We present the results of experiments in JET to study the effect of plasma shape on high density ELMy H-modes, with geometry of the magnetic boundary similar to that envisaged for the standard Q = 10 operation in ITER. The experiments described are single lower null plasmas, with standard q profile, neutral beam heating and gas fuelling, with average plasma triangularity δ calculated at the separatrix ∼0.45-0.5 and elongation κ ∼ 1.75. In agreement with the previous results obtained in JET and other divertor Tokamaks, the thermal energy confinement time and the maximum density achievable in steady state for a given confinement enhancement factor increase with δ. The new experiments have confirmed and extended the earlier results, achieving a maximum line average density n e ∼ 1.1n GR for H 98 ∼ 0.96. In this plasma configuration, at 2.5 MA/2.7 T (q 95 ∼ 2.8), a line average density ∼95% n GR with H 98 = 1 and β N ∼ 2 are obtained, with plasma thermal stored energy content W th being approximately constant with increasing density, as long as the discharge maintains Type I ELMs, up to n ped ∼ n GR (and n e ∼ 1.1n GR). A change in the Type I ELMs behaviour is observed for pedestal densities n ped 70% n GR , with their frequency decreasing with density (at constant P sep), enhanced divertor D α emission and increased inter-ELM losses. We show that this change in the ELM character at high pedestal density is due to a change in transport and/or stability in the pedestal region, with the ELMs changing from Type I to mixed Type I and Type II. The similarity of these observations with those in the Type II ELM regime in ASDEX Upgrade and

Results are presented from a series of dedicated experiments carried out on JET in tritium, DT, deuterium and hydrogen plasmas to determine the dependence of the H mode power threshold on the plasma isotopic mass. The Pthr ∝ Aeff-1 scaling is established over the whole isotopic range. This result makes it possible for a fusion reactor with a 50:50 DT mixture to access the H mode regime with about 20% less power than that needed in a DD mixture. Results on the first systematic measurements of the power necessary for the transition of the plasma to the type I ELM regime, which occurs after the transition to H mode, are also in agreement with the Aeff-1 scaling. For a subset of discharges, measurements of Te and Ti at the top of the profile pedestal have been obtained, indicating a weak influence of the isotopic mass on the critical edge temperature thought to be necessary for the H mode transition.

Density fluctuations in I-mode discharges in ASDEX Upgrade are studied. The I-mode specific weakly coherent mode (WCM) appears at the transition from L to I-mode. The WCM but also the turbulence in general are strongly modulated by a low frequency mode which can be related to the geodesic acoustic mode (GAM). The GAM induces an energy transfer away from the central WCM frequency, indicating an underlying instability responsible for the WCM. During the I-mode magnetic fluctuations close to the WCM frequency are intensified, which can be assigned to the geodesic Alfvénic oscillation. The geodesic Alfvénic oscillation is present already in L-mode, does not follow changes of frequency of the WCM, therefore it is not responsible for the WCM.

This paper describes experiments with highly shaped JET H-mode plasmas, which were directed to developing regimes where Type I ELMs are replaced by other edge relaxations, while maintaining the pedestal pressure of Type I ELMy H-modes. It was found that Type II ELMs coexisted with Type I, up to densities of the order of the Greenwald limit, where Type III ELMs appear, and the good confinement was lost. Only at the highest edge collisionality was it observed that Type II ELMs completely replace Type I. At high β p and q 95 , 'grassy' ELMs replace Type I completely. The MHD spectra characteristics for grassy ELMs are significantly different from those of Type II ELMs. This paper details the experiments, briefly compares the results to those obtained elsewhere and suggests open lines of investigations for the assessment of the potential of grassy ELM regimes as an ELM mitigation technique.

No abstract

This paper presents the results of JET experiments aimed at studying the operational space of plasmas with a Type III ELMy edge, in terms of both local and global plasma parameters. In JET, the Type III ELMy regime has a wide operational space in the pedestal n e-T e diagram, and Type III ELMs are observed in standard ELMy H-modes as well as in plasmas with an internal transport barrier (ITB). The transition from an H-mode with Type III ELMs to a steady state Type I ELMy H-mode requires a minimum loss power, P TypeI. P TypeI decreases with increasing plasma triangularity. In the pedestal n e-T e diagram, the critical pedestal temperature for the transition to Type I ELMs is found to be inversely proportional to the pedestal density (T crit ∝ 1/n) at a low density. In contrast, at a high density, T crit , does not depend strongly on density. In the density range where T crit ∝ 1/n, the critical power required for the transition to Type I ELMs decreases with increasing density. Experimental results are presented suggesting a common mechanism for Type III ELMs at low and high collisionality. A single model for the critical temperature for the transition from Type III to Type I ELMs, based on the resistive interchange instability with magnetic flutter, fits well the density and toroidal field dependence of the JET experimental data. On the other hand, this model fails to describe the variation of the Type III n e-T e operational space with isotopic mass and q 95. Other results are instead suggestive of a different physics for Type III ELMs. At low collisionality, plasma current ramp

We present state-of-the-art computations of propagation and absorption of electron cyclotron waves, retaining the effects of scattering due to electron density fluctuations. In ITER, injected microwaves are foreseen to suppress neoclassical tearing modes (NTMs) by driving current at the q = 2 and q = 3/2 resonant surfaces. Scattering of the beam can spoil the good localization of the absorption and thus impair NTM control capabilities. A novel tool, the WKBeam code, has been employed here in order to investigate this issue. The code is a Monte Carlo solver for the wave kinetic equation and retains diffraction, full axisymmetric tokamak geometry, determination of the absorption profile and an integral form of the scattering operator which describes the effects of turbulent density fluctuations within the limits of the Born scattering approximation. The approach has been benchmarked against the paraxial WKB code TORBEAM and the full-wave code IPF-FDMC. In particular, the Born approximation is found to be valid for ITER parameters. In this paper, we show that the transport in ITER is diffusive unlike in present experiments, thus causing up to a factor of 2 to 4 broadening in the absorption profile. However, the broadening depends strongly on the turbulence model assumed for the density fluctuations, which still has large uncertainties.

Abstract. The radial electric field (E r ) and its shear (∂E r /∂r) are believed to be fundamental for turbulence suppression in magnetically confined plasmas. Doppler reflectometry offers a direct method of obtaining E r and ∂E r /∂r measurements. It is a type of microwave radar technique which uses the back-scatter of microwaves from a radial position in the plasma where the refractive index equals zero. In this paper, the use of Doppler reflectometry for E r measurements is investigated. The technique is extended for ∂E r /∂r measurements by simultaneously probing the plasma with two microwave beams at different frequencies. E r and ∂E r /∂r measurements are presented for a wide variety of plasma conditions. The measurements show that E r and its associated shear are linked to plasma confinement. Their absolute values increase with confinement at the plasma edge. Furthermore, the measurements show dependence on the applied plasma auxiliary heating and the plasma shape. The E r shear results suggest that negative shear is more influential than positive shear, in contradiction with several theoretical models.