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We have conducted characterization of a scanning plasma enhanced chemical vapor deposition (PECVD) system for producing controlled non-uniform deposition or etching profiles. Both self-bias and plasma potential showed that the plasma conditions were disturbed significantly when the source was very close to the chamber wall but electron temperature and ion density were not affected significantly. It was found that a very thin but long tail of parasitic deposition was present over the entire large substrate. To eliminate the parasitic deposition an aperture (plasma guarding house) was constructed and was found to eliminate the parasitic plasma deposition. Deposited silicon nitride and silicon oxide thin films using the plasma guarding house in the scanning PECVD system showed very good optical properties similar to those obtained in conventional deposition methods. No multilayer structure was observed in TEM analysis on these films.

This paper discusses a novel method of plasma deposition or etching for producing uniform or controlled non-uniform profiles on large substrates. A plasma enhanced chemical vapour deposition (PECVD) system with dual electrodes was assembled using this method. An analytic expression for radial and angular movement of plasma source was given. Plasma diagnosis was carried out in order to understand the effect of the position of the plasma electrodes. We found that for a given phase difference between the two electrodes, the plasma potential was independent of the distance between the electrodes and the nearest wall when the distance was greater than 80 mm. When the distance was less than 80 mm, the floating potential varied significantly for a given phase difference. The nearest distance of the electrodes to chamber walls had a similar effect to the RF self-bias. PECVD thin film deposition was carried out for both uniform and controlled non-uniform profiles. The examples presented included linearly graded and parabolic profiles along the radius of the substrates. The results show that this method is capable of producing controlled thickness profiles on large substrates. The advantages of this method include simplicity, flexibility, and scalability.

This paper presents the results of measurements carried out on plasmas created in five different RF discharge systems. These systems all have two separately powered RF (13.56 MHz) electrodes, but differ in overall size and in the geometry of both vacuum chambers and RF electrodes or antennae. The two power supplies were synchronized with a phase-shift controller. We investigated the influence of the phase difference between the two RF electrodes on plasma parameters and compared the different system geometries. Single Langmuir probes were used to measure the plasma parameters in a region between the electrodes. Floating potential and ion density were affected by the phase difference and we found a strong influence of the system geometry on the observed phase difference dependence. Both ion density and floating potential curves show asymmetries around maxima and minima. These asymmetries can be explained by a phase dependence of the time evolution of the electrode–wall coupling within an RF-cycle resulting from the asymmetric system geometry.

Helium neutral beams of 29.5 keV amu−1 have been injected into helium plasmas on the JET tokamak. Thermal neutral helium in its ground state forms along the beam trajectory due to resonant double charge exchange. The helium neutrals form a halo about the neutral beam axis, and the latter is observed spectroscopically. The cross section for double charge exchange is derived relative to the cross section for beam emission. The double charge exchange spectrum is used to measure the plasma ion temperature and the frequency of toroidal rotation and can be used to infer the helium ion density. The potential for a He ash diagnostic based on observation of the helium halo is discussed.

Energetic neutral particles from neutral hydrogenic beam heating systems are widely used for active spectroscopic measurements of key plasma parameters in fusion experiments. Helium beams are used in dedicated diagnostic beamlines offering deeper penetration and resonant double charge exchange with alpha particles. Neutral beam systems using pure helium either require specialised helium gas pumping with a pumping speed in excess of 1000 m3/h or are restricted to short pulses (normally less than 1 s). A doped hydrogen/helium beam combines the requirements for plasma heating and diagnostics without the need for sophisticated helium pumping. A small flow of helium gas is injected into the plasma source for the time helium particles are required. The helium current is typically 10% of the total extracted current. The reduction in heating power of the doped beam can be kept below 5%. Doped deuterium/helium beams have been successfully tested and routinely used at JET. HeI beam emission spectra obtained with a doped deuterium/helium beam offer sufficiently strong visible lines for spectroscopic applications.

Modelling calculations for a fast He beam on its way through a tokamak plasma have been performed at TU Wien. They are based on the collisional-radiative ADAS model for He beams which delivers radially resolved population density and line intensity profiles for any initial population of a diagnostic He beam and any plasma structure. The effect of the metastable triplet population in the incident beam on the line emission profiles is shown. A sensitivity study of the optical lines with respect to electron temperature and density has been performed in the plasma edge and core regions. To validate these model calculations, experiments using fast He beams and beam emission spectroscopy have been performed at ASDEX Upgrade (IPP Garching) and JET by injecting He gas into the ion sources of the H- or D-heating beam systems.

Recent results from ASDEX Upgrade are presented. An improved understanding of energy and particle transport emerges in terms of a 'critical gradient' model for the temperature gradients. Coupling this to particle diffusion explains most of the observed behaviour of the density profiles, in particular the finding that strong central heating reduces the tendency for density profile peaking. Internal transport barriers with Te and Ti in excess of 20 keV (but not simultaneously) have been achieved. By shaping the plasma, a regime with small type II ELMs has been established. Here, the maximum power deposited on the target plates was greatly reduced at constant average power. Also, an increase of the ELM frequency by injection of shallow pellets was demonstrated. ELMfree operation is possible in the QH-mode regime previously found in DIII-D which has also been established on ASDEX Upgrade. Regarding stability, a regime with benign NTMs was found. During ECCD stabilisation of NTMs, bN could be increased well above the usual onset level without a reappearance of the NTM. ECRH and ECCD have also been used to control the sawtooth repetition frequency at a moderate fraction of the total heating power. The inner wall of the ASDEX Upgrade vessel has increasingly been covered with tungsten without detrimental effects on plasma performance. Regarding scenario integration, a scenario with a large fraction of noninductively driven current ( 50%), but without internal tranport barrier has been established. It combines improved confinement (tE=tITER98 1:2) and stability (bN 3:5) at high Greenwald fraction (ne=nGW 0:85) in steady state and with type II ELMy edge

Reducing the activation of the vessel and its components during the early phase of ITER operation is important. One scheme of minimising the activation is to use helium as the heating neutral beam species. This gives rise to a concern about how effective diagnostic spectroscopy will be with such beams. The diagnostic use of deuterium heating beams is well established yielding, for example, ion temperature profiles, plasma rotation, magnetic fields, impurity concentration, Zeff and beam penetration measurements. The experience with helium beams is more limited and there are greater uncertainties in the fundamental atomic data. A helium neutral beam has certain potential advantages: a reduced beam halo, no fractional energy components, greater penetration than deuterium beams and possibly more efficient charge exchange donation in certain energy regions.

Energetic neutral particles from neutral beam heating systems are widely used for active spectroscopic measurements of key plasma parameters in fusion experiments. Both the plasma discharges and the neutral beam systems are normally operated with hydrogen or deuterium. Helium beams are used in dedicated diagnostic beam lines as they offer deeper penetration and are subject to less background radiation and enable resonant double charge exchange with alpha particles. Neutral beam systems using pure helium either require specialized helium gas pumping with a pumping speed in excess of 1000 m3/h or are restricted to short pulses (normally less than 1 s). A doped hydrogen/helium beam combines the requirements for plasma heating and diagnostics without the need for sophisticated helium pumping. A small flow of helium gas is injected into the plasma source for the time helium particles are required. The helium current is typically 10% of the total extracted current. The reduction in heating power of the doped be...

Recent experiments at ASDEX Upgrade have achieved advanced scenarios with high β N (>3) and confinement enhancement over ITER98(y, 2) scaling, H H98y2 = 1.1-1.5, in steady state. These discharges have been obtained in a modified divertor configuration for ASDEX Upgrade, allowing operation at higher triangularity, and with a changed neutral beam injection (NBI) system, for a more tangential, off-axis beam deposition. The figure of merit, β N H ITER89-P , reaches up to 7.5 for several seconds in plasmas approaching stationary conditions. These advanced tokamak discharges have low magnetic shear in the centre, with q on-axis near 1, and edge safety factor, q 95 in the range 3.3-4.5. This q-profile is sustained by the bootstrap current, NBI-driven current and fishbone activity in the core. The off-axis heating leads to a strong peaking of the density profile and impurity accumulation in the core. This can be avoided by adding some central heating from ion cyclotron resonance heating or electron cyclotron resonance heating, since the temperature profiles are stiff in this advanced scenario (no internal transport barrier). Using a combination of NBI and gas fuelling line, average densities up to 80-90% of the Greenwald density are achieved, maintaining good confinement. The best integrated results in terms of confinement, stability and ability to operate at high density are obtained in highly shaped configurations, near double null, with δ = 0.43. At the highest densities, a strong reduction of the edge localized mode activity similar to type II activity is observed, providing a steady power load on the divertor, in the range of 6 MW m -2 , despite the high input power used (> 10 MW).