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Alpha particles production and their contribution to plasma heating effect in fusion devices

Daponta Chrysovalanti

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Summary

Fusion of light nuclei (of small atomic number) is the method of clean energy production for large scale power generation. The European fusion activity is implemented through Euratom and more recently, through Eurofusion, which is a world leader in research and technology, due to the construction and operation of large Magnetic Confinement Fusion (MCF) devices, like ITER (International Thermonuclear Experimental Reactor) and JET (Joint European Torus), both costing 10 - 12 billions of euros.The last few years and especially in the USA, there is an increasing interest for the development of Compact Magnetic Fusion Devices (CMFDs), operating with intermediate plasma densities: 1020 m-3 - 1024 m-3, compared to magnetic (MCF) or inertial (ICF) confinement fusion devices: 1019 m-3 και 1029 m-3, correspondingly. Compact Magnetic Fusion Devices are suitable for industrial applications, including magnetic confinement fusion plasma studies, fusion energy generation tests, space propulsion and blanket material construction studies for future MCF and ICF devices. It is noteworthy to mention that the final CMFD construction and operating cost is at least 100 times lower than that of conventional Tokamak and ICF machines.Conventional MCF and ICF machines, as well as CMFDs use mostly the nuclear fuels of Deuterium - Tritium (D-T) or Hydrogen - 11Boron (p-11B). However, D-T nuclear fusion reaction: D + T → 𝐻𝑒24+ n, isn’t the ideal candidate for clean energy generation, as it releases most of its energy in the form of high-energy neutrons (1 neutron of 14.1 MeV energy, per fusion reaction). p-11B nuclear fusion reaction: p - 11B → 3 𝐻𝑒24 on the other hand, not only emits less than 1.0% of its total energy in the form of neutrons, but also produces three (3) iso-energetic alpha particles (𝐻𝑒24) with 8.7 MeV total energy. The latter energy amount can be converted intoelectricity with a 60 % - 70 % efficiency, without passing through a thermodynamic cycle.Recent experiments of the international literature measure alpha particle generation from p – 11B nuclear reactions, arising from the interaction of a laser-driven high-energy proton beam with a solid 11Boron target or plasma (beam-target schemes). Since the generation of alpha particles depends on the initial proton beam energy, the initial density and temperature of 11Boron plasma, as well as the temporal evolution of both of these parameters, beam – solid target (or plasma) configurations produce a relatively small number of alpha particles.The major difficulty in the research conducted around p-11B fusion is its nuclear fusion cross section efficiency, at higher than 250 keV reactant energies. At these high energies, p-11B fusion plasma electrons emit intense Bremsstrahlung radiation, as a consequence of their acceleration from the electric field of other charged particles (protons or atomic nuclei). Bremsstrahlung radiation isn’t absorbed by the thermonuclear plasma and thus, is an unavoidable energy loss from it. For the overcome of the difficulty related to nuclear cross section efficiency, the formation of the proton - 11Boron plasma is suggested inside a Compact Magnetic Fusion Device (CMFD). The proton - 11Boron plasma will arise from the interaction of two high-energy particle beams (one proton fluid plasma beam and one Boron ions plasma beam), in the energy range of 100 keV – 300 keV. Inside the formed p-11B fusion plasma, the fusion produced alpha particles will transfer most of their energy to the protons and 11Boron ions, through elastic binary Coulomb collisions (chain reactions effect and related avalanche alpha heating effect, according to the international literature). The type of Compact Magnetic Fusion Device described above, will allow a relatively long plasma energy confinement type (τΕ ~ 10 s), so that the total number of produced alpha particles can contribute to “clean” energy generation.In the context of the present PhD dissertation, the following topics will be discussed: i) The plasma formation processes, ii) The p-11B fusion process inside a Compact Magnetic Fusion Device (CMFD), iii) The interactions of the fusion produced alpha particles inside the fusion plasma, iv) The result of the binary Coulomb collisions of the fusion produced alpha particles with the plasma charged particles (electrons, protons, Boron ions), v) The induced plasma heating plasma heating by the fusion produced alpha particles. The plasma formation by the high-energy particle beams will be discussed for the cases of beam production from: i) Short pulse high-intensity laser beam interactions with solid targets, and ii) Magnetically Insulated Diodes (MIDs). The results of the research will allow in a final stage, the proposal of a device, for the conversion of alpha particles energy into electrical energy.

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