Table of contents

An overview of the most recent experimental and modelling efforts on powder formation in reactive plasmas is given. The physics and chemistry of these dusty plasmas and their fundamental mechanisms leading to the production of nanometre-sized particles and their successive agglomeration leading to micrometre-sized particles are reviewed. The central role of particle charging and of charge fluctuations regarding the particle agglomeration is emphasised. Finally, the influence of the dust particles on the plasma parameters is described and an outlook on the most eminent problems towards the understanding of the reactive, dusty plasmas is given.

Recovery of the pressure profile is examined for the W7-AS stellarator on an equilibrium database using only simulated diagnostic information from external magnetic probes. This analysis establishes the extent of profile-relevant information contained in an extensive set of external magnetic data in isolation from measurement error, hitherto a subject of some debate. It is demonstrated that the profile can be recovered up to and including an average profile and a peaking factor. The implications for magnetic-based stellarator equilibrium reconstructions are considered.

A magnetic spectrometer positioned on the axis of a 3 kJ plasma focus device has been used to measure energy spectra of deuterons emitted from the pinch region of the focus, while an indium activation detector was employed to measure the associated neutron flux. Solid-state nuclear track detectors (PM-355) were used in the focal plane of the spectrometer to detect the analysed ions, with multiple spectra being recorded on each detector. An automated track counting system has been developed to read the spectral information from the detectors, which has facilitated the analysis of a far greater number of energy spectra, with greater accuracy, than has been possible in previous work. This automated system comprises an optical microscope with CCD camera, a motorized microscope stage, and image processing and control software. The experimental results show that the deuterons emitted from the pinch region have a continuous spectrum of energies which follow the empirical relationship dN/dE∝E^-n, where n is typically in the range 6-7. A clear correlation between the measured deuteron energy spectra and neutron flux is observed at each of three working gas pressures investigated.

The correlation version of the enhanced-scattering diagnostic is theoretically analysed and applied to diagnose ion acoustic turbulence, revealing its ability to measure the wavenumber and frequency spectra of small-scale fluctuations in a magnetized plasma.

The stability of the ideal internal kink mode is calculated, taking into account the kinetic response of thermal ions in the external region and the singular layer. By extending the collisionless dispersion relation to include the equilibrium radial electric field it is found that the stability of the internal kink mode depends sensitively on sheared toroidal plasma rotation. The sheared toroidal plasma rotation can increase the critical pressure for internal kink mode displacements typically by a factor of two.

Edge cooling experiments have been performed in Tore Supra by impurity injection and oblique pellet injection. Non-local transport (NLT) features have been confirmed in both cases in the ohmic regime. The pellet injection experiments have shown the existence of a threshold on the plasma density ⟨n_e⟩ and current I_p for the observation of this effect: ⟨n_e⟩/I_p^1/2⩽1.0×10¹⁶ (m^-3 A^-1/2). A second threshold on the relative density variation caused by the pellet injection has also been observed: Δ⟨n_e⟩/⟨n_e⟩⩽0.5. It has been observed that in the L regime during lower hybrid current drive (LHCD) experiments these limits are extended: [⟨n_e⟩/I_p^1/2]_crit = 1.4×10¹⁶ (m^-3 A^-1/2) and [Δ⟨n_e⟩/⟨n_e⟩]_crit⩾0.65. Observations in a LHCD scheme with fully non-inductive plasma current seem to indicate that the mechanism governing the NLT is unlikely to be linked to the plasma current profile effects including the spatial redistribution, the magnetic shear, and the low-m MHD modes.

The fast ions produced inside a mirror trap by neutral beam injection could form periodic short-lived density peaks near a turning point if the injection energy is properly modulated in time. Achievable parameters of fast ion bunches thus formed are analysed in this paper. The theory is illustrated by estimates for the neutron source based on a gas-dynamic trap. The bunching of deuterium and tritium ions can produce periodic short bursts of neutron radiation with intensity 1.5 times higher than the average level. The modulation of the neutron flux could extend the field of application for the neutron source. Also, the bunching could serve as a precise plasma diagnostic in mirror traps.

The atoms dissociated from desorbed molecules occupy a large fraction of the neutrals in edge plasmas. Experiments of Balmer line emission, both in our linear device MAP [1] and in TEXTOR [2], show a group of atoms at very low energy (<1 eV). This paper evaluates the energy distributions of dissociated atoms of the most important reaction channels of H₂. It is shown that the vibrational excitation can lead to a broad range energy distribution for the H₂ dissociated into ground state atoms. However, the contribution to the dissociated atoms at energies below 0.5 eV, is not so significant. The photon emission rate starting from H(2s) excitation is calculated and implemented in the DEGAS 2 modelling for our experiment case. From Hα modelling via DEGAS 2, it is concluded that the reaction channel, H₂ + e→ H₂(1s σ, n lλ|¹Λ)→ e + H (1s) + H (2s), and especially one of the dissociated products H(2s) may be the main source contributing to the Balmer line emissions where the low-energy component was observed.

The helical distortion of a compact tokamak plasma with currents in modular Furth-Hartman type coils is investigated with respect to the impact on the magnetic field structure and to the ideal magnetohydrodynamic stability. A sequence of configurations, in which the current in the helical coils is increased up to the level of that of the main toroidal coils, is generated. The magnetic field structure acquires a ripple along the field lines when the currents in both sets of coils becomes comparable. The magnitude of this ripple depends on the modulation of the Furth-Hartman coils on their winding surface. The local ideal ballooning stability properties deteriorate with the increasing helical deformation of the plasma shape because the normal magnetic field line curvature becomes larger in regions where the helical contribution enhances the toroidicity component while the local magnetic shear vanishes. The global external kink stability properties, in contrast, improve because the normal curvature on average does not change while the local magnetic shear acquires a noticeable helical modulation.

Radially peaked density profiles were found in discharges heated purely with ECH in the currentless W7-AS stellarator. Changes in the central particle source are not significant and do not explain the density peaking. This is evidence for inward convection in W7-AS. The existence of the inward convection is confirmed with two independent transient transport studies. Flatness of the temperature profiles is essential for the density peaking, and an anti-correlation was found between the density and temperature gradients. The density peaks as the ECH-deposition is moved outwards. The density peaking parameter was found to decrease with increasing heating power. No clear density dependence was found. Strong gas puffing, or the corresponding high edge density prevents the formation of the peaked profiles.

In the Rijnhuizen Tokamak Project (RTP) plasmas with electron cyclotron heating (ECH), a transient rise of the core T_e is observed when hydrogen pellets are injected tangentially to induce fast cooling of the peripheral region. The core T_e rise is a sharp function of the normalized power deposition radius (ρ_dep). The strongest effect (δT_e up to 80%) is observed when ρ_dep is just inside the q = 1 surface, whereas only modest effects are seen with larger or smaller ρ_dep. The rise is due to both a steepening of the T_e profile between the q = 1 and q = 2 surfaces, and a peaking inside ρ_dep.