Cryogenic and Superconductive Techniques Laboratory

There is a consensus that the hydrocarbon energetic will eventually be superceded by the thermonuclear one. The principal aspects of the thermonuclear fusion problem include: (i) understanding nuclear physics aspects of the fundamental fusion reactions at low energies, (ii) confinement of the hot and dense plasma, (iii) extraction of energy from plasma. Standard candidates for the fusion fuel include the hydrogen isotopes (the protium, deuterium, tritium) and the light isotope of Helium – ³He. As an input to practical realization of the fusion reactors one needs the nuclear cross sections of interactions of light nuclei; the same cross sections enter the analysis of energy production in stellar interior and the primordial nucleosynthesis at early stages of Big Bang.

The cross sections of nuclear interactions of light nuclei have been extensively studied, still not all of them are well known. This is especially true for the spin sector, although the spin effects in few-body reactions are gigantic – here we cite just several classical examples. First, in the nucleon-nucleon system the bound state, the deuteron, exists only in the spin-triplet channel, and the spin-singlet scattering length has a sign opposite to, and about four times larger in magnitude than, the spin-triplet scattering length. The second classical example: a broad 0⁺ resonance in the ⁴He nucleus entails a gigantic cross section of the neutron – ³He scattering in the spin-singlet channel. Such a strong spin-singlet absorption entails an almost 100% spin filtering of neutrons with an excellent transmission rate of 50% in the polarized ³He target [1]. Such a technique has become one of the methods to generate intense polarized neutron beams [2]. Finally, the amplitudes of important fusion reactions: d(³H,n)⁴He and d(³He,p)⁴He, are dominated by the S-wave J^P=3/2⁺ resonance. Such a resonance greatly simplifies a treatment of double-polarization effects: a simple counting of spin states implies that in an unpolarized plasma only 2/3 of nuclei can undergo the fusion. Alternatively, a full polarization of the deuteron and ³He would enhance the fusion cross section by 50%. Such a strong polarization effect has been confirmed experimentally to a good accuracy [3].

Based on the scrutiny of the available experimental data on the deuteron-deuteron interactions, as early as in 1969 Ad’yasevich and Fomenko suggested a possibility of the polarization enhancement of the DD fusion rate by a factor of two [4]. The first proposal by the Kurchatov Institute group of the experimental study of the polarization coefficients in doubly-polarized d(d,p)t and d(d,n)³He collisions in vacuum dates back to 1976 [5]. The proposal was not pursued as the available at that time technique of polarized atomic and ion beams does not allow producing the beams of adequate intensity.

A substantial step forward was made in 1982 in a theoretical study of depolarization of nuclei in magnetically confined plasma [6]. The principal conclusion was that the depolarization time greatly exceeds the fusion reaction time. Presently, it is considered feasible to confine ³He with nuclear polarization reaching 55% and to inject neutral deuterium with nuclear polarization circa 55%. The estimated enhancement of the fusion yield of 15% is anticipated. The experiment at DIII-D Tokamak at San Diego is planned [7]. In the case the polarization retention is confirmed, it is planned to look at a possibility of a nuclear polarization of tritium, which would offer improved prospects of ignition in the ITER program and possible important cost savings. The alternative approach to the experimental confirmation of persistence of nuclear polarization in a fusion process has been suggested at Orsay [8]: looking for nuclear reactions in a plasma generated by a petawatt laser hitting a polarized frozen HD target. Detecting the final state gamma’s and neutrons one would have an experimental access to both reactions p(D,³He)γ and D(D,³He)n.

The crucial milestone of the investigation of the practical use of polarized deuterium as a fuel for the thermonuclear reactors will be a systematic measurement of the spin-correlation coefficients for polarized deuteron interaction in the energy range of 10-100 keV. It is supposed to investigate the angular distribution of the reaction products at various energies:
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Due to a very low energy of initial particles their penetrating power is exceedingly small – less than a micron. Therefore experiments with solid targets would give large error bars because of an unknown surface polarization and influence of the multiple scattering on the energy of the reaction products. We propose to use direct interaction of the polarized deuteron beam with the polarized neutral deuterium atoms in the vacuum:

[1] R. M. Moon, T. Riste, and W. C. Koehler, Phys. Rev. A 181, 920-931 (1969).
[2] T.E. Chupp еt al., Nuclear Instruments and Methods in Physics Research. Section A 574, 500-509 (2007).
[3] H. Paetz gen. Schieck, Eur. Phys. J. A 44, 321–354 (2010).
[4] B.P. Ad’yasevich, D.E. Fomenko, Sov. J. Nucl. Phys. 9, 167 (1969)
[5] B. Ad’jasevich, V. Antonenko. Measurements of the polarization correlation coefficients in reactions d(d,p)t and d(d,n)³He. Preprint IEA-2704, Moscow (1976).
[6] R.M.Kulsrud et al., Phys. Rev. Lett. 49, 1248 (1982).
[7] A. Honig, A. Sandorfi, Proceedings of the 17th International Spin Physics Symposium. AIP Conference Proceedings 915, pp. 1010-1018 (2007).
[8] J.-P. Didelez and C. Deutsch. Persistence of Polarization in a Fusion Process. EPJ Web of Conferences 3, 04018 (2010).