Since the 1960's it is undisputed that the energy production of a coming fusion reactor can be increased for the DT or the D3He reaction if the nuclear fuel is polarized. The experimental proof for the D3He reaction was already done in 1971 at the Paul Scherrer Institute in Switzerland. However, before this option can be used, a number of questions must be answered.
The effective cross sections of the main fusion reactions d + t → 4He + n and d + 3He → 4He + p, which can be used for energy production, strongly depend on the nuclear spin polarization of the particles involved. Both reactions proceed via a so-called J=3/2+ resonance dominated by s-waves. That is, when a deuteron and a triton fuse, an intermediate 5He nucleus is formed for a short time, which subsequently decays into 4He and a neutron with considerable energy release. This unstable 5He nucleus has a nuclear spin of S=3/2, so nuclear fusion is only possible if the spins of the deuteron (S=1) and the triton (S=1/2) can be combined to form S=3/2. If both spins are antiparallel to each other this is not possible and the fusion reaction is suppressed. If both spins are aligned parallel from the beginning, i.e. if the fuel is polarized, the total effective cross section increases by a factor of 1.5. In addition, the polarization can be used to influence the differential cross sections and thus control the trajectories of the generated particles, especially the neutrons. This would be another way to optimize the technical design of a fusion reactor and could simplify a future fusion power plant and reduce operating costs.
However, before the concept of "polarized fuel" can be used for nuclear fusion power generation, at least 3 questions must be answered:
1) How to produce sufficient polarized fuel?
Tokamaks or stellerators are filled either with intense (up to 100 A) and high-energy (several 10 to 200 keV) atomic beams or with small pellets of the propellants, which would have to be polarized beforehand.
To solve this problem, several ideas have been developed at IKP that are also useful for optimizing polarized ion beams or polarized targets for accelerators. Since polarized hydrogen and deuterium atoms cannot be stored directly after their generation with an ABS as chemical radicals, the production and storage of polarized molecules after their recombination is a possible option. In addition, a polarized H2/D2 source based on the Stern-Gerlach principle was built and tested in collaboration with the Peter Grünberg Institute, the University of Düsseldorf, and the Budker Institute in Novosibirsk.
At the end of 2022, the IKP achieved a decisive breakthrough on this issue, which has since been patented (official file number: DE 10 2022 213 860.0).
When an unpolarized beam of metastable hydrogen or deuterium atoms flies with a fixed velocity v through a sinusoidal magnetic field (see also "Precision Spectroscopy of the Hyperfine Structure of Hydrogen"), the atoms experience an incoming electromagnetic wave in their inertial frame. The corresponding photons are coherent and monochromatic and trigger quantum transitions that can interfere with each other. Thus, at certain magnetic field strengths, it is possible to pump many of the atoms into a single hyperfine structure state with fixed nuclear and electron spin, i.e., the beam is polarized in the process. Simulations have shown that up to 95% of the nuclear spins of the hydrogen atoms are aligned in the desired direction, and for deuterium it is at least 90%. Since the energy of these photons is only 10-9 eV, a power of one watt is sufficient to polarize about 1028 atoms per second.
Experimental evidence has so far been obtained for metastable hydrogen and deuterium atoms in the 2S state. The results follow very closely simulations based on the solution of the Schrödinger equation for the corresponding Hamiltonian operator. A Dirac relativistic approach is not necessary since the velocity of the atoms did not exceed v~106 m/s (corresponding to a beam energy of ~5 keV) in the experiments. Other simulations for ground state deuterium atoms or even 3He+ ions were also successful and will be tested experimentally in the fall.
Theoretically, this method is extensible to many atoms, molecules and their ions and can be used for many possible applications. These include new polarized sources and targets for experiments at particle accelerators, materials research by nuclear spin tomography, nuclear spin-polarized tracers in medicine up to significantly improved and cheaper magnetic resonance imaging and, precisely, polarized fuel for nuclear fusion.
2) Does the nuclear polarization remain in the fusion plasma?
This question is very important for the use of polarized fuel for power generation. If the lifetime of polarization in the plasma is shorter than the average time required for a nucleus to undergo fusion, the polarized fuel will have little effect on fusion rates or energy production. This question has also been debated since the 1980s, but no experiments with polarized fuel have yet been possible. Moreover, this question must be answered separately for each reactor, since the amount of depolarizing wall collisions or the density of the plasma can have a large influence.
Besides the fusion concept of "magnetic confinement" of the plasma as in a tokamak or a stellarator, there are other possibilities, e.g. laser-induced nuclear fusion. But here, too, the question arises whether nuclear polarization will survive under the extreme magnetic and electric conditions of these laser beams. In this case, the IKP is participating in a collaboration with the Peter Grünberg Institute (PGI-6, group of Prof. Markus Büscher, HHU) in corresponding measurements at the PHELIX laser of GSI in Darmstadt to generate polarized 3He2+ ions by laser acceleration from polarized 3He atoms. If this is successful, it will be shown in parallel that nuclear polarization is preserved in the laser-induced plasma. Further experiments of this kind, e.g. with polarized D2 or HD ice as a substitute for the radioactive DT as target material, might be possible within the JuSPARC project.
3) What happens if only polarized deuterium is used for fusion?
In all scientific reactors for nuclear fusion experiments, the use of radioactive tritium is avoided and only the D+D reactions (d + d → 3He + n or d + d → t + p) are used. However, in these reactions, so-called direct nuclear reactions, the influence of nuclear spin on the reaction rates is much more complicated. Theoretical predictions for parallel spins range from a suppression of the reaction d + d → 3He + n by a factor of 10 to an increase of the reaction d + d → t + p by a factor of 2.5 in the energy range of a coming fusion reactor (marked in green in the figure). Only a measurement of this so-called "quintet suppression factor" can substantiate the different predictions of the various models and show which spin combination can increase the reaction rate or suppress neutron production.
The experiment itself is currently being set up at the St. Petersburg Nuclear Physics Institute (PNPI) in Gatchina, Russia, in collaboration with the University of Ferrara, Italy, and the IKP Jülich (as of 2021).
Contact: R. Engels