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Proton-boron-11 (p-11B) fusion is an attractive concept for advanced fusion energy because of its nearly aneutronic characteristics. However, efficient use of beam-injected fast ions may be limited by their slowing down through Coulomb collisions, which lead to lower fusion reactivity. In this study we investigate whether ion cyclotron range of frequencies (ICRFs) heating can ameliorate this effect by resonantly accelerating fast ions in the Large Helical Device of a stellarator. Using the tangential neutral beam (NB) injection to supply energetic protons at ∼160–170 keV, together with boron seeding, we conducted a series of experiments with superimposed hydrogen minority ICRF heating for beam proton acceleration. Three complementary experimental methods, ICRF power modulation, NB termination, and NB energy scan, consistently showed that ICRF waves accelerate beam ions and enhance the p-11B fusion yield. This led to measurable enhancements in the p-11B fusion alpha-particle yield, as observed with a dedicated fusion-product detector. These findings provide the first direct experimental evidence that ICRF heating can improve the efficiency of beam-driven p-11B fusion reactions.

The results of the first experimental series to produce a plasma using the radio frequency discharge (RF) above the ion cyclotron frequency at relatively low magnetic fields (0.5–0.6 T) in the Large Helical Device (LHD) are presented 1.43 MW of RF power produced target plasma with density up to 6 × 1018 m−3 to deuterium. Tangential NBI application into such a plasma increases plasma parameters. Electron temperatures up to ≈0.9 keV and densities up to ≈2.4 × 1019 m−3 had been achieved, and the maximum value of <βdia> was 2.6%. These experiments open possibilities for new regimes of LHD operation which are also interested to W7-X.

This study aim is to develop further an ion cyclotron range of frequencies (ICRF) method of plasma production in stellarators based on the minority heating. The previous studies demonstrate production of low density plasma (9.5 × 10 17 m −3 ) at low power of up to 0.2 MW. The higher ICRF heating power experiments become possible after introducing a programmable ICRF power ramp up at the front of the ICRF pulse. With this trick, all the shots went with the antenna voltage within the safe range. Increase of the ICRF power predictably results in increase of the density of produced plasma. Without pre-ionization the plasma density achieved was 6 × 10 18 m −3 which is 6 times higher than in previous experiments. However, the electron temperature was not high, the light impurities were hot fully stripped, and there were no recombination peaks after termination of the ICRF pulse. Plasma density is too low to provide good conditions for efficient plasma heating. For the reference, the ICRF heating of high density cold plasma prepared by electron cyclotron resonance heating is performed. Both electrons and ions were heated to high temperatures, and this plasma state is sustained. The antenna–plasma coupling was much better which result in larger heating power with the lower antenna voltage.

Energetic-particle driven Alfvén eigenmodes (AEs) and associated redistribution of energetic particles in phase space are investigated using kinetic-magnetohydrodynamic (MHD) hybrid simulations for a tokamak burning plasma with weakly reversed magnetic shear. The initial MHD equilibrium and plasma profile are those of ITER, which was studied in Todo and Bierwage (2014 Plasma Fusion Res. 9 3403068). Two types of kinetic-MHD hybrid simulations are performed and the results are compared. In the first type of simulation (KEP-MHD simulation), energetic particles are simulated using the gyrokinetic particle method, while in the second type (KEPTI-MHD simulation), both energetic particles and bulk ions are simulated using the gyrokinetic particle method. In the KEP-MHD simulation result, it is observed that multiple toroidal AEs (TAEs) with toroidal mode number n∼15 are the most unstable in the linear growth phase, while reversed shear AEs (RSAEs) with low-n dominate in the nonlinear phase. In the KEPTI-MHD simulation result, it is observed that beta-induced AEs with n∼15 have the largest linear growth rate, while TAEs with n∼10 dominate in the nonlinear phase. The redistribution of energetic particles is substantial and comparable between the two simulations. Redistribution of energetic alpha particles and beam deuterons in phase space is analyzed for the KEP-MHD simulation. In the early nonlinear phase, local flattening regions are formed around the resonances with the AEs. Staircase-like structures are formed along the E′=const. lines for the dominant AEs with amplitudes of radial MHD velocity normalized by the Alfvén velocity vr/vA∼4×10−4, where E′ is the conserved quantity of the wave-particle interaction. As the amplitudes of the AEs increase to vr/vA∼10−3, the resonance regions broaden and overlap each other leading to the formation of a single flattened region. In the KEP-MHD simulation, it is observed that the steep gradient regions in phase space formed by the multiple TAEs excite the low-n RSAE that transports energetic alpha particles near the plasma center. This process extends the flattened region of energetic alpha particle distribution to the plasma center. The stabilizing effect of thermal ions on AEs is demonstrated by analyzing redistribution in phase space for the KEPTI-MHD simulation.

Tritium yields in all six Large Helical Device deuterium experiment campaigns, or 41 064 discharges, have been numerically estimated. Usually, tritium yields are estimated from neutron yields by assuming that they are the same. In present fusion devices, this assumption is inaccurate because a fusion reaction between thermal-deuterons and fast-deuterons is dominant. In this paper, the energy distribution of fast-deuterons injected by neutral beam injectors is taken into account for the estimation of the ratio of the tritium yields to the neutron yields, Yt/Yn. By integrated simulation, the Yt/Yn is approximately 0.94 in each campaign. Because assumptions applied in the simulation aim to avoid under-estimation of the tritium yields compared to the actual value, the Yt/Yn should be 0.85

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