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The confinement of fast particles is of crucial importance for the success of future burning plasma experiments. On JET, the confinement of ion cyclotron resonant frequency (ICRF) accelerated fast hydrogen ions with energies exceeding 5?MeV has been measured using the characteristic ?-rays emitted through their inelastic scattering with carbon impurities, 12C(p,p'?)12C. Recent experiments have shown a significant decrease in this ?-ray emission (by a factor of 2) during so-called tornado mode activity (core-localized toroidal Alfvén eigenmodes (TAEs) within the q= 1 surface) in sawtoothing plasmas. This is indicative of a significant loss or extensive re-distribution of these (>5?MeV) particles from the plasma core. In this paper, mechanisms responsible for the radial transport and loss of these fast ions are investigated and identified using the HAGIS code, which describes the interaction of the fast ions and the TAE observed. The calculations show that the overlap of wave-particle resonances in phase-space leads to an enhanced radial transport and loss. On both JET and ASDEX Upgrade, new fast ion loss detectors have been installed to further investigate the loss of such particles. On JET, fast ion loss detectors based around an array of Faraday cups and a scintillator probe have been installed as part of a suite of diagnostic enhancements. On ASDEX Upgrade, a new fast ion loss detector has been mounted on the mid-plane manipulator allowing high resolution measurements in pitch angle, energy and time. This has enabled the direct observation of fast ion losses during various magnetohydrodynamics (MHD) phenomena to be studied in detail. Edge localised mode (ELM) induced fast ion losses have been directly observed along with the enhancement of fast ion losses from specific areas of phase-space in the presence of neoclassical tearing modes (NTMs) and TAEs.

The existence of an electron heat pinch has been experimentally investigated in ASDEX-Upgrade in plasmas with strong off-axis electron cyclotron heating (ECH). Localization of 1.6?MW of ECH power at ?dep= 0.65 in plasmas with low Ohmic power (<100?kW) still resulted in peaked electron temperature (Te) profiles at densities ne0~ 2.7 × 1019?m?3. Two out of four gyrotrons were modulated in order to study convective effects also on the Tetransient response at different frequencies. Peculiar profiles of the modulation amplitude have been measured, very flat in the region just inside ?dep(0.3 < ?tor< 0.65) or even increasing rather than decreasing away from ?depon the inner side. The phase profiles instead regularly indicate the propagation of the heat wave in both directions away from ?dep. Power balance analysis shows that the heat flux in the core region is very close to zero, but uncertainties do not allow a clear determination of its sign. Modelling steady-state and modulation results with an empirical model featuring a diffusive term characterized by a critical gradient length (LTe= ?Te/?Te) behaviour and a constant Gaussian shaped heat pinch profile allows good reproduction of the data when the region just inside ?depis close to the critical threshold and oscillates around it, and a small convective term U~ 1-2?m?s?1is present in the same region. At higher density the Teprofile becomes flatter and the amplitude profiles recover the usual shape with stronger decay inside with respect to outside ?dep. In this case no heat pinch is needed and the plasma goes below the critical threshold inside ?dep. The results are consistent with the theoretical prediction of a turbulence regulated by a threshold in R/LTeand a small turbulence generated electron heat pinch term. However initial attempts to perform first principle-based simulations using the quasi-linear electrostatic drift wave Weiland model have failed to reproduce the experimental results.

Plasma operation with high-Z plasma facing components is investigated with regard to sputtering, core impurity contamination and scenario restrictions. A simple model based on dimensionless quantities for fuel and high-Z ion sources and transport to describe the high-Z concentration in the plasma core is introduced. The impurity release and further transport is factorized into the sputtering yield, the relative pedestal penetration probability and a core confinement enhancement factor. Since there are quite large uncertainties, in particular, in the sputtering source and the edge transport of high-Z impurities, very different scenarios covering a wide parameter range are taken into account in order to resolve the experimental trends. Sputtering of tungsten by charge exchange neutrals in the energy range 0.5–2 keV is comparable to the effect of impurity ion sputtering, while the impact of thermal fuel ions is negligible. Fast ions produced by neutral beam injection as well as sheath acceleration during ICR heating may cause considerable high-Z sources if the limiters on the low-field side have high-Z surfaces. The critical behaviour of the central high-Z concentration in some experimental scenarios could be attributed to edge and core transport parameters in the concentration model. The improved H-mode with off-central heating turns out to be the most critical one, since a hot edge is combined with peaked density profiles.

Plasma operation with high-Zplasma facing components is investigated with regard to sputtering, core impurity contamination and scenario restrictions. A simple model based on dimensionless quantities for fuel and high-Zion sources and transport to describe the high-Zconcentration in the plasma core is introduced. The impurity release and further transport is factorized into the sputtering yield, the relative pedestal penetration probability and a core confinement enhancement factor. Since there are quite large uncertainties, in particular, in the sputtering source and the edge transport of high-Zimpurities, very different scenarios covering a wide parameter range are taken into account in order to resolve the experimental trends. Sputtering of tungsten by charge exchange neutrals in the energy range 0.5-2?keV is comparable to the effect of impurity ion sputtering, while the impact of thermal fuel ions is negligible. Fast ions produced by neutral beam injection as well as sheath acceleration during ICR heating may cause considerable high-Zsources if the limiters on the low-field side have high-Zsurfaces. The critical behaviour of the central high-Zconcentration in some experimental scenarios could be attributed to edge and core transport parameters in the concentration model. The improved H-mode with off-central heating turns out to be the most critical one, since a hot edge is combined with peaked density profiles.

Significant progress has been made on ASDEX Upgrade during the last two years in the basic understanding of transport, in the extension of the improved H-mode in parameter space and towards an integrated operating scenario and in the development of control methods for major performance limiting instabilities. The important features were the understanding of particle transport and the control of impurity accumulation based on it, the satisfactory operation with predominantly tungsten-clad walls, the improved H-mode operation over density ranges and for temperature ratios covering (non-simultaneously) the ITER requirements on ν*, n/nGW and Te/Ti, the ELM frequency control by pellet injection and the optimization of NTM suppression by DC-ECCD through variation of the launching angle. From these experiments an integrated scenario has emerged which extrapolates to a 50% improvement in n T τ or a 30% reduction of the required current when compared with the ITER base-line assumptions, with moderately peaked electron and controllable high-Z density profiles.

Recent results from the ASDEX Upgrade experimental campaigns 2001 and 2002 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 (ITBs) with electron and ion temperatures in excess of 20 keV (but not simultaneously) have been achieved. By shaping the plasma, a regime with small type II edge localized modes (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. ELM free operation is possible in the quiescent H-mode regime previously found in DIII-D which has also been established on ASDEX Upgrade. Regarding stability, a regime with benign neoclassical tearing modes (NTMs) was found. During electron cyclotron current drive (ECCD) stabilization of NTMs, β_N could be increased well above the usual onset level without a reappearance of the NTM. Electron cyclotron resonance heating 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 causing detrimental effects on the plasma performance. Regarding scenario integration, a scenario with a large fraction of noninductively driven current (50%), but without ITB has been established. It combines improved confinement (τ_E/τ_ITER98 ≈ 1.2) and stability (β_N  3.5) at high Greenwald fraction (n_e/n_GW ≈ 0.85) in steady state and with type II ELMy edge and would offer the possibility for long pulses with high fusion power at reduced current in ITER.

Recent experiments at ASDEX Upgrade have achieved advanced scenarios with high β_N (>3) and confinement enhancement over ITER98(y, 2) scaling, H^H98y2&equal;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_NH^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, q₉₅ 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&percent; 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&equal;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).

The elements of transport into and across the scrape-off layer in the poloidal divertor tokamak ASDEX Upgrade are analysed for different operational regimes with emphasis on enhanced confinement regimes with an edge barrier. Utilizing the existing set of edge diagnostics, especially the high-resolution multi-pulse edge Thomson scattering system, in combination with long discharge plateaus, radial sweeps and advanced averaging techniques, detailed radial mid-plane profiles of diverted plasmas are obtained. Profiles are smooth across the separatrix, indicating strong radial correlation, and there is no remarkable variation across the second separatrix either. Together with measured input, recycling, pumping and bypass fluxes, a corrected separatrix position is determined and transport characteristics are derived in the different radial zones generally identified in the profile structure. Transport in the steep gradient region inside and across the separatrix shows typical ballooning-type critical electron pressure gradient scaling and, in parallel, even a clear correlation between radial electron density and temperature decay lengths (e.g. η_e = d(ln T)/d(ln n)~2 for type-I ELMy H-modes). These findings indicate the importance of stiff profiles in this region, while diffusion coefficients are secondary parameters, determined essentially by the source distribution. The outer scrape-off layer wing exhibits a more filamentary structure with preferential outward drift especially in high-performance discharges, with formal diffusion coefficients far above the Bohm value in agreement with results on the old ASDEX experiment. A basic mechanism involved there seems to be partial loss of equilibrium and fast curvature-driven outward acceleration, in principle well known from theory, investigated decades ago in pinch experiments and utilized recently in high-field-side pellet fuelling.

The H-mode standard shot of ASDEX Upgrade provides an overview of the long term evolution of machine conditions and scatter of H-mode power threshold and confinement. This type-I ELMy H-mode is run every day of plasma operation since 1999. After a long opening for in-vessel work, the power threshold drops rapidly by about 50% over 100 discharges and then much slower by about 20% over the remaining 1000 plasma discharges made in a campaign. At the natural density without gas puffing, confinement increases by about 30% over about 500 discharges. Afterwards the peak-to-peak scatter is about 20%, smaller at high density. Conditioning causes changes of edge density and temperature, similar to the confinement degradation observed with gas puffing.

A detailed Monte Carlo-based numerical analysis of ASDEX Upgrade discharges is presented, examining the applicability of neutral particle analysers (NPAs) for measuring the central ion temperature from the neutral particle spectra above the neutral beam injection energy. The analysis is motivated by an earlier work carried out for TFTR (Fiore C L et al 1988 Nucl. Fusion 28 1315), where the beams were quite tangential. This work is now extended to investigate the applicability of the method when the beams are perpendicular. The Monte Carlo simulations take fully into account the effects due to finite orbits and realistic magnetic geometry, and reveal that the fast ion distribution above the beam energy, generated by collisions, reflects the background temperature with good accuracy. However, it is found that the use of NPA tends to give temperature values that are up to 30% lower than the on-axis temperature T_i(0) assumed for the background. This is because the neutral fluxes measured by NPA originate from a wide radial region, and thus the measured spectrum is a chord-averaged one. A fairly tangential viewing is found to be optimal because in that case the signal originating from the centre of the plasma is enhanced relative to the signal from outer radii. It is concluded that, in discharges where the slowing down from electrons does not dominate the beam ion dynamics, NPA can give a reliable estimate of an average central temperature, T_i(ρ = 0,...,0.3).

The transmission of fused and crystalline quartz, teflon, polyethylene and TPX was measured at mm- and sub-mm-wavelengths after neutron and γ-irradiation of the samples at doses comparable with typical values for vessel windows in ignited fusion experiments. Whereas fused and crystalline quartz and — with some restriction —teflon and polyethylene retained their transparency even after such extreme exposures, the transmission of TPX was degraded by irradiation doses one or two orders of magnitude below those expected for such experimental conditions.

Information concerning the effect of irradiation on the optical properties of materials in the infrared, sub-mm-and mm-wavelength regions is of increasing importance in fusion plasma diagnostics. The radiation induced transmission loss of a number of materials has therefore been investigated at wavelengths in the ranges 200 nm to 40 μm and 0.23 to 2.0 mm. The samples were irradiated with doses of up to 1010 rad in a nuclear reactor. While germanium shows considerable transmission loss at doses as low as 106 rad, and the transmission of TPX decreases at 109 rad, other materials, e.g. fused quartz and possibly ZnSe, exhibit sufficient radiation hardness for use in fusion plasma diagnostics.

Ion temperature profiles from a neutral particle analyser are used for a thorough analysis of the ion thermal conductivity. In Ohmic ASDEX plasmas, two distinct regimes of energy confinement are possible which differ in the ion heat transport properties. The Improved Ohmic Confinement regime is reached through a transition from anomalous to neoclassical ion heat transport. The transition could be caused by a modification of the sawtooth behaviour.

The gamma decay mode and the spin value of the 1.89 MeV state of42Sc were investigated by studying the40Ca(3He,p γ)42Sc reaction at 10.0 MeV energy of the incident3He beam. The analysis of thep-γangular correlation of the 1.89 MeV ground state transition — using method II of Litherland and Ferguson — yields the spin assignmentJ=1 for this level.