On the mechanisms governing gas penetration into a tokamak plasma during a massive gas injection

A new 1D radial fluid code, IMAGINE, is used to simulate the penetration of gas into a tokamak plasma during a massive gas injection (MGI). The main result is that the gas is in general strongly braked as it reaches the plasma, due to mechanisms related to charge exchange and (to a smaller extent) recombination. As a result, only a fraction of the gas penetrates into the plasma. Also, a shock wave is created in the gas which propagates away from the plasma, braking and compressing the incoming gas. Simulation results are quantitatively consistent, at least in terms of orders of magnitude, with experimental data for a D 2 MGI into a JET Ohmic plasma. Simulations of MGI into the background plasma surrounding a runaway electron beam show that if the background electron density is too high, the gas may not penetrate, suggesting a possible explanation for the recent results of Reux et al in JET (2015 Nucl. Fusion 55 093013)

Velocity-space sensitivity of the time-of-flight neutron spectrometer at JET

The velocity-space sensitivities of fast-ion diagnostics are often described by so-called weight functions. Recently, we formulated weight functions showing the velocity-space sensitivity of the often dominant beam-target part of neutron energy spectra. These weight functions for neutron emission spectrometry (NES) are independent of the particular NES diagnostic. Here we apply these NES weight functions to the time-of-flight spectrometer TOFOR at JET. By taking the instrumental response function of TOFOR into account, we calculate time-of-flight NES weight functions that enable us to directly determine the velocity-space sensitivity of a given part of a measured time-of-flight spectrum from TOFOR

Relationship of edge localized mode burst times with divertor flux loop signal phase in JET

A phase relationship is identified between sequential edge localized modes (ELMs) occurrence times in a set of H-mode tokamak plasmas to the voltage measured in full flux azimuthal loops in the divertor region. We focus on plasmas in the Joint European Torus where a steady H-mode is sustained over several seconds, during which ELMs are observed in the Be II emission at the divertor. The ELMs analysed arise from intrinsic ELMing, in that there is no deliberate intent to control the ELMing process by external means. We use ELM timings derived from the Be II signal to perform direct time domain analysis of the full flux loop VLD2 and VLD3 signals, which provide a high cadence global measurement proportional to the voltage induced by changes in poloidal magnetic flux. Specifically, we examine how the time interval between pairs of successive ELMs is linked to the time-evolving phase of the full flux loop signals. Each ELM produces a clear early pulse in the full flux loop signals, whose peak time is used to condition our analysis. The arrival time of the following ELM, relative to this pulse, is found to fall into one of two categories: (i) prompt ELMs, which are directly paced by the initial response seen in the flux loop signals; and (ii) all other ELMs, which occur after the initial response of the full flux loop signals has decayed in amplitude. The times at which ELMs in category (ii) occur, relative to the first ELM of the pair, are clustered at times when the instantaneous phase of the full flux loop signal is close to its value at the time of the first ELM

Optimization of the Layout of the ITER ICRF Antenna Port Plug and its Performance Assessment

The ITER ICRF antenna's coupled power to plasma is determined not only by the plasma scrape‐off layer profiles and shaping of the front strap array, but also by assuring optimal excitation of the array by the overall layout of the feed network and the detailed shaping of the RF components therein. This paper describes the optimization of the feed network layout inside the port plug with 4‐port junction, vacuum window and service stub components under RF, thermo‐mechanical, manufacturing and assembly constraint

Performance assessment of the ITER ICRF antenna

ITER's Ion Cyclotron Range of Frequencies (ICRF) system [1] comprises two antenna launchers designed by CYCLE (a consortium of European associations listed in the author affiliations above) on behalf F4E for the ITER Organisation (IO), each inserted as a Port Plug (PP) into one of ITER's Vacuum Vessel (VV) ports. Each launcher is an array of 4 toroidal by 6 poloidal RF current straps specified to couple up to 20 MW in total to the plasma in the frequency range of 40 to 55 MHz but limited to a maximum system voltage of 45 kV and limits on RF electric fields depending on their location and direction with respect to respectively the torus vacuum and the toroidal magnetic field. A crucial aspect of coupling ICRF power to plasmas is the knowledge of the plasma density profiles in the Scrape-Off Layer (SOL) and the location of the RF current straps with respect to the SOL. The launcher layout and details were optimized and its performance estimated for a worst case SOL provided by the IO. The paper summarizes the estimated performance obtained within the operational parameter space specified by IO. Aspects of the RF grounding of the whole antenna PP to the VV port and the effect of the voids between the PP and the Blanket Shielding Modules (BSM) surrounding the antenna front are discusse

Design, performance, and grounding aspects of the International Thermonuclear Experimental Reactor ion cyclotron range of frequencies antenna

ITER's Ion Cyclotron Range of Frequencies (ICRF) system comprises two antenna launchers designed by CYCLE (a consortium of European associations listed in the author affiliations above) on behalf of ITER Organisation (IO), each inserted as a Port Plug (PP) into one of ITER's Vacuum Vessel (VV) ports. Each launcher is an array of 4 toroidal by 6 poloidal RF current straps specified to couple up to 20 MW in total to the plasma in the frequency range of 40 to 55 MHz but limited to a maximum system voltage of 45 kV and limits on RF electric fields depending on their location and direction with respect to, respectively, the torus vacuum and the toroidal magnetic field. A crucial aspect of coupling ICRF power to plasmas is the knowledge of the plasma density profiles in the Scrape-Off Layer (SOL) and the location of the RF current straps with respect to the SOL. The launcher layout and details were optimized and its performance estimated for a worst case SOL provided by the IO. The paper summarizes the estimated performance obtained within the operational parameter space specified by IO. Aspects of the RF grounding of the whole antenna PP to the VV port and the effect of the voids between the PP and the Blanket Shielding Modules (BSM) surrounding the antenna front are discussed. These blanket modules, whose dimensions are of the order of the ICRF wavelengths, together with the clearance gaps between them will constitute a corrugated structure which will interact with the electromagnetic waves launched by ICRF antennas. The conditions in which the grooves constituted by the clearance gaps between the blanket modules can become resonant are studied. Simple analytical models and numerical simulations show that mushroom type structures (with larger gaps at the back than at the front) can bring down the resonance frequencies, which could lead to large voltages in the gaps between the blanket modules and perturb the RF properties of the antenna if they are in the ICRF operating range. The effect on the wave propagation along the wall structure, which is acting as a spatially periodic (toroidally and poloidally) corrugated structure, and hence constitutes a slow wave structure modifying the wall boundary condition, is examine

RF optimisation of the port plug layout and performance assessment of the ITER ICRF antenna

The ITER Ion Cyclotron Range of Frequencies (ICRF) antenna's capacity to couple power to plasma is determined by the plasma Scrape-Off Layer (SOL) profile, shaping of the front strap array, layout of the Port Plug (PP) and detailed design of its RF components. The first two factors are taken into account by the Torino Polytechnic Ion Cyclotron Antenna (TOPICA) calculated strap array Scattering/Impedance 24-port (S24 × 24−/Z24 × 24−) matrices, while this paper deals with the optimisation of the PP layout and components. The RF modeling techniques are explained and used to maximise the coupled power under a set of constraints on RF quantities inside the PP. The total PP RF surface conductive and volumetric dielectric losses are calculated. The resulting S-parameters at the rear RF PP flanges are evaluated as input for the design of the pre-match, decoupling and matching network outside the PP. A discussion of the effect of errors on the PP excitation on the coupled power is also included

L-H transition in the mega-Amp spherical tokamak

H-mode plasmas have been achieved on the MAST spherical tokamak at input power considerably higher than predicted by conventional threshold scalings. Following L-H transition, a clear improvement in energy confinement is obtained, exceeding recent international scalings even at densities approaching the Greenwald density limit. Transition is accompanied by an order-of-magnitude increase in edge-density gradient, a marked decrease in turbulence, the efficient conversion of internal electron Bernstein waves into free space waves, and the onset and saturation of edge poloidal rotation

Impact of nitrogen seeding on confinement and power load control of a high-triangularity JET ELMy H-mode plasma with a metal wall

This paper reports the impact on confinement and power load of the high-shape 2.5MA ELMy H-mode scenario at JET of a change from an all carbon plasma facing components to an all metal wall. In preparation to this change, systematic studies of power load reduction and impact on confinement as a result of fuelling in combination with nitrogen seeding were carried out in JET-C and are compared to their counterpart in JET with a metallic wall. An unexpected and significant change is reported on the decrease of the pedestal confinement but is partially recovered with the injection of nitrogen.Comment: 30 pages, 16 figure

Overview of JET results

Since the last IAEA conference, the scientific programme of JET has focused on the qualification of the integrated operating scenarios for ITER and on physics issues essential for the consolidation of design choices and the efficient exploitation of ITER. Particular attention has been given to the characterization of the edge plasma, pedestal energy and edge localized modes (ELMs), and their impact on plasma facing components (PFCs). Various ELM mitigation techniques have been assessed for all ITER operating scenarios using active methods such as resonant magnetic field perturbation, rapid variation of the radial field and pellet pacing. In particular, the amplitude and frequency of type I ELMs have been actively controlled over a wide parameter range (q95 = 3-4.8, βN ≥ 3.0) by adjusting the amplitude of the n = 1 external perturbation field induced by error field correction coils. The study of disruption induced heat loads on PFCs has taken advantage of a new wide-angle viewing infrared system and a fast bolometer to provide a detailed account of time, localization and form of the energy deposition. Specific ITER-relevant studies have used the unique JET capability of varying the toroidal field (TF) ripple from its normal low value δBT = 0.08% up to δBT = 1% to study the effect of TF ripple on high confinement-mode plasmas. The results suggest that δBT < 0.5% is required on ITER to maintain adequate confinement to allow QDT = 10 at full field. Physics issues of direct relevance to ITER include heat and toroidal momentum transport, with experiments using power modulation to decouple power input and torque to achieve first experimental evidence of inward momentum pinch in JET and determine the threshold for ion temperature gradient driven modes. Within the longer term JET programme in support of ITER, activities aiming at the modification of the JET first wall and divertor and the upgrade of the neutral beam and plasma control systems are being conducted. The procurement of all components will be completed by 2009 with the shutdown for the installation of the beryllium wall and tungsten divertor extending from summer 2009 to summer 2010