Wave-Driven Torques in Fluid and Oscillation Center Theories

The evaluation of ExB rotation drive by waves in laboratory plasmas requires calculating the torques induced by the waves. Such calculations are plagued with subtleties, and ensuring the self-consistency (and particularly momentum conservation) of the underlying theory is paramount to obtaining correct results. Recently, using a theory that treated nonresonant particles as a fluid and resonant particles kinetically, it was shown that there was an important difference between quasi-electrostatic plane waves which grow or decay in time, which do not apply a torque, and spatially structured waves, which do [1-3]. However, there are other, completely different approaches to calculating ponderomotive forces, which focus on the motion of the oscillation center, and which have their own momentum and energy conservation theorems. Thus, both theories should lead to the same conclusions—though this turns out to be far from straightforward to show. Here, we discuss the relationship between the two types of theory, showing the different subtleties that affect each in practical calculations, and demonstrate how the two theories can ultimately be shown to agree.

[1] Ochs, I. E. & Fisch, N. J. Physical Review Letters 127, 025003 (2021).

[2] Ochs, I. E. & Fisch, N. J. Physics of Plasmas 28, 102506 (2021).

[3] Ochs, I. E. & Fisch, N. J. Physics of Plasmas 29, 062106 (2022).