Polarized Fusion
Since 50 years it is undisputed that the energy production of a coming fusion reactor for the DT or the D3He reaction is increased when the nuclear fuel will be polarized. But before this option can be used a bunch of questions must be answered.
The total cross sections of the main fusion reactions d + t → 4He + n and d + 3He → 4He + p, which can be used for energy production, depend strongly on the polarization of the particles. Both reactions are going through a J=3/2+ resonance that is s-wave dominated. I.e., if a deuteron and a triton will fuse an intermediate 5He nucleus is built for a short time. This unstable 5He nucleus has a nuclear spin of S=3/2 and, therefore, a nuclear fusion is only allowed if the spins of the deuteron (S=1) and the triton (S=1/2) can be combined to S=3/2. When both spins are antiparallel 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 cross section is increased by about a factor 1.5. In addition, with the help of polarization the differential cross sections are influenced and the trajectories of the produced particles, especially of the neutrons, can be controlled. This would be another option to optimize the technical design of a fusion reactor and may simplify a future fusion power station and decrease the running costs.
But before the concept of “polarized fuel” can be used for the energy production by nuclear fusion, at least 3 questions must be answered:
1) How to produce enough polarized fuel?
To solve this question different ideas are under development at IKP that are helpful even for the optimization of polarized ion beams or polarized targets for accelerators. One option is the production and storage of polarized molecules after the recombination of polarized deuterium atoms produced by a polarized atomic beam source. In collaboration with the Peter-Grünberg-Institute, the University of Düsseldorf and the Budker Institute in Novosibirsk a polarized H2/D2 source following the Stern-Gerlach effect was built and tested. In addition, the development of polarized ion sources for COSY at IKP is very helpful, because the plasma of several fusion reactors is heated and fed with intense deuteron beams.
2) Is the nuclear polarization preserved in fusion plasma?
This question is very essential for the use of polarized fuel for energy production. If the lifetime of the polarization in the plasma is shorter than the average time a nucleus needs for fusion, polarized fuel will have no effect for the fusion rates or the energy production. Even this question has been discussed since the 80’, but no experiments with polarized fuel were possible up to now. In addition, this question must be answered for every reactor separately, because the amount of depolarizing wall collisions or the density of the plasma can have a huge influence. A first experiment with polarized 3He and HD ice is supposed for the DIII tokomak in San Diego.
There are other options beside the fusion concept of “magnetic confinement” of the plasma like in a tokomak or a stellarator, e.g. the laser-induced nuclear fusion. But again the same question will appear if the nuclear polarization will survive under the extreme magnetic and electric conditions of these laser beams. In this case, the IKP takes part in a collaboration with the Peter-Grünberg-Institute for measurements at the PHELIX laser at the GSI in Darmstadt to produce polarized 3He2+ ions by laser acceleration from polarized 3He atoms. If this will work out it is shown in parallel that the nuclear polarization is preserved in the laser-induced plasma. Further experiments of this type, e.g. with polarized HD ice as replacement for the radioactive DT as target material, might be possible within the framework of the JuSPARC project.
3) What will happen if only polarized deuterium will be used for fusion?
In all scientific reactors for nuclear fusion the use of radioactive tritium is avoided and only the DD reactions (d + d → 3He + n or d + d → t + p) are applied. But for them the influence of the nuclear spin on the reaction rates is unknown. The theoretical predictions for parallel spins reach from a suppression of the reaction d + d → 3He + n by a factor 10 to an increase of the reaction d + d → t + p by a factor 2.5 in the energy range of a coming fusion reactor. Only a measurement of this so called “quintet suppression factor” can prove the different predictions given by different models and can show which spin combination will increase the reaction rate or may help to suppress the neutron production.
The picture above shows the different predictions for the "Quintet-Suppression Factor", i.e. the modification of the total cross sections for different energies, when the nuclear spins of the deuterons are aligned parallel to the magnetic field.
The experiment itself is under construction at the St. Petersburg Nuclear Physics Institute (PNPI) in Gatchina, Russia, in collaboration with the University of Ferrara, Italy, and the IKP Jülich.
Contact: R. Engels