Axial plugging is a critical challenge for fusion in open-ended magnetic confinement systems. Unlike simple magnetic mirrors, which suffer from direct axial flow, multi-mirror systems utilise a series of aligned magnetic cells to suppress plasma loss; however, the resulting confinement still requires additional plugging to reach Lawson criterion levels. In Miller et al. (Phys. Plasmas, 2023, vol. 30, p. 072510), it was found that applying a travelling and rotating electric field in multi-mirror machines can significantly suppress axial loss due to a selectivity effect induced by the Doppler shift of the ion cyclotron resonance. However, this method is energetically expensive and vulnerable to plasma screening effects. Here, we show that using a travelling, rotating magnetic field can achieve comparable plugging effectiveness while offering better penetration and lower energy costs. Two limiting scenarios, with and without an induced electric field, were considered. The confinement enhancement is calculated using a semi-kinetic rate-equation model, in which the rate coefficients are determined from single-particle simulations. While both scenarios exhibit significant confinement enhancement, the scenario without an induced electric field is much more energetically efficient, as it relies on phase-space mixing rather than on energy deposition in the escaping particles. The decoupling of confinement from plasma collisionality enables fusion conditions in the central cell while allowing affordable and efficient confinement enhancement in the multi-mirror sections.

Collisionless zonal flow (ZF) saturation is analytically investigated using a reduced two-field fluid model for ion temperature gradient-driven turbulence subject to a three-wavevector, five-mode truncation that includes the interaction of an unstable mode, the ZF and a stable mode. Using weak-turbulence closure theory, a wave kinetic equation (WKE) is derived describing the temporal evolution of the ZF energy spectrum for this system according to linear and nonlinear dynamics. The terms in the nonlinear time evolution operator, which describe resonant energy transfer among triads of fluctuations, are expressed as matrices whose elements are arithmetic combinations of the linear eigenvalues of the individual modes participating in a given interaction. In the collisionless limit, the matrices characterising ZF drive and damping become highly symmetric, and conditions are found for spectral saturation in this regime. A set of stationary solutions to the WKE are found that describe a state of turbulence in which the nonlinearly driven stable modes reach identical levels as the linearly driven unstable modes, producing identical rates of up- and down-gradient thermal energy transport at each fluctuation length scale. Generalisation to the non-truncated system is briefly discussed.

Ion energy–angle distributions (IEADs) at material surfaces are a critical input for plasma–material interaction (PMI) studies in fusion devices, yet they are computationally expensive to obtain using particle-in-cell (PIC) simulations. In this work, we develop a machine learning surrogate based on a deep deconvolutional neural network (DDeCNN) trained on large databases generated with the hPIC2 code. The surrogate is capable of reconstructing IEADs from sheath parameters for both thermal and radio-frequency (RF) plasmas, including cases with multiple ion species. Across thousands of test cases, the model achieves high accuracy, with over 97 % of predictions classified as good or average based on standard error metrics (MAE, MSE, L2). Even in the more challenging RF and multi-species regimes, the surrogate reliably captures the multi-peak structure of PIC results. Once trained, the surrogate produces IEADs in milliseconds on a common workstation, yielding speedups of six to seven orders of magnitude compared with running a full PIC simulation. This computational gain enables dense parameter scans and direct coupling of IEAD predictions with PMI and erosion models on whole-device scales in fusion-relevant conditions.

In this work, ARC and SPARC equilibria with randomly generated coil misalignments are analysed using the metric of quasisymmetry error, frequently used in stellarator theory as a proxy for fast ion confinement, and a strong correlation between this metric and ripple-induced alpha power loss is found. In discussions of magnetic confinement devices, it is often taken for granted that tokamaks, unlike stellarators, are devices with axisymmetric magnetic fields. However, any tokamak built from discrete toroidal field (TF) coils will have non-axisymmetric components of the magnetic field due to the effect of TF ripple, and these components only grow when inevitable assembly errors are taken into account. Physically constructed tokamaks therefore have magnetic fields which can best be classified as near axisymmetric, and which share many characteristics with quasi-axisymmetric stellarators, allowing both classes of devices to be analysed using similar methods.

MIDAS-1D2V is a plasma simulation code for axially symmetric open traps. It solves the non-stationary kinetic equation for the ion distribution function, accounting for Coulomb collisions. This paper presents two key modifications to the model. The first modification enables the calculation of fusion reactions in the plasma, with charged reaction products treated as an additional source term in the kinetic equation. The second modification incorporates beam capture dynamics, including ionisation and charge-exchange processes. Both modifications have been implemented for hydrogen isotopes.

The non-perturbative guiding-centre model provides an exact alternative to full-orbit simulations of charged particle dynamics in situations where traditional guiding-centre theory may fail. We demonstrate that the charged particle motion in a homogeneous, time-varying magnetic field is a solvable example of the non-perturbative guiding-centre model. This entails showing that the exact magnetic moment of Qin and Davidson can be constructed to be asymptotic to the adiabatic invariant series of Kruskal. In contrast to the perturbative invariant, the exact invariant contains information about parametric resonances. These resonances destroy the conservation of the usual magnetic moment over very long times. This refutes some previous claims about the all-time invariance of the magnetic moment.

The space of high-beta, approximately quasiaxisymmetric, large-aspect-ratio stellarator configurations is explored using an inverse coordinate approach and a quadratic polynomial ansatz for the flux function, following the method of Palumbo, extended by Hernandes and Clemente. This approach yields a system of nonlinear ordinary differential equations (ODEs) that, when solved, give equilibria exhibiting positive or negative triangularity, cusps and (in an extreme limit) current singularities. It is shown that a cubic ansatz may also be used, but that polynomials of degree four or higher will lead to overdetermination.

We present a data-driven plasma fluid closure for both linear and nonlinear electrostatic Landau damping in one dimension. A Fourier neural operator (FNO) is trained online within a differentiable fluid solver, with the loss computed on trajectories produced by the closed fluid simulation rather than on individual kinetic snapshots. The closure is non-Markovian, acting on a trailing window of the resolved-moment history so as to represent the memory of the unresolved dynamics. We demonstrate that a single FNO trained in this way reproduces both linear and nonlinear Landau damping, generalises to unseen initial perturbation amplitudes lying between those used in training, and remains numerically stable when deployed in independent fluid simulations. In the nonlinear regime the learned heat flux reproduces the resolved-moment dynamics without matching the kinetic heat flux pointwise, behaving as an effective closure that carries the influence of the unresolved dynamics through the resolved-field history, though the learned specific flux is expected to depend on the numerical scheme and training data. A sensitivity analysis of the trained model shows that it computes a genuine moment-to-flux relation whose reliance on the memory window is structured rather than uniform, concentrating at lags spaced by the oscillation period.

Momentum transport caused by spatial channelling – spatial transport of the energy of energetic ions by eigenmodes destabilised by these ions – is considered. The torques that result from the resonant interaction of trapped and passing energetic ions with an eigenmode are calculated and compared. The shearing rate of the toroidal flow that arises when an eigenmode transports toroidal angular momentum from the excitation region to the damping region is evaluated. The possibility that this flow can contribute to turbulence suppression is discussed.

Using a two-fluid approach, we consider the properties of relativistically nonlinear (arbitrary $a_0$), circularly polarised electromagnetic waves propagating along a magnetic field in electron–ion and pair plasmas. Dispersion relations depend on how wave intensity scales with frequency, e.g. $a_0 (\omega )$. For superluminal branches, the nonlinear effects reduce the cutoff frequency, while the general form of the dispersion relations $\omega (k)$ remains similar to the linear case. For subluminal waves, whistlers and Alfvén, a new effect appears: dispersion curves effectively terminate at finite $\omega ^\ast {-} k^\ast$, where the group velocity becomes zero. Qualitatively, subluminal modes with fluctuating electric field larger than the guide field, $E_w (\omega ) \geqslant B_0$, cannot propagate. In extended systems, e.g. within magnetospheres of neutron stars, this leads to opening of the magnetosphere by a strong wave.

By a detailed comparison of leaky magnetohydrodynamic waves in coronal magnetic flux tubes with leaky electromagnetic waves in dielectric media, it is shown that the latter kind may be called quasi-normal modes, since they can be regularised by a normalisation which systematically cuts off the contribution of the external homogeneous region, whereas such a possibility is forbidden for the former kind by the conservation of magnetic flux. Consequently, leaky magnetohydrodynamic waves cannot be systematically applied to coronal seismology, i.e. to the inverse spectral problem of determining the different equilibrium distributions of the fields by comparing the spectra they produce with the observed ones.

The solar wind is observed to undergo substantial heating as it expands through the heliosphere, with measured temperature profiles exceeding those expected from adiabatic cooling. A plausible source of this heating is reflection-driven turbulence (RDT), in which gradients in the background Alfvén speed partially reflect outward-propagating Alfvén waves, seeding counter-propagating fluctuations that interact and dissipate via turbulence. Previous RDT models assume a radial-background magnetic field, but at larger radii the interplanetary field is known to be twisted into the Parker spiral (PS). Here, we generalise RDT phenomenology to include a PS, using three-dimensional expanding-box magnetohydrodynamic simulations to test the ideas and compare the resulting turbulence with the radial-background-field case. We argue that the underlying RDT dynamics remains broadly similar with a PS, but the controlling scales change: as the azimuthal field grows it ‘cuts across’ perpendicularly stretched, pancake-like eddies, producing outer scales perpendicular to the magnetic field that are much smaller than in the radial-background case. Consequently, the outer-scale nonlinear turnover time increases more slowly with heliocentric distance in PS geometry, weakening the tendency (seen in radial-background models) for the cascade to ‘freeze’ into quasi-static, magnetically dominated structures. This allows the system to dissipate a larger fraction of the fluctuation energy as heat, also implying that the turbulence remains strongly imbalanced (with high normalised cross-helicity) out to larger heliocentric distances. We complement our heating results with a detailed characterisation of the turbulence (e.g. spectra, switchbacks and compressive fractions), providing a set of concrete predictions for comparison with spacecraft observations.

In stellarators, achieving effective divertor configurations is challenging due to the three-dimensional nature of the magnetic fields, which often leads to chaotic field lines and separatrices surrounded by a thick stochastic layer. This work presents a novel approach to directly optimise modular stellarator coils for a sharp X-point divertor topology akin to the Large Helical Device’s (LHD) helical divertor using a target plasma surface with sharp corners. By minimising the normal magnetic field component on this surface, we construct a clean separatrix with minimal chaos. Notably, this approach demonstrates the first LHD-like helical divertor design using optimised modular coils instead of helical coils. Separatrices are produced with significantly lower chaos than in LHD, demonstrating that a wide chaotic layer is not intrinsic to the helical divertor. Additional optimisation methods are implemented to improve engineering feasibility of the coils and reduce chaos, including weighted quadrature and manifold optimisation, a method which does not rely on normal field minimisation. The results outline several new strategies for divertor design in stellarators, although it remains to achieve these edge divertor features at the same time as internal field qualities like quasisymmetry.

An overview of new experimental results obtained at the GOL-NB multiple-mirror trap over the past two years is presented. The main scientific objectives of GOL-NB are the direct demonstration of the multiple-mirror confinement efficiency with hydrogen plasma. The device has a configuration that simulates, at reduced plasma parameters, the configuration of a reactor-class facility with a central gas dynamic trap and sections with a strong magnetic field attached to it, which can be turned on both as long solenoids or as multiple mirrors. Plasma is heated by 1 MW neutral beam injection. The following elements are discussed in the paper: magnetohydrodynamic (MHD) stability, plasma parameters in the central trap, neutral beam trapping and fast ion spectrum, comparison of the multiple-mirror and solenoidal configurations at high collisionality, and measures for the reduction in neutral density. The main goals of the experiments were plasma properties characterisation in the central trap and studies of plasma flow in the strong-field sections. We also introduce two new systems for additional plasma heating, a low-energy electron beam mounted at the exit plasma receiver and a 13.56 MHz ICRH system, which is under tests at several kilowatts of input power.

The Gas Dynamic Trap (GDT) facility at the Budker Institute of Nuclear Physics continues experimental studies in support of the Gas-Dynamic Multiple-Mirror Trap (GDMT) project – a prospective neutron source and thermonuclear reactor concept based on open-ended magnetic confinement. During the 2024–2025 upgrade campaign, key engineering systems and diagnostic suites were substantially modernised to address critical physics and technology challenges for the GDMT development. Three key research and development directions were pursued. First, a conductive wall stabilisation system was designed to suppress magnetohydrodynamic (MHD) instabilities at high plasma beta. Second, a new second-harmonic X-mode electron cyclotron resonance heating (ECRH) system operating at 54.5 GHz with 0.8 MW power was commissioned. Preliminary experiments demonstrated increased electron temperature compared with non-ECRH heated plasmas. Third, proof-of-principle experiments on bulk plasma fuelling using a Marshall gun injector were successfully performed, supporting further development towards a high-repetition-rate injector. These results provide essential experimental validation and technological groundwork for the GDMT project, demonstrating progress towards high-beta plasma operation, efficient heating schemes and particle balance control in open magnetic traps.