Physical nature of experimentally observed oscillations

Determine the physical nature of the oscillations observed in the SMOLa helical-mirror experiments, whose frequencies and longitudinal wavenumbers satisfy the experimentally established relationships associated with resonant interaction between rotating plasma and trapped ions.

Background

Experiments on the SMOLa device observed oscillations with frequencies much smaller than the ion-cyclotron frequency, including oscillations with several eigenfrequencies and a continuous frequency spectrum. For a wave proportional to $e^{ik_\|z+im\theta-i\omega t}$, the paper describes a resonance condition with trapped ions, $\omega=\omega_E(m+k_\|h/(2\pi))$, and notes that low-frequency oscillations may be unstable when $k_\|h\approx 2\pi m$.

Although the relationships between the observed oscillation frequencies and longitudinal wavenumbers had been established experimentally, the underlying physical mechanism responsible for the oscillations had not been identified. The paper studies ion-acoustic eigenmodes generated by plasma rotation and helical magnetic-field corrugation as a possible explanation, but it does not resolve the experimental question definitively.