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Inertial confinement fusion approaches involve the creation of high-energy-density states through compression. High gain scenarios may be enabled by the beneficial heating from fast electrons produced with an intense laser and by energy containment with a high-strength magnetic field. Here, we report experimental measurements from a configuration integrating a magnetized, imploded cylindrical plasma and intense laser-driven electrons as well as multi-stage simulations that show fast electrons transport pathways at different times during the implosion and quantify their energy deposition contribution. The experiment consisted of a CH foam cylinder, inside an external coaxial magnetic field of 5 T, that was imploded using 36 OMEGA laser beams. Two-dimensional (2D) hydrodynamic modelling predicts the CH density reaches [Formula: see text], the temperature reaches 920 eV and the external B-field is amplified at maximum compression to 580 T. At pre-determined times during the compression, the intense OMEGA EP laser irradiated one end of the cylinder to accelerate relativistic electrons into the dense imploded plasma providing additional heating. The relativistic electron beam generation was simulated using a 2D particle-in-cell (PIC) code. Finally, three-dimensional hybrid-PIC simulations calculated the electron propagation and energy deposition inside the target and revealed the roles the compressed and self-generated B-fields play in transport. During a time window before the maximum compression time, the self-generated B-field on the compression front confines the injected electrons inside the target, increasing the temperature through Joule heating. For a stronger B-field seed of 20 T, the electrons are predicted to be guided into the compressed target and provide additional collisional heating. This article is part of a discussion meeting issue 'Prospects for high gain inertial fusion energy (part 2)'.

Microwave diagnostics, such as reflectometry and electron cyclotron emission, are widely employed in tokamak fusion plasmas, and are also particularly well suited to the burning plasma environment expected in the upcoming ITER device. In existing fusion plasmas, reflectometry has been used to measure electron density profile, density fluctuations, turbulent flow and internal magnetic field strength. The measurement of electron density profile via reflectometry is a particularly high priority in ITER, and so an assessment of electromagnetic wave propagation is important in estimating measurement capability. In ITER, the ratio of plasma size to the required microwave signal wavelength is significantly larger than in current fusion experiments ensuring a more realistic analysis of reflectometry via ray-tracing techniques. The analytical and numerical studies presented in this paper highlight the fact that the group velocity ( or power flow) is strongly dependent on the direction of wave propagation relative to the magnetic field. It is shown that this dependence strongly modifies power flow near the cut-off layer in a manner that embeds the local magnetic field direction in the 'footprint' of the returned power at the launch antenna. It will be shown that this can potentially be utilized to determine the magnetic field pitch angle at the cut-off location. The resultant beam drift and distortion due to the magnetic field also have consequences on the design of reflectometry systems for large, high-field fusion experiments such as ITER.

A new approach to high pressure magnetically-confined plasmas is necessary to design efficient fusion devices. This Letter presents a new sort of equilibrium combining two solutions of the Grad–Shafranov equation, which describes the magnetohydrodynamic equilibrium in toroidal geometry. The outer equilibrium is paramagnetic and confines the inner equilibrium, whose strong diamagnetism permits to balance large pressure gradients. The existence of both equilibria in the same volume yields a dual equilibrium structure. This combination improves free-boundary mode stability.

High beta poloidal tokamaks can confine plasma pressures an order of magnitude higher than their low beta poloidal counterparts. The theoretical stability of these high beta poloidal magnetohydrodynamics equilibria was left unresolved for many years. Using modern computational tools, such configurations are now found stable to Mercier, resistive and high-n (ideal and resistive) ballooning criteria as well as fixed and free-boundary modes for a wide range of current density profiles in the framework of a low field large-aspect-ratio machine.

There is great interest in the properties of extremely high-β magnetohydrodynamic equilibria in axisymmetric toroidal geometry and the stability of such equilibria. However, few equilibrium codes maintain solid numerical behavior as beta approaches unity. The free-boundary algorithm presented herein utilizes a numerically stabilized multigrid method, current density input, position control, magnetic axis search, and dynamically adjusted simulated annealing. This approach yields numerically robust behavior in the spectrum of cases ranging from low to very high-β configurations. As the convergence time depends linearly on the total number of grid points, the production of extremely fine, low-error equilibria becomes possible. Such a code facilitates a variety of intriguing applications which include the exploration of the stability of extreme Shafranov shift equilibria.

The Electric Tokamak [R. J. Taylor, T. A. Carter, J.-L. Gauvreau et al., Nucl. Fusion 45, 1634 (2005)] operates at high plasma density (one and a half times the Greenwald limit) due to a strong particle pinch. However, particle accumulation causes several problems. The operation of the machine can suffer several violent disruptions which hinder the study of many plasma phenomena. Plasma motion and large density swings are undesirable because they alter continuous processes, leaving only transient regimes to study. Particle source and local temperature control can defeat the fundamental mechanisms of this electric pinch. If edge fueling feedback is not sufficient to induce quiescent behavior, the fast ion loss caused by second harmonic ion-cyclotron radio-frequency injection functions as a particle sink deep within the outer plasma cross-section. By linking these strong effects to the fueling feedback, stable medium density (2x10^18 particles/m3) plasmas can be sustained for several seconds. This new regime yields surprisingly long and calm discharges.

Density profile information is invaluable to plasma experiments. In particular, the edge and mantle plasma density profile is believed to influence core plasma behavior of fusion devices such as tokamaks. In low density plasmas, light emission caused by neutral hydrogen gas injection and subsequent excitation can be used to measure the local plasma parameters. A neutral penetration model is required to evaluate plasma density from H alpha light emission. This paper verifies that a simple collisionless three-dimensional neutral penetration model can be utilized in low-density, low-temperature plasmas. Since the H a emission intensity can be linked directly to the local electron density, this model permits the reconstruction of the density profile well beyond the last closed flux surface of the plasma.

Numerous methods exist to solve the Grad–Shafranov equation, describing the equilibrium of a plasma confined by an axisymmetric magnetic field. Nevertheless, they are limited to low beta or small plasma pressure. Combining a nonconservative variational principle with a contour dynamics approach, the approach presented in this paper converges for extreme high beta configurations. By reducing the dimension of the problem from two to one, a compact and efficient numerical algorithm can be developed, and a wide range of boundary shapes can be utilized. Furthermore, the iterative nature of this technique greatly facilitates convergence at high beta while minimizing computation times.