D. Dodt

Heat loads on JET plasma facing components from ICRF and LH wave absorption in the SOL

In JET, lower hybrid (LH) and ion cyclotron resonance frequency (ICRF) wave absorption in the scrape-off layer can lead to enhanced heat fluxes on some plasma facing components (PFCs). Experiments have been carried out to characterize these heat loads in order to: (i) prepare JET operation with the Be wall which has a reduced power handling capability as compared with the carbon wall and (ii) better understand the physics driving these wave absorption phenomena and propose solutions for next generation systems to reduce them. When using ICRF, hot spots are observed on the antenna structures and on limiters close to the powered antennas and are explained by acceleration of ions in RF-rectified sheath potentials. High temperatures up to 800??C can be reached on locations where a deposit has built up on tile surfaces. Modelling which takes into account the fast thermal response of surface layers can reproduce well the surface temperature measurements via infrared (IR) imaging, and allow evaluation of the heat fluxes local to active ICRF antennas. The flux scales linearly with the density at the antenna radius and with the antenna voltage. Strap phasing corresponding to wave spectra with lower k? values can lead to a significant increase in hot spot intensity in agreement with antenna modelling that predicts, in that case, an increase in RF sheath rectification. LH absorption in front of the antenna through electron Landau damping of the wave with high N? components generates hot spots precisely located on PFCs magnetically connected to the launcher. Analysis of the LH hot spot surface temperature from IR measurements allows a quantification of the power flux along the field lines: in the worst case scenario it is in the range 15?30?MW?m?2. The main driving parameter is the LH power density along the horizontal rows of the launcher, the heat fluxes scaling roughly with the square of the LH power density. The local electron density in front of the grill increases with the LH launched power; this also enhances the intensity of the LH hot spots.

Integration of a radiative divertor for heat load control into JET high triangularity ELMy H-mode plasmas

Experiments on JET with a carbon-fibre composite wall have explored the reduction of steady-state power load in an ELMy H-mode scenario at high Greenwald fraction similar to 0.8, constant power and close to the L to H transition. This paper reports a systematic study of power load reduction due to the effect of fuelling in combination with seeding over a wide range of pedestal density ((4-8) x 10(19) m(-3)) with detailed documentation of divertor, pedestal and main plasma conditions, as well as a comparative study of two extrinsic impurity nitrogen and neon. It also reports the impact of steady-state power load reduction on the overall plasma behaviour, as well as possible control parameters to increase fuel purity. Conditions from attached to fully detached divertor were obtained during this study. These experiments provide reference plasmas for comparison with a future JET Be first wall and an all W divertor where the power load reduction is mandatory for operation.

Lower hybrid current drive for the steady-state scenario

Lower hybrid current drive (LHCD) experiments performed at a density close to that required for the steady-state (SS) scenario are reported. On C-Mod, FTU and Tore Supra, a strong decay of the bremsstrahlung emission is observed when the density is increased, much faster that the prediction of LHCD modelling. On JET, LH power deposition is also found to be sensitive to the plasma density: LH power modulation indicates that the power deposition moves to the very edge of the plasma (r/a ~ 0.9) when the density approaches the requirement of the JET SS scenario. From this experiment but also from the reconstruction of the electron cyclotron emission spectrum, the decrease in the LHCD efficiency with density is also found. From LHCD modelling of different JET pulses performed at different densities and wave parallel refraction indexes, it is concluded that the wave accessibility condition is not the key parameter for explaining the decrease in the efficiency. C-Mod, FTU and Tore Supra experiments indicate that the plasma edge parameters, namely density and temperature but also fluctuations, are affecting the efficiency via loss mechanisms which are likely to be collisional damping (C-Mod), parametric decay instabilities or wave scattering (FTU/Tore Supra).

Effect of nitrogen seeding on the energy losses and on the time scales of the electron temperature and density collapse of type-I ELMs in JET with the ITER-like wall

The baseline type-I ELMy H-mode scenario has been re-established in JET with the new tungsten MKII-HD divertor and beryllium on the main wall (hereafter called the ITER-like wall, JET-ILW).The first JET-ILW results show that the confinement is degraded by 20–30% in the baseline scenarios compared to the previous carbon wall JET (JET-C) plasmas. The degradation is mainly driven by the reduction in the pedestal temperature. Stored energies and pedestal temperature comparable to the JET-C have been obtained to date in JET-ILW baseline plasmas only in the high triangularity shape using N2 seeding.This work compares the energy losses during ELMs and the corresponding time scales of the temperature and density collapse in JET-ILW baseline plasmas with and without N2 seeding with similar JET-C baseline plasmas. ELMs in the JET-ILW differ from those with the carbon wall both in terms of time scales and energy losses. The ELM time scale, defined as the time to reach the minimum pedestal temperature soon after the ELM collapse, is ~2 ms in the JET-ILW and lower than 1 ms in the JET-C. The energy losses are in the range ΔWELM/Wped ≈ 7–12% in the JET-ILW and ΔWELM/Wped ≈ 10–20% in JET-C, and fit relatively well with earlier multi-machine empirical scalings of ΔWELM/Wped with collisionality. The time scale of the ELM collapse seems to be related to the pedestal collisionality. Most of the non-seeded JET-ILW ELMs are followed by a further energy drop characterized by a slower time scale ~8–10 ms (hereafter called slow transport events), that can lead to losses in the range ΔWslow/Wped ≈ 15–22%, slightly larger than the losses in JET-C. The N2 seeding in JET-ILW significantly affects the ELMs. The JET-ILW plasmas with N2 seeding are characterized by ELM energy losses and time scales similar to the JET-C and by the absence of the slow transport events.

Upgrade of the lithium beam diagnostic at JET

A 60 kV neutral Li beam is injected into the edge plasma of JET to measure the electron density. The beam observation system has been improved by replacing a Czerny-Turner spectrometer with a high-resolution transmission-grating spectrometer and a fast back-illuminated frame-transfer camera. The larger throughput of the spectrometer, the increased sensitivity, and the faster readout of the new camera allow inter-ELM (edge localized mode) measurements (frame rate of 100 Hz). The calibration of the setup, as well as an improved spectral fitting technique in the presence of carbon background radiation, is discussed in detail. The density calculation is based on a statistical analysis method. Results are presented for different plasma scenarios.

Reconstruction of an electron energy distribution function using integrated data analysis

The non-Maxwellian electron energy distribution function (EEDF) in the positive column of a neon dc-discharge was reconstructed from the visible emission spectrum obtained with an overview spectrometer. The analysis is based on Bayesian probability theory (integrated data analysis), which allows for the use of the full information in the spectral data by incorporating all important underlying physical mechanisms. The data are described by a collisional–radiative model and a statistical description of the spectroscopic measurement. An extensive and consistent set of electron impact excitation cross sections and Einstein coefficients obtained through semi-relativistic B-Spline R-Matrix calculations is employed, and estimates for uncertainties are included in the analysis. The results are consistent with theoretical modelling and reference measurements.

Recent improvements of the JET lithium beam diagnostic

A 60 kV neutral lithium diagnostic beam probes the edge plasma of JET for the measurement of electron density profiles. This paper describes recent enhancements of the diagnostic setup, new procedures for calibration and protection measures for the lithium ion gun during massive gas puffs for disruption mitigation. New light splitting optics allow in parallel beam emission measurements with a new double entrance slit CCD spectrometer (spectrally resolved) and a new interference filter avalanche photodiode camera (fast density and fluctuation studies).

Validation of atomic data using a plasma discharge

Using a neon discharge as a well-assessed reference, we demonstrate the validation of atomic data for discharge modeling with data from emission spectroscopy. Specifically, a collisional radiative model of a neon dc discharge was set up using a set of structure and collision data from a semirelativistic B-spline R-matrix calculation by Zatsarinny and Bartschat (2004 J. Phys. B: At. Mol. Opt. Phys. 37 2173) and the electron-energy distribution function of the plasma was determined from the spectroscopic measurement by Dodt et al (2008 J. Phys. D: Appl. Phys. 41 205207). Since the model covers almost the entire visible spectrum, considering a large number of emission lines and many important collisional coupling mechanisms, it enables us to thoroughly test the consistency of the modeled excited-state populations. Inconsistencies, which appear as correction factors for the rate coefficients, are extracted by means of Bayesian probability theory. Despite its limitations, the sensitivity of the approach was sufficient to provide critical information about the collision data, especially in cases where standard cross-section measurements using merged electron and atom beams are difficult to perform. The present approach thus complements existing experimental techniques to test theoretical predictions.

Bayesian modelling of a thermal helium beam for measurement of electron density and temperature in the W7-X divertor plasma

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 ne = 1018–1019 m−3 and Te = 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 ne and Te errors as well as input for potential improvements of the diagnostic setup. For example, the temperature error at Te = 5 eV and ne = 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.

Axisymmetric oscillations at L-H transitions in JET: M-mode

L to H transition studies at JET have revealed an n = 0, m = 1 magnetic oscillation starting immediately at the L to H transition (called M-mode for brevity). While the magnetic oscillation is pres ...