Table of contents

A helium plasma is produced by electron–cyclotron resonance heating in a cusp-configuration magnetic field. Several neutral helium lines are found polarized in the direction perpendicular to the magnetic field; the maximum polarization degree exceeds 10%. The polarization degree and intensity of the emission lines yield, respectively, the alignment and population of the upper levels. The population-alignment collisional-radiative model is developed, and the experimental result is interpreted in terms of an anisotropic electron velocity distribution; it is of a Saturn-type with the central thermal component of 14 eV and the 'ring' component displaced by 9.2 eV from the central component. The relative number of 'ring' electrons is 40%.

Scrape-off layer (SOL) data from DIII-D and C-Mod have been acquired and analysed for radial particle transport based on a particle balance model. This has allowed a detailed comparison for L-mode plasmas. The inferred radial particle flux, Γ_⊥(r), is parameterized in terms of diffusive [D_eff(r) ≡ Γ_⊥(r)/∇n(r)] and convective particle transport [v_eff(r) ≡ Γ_⊥(r)/n(r)]. The magnitude of the inferred D_eff or v_eff increases across the SOL for both tokamaks. The inferred D_eff or v_eff in the 'far' SOL (one density e-folding length from the separatrix and beyond) are essentially unchanged by changes in core density by factors of 2–3. This corresponds to changes in the far SOL density, collisionality (ν^*) and radial fluxes of a factor of 10 or more. Thus ν^* does not appear to be an important parameter in determining the radial particle transport in that region.

The dimensionlessly-scaled SOL plasma profiles from the two tokamaks overlay for similar dimensionless plasma parameters. The SOL density profile near the separatrix is steeper than in the 'far' SOL. The scaled D_eff and v_eff are slightly larger on DIII-D than C-Mod. This difference appears to be within experimental uncertainties.

Neutral ionization in the SOL does not appear to affect radial transport but may be related to the observed flattening of the density profiles with increasing .

The magnitude of plasma contact with main-wall surfaces is examined on the DIII-D poloidal divertor tokamak. A 'window-frame' technique has been developed for axisymmetric surfaces to provide measurements of total plasma flux (ions s⁻¹) to the walls, I_wall. Despite the use of a separatrix-wall gap that is 2–3 times the radial e-folding length of the plasma parameters near the separatrix, increasing e-folding lengths away from the separatrix result in an I_wall of similar magnitude to the ion flux received by the divertor plate, I_div. The I_wall/I_div ratio increases strongly with line-averaged density and ranges from ∼0.1–0.2 with attached outer divertor plasmas to ∼1 with detached divertor plasmas. These observations hold during core density scans in both low (L-mode) and high (H-mode) confinement energy confinement regimes, and their importance to core fuelling and impurities is discussed. It is found that the magnitude of I_wall cannot be accurately measured by arbitrary main chamber D-α views due to the strong poloidal and toroidal asymmetry of the plasma contact. However D-α measurements reflect the relative trends of main-chamber recycling. Based on scrape-off layer (SOL) profiles in the shadow of the main-wall baffle, the far SOL cross-field particle transport is best described as convective with an effective velocity ∼100 m s⁻¹.

The ergodic magnetic limiter is a device designed to generate a cold boundary layer of chaotic magnetic field lines at the peripheral region of a tokamak, with the main purpose of reducing the deleterious effects of the plasma–wall interaction. In the TCABR tokamak an ergodic limiter was constructed and recently installed inside the vacuum chamber. We developed a theoretical model for the action of an ergodic magnetic limiter in a large aspect-ratio tokamak taking into account the finite width of the limiter. The theoretical results are in good agreement with measurements of the vacuum magnetic field created by the limiter. Poincaré maps of field line flow are computed to reveal the resulting magnetic field line structure due to the ergodic limiter and show that the operation of the ergodic limiter in the TCABR tokamak is feasible and results in a chaotic boundary layer for limiter currents of about 6% of the plasma current.

In a fusion device, the so-called sawtooth instability can lead to the triggering of confinement limiting neoclassical tearing modes. On the other hand, the existence of sawteeth is desirable for the removal of one fusion product, i.e. helium ash from the plasma core. This has led to great interest in the control of sawteeth. The sawtooth period can be changed drastically by local modification of the q-profile. In this paper, the influence of the beam line geometry of the neutral beam injection in the ASDEX Upgrade tokamak will be presented as well as the effect of local electron cyclotron current drive. Systematic scans in the electron cyclotron current deposition from the high-field side to the low-field side resolve areas with sawtooth stabilization and destabilization. These observations will be discussed, including modelling of the main results, with the ASTRA transport code constrained by experimental data.

We study turbulence on closed and open flux surfaces in a comparative manner using the three-dimensional electromagnetic gyrofluid turbulence code GEM. A magnetic field on a tokamak is doubly periodic and sheared. This leads to the so-called field line connection, which ensures a finite parallel response for every degree of freedom. In contrast, in the scrape-off layer (SOL), the field lines end on plates, breaking this constraint and allowing the existence of convective cell modes. Since the parallel electron response provides a path to dissipation, whether or not it is allowed to vanish is important. For the SOL case, a standard Debye sheath model is used to provide the parallel boundary conditions. A zero loss model (no fluxes into the plates) is also used to assess the importance of the Debye currents. Turbulence on closed and open flux surfaces at the same parameters is found to be very different, a property which basic transport models should take into account.

The subject of this paper is the design of two types of stationary multiphase ac plasma generators, developed for plasma chemical methods of waste destruction and processing (including syngas production). This paper presents plasma generators of average power (up to 50 kW) and high power (up to 500 kW) working on oxidizing media and describes the basic physical processes in the discharge chamber of a multiphase low-temperature (thermal) plasma generator. The presence of diffuse mode of arc burning at n_e ∼ 10¹⁴–10¹⁵ cm⁻³ and contracted mode n_e ⩾ 10¹⁶ cm⁻³ is detected. The external characteristics (dependence of working gas heat content, power in arcs and efficiency on flow rate) based on experimental data are presented. The influence of plasma forming gas variation on electric parameters is demonstrated. The powerful multiphase plasma generator works at atmospheric pressure on oxidizing media (air) in the power range 100–500 kW and the flow rates 10–70 g s⁻¹ with thermal efficiency of 70–90% and electrode lifetime of more than a hundred hours. The thermal efficiency of an average power (up to 50 kW) plasma generator in the range of air flow rate of 2–25 g s⁻¹ is 80–95%, while the electrode lifetime is hundreds of hours. The described multiphase plasma generators allow the working gas heat content to be controlled in a wide range at the outlet (for air—from 1.5 MJ kg⁻¹ up to 12.5 MJ kg⁻¹), which is important for the realization of plasma technologies, including syngas production.

An investigation of the ions induced diocotron instability in an electron plasma confined in a Malmberg–Penning trap is presented. The detection of the instability is based on the spectral analysis of the induced charge signals on the walls of the confining electrodes, which allows tracking of the plasma displacement from the axis. The dependence of the instability on the electron energy is analysed by three different methods: (i) injecting electrons with different energies, (ii) heating the electrons with a single radio frequency burst, (iii) varying the ramp-up time of the confining voltage. An experimental technique to limit the ion resonance instability, based on the application of suitable potentials on a set of electrodes, is presented.

The control of the current, position and shape of an elongated cross-section tokamak plasma is complicated by the so-called instability of the current vertical position. Linearized models all share the feature of a single unstable eigenmode, attributable to this vertical instability of the plasma equilibrium movement, and a large number of stable or marginally stable eigenmodes, attributable to zero or positive resistance in all other model circuit equations. Due to the size and therefore cost of the ITER tokamak, there will naturally be smaller margins in the poloidal field coil power supplies, implying that the feedback control will experience actuator saturation during large transients due to a variety of plasma disturbances. Current saturation is relatively benign, due to the integrating nature of the tokamak, resulting in a reasonable time horizon for strategically handling the approach to saturation which leads to the loss of one degree of freedom in the feedback control for each saturated coil. On the other hand, voltage saturation is produced by the feedback controller itself, with no intrinsic delay. This paper presents a feedback controller design approach which explicitly takes saturation of the power supply voltage into account when producing the power supply demand signals. We consider the vertically stabilizing part of the ITER controller (fast controller) with one power supply and therefore a single saturated input. We consider an existing ITER controller and enlarge its region of attraction to the full null controllable region by adding a continuous nonlinearity into the control. In a system with a single unstable eigenmode and a single stable eigenmode we have already provided a proof of the asymptotical stability of the closed loop system, and we have examined the performance of this new continuous nonlinear controller. We have subsequently extended this analysis to a system with a single eigenmode and multiple stable eigenmodes. The method requires state feedback control, and therefore a reconstruction of the states is indispensable. We discuss the feasibility of extracting these states from the available diagnostic information as well as other implementation details. As a complement to our ITER simulations we confirm the enlargement of the region of attraction by the new controller by a JET simulation.

The ideal stability of the internal kink mode is analysed for realistic tokamak geometry. Accurate numerical results demonstrate convergence with Bussac's toroidal solution (Bussac M N, Pellat R, Edery D and Soulé J L 1975 Phys. Rev. Lett.35 1638) for inverse aspect ratio ₁ ∼ 0.01 at the q = 1 rational surface. For realistic inverse aspect ratio (e.g. ₁ ∼ 0.1) the growth rate is found to scale linearly with the poloidal beta and to be smaller than the analytical prediction of the toroidal growth rate. The effect of the shaping of the plasma cross-section is also analysed. Analytical results are found to disagree with numerical results at realistic inverse aspect ratio primarily because of the toroidal nature of the Mercier mode shaping terms. Furthermore, in addition to the Mercier shaping terms, there are quasicylindrical contributions to the kink mode which are quadratic and stabilizing in both triangularity and ellipticity. To verify this empirically and to further demonstrate the importance of the ideal internal kink stability boundary, discharges in the tokamak à configuration variable are shown to display longer sawteeth for both very positive and negative triangularity. Finally, a parameter scan in the triangularity, elongation, aspect ratio and poloidal beta has been undertaken using the code KINX. Versatile predictions of the ideal internal kink stability in future tokamaks should be assisted by the functional fitting presented here of the parameter scans.

Neutron emission spectroscopy (NES) has been used at JET to study deuterium–tritium discharges heated with neutral beam (NB) injection of tritium and deuterium at energies of 150 keV and 75 keV, respectively. The measurements were performed using the magnetic proton recoil neutron spectrometer and the data analysed with several contributing spectral components. The analysis model considers neutrons from reactions involving high-energy NB ions in (co and counter) passing and trapped orbits and NB ions slowed down to epithermal energies besides ion reactions within the thermal bulk. This study was undertaken to explore the level of fuel ion kinetic information that can be obtained for different NB heating scenarios. Time dependent information was extracted to study how the plasma response to NB injection is manifested in the neutron emission for apparent quiescent periods, during spontaneous plasma instabilities (such as sawteeth) and as a result of intentional changes of the plasma conditions such as the NB injection. The discharges for this study were selected to explore the systematic features that neutron emission spectra display for NB heated discharges and the possibility to interpret them in terms of the underlying kinetic state of the fuel ions. The results obtained on the ion temperature, the toroidal rotation and the supra-thermal fraction of the total neutron yield are compared with those of charge exchange recombination spectroscopy and, in some cases, with plasma equilibrium model calculations. A phenomenological analysis of the results is performed from studies of systematics with the object of exploring the uses of the NES diagnostic and their dependence on data quality.

T-10 experiments with electron internal transport barrier (ITB) formation in discharges with reverse shear q(r) profile are described. Reverse magnetic shear was formed in the central region characterized by r/a_L ⩽ 0.3. It is shown that electron thermal conductivity decreases essentially in comparison with the value typical for the L-mode. It is found that degradation of the ITB correlates with development of MHD activity in the internal part of the plasma column.

The field distribution of a symmetric surface wave (SW) propagating on a weakly ionized unmagnetized plasma placed in a cylindrical ideal waveguide, in which a quartz tube is surrounded by vacuum, is presented in the diffusion-controlled regime. This is done to better understand the effect of some physical parameters such as electron temperature, SW frequency, etc on the plasma column. The graphs of electric and magnetic fields versus the plasma column radius under the radial electron density profile dependency are obtained in the commercial radio frequency region where spatial dispersion might be important.

Non-linear gyrokinetic simulations of the interchange instability are discussed. The semi-Lagrangian numerical scheme allows one to address two critical points achieved with simulations lasting several confinement times: an accurate statistical analysis of the fluctuations and the back reaction of the turbulence on equilibrium profiles. Zonal flows are found to quench a 2D + 1D interchange turbulence when one of the species has a vanishing response to zonal modes. Conversely, when streamers dominate, the equilibrium profiles are found to be stiff. In the non-linear regime and steady-state turbulence, the distribution function exhibits a significant departure from a Maxwellian distribution. This property is characterized by an expansion on generalized Laguerre functions with a slow decay of the series of moments. This justifies the use of gyrokinetic simulations since a standard fluid approach, based on a limited number of moments, would certainly require a complex closure so as to take into account the impact of these non-vanishing high order moments.

Intermittent convective transport has been investigated in the edge and the scrape-off layer (SOL) of TEXTOR using Langmuir probe signals. The probability distribution function (PDF) of the density fluctuations and the turbulence-induced flux are all positively skewed, while a Gaussian shape is recorded for the negative fluctuations. The deviation of the signals from Gaussian statistics clearly increases from the plasma edge to the SOL. Conditional averaging reveals that in the SOL region the waveform of intermittent structures is asymmetric in time and the burst events move radially outwards with E_θ × B_T/B² velocities of ∼ 450 m s⁻¹. It is found that the large burst fluctuations (⩾2.5 × rms) account for nearly 40% of the total transport in the SOL. Statistics of the waiting-time between successive bursts indicate that the PDF of the time interval follows a Poisson-distribution for small-duration events (selected by size ⩽2.5 × rms) and changes into a power-law form for larger ones. Moreover, the intermittency density fluctuation data clearly show self-similar characters and long-range time correlations through the presence of (1) sandpile-like frequency spectra and existence of the f⁻¹ region; (2) a long tail in the autocorrelation function and (3) Hurst exponents H > 0.5 from R/S analysis, suggesting a possible role of avalanche-like transport in the turbulence intermittency.