Aneutronic fusion

Some proponents see a potential for dramatic cost reductions by converting energy directly to electricity, as well as in eliminating the radiation from neutrons, which are difficult to shield against.

He proposed to capture the kinetic energy of charged particles as they were exhausted from a fusion reactor and convert this into voltage to drive current.

[3] Post helped develop the theoretical underpinnings of direct conversion, later demonstrated by Barr and Moir.

[14] In 2021, TAE Technologies field-reversed configuration announced that its Norman device was regularly producing a stable plasma at temperatures over 50 million degrees.

[15] In 2021, a Russian team reported experimental results in a miniature device with electrodynamic (oscillatory) plasma confinement.

The Coulomb barrier is the minimum energy required for the nuclei in a fusion reaction to overcome their mutual electrostatic repulsion.

In most fusion concepts, the energy needed to overcome the Coulomb barrier is provided by collisions with other fuel ions.

A plasma is "ignited" if the fusion reactions produce enough power to maintain the temperature without external heating.

Those with the largest cross sections are: The 3He–D reaction has been studied as an alternative fusion plasma because it has the lowest energy threshold.

Detailed analyses, however, do not show sufficient reactivity enhancement to overcome the inherently low cross section.

3He occurs in only minuscule amounts on Earth, so it would either have to be bred from neutron reactions (counteracting the potential advantage of aneutronic fusion)[clarification needed] or mined from extraterrestrial sources.

Extracting that amount of pure 3He would entail processing 2 billion tonnes of lunar material per year, even assuming a recovery rate of 100%.

The fusion of the boron nucleus with a proton produces energetic alpha particles (helium nuclei).

Earlier methods used a solid boron target, "protected" by its electrons, which reduced the fusion rate.

[28] Experiments suggest that a petawatt-scale laser pulse could launch an 'avalanche' fusion reaction,[27][29] although this remains controversial.

A clever magnetic confinement scheme could in principle suppress the first reaction by extracting the alphas as they are created, but then their energy would not be available to keep the plasma hot.

[note 1] The hydrogen must be isotopically pure and the influx of impurities into the plasma must be controlled to prevent neutron-producing side reactions such as: The shielding design reduces the occupational dose of both neutron and gamma radiation to a negligible level.

Advancements include:[34] Aneutronic fusion produces energy in the form of charged particles instead of neutrons.

This means that energy from aneutronic fusion could be captured directly instead of blasting neutrons at a target to boil something.

Because of the photoelectric effect, X-rays passing through an array of conducting foils transfer some of their energy to electrons, which can then be captured electrostatically.

Experimenting with D–T fusion is more difficult because tritium is expensive and radioactive, requiring additional environmental protection and safety measures.

The combination of lower cross-section and higher loss rates in D–3He fusion is offset to a degree because the reactants are mainly charged particles that deposit their energy in the plasma.

For pressure-limited confinement concepts, optimum operating temperatures are about 5 times lower, but the ratio is still roughly ten-to-one.

Confinement is usually characterized by the time τ the energy is retained so that the power released exceeds that required to heat the plasma.

In conventional reactor designs, whether based on magnetic or inertial confinement, the bremsstrahlung can easily escape the plasma and is considered a pure energy loss term.

[41] This is considerably higher than the Lawson criterion of ρR > 1 g/cm2, which is already difficult to attain, but might be achievable in inertial confinement systems.

[42] In megatesla magnetic fields a quantum mechanical effect might suppress energy transfer from the ions to the electrons.

In this well-known case, the cyclotron radiation is trapped inside the plasmoid and cannot escape, except from a very thin surface layer.

The probability of a feasible reactor based solely on this effect remains low, however, because the gain is predicted to be less than 20, while more than 200 is usually considered to be necessary.

However, relaxing these assumptions, for example by considering hot electrons, by allowing the D–T reaction to run at a lower temperature or by including the energy of the neutrons in the calculation shifts the power density advantage to D–T.