[en] Internal transport barriers (ITBs) can provide high tokamak confinement at modest plasma current. This is desirable for operation with most of the current driven non-inductively by the bootstrap mechanism, as currently envisaged for steady-state power plants. Maintaining such plasmas in steady conditions with high plasma purity is challenging, however, due to MHD instabilities and impurity transport effects. Significant progress has been made in the control of ITB plasmas: the pressure profile has been varied using the barrier location; q-profile modification has been achieved with non-inductive current drive, and means have been found to affect density peaking and impurity accumulation. All these features are, to some extent, interdependent and must be integrated self-consistently to demonstrate a sound basis for extrapolation to future devices

[en] Simultaneous current ramping and application of lower hybrid heating and current drive (LHCD) have produced a region with zero current density within measurement errors in the core (r/a≤0.2) of JET tokamak optimized shear discharges. The reduction of core current density is consistent with a simple physical explanation and numerical simulations of radial current diffusion including the effects of LHCD. However, the core current density is clamped at zero, indicating the existence of a physical mechanism which prevents it from becoming negative

[en] Mechanisms of arc formation have been analyzed and the critical electric fields for the multipactor effect calculated, compared to the experimental values and found to be within the normal operational space of the LH system on JET. It has been shown that the characteristic electron energy (20-1000)eV for the highest multipactor resonances (N = 4-9) are within the limits of secondary electron yield above 1 required for multipactoring. Electrons with these energies provide the highest gas desorption efficiency when hitting the waveguide walls. The effect of higher waveguide modes and magnetic field on the multipactor was also considered. The distribution function for electrons accelerated by LH waves in front of the launcher has been calculated. The field emission currents have been estimated and found to be small. It is proposed that emission of Fel5, 16 lines, which can be obtained with improved diagnostics, could be used to detect arcs that are missed by a protection system based on the reflected power. The reliability and time response of these signals are discussed. A similar technique based on the observation of the emission of low ionized atoms can be used for a fast detection of other undesirable events to avoid sputtering or melting of the plasma facing components such as RF antenna. These techniques are especially powerful if they are based on emission uniquely associated with specific locations and components.

[en] A pedestal prediction model Europed is built on the existing EPED1 model by coupling it with core transport simulation using a Bohm-gyroBohm transport model to self-consistently predict JET-ILW power scan for hybrid plasmas that display weaker power degradation than the IPB98(y, 2) scaling of the energy confinement time. The weak power degradation is reproduced in the coupled core-pedestal simulation. The coupled core-pedestal model is further tested for a 3.0 MA plasma with the highest stored energy achieved in JET-ILW so far, giving a prediction of the stored plasma energy within the error margins of the measured experimental value. A pedestal density prediction model based on the neutral penetration is tested on a JET-ILW database giving a prediction with an average error of 17% from the experimental data when a parameter taking into account the fuelling rate is added into the model. However the model fails to reproduce the power dependence of the pedestal density implying missing transport physics in the model. The future JET-ILW deuterium campaign with increased heating power is predicted to reach plasma energy of 11 MJ, which would correspond to 11–13 MW of fusion power in equivalent deuterium–tritium plasma but with isotope effects on pedestal stability and core transport ignored. (paper)

[en] The paper presents results on the use of resonant field amplification for experimental probing of stability and β-limits (β is the ratio of the plasma pressure to the magnetic field pressure) in JET. It is found that an externally applied helical magnetic field is strongly enhanced when the plasma exceeds the ideal no-wall stability limit or approaches proximity to other marginally stable (i.e. current-driven) modes. This effect is known as the resonant field amplification (RFA) and was used for the systematic probing of stability in different advanced regimes on JET. The application of this technique on JET is discussed in the paper and the results of the RFA measurements are presented and related to the observed limitations in β.

[en] The full scale realisation of nuclear fusion as an energy source requires a detailed understanding of power and energy balance in current experimental devices. In this we explore whether a global power balance model in which some of the calibration factors applied to the source or sink terms are fitted to the data can provide insight into possible causes of any discrepancies in power and energy balance seen in the JET tokamak. We show that the dynamics in the power balance can only be properly reproduced by including the changes in the thermal stored energy which therefore provides an additional opportunity to cross calibrate other terms in the power balance equation. Although the results are inconclusive with respect to the original goal of identifying the source of the discrepancies in the energy balance, we do find that with optimised parameters an extremely good prediction of the total power measured at the outer divertor target can be obtained over a wide range of pulses with time resolution up to ∼25 ms. (paper)

[en] The ITER hybrid scenario aims to exploit non-inductive current drive to enable burn times in excess of 1000 s. To achieve this, and optimize fusion performance, requires high β_N (the plasma pressure normalized to a stability scaling) and energy confinement equal to or greater than that predicted for the baseline scenario. This paper discusses results from the JET candidate hybrid scenario, where β_N,MHD ≤ 3.6 plasmas have been produced. Despite a different initial phase, confinement relevant plasma parameters evolve rapidly towards those of equivalent ELMy H-modes and are well described by IPB98(y, 2). In contrast to previous ELMy H-mode studies, a dedicated β scan experiment in the JET hybrid candidate scenario shows a strong negative dependence of global confinement on β_N. Analysis indicates that the core transport remains consistent with weakly dependent electrostatic transport, whilst the edge confinement decreases strongly with increasing β_N. By combining global confinement data from ASDEX Upgrade, DIII-D and JET hybrid scenario discharges, a multi-machine database is produced. In contrast to the JET case, confinement in ASDEX Upgrade and DIII-D is shown to be inconsistent with IPB98(y, 2) and alternative dependences are explored.

[en] RF heating and current drive with ion cyclotron waves and Lower Hybrid waves have been crucial for the development of the Optimized Shear scenario on JET to high performance. Peaked electron temperature profiles and improved energy confinement could be obtained with electron heating both from LHCD and ICRH during plasma current ramp up. ICRH and NBI comparisons allow to separate heating and fueling and suggest a dominant role of core heating in the formation of an Internal Transport Barrier (ITB). ICRH and NBI powers are there equivalent. Pressure profile control by varying the composition of centrally peaked ICRF and broader NBI deposition improves MHD stability. Current profile modifications in a wide range have been obtained with LHCD and in combination with NBI during current ramp up. During the high performance phase, however, LH coupling degrades strongly due to the steep edge density gradient resulting in a drop of the density in front of the LH antenna to the cut-off density. High fusion performance achieving simultaneously high beta values and bootstrap currents is predicted in scenario modeling using pressure and current profile control with ICRF and LHCD

[en] Trace tritium experiments (TTE) on JET were analysed using Monte Carlo modelling of the neutron emission resulting from the neutral beam injection (NBI) of short (∼300 ms) tritium (T) beam blips into reversed shear, hybrid ELMy H-mode and L-mode deuterium plasmas for a wide range of plasma parameters. The calculated neutron fluxes from deuterium-tritium (DT) reactions could only be made consistent with all plasmas by applying an artificial reduction of the T beam power in the modelling of between 20% and 40%. A similar discrepancy has previously been observed in both JET (Gorini et al 2004 Proc. 31st EPS Conf. on Plasma Physics (London, UK) vol 28G (ECA)) and TFTR (Ruskov et al 1999 Phys. Rev. Lett. 82 924), although no mechanism has yet been found that could explain such a difference in the measured T beam power. Applying this correction in the T beam power, good agreement between calculated and measured DT neutron emission profiles was obtained in low to moderate line averaged density (n-bar_e) < 4x10¹⁹m^-3) ELMy H-Mode plasmas assuming that the fast beam ions experience no, or relatively small, anomalous diffusion (D_an << 0.5 m² s^-1). However, the modelled neutron profiles do not agree with measurements in higher density plasmas using the same assumption and the disagreement between the measured and calculated shape of the neutron profile increases with plasma density. In this paper it is demonstrated that large anomalous losses of fast ions have to be assumed in the simulations to improve agreement between experimental and simulated neutron profiles, characterized by the goodness of fit. Various types of fast ion losses are modelled to explain aspects of the data, though further investigation will be required in order to gain a more detailed understanding of the nature of those anomalous losses.

[en] Exhaust power components due to ELMs, radiation and heat transport across the edge transport barrier (ETB) between ELMs are quantifed for H-mode plasmas in JET-C and JET-ILW for comparison with simulations of pedestal heat transport. In low-current, JET-ILW pulses with a low rate of gas fuelling, the pedestal heat transport is found not to be stiff, i.e. the effective, mean heat diffusivity $⟨<!--\; ???\; -->{\mathrm{\chi <!--\; ??\; -->}}_{eff}⟩<!--\; ???\; -->$ does not increase with the electron temperature gradient $⟨<!--\; ???\; -->{dT}_e/dR{⟩<!--\; ???\; -->}_{ped}$ across the pedestal and the parameter ${\mathrm{\eta <!--\; ??\; -->}}_e=L_{n_e}/L_{T_e}$ increases with the conducted loss power across the pedestal, with the latter saturating at mean values $⟨<!--\; ???\; -->{\mathrm{\eta <!--\; ??\; -->}}_e{⟩<!--\; ???\; -->}_{ped}\lesssim <!--\; ???\; -->2$. This increase in pedestal temperature gradient is partly due to a relative reduction of the ion neo-classical heat transport (which is more significant at low plasma current) with decreasing collisionality at higher power. In JET-ILW pulses, significantly more power is required at a high gas puffing rate to achieve a similar pedestal pressure and normalised confinement to that in otherwise similar JET-C pulses without gas-puffing. The increased heat transport across the JET-ILW pedestals is caused by changes to the pedestal structure induced by the gas puffing, which is required to mitigate contamination by W impurities sputtered from the target plates. In high-power JET-ILW pulses, the radiated power is dominated by that from W, which exhibits a highly asymmetric poloidal distribution due to toroidal rotation. During the ELMy H-mode phase, the W is concentrated in the outer ‘mantle’ region ($0.7⩽<!--\; ???\; -->{\mathrm{\rho <!--\; ??\; -->}}_N⩽<!--\; ???\; -->0.96$) inside the pedestal top by a favourable alignment of profile gradients, where it can be effectively flushed by ELMs. Transport analysis reveals that the strong mantle radiation cools the outer region of the plasma, causing more of the heat to be lost through the electron channel. However, direct cooling by W radiation from the ETB region is shown to be insignificant compared to the power conducted through the pedestal. (paper)