Table of contents

It is shown that a simple algorithm which exactly segregates between adiabatic and nonadiabatic electrons in particle-in-cell simulations of drift modes yields excellent conservation properties (e.g. particle number, energy) compared to the conventional δf scheme. The removal of the free streaming term in the evolution of the marker weight is shown to be responsible for the improved linear and nonlinear properties of the simulated plasma.

A successful demonstration of real-time, model-based control of the plasma current density profile has been made in JET. The safety factor profile was reconstructed using magnetic and polarimetric signals. Various predefined q-profile targets have been reached—in the least square approximation—using lower hybrid current drive as the only actuator, with a feedback control loop using the measurements from five fixed normalized radii.

This paper presents recent space resolved experimental study of polarization of He-like argon spectra in plasma focus. Consideration is made on dominant anisotropy mechanisms responsible for preferential orientation of different excited states at the moment of de-excitation in dense hot plasmas. Essential attention is given to alignment along the resulting electromagnetic field created as by exciting electron and also by the local electromagnetic field existing inside the plasma. Such study would provide the tool to diagnose distribution of electromagnetic fields and to count for anisotropy effects when calculating plasma parameters.

Silicon photodiodes of different types were tested for the possibility of measurement of high-intensity x-ray pulses from the plasmas produced by the 1 kJ PALS laser system in Prague. The x-ray energy range of the operation of the detectors was 1–20 keV. The tests were done with the use of different targets at the laser energy up to 740 J in the case of fundamental frequency (1ω, 1315 nm) and up to 230 J in the case of the 3rd harmonics (3ω). The detectors that were assigned for the measurement of the harder part of the spectrum (5–20 keV) can operate without overloading at all laser energies used, but the detectors for soft radiation can be easily overloaded above 200 J (at the fundamental frequency). Some problems with noise compatibility were encountered, as well. The possible improvements of detection systems for the future experiments are proposed. The detectors investigated in the experiment can be applied for finding the optimal condition for producing the hottest plasmas and for a precise calibration of the target positioning system.

Efficient production of highly charged (z>45) high-energy (up to ∼20 MeV) Ta ions by the interaction of a short-wavelength subnanosecond laser pulse with a solid target has been demonstrated by experimental studies of ion emission from plasma generated by the PALS high-energy short-wavelength (3ω₀) iodine laser system (⩽0.25 kJ, 0.4 ns, 0.438 μm, ⩽10¹⁶ W cm⁻²) in Prague. The plasma was investigated by means of ion diagnostics based on the time-of-flight method, i.e. ion collectors (ICs) and an electrostatic ion-energy analyser equipped with a windowless electron multiplier serving as an ion current detector. Five ICs were located at different angles with respect to the target normal. The existence of fast highly charged ions, as well as thermal and slow ions, has now been demonstrated for plasma produced by short-wavelength high-energy laser pulses, as it previously was for long-wavelength lasers

Integrated data analysis of fusion diagnostics is the combination of different, heterogeneous diagnostics in order to improve physics knowledge and reduce the uncertainties of results. One example is the validation of profiles of plasma quantities. Integration of different diagnostics requires systematic and formalized error analysis for all uncertainties involved. The Bayesian probability theory (BPT) allows a systematic combination of all information entering the measurement descriptive model that considers all uncertainties of the measured data, calibration measurements, physical model parameters and measurement nuisance parameters. A sensitivity analysis of model parameters allows crucial uncertainties to be found, which has an impact on both diagnostic improvement and design. The systematic statistical modelling within the BPT is used for reconstructing electron density and electron temperature profiles from Thomson scattering data from the Wendelstein 7-AS stellarator. The inclusion of different diagnostics and first-principle information is discussed in terms of improvements.

A correct estimation of plasma parameters in fusion experiments cannot be done unless the role of polarization of x-ray line emission is recognized.

An x-ray polarimeter is presented, based on single hexagonal crystal. The polarimeter selects two polarized components simultaneously. Simple analytical formulae are suggested to determine Bragg angle for analysed x-rays and for the directions of reflected polarized components. The polarimeter is expected to be especially useful to study polarization of x-rays emitted from plasmas with thermonuclear parameters.

In our previous experiments it has been found that bombardment of solid substrates, especially those having low heat conductivity, with plasma pulses of power density in MW cm⁻² range, produced by a rod plasma injector type of device leads to some loss of their mass. The aim of this paper is to clarify whether this mass loss is due to evaporation, sputtering, or ablation effects. In addition, we intended also to cast some light on the coating formation mechanism in the process referred to as `deposition by pulsed erosion (DPE)'. The dynamics of plasma–solid interaction was observed with the use of time-resolved optical spectroscopy technique.

The obtained results lead to two main conclusions. First, an interaction of gaseous plasma pulse of an energy density of 5 J cm⁻² with the surface of a solid sample (Al₂O₃ containing alloyed Ti) causes its ablation and forms (in case of Ti electrodes) an Al–Ti plasma plume of a temperature of (2–3)×10⁴ K. Second, the duration of a gaseous plasma pulse in DPE operation mode is approximately 1 μs.

The recent diagnostic developments on the large helical device (LHD) are described briefly. LHD is the largest helical machine with all superconducting coils, and its purpose is to prove the ability of a helical system to confine a fusion-relevant plasma in steady state. According to the missions of LHD research, the diagnostic devices are categorized as follows: diagnostics for (i) high nτ_ET plasmas and transport physics; (ii) magnetohydrodynamic stability; (iii) long pulse operation and divertor function; and (iv) energetic particles. These are briefly described focusing on the recent developments of the devices. Since the LHD experiment started in March 1998, five series of experimental campaigns have been carried out. The LHD diagnostics during these periods were operated successfully, and contributed to the analysis of the experimental results.

We describe methods for time-resolved imaging of the complex coherence of a spectral scene at one or more optical delays using electro-optically modulated polarization interferometers. By encoding the coherence information at harmonics of the electro-optic modulation frequency, it can be possible to obtain unambiguous spatio-temporal information about important physical parameters that govern the spectral content of the scene. We discuss the physical principles upon which the instrument is based and describe some applications in plasma spectroscopy, including Doppler tomography, Zeeman spectroscopy and relative line intensity measurements.

Bolometer diagnostics are installed on the Large Helical Device (LHD) to provide measurements of the total (broad spectrum) electromagnetic radiation emitted by the plasma. Three types of detectors are used: resistive metal film bolometers, absolute extreme ultraviolet photodiodes and infrared imaging video bolometers. Details of the installation in LHD are given for each type of detector. The detector calibration, data analysis and tomography techniques are described and compared and sample results are shown.

Internal transport barriers with respect to electron thermal transport (eITB) were observed in the large helical device, when the electron cyclotron resonance heating (ECH) power was highly localized on the centre of a plasma sustained by neutral beam injection. The eITB is characterized by a high central electron temperature of 6–8 keV with an extremely steep gradient, as high as 55 keV m⁻¹ and a low electron thermal diffusivity within a normalized average radius ρ≈0.3 as well as by the existence of clear thresholds for the ECH power and plasma collisionality.

Toroidal plasmas created with negative magnetic shear in the core region offer advantages in terms of MHD stability properties. These plasmas, transiently created in several tokamaks, have exhibited high-performance as measured by normalized stored energy and neutron production rates. A critical issue with extending the duration of these plasmas is the need to maintain the off-axis-peaked current distribution required to support the minimum in the safety factor q at large radii. We present equilibrium and transport simulations that explore the use of electron cyclotron heating and current drive to maintain this negative shear configuration. Using parameters consistent with DIII-D tokamak operation (Strait E et al 1995 Phys. Rev. Lett.75 4421, Rice B W et al 1996 Nucl. Fusion36 1271), we find that with sufficiently high injected power, it is possible to achieve steady-state conditions employing well aligned electron cyclotron and bootstrap current drive in fully non-inductively current-driven configurations.

The first measurement of an elevated temperature of the target gas in a neutralizer cell, resulting from interaction with the beam, is reported. Effects unique to the beam–plasma system in the neutralizer are considered with particular reference to the relevance of the experimental spectroscopic analysis technique. The gas is found to reach temperatures up to 900 K for operation with multi-megawatt positive ion beams.

The use of microwave reflectometry in the estimation of the bicoherency of the turbulent modes in plasmas is theoretically investigated. The full-wave two-dimensional model of microwave scattering in plasma is applied to simulate the reflectometer response to the turbulent fluctuations, consisting of random and coherent modes, and the simulated data ensemble is further processed to obtain the auto-bicoherence function. The sensitivity of the method was analysed for different ambient plasma profiles, geometry, fluctuation spectra and amplitudes.

It was found that for plasma conditions similar to the edge JT-60U tokamak plasma, reflectometry-based estimations of the bicoherency can prove the existence of the phase-coupled modes only if the ratio of coherent to random amplitudes is larger than unity. Reflectometry in plasmas with stronger curvature of the cut-off layer has a better sensitivity to coherent fluctuations as compared to weak-curvature profiles.

As an illustration, the results obtained were applied to the data ensemble acquired by the O-mode reflectometer on the JT-60U tokamak to demonstrate the existence of strong phase coupling in the H-mode plasma near the separatrix.

The effect of the q(r) profile on transport barrier formation has been investigated in the T-10 tokamak using rapid current ramp-up with electron cyclotron resonance heating (ECRH). The enhanced core confinement (formation of the internal transport barrier (ITB)) arises earlier than the additional current penetrates in the core region. It is suggested that this process takes place due to decrease of magnetic surface density in the region of the rational q = 2 surface due to β_p decrease, which is equivalent to dq/dr decrease near q = 2 surface. At various q-profiles, internal and external barriers were observed. Transient confinement improvement inside the ITB after the ECRH switch-off was also observed.

CO₂-laser-produced plasma ion component parameters were studied for aluminium and lead targets at laser intensity of P = 4×10¹³ W cm⁻² and pulse duration of τ = 15 ns experimentally and numerically. Angular dependences of ion number density for different charge states, average velocity and its spread were measured by time-of-flight method. Ion charge state distribution shows high-charge and low-charge state groups at normal expansion direction. Ions in these groups have different average expansion velocity and longitudinal velocity spread. Angular distribution of high-charge states is narrower than that of the low-charge state ion group, maximum yield of low-charge states occur at some angle from normal. For Al target results show similar trends as for Pb target, but simulations have indicated that the effect of laser ponderomotive force is more pronounced in this case.

The helicity injected torus (HIT-II) device is a spherical tokamak capable of both inductive (Ohmic) and co-axial helicity injection (CHI) current drive. The HIT-II plasma edge, in both Ohmic and CHI discharges, has been characterized using a triple Langmuir probe. An Ohmic discharge develops in two phases, a slide-away phase followed by a normal Ohmic discharge. During the normal Ohmic discharge, the floating potential is negative, just as in a conventional large-aspect-ratio tokamak. The plasma density increases sharply from the plasma edge into the centre. The auto-power spectrum of Ohmic plasma edge fluctuations shows a nearly constant auto-power at low frequencies, with auto-power decreasing at higher frequencies, similar to observations in conventional large-aspect-ratio tokamaks. In HIT-II CHI discharges, the magnetic field lines at the plasma edge are clearly connected to the injector electrodes, as expected. However, the time-evolution of the floating potential in the core plasma is significantly different from that of the edge, which may indicate a decoupling of the core plasma from the CHI electrodes. Finally, the fluctuations at the edge of high-performance CHI discharges exhibit a coherent oscillation at a frequency similar to that of the observed n = 1 mode.

Feedback reduction of nonlinear dissipative drift wave fluctuations is studied numerically. A linear feedback term is included in the Hasegawa–Wakatani model and a subsequent Galerkin projection provides a low dimensional model for the nonlinear dynamics near the instability threshold. We show that chaotic fluctuations can be completely suppressed provided that the time delay of the feedback is smaller than the correlation time of the fluctuations. Secondly, the full Hasegawa–Wakatani model is numerically simulated with feedback and it is found that the fluctuation energy of the fully developed drift wave turbulence can be reduced substantially.

Electron Bernstein waves (EBWs) produced via the linear conversion of incident electromagnetic modes have moderate parallel refractive index n_|| ⩽ 1 during their lifetime. Because of this they acquire large n_⊥ in the electron cyclotron resonance vicinity. Using this fact, an approximate dispersion relation valid regardless of the resonance harmonic number is proposed. It is expressed in terms of a modified plasma dispersion function whose real part is readily calculated numerically and the imaginary part is given by a simple analytical formula.

The relativistic case (n_|| ⩽ β) of EBWs is analysed in detail. It is shown that the waves with extremely small n_|| propagate with no damping and terminate abruptly at a surface shifted by a distance ∼n_||²R₀ off the resonance.

It is demonstrated both analytically and numerically that propagation of EBWs with moderate n_|| is satisfactorily described by the dispersion relation with n_|| = 0. In the large k_⊥ approximation this relation is rather simple and convenient for tracing waves, however, it breaks down far off the resonance and must be replaced by the exact dispersion relation with n_|| = 0. An effective algorithm for its fast computing is suggested.

Formation of a centrally peaked profile of the ion temperature by impurity seeding has been observed in all the largest tokamaks: JET, JT-60U, TFTR. A possible explanation of this phenomenon is proposed, which takes into account the suppression of ion temperature gradient instability induced anomalous transport through increasing ion effective charge Z_eff and Doppler shift of unstable modes, S_⊥. The particular importance of the S_⊥ component proportional to the temperature gradient, which stems from the pressure gradient and neoclassical poloidal rotation, is elucidated. It is demonstrated that this contribution grows significantly by impurity seeding when the rotation regime changes due to increased collisionality. An interrelation between the heating power density and Z_eff necessary for the transition to a peaked ion temperature is found. The modification of radial profiles by impurity seeding is modelled numerically and the results are in agreement with the experimental data from JET.

The experiments studying the electron heating in a magnetic trap by an incident wave at half gyrofrequency showed that there is an essential increase in electron energy and the intensity of the produced x-rays compared to the acceleration at gyrofrequency. The estimates of the influence of various non-linearities do not allow for a satisfactory explanation of this phenomenon. Proposed in this paper is the interpretation of the experimental results in the framework of a non-neutral plasma model. We show the existence of a strong resonance of an incident wave at half gyrofrequency in non-neutral plasma created by an excess of electron density. The analysis of confinement of a cylindrical electron layer is carried out both in radial and in axial directions; we have taken into account several factors such as mirror geometry, pulse mode and relativistic influence. We made separate estimates for each mechanism independently. They show that the proposed mechanisms are quite adequate for the explanation of the increase in x-ray energy and intensity.

For the stellarator-type devices of today, which are very different from the quasi-symmetric ones, the analytical conditions for the existence of two kinds of ripple wells along field lines are derived taking into account the multiple helicity and multiple toroidicity of the magnetic field. It is shown that a typical feature of the above-mentioned devices, in contrast to a classical heliotron, is a considerable deformation of magnetic wells, both ordinary and secondary, by additional ripple fields. When this deformation exceeds a critical level, one kind of magnetic ripple well can be suppressed in some toroidal region. This happens if the amplitudes of the helical and toroidal magnetic fields and the amplitudes of the ripple fields are correlated with each other through the relations known as conditions for parametric resonance. When the parametric resonance conditions are fulfilled, charged particle confinement is improved because of a suppression of the toroidal drift.

Based on the models describing the behaviour of neoclassical tearing modes (NTMs), fits for the seed-island triggered excitation and disappearance of these modes at small β-values have been tested for an extended parameter range in the ρ_pi*–_ii plane in ASDEX Upgrade.

The disappearance of NTMs at the marginal β_p,marg during power ramp-down phases provides input for a direct comparison with models without the influence of the seed-island size. It has been found that for low q₉₅ the NTM remains present until the HL-transition, whereas for higher q₉₅ the mode decays well in H-mode. Scalings for the onset and the marginal β_p for the (3/2)-NTM and the analysis of marginal β_p for the (2/1)-NTM are presented.