Comments on the Paper “Fundamental Scaling of Adiabatic Compression of Field Reversed Configuration Thermonuclear Fusion Plasmas”

Abstract In a recent paper Kirtley and Milroy [D. Kirtley, R. Milroy, J. of Fusion Energy (2023) 42:30] discuss the Helion concept for a pulsed reactor, in which two supersonic field-reversed configurations (FRCs) merge and the resulting plasmoid is adiabatically compressed to fusion conditions. Using D-He3 as fuel this would allow for efficient electromagnetic energy conversion during the subsequent expansion phase. Essential for their very promising projections of energy gain is the assumed large temperature ratio between ions and electrons ( $$\:{T}_{i}/{T}_{e}\approx\:10$$ ) resulting initially from the supersonic plasmoid collision and the following adiabatic compression. For reactor-relevant conditions, however, the collisional ion to electron power transfer exceeds the fusion power by more than an order of magnitude at $$\:{T}_{i}$$ =100 keV (and by over two orders at $$\:{T}_{i}$$ =40 keV). Even if this exchange does not change total pressure, it rapidly erodes $$\:{T}_{i}/{T}_{e}\gg\:1$$ and causes the time-integrated fusion energy yield to fall well below values inferred from the initial, non-equilibrated state. While this does not preclude a D-He3 FRC reactor our analysis implies much tighter requirements on plasma lifetime, anomalous losses, and direct-conversion efficiency than suggested in D. Kirtley, R. Milroy, J. of Fusion Energy (2023) 42:30.