J. Ehrenberg

Tritium recycling and retention in JET

Abstract JETs 1997 Deuterium Tritium Experiment (DTE1) allows a detailed study of hydrogenic isotope recycling and retention in a pumped divertor configuration relevant to ITER. There appear to be two distinct forms of retained tritium. (1) A dynamic inventory which controls the fueling behaviour of a single discharge, and in particular determines the isotopic composition. This is shown to be consistent with neutral particle implantation over the whole vessel surface area. (2) A continually growing inventory, which plays a small role in the particle balance of a single discharge, but ultimately dominates the hydrogenic inventory for an experimental campaign comprising thousands of pulses. This will be the dominant retention mechanism in long-pulse devices like ITER. The JET retention scaled-up to ITER proportions suggests that ITER may reach its tritium inventory limit in less than 100 pulses.

Hydrogen and helium recycling in tokamaks with carbon walls

Abstract 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.

Interpretation of ion flux and electron temperature profiles at the JET divertor target during high recycling and detached discharges

Abstract Detailed experiments have been carried out with the JET Mark 1 pumped divertor to characterise high recycling and detached plasma regimes. This paper presents new measurements of high resolution divertor ion flux profiles that identify the growth of additional peaks during high recycling discharges. These ion flux profiles are used in conjunction with Dα and neutral flux measurements to examine the physics of divertor detachment and compare against simple analytic models. Finally, problems are highlighted with conventional methods of single and triple probe interpretation under high recycling conditions. By assuming that the single probe behaves as an asymmetric double probe the whole characteristic may be fitted and significantly lower electron temperatures may be derived when the electron to ion saturation current ratio is reduced. The results from the asymmetric double probe fit are shown to be consistent with independent diagnostic measurements.

The X-point scrape-off plasma in jet with L- and H-modes

Abstract 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.

Studies in JET divertors of varied geometry. I: Non-seeded plasma operation

Results of experiments investigating the performance of the JET Mark IIA divertor are reported and compared with the performance of its Mark I predecessor. The principal effect of reducing the divertor width (increasing closure) was to increase pumping for both deuterium and impurities while reducing upstream neutral pressure. Neither the orientation of the divertor target relative to the divertor plasma nor the width of the divertor had a major influence on core plasma performance in ELMy H modes. Changing the core triangularity and thus the edge magnetic shear modifies the ELM frequency in ELMy H mode plasmas, thereby changing the peak divertor power loading. The integrated performance of the core and divertor plasmas is reviewed with a view to extrapolation to the requirements of ITER. The confinement of JET ELMy H modes with hot, medium density edges is good (H97 ≈ 1) and follows a gyro-Bohm scaling. The impurity content of these discharges is low and within the ITER requirements. When an attempt is made to raise the density with deuterium gas fuelling, the ELM frequency increases and the confinement, especially in the edge, decreases. Good confinement can be achieved in JET either by producing a large edge pedestal, typically in discharges with NB heating or by centrally peaked heating with ICRH schemes. Large amplitude type I ELMs, which are present in all discharges with a large edge pedestal, would result in unacceptable divertor plate erosion when scaled to ITER. Since the power deposition profile due to α heating in ITER is calculated to be intermediate between the JET NB and RF heating profiles, it is likely that operation in ITER with small ELMs in order to reduce first wall loading will result in degraded confinement compared with present day scaling laws.

Hydrogen and deuterium retention in wall samples of JET

Abstract The amounts of H and D trapped in the surface layers of samples made out of C, Ni, Inconel and Si, installed at the vessel walls of JET and removed after the 1984 and 1985 periods of operation were measured. After the 1984 operation period the D concentration in carbon probes was of the order of 5 × 10 19 /m 2 . while the hydrogen was about a factor of 40 higher. This can be attributed to a final glow discharge cleaning in hydrogen at the end of the operation period. After the 1984 period of operation the amounts of deuterium and hydrogen were nearly equal at about 5 × 10 21 /m 2 . A comparison of the amounts of deuterium and hydrogen with the amount of carbon deposition on the Inconel wall samples shows a ratio of about 0.3 to 0.4 (H + D) per C-atom. The hydrogen concentrations trapped in the vessel wall correspond to more than 100 times the amount of hydrogen isotopes in the plasma during a discharge.

Hydrogen Isotope Retention in the JET Limiters

Abstract In order to evaluate the deuterium and hydrogen distributions in JET after long term operation in deuterium in 1985, as well as after a few weeks with deuterium at the end of the hydrogen-based operations in 1984, limiters have been analysed using nuclear reaction techniques and laser induced mass analysis (LIMA). The results show the presence of very high amounts of deuterium and 1H (up to more than 1022 atoms/m2 on the sides and edges of the limiters). If this amount were the result of the implantation of deuterium into the limiter graphite, it would require ion energies far higher than those expected in the boundary layer. The deuterium concentration shows a broad minimum near the centre of the limiter, where the heat load from the plasma is a maximum. The depth distributions, which are obtained by the 3He(d, p)4He nuclear reaction, indicate that deuterium is present to a considerable depth on the sides of the limiters. Similar amounts have been seen at the inner wall of the vessel — either on small samples mounted on the wall, or on the carbon tiles which covered the wall in 1985 — and on the RF antennae. It appears that co-deposition of carbon and deuterium enables significant trapping of hydrogen isotopes on all exposed surfaces.

Tritium concentration measurements in the Joint European Torus divertor by optical spectroscopy of a Penning discharge

Obtaining precision measurements of the relative concentrations of hydrogen, deuterium, tritium, and helium in the divertor of a tokamak is 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. A diagnostic technique has been developed to measure the deuterium–tritium isotopic ratio in the divertor of the Joint European Torus 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 camera for optical detection...

Diagnostic experience during deuterium-tritium experiments in JET, techniques and measurements

Abstract During 1997 JET was operated for an extensive period using a D–T mixture (DTE1). Changes in the design and operation of diagnostic systems made over the years in preparation for this phase are described. A number of diagnostic techniques have been deployed to measure the deuterium and tritium content of the plasma during DTE1 and their results are compared. All diagnostics with a direct vacuum interface with the main vessel have been fitted with tritium compatible pumps and their exhaust gases have been re-routed to the active gas handling plant. All items on the torus which could lead to a significant leak in the event of failure, were required to have double containment. Therefore, all windows, and a majority of bellows and feedthroughs, were designed and installed with a double barrier. Heated fibre hoses were installed to transmit plasma light beyond the biological shield for spectroscopic purposes. Blind fibres and fibre loops were also installed to study the effects of higher neutron fluxes on these fibres. A radiation-hardened video camera was installed to monitor the plasma during the DTE1 discharges. Extra shielding was installed on a number of diagnostics to deal with the higher neutron fluxes during DTE1. The effect of neutron radiation on electronics in the Torus Hall was studied. During DTE1 the tritium fraction was measured at the edge and in the core using several diagnostic methods. High resolution Balmer α line spectroscopy gave a measurement typical of the plasma edge region. In the JET sub-divertor volume the tritium concentration of the neutral gas was measured using Balmer α spectroscopy of a Penning gauge discharge. Using Neutral Particle Analysis, the tritium concentration was measured typically in a zone 20–40 cm from the plasma edge. Local core measurements of the tritium fraction have been made using active Balmer α charge exchange spectroscopy. The error on this measurement is, however, large,∼30%. After the discharge the tritium fraction of the exhaust was measured using the exhaust monitoring system. Using short deuterium neutral injection pulses allowed neutron rate measurements of the tritium concentration in the core region. A further technique used the measured neutron rate and calculated neutron rate from other plasma parameters to determine the tritium concentration.

Analysis of deuterium recycling in JET under beryllium first wall conditions

The aim of the paper is to give a summarising account of the main experimental results concerning pumping, fuelling and outgassing of deuterium during this beryllium (Be)-phase in comparison with the earlier carbon (C)-phase. From the pumping and fuelling data of the Be-phase the effective ratio of the diffusion to the recombination coefficient for deuterium on JET tokamak surfaces is derived. This is compared with results from laboratory measurements on beryllium samples