Showing papers by "Jet-Efda Contributors published in 2012"
TL;DR: In this paper, the effect of plasma current sharing with the wall was introduced into theory, and the toroidal asymmetry of the plasma current measurements during vertical disruption event (VDE) on the Joint European Torus was explained.
Abstract: This paper describes progress achieved since 2007 in understanding disruptions in tokamaks, when the effect of plasma current sharing with the wall was introduced into theory. As a result, the toroidal asymmetry of the plasma current measurements during vertical disruption event (VDE) on the Joint European Torus was explained. A new kind of plasma equilibria and mode coupling was introduced into theory, which can explain the duration of the external kink 1/1 mode during VDE. The paper presents first results of numerical simulations using a free boundary plasma model, relevant to disruptions.
81 citations
TL;DR: The full-metal ITER-like wall (ILW) at JET was found to have a profound impact on the physics of disruptions, yielding higher plasma temperatures after the thermal quench and thus longer current quench times.
Abstract: The new full-metal ITER-like wall (ILW) at JET was found to have a profound impact on the physics of disruptions. The main difference is a significantly lower fraction (by up to a factor of 5) of energy radiated during the disruption process, yielding higher plasma temperatures after the thermal quench and thus longer current quench times. Thus, a larger fraction of the total energy was conducted to the wall resulting in larger heat loads. Active mitigation by means of massive gas injection became a necessity to avoid beryllium melting already at moderate levels of thermal and magnetic energy (i.e. already at plasma currents of 2 MA). A slower current quench, however, reduced the risk of runaway generation. Another beneficial effect of the ILW is that disruptions have a negligible impact on the formation and performance of the subsequent discharge.
79 citations
TL;DR: In this paper, a new technique has been developed to produce plasmas with improved confinement relative to the H-98,H-y2 scaling law on the JET tokamak.
Abstract: A new technique has been developed to produce plasmas with improved confinement relative to the H-98,H-y2 scaling law (ITER Physics Expert Groups on Confinement and Transport and Confinement Modelling and Database ITER Physics Basics Editors and ITER EDA 1999 Nucl. Fusion 39 2175) on the JET tokamak. In the mid-size tokamaks ASDEX upgrade and DIII-D heating during the current formation is used to produce a flat q-profile with a minimum close to 1. On JET this technique leads to q-profiles with similar minimum q but opposite to the other tokamaks not to an improved confinement state. By changing the method utilizing a faster current ramp with temporary higher current than in the flattop (current overshoot) plasmas with improved confinement (H-98,H-y2 = 1.35) and good stability (beta(N) approximate to 3) have been produced and extended to many confinement times only limited by technical constraints. The increase in H-98,H-y2-factor is stronger with more heating power as can be seen in a power scan. The q-profile development during the high power phase in JET is reproduced by current diffusion calculated by TRANSP and CRONOS. Therefore the modifications produced by the current overshoot disappear quickly from the edge but the confinement improvement lasts longer, in some cases up to the end of the heating phase.
66 citations
TL;DR: In this article, a systematic study of power load reduction due to the effect of fuelling in combination with seeding over a wide range of pedestal density ((4-8) x 10(19) m(-3)) with detailed documentation of divertor, pedestal and main plasma conditions, as well as a comparative study of two extrinsic impurity nitrogen and neon.
Abstract: 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 similar to 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 ((4-8) x 10(19) m(-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.
65 citations
TL;DR: A new endoscope with optimised divertor view has been developed in order to survey and monitor the emission of specific impurities such as tungsten and the remaining carbon as well as beryllium in the tungsen divertor of JET after the implementation of the ITER-like wall in 2011.
Abstract: A new endoscope with optimised divertor view has been developed in order to survey and monitor the emission of specific impurities such as tungsten and the remaining carbon as well as beryllium in the tungsten divertor of JET after the implementation of the ITER-like wall in 2011. The endoscope is a prototype for testing an ITER relevant design concept based on reflective optics only. It may be subject to high neutron fluxes as expected in ITER. The operating wavelength range, from 390 nm to 2500 nm, allows the measurements of the emission of all expected impurities (W I, Be II, C I, C II, C III) with high optical transmittance (≥30% in the designed wavelength range) as well as high spatial resolution that is ≤2 mm at the object plane and ≤3 mm for the full depth of field (±0.7 m). The new optical design includes options for in situ calibration of the endoscope transmittance during the experimental campaign, which allows the continuous tracing of possible transmittance degradation with time due to impurit...
57 citations
TL;DR: A pedestal database was built using data from type-I ELMy H-modes of ASDEX Upgrade, DIII-D and JET as discussed by the authors, and the two-line method is a bilinear fit which shows better reproducibility of pedestal parameters than a modified hyperbolic tangent fit.
Abstract: A pedestal database was built using data from type-I ELMy H-modes of ASDEX Upgrade, DIII-D and JET. ELM synchronized pedestal data were analysed with the two-line method. The two-line method is a bilinear fit which shows better reproducibility of pedestal parameters than a modified hyperbolic tangent fit. This was tested with simulated and experimental data. The influence of the equilibrium reconstruction on pedestal parameters was investigated with sophisticated reconstructions from CLISTE and EFIT including edge kinetic profiles. No systematic deviation between the codes could be observed. The flux coordinate system is influenced by machine size, poloidal field and plasma shape. This will change the representation of the width in different coordinates, in particular, the two normalized coordinates ΨN and r/a show a very different dependence on the plasma shape. The scalings derived for the pedestal width, Δ, of all machines suggest a different scaling for the electron temperature and the electron density. Both cases show similar dependence with machine size, poloidal magnetic field and pedestal electron temperature and density. The influence of ion temperature and toroidal magnetic field is different on each of and . In dimensionless form the density pedestal width in ΨN scales with , the temperature pedestal width with . Both widths also show a strong correlation with the plasma shape. The shape dependence originates from the coordinate transformation and is not visible in real space. The presented scalings predict that in ITER the temperature pedestal will be appreciably wider than the density pedestal.
51 citations
TL;DR: In this article, high-resolution?-ray measurements were carried out on the Joint European Torus (JET) in an experiment aimed at accelerating 4He ions in the MeV range by coupling third harmonic radio frequency heating to an injected 4He beam.
Abstract: High-resolution ?-ray measurements were carried out on the Joint European Torus (JET) in an experiment aimed at accelerating 4He ions in the MeV range by coupling third harmonic radio frequency heating to an injected 4He beam. For the first time, Doppler broadening of ?-ray peaks from the 12C(d, p?)13C and 9Be(?, n?)12C reactions was observed and interpreted with dedicated Monte Carlo codes based on the detailed nuclear physics of the processes. Information on the confined 4He and deuteron energy distribution was inferred, and confined 4He ions with energies as high as 6?MeV were assessed. A signature of MHD activity in ?-ray traces was also detected. The reported results have a bearing on diagnostics for fast ions in the MeV range in next step fusion devices.
49 citations
TL;DR: It is demonstrated that a protection system based on CCD cameras can work and examples are shown of hot spot detections that stop the plasma pulse.
Abstract: The new JET ITER-like wall (made of beryllium and tungsten) is more fragile than the former carbon fiber composite wall and requires active protection to prevent excessive heat loads on the plasma facing components (PFC). Analog CCD cameras operating in the near infrared wavelength are used to measure surface temperature of the PFCs. Region of interest (ROI) analysis is performed in real time and the maximum temperature measured in each ROI is sent to the vessel thermal map. The protection of the ITER-like wall system started in October 2011 and has already successfully led to a safe landing of the plasma when hot spots were observed on the Be main chamber PFCs. Divertor protection is more of a challenge due to dust deposits that often generate false hot spots. In this contribution we describe the camera, data capture and real time processing systems. We discuss the calibration strategy for the temperature measurements with cross validation with thermal IR cameras and bi-color pyrometers. Most importantly...
49 citations
TL;DR: In this article, the operational experience of the ion cyclotron resonant frequency (ICRF) ITER-like antenna on JET is summarized, and an in-depth description of its commissioning, operational aspects and achieved performances is presented.
Abstract: This paper summarizes the operational experience of the ion cyclotron resonant frequency (ICRF) ITER-like antenna on JET aiming at substantially increasing the power density in the range of the requirements for ITER combined with load resiliency. An in-depth description of its commissioning, operational aspects and achieved performances is presented.
46 citations
11 Sep 2012-Nuclear Instruments & Methods in Physics Research Section A-accelerators Spectrometers Detectors and Associated Equipment
TL;DR: In this article, an optimised version of robust and fast tomographic algorithm based on the Tikhonov regularisation constrained to minimum Fisher information is presented. And the resulting speed of reconstruction is compared to other referenced tomographic algorithms.
Abstract: Tomography is a technique which is widely applied to fusion plasmas as it can provide improved understanding of plasma emissivity distributions It is challenging because of the sparse nature of data available from the measured plasma projections An optimised version of robust and fast tomographic algorithm based on the Tikhonov regularisation constrained to Minimum Fisher Information is presented in this contribution A new regularisation matrix enforcing preferential emissivity smoothness along magnetic flux surfaces is introduced The paper also details application of advanced numerical methods which lead to a substantial decrease in computation time Subsequent implementation of fast presolvers of the inverse problem further contributes to the algorithm's efficiency and also an improved stability of the tomography reconstruction Finally, reliability and performance of the tomography algorithm is exemplified by the reconstruction of soft X-ray data evolution following tungsten ablation into a JET plasma The resulting speed of reconstruction is compared to other referenced tomographic algorithms
40 citations
TL;DR: Stability of high-beta plasmas is studied on discharges from a series of JET experiments on steady-state and hybrid advanced scenarios, with a wide range of safety factor (q) profiles and normalized beta values extending to βN = 4.
Abstract: Stability of high-beta plasmas is studied on discharges from a series of JET experiments on steady-state and hybrid advanced scenarios, with a wide range of safety factor (q) profiles and normalized beta values extending to βN = 4. Bursting and continuous forms of global n = 1 instabilities are encountered that degrade confinement or, in some cases, give rise to disruptions. Mode frequencies are well above the inverse wall time and correspond to plasma rotation at around mid-radius. Stability boundaries in terms of qmin and pressure peaking are examined. For relatively broad pressure profiles the stability limit decreases from βN = 4 at qmin = 1 to βN = 2 at qmin = 3, while at fixed qmin it decreases with increasing pressure peaking. Metastable and unstable regions are identified in the βN–qmin diagram by mode-trigger analysis. Tearing and kink mode structures are found from phase analysis of temperature profile oscillations; for a selection of kink cases, instability conditions and mode structure are compared with ideal stability calculations.
TL;DR: In this article, Liang et al. observed a strike point splitting by resonant magnetic perturbations (RMPs) at JET and showed that the formation of a secondary strike point (strike point splitting) by RMPs observed in other machines has not been observed before.
Abstract: At JET the error field correction coils can be used to generate an n = 1 or n = 2 magnetic perturbation field (Liang et al 2007 Plasma Phys. Control. Fusion 49 B581). Various experiments at JET have already been carried out to investigate the mitigation of ELMs by resonant magnetic perturbations (RMPs) (Liang et al 2010 Nucl. Fusion 50 025013, Liang et al 2011 Nucl. Fusion 51 073001). However, the typical formation of a secondary strike point (strike point splitting) by RMPs observed in other machines (Jakubowski et al 2010 Contrib. Plasma Phys. 50 701–7, Jakubowski et al 2004 Nucl. Fusion 44 S1–11, Nardon et al 2011 J. Nucl. Mater. 415 S914–7, Eich et al 2003 Phys. Rev. Lett. 91 195003, Evans et al 2007 J. Nucl. Mater. 363–365 570–4, Evans et al 2005 J. Phys.: Conf. Ser. 7 174–90, Watkins et al 2009 J. Nucl. Mater. 390–391 839–42) has never been observed at JET before. In this work we will present discharges where for the first time a strike point splitting by RMPs at JET has been observed. We will show that in these particular cases the strike point splitting matches the vacuum edge magnetic field topology. This is done by comparing heat and particle flux profiles on the outer divertor plate with the magnetic footprint pattern obtained by field line tracing. Further the evolution of the strike point splitting during the ramp up phase of the perturbation field and during a q95-scan is investigated, and it will be shown that the spontaneous appearance of the strike point splitting is only related to some geometrical effects of the toroidal asymmetric magnetic topology.
TL;DR: In this article, the authors investigated the non-resonant magnetic braking effect induced by a non-axisymmetric magnetic perturbation on JET and TEXTOR.
Abstract: The non-resonant magnetic braking effect induced by a non-axisymmetric magnetic perturbation is investigated on JET and TEXTOR. The collisionality dependence of the torque induced by the n = 1, where n is the toroidal mode number, magnetic perturbation generated by the error field correction coils on JET is observed. The observed torque is located mainly in the plasma core (normalized radius ρ < 0.4) and increases with decreasing collisionality. The neoclassical toroidal plasma viscosity (NTV) torque in the collisionless regime is modelled using the numerical solution of the bounce-averaged drift kinetic equation. The calculated collisionality dependence of the NTV torque is in good agreement with the experimental observation on JET. The reason for this collisionality dependence is that the torque in the plasma core on JET mainly comes from the flux of the trapped electrons, which are still mainly in the 1/ν regime. The strongest NTV torque on JET is also located near the plasma core. The magnitude of the NTV torque strongly depends on the plasma response, which is also discussed in this paper. There is no obvious braking effect with n = 2 magnetic perturbation generated by the dynamic ergodic divertor on TEXTOR, which is consistent with the NTV modelling.
TL;DR: A new methodology to select the set of the signals’ parameters in order to maximize the performance of the predictor is reported, based on “Genetic Algorithms” (GAs).
TL;DR: Pinches et al. as mentioned in this paper showed that monster sawtooth crashes are strongly facilitated by the AE-induced re-distribution of the fast D ions from inside the q'='1 radius to the plasma edge.
Abstract: Experiments on accelerating NBI-produced deuterium (D) beam ions from their injection energy of ∼110 keV up to the MeV energy range with 3rd harmonic ion cyclotron resonance heating were performed on the Joint European Torus [P H Rebut and B E Keen, Fusion Technol 11, 13 (1987)] A renewed set of nuclear diagnostics was used for analysing fast D ions during sawtooth stabilization, monster sawtooth crashes, and during excitation of Alfven eigenmodes (AEs) residing inside the q = 1 radius The measurements and modeling of the fast ions with the nonlinear HAGIS code [S D Pinches et al, Comput Phys Commun 111, 133 (1998)] show that monster sawtooth crashes are strongly facilitated by the AE-induced re-distribution of the fast D ions from inside the q = 1 radius to the plasma edge
TL;DR: Recent enhancements of the diagnostic setup are described, new procedures for calibration and protection measures for the lithium ion gun during massive gas puffs for disruption mitigation are described.
Abstract: A 60 kV neutral lithium diagnostic beam probes the edge plasma of JET for the measurement of electron density profiles. This paper describes recent enhancements of the diagnostic setup, new procedures for calibration and protection measures for the lithium ion gun during massive gas puffs for disruption mitigation. New light splitting optics allow in parallel beam emission measurements with a new double entrance slit CCD spectrometer (spectrally resolved) and a new interference filter avalanche photodiode camera (fast density and fluctuation studies).
TL;DR: In this paper, the authors summarized the continuous developments made to the ion cyclotron resonant frequency (ICRF) system at JET in order to improve the reliability of the power coupled to plasma.
Abstract: The paper summarizes the continuous developments made to the ion cyclotron resonant frequency (ICRF) system at JET in order to improve the reliability of the power coupled to plasma. It details the changes and improvements made to the system so that more power is coupled during ELMy plasmas as well as increasing the power density to demonstrate reliable operation in the range of the requirements for ITER. Results obtained using the conventional matching (stubs and trombones) system, 3 dB couplers and the conjugate-T scheme with variable matching elements outside the wave launching structure (external conjugate-T) and inside the wave launching structure (ITER-like antenna) are described. The presence of the three different approaches to load resilient ICRF systems at JET creates a unique opportunity to compare these methods under very similar plasma conditions and to assess the results of ICRF power delivery to ELMy plasmas, an important issue for ITER. The impact of the availability of increased levels of reliable ICRF power on plasma physics studies in JET is illustrated.
TL;DR: In this article, a detailed study of the influence of ion cyclotron resonance heating (ICRH) on the impurity content of JET plasmas is presented, and the contribution of Ni impurity to Zeff and to plasma dilution is estimated.
Abstract: Additional heating systems can be a source of impurities in fusion plasmas. Studying the behaviour of such impurities is important to understand and minimize their effects on tokamak plasma performance. In this paper we present a detailed study of the influence of ion cyclotron resonance heating (ICRH) on the impurity content of JET plasmas. Using spectroscopic methods we monitor the Ni impurity release by the two ICRH antenna types in JET: the so-called ITER-like antenna and the A2 antennas. The release of Ni during ICRH is presented as a function of the power density, total ICRH power applied, the relative phasing of the A2 antenna straps, the D2 gas injection level and the plasma–strap distance. We also estimate the contribution of Ni impurity to Zeff and to plasma dilution. L-mode discharges are compared for different relative phasings of the four antenna straps of the A2 antennas: (0 π 0 π), (0 π π 0), (0 0 π π), (00 π/2 π/2), (0 π/2 π 3π/2), (− − π 0) and (0 π − −). We observe that for antenna phasings that allow the best coupling, the Ni impurity concentration is reduced in the central part of the plasma. The impurity content is also monitored in H-mode experiments aiming at improving the ICRH coupling at large antenna strap–separatrix distances, up to 19 cm, by injecting gas from different inlets. Interestingly, with increasing gas injection rate the coupling improves and the Ni concentration decreases, and the actual location of gas injection is found to influence the Ni concentration.
TL;DR: 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.
Abstract: 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.
TL;DR: In this article, an analysis of the radio frequency (RF) physics of ICWC discharges, the optimization of the operation of ICRF antennas for plasma startup and an outlook for ICWC in ITER using the ITER ICWC heating system is provided.
Abstract: ITER as a superconducting fusion machine needs efficient wall conditioning techniques for application in the presence of the permanent high toroidal magnetic field for (i) reducing the in-vessel impurity content, (ii) controlling the surface hydrogen isotopic ratio and (iii) mitigating the in-vessel long-term tritium inventory build-up. Encouraging results recently obtained with ion-cyclotron wall conditioning (ICWC) in the present-day tokamaks and stellarators have raised ICWC to the status of one of the most promising techniques available to ITER for routine inter-pulse and overnight conditioning with the ITER main ICRF heating system in the presence of the permanent high toroidal magnetic field. This paper is dedicated to a milestone experiment in ICWC research: the first simulation of ICWC operation in an equivalent ITER full-field scenario and the assessment of the wall conditioning effect on the carbon wall in the largest present-day tokamak JET. In addition, we address in this paper the following topics: (i) an analysis of the radio frequency (RF) physics of ICWC discharges, (ii) the optimization of the operation of ICRF antennas for plasma startup and (iii) an outlook for the performance of ICWC in ITER using the ICRF heating system. Important operational aspects of the conventional ICRF heating system in JET (the so-called A2 antenna system) for use in the ICWC mode are highlighted: (i) the ability of the antenna to ignite the cleaning discharge safely and reliably in different gases, (ii) the capacity of the antennas to couple a large fraction of the RF generator power (>50%) to low-density (≈1016–1018 m−3) plasmas and (iii) the ICRF absorption schemes aimed at improved RF plasma homogeneity and enhanced conditioning effect. Successful optimization of the JET-ICWC discharge parameters (BT = 3.3 T, f = 25 MHz) resulted in a reliable operation of the JET A2 antennas and a high conditioning efficiency in a scenario imitating closely ITER full-field operation (BT = 5.3 T, f = 40 MHz) with the fundamental ion-cyclotron resonance for deuterium located on-axis. Numerical modelling with the 3D electromagnetic code Micro Wave Studio, a 1D RF full wave code and a 0D plasma code allows extrapolating the results obtained on JET and other present-day tokamaks to ITER and provides good prospects for the use of the ITER ICRF antennas for ICWC purposes.
TL;DR: In this paper, material research and developments for the materials of the JET ITER-like wall with respect to mechanical and thermal properties are discussed. And the impact of these materials and components on the ITERlike wall and implications for the ongoing scientific program will be summarised.
Abstract: The chosen materials for plasma facing components for the deuterium/tritium phase of ITER are beryllium and tungsten. These materials have already been widely investigated in various devices like ion beam or electron beam tests. However, the operation of this material combination in a large tokamak including plasma wall interaction, material degradation, erosion and material mixing has not been proven yet. The ITER-like Wall, which has been recently installed in JET, consists of a combination of bulk tungsten and tungsten coated CFC divertor tiles as well as bulk beryllium and beryllium coated INCONEL in the main chamber. The experiments in JET will provide the first fully representative test of the ITER material choice under relevant conditions. This paper concentrates on material research and developments for the materials of the JET ITER-like Wall with respect to mechanical and thermal properties. The impact of these materials and components on the JET operating limits with the ITER-like Wall and implications for the ongoing scientific program will be summarised.
TL;DR: In this paper, the authors studied the heating performance in (He-3)-H plasmas at full field, with fundamental cyclotron heating of He-3 as the only possible ion heating scheme in view of the foreseen ITER antenna frequency bandwidth.
Abstract: Radio frequency (RF) heating experiments have recently been conducted in JET (He-3)-H plasmas. This type of plasmas will be used in ITER's non-activated operation phase. Whereas a companion paper in this same PPCF issue will discuss the RF heating scenario's at half the nominal magnetic field, this paper documents the heating performance in (He-3)-H plasmas at full field, with fundamental cyclotron heating of He-3 as the only possible ion heating scheme in view of the foreseen ITER antenna frequency bandwidth. Dominant electron heating with global heating efficiencies between 30% and 70% depending on the He-3 concentration were observed and mode conversion (MC) heating proved to be as efficient as He-3 minority heating. The unwanted presence of both He-4 and D in the discharges gave rise to 2 MC layers rather than a single one. This together with the fact that the location of the high-field side fast wave (FW) cutoff is a sensitive function of the parallel wave number and that one of the locations of the wave confluences critically depends on the He-3 concentration made the interpretation of the results, although more complex, very interesting: three regimes could be distinguished as a function of X[He-3]: (i) a regime at low concentration (X[He-3] < 1.8%) at which ion cyclotron resonance frequency (ICRF) heating is efficient, (ii) a regime at intermediate concentrations (1.8 < X[He-3] < 5%) in which the RF performance is degrading and ultimately becoming very poor, and finally (iii) a good heating regime at He-3 concentrations beyond 6%. In this latter regime, the heating efficiency did not critically depend on the actual concentration while at lower concentrations (X[He-3] < 4%) a bigger excursion in heating efficiency is observed and the estimates differ somewhat from shot to shot, also depending on whether local or global signals are chosen for the analysis. The different dynamics at the various concentrations can be traced back to the presence of 2 MC layers and their associated FW cutoffs residing inside the plasma at low He-3 concentration. One of these layers is approaching and crossing the low-field side plasma edge when 1.8 < X[He-3] < 5%. Adopting a minimization procedure to correlate the MC positions with the plasma composition reveals that the different behaviors observed are due to contamination of the plasma. Wave modeling not only supports this interpretation but also shows that moderate concentrations of D-like species significantly alter the overall wave behavior in He-3-H plasmas. Whereas numerical modeling yields quantitative information on the heating efficiency, analytical work gives a good description of the dominant underlying wave interaction physics.
TL;DR: In this article, the impact of increased s/q at outer radii on core confinement in low-triangularity JET and ASDEX Upgrade (AUG) experiments was investigated by predictive heat and particle transport modelling using the integrated modelling code CRONOS coupled to the GLF23 turbulent transport model.
Abstract: Hybrid scenarios in present machines are often characterized by improved confinement compared with the IPB98(y,2) empirical scaling law expectations. This work concentrates on isolating the impact of increased s/q at outer radii (where s is the magnetic shear) on core confinement in low-triangularity JET and ASDEX Upgrade (AUG) experiments. This is carried out by predictive heat and particle transport modelling using the integrated modelling code CRONOS coupled to the GLF23 turbulent transport model. For both machines, discharge pairs were analysed displaying similar pedestal confinement yet significant differences in core confinement. From these comparisons, it is found that s/q shaping at outer radii may be responsible for up to ∼50% of the relative core confinement improvement observed in these specific discharges. This relative improvement is independent of the degree of rotational shear turbulence suppression assumed in the GLF23 model. However, employing the full GLF23 rotational shear model leads to an overprediction of the ion temperatures in all discharges analysed. Additional mechanisms for core confinement improvement are discussed and estimated. Further linear threshold analysis with QuaLiKiz is carried out on both pairs of discharges. This work aims to validate recent predictions of the ITER hybrid scenario also employing CRONOS/GLF23, where a high level of confinement and resultant fusion power sensitivity to the s/q profile was found.
TL;DR: N(t)/n(d)-values obtained using neutron spectroscopy are in qualitative agreement with those from other diagnostics measuring the isotopic composition of the exhaust in the divertor.
Abstract: In this paper, we present a method to derive nt/nd using the ratio of the thermonuclear neutron emission to the beam-target neutron emission. We apply it to neutron spectroscopy data from the magnetic proton recoil spectrometer taken during the deuterium tritium experiment at JET. nt/nd-values obtained using neutron spectroscopy are in qualitative agreement with those from other diagnostics measuring the isotopic composition of the exhaust in the divertor.
TL;DR: The MFR algorithm applied on data from the recently commissioned COMPASS tokamak demonstrated that a high resolution plasma positioning independent of the magnetic diagnostics would be possible provided that a proper calibration of the cameras on an x-ray source is undertaken.
Abstract: The contribution focuses on plasma tomography via the minimum Fisher regularisation (MFR) algorithm applied on data from the recently commissioned tomographic diagnostics on the COMPASS tokamak. The MFR expertise is based on previous applications at Joint European Torus (JET), as exemplified in a new case study of the plasma position analyses based on JET soft x-ray (SXR) tomographic reconstruction. Subsequent application of the MFR algorithm on COMPASS data from cameras with absolute extreme ultraviolet (AXUV) photodiodes disclosed a peaked radiating region near the limiter. Moreover, its time evolution indicates transient plasma edge cooling following a radial plasma shift. In the SXR data, MFR demonstrated that a high resolution plasma positioning independent of the magnetic diagnostics would be possible provided that a proper calibration of the cameras on an x-ray source is undertaken.
TL;DR: Nave et al. as discussed by the authors measured the angular velocity of the plasma column in JET H-mode plasmas with pure ICRF heating both for the standard low toroidal magnetic ripple configuration, of about similar to 0.08% and, for increased ripple values up to 1.5%.
Abstract: Understanding the origin of rotation in ion cyclotron resonance frequency (ICRF) heated plasmas is important for predictions for burning plasmas sustained by alpha particles, being characterized by a large population of fast ions and no external momentum input. The angular velocity of the plasma column has been measured in JET H-mode plasmas with pure ICRF heating both for the standard low toroidal magnetic ripple configuration, of about similar to 0.08% and, for increased ripple values up to 1.5% (Nave et al 2010 Phys. Rev. Lett. 105 105005). These new JET rotation data were compared with the multi-machine scaling of Rice et al (2007 Nucl. Fusion 47 1618) for the Alfven-Mach number and with the scaling for the velocity change from L-mode into H-mode. The JET data do not fit well any of these scalings that were derived for plasmas that are co-rotating with respect to the plasma current. With the standard low ripple configuration, JET plasmas with large ICRF heating power and normalized beta, beta(N) approximate to 1.3, have a very small co-current rotation, with Alfven-Mach numbers significantly below those given by the rotation scaling of Rice et al (2007 Nucl. Fusion 47 1618). In some cases the plasmas are actually counter-rotating. No significant difference between the H-mode and L-mode rotation is observed. Typically the H-mode velocities near the edge are lower than those in L-modes. With ripple values larger than the standard JET value, between 1% and 1.5%, H-mode plasmas were obtained where both the edge and the core counter-rotated.
TL;DR: A full predictive photonic simulation able to assess accurately the surface temperature measurement with classical IR thermography from a given plasma scenario and by taking into account the optical properties of PFCs materials is presented.
Abstract: Infrared (IR) thermography systems are mandatory to ensure safe plasma operation in fusion devices However, IR measurements are made much more complicated in metallic environment because of the spurious contributions of the reflected fluxes This paper presents a full predictive photonic simulation able to assess accurately the surface temperature measurement with classical IR thermography from a given plasma scenario and by taking into account the optical properties of PFCs materials This simulation has been carried out the ITER-like wide angle infrared camera view of JET in comparing with experimental data The consequences and the effects of the low emissivity and the bidirectional reflectivity distribution function used in the model for the metallic PFCs on the contribution of the reflected flux in the analysis are discussed
TL;DR: A major diagnostic upgrade to measure the neutral gas pressure and composition in the sub-divertor region has been completed, to characterise retention and outgassing of the new metallic first wall in JET.
Abstract: With installation of the ITER-like wall in JET a major diagnostic upgrade to measure the neutral gas pressure and composition in the sub-divertor region has been completed, to characterise retention and outgassing of the new metallic first wall. The upgrade includes two new magnetically shielded systems consisting of sensitive capacitance manometers and residual gas analysers, both capable of providing data during plasma operation. These enable absolute pressure and gas composition measurements (pressure range: 10−5–10−1 mbar, mass range: 1–200 amu, respectively) and have been used to characterise the neutral gas behaviour under various plasma conditions.
TL;DR: The rotation of L-mode plasmas in the JET tokamak heated by waves in the ion cyclotron range of frequencies (ICRF) damped on electrons, is reported in this paper.
Abstract: The rotation of L-mode plasmas in the JET tokamak heated by waves in the ion cyclotron range of frequencies (ICRF) damped on electrons, is reported. The plasma in the core is found to rotate in the counter-current direction with a high shear and in the outer part of the plasma with an almost constant angular rotation. The core rotation is stronger in magnitude than observed for scenarios with dominating ion cyclotron absorption. Two scenarios are considered: the inverted mode conversion scenarios and heating at the second harmonic He-3 cyclotron resonance in H plasmas. In the latter case, electron absorption of the fast magnetosonic wave by transit time magnetic pumping and electron Landau damping (TTMP/ELD) is the dominating absorption mechanism. Inverted mode conversion is done in (He-3)-H plasmas where the mode converted waves are essentially absorbed by electron Landau damping. Similar rotation profiles are seen when heating at the second harmonic cyclotron frequency of He-3 and with mode conversion at high concentrations of He-3. The magnitude of the counter-rotation is found to decrease with an increasing plasma current. The correlation of the rotation with the electron temperature is better than with coupled power, indicating that for these types of discharges the dominating mechanism for the rotation is related to indirect effects of electron heat transport, rather than to direct effects of ICRF heating. There is no conclusive evidence that mode conversion in itself affects rotation for these discharges.
TL;DR: In recent years there has been a major upgrade of the JET far infrared diagnostic system consisting of a new laser system with the wavelength at 118.8 μm at and more advanced processing electronics for phase counting, which provides a second colour measurement of the electron plasma density on the vertical system.
Abstract: In recent years there has been a major upgrade of the JET far infrared diagnostic system consisting of a new laser system with the wavelength at 118.8 μm at and more advanced processing electronics for phase counting. This provides a second colour measurement of the electron plasma density on the vertical system. Due to the shorter wavelength, the plasma induced laser beam refraction is reduced by a factor of three alleviating density errors caused by loss of signal (so-called “fringe jumps” [A. Murari et al., Rev. Sci. Instrum. 77, 073505 (2006)]10.1063/1.2219731), in particular during high performance plasmas experiments in JET.