Fusion Enhancement from Bunched Ion Beams

Using a 1D PIC simulation, the potential for fusion enhancement through injection of an ion beam with density modulation (“bunching”) is demonstrated. The beam is composed of protons, and the background is composed of homogeneous deuterons and electrons in a uniform magnetic field, approximating the scrape-off-layer (β ≈ 0.1) in the FRC (field-reversed configuration) of the C-2U experiment. First, near-perpendicular angles of beam injection and wave propagation are identified as best for producing fast ions. This fast-ion tail is created by beam-driven Ion-Bernstein (IB) modes through a coherent Tajima-Dawson wakefield-like mechanism. The DD fusion reactivity is calculated from the deuteron velocity distribution and normalized with respect to the initial thermonuclear value to quantify fusion enhancement. Next, the fusion enhancement is calculated for an array of beam energies, revealing a power-law relationship. Finally, the beam is initialized with density bunching with wavelength matching that of the fastest-growing IB mode, and the scan over beam energies is repeated. The resulting fusion enhancement is found to exceed that of the un-bunched case by as much as a factor of ≈ 30 for roughly v_b < 32v_th, beyond which the mode structure evolves away from IB modes.