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Recent experimental studies of H-mode plasmas in 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 oscillation is present a weak ELM-less H-mode regime is obtained (medium confinement), with a clear increase of pedestal density (ne,ped) and a weak temperature pedestal (Te,ped). In ICRH heated plasmas or low power, low density NBI plasmas the mode and the pedestal pressure can remain steady for the duration of the heating phase, of order 10s or more. The observed axisymmetric MHD oscillation has period ~1­2ms, and odd parity across the mid-plane: m = 1 with sin(q) dependency. Electron Cyclotron Emission, interferometry, reflectometry, SXR and fast Li beam measurements locate the mode in the pedestal region. Da, fast infrared camera and Langmuir probe measurements show that the mode modulates heat and particle flux to the target. Similar phenomena are described elsewhere, lately described as an "I-phase". From a physics point of view, our fundamental observation is that the M-mode is magnetic, not electrostatic, and its frequency scales with pedestal poloidal Alfvén velocity.

After the change over from the C-wall to the ITER-like Be/W wall (ILW) in JET, the radiation losses during ICRF heating have increased and are now substantially larger than those observed with NBI at the same power levels, in spite of the similar global plasma energies reached with the two heating systems. A comparison of the NBI and ICRF performances in the JET-ILW experiments, based on a statistical analysis of ~3000 L-mode discharges, will be presented.

E. Joffrin, M. Baruzzo, M. Beurskens, Bourdelle, S. Brezinsek, J. Bucalossi, P. Buratti, G. Calabro, C.D. Challis, M. Clever, J. Coenen, E. Delabie, R. Dux, P. Lomas, E. de la Luna, P. de Vries, J. Flanagan, L. Frassinetti, D. Frigione, C. Giroud, M. Groth, N. Hawkes, J. Hobirk, M. Lehnen, G. Maddison, J. Mailloux, C.F. Maggi, G. Matthews, M. Mayoral, A. Meigs, R. Neu, I. Nunes, T. Puetterich, F. Rimini, M. Sertoli, B. Sieglin, A.C.C. Sips, G. van Rooij, I. Voitsekhovitch and JET EFDA contributors In the recent JET experimental campaigns with the new ITER-like Wall (JET-ILW), major progress has been achieved in the characterisation and operation of the H-mode regime in metallic environment: i) plasma breakdown has been achieved at the first attempt and X-point L-mode operation recovered in a few days of operation, ii) stationary and stable type I ELMy Hmodes with bN~1.4 have been achieved in low and high triangularity ITER­like shape plasmas and are showing that their operational domain at H = 1 is significantly reduced with the JET-ILW mainly because of the need to inject large amount of gas (above 1022D/s) to control core radiation. iii) in contrast the hybrid H-mode scenario has reached H factor of 1.2 to 1.3 at bN of 3 for 2 to 3s. iv) in comparison to carbon equivalent discharges, total radiation is similar but the edge radiation is lower and Zeff of the order of 1.3-1.4. At low gas fuelling rate (below 0.5 1022D/s) and low ELM frequency (typically less than 10Hz), strong core radiation peaking is observed in H-mode discharges even when tungsten influx from the divertor is constant. High Z impurity transport from the plasma edge to the core appears to be the dominant factor to explain is paper reviews the major physics and operational achievements and challenges that an ITERlike wall configuration has to face to produce stable plasma scenarios with maximised performance.

The most recent JET campaign has focused on characterizing operation with the "ITER-like" wall. One of the questions that needed to be answered is whether the auxiliary heating methods do not lead to unacceptably high levels of impurity influx, preventing fusion-relevant operation. In view of its high single pass absorption, hydrogen minority fundamental cyclotron heating in a deuterium plasma was chosen as the reference wave heating scheme in the ion cyclotron domain of frequencies. The present paper discusses the plasma behavior as a function of the minority concentration X[H] in L-mode with up to 4MW of RF power. It was found that the tungsten concentration decreases by a factor of 4 when the minority concentration is increased from X[H] 5% to X[H] 20% and that it remains at a similar level when X[H] is further increased to 30%; a monotonic decrease in Beryllium emission is simultaneously observed. The radiated power drops by a factor of 2 and reaches a minimum at X[H] 20%. It is discussed that poor single pass absorption at too high minority concentrations ultimately tailors the avoidance of the RF induced impurity influx. The edge density being different for different minority concentrations, it is argued that the impact ICRH has on the fate of heavy ions is not only a result of core (wave and transport) physics but also of edge dynamics and fueling.

C.F. Maggi, E. Delabie, T.M. Biewer, M. Groth, N.C. Hawkes, M. Lehnen, E. de la Luna, K. McCormick, C. Reux, F. Rimini, E.R. Solano, Y. Andrew, C. Bourdelle, V. Bobkov, M. Brix, G. Calabro, A. Czarnecka, J. Flanagan, E. Lerche, S. Marsen, I. Nunes, D. Van Eester, M.F. Stamp and JET EFDA contributors A comparison of the L-H power threshold (Pthr) in JET with all carbon, JET-C, and beryllium/tungsten wall (the ITER-like choice), JET-ILW, has been carried out in experiments with slow input power ramps and matched plasma shapes, divertor configuration and IP/BT pairs. The low density dependence of the L-H power threshold, namely an increase below a minimum density ne,min, which was first observed in JET with the MkII-GB divertor and C wall and subsequently not observed with the current MkII-HD geometry, is observed again with JET-ILW. At plasma densities above ne,min, Pthr is reduced by ~ 30%, and by ~ 40% when the radiation from the bulk plasma is subtracted (Psep), with JET-ILW compared to JET-C. At the L-H transition the electron temperature at the edge, where the pedestal later develops, is also lower with JET-ILW, for a given edge density. With JET-ILW the minimum density is found to increase roughly linearly with magnetic field, ne,min ~ BT4/5, while the power threshold at the minimum density scales as Psep,min ~ BT5/2. The H-mode power threshold in JET-ILW is found to be sensitive both to variations in main plasma shape (Psep decreases with increasing lower triangularity and increases with upper triangularity) and in divertor configuration. When the data are recast in terms of Psep and Zeff or subdivertor neutral pressure a linear correlation is found, pointing to a possible role of Zeff and/or subdivertor neutral pressure in the L-H transition physics. Depending on the chosen divertor configuration, Pthr can be up to a factor of two lower than the ITPA scaling law for densities above ne,min. A shallow edge radial electric field well is observed at the L-H transition. The edge impurity ion poloidal velocity remains low, close to its L-mode values, 5 km/s ±2­3km/s, at the L-H transition and throughout the H-mode phase, with no measureable increase within the experimental uncertainties. The edge toroidal rotation profile does not contribute to the depth of the negative Er well and thus may not be correlated with the formation of the edge transport barrier in JET.

In tokamaks, the H-mode is characterized by density and temperature pedestals that build up at the L-H transition. During JET 2012 campaign, discharges were carried out with a smooth increase of heating power to compare the L-H transition threshold in JET-C and JET-ILW. In these discharges, plasma passed through intermediate regimes between the L-mode and the H-mode: divertor oscillation and M-mode. The divertor oscillation (DO) is characterized by a periodic (typ. 10ms) oscillation of plasma radiation with an opposition of phase between the inner and outer side Da signal. Although reminiscent of the dithering L-H transitions, several features distinguish the divertor oscillations. The DO is an oscillation between two different plasma states: firstly the edge plasma density increases with low divertor radiation, secondly follows a sudden decrease of the plasma density with increased divertor radiation. During the high plasma density phase, the plasma stored energy increases slightly whereas dithering cycles are periodic L-H-L transitions at frequency an order of magnitude higher. Improvement of reflectometry data processing to reduce noise and radial jitter allows to follow the dynamic of the density profile during the DO. This study is preliminary; a very limited number of shots were processed. The observations lead us to propose a simple model to explain the periodic density oscillations.

ITER will start its operation with non-activated Hydrogen and Helium plasmas at a reduced magnetic field of B0 = 2.65T. In Hydrogen plasmas, the two Ion Cyclotron Resonance Frequency (ICRF) heating schemes available for central plasma heating (fundamental H majority and 2nd harmonic 3He minority ICRF heating) are likely to suffer from relatively low RF wave absorption, as suggested by numerical modelling and confirmed by previous JET experiments conducted in conditions similar to those expected in ITER's initial phase. With 4He plasmas, the commonly adopted fundamental H minority heating scheme will be used and its performance is expected to be much better. However, one important question that remains to be answered is whether increased levels of Hydrogen (due to e.g. H pellet injection) jeopardize the high performance usually observed with this heating scheme, in particular in a full-metal environment. Recent JET experiments performed with the ITER-like wall (ILW) shed some light into this question and the main results concerning ICRF heating performance in L-mode discharges are summarized here.

An overview of the recent results of Lower Hybrid (LH) experiments at JET with the ITER-like wall (ILW) is presented. Topics relevant to LH wave coupling are addressed as well as issues related to ILW and LH system protections. LH wave coupling was studied in conditions determined by ILW recycling and operational constraints. It was concluded that LH wave coupling was not significantly affected and the pre-ILW performance could be recovered after optimising the launcher position and local gas puffing. SOL density measurements were performed using a Li-beam diagnostic. Dependencies on the D2 injection rate from the dedicated gas valve, the LH power and the LH launcher position were analysed. SOL density modifications due to LH were modelled by the EDGE2D code assuming SOL heating by collisional dissipation of the LH wave and/or possible E¥B drifts in the SOL. The simulations matched reasonably well the measured SOL profiles. Observations of arcs and hotspots with visible and IR cameras viewing the LH launcher are presented.

An all-metal ITER-Like Wall (JET-ILW), consisting of beryllium in the main chamber and tungsten surfaces in the divertor, has now been installed in JET to pursue low retention of fuel species and to explore the impact on next-step-relevant plasmas. Its implementation has offered a unique opportunity to compare behaviour with that in the previous all-Carbon lining (JET-C), notably for high-triangularity Type I H-modes with impurity seeding. This technique is recognised to be necessary for power handling both in ITER and in JET at full performance. Contrasting results are reported for closely-matched deuterium-fuelling plus nitrogen-seeding scans in each JET environment. Attention is focused upon neutral-beam-heated plasmas with total input power 15­17MW at 2.65T, 2.5MA, q95 3.5 , average triangularity d 0.4 , elongation k 1.7 and gas inputs spanning ranges 0.75 FD 3.3 , 0 FD 4.7 (1022 electrons / s assuming full ionisation). JET-C cases also included 1­2MW of central ion-cyclotron-resonance-frequency heating, so far absent from JET-ILW pulses, with possible consequences for respective core sawtooth and impurity-concentration results.

The major aspects linked to the use of the JET auxiliary heating systems: NBI, ICRF and LHCD, in the new JET ITER-like wall (JET-ILW) are presented. We show that although there were issues related to the operation of each system, efficient and safe plasma heating was obtained with room for higher power. For the NBI up to 25.7MW was safely injected; issues that had to be tackled were mainly the beam shine-through and beam re-ionisation before its entrance into the plasma. For the ICRF system, 5MW were coupled in L-mode and 4MW in H-mode; the main areas of concern were RF-sheaths related heat loads and impurities production. For the LH, 2.5MW were delivered without problems; arcing and generation of fast electron beams in front of the launcher that can lead to high heat loads were the keys issues. For each system, an overview will be given of: the main modifications implemented for safe use, their compatibility with the new metallic wall, the differences in behavior compared with the previous carbon wall, with emphasis on heat loads and impurity content in the plasma.

In JET C and W/Be wall experiments, the use of ELM pacing at high ELM frequency as a tool to mitigate the ELM impact on plasma facing components and for impurity control has been successfully investigated. To improve the understanding of ELM mitigation mechanisms at JET in view of ITER and to examine dependencies of the ELM trigger efficiency on available actuators, modelling schemes are required, combining the evolution of the plasma core, edge and SOL and the analysis of MHD stability in an integrated way. The JET transport suite of codes JINTRAC has been used in fully integrated modeto study and assess physical processes that appear to be relevant in case of ELMs triggered (a) by application of magnetic field perturbations that cause a sudden displacement of the plasma ("kicks") and (b) by injection of pellets. Whereas induced edge current perturbations were found to be relevant to destabilise the plasma edge in case (a), local and plasmoid-driven pressure perturbations are held responsible for the appearance of an ELM in case (b). In the context of previous integrated simulation studies of ELM pacing / mitigation by kicks and pellets at JET, recent advances are reported on the following subjects: ­ ELM mitigated regimes are analysed with JINTRAC in fully integrated mode in a refined way using a new tool for the evaluation of the MHD stability at runtime with HELENA+MISHKA. ­ The effect of kicks at low vs. high kick amplitude / gas fuelling rate in JET C and W/Be wall discharges on MHD stability is analysed and compared against experimental findings with JINTRAC+CREATE-NL.

ITER operation scenarios foresee the access to H-mode with limited additional heating at low density, followed by an increase in density and power while remaining in H-mode to the required operating conditions. Such scheme is thus sensitive to the level of hysteresis in the H-mode power and to the competing evolution of input power, density and radiation losses from the bulk plasma. Whereas most studies focus on the access to H-mode, the H-L back transition is less explored. This work examines the evolution and properties of JET plasmas with the ITER-like wall at the H-L back transition in relation to the forward L-H transition. The influence of the wall composition (C versus Be/W), in particular on the density evolution, radiation losses and impurity composition, is also analysed, through direct comparison of forward and back transitions in JET-C and JET-ILW plasmas.