How to Build a Farnsworth Fusion Reactor and Become Part of the Nuclear Culture Canon




Introduction: How to Build a Farnsworth Fusion Reactor and Become Part of the Nuclear Culture Canon

About: INSTA: WEB:

With the hopes of decentralizing knowledge power hierarchies and empowering the individual, we will be going through the steps necessary in order to build a device which will ionize particles into plasma using electricity. This device will demonstrate basic principles that, when scaled, can be used for more robust (and possibly nuclear) fusion reactions.

A Farnsworth Fusion Reactor (or Fusor) is a device that uses an electric field to heat ions to nuclear fusion conditions. The machine induces a voltage between two metal cages, inside a vacuum (learn more HERE).

My design is loosely based on a Fusor Design published in Make Magazine Vol 36. I highly recommend checking out this project.


-This device uses high current and high voltage, a very dangerous combination!

-A high-vacuum apparatus may implode if improperly handled.

-This device may produce ultraviolet and x-ray radiation.

If you are serious about building one of these devices DO MORE RESEARCH, get multiple opinions, practice cautious making, and be sure you are comfortable working with glass, high voltage electricity and vacuum chambers.

A great place to do more research is among the already existing online Fusor community at

The Make Magazine article I referenced earlier is also a great outline (written by people who have been doing this way longer than I have!)

I also strongly recommend checking out this video playlist of other models people have made (I've also included some geiger counter builds at the end).

Step 2: Basic Components

-Vacuum system 

-pump and chamber

-Voltage system 

-120-220 AC volts from wall  

- ~20,000 DC volts in chamber


-for conducting electricity through chamber


-I got my pump online but have been having a lot of issues with my model. Essentially you will need a 2 stage Vacuum pump, 0.025mm Hg (25 micron) minimum vacuum rating. The higher the cubic-feet-per-minute (CFM) rating, the better. This is definitely the most expensive element of the project but is worth the investment! The price tag on my cheap pump does not outweigh the headaches.

-jb weld can be found at most hardware stores or amazon

-microwave transformers can be bought on eBay (expensive!) or sourced from microwaves. (these things are pretty tough so even if you find a broken microwave, chances are these things will still work)

-Diodes can be sourced from microwaves or bought in bulk from ebay

-I make probes out of different gauge steel wire but I highly recommend experimenting with other wire types

-Vacuum containers can be made out of a jar (I prefer ones with sealable lids but you can make gaskets for jars without lids).

- Hose and hose adapters and can be bought from hardware stores (sizes don't really matter just make sure you get parts that match/fit!)

-Variac alternative can be made out of re-purposed plastic containers (more on this later)

Step 3: Vacuum System

Vacuum chambers can be made from recycled glass containers like wine bottles and mason jars. Plastic tends to collapse on itself under the pressures we need however glass can be dangerous to work with so be careful!!!

Another note on this is I have seen people make chambers out of thick acrylic tubing which is much easier/safer to craft a chamber around than glass, but I would suggest researching this method more on your own before commiting (plastics can yield strange results when it comes to de-gassing).

The vacuum pump needs to be able to bring our chamber down to between 100 and 10 millitorr. [1 Torr ~ .001 Atmospheric] 

The lower the pressure, the easier it is for particles to move around

I borrowed a pump from a friend who was using it for removing air bubbles from silicone casting materials. It works fine for my needs and cut my spending in half [the two most expensive elements to this system are the pump and the variac]

I have seen some systems use multiple pumps to get the pressure down even lower but for my needs the system stated above was fine

Step 4: Building the Vacuum Chamber

For the chamber, I needed 3 holes drilled: 

One for the cathode (this one will be in the glass so be careful!) 

One for the vacuum pump adapter 

One for the anode

For my chamber, I used a small glass pickle jar I recycled. It had a metal lid which I drilled the vacuum adapter hole and the anode hole into.

In order to seal everything up I used JB Weld [a two part epoxy which has been refered to me as the "duct tape of the vaccum world"]

Step 5: Voltage System

Using a microwave transformer, we can step up the 120-220AC volts from a wall socket to about 2,000 volts with a minor loss of current [a wall socket provides enough amps that we don't have to worry about the drop in current in the transformer].

The alternating current (ac) provided by the wall can be converted to direct current (dc) using a dymond of high voltage diodes. These can be sourced from multiple microwaves or bought in bulk online. When I first built this system, I tried a circuit with a capacitor from the microwave as witnessed in a video. For me, this circuit only produced arcs which, while still very exciting, didnt emit the plasma I was after. After ditching it and trying a new diode setup I had much better results. [NOTE: capacitors can still hold charge so make sure you ground them before touching!]

Step 6: How to Control the Voltage

In order to control the voltage from the wall we need a variable system called a variac. However, these can be expensive and hard to find so we will use an alternative called a scariac

Two copper plates suspended in a tub of baking soda and water will work just as well

By putting one of the suspended copper pieces on a hinge, you can move it towards the other and increase the output voltage (don't touch the copper! Clamp it to a stick or something. I drilled some holes into some scrap plywood and mounted the entire setup on the tub).

Some advice: When I was trying to find a cheaper alternative to a variac, I thought a dimmer switch could solve my problem! On principle, a dimmer switch seems to limit the amount of electricity flowing to a light bulb or device, so why not use it to control the output of electricity to my transformer? THIS WILL NOT WORK! Here is a great video that explains the difference between a variac and a dimmer switch.

Step 7: Before Plugging Anything In...

Always have a fail safe!

Emergency switches should be easily accessible

A system of multiple checks can lead to a safer practice

I like to use power strips with switches built in.

Some of these have fuses that could pop if you draw too muchpower which is a nice and cheap failsafe.

Step 8: Wire Everything Up

Plug in your vacuum pump and connect to you chamber

Plug your transformer into your varian

Attach the diode and capacitor to the secondary on the transformer

Connect the positive output to the anode and the negative output to the cathode from the diode converter to the vacuum chamber

Plug your variac/ scariac into the wall.

Step 9: Testing the System

After making sure all connections are wired correctly, we can turn on the vacuum chamber and wait for it to reduce the pressure inside the chamber (for me this took about a minute).  If the pressure doesn't drop, you have a leak (in some cases you can hear the leak)

Once this is done and you chamber is at the propper pressure, we can turn on our high voltage system and slowly increase the power until our anode begins to glow.

Step 10: Improvements

Vacuum system improvements ---> Vacuum chamber is pretty makeshift. Minor leaks leave more atmosphere for particles to move through which means we need more power to run our device.

Electrical systems improvements ---> Could use an actual variac for a more reliable current management

Since writing this tutorial at the beginning of 2018, I've continued to work on this system improving the circuit, chambers and trying out different ways to connect multiple chambers. More updates will come soon.

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    3 years ago

    Attention all,

    As a concerned Instructables member I have to say:
    1) are you aware that this setup will generate x-ray radiation? Someone who doesn't already know that could potentially be at risk by not taking precautions.
    2) this is NOT a nuclear fusion reactor. 20kV will generate x-rays when the electrons strike a target. X-rays are not nuclear fusion, and this would have to be seriously upscaled to get anywhere close.

    I know this because I built something very similar for the purpose of generating x-rays. It worked. I even used less voltage. I researched heavily beforehand and took safety precautions. I talked to qualified scientists and safety officials.

    Don't get me wrong, this is a very interesting instructable. But please, DO NOT MAKE ONE if you lack the background knowledge and want to stay safe. You can do this if you understand the science, method, risks and how to handle high-voltage electricity. I did all of this stuff and more as an inexperienced researcher, but I had counsel and guidance in the form of qualified professionals. I would hate to see someone get hurt out of inexperience trying to do this on their own.

    Apart from this, I find the title misleading. This is a plasma generator and potentially x-ray generator. Not a fusion reactor.

    There is not sufficient safety guidance in this instructable for handling high voltage or proper shielding. The jar used as a vacuum chamber is also unsafe. A high enough vacuum CAN NOT be made from any sealable jar. The author has not shared and could not share all the necessary background information and knowledge to you readers.

    a concerned Instructables member.

    If you want to detect x-rays, check out
    If you want to build vacuums, check out


    Reply 3 years ago

    Yes, you're completely right here. 20 kV is not sufficient for fusion it more like 50-60 kV and high current. I am a bit concerned about the X-ray risk but 20 kV should produce soft x-rays you need 30 to 45 kV for that to be effective. Still if your not experienced a shock of that voltage could kill besides the "X-rays" Risk.


    3 years ago

    Glass vessels with flat bottoms are notoriously prone to implosion under vacuum, especially if the glass has any scratches or moulding flaws. I would strongly recommend a safety cage around the vacuum vessel.


    Reply 3 years ago

    You are right. A convex glass vessel (bell jar) would be much safer. I have an instructable on building vacuum chambers.


    3 years ago

    OK that's quite a bit of work for a visual show of electrons moving in a vacuum. So what do you do with it then?
    Quote"This device will demonstrate basic principles that, when scaled, can be
    used for more robust (and possibly nuclear) fusion reactions."
    I don't see where fusion comes into anything . Is it a plasma ? I don't know but my plasma cutter probably demonstrates that principle fairly clearly and I have something I can use to do work.

    Penolopy Bulnick
    Penolopy Bulnick

    3 years ago

    That is quite the setup! Thanks for sharing :)