Table of contents

The effect of plasma shape on sawtooth oscillations in the DIII-D tokamak plasmas is investigated by comparing discharges with cross-sections shaped like a bean and an oval. The two shapes are designed so that the Mercier instability threshold is reached when the axial safety factor is below unity for the bean and above unity for the oval cross-sections. This allows the role of interchange modes to be differentiated from that of the kink-tearing mode. The differences in the nature of the sawtooth oscillations in the bean and oval discharges are found to be determined primarily by extreme differences in the electron heat transport during the reheat. In both cases, the axial safety factor is found to be near unity following the crash.

A review of the use of gamma ray spectrometry as a diagnostic of nuclear reaction rates and nuclear reaction product densities in high temperature fusion plasmas is presented. In this review we will discuss the historic genesis of the concept, a brief overview of the relevant nuclear physics, the experimental techniques utilized in the measurements and some of the analytical techniques required to extract the diagnostic information from the basic measurements. Of particular interest is the ability to measure the population of confined fast alpha particles in future burning plasma experiments.

The plasma particle source resulting from ionization by microwaves in the electron-cyclotron range of frequencies is investigated in the simple magnetized torus TORPEX (Fasoli A et al 2006 Phys. Plasmas13 055902), together with its dependence upon experimental parameters, such as the absorbed microwave power, the magnetic field configuration and the neutral gas pressure. The relative contributions to the ionization rate from thermal electrons and from suprathermal electrons accelerated at the plasma resonances are quantified through a global particle balance. An expression for the spatial profile of the particle source is derived from the experimental results and a simple numerical code.

The propagation of relativistic (fast) electrons produced by ultraintense laser irradiation into solid targets is crucial to important fields of laser–plasma study including Fast Ignitor ICF and ion acceleration. In current experiments, targets are initially at room temperature, and there is a need to determine the extent to which the solid–plasma transition affects fast electron transport. A Vlasov–Fokker–Planck code with ionization physics is used to simulate laser–solid interactions around 10¹⁸ W cm⁻² µm². Both field and collisional ionization physics is included, and the target is initially unionized. We find that both field and collisional ionization are important to the initial breakdown. After the initial breakdown, collisional ionization continues to ionize the target, and this affects the electric field structure. The effect of continual ionization is for the cold electron distribution to be non-Maxwellian, and for the ionization state to vary throughout the target. This will be important for 'interior' ion acceleration, magnetic field generation and the transverse structure of the fast electron beam.

A new data-set of outboard mid-plane scrape-off layer (SOL) heat flux widths, Δ_h, has been constructed for L-mode plasmas in the MAST spherical tokamak (ST). The scaling with key plasma parameters such as density, toroidal magnetic field, parallel connection length in the SOL and surface heat flux at the separatrix is investigated. An empirical scaling is developed for the Δ_h data-set, which exhibits a strong positive dependence on both the connection length (or edge safety factor) and density and weak or moderate inverse dependences on the surface heat flux and magnetic field, respectively. The empirical scaling is compared with earlier results for a range of tokamaks with conventional geometry, which show weaker dependence on the density and edge safety factor. Importantly, however, the weak negative dependence on the surface heat flux (and thus heating power) is common in both conventional and ST geometries. The experimental data are also used to test a number of dimensionally correct Δ_h scalings developed from theoretical models for perpendicular transport in the SOL coupled with classical transport parallel to the magnetic field. A scaling based on perpendicular transport driven by resistive MHD interchange provides the best fit, although several models are close. A subset of the better fitting theoretical scalings are used to extrapolate for Δ_h in one design for a future burning ST machine and finally to predict the peak heat loading on the outboard divertor target plate.

This paper investigates the effect of ion- and electron-induced electron emission from a material wall on the voltage drop across the adjacent plasma sheath ('plasma sheath voltage' (PSV)). For this purpose, a new model involving a collisionless kinetic sheath consistently coupled to a fluid presheath is developed. The underlying analysis is valid for plasmas (both magnetized and unmagnetized) in which the Debye length is much smaller than the relevant characteristic presheath length ('asymptotic two-scale limit'). Material boundaries of particular interest are first walls and divertor target plates bounding magnetically confined fusion plasmas. Majority and impurity ions accelerated from the bulk plasma towards the material boundary release electrons flowing back into the plasma, thus giving rise to a lower PSV than without electron emission. In addition, sufficiently fast electrons from the plasma impinging on the bounding wall produce secondary electrons and are also partially reflected. The present work represents a first step in which the unmagnetized case is considered and electron reflection at the wall is still neglected. Considering typical boundary–plasma conditions and characteristic particle-induced electron emission (PIEE) data (i.e. electron yields and energy distributions), the PSV is self-consistently calculated by means of the new sheath model, showing appreciable effects of PIEE.

The dispersion relation of azimuthal electromagnetic surface waves in a rod dielectric magnetized plasma waveguide is obtained. It is investigated that these waves in E-type can be excited by a thin annular relativistic rotating electron beam (TARREB). The effects of the rotating velocity of beam, radius TARREB, on the frequency spectra and growth-rate coefficient are presented.

Neutral particle dynamics is measured in the edge plasma of the TRIAM-1M tokamak by means of the Zeeman patterns in the spectral shape. This measurement can provide the emission position with high spatial resolution, as well as the local drift velocity and temperature. In the TRIAM-1M, the inward neutral flow velocity driven by the radial neutral pressure gradient is observed. The temporal evolution of the flow velocity suggests that the distance between the last closed flux surface and the first wall changes the recycling flux. Thereby, the neutral pressure gradient and the neutral flow velocity are varied. This technique can be an effective way for the measurement of the local neutral behaviour in the edge region.

Recent experimental results from the Trident laser confirm the importance of kinetic effects in determining laser reflectivities at high intensities. Examples observed include scattering from low frequency electron acoustic waves (EAWs) and the first few stages of a cascade towards turbulence through the Langmuir decay instability. Interpretive and predictive computational capability in this area is assisted by the development of Vlasov codes, which offer high velocity space resolution in high energy regions of particle phase space and do not require analytical pre-processing of the fundamental equations. A direct Vlasov solver, capable of resolving these kinetic processes, is used here to address fundamental aspects of the existence and stability of the electron acoustic wave, together with its collective scattering properties. These simulations are extended to realistic laser and plasma parameters characteristic of single hot-spot experiments. Results are in qualitative agreement with experiments displaying both stimulated Raman and stimulated electron acoustic scattering. The amplitude of simulated EAWs is greater than that observed experimentally and is accompanied by a higher phase velocity. These minor differences can be attributed to the limitations of a one-dimensional collisionless model.

The formation of eigenmodes with the m = 1 fast Alfvén waves in the ion-cyclotron range of frequency are investigated in the axisymmetric central cell of the GAMMA 10 tandem mirror. When the fast waves with frequencies near the fundamental ion-cyclotron frequency have been used for the plasma production, the saturation in the density has been observed. The spatial structure of the excited wave field is calculated in the central cell using a two-dimensional full wave code. The results of numerical analysis indicate that the increase in plasma density depends strongly on the eigenmode formations associated with the boundary conditions. The results of numerical analysis are compared with the results of measurements of the waves with magnetic probes. A very good degree of agreement is found between the theoretical results and the experimental results. It is suggested that the simultaneous excitation of several radial eigenmodes with high-harmonic fast waves is effective for higher density plasma production.

Molecular deuterium fluxes into the edge of deuterium-fuelled L-mode discharges are measured using passive visible spectroscopy of D₂ emission lines. Comparison with the atomic deuterium influx measured using D_α emission suggests that a significant fraction of the plasma edge fuelling from the walls is in the form of D₂. Molecular deuterium flux is observed in both the divertor and main chamber regions but is roughly a factor 100 smaller near the inner main chamber wall and roughly a factor 1000 smaller near the outer main chamber wall, when compared with the divertor region. Very high levels of molecular D₂ excitation are measured, with ground state D₂ rotational population temperatures T_rot up to 10 000 K and vibrational population temperatures T_vib up to 30 000 K. Comparisons between rotational population temperatures and the local electron density suggest that T_rot can be used as a reasonably good indicator of electron density in the D₂ line emission region. In recombining, detached divertor operation, estimates of the enhanced volume recombination rate due to the presence of vibrationally-excited D₂ suggest that the effect of molecular-assisted volume recombination could be comparable in magnitude to that of normal D⁺ volume recombination (EIR).

Electron beam hollowing in a plasma is investigated using an analytical, rigid beam model and two different hybrid codes in an attempt to explain observations of hollow plasma formations on the back of plastic targets in experiments carried out on the Vulcan terawatt laser. The relevance of the results to electron transport in fast ignition inertial confinement fusion is considered using dimensionless scaling parameters.

The paper investigates the momentum balance of a multi-species plasma in a cylinder with helical magnetic field. The main magnetic field is oriented in the z-direction. Starting from the macroscopic equations, a steady-state solution is obtained which includes the effect of centrifugal forces and Coriolis forces. The poloidal rotation and axial flow of each particle species is governed by the balance between spin-up, viscous damping, turbulent forces and the v × B driving term. In cylindrical geometry, classical viscous effects are very small and can be easily dominated by the turbulent viscosity. The effect of turbulent Reynolds stresses and the turbulent viscosity is investigated. Under quite general assumptions about the dependence of the anomalous transport coefficients on the velocity shear, criteria on the existence of the bifurcation points and multiple solution can be formulated. The shaping of the velocity shear and the transport barrier depend strongly on the eddy viscosity. Numerical solutions of the differential equations for a two-component plasma support the analytical results. The viscosity is considered as a variable parameter simulating the effect of turbulent eddy viscosity. These computations will be compared with the experimental results found in the HDH-mode experiments in Wendelstein 7-AS.

Fast protons can react with tritons in an endothermic nuclear reaction which can act as a source of neutrons in magnetically confined fusion plasmas. We have performed an experiment to systematically study this reaction in low tritium concentration (≈1%) plasmas in the Joint European Torus. A linear dependence is found between excess neutron rate and tritium concentration when the DT fusion rate is low. We discuss the properties of the neutron emission, including anisotropy, from the proton–triton reaction in a fusion reactor environment and derive simple models for the calculation of the neutron yield from this reaction in terms of tritium density, fast ion temperature and fast ion energy content.

Ideal MHD equations employed in the NOVA code are analysed analytically and numerically in order to investigate the role of the pressure gradient on global reversed shear Alfvén eigenmodes (RSAEs) or Alfvén cascades. We confirm both numerically and analytically conclusions obtained earlier using the ideal MHD code NOVA [1] and analytically [10] that the plasma pressure gradient plays a key role in the existence condition and in the dispersion relation for the mode. The effect of the plasma pressure gradient is to shift the mode frequency up at the low part of the RSAE frequency chirp and downshift the mode frequency when the frequency approaches the TAE gap. This finding is contrary to predictions in a recent publication [2], where the pressure gradient is found to be always stabilizing by means of downshifting the RSAE frequency and enhancing its interaction with the continuum. We resolve this discrepancy by showing that neglecting the pressure gradient effect on the plasma equilibrium (modification of the Shafranov shift and the averaged curvature) leads to conclusions at variance with the numerical and analytical results presented here. A new variational approximation of the RSAE is introduced which compares remarkably well with NOVA solutions. With this new approximation we clearly demonstrate the diagnostic potential and limitations of the RSAE frequency measurement for MHD spectroscopy.