Recent advances in stellarator optimisation have found nearly precise quasisymmetric configurations. These are expected to reduce the non-turbulent background plasma transport to acceptable neoclassical levels while removing nearly all collisionless direct orbit losses of alpha particles. Yet, alpha particles under resonant conditions can be very sensitive to collisions, causing concerning energy losses and damaging plasma facing components. For the passing alphas such resonances can happen near rational surfaces in the presence of helical error field departures from quasisymmetry that change the magnetic field direction and magnitude. The cancellation between streaming motion and tangential drift of the alphas enhances the effective collision frequency, allowing the formation of a collisional boundary layer and giving rise to a perturbed distribution function. We develop an analytical model to illustrate and evaluate the resonant plateau transport this mechanism causes by formulating a drift kinetic treatment. The results indicate the associated particle and energy losses can become significant in the vicinity of rational surfaces at values of $q=m/n$ when error fields with poloidal and toroidal numbers $m$ and $n$ are present. In addition, we investigate the validity of the quasilinear approximation to the energy flux to show that it imposes a restriction on the error field amplitude that can be considered.

A recent paper (Hazeltine et al. 2026 Phys. Plasmas, vol. 33, 072501) has questioned whether the standard result, (ultimately) due to Ferraro, that the plasma angular velocity is approximately constant along individual equilibrium magnetic field lines in a rotating axisymmetric magnetic mirror machine, continues to hold when the rotation becomes sonic or supersonic. In order to resolve this issue, the equilibrium of a rapidly rotating mirror is investigated, starting from first principles, using an ideal two-fluid model with anisotropic pressure. It is found that, as long as the ion gyro-radius is much less than the machine size, and the angular velocity of the plasma is much less than the ion gyro-frequency, the Ferraro result holds good.

Kinetic instabilities, notably the whistler heat-flux instability (WHFI), are known to suppress thermal transport significantly in the moderate- to high-$\beta$ plasmas relevant to many astrophysical systems. This paper explores WHFI-regulated heat transport in a new regime: ultra-high-$\beta$ plasmas with $\beta _{e} \gtrsim L_{\mathrm{T}}/\rho _e$ (where $\beta _e$ is the ratio of electron thermal to magnetic pressure, $\rho _{e}$ is the electron Larmor radius and $L_{\mathrm{T}}$ is the macroscopic temperature-gradient length scale). Extrapolating previous theories of the WHFI to ultra-high-$\beta$ plasmas, we propose that the magnetic energy in unstable whistler fluctuations becomes comparable to that of the background magnetic field at saturation. We corroborate this hypothesis with kinetic simulations using the particle-in-cell code OSIRIS in one spatial dimension and three velocity dimensions (1D3V) and two spatial dimensions and three velocity dimensions (2D3V). We also find that, in ultra-high-$\beta$ plasmas, the heat flux is localised and no longer regulated primarily by resonant pitch-angle scattering of electrons; instead, thermal energy is transported predominantly by advection at the phase velocity of energetically dominant whistlers. Heat-flux suppression is observed in both 1D3V and 2D3V simulations; however, we show that the saturation of the WHFI and the regulation of heat flux are sensitive to dimensionality in the ultra-high-$\beta$ regime. Both the amplitude and phase velocity of the heat-flux-regulating whistler waves scale differently with $\beta _e$, yielding parallel heat fluxes, normalised to the free-streaming value, of $q_{e\parallel } / q_{\mathrm{fs}} \approx 4.7 \beta _{e}^{-1}$ and $q_{e\parallel } / q_{\mathrm{fs}} \approx 0.3 \beta _{e}^{-1/2}$ in 2D3V and 1D3V simulations, respectively. We also perform 2D3V simulations with background magnetic fields inclined to the temperature gradient, showing that perpendicular heat transport remains negligible in ultra-high-$\beta$ plasmas; this indicates that anomalous diffusive processes contribute negligibly to cross-field transport. We develop an heuristic theory from kinetic equations that explains these phenomena. Our work extends our understanding of how the WHFI modifies thermal transport to regimes applicable to high-energy-density physics and the reionised intergalactic medium.