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This paper summarizes results of dimensionless identity experiments in JT-60U and JET, aimed at the comparison of the H-mode pedestal and ELM behaviour in the two devices. Given their similar size, dimensionless matched plasmas are also similar in their dimensional parameters (in particular, the plasma minor radius a is the same in JET and JT-60U). Power and density scans were carried out at two values of Ip, providing a q scan (q95 = 3.1 and 5.1) with fixed (and matched) toroidal field. Contrary to initial expectations, a dimensionless match between the two devices was quite difficult to achieve. In general, pped in JT-60U is lower than in JET and, at low q, the pedestal pressure of JT-60U with a Type I ELMy edge is matched in JET only in the Type III ELM regime. At q95 = 5.1, a dimensionless match in ?*, ?* and ?p,ped is obtained with Type I ELMs, but only with low power JET H-modes. These results motivated a closer investigation of experimental conditions in the two devices, to identify possible 'hidden' physics that prevents obtaining a good match of pedestal values over a large range of plasmas parameters. Ripple-induced ion losses of the medium bore plasma used in JT-60U for the similarity experiments are identified as the main difference with JET. The magnitude of the JT-60U ripple losses is sufficient to induce counter-toroidal rotation in co-injected plasma. The influence of ripple losses was demonstrated at q95 = 5.1: reducing ripple losses by ?2 (from 4.3 to 1.9?MW) by replacing positive with negative neutral beam injection at approximately constant Pin resulted in an increased pped in JT-60U, providing a good match to full power JET H-modes. At the same time, the counter-toroidal rotation decreased. Physics mechanisms relating ripple losses to pedestal performance are not yet identified, and the possible role of velocity shear in the pedestal stability, as well as the possible influence of ripple on thermal ion transport are briefly discussed. Toroidal rotation of the ITER reference inductive Q = 10 H-mode is predicted to be rather low, of the order of ~1/10 of the frequency of typical JET H-modes. Nonetheless, fast ion ripple losses in that scenario are also predicted to be negligible (1%), and therefore plasma toroidal rotation slow-down or ripple-induced counter-rotation should not affect pedestal parameters and stability in ITER. Finally, the possible effect of ripple on thermal transport may deserve more attention in future experiments and modelling, since the ripple magnitude of ITER is intermediate between that of JET and JT-60U.

Pedestal and global plasma parameters are compared in conventional ELMy H-modes and improved confinement discharges from ASDEX Upgrade (AUG), DIII-D, JET and JT-60U with varying net input power. Both electron and ion pedestal pressures are studied. The pedestal top pressure pPED increases moderately with power in all tokamaks, in broad agreement with the power dependence of the IPB98(y, 2) scaling. Higher pedestal pressures are observed in AUG improved H-modes and in JT-60U high βpol discharges at q95 ~ 6.5 and high triangularity. For all machines and all scenarios a robust correlation between the total and the pedestal thermal stored energy is observed, with the ratio of the two varying between ~0.3 and 0.5. However the relative importance of pedestal and core confinement varies from regime to regime. In AUG the confinement improvement with respect to the IPB98(y, 2) scaling is due to improved pedestal confinement in improved H-modes with early heating and to both improved pedestal and core confinement in improved H-modes with late heating. In DIII-D hybrid discharges the increase in confinement factor compared with conventional H-modes is due to improved confinement in the plasma core. JT-60U reversed shear H-modes have strong internal transport barriers and thus improved core performance. In all four tokamaks improved edge stability is correlated with increasing total βpol and H98(y,2) increases with pedestal βpol. The analysed multimachine data set supports a scaling expression for the pedestal stored energy derived under the assumption that the dominant loss term for the pedestal is by thermal conduction in the edge transport barrier region.

Alfven cascade (AC) eigenmodes excited by energetic ions accelerated with ion-cyclotron resonance heating in JET reversed-shear discharges are studied experimentally in high-density plasmas fuelled by neutral beam injection (NBI) and by deuterium pellets. The recently developed O-mode interferometry technique and Mirnov coils are employed for detecting ACs. The spontaneous improvements in plasma confinement (internal transport barrier (ITB) triggering events) and grand ACs are found to correlate within 0.2 s in JET plasmas with densities up to ∼5 × 10^19 m−3. Measurements with high time resolution show that ITB triggering events happen before ‘grand’ ACs in the majority of JET discharges, indicating that this improvement in confinement is likely to be associated with the decrease in the density of rational magnetic surfaces just before qmin(t) passes an integer value. Experimentally observed ACs excited by sub-Alfvenic NBI-produced ions with parallel velocities as low as V_||NBI ≈ 0.2 · V_A are found to be most likely associated with the geodesic acoustic effect that significantly modifies the shear-Alfven dispersion relation at low frequency. Experiments were performed with a tritium NBI-blip (short time pulse) into JET plasmas with NBI-driven ACs. Although considerable NBI-driven AC activity was present, good agreement was found both in the radial profile and in the time evolution of DT neutrons between the neutron measurements and the TRANSP code modelling based on the Coulomb collision model, indicating the ACs have at most a small effect on fast particle confinement in this case.

During the initial operation of the International Thermonuclear Experimental Reactor (ITER), it is envisaged that activation will be minimized by using hydrogen (H) plasmas where the reference ion ...

Edge localized mode (ELM) events are typically observed in a broad variety of tokamaks and stellarators. In the TJ-II Heliac, ELM-like events have been observed in experiments at low plasma parameters (Te ~ Ti ~ 100 eV, n ~ 6 × 1012 cm−3, where Ti, Te and n are the ion and the electron temperatures and the plasma density, respectively) away from the electron cyclotron resonance heating power deposition region. Moreover, numerical calculations have shown that the experimental values for plasma pressure are more than an order of magnitude lower than the theoretical pressure threshold for both ideal and resistive magnetohydrodynamic modes. These transport events, now called localized electromagnetic modes, are found to be linked to the presence of rational magnetic surfaces in the vicinity of the steep density gradient region located at 0.5 ≤ r ≤ 0.7, r = reff/ap, where reff and ap are, respectively, the average radius of the magnetic surface and the plasma boundary; so, while the morphology of the time signals is similar to that of the type-III ELMs, the mode in TJ-II is localized at the edge of the confinement region. The characteristic frequency of the oscillations observed by the Mirnov coils is f = 15–50 kHz. The position of the rational magnetic surface is determined by the channel of the ECE diagnostic with the highest correlation to Mirnov probes and matches, within the experimental error, the position obtained from numerical calculations when the modification of the rotational transform due to toroidal currents is taken into account. These two coupled observations are attributed to the coupling of drift modes with Alfven and acoustic waves.

1 H-mode access studies under conditions of very low plasma density provide valuable information on the, physics of the edge transport barrier formation and are an important part of the operational scenario development for the next step fusion device, ITER. JET operation with a very low edge electron density, (n) over bar (e) = 0.8-1.4 x 10(19) m(-3), with the MkII gas box septum divertor was found to have a significant effect on the power threshold for the L-H transition, P-th. A sharp `turning point' below which P-th increases with decreasing (n) over bar (e) is found for I-p/B-t values of 2.5 MA/2.7 T, 2.5 MA/2.6T 2.5 MA/2.4T and 2.2 MA/2.4T. The pedestal electron temperature demonstrates a very similar dependence on the edge (n) over tilde (e) as P-th. The role of Z(eff) in the variation of the H-mode power threshold with density is also considered. Shots with the divertor septum present are compared with an identical (n) over bar (e) scan, run with the MkII GB septum replacement plate divertor at 2.5 MA/2.7 T and 2.5 MA/2.6 T. The results from this investigation indicate that removal of the septum may have lowered the (n) over tilde (e) `turning point' for P-th.

Extensive analysis of disruptions in JET has helped advance the understanding of trends of disruption-generated runaway electrons. Tomographic reconstruction of the soft x-ray emission has made possible a detailed observation of the magnetic flux geometry evolution during disruptions. With the aid of soft and hard x-ray diagnostics runaway electrons have been detected at the very beginning of disruptions. A study of runaway electron parameters has shown that an approximate upper bound for the conversion efficiency of pre-disruptive plasma currents into runaways is about 60% over a wide range of plasma currents in JET. Runaway generation has been simulated with a test particle model in order to verify the results of experimental data analysis and to obtain the background for extrapolation of the existing results onto larger devices such as ITER. It was found that close agreement between the modelling results and experimental data could be achieved if in the calculations the post-disruption plasma electron temperature was assumed equal to 10 eV and if the plasma column geometry evolution is taken into account in calculations. The experimental trends and numerical simulations show that runaway electrons are a critical issue for ITER and, therefore, the development of mitigation methods, which suppress runaway generation, is an essential task.

The development of scenarios characterized by H-mode confinement and internal transport barriers (ITBs) in high triangularity, δ ~ 0.4–0.5, discharges is of particular interest for ITER advanced tokamak operation. Previous JET experiments have shown that high triangularity favours H-modes which are ELM-free or develop type I edge localized mode (ELM) activity, which inhibits long lasting ITBs. The recent experiments reported here concentrate on integrated optimization of edge and core conditions. The stability of the edge pedestal was controlled using gas injection, deuterium or light impurities, and plasma current ramps. Both methods yield more ITB-friendly edge pedestal conditions, varying from small type I to type III ELMs and, in extreme cases, resulting in L-mode. In parallel, the conditions for triggering and sustaining ITBs encompassing a large proportion of the plasma volume (outer ITBs) were optimized, as opposed to less performing ITBs located closer to the plasma centre (inner ITB). These plasmas have deeply reversed target current profiles with qmin ~ 3 and a narrow inner ITB, located typically at a small normalized radius ρ < 0.5 and close to the reversed shear region, is routinely observed. Large radius outer ITBs are only triggered at an input power in excess of 20 MW, but they do not usually survive the transition into H-mode. The best results, in terms of sustained high confinement, have been obtained with neon injection; an outer ITB is triggered during the phase with L-mode edge and survives into H-mode for about 2 s, corresponding to ~10 times the global energy confinement time τE, at q95 = 7.5, H89βN ~ 3.5–4 and ~60% of the Greenwald density limit. In summary, a high triangularity scenario has been developed, which combines the desirable characteristics of controlled edge, long lasting wide ITBs and high performance at density higher than the low triangularity JET scenarios.

Over the past few years, the preparation of ITER-relevant plasma scenarios has been the main focus of experimental activity on tokamaks. The development of integrated, simultaneous, real-time controls of plasma shape, current, pressure, temperature, radiation, neutron profiles and also impurities, ELMs and MHD are now seen to be essential for further development of quasi-steady state conditions with feedback, or the stabilisation of transient phenomena with event-driven actions. For this thrust, the EFDA JET Real-Time Project has developed a set of real-time plasma measurements, experiment control and communication facilities. The plasma diagnostics used for real-time experiments are Far Infra Red interferometry, polarimetry, visible, UV and X-ray spectroscopy, LIDAR, bolometry, neutron and magnetics. Further analysis systems produce integrated results, such as temperature profiles on geometry derived from MHD equilibrium solutions. The actuators include toroidal, poloidal and divertor coils, gas and pellet fuelling, neutral beam injection, radio frequency (ICRH) waves and microwaves (LH). The heating/fuelling operators can either define a power or gas request waveform or select the real-time instantaneous power/gas request from the real-time experiment central control (RTCC) system. The real-time experiment control system provides both a high-level, control-programming environment and interlocks with the actuators. A MATLAB facility is being developed for the development of more complex controllers. The plasma measurement, controller and plant control systems communicate in ATM network. The EFDA Real-Time Project is essential groundwork for future reactors such as ITER. It involves many staff from several institutions. The facility is now frequently used in experiments. This work has been conducted under the European Fusion Development Agreement and is partly funded by Euratom and the UK Engineering and Physical Sciences Research Council.

Millimeter-wave diagnostics at JET, mainly reflectometry, are employing state of art electronics, but are limited in performance by the existing waveguides and antenna system that are inadequate and obsolete. The use of long run waveguides with high losses and non-optimized antennas (in some cases, not pointing to relevant plasma positions) lead to difficult measurement conditions for reflectometry. The new access system presented in this article has been designed to improve the performance of reflectometry measurements and enable the installation of antennas for oblique viewing ECE. These two new antennae will allow the ECE radiation to be collected at different angles with respect to the magnetic field. This set-up, known as oblique ECE [1] , is expected to be extremely useful in improving the interpretation of ECE temperature measurements in all fusion experiments with significant additional heating. For reflectometry, there is an urgent need to improve the edge density measurements as both the lithium beam and Thomson scattering exhibit limitations of resolution at lower densities. The project proposal states that the expected improvement in reflectometry S/N ratio is 30 dB. If realized, this will allow broad band reflectometry, for the measurement of the electron density profile, for the first time in JET.

One of the scenarios actively studied during the 2003 trace tritium experimental campaign performed on JET includes a gas puff of small amounts of tritium into sawtooth-unstable H-mode plasmas heated by deuterium beams. The sawtooth crashes observed in these discharges, with the electron cyclotron emission diagnostics, are frequently accompanied by the oscillations of the 14 MeV neutron emission, produced mainly due to the reaction between the deuterium beam and the thermal tritium. Such oscillations are clearly seen in low density plasmas while they are weak or absent at high density. The goal of this work is to explain the different behaviour of the oscillations of neutron emission with density. For this purpose, the dynamics of the trace tritium and beam deuterium during the sawtooth crash in discharges with different plasma density is studied numerically using the TRANSP code. In addition, a possibility of using the neutron measurements in sawtoothing low density plasmas for the test of the full and partial reconnection models is discussed and the sensitivity of the simulations to the key parameters of these models is presented.

The plasma edge MHD stability is analysed for several JET discharges in the diagnostic optimized configuration. The stability analysis of Type I ELMy plasmas shows how after an edge localized mode (ELM) crash the plasma edge is deep in the stable region against low- to intermediate-n peeling?ballooning modes. As the pressure gradient steepens and the edge current builds up, the plasma reaches the low- to intermediate-n peeling?ballooning mode stability boundary just before the ELM crash. Increasing the plasma fuelling by gas puffing makes the second stability access against high-n ballooning modes narrower until it closes completely and the ELMs change from Type I to Type III. Reducing the plasma heating has a similar effect. Increasing the safety factor at the plasma edge improves the stability against low- to intermediate-n modes allowing steeper pressure gradients to develop before an ELM crash.

JET plasmas show fluctuations from a large variety of MHD instabilities. A combination of spectral analysis, mode number analysis, and determination of mode radial position has the potential as a diagnostic for the plasma q profile. Mode frequencies and mode numbers are obtained from spectral and phase analysis from fast magnetic pickup signals. Cross correlation of magnetic and multiarray signals from ECE, SXR, and reflectometer systems provide the mode localization. The present status of the diagnostic systems used for fluctuation analysis is summarized, with particular emphasis on the fast magnetic pickup system. A new data collection system allows the magnetic signals to be recorded at 2 MHz for up to 32 s in each pulse. An example of q-profile validation from the observation of Alfven eigenmode cascades and a snake is given. At JET, Alfven cascades are used to determine q(min) in optimized shear discharges, while a variety of low frequency modes (such as the snake) allow the determination of the radial position of q integer values in monotonic and reversed shear q profiles. (C) 2004 American Institute of Physics.

This paper presents the experimental characterization of pedestal parameters, edge localized mode (ELM) energy, and particle losses from the main plasma and the corresponding ELM energy fluxes on plasma facing components for a series of dedicated experiments in the Joint European Torus (JET). From these experiments, it is demonstrated that the simple hypothesis relating the peeling-ballooning linear instability to ELM energy losses is not valid. Contrary to previous observations at lower triangularities, small energy losses at low collisionality have been obtained in regimes at high plasma triangularity and q95∼4.5, indicating that the edge plasma magnetohydrodynamic stability is linked with the transport mechanisms that lead to the loss of energy by conduction during type I ELMs. Measurements of the ELM energy fluxes on the divertor target show that their time scale is linked to the ion transport along the field and the formation of a high energy sheath, in agreement with kinetic modeling of ELMs. Higher...

In neutral beam injection and ion cyclotron resonance frequency (NBI+ICRF) heated high-temperature Joint European Torus (JET) plasmas, electron cyclotron emission (ECE) spectra measured by the Michelson interferometer are inconsistent with the electron bulk distribution being Maxwellian. This conclusion follows from new, accurate modeling of the ECE spectra over the full measured frequency range, covering the first four harmonics of the cyclotron frequency. A model distribution function obtained by fitting a measured ECE spectrum reveals a distortion of the Maxwellian that is sharply localized at ∼1.5×thermal momentum. A consequence deduced from this distribution is that the electron temperature measured by JET’s Thomson scattering diagnostic, which weighs the bulk electrons differently, should be lower than the ECE one in the spatial region where this distortion exists. This prediction appears to be confirmed by temperature profile comparisons showing that core Thomson scattering electron temperatures on JET are up to 20% lower than ECE values for plasmas with strong NBI+ICRF heating. The possibility of addressing experimentally the question of whether the plasma has a Maxwellian bulk is also discussed. This goal can be accomplished by observing the emission at different angles with respect to the magnetic field (oblique ECE). Simulations of oblique ECE spectra using the model distribution function obtained previously show an angular variation of the emission that is characteristic of the distorted bulk.

Heat wave experiments are performed on TJ-II stellarator plasmas to estimate both heat diffusivity and power deposition profiles. High frequency electron cyclotron resonance heating (ECRH) modulation experiments are used to obtain the power deposition profile, which is observed to be wider and duller than that estimated by ray tracing techniques. The causes of this difference are discussed in the paper. Fourier analysis techniques are used to estimate the heat diffusivity in low frequency ECRH modulation experiments. This includes the power deposition profile as a new ingredient. ECRH switch-on/off experiments are exploited to obtain power deposition and heat diffusivities profiles. These quantities are compared with those obtained by modulation experiments and transport analysis, showing good agreement.

The influence of the ITER-like Wall (ILW) with divertor target plate made of tungsten (W), on plasma performance in JET H-mode is being investigated since 2011 (see F. Romanelli and references therein). One of the key issues in discharges with low level of D fuelling is observed accumulation of W in the plasma core, which leads to a reduction in plasma performance. To study the interplay between W sputtering on the target plate, penetration of W through the SOL and edge transport barrier (ETB) and its further accumulation in plasma core predictive modelling was launched using a coupled 1.5D core and 2D SOL code JINTRAC (Romanelli, 2014; Cenacchi and Taroni, 1988; Taroni et al., 1992; Wiesen et al., 2006). Simulations reveal the important role of ELMs in W sputtering and plasma density control. Analyses also confirm pivotal role played by the neo-classical pinch of heavy impurities within the ETB.

A set of discharges performed with the JET ITER-like wall is investigated with respect to control capabilities on tungsten sources and transport. In attached divertor regimes, increasing fueling by gas puff results in higher divertor recycling ion flux, lower divertor tungsten source, higher ELM frequency and lower core plasma radiation, dominated by tungsten ions. Both pedestal flushing by ELMs and divertor screening (including redeposition) are possibly responsible. For specific scenarios, kicks in plasma vertical position can be employed to increase the ELM frequency, which results in slightly lower core radiation. The application of ion cyclotron radio frequency heating at the very center of the plasma is efficient to increase the core electron temperature gradient and flatten electron density profile, resulting in a significantly lower central tungsten peaking. Beryllium evaporation in the main chamber did not reduce the local divertor tungsten source whereas core radiation was reduced by approximately 50%.

This paper describes a series of experiments performed in TJ-II stellarator with the aim of studying the influence of magnetic configuration on stability and transport properties of TJ-II plasmas. Plasma potential profiles have been measured in several configurations up to the plasma core with the heavy ion beam probe diagnostic. Low-order rational surfaces have been positioned at different plasma radii observing the effect on transport features. Plasma edge turbulent transport has been studied in configurations that are marginally stable due to decreased magnetic well. Results show a dynamical coupling between gradients and turbulent transport. Experiments on the influence of magnetic shear on confinement are reported. Global confinement issues as well as enhanced confinement regimes found in TJ-II are discussed as well.

A transient behaviour is observed in the plasma core of TJ-II stellarator with fast drops in the electron temperature. Changes in the line-averaged density are observed synchronized with temperature drops. This phenomenon appears in plasmas created and heated using 300 kW of electron cyclotron heating with high power density. The transient behaviour resembles both, the electric pulsation discovered in CHS and the `electron root' feature reported by the W7-AS team. The flexibility and low magnetic shear of TJ-II have permitted the identification of the plasma current as the control parameter for the appearance of this phenomenon. The results obtained during the magnetic configuration scans carried out in TJ-II points to the hypothesis that the transient behaviour is connected with the presence of a rational surface close to the plasma centre. Equilibrium calculations performed with the VMEC code reinforce this hypothesis.

Recent experimental results show that the core electron temperature in the TJ-II stellarator almost doubles previously obtained values for the same heating power. These plasmas, heated with electron cyclotron waves, are characterized by their low density, and by having highly peaked electron temperature profiles and flat, or even hollow, density profiles. The conditions for obtaining these high electron temperature discharges regarding their density, injected power and dependence on plasma species are described. Neoclassical and experimental transport analyses are performed for these discharges, showing a reduction in the electron heat conductivity at the plasma core. The relations of this observed confinement enhancement to the CHS internal transport barrier and the W7-AS neoclassical electron root feature are discussed.

The TJ-II is a four period, low magnetic shear stellarator, with high degree of configuration flexibility (rotational transform from 0.9 to 2.5) which has been operating in Madrid since 1998 (R=1.5 m, a

TJ-II is a four period, low magnetic shear stellarator (R = 1.5 m, a < 0.22 m, B0 ≤ 1.2 T) which was designed to have a high degree of magnetic configuration flexibility. In the most recent experimental campaign, coupling of the full ECRH power (PECRH ≤ 600 kW) to the plasma has been possible using two ECRH transmission lines which have different power densities. Both helium and hydrogen fuelled plasmas have been investigated. The article reviews the latest physics results in particle control, configuration effects, and transport and fluctuation studies.

Significant features, consisting of flats and humps, have been observed in impurity-ion ultraviolet line-emission profiles measured on the TJ-II flexible heliac, thereby providing evidence for the existence of topological structures in the plasma interior. The data for this study were collected using a fast spectroscopic scanning system with good spatial resolution capabilities; they were analysed using an in-house developed pattern recognition algorithm. This algorithm is capable of separating effects due to plasma fluctuations and noise from those caused by two-dimensional structures. In the paper we describe the data collection system and the pattern recognition algorithm and we analyse plasma radiation profiles from standard TJ-II configurations to demonstrate that this pattern recognition algorithm can identify real features in these. Next, we analyse profile data from a configuration scan made on the TJ-II and finally we report on the features found, their typical widths and symmetry, as well as on the time evolution and correlation with the electron density.

Electron cyclotron emission measurements are routinely performed in TJ-II stellarator by means of a multichannel heterodyne radiometer. The radiometer is absolutely calibrated and measures the temperature profile with high temporal resolution. The description of the radiometer, the calibration procedure, and some data taken during the occurrence of fast phenomena in the plasma (edge localized mode-like events and central temperature crashes) are presented in the article.

Electron cyclotron emission (ECE) measurements are used to investigate the changes in the electron temperature profile during on axis and off axis EC-heating in TJ-II stellarator. The modifications of the electron temperature profiles are discussed and compared with the predictions of ray tracing calculations. Recently, the two gyrotrons (PECRH ≤600 kW) were simultaneously applied for the plasma heating in TJ-II. ELM-like events are clearly observed in these plasmas. A first description of these phenomena is also presented.

Real-time simultaneous control of several radially distributed magnetic and kinetic plasma parameters is being investigated on JET, in view of developing integrated control of advanced tokamak scenarios. This paper describes the new model-based profile controller which has been implemented during the 2006–2007 experimental campaigns. The controller aims to use the combination of heating and current drive (H&CD) systems—and optionally the poloidal field (PF) system—in an optimal way to regulate the evolution of plasma parameter profiles such as the safety factor, q(x), and gyro-normalized temperature gradient, . In the first part of the paper, a technique for the experimental identification of a minimal dynamic plasma model is described, taking into account the physical structure and couplings of the transport equations, but making no quantitative assumptions on the transport coefficients or on their dependences. To cope with the high dimensionality of the state space and the large ratio between the time scales involved, the model identification procedure and the controller design both make use of the theory of singularly perturbed systems by means of a two-time-scale approximation. The second part of the paper provides the theoretical basis for the controller design. The profile controller is articulated around two composite feedback loops operating on the magnetic and kinetic time scales, respectively, and supplemented by a feedforward compensation of density variations. For any chosen set of target profiles, the closest self-consistent state achievable with the available actuators is uniquely defined. It is reached, with no steady state offset, through a near-optimal proportional-integral control algorithm. Conventional optimal control is recovered in the limiting case where the ratio of the plasma confinement time to the resistive diffusion time tends to zero. Closed-loop simulations of the controller response have been performed in preparation for experiments, and typical results are shown. Finally, in the last section of the paper, the first experimental results using this dynamic-model approach to control the plasma current and the safety factor profile on JET, either with the three H&CD systems or also with the PF system as an additional actuator, are presented and discussed.

ELM-like activity has been recently observed in TJ-II, in plasmas with stored energy above 1 kJ. The plasma is observed to develop bursts of magnetic activity (seen in Mirnov coil signals) which are followed by a large and distinct spike in the Hα signal. An increase in electrostatic and magnetic fluctuations at the plasma edge and a cold pulse towards the plasma centre are also characteristics of these events. In addition, the electron temperature profile locally flattens at the plasma radius where the temperature is in the range 100-200 eV. This flattening can be explained in terms of enhanced electron heat conductivity. Between ELM-like events the electromagnetic turbulence at the edge decreases and the Te profiles recover their former shapes. This activity is probably triggered by a resonant m = 2, n = 3 mode.

If the ion Bernstein wave (IBW) heating power in an H mode discharge of the PBX-M experiment exceeds a threshold power of about 200 kW, a core transport barrier is created in the central region of the plasma. At lower neutral beam injection (NBI) powers, the core barrier is accompanied by an edge L mode. The high edge localized mode (ELM) repetition frequency (1 kHz) prevents the creation of a strong barrier, so the edge first has to make an H-to-L transition before a strong core transport barrier can be created. At higher NBI powers, the ELM repetition frequency is lowered to less than 200 Hz, which allows the immediate creation of a strong core barrier. Edge localized mode loss, which propagates radially first on a fast (non-diffusive) and then on a slow (diffusive) time-scale all the way to the plasma core, is strongly reduced in the core barrier region. Correlated with the reduced ELM loss, the fluctuations in the core barrier region are also strongly reduced, both during the ELM and during the quiet periods between the ELMs. There is strong evidence that the IBW induced poloidal flow shear is responsible for the stabilization of core turbulence and the creation of the core transport barrier. The large perpendicular E ? B flow shear component of the measured toroidal velocity in co-injection neutral beam heated discharges seems to be largely cancelled by the ion diamagnetic drift shear produced by large ion pressure gradients in the core barrier region. The value of IBW induced poloidal flow has not been experimentally determined, but its numerical value is found to be a factor of 4 larger than either the toroidal velocity or the ion diamagnetic drift shear components, leaving only IBW induced flow shear as the most probable cause for the turbulence stabilization. The core turbulence suppression and the creation of the core transport barrier is also consistent with expectations from a comparison between the E ? B flow shear rate and a rough estimate of the linear ion temperature gradient (ITG) growth rate. The presence of the core barrier region also strongly modifies the other MHD events: crashes on the q = 1.5, 2 surfaces and the disruption.

Two new types of high frequency (HF) MHD mode activity are observed when a core transport barrier is formed during ion Bernstein wave (IBW) heating in PBX-M. These modes, localized in the core transport barrier, seem to be mainly responsible for the soft saturation in the strength of the core transport barrier. The two types of HF mode activity are determined by the presence or absence of a strong 1/1 mode. The n = m modes are observed with the 1/1 mode, while the n = m - 1 modes are seen when the 1/1 mode is absent. The m numbers of both of these modes vary between 4 and 13. They all seem to be coupled and rotate as a rigid body. The n = m - 1 modes seemed to be narrowly localized on their corresponding rational surface, i.e. q = m/(m - 1). The modes are ballooning-like, and their amplitudes increase with increasing β. Calculations with the CASTOR code show that the n = m - 1 modes are most probably pressure driven tearing modes, but the possibility of kinetic ballooning modes (KBMs) cannot be fully excluded. The n = m modes appear on or around the q = 1 surface, and are localized at the X point of the 1/1 mode island. The frequency offset of the n = m modes, which is too low for a TAE, is shown to be consistent with β-induced toroidal Alfven eigenmodes (BAEs).

Type I ELMy H-mode operation in JET with the ITER-like Be/W wall (JET-ILW) generally occurs at lower pedestal pressures compared to those with the full carbon wall (JET-C). The pedestal density is similar but the pedestal temperature where type I ELMs occur is reduced and below to the so-called critical type I-type III transition temperature reported in JET-C experiments. Furthermore, the confinement factor H 98( y ,2) in type I ELMy H-mode baseline plasmas is generally lower in JET-ILW compared to JET-C at low power fractions P loss / P thr,08 2, the confinement in JET-ILW hybrid plasmas is similar to that in JET-C. A reduction in pedestal pressure is the main reason for the reduced confinement in JET-ILW baseline ELMy H-mode plasmas where typically H 98( y ,2) = 0.8 is obtained, compared to H 98( y ,2) = 1.0 in JET-C. In JET-ILW hybrid plasmas a similarly reduced pedestal pressure is compensated by an increased peaking of the core pressure profile resulting in H 98( y ,2) ⩽ 1.25. The pedestal stability has significantly changed in high triangularity baseline plasmas where the confinement loss is also most apparent. Applying the same stability analysis for JET-C and JET-ILW, the measured pedestal in JET-ILW is stable with respect to the calculated peeling ballooning stability limit and the ELM collapse time has increased to 2 ms from typically 200 µ s in JET-C. This indicates that changes in the pedestal stability may have contributed to the reduced pedestal confinement in JET-ILW plasmas. A comparison of EPED1 pedestal pressure prediction with JET-ILW experimental data in over 500 JET-C and JET-ILW baseline and hybrid plasmas shows a good agreement with 0.8

A comparison of the L-H power threshold (P-thr) 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 I-P/B-T pairs. The low density dependence of the L-H power threshold, namely an increase below a minimum density n(e,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 n(e,min), P-thr is reduced by similar to 30%, and by similar to 40% when the radiation from the bulk plasma is subtracted (P-sep), 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, n(e,min) similar to B-T(4/5), while the power threshold at the minimum density scales as P-sep,P-min similar to B-T(5/2). The H-mode power threshold in JET-ILWis found to be sensitive both to variations in main plasma shape (P-sep decreases with increasing lower triangularity and increases with upper triangularity) and in divertor configuration. When the data are recast in terms of P-sep and Z(eff) or subdivertor neutral pressure a linear correlation is found, pointing to a possible role of Z(eff) and/or subdivertor neutral pressure in the L-H transition physics. Depending on the chosen divertor configuration, P-thr can be up to a factor of two lower than the ITPA scaling law for densities above n(e,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

Amplitude modulation (AM) reflectometry is a technique for density profile measurements in magnetic fusion plasmas based on the measurement of the phase delay of the modulation in the amplitude of a microwave beam launched and reflected at the plasma. Results from AM experiments in the PBX-M tokamak and the W7-AS stellarator are presented. A general analysis of the capabilities of the technique is performed, particularly centered in the effects of spatial turbulence. Simulations of the effects of two-dimensional turbulence have been performed for medium size (W7-AS) and large devices (LHD stellarator, ITER), showing the capability of the AM technique to operate in turbulent plasmas. Finally, possible solutions to the problem of parasitic reflections in AM systems are presented as development options.

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.

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.

An extensive upgrade of the JET radiometer was implemented in 2008 in order to further enhance the performance of the system. Six new high frequency mixers were added, bringing the total number of receivers to twelve. In this way the radiometer provides continuos frequency coverage from 69GHz to 207GHz, enabling the diagnostic to operate over a wider range of plasma conditions. This report describes the changes introduced in the system architecture in order to add the new hardware and gives some illustrative examples of the improved performance of the diagnostic.

A short review of prior results on W-erosion, W-screening of the divertor and W-transport in the pedestal and the plasma core is given. The W-transport in the core plasma of JET is investigated experimentally by deriving the W-concentration profiles from modelling of the signals of the soft X-ray cameras. For the case of pure neutral beam heating W-accumulates in the core to W-concentrations of 10­3 in between the sawtooth crashes, which flatten the W-profile to a concentration of about 3¥10­5. When central ICRH heating is additionally applied the core Wconcentration decays in phases that exhibit a changed mode activity, while also the electron temperature increases and the density profile becomes less peaked. The immediate correlation between the change of MHD and the removal of W from the plasma core supports the theory that the change of the MHD activity is the underlying cause for the change of transport. Furthermore, two discharge scenarios from ASDEX Upgrade are investigated. In the first case the central W-radiation influences the core W-transport such that slowly increasing W-radiation leads to a steepening of the W-concentration profiles. In the second case two discharges are compared in which a small change of the heating trajectory leads to a change of the W-transport and a change of the sawtooth behaviour. It is speculated that the W-accumulation is the underlying reason for the irregular sawteeth, which in turn aids the W-accumulation.

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.

Reflectometry is a very attractive diagnostic for measurements of density profile and plasma turbulence in reactor conditions, but the problem of interpretation of the results is not solved utterly. The reason for this is the plasma turbulence which leads to interference of signals, reflected simultaneously on different turbulent waves and back scattering of signal. Other source of perturbations is the multiple reflection of the signal between plasma and chamber wall. These effects can be simulated completely only in a 3-d full-wave code, though simulation of effects of scattering and interference may be reduced to 2-d taking advantage of the homogeneity of the plasma in the toroidal direction. Full-wave 2-d codes have been developed in recent time 1, but the use of this codes is limited due to the very complicated computations involved (a realistic simulation of a density profile measurement involves typically the calculation of the E-field detected at the antenna for a set of 2000 frequency values). In this work, a simple WKB 2-d code was used for qualitative simulation of effects of interference in situations where the turbulence size is larger than the signal wavelength.

The capabilities of the Joint European Torus (JET) electron cyclotron emission (ECE) diagnostics have recently been extended with an upgrading of the heterodyne radiometer. The number of channels has been doubled to 96 channels, with a frequency separation corresponding to 2.6 m for the X-mode due to harmonic overlap) at almost all magnetic field values used at JET (1.7 T

The size and capability of JET to reach high plasma current and field enables a study of the plasma behaviour at ion Larmor radius and collisionality values approaching those of ITER. In this paper such study is presented. The achievement of stationary type I ELMy H-modes at high current proved to be quite challenging. As the plasma current was increased, it became more difficult to achieve stationary conditions. Nevertheless, it was possible to achieve stable operation at high plasma current (up to 4.5 MA) and low q 95 (2.65–3) at JET. One of the main reasons to revisit the high plasma current experiments done in 1997 is the higher power available and the improvement of the pedestal diagnostics. Indeed, compared with previous results, higher stored energy was achieved but confinement was still degraded. The causes of this confinement degradation are discussed in the paper.

To consolidate International Thermonuclear Experimental Reactor (ITER) design choices and prepare for its operation, Joint European Torus (JET) has implemented ITER's plasma facing materials, namely, Be for the main wall and W in the divertor. In addition, protection systems, diagnostics, and the vertical stability control were upgraded and the heating capability of the neutral beams was increased to over 30 MW. First results confirm the expected benefits and the limitations of all metal plasma facing components (PFCs) but also yield understanding of operational issues directly relating to ITER. H-retention is lower by at least a factor of 10 in all operational scenarios compared to that with C PFCs. The lower C content (≈ factor 10) has led to much lower radiation during the plasma burn-through phase eliminating breakdown failures. Similarly, the intrinsic radiation observed during disruptions is very low, leading to high power loads and to a slow current quench. Massive gas injection using a D2/Ar mixture restores levels of radiation and vessel forces similar to those of mitigated disruptions with the C wall. Dedicated L-H transition experiments indicate a 30% power threshold reduction, a distinct minimum density, and a pronounced shape dependence. The L-mode density limit was found to be up to 30% higher than for C allowing stable detached divertor operation over a larger density range. Stable H-modes as well as the hybrid scenario could be re-established only when using gas puff levels of a few 1021 es-1. On average, the confinement is lower with the new PFCs, but nevertheless, H factors up to 1 (H-Mode) and 1.3 (at β N ≈ 3, hybrids) have been achieved with W concentrations well below the maximum acceptable level. © 2013 EURATOM.

A reflectometer, based on the amplitude modulation technique, has been developed and operated on a PBX-M for the measurement of electron density profile. The system, operating with the extraordinary mode in the range of 32-50 GHz, is able to measure the density profile in the plasma edge from the scrape-off layer up to typically r/a=0.7. The determination of the time delay for each frequency is achieved by measuring the phase delay of a 200 MHz amplitude modulation envelope superimposed on the millimetre wave probing signal. The system has a final bandwidth of 40 kHz and is able to obtain the edge profile during a 1 ms sweep of the microwave source. High quality profiles are obtained in systematic good agreement with Thomson scattering measurements. The profile reconstruction from the raw data is direct, with only a need for minimal data processing. Profiles have been measured for ohmic, RF and NBI heated discharges. Features of the profile changes in the kH transition are shown. One of the goals of the instrument has been the measurement of the slight modifications to the edge density profile produced by the injection of ion Bernstein waves. These changes have been clearly observed and are in agreement with theoretical expectations.

In April 1994 first ECRH plasmas (28 GHz, 2nd harmonic) have been produced in the TJ-I U torsatron (R=0.6m, [a]=0.1m, B(0)=0.52 T, E(0)=0.21). In November 1994 a second experimental campaign using a 37.5 GHz gyrotron (B(o)=0.69 T) has started. The main parameters of the device, the basic characteristics of the heating system and some results concerning magnetic surface mapping and first plasma discharges, are reported. 4 refs., 4 figs.

Application of Ion Bernstein Wave Heating (IBWH) into the Princeton Beta Experiment‐Modification (PBX‐M) [Phys. Fluids B 2, 1271 (1990)] tokamak stabilizes sawtooth oscillations and generates peaked density profiles. A transport barrier, spatially correlated with the IBWH power deposition profile, is observed in the core of IBWH‐assisted neutral beam injection (NBI) discharges. A precursor to the fully developed barrier is seen in the soft x‐ray data during edge localized mode (ELM) activity. Sustained IBWH operation is conducive to a regime where the barrier supports large ∇ne, ∇Te, ∇νφ, and ∇Ti, delimiting the confinement zone. This regime is reminiscent of the H(high) mode, but with a confinement zone moved inward. The core region has better than H‐mode confinement while the peripheral region is L(low)‐mode‐like. The peaked profile enhances NBI core deposition and increases nuclear reactivity. An increase in central Ti results from χi reduction (compared to the H mode) and better beam penetration. Boot...

At the fusion experiment JET, a Michelson interferometer is used to measure the spectrum of the electron cyclotron emission in the spectral range 70-500 GHz. The interferometer is absolutely calibrated using the hot/cold technique and, in consequence, the spatial profile of the plasma electron temperature is determined from the measurements. The current state of the interferometer hardware, the calibration setup, and the analysis technique for calibration and plasma operation are described. A new, full-system, absolute calibration employing continuous data acquisition has been performed recently and the calibration method and results are presented. The noise level in the measurement is very low and as a result the electron cyclotron emission spectrum and thus the spatial profile of the electron temperature are determined to within +/- 5% and in the most relevant region to within +/- 2%. The new calibration shows that the absolute response of the system has decreased by about 15% compared to that measured previously and possible reasons for this change are presented. Temperature profiles measured with the Michelson interferometer are compared with profiles measured independently using Thomson scattering diagnostics, which have also been recently refurbished and recalibrated, and agreement within experimental uncertainties is obtained. [http://dx.doi.org/10.1063/1.4768246] QC 20220509

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 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 ((48)×10 19m 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.