Table of contents

Temporally and spatially resolved measurements of carbon and boron impurity density are obtained in the reversed field pinch (RFP) for the first time. It is observed that, unlike in tokamaks and stellarators, the RFP does not exhibit a centrally peaked impurity profile in either standard plasmas where field lines have some degree of stochasticity, or improved confinement discharges where there exist well-nested flux surfaces for a substantial fraction of the plasma volume. Results from improved confinement discharges also indicate an outward convection of impurities from the core of the plasma.

Carbon fibre composites (CFCs) planned to be used in the International Thermonuclear Experimental Reactor (ITER) divertor can be eroded due to hydrogen species from the plasma and the resulting hydrocarbons can be redeposited in other locations. Tritium retention in redeposited materials is the major concern due to the limits imposed for safety reasons by the nuclear licensing authorities. The scavenging effect has already been proposed to decrease the carbon redeposited materials using nitrogen as a scavenger. To date, only fairly limited data are available on the use of NH₃ as a scavenger. In this brief communication, the possibility of performing the inhibition of carbon-film formation using NH₃ was examined.

We report on the results of a recent experiment at the Rutherford Appleton Laboratory investigating fast electron propagation in cylindrically compressed targets; a subject of interest for fast ignition. This experiment was performed within the framework of the road map of HiPER (the European High Power laser Energy Research facility Project). Protons accelerated by a ps-laser pulse are used to radiograph a 220 µm diameter, imploded with ∼200 J of laser light (1 ns λ = 0.53 µm) in four symmetrically incident beams. Results are also compared with those from hard x-ray radiography. Detailed comparison with 2D radiation hydrodyamics simulations is performed with the aid of a Monte Carlo code adapted to describe plasma effects. Finally, a simple analytical model is developed to estimate the performance of proton radiography for given implosion conditions.

The paper deals with JET polarimeter measurements and in particular it presents a study of the Faraday rotation angle, which is used as a constraint in equilibrium codes. This angle can be calculated by means of the rigorous numerical solution of Stokes equations. A detailed comparison of calculations is carried out with the time traces of measurements, inside a limited dataset representative of JET discharges: in general, it is found that the Faraday rotation angle and Cotton–Mouton phase shift measurements can be represented by the numerical solution to Stokes equations. To obtain this agreement in particular for Faraday rotation, a shift of the magnetic surfaces must be included. This results in an improvement of the position of the magnetic surfaces as calculated by the EFIT equilibrium code. The approximated linear models normally used can be applied only at low density and current. The Cotton–Mouton is calculated at high plasma density including the contribution by the Faraday rotation angle. For high plasma current the non-linear terms in the propagation equations can be important. These conclusions have some impact on the mathematical form of the polarimetric constraints (Faraday and Cotton–Mouton) in equilibrium codes.

The influence of passive edge emission on the charge exchange (CX) spectra of carbon (C⁵⁺, n = 8 → 7 at 529 nm) measured in fusion plasmas at the ASDEX Upgrade tokamak is investigated. The spectra are obtained viewing the plasma edge tangentially with eight lines of sight while the plasma is swept to enhance the spatial density of the measurements. A forward model to deconvolute the measured line-integrals is employed. The local emissions are then compared with the simulated radiation obtained with the 1D impurity transport code STRAHL using transport coefficients which are determined independently. Depending on the background neutral deuterium densities the simulation predicts the absolute line intensities and the relative contributions of electron impact excitation and thermal CX to the measured signals. Therefore, a background neutral deuterium density profile has been determined. For the passive emission line of C⁵⁺, the comparison between forward model and simulations yields that electron impact excitation and thermal CX are both important for understanding the passive line. Indeed, thermal CX proves to be affecting the passive emission line considerably via two mechanisms, i.e. change in the ionization equilibrium through CX recombination and radiation due to local CX reactions.

The radial propagation of type-I edge localized modes (ELMs) in ASDEX Upgrade has been measured using a number of techniques. The most reliable technique uses a filament probe to measure the time difference between two separated probes and yields a mean radial velocity obtained of ∼1.5 km s⁻¹. The radial velocity calculated using a time of flight technique suffers from uncertainties in the start time and yields radial velocities which are 3–4 times lower than those based on the filament probe. The filament probe data show that the filaments leave the last closed flux surface (LCFS) over a period of up to 400 µs. The velocity derived from the floating potential at two poloidally separated probes suffers from the neglect of temperature differences, local turbulence effects and the smoothing used. The mean radial velocity calculated is ∼1100 m s⁻¹ for a smoothing of 0.5 µs (the data acquisition time) decreasing to ∼850 m s⁻¹ for a 5 µs smoothing. In spite of the differences in the size of the radial velocity all the methods suggest that V_r is independent on distance from the LCFS for 20 < ΔR_LCFS < 100 mm and on ELM size. The ion saturation current e-folding length scales with q₉₅ and inversely with the square root of the temperature pedestal. Once these dependences are accounted for the evolution of the e-folding length with ΔW_ELM/W_ped is consistent with V_r being independent of ELM size.

The integral energy balance is analysed for the fast-heated plasmas in tokamaks and stellarators. This is an approach to resolving the so-called 'missing power' problem (Andreev et al2004 Plasma Phys. Control. Fusion46 319). The analysis is based on ideal MHD and includes plasma interaction with the magnetic field, which is traditionally disregarded in similar transport studies. It is shown that this interaction provides such constraints on the energy fluxes that the main part of the injected power must be absorbed by the bulk plasma. The plasma heating is modelled as flux-conserving evolution of isotropic plasma.

A multi-energy soft x-ray (ME-SXR) array is used for the characterization of resistive wall modes (RWMs) in the National Spherical Torus Experiment (NSTX). Modulations in the time history of the ME-SXR emissivity profiles indicate the existence of edge density and core temperature fluctuations in good agreement with the slow evolution of the n = 1 magnetic perturbation measured by the poloidal and radial RWM coils. The characteristic 20–25 Hz frequency in the SXR diagnostics is approximately that of the n = 1 stable RWM, which is also near the measured peak of the resonant field amplification (RFA) and inversely proportional to the wall time. Together with the magnetics, the ME-SXR measurements suggest that in NSTX the RWM is not restricted exclusively to the reactor wall and edge, and that acting with the stabilizing coils on its global structure may result in density and temperature fluctuations that can be taken into account when designing the feedback process.

The stability of a plasma produced due to the interaction of a high-frequency, circularly polarized microwave (MW) field with a dilute neutral gas is investigated in the presence of an axial external magnetic field. Here the investigation is extended for excited electromagnetic waves, which propagate parallel to the external magnetic field, in the short and long wavelength limits. It is shown that the unstable Weibel mode grows under the competition of MW and external magnetic fields. However, increasing the MW field amplitude increases the Weibel instability growth rate, and the instability disappears with a sufficiently strong magnetic field. It has already been revealed that the Weibel mode oscillates very slowly on time (ℜ(ω) ≪ ω_pe) in a magnetized plasma, in contrast to the case of a non-magnetized plasma where that mode does not oscillate. The numerical calculations indicate that a different unstable mode is generated during the plasma production. This unstable mode is resonant (ℜ(ω) > ℑ(ω_max), where ℑ(ω_max) denotes the maximum imaginary part of the mode frequency), oscillates very fast (ℜ(ω) > ω_pe) and has a large increment compared with the Weibel mode. Furthermore, a low-frequency Alfvén mode spectrum analysis predicts that the Alfvén mode velocity increases slightly for such and anisotropic plasma. The analytical and numerical results are in good agreement for a weakly magnetized plasma in the long wavelength limit.

Core toroidal rotation behavior and momentum transport have been examined in neutral beam injection (NBI) heated plasmas with and without electron cyclotron resonance heating (ECRH) in ASDEX Upgrade (AUG). The impurity ion temperature and rotation are measured by means of charge exchange recombination spectroscopy (CXRS) and the main ion rotation is calculated neoclassically based on the measured impurity ion temperature and rotation profiles. In purely NBI heated discharges the plasma spins up in the co-current direction and forms peaked rotation profiles. However, when ECRH power is added to these discharges the rotation decreases significantly leading to flat and occasionally slightly hollow profiles. The rotation at the edge of the plasma (ρ > 0.65) is largely unaffected by the application of the ECRH. During ECRH phases the electron temperature in the core increases dramatically concomitant with a flattening of the ion temperature profile. In these phases peaking of the impurity ion and electron density profiles is also observed. The change in toroidal rotation is sensitive to both the amount of ECRH power deposited as well as to the deposition location. The measured rotation changes cannot be explained by a modification to the NBI torque deposition profile, a preferential loss of fast ions from the plasma core, or by a simple increase in momentum diffusivity. In addition, the inward directed Coriolis momentum pinch is predicted to increase, not decrease, during the ECRH phases. Altogether, the data suggest the presence of either an outward convection of momentum or an intrinsic, counter-current directed torque. Although the physics behind these possibilities remains unclear, they are presumably related to either a convective or residual stress driven momentum flux, which responds to the ECRH-induced changes in the plasma profiles and turbulence.

We provide numerical evidence of the role of finite Larmor radius effects in the nonlinear dynamics of magnetic field line reconnection in high-temperature, strong guide field plasmas in a slab configuration, in the large Δ' regime. Both ion and electron temperature effects introduce internal energy variations related to mechanical compression terms in the energy balance, thus contributing to regularize the gradients of the ion density with respect to the cold regimes. For values of the Larmor radii that are not asymptotically small, the two temperature effects are no longer interchangeable, in contrast to what is expected from linear theory, and the differences are measurable in the numerical growth rates and in the nonlinear evolution of the density layers. We interpret such differences in terms of the change, due to ion temperature effects, of the Lagrangian advection of the 'plasma invariants' that are encountered in the cold-ion, warm-electron regime. The different roles of the ion and ion-sound Larmor radii in the reconnection dynamics near the X- and O-points are evidenced by means of a local quadratic expansion of the fields.

The provision of measurements of metallic impurity densities, Z_eff and dilution for a large number of discharges within a campaign facilitates the analysis of impurity trends. Such trends are of increasing importance as additional heating power and pulse length increase. This is particularly important for RF heating and therefore it is in particular relevant to the assessment of the ITER-like ICRF antenna (ILA) on JET. To this end, a method is presented for determining the metal impurity density, ΔZ_eff and dilution in steady-state JET plasmas using passive VUV emission. The method is based on the combination of absolutely calibrated VUV transition intensity measurements with Universal Transport Code (UTC) simulations. In the analysis the line-integrated measurements of transitions in Li-like Ni, Fe and Cu have been used for test discharges characterized by widely varied plasma profiles. The simulations use a wide class of transport coefficients for diffusion D(r) and convection V(r). For a given pair of D(r) and V(r), the simulated line intensity has been matched to the line intensity measured in the experiment. An approximately linear dependence of the derived metal densities, ΔZ_eff and dilution normalized to a Li-like line intensity on electron temperature has been obtained which is valid in a localized, mid-radius plasma region. These linear dependences are exploited to derive local metal densities for JET discharges.

Several aspects of the sensitivity of a shock-ignited inertial fusion target to variation of parameters and errors or imperfections are studied by means of one-dimensional and two-dimensional numerical simulations. The study refers to a simple all-DT target, initially proposed for fast ignition (Atzeni et al 2007 Phys. Plasmas7 052702) and subsequently shown to be also suitable for shock ignition (Ribeyre et al2009 Plasma Phys. Control. Fusion51 015013). It is shown that the growth of both Richtmyer–Meshkov and Rayleigh–Taylor instability (RTI) at the ablation front is reduced by laser pulses with an adiabat-shaping picket. An operating window for the parameters of the ignition laser spike is described; the threshold power depends on beam focusing and synchronization with the compression pulse. The time window for spike launch widens with beam power, while the minimum spike energy is independent of spike power. A large parametric scan indicates good tolerance (at the level of a few percent) to target mass and laser power errors. 2D simulations indicate that the strong igniting shock wave plays an important role in reducing deceleration-phase RTI growth. Instead, the high hot-spot convergence ratio (ratio of initial target radius to hot-spot radius at ignition) makes ignition highly sensitive to target mispositioning.

The results of the experimental study of confinement mode bifurcation performed on the TUMAN-3M tokamak are reported. As a trigger of confinement mode switching, plasma current ramp-up/-down or magnetic compression/decompression is used. It is found that the possibility and direction of confinement mode switching are correlated not with plasma current profile perturbation (peaking or broadening) but with the sign of toroidal electric field perturbation. A model connecting confinement bifurcation and toroidal electric field perturbation through the perturbation of the radial electric field is used to describe the phenomena observed in all eight scenarios investigated. This model ascribes the radial electric field generation to the non-compensated Ware drift of banana electrons at the TUMAN-3M peripheral plasma, where . Measurements of turbulence level and poloidal plasma rotation using microwave reflectometry are used to validate the model.

The effects of the equilibrium density gradient on non-axisymmetric magnetorotational instability are investigated in a pure axial magnetic field for ideal incompressible plasmas. A second-order ordinary differential equation is employed to determine the magnetic field perturbation and the full dispersion relationship regarding the non-axisymmetric magnetohydrodynamic instability in the presence of gravitation and density gradient effects. By means of local linear analysis, the reduced dispersion relationship is derived with a small azimuthal wavenumber. Spatial variations in the radial field perturbation magnitude cannot be neglected in the calculation since this term has the same order of magnitude as L_D, which is the scale length of radial density gradient. The analytical expression of the instability growth rate is presented. Our analysis shows that the instability criterion is modified by the density gradient which has a stabilizing effect when increasing outwards and conversely a destabilizing effect when decreasing outwards. The growth rate increases with L_D when L_D is small. For a sufficiently large L_D, the growth rate decreases with increasing L_D. The magnetic field exerts a similar effect on the growth rate and can totally quench the instability. The non-axisymmetric effect introduces a frequency shift and increases the growth rate but does not affect the instability criterion.

Gyrokinetic calculations indicate microtearing modes below the ion gyroscale are linearly unstable in a National Spherical Torus Experiment (NSTX) plasma. The modes are robustly unstable with respect to simulation parameters, radial location and discharge time. The modes exist at higher wavenumbers and exhibit narrower electric potential mode structures than conventional microtearing modes, but both modes extend to similar normalized radial wavenumbers. Mode growth rates increase with higher electron temperature gradients and higher collisionality. Finally, microtearing modes below the ion gyroscale are the most unstable modes near the magnetic axis, but electron temperature gradient modes are the most unstable modes in the outer plasma region.

Disruptions in a large tokamak can cause serious damage to the device and should be avoided or mitigated. Massive gas or killer pellet injection are possible ways to obtain a controlled fast plasma shutdown before a natural disruption occurs. In this work, plasma shutdown scenarios with different types of impurities are studied for an ITER-like plasma. Plasma cooling, runaway generation and the associated electric field diffusion are calculated with a 1D-code taking the Dreicer, hot-tail and avalanche runaway generation processes into account. Thin, radially localized sheets with high temperature can be created after the thermal quench, and the Dreicer and avalanche processes produce a high runaway current inside these sheets. At high impurity concentration the Dreicer process is suppressed but hot-tail runaways are created. Favorable thermal and current quench times can be achieved with a mixture of deuterium and neon or argon. However, to prevent the avalanche process from creating a significant runaway current fraction, it is found to be necessary to include runaway losses in the model.

Single helical axis configurations are emerging as the natural state for high-current reversed field pinch plasmas. These states feature the presence of transport barriers in the core plasma. Here we present a method for computing the equilibrium magnetic surfaces for these states in the force-free approximation, which has been implemented in the SHEq code. The method is based on the superposition of a zeroth-order axisymmetric equilibrium and of a first-order helical perturbation computed according to Newcomb's equation supplemented with edge magnetic field measurements. The mapping of the measured electron temperature profiles, soft x-ray emission and interferometric density measurements on the computed magnetic surfaces demonstrates the quality of the equilibrium reconstruction. The procedure for computing flux surface averages is illustrated, and applied to the evaluation of the thermal conductivity profile. The consistency of the evaluated equilibria with Ohm's law is also discussed.

We show that in perfectly quasi-isodynamic magnetic fields, which are generally non-quasisymmetric and which can approximate fields of experimental interest, concise expressions can be obtained for the flow, current and electric field. Here, we define a quasi-isodynamic magnetic field to be one in which the longitudinal adiabatic invariant is a flux function and in which the constant-B contours close poloidally. We first derive several geometric relations among the magnetic field components and the field strength. Using these relations, the forms of the flow and current are obtained for arbitrary collisionality. The flow, radial electric field and bootstrap current are then determined explicitly for the long-mean-free-path regime.

In previous experiments by the authors on a magnetic dipole interacting with a laser-produced plasma the generation of an intense field-aligned current (FAC) system on terrella poles was observed. In this paper the question of the origin of these currents in a low-latitude boundary layer of magnetosphere is investigated. Experimental evidence of such a link was obtained by measurements of the magnetic field generated by tangential drag and sheared stress. This specific azimuthal field was found to have quadruple symmetry and local maxima inside the magnetosphere adjacent to the boundary layer. Cases of metallic and dielectric dipole covers modeling good conductive and non-conductive ionosphere revealed that the presence or absence of FACs results in different amplitudes and spatial structures of the sheared field. The current associated with the azimuthal field flows upward at the dawnside, and toward the equator plane at the duskside. It was found to coincide by direction and to correspond by amplitude to a total cross-polar current measured independently. The results suggest that compressional and Alfvén waves are responsible for FAC generation. The study is most relevant to FAC generation in the magnetospheres of Earth and Mercury following pressure jumps in solar wind.

Data from forced Vertical Displacement Event (VDE) experiments in the Mega Ampère Spherical Tokamak (MAST) indicate that the plasma is highly destabilized above z displacements of ∼0.4 m. Previous work investigating the plasma response to vertical plasma perturbations in MAST had found it to be more non-linear than an equivalent conventionally shaped tokamak. Further investigation into the stability of the plasma at high z displacements is done using a linear vertical stability code. Theoretically it is found that the plasma becomes ideally unstable from z ∼ 0.8 m, corresponding to a significant acceleration of the vertical position experimentally.

In Greifswald/Germany W7-X, a new stellarator-type fusion plasma experiment, is currently being built. For the investigation of the divertor plasma two thermal helium beams are foreseen. This diagnostic is routinely used on several fusion plasma experiments and is capable of measuring radial profiles of electron density and temperature with good spatial and temporal resolution in the range of typical edge plasma parameters n_e = 10¹⁸–10¹⁹ m⁻³ and T_e = 20–200 eV. The penetration depth of the beam is limited by electron collisional ionization of the helium atoms and amounts to 3–8 cm in this parameter range. In this paper we investigate the beam propagation for detached plasma conditions in the W7-X divertor region (based on a background plasma simulated with a 3D plasma and neutral transport code EMC3/EIRENE), in which the electron density in the divertor may well exceed 10²⁰ m⁻³, as observed in the predecessor experiment W7-AS. In this regime the beam penetration drops to 1–2 cm. Through a Bayesian approach, we include uncertainties of all rate coefficients for electronic excitation and ionization used in the collisional–radiative model of atomic helium based on a steady-state approximation valid for a relaxed thermal or supersonic beam. Bayesian inversion of simulated signals for W7-X conditions provides a reliable quantitative estimation of the propagation of uncertainties of the atomic data to the n_e and T_e errors as well as input for potential improvements of the diagnostic setup. For example, the temperature error at T_e = 5 eV and n_e = 10²⁰ m⁻³ can be reduced from approximately 50% to 9% by absolute calibration of the observation system and fitting of three absolute line intensities instead of two line intensity ratios to the model.

The evolution of neoclassical tearing modes (NTMs) is usually described by the generalized Rutherford equation for a symmetric magnetic island. Despite the success of this representation, various experiments have found the evidence of asymmetries in the island geometry. A generalization of the model suggests that a number of effects, such as a quasi-linear correction of the constant-ψ approximation, a shear flow or a temperature gradient across the island, might be responsible for the deformation of the island geometry. In addition, it is noted that the symmetry is broken in the radial direction also by a finite third order derivative in the equilibrium helical flux function. This paper addresses the role of these asymmetries in the growth and suppression of NTMs in a slab geometry, with particular attention to the implications for the local current drive (ECCD) and resonant heating (ECRH) terms. The stabilizing contributions provided by electron cyclotron waves to NTMs are found to be largely unaffected by these perturbations. These results correct and extend some of the conclusions presented in Lazzaro and Nowak (2009 Plasma Phys. Control. Fusion51 035005).

The electromagnetic ion-cyclotron (EMIC) instabilities with isotropic ion beam and general loss-cone distribution of cold and hot core plasmas are discussed. The growth rate, parallel and perpendicular resonance energies of the electromagnetic ion-cyclotron waves in a low β (ratio of plasma pressure to magnetic pressure), homogeneous plasma have been obtained using the dispersion relation for cold and hot plasmas. The wave is assumed to propagate parallel to the static magnetic field. The whole plasma is considered to consist of resonant and non-resonant particles permeated by isotropic ion beam. It is assumed that resonant particles and ion beam participate in energy exchange with the wave whereas non-resonant particles support the oscillatory motion of the wave. We determined the variation in energies and growth rate in cold and hot plasmas by the energy conservation method with a general loss-cone distribution function. The thermal anisotropy of the core plasma acts as a source of free energy for EMIC wave and enhances the growth rate. It is noted that the EMIC wave emissions occur by extracting energy of perpendicularly heated ions in the presence of up flowing ion beam and steep loss-cone distribution in the anisotropic magnetosphere. The effect of the steep loss-cone distribution is to enhance the growth rate of the EMIC wave. The heating of ions perpendicular and parallel to the magnetic field is discussed along with EMIC wave emission in the auroral acceleration region. The results are interpreted for the space plasma parameters appropriate to the auroral acceleration region of the earth's magnetoplasma.

Fast ignition (FI) of a conically guided DT assembly by a laser-accelerated deuteron beam is proposed. The uniformly pre-compressed fuel of 300 g cm⁻³ is heated by the deuteron beam of a Maxwellian energy distribution with a temperature of 3 MeV. This scheme makes full use of the deposited energy of the alpha particles produced by the athermal nuclear reactions and can save about 4.5% ion-beam energy compared with the FI by fast proton or carbon ion beams. The ignition energy delivered by the external beam can be reduced appreciably.

The knock-on tail formations in fuel-ion velocity distribution functions by energetic alpha particles (by the T(d,n)⁴He reaction) and protons (by the D(d,p)T and ³He(d,p)⁴He reactions) are investigated by simultaneously solving the Boltzmann–Fokker–Planck (BFP) equations for deuteron, triton, ³He, alpha particle and proton in an ITER-like (DT) plasma admixed with a small amount of ³He. As a result of the ³He inclusion, a fraction of the transferred energy from energetic ions to thermal deuterons and tritons via nuclear plus interference (NI) scattering is reduced. Owing to the NI scattering of the energetic protons by fuel ions, the latter are knocked up to higher energies. The knocking-up effect of fuel ions is enhanced with increasing ³He concentration. It is shown that if ³He with relative concentration of 4.2%, i.e. , is included in T_e = 20 keV, n_e = 9.5 × 10¹⁹ m⁻³ plasma, the magnitude of the knock-on tail in deuteron distribution function in 300 keV–3 MeV energy range is reduced by about 15% from the value when ³He is not externally supplied. Such knock-on tail reduction also results in alternation of the non-Gaussian neutron emission spectrum with energies less than ∼13 MeV and above ∼15 MeV.

The transport of argon as trace impurity has been investigated in electron cyclotron resonance heated L-mode discharges at ASDEX Upgrade to test recent theories predicting the rise of an outward impurity convection. The profiles of the argon transport coefficients for r/a < 0.65 have been determined by analysing the linear flux–gradient dependence of the total argon ion density evolution after the puff. A new methodology to experimentally obtain the total impurity ion density from the integrated use of two spectroscopic diagnostic and the 1D impurity transport code STRAHL has been developed. Results confirm the enhancement in diffusivity and the rise of the positive convection observed in previous studies, but show for the first time how these effects are strongly localized around the electron cyclotron resonance heating deposition radius. These experimental results are found to be in promising qualitative agreement with a set of quasi-linear gyrokinetic simulations with the code GS2.