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In recent years, a strong effort has been dedicated to the development of tokamak plasma regimes alternative to the standard high confinement mode (H-mode) with type-I edge localized mode (ELM), i.e. ELM-free and small-ELM regimes, given the associated hardly sustainable energy and particle fluxes on plasma facing components. In this work, we will focus on new H-mode regimes with small-ELMs, the so-called baseline small-ELMs (BSE), characterized by high thermal confinement and low core impurity accumulation, which have been recently found at JET. In order to characterize the micro-turbulence at play at the top of the pedestal, an extensive local linear gyrokinetic analysis with the GKW code has been carried out. In particular, a comparison between a reference type-I ELM (#97395) and two BSE plasmas (#96994 and #94442) has been performed. The ion-scale (0.1 ≤ k θ ρ i ≤ 2) micro-turbulence is found to have different characteristics in the two regimes. Indeed, kinetic-ballooning modes (KBM) are destabilized in the type-I ELM regime at k θ ρ i ∼ 0.1, while they are stable in BSE regimes. In addition, negative (i.e. electron-diamagnetic-direction) frequency modes, identified as electron-temperature-gradient (ETG) modes, are destabilized at k θ ρ i ∼ 1.5 in the type-I ELM regime while BSE regimes are characterized by positive (i.e. ion-diamagnetic-direction) frequency modes. Meanwhile, at electron-scale (10 ≤ k θ ρ i ≤ 700) ETG modes are the dominant micro-instabilities in both regimes. Then, since BSE regimes are characterized by a higher impurity concentration at the pedestal, particular attention has been given to the role played by them. We found that impurities represent a critical player in the linear dynamics, strongly affecting the nature of micro-instabilities at ion-scale.

Recent experiments performed in JET at high level of plasma heating, in preparation of, and during the DT campaign have shown significant discrepancies between electron temperature measurements by Thomson Scattering (TS) and Electron Cyclotron Emission (ECE). In order to perform a systematic analysis of this phenomenon, a simple model of bipolar distortion of the electron distribution function has been developed, allowing analytic calculation of the EC emission and absorption coefficients. Extensive comparisons of the modelled ECE spectra (at both the 2 nd and the 3 rd harmonic extraordinary mode) with experimental measurements display good agreement when bulk electron distribution distortions around 1-2 times the electron thermal velocity are used and prove useful for a first level of analysis of this effect.

For high-temperature JET and TFTR discharges, electron cyclotron emission (ECE) measurements of central electron temperature were systematically found to be up to 20% higher than those taken with Thomson scattering. In recent high-performance JET discharges, central Te measurements, performed with LIDAR Thomson scattering and the X-mode ECE interferometer, have been studied in a large database, including deuterium (DD), and deuterium-tritium plasmas (DT). Discrepancies between Te measurements have been observed outside of the experimental uncertainties. ECE measurements, at high Te, have been found to be higher or lower than those of LIDAR, depending on the specific plasma scenario. In addition, discrepancies between the peaks of the second and third harmonic ranges of the ECE spectrum have been interpreted as evidence for the presence of non-Maxwellian features in the electron distribution function. These comparisons seem to suggest that such features can be found in most of the high-performance scenarios selected in this JET database.

The eXtreme Shape Controller (XSC) has been originally designed to control the plasma shape at JET during the flat-top phase, when the plasma current has a constant value. During the JET 2012 experimental campaigns, the XSC has been used to improve the shape control during the transient phases of plasma current ramp-up and ramp-down. In order to avoid the saturation of the actuators with these transient phases, a current limit avoidance system has been designed and implemented. This paper presents the experimental results achieved at JET during the 2012 campaigns using the XSC.

ICRH was extensively used in the 2015-16 JET-ILW (ITER like wall) experimental campaign; bulk heating together with high-Z impurity chase-out from plasma centre importantly contributed to the good DD fusion performance obtained recently in JET. Power up to 6 MW was launched in H-mode deuterium plasmas and 8 MW during the hydrogen campaign. The ILA was re-installed and contributed positively to the availability of ICRH power. The ILA produces slightly less high-Z impurities than the A2's and the PWI measured via Be line emission on limiters is in the same ballpark. Specific experiments were conducted to optimise ICRH scenarios in preparation for DT in particular the dual frequency scheme, (H)D and (He)D were tested. In addition, it was confirmed that the (D)H scenario is accessible in a ILW environment and the novel 3-ions ICRH scheme was validated experimentally.

For high-temperature JET and TFTR discharges, electron cyclotron emission (ECE) measurements of central electron temperature were systematically found to be up to 20% higher than those taken with Thomson scattering. In recent high-performance JET discharges, central T e measurements, performed with LIDAR Thomson scattering and the X-mode ECE interferometer, have been studied in a large database, including deuterium (DD), and deuterium-tritium plasmas (DT). Discrepancies between T e measurements have been observed outside of the experimental uncertainties. ECE measurements, at high T e , have been found to be higher or lower than those of LIDAR, depending on the specific plasma scenario. In addition, discrepancies between the peaks of the second and third harmonic ranges of the ECE spectrum have been interpreted as evidence for the presence of non-Maxwellian features in the electron distribution function. These comparisons seem to suggest that such features can be found in most of the high-performance scenarios selected in this JET database.

The former all-carbon wall on JET has been replaced with beryllium in the main torus and tungsten in the divertor to mimic the surface materials envisaged for ITER. Comparisons are presented between Type I H-mode characteristics in each design by examining respective scans over deuterium fuelling and impurity seeding, required to ameliorate exhaust loads both in JET at full capability and in ITER.

New plasma regimes with high confinement, low core impurity accumulation and small Edge localized mode (ELMs) perturbations have been obtained close to ITER conditions in magnetically confined plasmas from the Joint European torus (JET) tokamak. Such regimes are achieved by means of optimized particle fuelling conditions which trigger a self-organize state with a strong increase in rotation and ion temperature and a decrease of the edge density. An interplay between core and edge plasma regions leads to reduced turbulence levels and outward impurity convection. These results pave the way to an attractive alternative to the standard plasmas considered for fusion energy generation in a tokamak with metallic wall environment such as the ones expected in ITER

In this paper important aspects of Lower Hybrid (LH) operation with the ITER Like Wall (ILW) [1] at JET are reported. Impurity release during LH operation was investigated and it was found that there is no significant Be increase with LH power. Concentration of W was analysed in more detail and it was concluded that LH contributes negligibly to its increase. No cases of W accumulation in LH-only heating experiments were observed so far. LH wave coupling was studied and optimised to achieve the level of system performance similar to before ILW installation. Measurements by Li-beam were used to study systematic dependencies of the SOL density on the gas injection rate from a dedicated gas introduction module and the LH power and launcher position. Experimental results are supported by SOL transport modelling. Observations of arcs in front of the LH launcher and hotspots on magnetically connected sections of the vessel are reported. Overall, a relatively troublefree operation of the LH system up to 2.5MW of coupled Radio Frequency (RF) power in L-mode plasma was achieved with no indication that the power cannot be increased further.

When using Ion Cyclotron Range of Frequency (ICRF) heating, enhanced heat-fluxes are commonly observed on some plasma facing components close to the antennas. Experiments have recently been carried out on JET with the new ITER-Like-Wall (ILW) to characterize the heat flux to the JET ICRF antennas. Using Infra-Red thermography and thermal models of the tiles, heat-fluxes were evaluated from the surface temperature increase during the RF phase of L-mode plasmas. The maximum observed heat-flux intensity was ~ 4.5 MW/m2 when operating with -{\\pi}/2 current drive strap phasing at power level of 2MW per antenna and with a 4 cm distance between the plasma and the outer limiters. Heat-fluxes are reduced when using dipole strap phasing. The fraction of ICRF power deposited on the antenna limiters or septa was in the range 2-10% for dipole phasing and 10-20% with +/-{\\pi}/2 phasing.