Page 5 - Demo

800 peaked 700
1.8 1.5
1.2
1.0 flat
0.8 0.6 0.4 0.2 0.0
0.0 0.5 1.0 1.5 2.0 velocity (106 m/s)
1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0
01234 t (ms)
FIG. 6. The magnetic fluctuations (top right) and neutron rates (bot- tom) are compared from discharges with ”peaked“ and ”flat“ initial NB velocity distributions (top left).
integral is performed over regions of positive gradient. For the peaked case, α = 2.5 and in the flat case, α = 1.8, a reduction of 1.4x.
The result of the experiment is summarized in Figure 6. Shown in the upper left panel are the calculated initial velocity distributions for the 2 cases. It can be seen that in the ”peaked” case (black curve), larger positive gradients are present than in the “flat” (or less peaked) case (red curve). The magnetic fluc- tuation spectrograms are compared in the upper right panel. The “peaked” case clearly has more activity in the ion cy- clotron frequency band (in both cases, the fluctuations die out around t = 3 ms). Finally, in the bottom panel the measured neutron rate in the two cases are compared. The fusion en- hancement is larger in the peaked case, by a factor of almost 2.
SUMMARY AND CONCLUSION
We have shown that in C-2U, the injection of high power hydrogen neutral beams results in an increase of the fusion re- action rate over the expected thermonuclear rate. The rise in reactivity occurs on a time scale much shorter than the classi- cal collisional time scale and is sustained for several ion slow- ing down times. This fusion enhancement is due to a tail gen- erated in the energy distribution of the bulk deuterium plasma and is often accompanied by increased magnetic and density fluctuations near the ion cyclotron frequency. These observa- tions are consistent with a model of ion acceleration due to a large amplitude beam-driven ion Bernstein wave. This would be, to our knowledge, the first observation of ion acceleration from a beam-driven mode in a magnetic fusion experiment, made possible by the unique geometry of the advanced, beam- driven FRC.
Finally, we emphasize that perhaps the most significant as- pect of this phenomenon is that which is not observed - the de-
struction of the plasma. A common criticism of beam-based fusion concepts is that since fast particle populations are a source of free energy, any plasma which relies too heavily on them will be driven unstable. The robustness of the C-2U plasma during intense neutral beam drive is a counter exam- ple to this prevailing wisdom, and is understood here in terms of the phase space separation of the driven wave and the bulk plasma.
ACKNOWLEDGEMENTS
We thank our investors for their support of Tri Alpha En- ergy and the TAE and Budker teams for their contributions to this project. Special thanks to Erik Granstedt and Erik Trask for help with the design of the experiment. This research used resources of the Oak Ridge Leadership Computing Facility, which is a DOE Office of Science User Facility supported un- der Contract DE-AC05-00OR22725.
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