Table of contents

An analytical approach is developed for assessing the confinement of fusion alpha particles born in the plasma core of a tokamak reactor with a toroidal current hole (CH). Confinement criteria determining the minimum plasma current requirements with respect to the alpha particle energy are derived. It is shown that the enhancement of first orbit alpha losses induced by a CH of a radius r_* can be recompensed by an increase of the total plasma current by the factor , where a is the minor plasma radius. Numerical modelling of alpha losses in an equilibrium magnetic configuration reconstructed for a high-performance JET CH plasma validates the analytical results.

Anomalous momentum transport has been observed in Alcator C-Mod tokamak plasmas. The time evolution of core impurity toroidal rotation velocity profiles has been measured with a tangentially viewing crystal x-ray spectrometer array. Following the L-mode to EDA (enhanced D_α) H-mode transition in both Ohmic and ion cyclotron range of frequencies heated discharges, the ensuing co-current toroidal rotation velocity, which is generated in the absence of any external momentum source, is observed to propagate in from the edge plasma to the core with a timescale of the order of the observed energy confinement time, but much less than the neo-classical momentum confinement time. The ensuing steady state toroidal rotation velocity profiles in EDA H-mode plasmas are relatively flat, with V_ϕ ∼ 50 km s⁻¹, and the momentum transport can be simulated using a simple diffusion model. Assuming that the L–H transition produces an instantaneous edge source of toroidal torque (which disappears at the H- to L-mode transition), the momentum transport may be characterized by a diffusivity, with values of ∼0.07 m² s⁻¹ during EDA H-mode and ∼0.2 m² s⁻¹ in L-mode. These values are large compared to the calculated neo-classical momentum diffusivities, which are of the order of 0.003 m² s⁻¹. Velocity profiles of ELM-free H-mode plasmas are centrally peaked (with V_ϕ(0) exceeding 100 km s⁻¹ in some cases), which suggests the presence of an inward momentum pinch; the observed profiles are consistent with simulations including an edge inward convection velocity of ∼10 m s⁻¹. In EDA H-mode discharges which develop internal transport barriers, the velocity profiles become hollow in the centre, indicating the presence of a negative radial electric field well in the vicinity of the barrier foot. Upper single null diverted and inner wall limited L-mode discharges exhibit strong counter-current rotation (with V_ϕ(0)∼−60 km s⁻¹ in some cases), which may be related to the observed higher H-mode power threshold in these configurations. For plasmas with locked modes, the toroidal rotation is observed to cease (V_ϕ ⩽ 5 km s⁻¹).

Free boundary MHD equilibrium properties of Heliotron J are investigated by VMEC, HINT and PIES codes, and ideal MHD stability properties are studied by the Mercier criterion, the ballooning mode equation and the CAS3D global stability code. It is shown by the equilibrium calculations that the change of the plasma boundary shape is substantial in a low shear helical system even if β is relatively low. Preliminary comparison between PIES results and HINT results shows that the β value at which the magnetic island begins to be perceptible is almost the same in both codes, but the island width seems to be different. It is considered that the difference comes from a difference of pressure distribution in real space which is determined through each numerical algorithm. In the stability analysis, the effect of the global magnetic shear on the three-dimensional or helical ballooning mode whose mode structure shows strong poloidal and toroidal mode (helical mode) coupling is investigated. It is found that the positive shear of the rotational transform is favourable for the three-dimensional ballooning mode stability in a low shear helical system.

In high current pulse discharges of the plasma focus (PF) type, inside the collapsing pinch column, there are formed local micro-regions of high-density and high-temperature plasma, so-called hot spots. Individual hot spots are separated in space and time. Each hot spot is characterized by its specific electron concentration and temperature, as well as by the emission of x-ray lines with different polarization. When numerous hot spots are produced it is impossible to determine local plasma parameters and to interpret the polarization effects. To eliminate this problem this study was devoted to the realization of PF-type discharges with single hot spot only. It has been achieved by a choice of the electrode configuration, which facilitated the formation of a single hot spot emitting intense x-ray lines. At the chosen experimental conditions it was possible to determine local plasma parameters and to demonstrate evident differences in the polarization of the observed x-ray lines.

Experiments of simultaneous injection of LHWs and IBWs into plasmas have been conducted in the HT-7 tokamak. Evidence that IBW can control the plasma pressure profile was observed in LHCD plasmas. It was found that electron Landau damping (ELD) of IBWs occurred around the maxima of parallel refractive index, n_||, and that the ion cyclotron resonant layer can enhance off-axis LHW power deposition and create a localized fast electron current channel. But the global LHCD efficiency during an IBW pulse was not improved obviously. As a result of the localization of LHCD during an IBW pulse, the global current density profile was modified, which was indicated by the internal inductance, l_i. Optimization of this operation mode utilizing the features of the interaction of IBWs and LHCD provides a new way of obtaining a high performance plasma under steady-state conditions.

In this paper, the effect of electron–electron (e–e) collisions on second harmonic electron cyclotron resonance plasma breakdown is studied. A previously developed kinetic model for stellarators has been completed by means of a collision operator derived from the Fokker–Planck equation. It is shown that, although e–e collisions are not important for initial growth rate predictions and play a minor role in the breakdown time calculation, they must be considered if a realistic evolution of the electron energy distribution function is desired. Moreover, it is seen that the introduction of the e–e collision processes is needed in order to obtain a correct description of the power balance during breakdown.

We investigate a classical analogue of electromagnetically induced transparency (EIT) currently popular in quantum electronics. We consider EIT for electron cyclotron waves in a finite-temperature plasma. We derive an expression for the effective refractive index of an electromagnetic wave and study the dispersion and absorption of this wave under EIT conditions. Allowance for thermal motion is shown to change radically the behaviour of the dispersion curves for the probe wave in the EIT region compared with the case of a cold plasma. The influence of possible parametric instability on the EIT effect is discussed.

Supersonic molecular beam injection (SMBI) was first proposed and demonstrated on the HL-1 tokamak and was successfully developed and used on HL-1M. Recently, new results of SMBI experiments were obtained by increasing the gas pressure from 0.5 to over 1.0 MPa. A stair-shaped density increment was obtained with high-pressure multi-pulse SMBI that was similar to the density evolution behaviour during multi-pellet injection. This demonstrated the effectiveness of SMBI as a promising fuelling tool for steady-state operation. The penetration depth and injection speed of the high-pressure SMBI were roughly measured from the contour plot of the H_α emission intensity. It was shown that injected particles could penetrate into the core region of the plasma. The penetration speed of high-pressure SMBI particles in the plasma was estimated to be about 1200 m s⁻¹. In addition, clusters within the beam may play an important role in the deeper injection.

A new type of boundary element method (BEM) has been applied to solve the Grad–Shafranov equation and to give a distribution of magnetic flux function in a tokamak nuclear fusion device. The quantity μ₀rj_φ related to the plasma current profile is expanded into a two-dimensional polynomial. Using the particular solution of the Grad–Shafranov equation with this inhomogeneous polynomial source and applying Green's second identity, the domain integral related to the plasma current is transformed into an equivalent boundary integral. Domain discretization is not required in this formulation, thus preserving all the advantages of the BEM. Numerical computations of all boundary integrals are only required in the initial stage of the eigenvalue iteration, so that the number of eigenvalue iterations does not hamper the total computing time. Test calculations demonstrated that the present method provides stable and accurate solutions.

Due to the unique properties of the recently discovered sheath laser-ion source the investigation of super-intense, quasi-neutral ion beams interacting with plasma has become of substantial interest. Novel experiments in parameter regimes that bear relevance for future ion based fast ignition concepts can be envisioned. Simulations in three spatial dimensions of protons, charge and current neutralized by co-moving electrons, interacting with a low density plasma are presented. Quasi-neutral beam propagation and normalized proton beam momentum growth are discussed.

The Tore Supra gigajoule (GJ) scenario was developed during the 2002 experimental campaign, with the goal of realizing long pulses in a stationary state, at a loop voltage therefore close to zero. This challenge has led to very successful results, with a record injected energy of 0.75 GJ and perfect control of the plasma density over 4 min 25 s. The performance is however limited by the lower hybrid system reliability and MHD limits. This paper focuses on the three kinds of MHD activity that were observed in the development of this scenario. The first one, probably a manifestation of the feedback control on the plasma position, is benign and, once triggered, stays at a low level. The second one is the triggering of a fast growing instability, which leads to a strong degradation of the confinement in the core region and to a redistribution of the plasma current. After such events, a regime with a persistent MHD activity sets in and represents the third kind of MHD activity in the GJ experiments. This paper reports on the analysis of this MHD activity. Perspectives for the future development of the GJ scenario in Tore Supra are drawn.

A spherical torus (ST) fusion energy development path which is complementary to the proposed tokamak burning plasma experiments such as ITER is described. The ST strategy focuses on a compact component test facility (CTF) and high performance advanced regimes leading to more attractive Demo and power plant scale reactors. To provide the physical basis for the CTF an intermediate step needs to be taken, which we refer to as the 'next-step spherical torus' (NSST) device and which we examine in some detail herein. NSST is a 'performance extension' stage ST with a plasma current of 5–10 MA, R = 1.5 m, B_T ⩽ 2.6 T and the possibility of varying physical parameters. The mission of NSST is to (1) provide a sufficient physical basis for the design of a CTF; (2) explore advanced operating scenarios with high bootstrap current fraction and high performance which can be utilized by CTF, Demo, and power plants; and (3) contribute to the general science of high β toroidal plasmas. The NSST is designed to utilize a TFTR-like site to minimize the cost and time required for design and construction.

In this paper, we present numerical tools convenient for the investigation of tokamak discharges with current holes. These tools allow the computation of fixed- and free-boundary equilibria with current holes, their representation in Boozer co-ordinates, the determination of the corresponding magnetic fields, field line tracing and also tracing of guiding centre orbits inside the hole. The numerical methods are tested by using two example configurations with different hole sizes. Furthermore, an equilibrium reconstruction of an ASDEX Upgrade discharge is performed. The motional Stark effect polarization angles of this discharge, which indicate a small current hole in the plasma centre, are reproduced by a numerical fit of various equilibrium parameters including pressure profile, rotational transform profile and poloidal coil currents. The resulting plasma parameters lie well inside the experimental error margins.