High Power (Breakeven?) Fusor

A fusor is a device which uses an electrostatic field to accelerate deuterium (or a mix of deuterium and tritium) ions and cause them to collide with sufficient energy to overcome the coulomb barrier and fuse.

A fusor consists of two concentric grids placed into vacuum chamber with deuterium introduced at a very low pressure. The outer grid is charged positively, the inner grid negatively. Deuterium ions are accelerated by the voltage gradient towards the inner negative grid. Most of them pass through the grid towards the center of the device where some of them collide and fuse.

The ions impacting the inner grid heat the grid. This heating limits the power level and fusion rate which can be obtained. The current amateur record is 5×10^6 neutrons per second, which equates to about .03 watts of fusion power using 4 kw input power.

The original inventor, Philo Farnsworth (also the inventor of the cathode ray tube) and his team created a device that achieved 10^12 neutrons using a deuterium tritium mix. Breakeven, requires on the order of 10^14 to 10^16 neutrons per second would be required. Even 10^12 neutrons per second requires shielding.

The present state of the fusor device makes it useful as a neutron source but not as a power generation device. To achieve breakeven the reaction level would have to increase from between 100 and 10,000 times the present record. Even if you replaced the grids with very heat resistant materials these power levels could not be achieved with the existing design.

For a deuterium-tritium mix the ideal energy of the colliding deutrons (neutron + proton) is approximately 60 KeV. The present fusor designs comes close to this, accelerating deutrons through fields approaching 30 KeV. Deutrons that hit head-on have enough energy to overcome the coulomb barrier and fuse. Present devices are operated at very low pressures so there are few collisions.

If the grid could be made to repel deutrons just as they were passing, then the ions would be deflected through the holes between the grid wires instead of colliding with the grid. If this could be achieved then all of the energy would go into accelerating the ions instead of heating the grid.

My idea is to operate the fusor more like a particle accelerator. Operate it in pulsed AC mode instead of DC. The frequency would be chosen so that just as the particles get to the outer grid, the outer grid is supplied with a positive voltage and the inner with a negative, but just as the pulse of particles is passing the inner grid, the polarity is reversed and the ions are repelled away from the grid and steered through the openings between the grid wires. The repulsive force after they pass they will accelerate them further.

Eliminating the collisions would allow you to operate at the optimal voltage and density necessary for maximum fusion yield. With an AC system you could even use multiple concentric grids in what would basically be a concentric version of a linear accelerator.

If the heating from the fusion reaction heats the grids excessively, you could build them out of small diameter tubing instead of wire and run a coolant through the tubing which would allow you to simultaneously cool the device and extract thermal energy.

I don’t have the means or skill to build such a device so I am throwing this idea out in hopes that someone experimenting with fusors might find this approach interesting and give it a try.

Here are some related links:

Brian McDermott’s device is probably the most well constructed amateur fusor. At this point he still hasn’t achieved anything close to Farnsworth record fusion rate but it’s still in process.

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