We present a simple, analytically solvable magnetohydrodynamics model of current sheet formation through X-point collapse under optically thin radiative cooling. Our results show that cooling accelerates the collapse of the X-point along the inflows, but strong cooling can arrest or even reverse the current sheet elongation in the outflow direction. Hence, we detail a modification to the radiatively cooled Sweet–Parker model developed by Uzdensky & McKinney (Phys. Plasmas, 1962, vol. 18, issue 4, p. 042105) to allow for varying current sheet length. The steady-state solution shows that, when radiative cooling dominates compressional heating, the current sheet length is shorter than the system size, with an increased reconnection rate compared with the classical Sweet–Parker rate. The model and subsequent results lay out the groundwork for a more complete theoretical understanding of magnetic reconnection in regimes dominated by optically thin radiative cooling.

The near-axis description of optimised stellarator fields has proven to be a powerful tool both for the design and understanding of this magnetic confinement concept. The description consists of an asymptotic model of the equilibrium in the distance from its centremost axis, and is thus only approximate. Any practical application therefore requires the eventual construction of a global equilibrium. This paper presents a novel way of constructing global equilibria using the DESC code that guarantees the correct asymptotic behaviour imposed by a given near-axis construction. The theoretical underpinnings of this construction are carefully presented, and benchmarking examples provided. This opens the door to an efficient coupling of the near-axis framework and that of global equilibria for future optimisation efforts.

We detail here a semi-analytical model for the pellet rocket effect, which describes the acceleration of pellets in a fusion plasma due to asymmetries in the heat flux reaching the pellet surface and the corresponding ablation rate. This effect was shown in experiments to significantly modify the pellet trajectory, and previously projected deceleration values of ${\sim} 10^6\,\textrm {m}\,\textrm{s}^{-2}$ for reactor-scale devices indicated that it may severely limit the effectiveness of pellet injection methods. We account for asymmetries stemming both from plasma parameter gradients and an asymmetric plasmoid shielding caused by the drift of the ionised pellet cloud. For high temperature, reactor relevant scenarios, we find a wide range of initial pellet sizes and speeds – particularly those relevant for large fragments of shattered pellet injection for disruption mitigation – where the rocket effect has a major impact on the penetration depth. In these cases, the plasma parameter profile variations dominate the rocket effect. We find that for small and fast pellets, where the rocket effect is less pronounced, plasmoid shielding-induced asymmetries dominate.

To better understand the dependence of the magnetic field structure in the plasma edge on the plasma boundary shape, in the context of X-point and island divertor designs, we define and develop a class of stellarators called umbilic stellarators. These equilibria are characterised by a single continuous high-curvature edge on the plasma boundary that goes around multiple times toroidally before meeting itself. We develop a technique that allows us to simultaneously optimise the plasma boundary along with a curve lying on the boundary on which we impose a high curvature while imposing omnigenity – a property of the magnetic field that ensures trapped particle confinement throughout the plasma volume. We find that umbilic stellarators naturally tend to favour piecewise omnigenity instead of omnigenity with a specific helicity. After generating omnigenous umbilic stellarators, we design coil sets for some of them and explore the fieldline structure in the edge and its sensitivity to small fluctuations in the plasma. Finally, using single-stage optimisation, we simultaneously modify the plasma and coil shape and propose an experiment to modify an existing tokamak to a finite-$\beta$ stellarator using this technique and explore a potentially simpler way to convert a limited tokamak into a diverted stellarator.

This study investigates the feasibility of reconstructing the last closed flux surface in the DIII-D tokamak using neural network models trained on reduced input feature sets, addressing an ill-posed task. Two models are compared: one trained solely on coil currents and another incorporating coil currents, plasma current and loop voltage. The model trained exclusively on coil currents achieved a mean point displacement of $0.04$ m on a held-out test set, while the inclusion of plasma current and loop voltage reduced the error to $0.03$ m. This comparison highlights the trade-offs between input feature complexity and reconstruction accuracy, demonstrating the potential of machine learning algorithms to perform effectively in data-limited environments, such as those expected in fusion power plants due to diagnostic constraints imposed by the presence of blankets and shielding.

This work demonstrates that magnetohydrodynamic (MHD) stable, quasi-isodynamic (QI) stellarator equilibria with reduced turbulence can be generated with an optimised coilset. We present one such equilibrium which, when being generated by coils, maintains the benefits of its excellent QI quality (low neoclassical transport at small particle collisionality net toroidal current and good fast-particle confinement) while demonstrating ideal-MHD stability and lower ion-temperature-gradient-driven turbulent heat flux than W7-X. As a consequence of its optimised rotational transform profile, this plasma equilibrium has nested flux surfaces and a chain of large islands at the plasma’s edge, for which we present an island divertor design. It additionally features an electron root – a large region in the plasma core in which the radial electric field points outwards, towards the plasma boundary – which provides a potential solution for preventing impurity accumulation in a fusion device.

An efficient novel mechanism of laser pulse focusing with the help of a shaped underdense plasma target immersed in an inhomogeneous magnetic field has been demonstrated. These studies have been carried out with the help of two-dimensional particle-in-cell simulation employing the OSIRIS 4.0 platform. It is shown that the divergent magnetic field profile compresses the electromagnetic wave pulse in the transverse direction. A comparative investigation with plane and lens-shaped plasma geometries has also been conducted to find an optimal configuration for focusing the laser at the desirable location. Furthermore, it is also demonstrated that, when the electron cyclotron resonance layer is placed at a suitable location where the laser is focused, a highly energetic electron beam is generated.

This study makes use of plasma-profile data from the EUROfusion pedestal database (Frassinetti et al. 2020 Nucl. Fusion vol. 61, p. 016001), focusing on the electron-temperature and electron-density profiles in the edge region of H-mode ELMy JET ITER-Like-Wall (ILW) pulses. We make systematic predictions of the electron-temperature pedestal, taking engineering parameters of the plasma pulses and the density profiles as inputs. We first present a machine-learning (ML) algorithm which, given more inputs than theory-based modelling, is able to reconstruct unseen temperature profiles within $20\,\%$ of the experimental values. We find a hierarchy of the most consequential engineering parameters for such predictions. This result confirms the conceptual possibility of accurate data-driven prediction. Next, taking a simple theoretical approach that assumes a definite local relationship between the electron-density ($R/L_{n_e}$) and electron-temperature ($R/L_{T_e}$) gradients, we find that a range of power-law scalings $R/L_{T_e}=A(R/L_{n_e})^\alpha$ with $\alpha\approx 0.4$ correctly capture the behaviour of the electron-temperature in the steep-gradient region. Fitting $A$ and $\alpha$ independently for each pedestal reveals a clear one-to-one correlation, suggesting an underlying constraint in pedestal physics. The measured $\eta_e = L_{n_e}/L_{T_e}$ values across the pedestal exhibit a wide distribution, significantly exceeding the slab-ETG linear stability threshold, implying either a non-linear threshold shift or a measurably supercritical saturated turbulent state. Finally, we fit parameters for scalings that relate the turbulent heat flux to the gradients $R/L_{T_e}$ and $R/L_{n_e}$, similarly to models extracted from gyrokinetic simulations. The inclusion of more experimental parameters is necessary for such models to match the accuracy of our ML results.

Based on the High Magnetic field Helicon eXperiment (HMHX), considering parabolic distribution and Gaussian distribution of radial plasma density, the HELIC code was used to study the parameter optimisation design of a helicon wave plasma (HWP) source. Some parameters (antenna type, radio frequency, discharge gas, plasma radius, magnetic field) were selected. The results show that a half-helix antenna is the excitation antenna and a frequency of 13.56 MHz is the most commonly used power source for HMHX. Argon and nitrogen are selected as discharge gases to achieve the best effect of power deposition. In order to realise hydrogen–HWP discharge, a new antenna with plasma radius of 10.5 mm and antenna radius of 13.5 mm can be designed. For the new antenna, when the magnetic field intensity is 1000 Gs, the best discharge effect can be achieved. The results of this paper can provide guidance for the design of a plasma source for HWP discharge under different conditions in the future.