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Estimations of the ITER first wall (FW) lifetime, previously made using the three-dimensional Monte-Carlo ERO code (Borodin et al 2011 Phys. Scr. T145 014008), depend strongly on the assumptions of the physical sputtering yield for beryllium (Be). It is of importance to validate the respective model and data at existing devices including the JET ITER-like wall (ILW) as most ITER-relevant experiments. Applying the same sputtering input data in ERO as those used before in the ITER-predictions, the ERO simulations for the Be light intensity (using up to date atomic data from ADAS and measured plasma conditions) reveal a factor of 2 overestimation in the assumed yield even if the low estimate assuming 50% D surface content is used. This result indicates the preference of this assumption for plasma-wetted areas. It points to a possible necessity to correct (reduce) the respective estimates for the Be sputtering yield and, accordingly, re-visit the ITER FW lifetime predictions.

Carbon is the principal impurity in the Joint European Torus (JET). Methane screening experiments quantify the ability of the scrape-off layer (SOL)/divertor system to ionize carbon and transport it to the divertor, preventing core plasma contamination. Previous JET publications studied edge-localized-mode-averaged high-confinement mode screening, and separately, evaluated the methodology of low-confinement mode (L-mode) screening measurements. This paper extends the L-mode measurements to include relevant plasma parameter scans and DIVIMP modeling of the L-mode screening.

RF heating and current drive with ion cyclotron waves and Lower Hybrid waves have been crucial for the development of the Optimized Shear scenario on JET to high performance. Peaked electron temperature profiles and improved energy confinement could be obtained with electron heating both from LHCD and ICRH during plasma current ramp up. ICRH and NBI comparisons allow to separate heating and fueling and suggest a dominant role of core heating in the formation of an Internal Transport Barrier (ITB). ICRH and NBI powers are there equivalent. Pressure profile control by varying the composition of centrally peaked ICRF and broader NBI deposition improves MHD stability. Current profile modifications in a wide range have been obtained with LHCD and in combination with NBI during current ramp up. During the high performance phase, however, LH coupling degrades strongly due to the steep edge density gradient resulting in a drop of the density in front of the LH antenna to the cut-off density. High fusion performance achieving simultaneously high beta values and bootstrap currents is predicted in scenario modeling using pressure and current profile control with ICRF and LHCD.

Obtaining precision measurements of the relative concentrations of hydrogen, deuterium, tritium, and helium in the divertor of a tokamak are an important task for nuclear fusion research. Control of the deuterium-tritium isotopic ratio while limiting the helium ash content in a fusion plasma are key factors for optimizing the fuel burn in a fusion reactor, like the International Tokamak Experimental Reactor (ITER). A diagnostic technique has been developed to measure the deuterium-tritium isotopic ratio in the divertor of the Joint European Torus (JET) with a species-selective Penning vacuum gauge. The Penning discharge provides a source of electrons to excite the neutral hydrogen isotopes in the pumping duct. Subsequently, the visible light from the hydrogen isotopes is collected in an optical fiber bundle, transferred away from the tokamak into a low radiation background area, and analyzed in a high resolution Czerny-Turner spectrometer, which is equipped with a fast charge coupled device (CCD) camera for optical detection. The intensity of the observed line emission (D{sub {alpha}} -- 6561.03 {angstrom}; and T{sub {alpha}} -- 6560.44 {angstrom}) is directly proportional to the partial pressure of each gas found in the divertor. The line intensity of each isotope is calibrated as a function of pressure.more » The ratio of the line intensities thus provides a direct measurement of the deuterium-tritium isotopic ratio. The lower limit for the determination of the deuterium-tritium isotopic ratio is about 0.5%. The applicable pressure range for this system is from 10{sup {minus}5} mbar to a few times 10{sup {minus}3} mbar.« less

Ion temperature and velocity distribution functions, ion densities, plasma rotation velocities and magnetic fields have been identified by the ITER design team as key parameters1, for which spatially-resolved profiles with medium time resolution are of vital importance in three major areas. These are: machine protection and plasma control, evaluation and optimisation of plasma performance, and investigation of the physical phenomena which might limit ITER’s performance. Only a diagnostic based on active-charge-exchange (CXRS) and beam-emission spectroscopy (BES), using a dedicated neutral beam, can provide all these data with the accuracy and resolution required (table 1).

Considerable attention in modern large fusion experiments is focused on neutral beam penetrated plasma and on divertor plasma. In the former, ion/atom reactions drive the population dynamics while in the latter, although electron processes predominate, there are significant ion/atom and atom/atom influences. Neutral atoms in beams, impurities in beam penetrated plasma and complex partially ionized impurities in divertor plasma find themselves in the collisional radiative regime where the simple stationary excitation/cascade picture of the coronal model is invalidated by secondary collisions. Thus atomic data needed in such fusion applications are not a few reaction cross-sections but large complete sets which support full collisional-radiative models. Also, experiment analysis does not make use directly of fundamental atomic data but rather effective coefficients deduced from such models. In the paper, we describe two cases. The first is neutral deuterium beam attenuation and beam emission and its linking...

Recent experiments at JET combining reciprocating probe measurements upstream and infrared thermography at the plasma-facing components (PFC) on plasmas in limiter configurations show that the common approach to predicting the power load on the limiter underestimates the heat flux at the contact point by a factor 1.5-3. The current model and scaling laws used for predicting the power load onto the first wall during limiter current ramp-up/down in ITER are uncertain and a better understanding of the heat transport to the PFCs is required. The heat loads on PFCs are usually predicted by projecting the parallel heat flux associated with scrape-off layer (SOL) properties at the outer mid-plane (upstream) along the magnetic field lines to the limiter surface and deducing the surface heat flux through a cosine law, thus ignoring any local effect of the PFC on transport within the SOL. The underestimate of the heat flux is systematic in inner wall limiter configurations, independent of the plasma parameters, whereas in outer limiter configuration this is not observed, probably because of the much shorter SOL power decay length. Models that can explain this enhanced heat flux around the contact point are proposed and discussed although no definitive conclusion can be drawn. © 2013 IAEA, Vienna.

The options and needs for a comprehensive spectroscopic diagnostic package on ITER are discussed. Recent JET results illustrate the role of key parameters leading to acceptable levels of data consistency. A high level of accuracy in bulk plasma data, magnetic surfaces and the inclusion of an active beam diagnostic appears to be indispensable for a prospective quantitative spectroscopy on ITER.

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, we demonstrate that a protection system based on CCD cameras can work and show examples of hot spot detections that stop the plasma pulse. The limits of such a design and the associated constraints on the operations are also presented.

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 impurity deposition and erosion by fast neutral particles. In parallel to the new optical design, a new type of possibly ITER relevant shutter system based on pneumatic techniques has been developed and integrated into the endoscope head. The endoscope is equipped with four digital CCD cameras, each combined with two filter wheels for narrow band interference and neutral density filters. Additionally, two protection cameras in the λ0.95 μm range have been integrated in the optical design for the real time wall protection during the plasma operation of JET.

Enhancements to the JET poloidally scanning spectrometers are presented, which will aid the exploitation of the recently installed ITER-like wall in JET. They include the installation of visible filter∕photomultiplier tube assemblies and spectrometers and the replacement of large rotating mirrors in the JET vacuum with small oscillating mirrors outside. The upgrade has resulted in a more robust and reliable diagnostic than before, which is described. Drifts in the mirror angle reconstructed from quadrature encoder signals are found, a reference signal being required. The use of the small scanning mirrors necessitated the inclusion of focusing mirrors to maintain throughput into the vacuum ultraviolet spectrometers. The mirror design has taken account of the extreme sensitivity of the focusing to the grazing angle of incidence, an aspect of importance in the design of grazing incidence focusing components on future machines, such as ITER. The visible system has been absolutely calibrated using an in-vessel light source.

An in-vessel calibration light source (ICLS) has been implemented for remote use during extended shutdown periods of the Joint European Torus (JET). The ICLS facilitated the in situ calibration of optical diagnostics, which previously were performed when the diagnostics were removed from JET. Since the ICLS is used to calibrate diagnostics over the entire, exact optical path as used when plasma discharge data are measured, the ICLS calibration implicitly accounts for any vignetting losses in the JET vessel viewports in addition to the vacuum window transmission. At least ten diagnostic systems have benefited from the ICLS during the extended ITER-like wall shutdown of 2009-2011. Examples of the use of the ICLS in JET are given.

We present in this paper modelling of the JET scrape-off layer (SOL) including direct SOL ionization by launched lower hybrid (LH) waves. For that purpose, the two-dimensional fluid code EDGE2D-NIMBUS was modified in order to account for possible enhanced ionization in the SOL, due to the LH power absorbed by the electrons in a layer extending in the radial direction from the launcher. By taking the direct LH SOL ionization into account, the observed modification of the density in front of the LH antenna during a LH power and gas injection, can be explained. The JET grill private SOL limiters, acting as sinks for the particles, are also included in the model. We compare the efficiency of gas puffing from the top and the outer mid-plane for SOL density enhancement and improvement in LH coupling. The observed reduction of the temporal variation of the LH wave reflection coefficient and of measured saturated currents in the SOL during ELMs, is explained. The density depletion, by ponderomotive forces in front of the grill, is estimated.

The code COREDIV, self-consistent with respect to both the interaction of plasma core-edge and main plasma impurities, is used to simulate nitrogen-seeded discharges in JET. The model is fully described and numerical results are compared with experimental data pertaining to two series of discharges differing in input power, confinement, the level of power radiated and the puffing rate of the main gas. Impurity sources, their transport and densities are considered as functions of the edge and core parameters and consistency of their radiated power is discussed. Special emphasis is given to analysis of the fluxes of carbon and recycled deuterium.

Strike-point sweeping and real-time-controlled (RTC) impurity seeding are both expected to be needed on JET following its upgrade to an all-metal wall with enhanced neutral-beam heating, thereby anticipating exhaust-control requirements plus the materials planned for ITER. Preliminary trials in the previous carbon device have combined these techniques in high-triangularity type I H-mode plasmas, using a VUV spectroscopic signal for feedback control of nitrogen injection. Compared with earlier unswept feedforward counterparts, similar strong mitigation of divertor heat load between ELMs was achieved in swept RTC cases for less than half the integrated nitrogen input and correspondingly less adverse effect upon other properties. Both sweeping and RT control contributed to this improvement. Time-average normalized energy confinement H98yt ~ 1, Greenwald density fraction fGwdt ~ 0.9 and particularly purity denoted by effective ionic charge Zefft ≈ 1.7, all remained closer to good reference levels. Transient effluxes in ELMs were also less affected, however, and would require separate active control.

Modification of the sawtooth period through ion cyclotron resonance frequency (ICRF) heating and current drive has been demonstrated in a number of experiments. The effect has been seen to depend critically on the location of the ICRF absorption region with respect to the q = 1 flux surface. Consequently, for ICRF to be a viable tool for sawtooth control, one must be able to control the ICRF absorption location in real time so as to follow variations in the location of the q = 1 surface. To achieve this, the JET ICRF system has been modified to allow the JET real time central controller to control the frequency of the ICRF generators. An algorithm for real time determination of the sawtooth period has been developed and a closed loop controller, which modifies the frequency of the ICRF generators to bring the measured sawtooth period to the desired reference value, has been implemented. This paper shows the first experimental demonstration of closed loop sawtooth period control by real time variation of the ICRF wave frequency.

In anticipation of revised operational limits accompanying upgrade of the JET tokamak to an all-metal, ITER-like wall, systematic studies have been conducted of moderating divertor heat load in type I ELMy H-mode by enhanced deuterium fuelling and seeding with the extrinsic impurities neon or nitrogen. These were found to differ mainly in the roughly ten times higher level of nitrogen than neon presently required to approach outboard detachment, and in their effects upon ELMs. With either species up to input power of ?17?MW, strongly reduced target loads between ELMs, compatible with the new wall, were sustained for longer than 10 energy confinement times while losing

Since 1994, JET has had a mirror-link spectroscopy system with a poloidal view of 150 mm of the outer divertor split into three ranges: near-ultraviolet (near-UV) (∼ 300–450 nm), visible (450–750 nm), and near-infrared (near-IR) (750–1200 nm). The system consists of three Czerny–Turner/charge coupled device (CCD) pairs: 1 m focal length for the near-UV, 0.75 m focal length for the visible, and 0.5 m focal length for the near-IR. All were aligned along the same optical path to the divertor. As part of the JET ITER-like wall enhancements, the diagnostic system will be upgraded in five areas: (1) frame rate, (2) quantum efficiency (QE), (3) radial coverage, (4) optical throughput, and (5) for the near-UV, spectral resolution and survey capability. New CCDs for the near-UV and visible will have increased QE and allow three times frame rate. The near-UV will benefit from a 0.75 m imaging spectrometer with three gratings. The optics have been redesigned to allow ∼ 360 mm view and greater than two times throughput. This paper will look at the design and implementation as well as the new diagnostic capabilities of the system.

The ELM filamentary structure in the JET scrape-off layer (SOL) has been investigated using a multi-pin reciprocating Langmuir probe system inserted at the top, low field side of the poloidal cross-section. These measurements clearly demonstrate that both Type I and III edge localized modes (ELMs) are composed of a number of filaments. The filament propagation in the SOL is investigated and the radial velocity estimated using different methods. The large radial velocities observed during Type I ELMs (up to 6 km s−1) imply that filaments have no time to drain significant energy along the field lines, reaching the wall with a reasonable fraction of the pedestal density and temperature. In spite of the time between Type I ELMs being much longer than the ELM duration, most of the particle flux to the main chamber plasma-facing components occurs in the latter period. Furthermore, filaments in Type I ELMs are significantly larger and faster than those observed in Type III ELMs. During Type I ELMs a strong parallel flow (M ~ 0.4) is measured towards the inner divertor, which increases during filaments. These observations are in agreement with the ballooning nature of the ELM losses.

The first experimental evidence showing the connection between blob/hole formation and zonal-flow generation was obtained in the edge plasma of the JET tokamak. Holes as well as blobs are observed to be born in the edge shear layer, where zonal-flows shear off meso-scale coherent structures, leading to disconnection of positive and negative pressure perturbations. The newly formed blobs transport azimuthal momentum up the gradient of the azimuthal flow and drive the zonal-flow shear while moving outwards. During this process energy is transferred from the meso-scale coherent structures to the zonal flows via the turbulent Reynolds stress, resulting in nonlinear saturation of edge turbulence and suppression of meso-scale fluctuations. These findings carry significant implications for the mechanism of structure formation in magnetically confined plasma turbulence.

The determination of a good relative and absolute sensitivity calibration for wideband VUV spectrometers is challenging. On JET, the possible T and Be contamination of the VUV spectrometer precludes its removal to a synchrotron source and, consequently, a range of alternative in situ techniques have been investigated in depth. This has resulted in a reliable calibration for the complete spectral range, the relative calibration at short wavelengths being particularly accurate. At these wavelengths, a novel approach is used, in which the calibration is extended using a number of Na- and Li-like metal doublets. At longer wavelengths, the Li-like doublets of Ar and Ne have been used in conjunction with CII, CIII and CIV line intensity ratios. Unexplained discrepancies between the measured and modelled C results have meant that the exceptional short wavelength accuracy has not be repeated at these longer wavelengths. The absolute sensitivity has been determined from branching ratios to an absolutely calibrated visible spectrometer. The long term stability of the calibration is discussed.

Radio frequency heating of majority ions is of prime importance for understanding the basic role of auxiliary heating in the activated D-T phase of ITER. Majority deuterium ion cyclotron resonance heating (ICRH) experiments at the fundamental cyclotron frequency were performed in JET. In spite of the poor antenna coupling at 25 MHz, this heating scheme proved promising when adopted in combination with D neutral beam injection (NBI). The effect of fundamental ICRH of a D population was clearly demonstrated in these experiments: by adding similar to 25% of heating power the fusion power was increased up to 30-50%, depending on the type of NBI adopted. At this power level, the ion and electron temperatures increased from T-i similar to 4.0 keV and T-e similar to 4.5 keV (NBI-only phase) to T-i similar to 5.5 keV and T-e similar to 5.2 keV (ICRH + NBI phase), respectively. The increase in the neutron yield was stronger when 80 keV rather than 130 keV deuterons were injected in the plasma. It is shown that the neutron rate, the diamagnetic energy and the electron as well as the ion temperature scale roughly linearly with the applied RF power. A synergistic effect of the combined use of ICRF and NBI heating was observed: (i) the number of neutron counts measured by the neutron camera during the combined ICRF + NBI phases of the discharges exceeded the sum of the individual counts of the NBI-only and ICRF-only phases; (ii) a substantial increase in the number of slowing-down beam ions was detected by the time of flight neutron spectrometer when ICRF power was switched on; (iii) a small D subpopulation with energies slightly above the NBI launch energy was detected by the neutral particle analyzer and gamma-ray spectroscopy.

An in-vessel calibration light source (ICLS) is proposed for use during extended shutdown periods of the Joint European Torus (JET). The ICLS is primarily a 12 in. integrating sphere (4 in. opening) with four lamps (of known radiance), which can be positioned inside the JET vacuum vessel via the remote handling arm during interventions in the JET operating schedule. This will facilitate the in situ calibration of optical diagnostics, which rely on absolute light intensity measurements currently made when the diagnostics are removed from JET. The ICLS could ultimately reduce/remove the mechanical stresses associated with the repositioning of diagnostics for calibration purposes. At least 10 diagnostic systems (approximately 20 diagnostic subsystems) could benefit from the ICLS; in some instances the ICLS provides the only viable absolute-calibration strategy. Moreover, the ICLS will be a broad-spectrum white light source, enabling intensity calibrations at all visible wavelengths. A secondary benefit of the ICLS is in its use as an illumination source for making measurements of the reflectance (over a broad spectral range and at multiple angles) from the tiles lining the JET vacuum vessel. During the ITER-like wall intervention new Be, C, and W tiles will be installed in JET and their reflectance measured. Measurements made in subsequent JET interventions will provide data on the effect of high-temperature plasma operation on the reflectance of these tiles.

The influence of the physical isolation of inner and outer divertor volumes by a septum plate of the Mk-II gas box divertor, thus increasing divertor closure and neutral compression, on the plasma and divertor performance has been studied at the Joint European Torus (JET). The septum plate was installed in 1999, together with the original Mk-II gas box divertor, and was then replaced by a simple protection plate in 2001. This removal reduced the closure of the divertor by opening a line of sight path for neutrals to travel between the inner to the outer divertor volumes. Comparison of identical discharges with and without the septum thus provides direct evidence of the effect of divertor closure on plasma behaviour. With this aim, following septum removal, several dedicated L-mode and H-mode discharges have been performed, in each case repeating earlier discharges when the septum was still in place. In each case, the fuelling location was varied between the inner/outer divertor and the main chamber, and differences in detachment in the inner and outer divertors were studied. Under L-mode conditions, differences in detachment dynamics were indeed observed between closed (with septum) and open (without septum) divertor configurations, although the differences were only significant in the medium density range. In contrast, the ultimate density limit was not affected, being determined in each case by the formation of a wall multifacedted asymmetric radiation from the edge (MARFE), rather than an X-point MARFE. Under H-mode conditions, the differences were more subtle. Although the ion fluxes to the targets were unaffected, the target electron temperatures were found to be lower in the closed divertor configuration. In this case, the fuelling efficiency was the largest when the gas injected from the inner divertor, with implications on global energy confinement and ELM frequency. Otherwise, no difference in the confinement of the discharges with and without septum was observed.

During X-point discharges in JET the wall region near the X-point has been studied with CCD cameras and an OMA system. Tile temperatures, carbon influx, recycled deuterium and a beryllium line were measured. Strike zones which occur only with additional heating receive from fast ions the highest power flux of up to 3 kW/cm 2 resulting in tile temperatures of 2400 ° C. This type of strike zone releases most of the carbon observed. The Carbon yield begins to rise significally at about 1850° C due to RES and continues to rise up to 2350 ° C when thermal sublimation takes over. The carbon influx is amplified by selfsputtering resulting in a carbon bloom almost immediately when the regime of thermal sublimation is entered. It is suggested that the presence of beryllium on the graphite target mitigates the carbon influx.

The paper focusses on the conversion of photon fluxes into particle fluxes by means of photon efficiencies in tokamaks. Both flux contributions to the chemical erosion yield, the deuterium flux, as well as the eroded hydrocarbon flux, are critically investigated, and possible uncertainties in the data evaluation are identified. Inverse photon efficiencies, so-called S/XB or D/XB values, are critical factors in the conversion of the fluxes. For deuterium, the flux determination is influenced by the presence of a molecular source. In contrast, the hydrocarbon flux is also uncertain due to the indirect conversion from photon to particle fluxes. In particular, the conversion of CD photon fluxes into CD4 particle fluxes can lead to misinterpretation, because C2Dy and CD4 can finally contribute to the flux of CD radicals. A correction formula for the total hydrocarbon flux which includes the contribution of different hydrocarbon sources is introduced. We also present results from spectra simulations of the A-X transition of CD, which show that the usual observation of the CD band (429.4 nm–430.9 nm) may lead to an underestimation of the total photon flux from the whole A-X transition of more than a factor of two. Up to now, the modelling with erosion and deposition codes only takes the complete electronic transition into account. A unification of the quantities is necessary to compare modelling and experiment.

Plasma facing components from JET and TEXTOR were studied. The emphasis was on the comparison of co-deposition, material mixing and fuel inventory on plasma facing and side surfaces of tiles, i.e. ...

The injection of hydrocarbons into the JET Gas Box divertor in ohmic and L-mode discharges has been used to calibrate the intrinsic hydrocarbon erosion yields and to determine the variation of the photon efficiency with the edge ion flux. Methane erosion yields of 5% for CD4 and 3% for CH4 were measured, with no sign of any flux dependence. The erosion yield of ethane/ethene was lower but increased with the edge ion flux (increasing edge plasma density, decreasing edge electron temperature) such that it dominated the molecular carbon source at high density. The methane and ethane photon efficiencies (the number of molecular dissociations per emitted photon) were both not constant, but were found to increase with increasing density (increasing edge ion flux, decreasing edge electron temperature).

JET has conducted experiments specifically designed to tackle the issue of CD4 redeposition at the divertor target plates. This paper reports the results obtained by these experiments, quantifies the percentage of prompt CD4 redeposition and evaluates the importance of chemical erosion at the divertor in an ITER relevant regime such as the steady state ELMy H-mode.

In the JET Mark I and Mark II divertors the in-situ chemical sputtering yield of the carbon target plates has been studied under a wide range of conditions. To evaluate the chemical yield, the incident hydrogenic flux is directly evaluated from Langmuir probe measurements while the methane production rate is inferred from the CD/CH molecular band emission calibrated using methane injection experiments. It is found that the chemical sputtering yield reduces at high flux densities (> 2 × 1022 m-2 s-1) and low impact energies (< 50 eV). In the Mark II divertor both the absolute yield and the flux dependence depend upon the tile temperature with Ychem ∝ Γ−0.4 at 450 K and Ychem ∝ Γ−0.7 at 360 K. In the Mark I divertor the yield exhibits an even stronger flux dependence (Ychem ∝ Γ−1.25) which is consistent with the relatively low tile temperature of 300 K. However, the increased flux densities are associated with reduced impact energies which may also contribute to the yield reduction. The chemical yield is also shown to depend strongly on the isotope mass (Ychem ∝ mion) and is consistent with thermal reactions enhanced by radiation damage.

Recent developments and results of the JET CXRS diagnostic are reported. The measurements of radial profiles of ion temperatures and densities are based on CXR spectra of fully stripped ions of either carbon or beryllium. Considerable effort has been expended in ensuring consistency between radial profiles of low Z impurity densities and those from other diagnostics. The contributions of the main light impurities are used to reconstruct radial profiles of Zeffwhich can be compared with Abel‐inverted signals from visible bremsstrahlung or soft x‐ray emission. Active Balmer‐Alpha spectroscopy (ABAS) is being introduced as a diagnostic tool providing data on local magnetic fields, neutral beam densities, and dilution factors. The effects of collision‐energy‐dependent CXR cross sections on observed CXR spectra are calculated. Corrections for the values of deduced ion temperatures, toroidal velocities, and impurity densities are discussed for the case of plasmas with high ion temperatures and high toroidal rot...

The energy confinement properties of ohmically heated JET discharges are discussed in detail, from both a local and a global point of view. Also, the plasma resistivity and poloidal field diffusion are discussed in some detail.

Wavelengths and transition probabilities are presented for the allowed and forbidden n=2-2 transitions belonging to the fluorine I to lithium I isoelectronic sequences which are expected to be observed in Tokamak discharges. The listing is compiled for the elements Ti, Cr, Fe, Co and Ni, these metals being likely contaminants. The emphasis is placed on an accurate compilation of wavelengths from laboratory and solar-flare observations. Where these are inadequate, predictions are given. In the case of the nitrogen-, carbon- and boron-like intercombination lines, these predictions represent a significant improvement on previous data. Theoretical line intensities calculated for conditions typical of a Tokamak plasma are given for each transition. Identifications in Ti of the five strongest forbidden transitions lying in the visible and near-UV spectral regions have been made in spectra from the DITE Tokamak. Such spectral features are useful diagnostic indicators of metallic ion impurity concentrations and mass motions.

Visible spectroscopy has been used to study the impurity influx behaviour in JET plasmas. The effects of density, toroidal magnetic field and plasma current have been examined. Chromium evaporation from the JET carbon limiters has been observed and enhanced chemical sputtering of carbon is found not to be important.

The large increase in radiation during neutral injection in DITE is identified as being primarily due to the charge-exchange recombination of the impurities on the fast neutrals. The time dependence of the increase in line eion and its dependence on injection power and plasma density is shown to fit the theory. – The line emission and the total radiated power are found to be toroidally asymmetric, being larger in the region of beam deposition. A theoretical explanation for this asymmetry is given. The recombination of impurities by thermal neutrals is found to be important in low-density Ohmic discharges, and the implications of this are discussed.

This paper presents a review of hydrogen and helium recycling phenomena in tokamaks with limiters and walls largely made out of carbon (graphite, a-C:H layers). The key points of interest are the plasma fuelling efficiency, the wall pumping phenomena as observed in JET, TFTR, TEXTOR and other machines under various fuelling schemes (gas, neutral beams, pellets), the release of hydrogen/helium from material surfaces during and after plasma discharges and the long term retention (total particle inventory) of hydrogen in graphite or carbonised structures in tokamaks. The effect of a combined hydrogen/helium plasma on recycling is also discussed. It is shown that only part of the above phenomena can be understood in terms of processes between hydrogen/helium and carbon as known from simulation experiments (ion beams, gas discharge facilities) and that others (in particular the JET wall pumping phenomenon) have still to be explained. Possible mechanisms are outlined and discussed by means of global models.

During JET operation in 3He and 4He, a higher density limit is observed than in deuterium, density control is easier and radiation losses are lower during ICRH. Also, there are lasting beneficial effects on subsequent D discharges. Impurity behaviour, particle confinement and residual deuterium content of these plasmas have been studied by visible and VUV spectroscopy. It has been found that He and D influxes are comparable in similar helium and deuterium discharges, respectively, i.e. τHe ≈ 0.5τD. Release of deuterium, but not helium, from carbon surfaces leads to higher deuterium fractions in ICRH helium plasmas and is responsible for a higher density and radiation increase in deuterium. Impurity diffusion coefficients and confinement times are the same in helium and deuterium plasmas. Nickel concentrations are higher in helium as expected from the higher sputtering yield. Oxygen is almost absent in pure helium discharges and is well correlated with the residual deuterium flux, suggesting a chemical production mechanism. Subsequent to longer periods of helium operation, a lasting reduction of oxygen has also been observed in deuterium plasmas. Carbon concentrations in both helium and deuterium show the signature of physical sputtering, i.e. they increase with Ip and decreases with ne. While carbon production and densities in helium can be explained by the theoretical sputtering yields, the situation in deuterium is more complicated owing to the presence of oxygen. The low oxygen levels and the trend of falling carbon concentrations with increasing ne are the reason for the high density limits of helium plasmas.

After five years of construction JET started its operation in June 1983. In December plasmas with peak currents of 3 MA were produced. Flat top times of a few seconds were obtained for currents of 2.4 MA. The plasmas were slightly elongated ( ~ 20%) with a minor radius of about 1 m. Line densities (peak values) reached 7 × 10 19 m −2 , electron temperatures on the axis 1.5–2 keV and energy confinement times 0.3 s. The paper describes the state of the machine, the methods employed for impurity control and summarises the experimental results obtained during 1983.

As a first step in the JET ICRF programme, two antenna generator units have been installed at JET and operated up to the design specification of 3 MW coupled in the Torus for 1 second. After a brief description of the system, the experimental results of wave coupling to the plasma and matching the plasma loaded antenna are discussed. The first heating results are presented concentrating on the analysis of a case where the total power to the plasma increases by a ratio of 2.5.

Ohmic heating studies in hydrogen and deuterium up to currents of Ip = 5 MA have been completed and additional heating experiments by means of ICRH and NBI are now in process. With ICRH powers up to 7 MW, the global energy confinement time τE is observed to deteriorate with increasing heating power. NBI experiments are at a more preliminary stage, but also show deterioration in τE. Detailed studies of confinement time scaling in ohmically heated discharges show a strong dependence of plasma size, τE ∼ R1.7a1.3, but a much weaker dependence on q and ne than seen in smaller experiments. Boundary phenomena have been studied from early 1984. Data on the impurity coverage of limiters and wall have been collected during three successive experimental campaigns. Substantial amounts of wall material have consistently been found on the limiters and a large inventory of hydrogen isotopes concentrated on the colder edges. The wall in the shadow of protective elements shows non-uniform erosion. Langmuir probe measurements of edge plasma parameters have been taken in OH, RF and NB discharges. Additional heating leads generally to a temperature increase of the scrape-off layer. Model calculations using data from these diagnostics seem to produce a consistent picture of impurity production in the scrape-off layer which agrees reasonably well with spectroscopic observations.

A visible spectrometer with an intensified linear diode array detector has been used to simultaneously measure D, He, C and O influxes from the JET belt limiters. The behaviour of impurity influxes in helium plasmas has been examined, and the ratio of D α /D γ line intensities has been used to measure the edge electron density.

The JET scrape-off plasma during X-point discharges with L- and H-modes is described. There is a flow of current in the scrape-off layer which can influence equilibrium, transport, heating and impurity generation in the scrape-off layer. A local measurement of D ⊥ is obtained in the divertor region. The saturated flux of ions flowing parallel to the magnetic field, I SAT is ∼10 A/cm2 at the divertor separatrix in ohmic discharges. In L-mode with NBI it increases by a factor ⩽ 2, depending on injected power level ( P NBI MW ) and decreases with H-mode. The e-folding decay length, λ I SAT , is the same in ohmic and L-mode but decreases with H-mode. The electron temperatures at the divertor target are ~ 30 eV (OH) and ~ 50–60 eV (H-mode). Measurements of T e during L-mode are not reported because of the high level of fluctuations. An enhancement in the erosion of carbon due to ion-stimulated desorption at the divertor target is discussed. Flow reversal in the scrape-off layer is demonstrated.

During the Ohmic Heating Phase of JET operation (June 1983 - September 1984), impurities in the plasma have been studied by visible and VUV spectroscopy, and from bolometer and soft X-ray signals.T ...

The flux of impurity atoms into plasmas from limiting surfaces is considered. It is shown how the flux of an impurity released from a surface can be derived from spectroscopic measurements along a line-of-sight directed at the surface. A theoretical atomic level population model is developed to obtain the 'ionization per emitted photon' quantities which link the spectroscopic measurement to the flux. Metastable states and finite density plasma effects are taken into account and observations at visible wavelengths are emphasized. Detailed studies and calculations are performed for C+1, C+2, O+1, O+2, Cr, Cr+1, Fe, Fe+1, Ni and Ni+1. Tabulations and graphs of relevant quantities are provided. The application of the theory to impurity influxes in the JET Tokamak is described.

The 1s2s3S−1s2p3PJ (J = 0, 1, 2) transitions in helium-like ions can be used to test QED, but only if the other contributions to the transition energy, and especially the relativistic contributions are known to high accuracy. This work tests the relativistic calculations by comparing the theoretical 3 P J splittings (which are nearly ndependent of QED) with each other and with experiment. An estimate of the largest uncalculated relativistic term, of relative order α4Z4, is also made. It is found that the calculations of Hata and Grant are not very accurate, and that the small discrepancy between Drake and experiment vanishes if the α4Z4 relativistic term is included.

Light from the Joint European Torus (JET) tokamak is relayed by optical fibers to detectors outside the biological shield, where personnel access is unrestricted. This arrangement, which is described in detail, will permit routine recording during the intense neutron emission associated with the production of D–T plasmas—scheduled for 1989. The diagnostic has been in routine use since JET began operation in mid‐1983. A selection of results obtained is presented. The likely degradation in fiber performance, due to irradiation, during the D–T phase of the program is assessed. Methods of overcoming the problems due to induced absorption are presented.

If an ELM is a peeling of flux surfaces from the plasma, due to a broken separatrix, current density is lost as well as particles and energy. The fast loss of a current-carrying plasma layer modifies the plasma equilibrium, leading to sudden shifts in the strike points at each ELM, towards the plasma centre. An experimental study of this conjectured model of the ELM has been made at JET, showing that in all cases of Type I ELMs studied, strike point shifts were observed. In two cases studied in detail, the estimated equilibrium changes provoked by flux surface peeling agree qualitatively with the observed strike point shifts. (c) 2004 Elsevier B.V. All rights reserved.

The deuterium and helium dynamics in the plasma and subdivertor regions of JET are compared during a sequence of similar ohmic and ICRH pulses where 100% He gas is injected into the JET vacuum vessel, whose graphite walls were previously saturated with deuterium. After the first six He fueled change-over discharges, only He plasma operation was performed. Following this investigation, the situation is reversed and the change-over from an initially saturated He wall is investigated when only D-2 plasma fuelling is used. The He concentration is measured in the subdivertor with a species selective Penning gauge. Comparison of the time dependence of the divertor concentrations with those at the edge and strike point shows significant differences during the first six discharges. This difference along with a global He particle balance is used to assess the status of the wall saturation over the initial 6-7 He change-over discharges. (C) 2003 Elsevier Science B.V. All rights reserved.

This paper examines the radiation profiles and corresponding ionization source profiles of carbon and main plasma ions in matched helium and deuterium. L-mode plasmas in JET. The radiation intensities from C1+, C2+ and C3+ in the helium plasmas showed reduction by factors of 8, 10 and 25, respectively along the inner SOL and divertor leg compared with the deuterium cases. However, the emission in the outer divertor leg was less than a factor of 2 lower in helium. Using photon efficiencies calculated by the UEDGE code for the spectrometer lines of sight, the calculated source rates of C1+ and C3+ along the inner SOL and inner divertor were reduced by factors of 4 and 20, respectively in the helium plasmas. In the outer divertor leg the source rate of C3+ was reduced by a factor of 10 but the C1+ source rate did not change in helium. These measurements are consistent with a model that chemical sputtering of carbon dominates the source from the inner wall and inner divertor in deuterium L-mode plasmas while physical sputtering appears to dominate the source from the outer divertor. (C) 2003 Elsevier Science B.V. All rights reserved.