could you use a 2 3/4" 6 way or 5 way vacuum chamber?

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