Table of contents

Predictive, time-dependent transport simulations with a semi-empirical plasma model have been used in closed-loop simulations to control the q-profile and the strength and location of the internal transport barrier (ITB). Five transport equations (T_e, T_i, q, n_e, v_Φ) are solved, and the power levels of lower hybrid current drive, NBI and ICRH are calculated in a feedback loop determined by the feedback controller matrix. The real-time control (RTC) technique and algorithms used in the transport simulations are identical to those implemented and used in JET experiments (Laborde L. et al2005 Plasma Phys. Control. Fusion47 155 and Moreau D. et al2003 Nucl. Fusion43 870). The closed-loop simulations with RTC demonstrate that varieties of q-profiles and pressure profiles in the ITB can be achieved and controlled simultaneously. The simulations also showed that with the same RTC technique as used in JET experiments, it is possible to sustain the q-profiles and pressure profiles close to their set-point profiles for longer than the current diffusion time. In addition, the importance of being able to handle the multiple time scales to control the location and strength of the ITB is pointed out. Several future improvements and perspectives of the RTC scheme are presented.

Progress in the theoretical and experimental development of the plasma jet source and injection of hydrogen plasma and neutral gas jets into the Globus-M spherical tokamak is discussed. An experimental test bed is described for investigation of intense plasma jets that are generated by a double-stage plasma gun consisting of an intense source for neutral gas production and a conventional pulsed coaxial accelerator. A procedure for optimizing the accelerator parameters so as to achieve the maximum possible flow velocity with a limited discharge current and a reasonable length of the coaxial electrodes is presented. The calculations are compared with experiment. Plasma jet parameters, among them pressure distribution across the jet, flow velocity, plasma density, etc, were measured. Plasma jets with densities of up to 10²² m⁻³, total numbers of accelerated particles (1–5) × 10¹⁹, and flow velocities of 50–100 km s⁻¹ were successfully injected into the plasma column of the Globus-M tokamak. Interferometric and Thomson scattering measurements confirmed deep jet penetration and a fast density rise (<0.5 ms) at all spatial points up to a radius r≈ 0.3a. The plasma particle inventory increase by ∼50% (from 0.65 × 10¹⁹ to 1 × 10¹⁹) did not result in plasma degradation.

Impurity deposition and mixing during gas jet-initiated plasma shutdown is studied using a rapid (∼5 ms), massive (∼10²² particles) injection of neon or argon into stationary DIII-D H-mode discharges. Fast-gated camera images indicate that the neutral deposition remains fairly localized near the injection port and does not penetrate far into the plasma pedestal. Nevertheless, fast bolometry indicates that high (50–100%) thermal quench radiated power fractions are achieved; this appears to be facilitated through a combination of fast ion mixing and fast heat transport, both driven by large-scale magnetohydrodynamic activity. These experiments suggest that massive gas injection might be viable for disruption mitigation in future tokamaks even if core penetration of jet neutrals is not achieved.

We present the first results from recent compact toroid (CT) injection experiments in the JFT-2M tokamak using the improved CT injector and diagnostics with fast time resolution. We have observed that the core line density increases rapidly at a maximum rate of ∼1.3 × 10²² m⁻³ s⁻¹ after a delay of 100–200 µs. This increment rate of the core density is about several times larger than that obtained so far. Interferometry measurement along the peripheral line chord of R = 1.1 m in the inboard side indicates that CT plasma reaches a region near the plasma core beyond the separatrix. Time-frequency and space distribution analyses of edge magnetic probe signals show that the magnetic fluctuation induced by the CT has the spectral peak at 250–350 kHz and propagates in the toroidal direction at the Alfvén speed of the order of 10⁶ m s⁻¹. These results indicate the excitation of Alfvén wave by CT injection. We have observed that the fluctuation level of the ion saturation current in the divertor and the D_α spectral line intensity decrease significantly after CT injection. Corresponding increase in the soft x-ray signals in the core region may suggest that the CT causes a transition to H-mode-like discharges.

Beryllium is considered to be a candidate material for the first wall of nuclear fusion plasma experiments, e.g. ITER. In this application, the interaction of beryllium with oxygen is important for two reasons. One aspect is the reaction of hot beryllium with air in case of a catastrophic leak, the other is the action of beryllium as a getter, binding oxygen impurities and thus helping to keep the level of contamination in the plasma low. We therefore investigated the interaction of beryllium with air at elevated temperatures up to 600°C on a microscopic level, using a high resolution Auger electron microscope. At 390°C, a thin protective oxide film is formed, while at 500°C oxidation starts to enter into the grain boundaries, leading to the loosening of small particles of beryllium already at 600°C. The expected diffusion of oxygen from the surface into the bulk has not been observed up to 390°C, the highest temperature to be safely applied inside the Auger microscope. Exposure to hydrogen atmosphere showed no change up to 600°C, exposure to hydrogen plasma resulted in an equilibrium thickness of the oxide layer, which is most likely due to impurities in the plasma. Thus, an operation of beryllium liners as a non-evaporable getter is not to be expected in this temperature range. Getter activity linked to the transport of beryllium from the liner to some deposition areas is, however, possible.

H-mode operation plays a crucial role in National Spherical Torus Experiment (NSTX) research, allowing higher beta limits due to reduced plasma pressure peaking, and long pulse operation due to high bootstrap current fraction. Here, new results are presented in the areas of edge localized modes (ELMs), H-mode pedestal physics and power threshold studies. ELMs of several types as reported by higher aspect ratio tokamaks have been observed: (1) large, Type I ELMs, (2) intermediate-sized Type III ELMs and (3) tiny ELMs. Many high performance discharges in NSTX have the tiny ELMs (recently termed Type V), which have some differences as compared with small-magnitude ELM types in the published literature. A divertor multifaceted axisymmetric radiation from the edge (MARFE) on the inboard leg provides an effective light source to examine the effect of the ELMs on the divertor plasma; it is clear that only the large ELMs burn through the MARFE. The time difference between observation of the ELM flux at the outer and inner targets is substantially longer for the smallest ELMs as compared with the large ELMs. In addition, the visible light patterns show 'finger-like' striations during the tiny ELMs. H-mode pedestal studies have commenced, with the observation that the pedestal contains between 25% and 33% of the total stored energy, and the NSTX pedestal energy agrees reasonably well with a recent international multi-machine scaling. A power threshold identity experiment between NSTX and the Mega-Amp Spherical Tokamak shows comparable loss power at the L–H transition in balanced double-null discharges. Both machines require more power for the L–H transition as the balance is shifted toward lower-single null. High-field side gas fuelling enables more reliable H-mode access in NSTX, but does not always lead to a lower power threshold, e.g. with a reduction of the duration of early heating.

The condition of the latest version of the ELMy H-mode database has been re-examined. It is shown that there is bias in the ordinary least squares regression for some of the variables. To address these shortcomings three different techniques are employed: (a) principal component regression, (b) an error in variables technique and (c) the selection of a better conditioned dataset with fewer variables. Scalings in terms of the dimensionless physics variables, as well as the standard set of engineering variables, are also derived. The new scalings give a very similar performance for existing scalings for ITER at the standard β_n of 1.6, but a much improved performance at higher β_n.

A prototype passive active multijunction (PAM) launcher for lower hybrid (LH) waves conceptually similar to that foreseen for ITER has been successfully tested on FTU at frequency f = 8 GHz. The power routinely and safely managed for the maximum time allowed by the LH power plant (0.9 s) without any fault in the transmission lines is 250 kW, corresponding to 75 MW m⁻² across the antenna active area and very close to the design value of 270 kW or 80 MW m⁻². The achieved value is at least 1.4 times larger than the ITER request, which would be only 52 MW m⁻², if the 33 MW m⁻² required to the ITER grill in order to couple 20 MW to the plasma, are scaled up linearly with f, from f_ITER = 5 GHz. This linear scaling of the power handling capability of the LH antennae is indeed conservative with respect to the available data. The test results validate also the other two main expectations relevant to ITER, foreseen by the codes, namely to operate with the grill entirely in the vessel shadow and to still preserve good current drive (CD) efficiency. Even with the grill mouth retracted 2 mm inside the port shadow and with density in front of the launcher very close or even lower than the cut-off value, the PAM reflection coefficient is always ⩽ 2.5%, if the antenna has been properly conditioned. The CD efficiency is comparable to that of a conventional grill, once the lower directivity is taken into account. Flexibility in determining the N_|| spectrum is also maintained, according to hard x-rays and electron cyclotron emission spectra. Conditioning the PAM in order to operate at the ITER equivalent power level required only one day of radio-frequency operation, without a previous baking of the waveguides.

In an additional heating experiment on translated theta-pinch produced field reversed configuration (FRC) plasma with density of 5 × 10¹⁹m⁻³ and pressure balance temperature of 100 eV by 10 keV hydrogen neutral beams, only electrons are observed to be heated; electron temperature is controlled independently of ion temperature for the first time ever in FRC plasma. So far, the confinement times of the FRC plasma were known to depend approximately on (r_s, separatrix radius, ρ_i0, ion gyro radius) and dependence on electron temperature was small; this is confirmed by the present experiment. The FRC plasma is also produced and sustained for longer than 4 ms by a rotating magnetic field (RMF) in a metal confinement chamber. The performance of the present RMF-produced FRC plasma is low with density and temperature less than 5 × 10¹⁷m⁻³ and 10 eV, respectively. The efficiency of the FRC plasma production by the RMF is found to be higher for higher RMF frequencies.

Evolution of the current density profile associated with magnetic island formation in a neoclassical tearing mode plasma is measured for the first time in JT-60U by using a motional Stark effect diagnostic. As the island grows, the current density profile turns flat at the radial region of the island and a hollow structure appears at the rational surface. As the island shrinks the deformed region becomes narrower and finally diminishes after the disappearance of the island. In a quiescent plasma without magnetohydrodynamic instabilities, on the other hand, no deformation is observed. The observed deformation in the current density profile associated with the tearing mode is reproduced in a time dependent transport simulation assuming the reduction of the bootstrap current in the radial region of the island. Comparison of the measurement with a calculated steady-state solution also shows that the reduction and recovery of the bootstrap current at the island explains the temporal behaviours of the current density and safety factor profiles. From the experimental observation and simulations, we reach the conclusion that the bootstrap current decreases within the island O-point.

A study is conducted on the influence of radio frequency (rf) waves upon interchange stability in HANBIT mirror plasmas. An emphasis is put on the interchange stability near the resonance region, ω₀ ≃ Ω_i, where ω₀ (Ω_i) is the angular frequency of the applied rf wave (ion cyclotron frequency). A strong nonlinear interaction between the rf wave and the interchange mode has been observed with the generation of sideband waves. A theory of rf-interchange mode interaction keeping effects of both the equilibrium ponderomotive force and the nonlinear sideband wave coupling is applied to interpret HANBIT experimental results, resulting in good agreement. Results have shown that the nonlinear coupling process is responsible for the rf stabilization of interchange modes near the resonance region.

It has been hypothesized that radial electron heat transport in low collisionality, current-carrying magnetically-confined toroidal plasmas results from paleoclassical Coulomb collision processes (parallel electron heat conduction and magnetic field diffusion). In such plasmas the electron temperature equilibrates along magnetic field lines a long length L, which is the minimum of the electron collision length and a maximum effective half length of helical field lines. Diffusing field lines carry this equilibrated electron temperature with them and thus induce a radial electron heat diffusivity M ≃ L/(πR₀q) ∼ 10 ≫ 1 times the resistivity-induced magnetic field diffusivity η/μ₀ ≃ ν_e (c/ω_p)². Interpretations of many features of 'anomalous' electron heat transport provided by the paleoclassical model are discussed: magnitude and radial profile of electron heat diffusivity (in tokamaks, spherical tokamaks and reversed field pinches), Alcator scaling in high density plasmas, electron heat transport barriers around low order rational surfaces and near a separatrix and a natural heat pinch (or minimum temperature gradient) electron heat flux form. Also, the context (relative to other transport models), regime of applicability and suggestions for experimental tests of the paleoclassical model are discussed.

Different models have been introduced in the stability code MARS-F in order to study the damping effect on resistive wall modes (RWM) in rotating plasmas. Benchmarks of MARS-F calculations with RWM experiments on JET and DIII-D indicate that the semi-kinetic damping model is a good candidate for explaining the damping mechanisms. Based on these results, the critical rotation speeds required for RWM stabilization in advanced ITER scenarios are predicted. Active feedback control of the n = 1 RWM in ITER is also studied using the MARS-F code.

In this paper several transport phenomena are described and explained through a (quasi) linear description of the micro-instabilities. This paper deals with the following phenomena: density peaking, electron heat transport, density pump-out, reactor density profiles and the stabilization of the ion temperature gradient (ITG) mode in transport barriers. Density peaking is observed to increase with decreasing collisionality—a phenomenon that can be explained by the influence of collisions on the trapped electron response. The density pump-out due to central electron heating is observed to occur only if the dominant instability is the trapped electron mode (TEM). The proposed explanation involves the thermo-diffusive contribution to the particle flux which is outwards for the TEM while it is inwards for the ITG. The insight in the density profile behaviour can be used to predict moderately peaked density profiles in a reactor. Collisionality is also found to influence the electron heat flux in discharges with dominant electron heating even at relatively small values of the density. The dominant instability under dominant electron heating is found to be the TEM, and the dependence of the electron heat flux on the electron temperature gradient is reasonably described by the quasi-linear results. At higher density (and collisionality) the growth rate of the TEM is reduced and a transition to a dominant ITG is found. This transition is reflected in the speed of the electron heat pulse propagation. Finally, it has been found that the ITG under experimentally relevant conditions is not stabilized by a uniform radial electric field.

In the 'metal liner' approach to magnetized target fusion (MTF), a preheated magnetized plasma target is compressed to thermonuclear temperature and high density by externally driving the implosion of a flux conserving metal enclosure, or liner, which contains the plasma target. As in inertial confinement fusion, the principal fusion fuel heating mechanism is pdV work by the imploding enclosure, called a pusher in ICF. One possible MTF target, the hard-core diffuse z pinch, has been studied in MAGO experiments at VNIIEF and is one possible target being considered for experiments on the Atlas pulsed power facility. Numerical MHD simulations show two intriguing and helpful features of the diffuse z pinch with respect to compressional heating. First, in two-dimensional simulations the m = 0 interchange modes, arising from an unstable pressure profile, result in turbulent motions and self-organization into a stable pressure profile. The turbulence also gives rise to convective thermal transport, but the level of turbulence saturates at a finite level, and simulations show substantial heating during liner compression despite the turbulence. The second helpful feature is that pressure profile evolution during compression tends towards improved stability rather than instability when analysed according to the Kadomtsev criteria. A liner experiment is planned for Atlas to study compression of magnetic flux without plasma, as a first step. The Atlas geometry is compatible with a diffuse z pinch, and simulations of possible future experiments show that kiloelectronvolt temperatures and useful neutron production for diagnostic purposes should be possible if a suitable plasma injector is added to the Atlas facility.

The linear magnetohydrodynamic (MHD) stability of ideal and resistive, axisymmetric toroidal equilibria is investigated with respect to various physical effects, such as differential toroidal rotation, viscosity, ideal and resistive external walls and current holes. For this purpose, the CASTOR code has been comprehensively extended. Static equilibria, equilibria with toroidal flow and equilibria with current holes serve as input to this code called CASTOR_FLOW code. ASDEX Upgrade type equilibria with toroidal flow are computed up to a toroidal Mach number of M_ta = 0.5, and compared with the static solution. Using these equilibria, the stabilizing effect of differential toroidal rotation on double tearing modes (DTMs) is investigated. The studies show that the computation of equilibria with flow is necessary for toroidally rotating plasma with M_ta ⩾ 0.2. The stability of DTMs is also studied for equilibria with current holes. Further, the stabilizing effect of a resistive wall on an external ideal kink mode is investigated.

In preparation for next step burning plasma devices such as ITER, experimental studies of instabilities and confinement of energetic ions were performed on Joint European Torus (JET) and on Mega-Amper Spherical Tokamak (MAST) with innovative diagnostic techniques, in conventional and shear-reversed plasmas, exploring a wide range of effects for energetic ions. A compendium of recent results testing capabilities of the present-day facilities for burning plasma relevant study is presented in this paper. 'Alpha tail' production using 3rd harmonic ion-cyclotron resonance heating (ICRH) of ⁴He beam ions has been employed on JET for studying ⁴He of the megaelectronvolt energy range in a 'neutron-free' environment. The evolution of ICRH-accelerated ions of ⁴He with E ⩾ 1.7 MeV and D with E ⩾ 500 keV was assessed from nuclear gamma-ray emission born by the fast ions colliding with Be and C impurities. A simultaneous measurement of spatial profiles of fast ⁴He and fast D ions relevant to ITER was performed for the first time in positive and strongly reversed magnetic shear discharges. Time-resolved gamma-ray diagnostics for ICRH-accelerated ³He and H minority ions allowed changes in the fast ion distribution function to be assessed in the presence of unstable toroidal Alfvén eigenmodes (TAEs) and sawteeth. A significant decrease of gamma-ray intensity from protons with E ⩾ 5 MeV was detected during the 'tornado' modes. This was interpreted as 'tornado'-induced loss of fast ions with the drift orbit width, Δ_f, comparable to the minor radius of tokamak a. Experiments performed in the opposite case, Δ_f/a ≪ 1, for ICRH-accelerated ³He ions with E ⩾ 500 keV, have shown excitation of numerous Alfvén eigenmodes without a significant degradation of the fast ion confinement. The stabilizing effect of fast particles on 'monster' sawteeth was experimentally found to fail in low-density plasmas with high power ion cyclotron resonance frequency (ICRF)-heating. The transition from the 'monster' to short-period 'grassy' sawteeth was investigated with different ICRF phasing, which controls the pinch-effect and radial distribution of ICRF-accelerated ions. Instabilities excited by super-Alfvénic beam ions were investigated on the spherical tokamak MAST. Due to higher values of β and a higher proportion of fast ions on MAST than on JET, a wider variety of modes and nonlinear regimes for the Alfvén instabilities were observed, including the explosive TAE-regimes leading to the formation of hole-clump pairs on the fast ion distribution function. The MAST and START data showed that TAE and chirping modes decrease both in their mode amplitudes and in the number of unstable modes with increasing β.

A theoretical categorization of the onset of tearing modes in tokamaks is presented using DIII-D equilibrium reconstructions as initial conditions in the NIMROD nonlinear three-dimensional resistive magnetohydrodynamic code (Sovinec C.R. et al 2004 J. Comput. Phys.195 355). The onset mechanism of tearing modes are categorized into three types: spontaneous, mixed and forced depending on the importance of linear instability versus forced reconnection. The physics of the early evolution of growing tearing modes in simulations with time varying linear instability drive, nonlinear coupling between modes and neoclassical bootstrap drive are compared to experimental data to explain this qualitative categorization. Important effects, such as rotational shear and thermal anisotropy, are included and confirm that this categorization is consistent with the experimental observations in DIII-D.

The diagnostics functions of neutron measurements as well as the roles played by neutron yield monitors, cameras and spectrometers are reviewed. The importance of recent developments in neutron emission spectroscopy (NES) diagnostics is emphasized. Results are presented from the NES diagnosis of the Joint European Torus (JET) plasmas performed with the magnetic proton recoil (MPR) spectrometer during the first deuterium tritium experiment of 1997 and the recent trace tritium experiment of 2003. The NES diagnostic capabilities at JET are presently being enhanced by an upgrade of the MPR (MPRu) and a new 2.5 MeV time-of-flight (TOF) neutron spectrometer (TOFOR). The principles of MPRu and TOFOR are described and illustrated with the diagnostic role they will play in the high performance fusion experiments in the forward programme of JET largely aimed at supporting the International Thermonuclear Experimental Reactor (ITER). The importance of the JET NES effort for ITER is discussed.