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Richard Hull
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Post by Richard Hull »

The classic amateur fusor in its simplest form is shown below. It is a modified Farnsworth (Hirsch-Meeks) design. The diagram may appear confusing and complicated, but all will be explained. In spite of the details below, this FAQ is still only a "quick rinse" in the basics of the fusor.

NOTE: Fine details and data related to all the arms in the schematic diagram are found in separate FAQs in the various forums dealing specifically with those schematic arms.

There are two main types of amateur fusor.

1. Demo fusor - no fusion, only plasma glow and possibly star mode in the best demo designs - High school science fair
2. Fusion capable fusor - d-d fusion occuring, neutron producing. Possible neutron activation experiments


The demo fusor

This is the first pass for most all amateur, would-be, fusioneers. It does not do fusion but is a fusion work-alike model that if built well, can easily become a real fusion machine.

Many builders start here to join the "Plasma Club". As there is little visual difference between a demo fusor and a fusing, operational fusor, Many young people only go this far. The cost differential between a demo fusor and a real fusor can be a full order of magnitude!! ($300.00 vs. $3000.00).

A demo fusor can and has been made in a bell jar with crude vacuum gear and kluded-up power supply. Most such systems are assembled by younger school age children (mostly boys save for one pair of girls). The demo fusor often serves as a science fair project in the 10th to 12th grades of high school. The better examples have won regional science fairs and a couple of national events!

WARNING! The use of standard glass vessels can be dangerous and deadly! Proper bell jars can also be hazardous. You must go to this posting as a critical example.


There are four variants of the demo fusor.

A. Raw minimal demo fusor - similar to the MAKE magazine device.
B. Public demo fusor
C. Science fair grade demo fusor
D. Research Demo fusor.

Raw Demo fusor -
This fusor is a plasma glow in a jar. Such demos are almost exclusively bell jar types. It is never metered for pressure or electrical voltage or current. A quick pass device that could never be a real fusion device as it construction will not allow higher voltages or data collection. It is assembled mostly for the optical glow or "poissor" within the inner grid structure. Simple DIY folks might make it only this far as they are not interested in the science.

Public demo fusor-
This fusor is a bit better as its construction is a little cleaner, and takes on some safety precautions for the operator/observer. This is also often presented in bell jar form. It often has electrical metering on the fusor and supply. It is designed to demonstrate a full working fusor, but without x-rays or other radiation output. This could be a low grade science fair fusor.

Science fair grade demo fusor -
This device is a lot more polished and can be a regional contender as it will often be a spherical metal device with a view port and full metering and maybe even a vacuum gauge and TV monitoring system. This will allow a minimal amount of plasma research to be available to the student.
***Note: This is the minimum fusor with metering and gauges needed to apply for membership in our Plasma club listing***

Research grade demo fusor-
This is a fusor on its way to being a working, fusing, neutron producing fusor and would be built most often by an older enthusiast who has a job with a certain amount of disposable income. This will definitely be a conductive, metal shelled version and most often be spherical in shape. Many 12th graders who are in a school with lots of faculty assistance and parents eager to contribute funds can also be in this class of demo fusor builders.

The research demo fusor will allow the aspiring fusioneer a platform to study a myriad of fusion related technologies and become familiar with a lot of principles of vacuum systems, electrical wiring and safety related procedures The amateur will surely have a Geiger counter to check for x-rays in a higher power demo device. Remote viewing by means of a TV camera is often used in this class machine with ever more sophisticated pressure reading instrumentation.

It would rather stupid to build this finest of demo fusors and not take the next step and turn it into a real full blown fusing reactor.

Farnsworth-Hirsch-Meeks fusor - Real fusing fusor - neutron producer - Nuclear fusion reactor

This fusor is the real Mc Coy. It was first proposed by Hirsch and Meeks as a team effort within the Farnsworth IEC fusion team at ITT in 1965.

It demands a metal shelled vacuum chamber!

It produces real fusion, most often based on the classic d-d reaction and produces radiation and high energy particles and isotopes. It is a real fusion nuclear reactor! The follwoing particles and radiations are common to the real fusing fusor.

Neutrons at approx 2.5 mev energy
Tritium nuclei (isotope of hydrogen) at ~1mev energy
Protons at ~3mev energy
Helium 3 nuclei (isotope) at ~0.8mev energy
X-radiation in a spectrum from zero to full operating voltage energies.

Such a machine involves great expense and operational knowledge and places a mantle of new responsibilities upon the operator. Many of the skills needed have already been acquired in the demo fusor construction and use phase due to the fusioneer's early efforts.

This machine differs from even the most elegant and expensive demo system in that it must have a very robust high voltage supply capable of voltages several times those of the demo machine. A power supply rated at 0-35 kilovolts that can supply 20 miliamps DC is none too beefy. The demo fusor is also now fitted with gas lines going to a deuterium gas supply tank with all associated metering valves and pressure gauges.

The full fledged fusor also demands a tighter vacuum system and often includes a deep pumping, secondary vacuum pump such as a diffusion or turbo molecular pump. These assure the operator that deuterium is, indeed, the dominant gas in the system.

Complex and often expensive neutron radiation detection systems are a must to demonstrate that fusion is taking place. A good fusioneer will often have three or four different counter systems, each performing a specific function or acting as a check or fail-safe against another.

Needless to say, these machines place the user in the "Neutron Club". Once there, the builder has, truly, earned his title of "fusioneer". It is exceedingly rare that a person, using his own means, talents and time can boast that they have done fusion in their home with their own hands and have solely owned and operated a fusion reactor.

The atmosphere in the fusion club is highly rarified, indeed.

There are two types of classic functional, fusing fusors.

1. Basic fusion device for real fusion demo or a college project
2. Research grade fusion system and neutron source.

The basic fusion fusor device -
This device produces fusion and has a minimal compliment of add-ons to the demo device to produce and verify fusion. Rarely do these devices exceed neutron rates of 500k neutrons/sec, isotropic emission.

Research grade fusion fusor and neutron source-
This device usually is in the $10,000 outlay category and is as well outfitted with auxiliary monitoring devices and plus ultra vacuum components as would be found in a good college or industry research lab. Such systems can be used to research design upgrades and act as weak neutron sources for activation studies.


Now that we have discussed the basic fusor variants, generally, we will elaborate on the technical details of the diagram below and explain the options that might be found on each type of fusor.

You will note that the diagram is color coded.

Devices in black, and whose designations are surrounded by the light blue boxes, are considered minimal components to be found in all systems.

The shell casing or outer grid work, the inner grid with HV insulator that is vacuum rated, the high voltage supply, the view port, the vacuum flange and connection to a working 2 stage vacuum pump are just bare minimums to create a glow with only visual appeal. Such a machine will transfer no fusion data or information to the user/operator. BUT....It is a demo fusor system.

The Basic Power Supply and Connections ...................

The basic supply must always have its negative high voltage lead UNGROUNDED, (that is not hooked to the metal supply case or cabinet in any way). The negative lead is the HOT lead.

The hot negative lead always gets connected directly to the inner grid via the high voltage insulator on the fusor.

The basic supply's postive lead WILL ALWAYS be grounded to the supply's metal case and a good electrical ground lead, no matter what!! A wire or ground lead is then taken, either directly to the fusor's outer grid, (in a glass vessel), or to the Fusor's metal, spherical shell if a metal vacuum chamber is used.

If the fusor's current metering option is used, the supply's positive lead is taken to ground directly at the supply case, as noted above. However, the fusor shell or outer grid is then taken to ground through a heavy current meter shunt, across which, the meter is placed. ******* IMPORTANT******** The heavy duty wire nature of this shunt can't be overstated! It must never burn out or be capable of being burned out. I use a length of stiff nichrome wire that allows a 50ua meter movement to deflect to indicate a 50ma current flow through the nichrome wire shunt. All connections to this must be robust and using significant bolting or hardware between the shell of the fusor and the positive ground of the power supply.

When running, the glow in the demo fusor is due to remnant atmospheric gases turning into plasma via high field and secondary electron emission ionization.

These black items, outlined in blue on the diagram are part of a


Now we discuss the various options, all in red boxes. We will define each option, in turn, and then give a formal listing of the various fusors with the option numbers to be found on them to make that grade or class of demo or fusing fusor.

$$$ Note: the options below with multiple stars, (***), are not optional but a must have option for any fusor where you wish to claim admittance into our Plasma or Neutron club listings!! $$$

Gas options
#1 Mass flow controller - a professional, electronically metered gas flow device. often costing up to $1500.00 with its controller. (Not needed or used by many fusioneers.)
#2 Micrometer needle or bellows valve - this is a manually adjusted alternative to the MFC listed above. if will cost around $50.00 - $200.00
#3 Micron filter - this is a rare option that will allow only gas to pass, filtering out solids and most liquids from the stream.
#4 flow restrictor line/reservoir - this is often a long, low conductance coil of hypodermic neeedle line and slows the gas flow to the valve. Small laser made, pin hole restrictors have also been used.

#5 D2 gas tank with a full pressure regulator assembly attached the regulator is a must have item to keep from blowing up the gas line. (note* Some few people have used the electrolysis of "heavy water", (D2O) to obtain their deuterium gas)

Vacuum options
#1 Gate or bellows valve - this controls the vacuum pumps rate and is traded off against the gas flow rate to determine the flowing gas pressure within the fusion reaction vessel.
#2 Diffusion or Turbopump - this option is critical for fast starts and repeatable operations where D2 must be the supreme, dominant gas. This device drags the vacuum in the vessel to several orders of magnitude below fusion pressures.
3# and #5 Bellows valve - These valves are demanded when the foreline trap is used.
#4 Foreline trap - this device allows a full order of magnitude deeper vacuum pressure over a common 2 stage mechanical pump.

Pressure options
*** #1Gauge tube or transducer (many choices, i.e., ion, thermocouple, capacitive)
#2 pressure electronic readout (relates to tranducer selection - above)

Current meter option
***This option demands a shunted 50ua meter movement or a specially adapter digital display voltmeter.

***Voltmeter option
This option uses a digital or analog readout meter that requires a special hi megohm divider string of resistors. (custom fabrication). Not optional on any fusor where you wish to claim admittance to the Plasma or Neutron club.

Optional HV supply
This 20-60 kilovolt option is needed only if fusion is being attempted and replaces the demo fusor supply.

Radiation detection options
#1 Geiger counter or ionization chamber meter. - allows the user to determine x-ray radiation levels when working at voltages over 10kv.
#2 Neutron counter of the BF3 or He3 type with rate meter/counter. Such detectors can easily cost hundreds of dollars, but are needed for real time indication of neutrons.
#3 BTI bubble detector for fast neutrons. - this amazing device is a very low cost fast neutron dosimeter that is perfect for fusors that produce in excess of 20,000 neutrons/sec.

View options
#1 TV camera for remote viewing color or B&W
#2 Monitor for viewing. Large for public showing. Small for operator use. Recording devices like video tape recorders or digital cameras can be added to save data for later viewing.

Now we will review the types of fusors with minimal required options.

Demo fusors - No fusion, but fusor work and look alike
1. Raw minimal demo fusor - no options required
2. Public demo fusor - view options #1,#2; Meter options volt and amp
3. Science fair demo fusor - view options #1,#2; Meter Options volt and amp; pressure options #1,#2
4. Research grade demo fusor - view options, #1,#2; meter options volt and amp; pressure options #1,#2; Vacuum options #3,#4,#5; power options HV supply to 30kv; Rad option #1

Full operational fusing fusor systems
1. Basic fusion device - Veiw options all, Pressure options all, metering options volts and amps; vacuum options ALL; Gas options #2,#5; radiation options #1,#3; power supply option will have a 0-30kvdc supply
2. Research fusor and neutron source - All view options, All meter options, All pressure options, Gas options; #1,#3,#4,#5, Vacuum options; ALL, Radiation options #1,#2,#3, power options will need a 0-50kv supply capable of 20 ma.

There you have it. A full range of fusors with a full range of options that will allow assembly of fusors in the range of $200.00 to $20,000.

Richard Hull
schematic - new fusor.JPG
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
User avatar
Richard Hull
Posts: 14949
Joined: Fri Jun 15, 2001 9:44 am
Real name: Richard Hull


Post by Richard Hull »

Upgraded 2023

Several additional things added for clarity

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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