# Why do tokamaks and most other fusion reactors use neutral plasmas?

Most experimental fusion reactors use quasi-neutral plasma as the working substance. However, there are numerous advantages to using a non-neutral plasma:

1. Ease of confinement. Pure proton gases have very long confinement times, and with special setups, can actually be confined indefinitely. Compare this to non-neutral plasmas, which have very short confinement times.
2. Reduction of bremsstrahlung losses. Bremsstrahlung losses are one of the prime sources of energy loss in fusion reactors. However, according to the Larmor formula, this loss is proportional to $$m^{-4}$$ in toroidal setups. Just by expelling all electrons, bremsstrahlung losses can be reduced by a factor of $$10^{13}$$. Wouldn't it be advantageous to increase the mass of your ions as much as possible - say, by using heavier atoms and expelling all electrons?
3. It seems that almost all the heating methods used for neutral plasmas can be used for non-neutral ones (microwave heating, neutral beam injection, etc.)
4. Creating non-neutral plasma isn't difficult - ionizing species is not a difficult task, and the energy investment isn't too high. Furthermore, these "waste electrons" could be reused in some type of direct energy conversion scheme.

While I understand that Coulomb forces would be great, leading to increased mechanical stress, I don't see how this invalidates the technology. Many places are experimenting with non-neutral plasmas, and they seem so attractive that I don't see why they are so unpopular.

• I imagine creating a non-neutral plasma of sufficient density requires a tremendous amount of energy. Then you add in the necessity to overcome the huge Coulomb barriers. In a typical quasi-neutral plasma, the Debye length is on the order of microns which is advantageous for charge-charge collisions. – honeste_vivere Apr 22 at 15:43

While I understand that Coulomb forces would be great, leading to increased mechanical stress, I don't see how this invalidates the technology

It's called beta.

First off, you promote bremsstrahlung to a unique position as a loss mechanism. Conduction and other radiative effects are more important in energy terms, and these generally increase with mass. This is why so much of modern design is devoted to various scrape-off systems and divertor designs that remove high-mass or low-energy particles from the plasma.

So even if you solve all of these problems, the issue is that you are spending more of your magnetic energy simply keeping the particles in place, as opposed to increasing their reaction rate. Beta is basically a direct measure of economic performance, as magnet power scales with size and thus expense. Anything that lowers the beta, which pure protonium would do, reduces the economic performance of the system.

Many places are experimenting with non-neutral plasmas

Most of them are woo. A good example is the polywell, where its proponents spend years stating that quasi-neutral considerations in Rider's work, only to find that the system never really worked.

• The polywell is presumably also where OPs focus on bremsstrahlung losses comes from. Polywell and other charged inertial confinement concepts have been shown to suffer majorly from bremsstrahlung. – AtmosphericPrisonEscape May 12 at 15:03
• Good point. As noted above, however, it appears this design simply doesn't work. – Maury Markowitz May 12 at 15:23