Deuterium-tritium Experiments Research Articles

New Safety and Technical Challenges and Operational Experience on the JET First Trace Tritium Experiment

‘Trace Tritium Experiments’ (TTE) were successfully performed on JET in 2003. The Campaign marked the first use of tritium in JET plasmas since the Deuterium-Tritium Experiment (DTE1) Campaign in 1997, and was the first use of tritium in experiments under the EFDA organisation with the UKAEA as JET Operator. The safety and regulatory preparations for the experiment were extensive. Since JET has been operated by the UKAEA the operations have followed the model of a licensed nuclear site. The safe operation of the JET torus is demonstrated in a safety case. Key Safety Management Requirement (KSMR) and Key Safety Related Equipment (KSRE) are identified in the Safety Case for DT operation. The safe operation of the torus is within the bounds of, and under the control of, an Authority to Operate (ATO). New technical challenges were presented by the need to inject and account for small quantities of tritium in very short pulses (~80ms), with an accurate time stamp. The safety and operational management of the campaign are described. Valuable lessons were learned which would help in running future experiments. It is concluded that JET is in a strong position to run future trace tritium and full DT discharges.

Gamma-diagnostics of alpha-particles in He4 and D–T plasmas

This article reports on γ-ray diagnosis of ICRF-accelerated He4 ions and fusion α particles used in the JET tokamak. This diagnostic based on the analysis of γ rays emitted in nuclear reaction Be9(α,nγ)12C. The results of the first observation of the 4.44-MeV γ-radiation due to the reaction Be9(α,nγ)12C in JET deuterium–tritium experiment (DTE1) are presented. Gamma-ray spectra have been recorded with collimated spectrometer, and the γ-ray emission radial profiles measured with the JET neutron/gamma profile monitor. Doppler broadening effects which could be used for the ICRF-accelerated He-ion4 diagnosis with high energy resolution γ-ray detectors are discussed. Capabilities of the γ-ray measurements in next step D–T devices are discussed.

Assessment of the performance of the JET exhaust detritiation system

The exhaust detritiation system (EDS) is one of the key safety related systems, which allows operation of the Joint European Torus (JET) machine. Its role is to detritiate the exhaust gases in case of an accident and to minimise the tritium release to the environment. EDS has also been used on JET during the last 3 years for maintenance purposes. During the main phase of the deuterium–tritium experiment (DTE1), the EDS was the final stage for detritiation of gases coming from the Torus vessel. There is a regulatory requirement to prepare an ‘Engineering Substantation’ of key safety related systems. Although this requirement applies to EDS when dealing with accidental tritium releases, it has been possible to investigate the efficiency of the system by analysing the experimental data obtained during maintenance. In 2000, during the shutdown, campaign measurements on EDS have been performed to determine its efficiency on a daily basis. Detritiation factors (DF) range between 20 and 80 000. Results are presented and discussed in relation to the requirements of the JET safety case and the factors, which cause apparent low DFs. An enhancement of the system for tritium control tracking is proposed.

A thin film device as a low energy, high flux charged particle spectrometer

We are continuing our investigation of the use of stacks of electrically isolated thin metal foils as spectrometers for lost ions from tokamak fusion plasmas. Devices of this type in which the foil thicknesses were a few micrometers were installed on the Joint European Torus during the recent first deuterium–tritium experiment in an effort to observe lost energetic alpha particles. While there was no convincing evidence of lost alpha particles in this experiment, we did observe significant fluxes of low energy (<500 keV) charged particles. In an effort to provide an instrument for the investigation of this phenomenon and of escaping relatively low energy (<100 keV) ions from other fusion plasma devices, we have developed alternative devices with very thin (few hundred nanometers) alternating layers of conductor and insulator. Four such devices have been fabricated and tested for protons with energies between 20 and 160 keV and demonstrated good energy resolution (typically about 10%) for proton bombarding energies between about 40 and 120 keV. One of the devices, consisting of deposited layers of Al, Ti, and SiO2 was operated up to a current density of about 100 m/cm2 at an energy of 100 keV, corresponding to a power volume density of 100 kW/cm3

Measured response of bubble neutron detectors and prospects for alpha knock-on diagnostics

Measurement of the neutron energy spectrum above ∼16 MeV will yield information on the spatial and energy distributions of confined fast alphas in deuterium–tritium (DT) tokamaks (Fisher, Nucl. Fusion; Gorini Rev. Sci. Instrum.). The energetic neutrons result from fusion reactions involving the energetic ions created by alpha-fuel ion knock-on collisions. Standard two-gas bubble neutron detectors, designed to only detect neutrons with energies above a selectable threshold determined by the gas mixture, were used in preliminary attempts to measure the knock-on neutrons from DT plasmas in the Tokamak Fusion Test Reactor and Joint European Torus (JET). Subsequent measurements at accelerator neutron sources showed an unexpected below-threshold detector response that prevented observations of the alpha-induced neutron tails. Spontaneous bubble nucleation measurements show that the majority of this below-threshold response is due to slight variations in the gas mixture, and is not present in single-gas detectors. Single-gas detectors will be tested at the University of California Berkeley to determine the neutron energy threshold as a function of detector operating temperature and to confirm their suitability for alpha knock-on tail measurements. An array of single-gas detectors operating at different temperatures should allow measurements of the alpha knock-on neutron tail during planned DT experiments on JET.

00/01880 Diagnostic experience during deuterium-tritium experiments in JET, techniques and measurements

00/01880 Diagnostic experience during deuterium-tritium experiments in JET, techniques and measurements

Overview of the performance of the JET Active Gas Handling System during and after DTE1

The JET Active Gas Handling System (AGHS) was designed, built and commissioned to handle radioactive tritium gas mixtures safely, to supply tritium (T 2) and deuterium (D 2) to the JET torus, to process the exhaust gases with the main purpose to enrich and re-use T 2 and D 2, to detritiate tritiated impurities and to keep discharges below the approved daily release limits. In addition, the AGHS had to supply the necessary ventilation air streams during maintenance or repair inside or outside of the AGHS building. During the first Deuterium–Tritium Experiment (DTE1) at JET in 1997 the AGHS fulfilled all these tasks in an excellent manner. No unauthorised or unplanned tritium releases occurred and no operational delays were caused by the AGHS. In fact, this was the first true demonstration that quantities of tritium in the tens of grams range can be recycled safely and efficiently in a large fusion device. At the start of DTE1 20 g of tritium were available on the JET site. About 100 g of tritium were supplied from the AGHS to the users which necessitated the recycling of tritium at least five times. Approximately 220 tritium plasma shots were performed during DTE1. Large amounts of tritium were temporarily trapped in the torus. This overview presents the performance of the whole AGHS during DTE1 as well as general aspects such as the preparation for DTE1; the quantities of gases supplied from the AGHS to the users and pumped back to the AGHS; tritium accountancy; interlock systems; failure of equipment; and gives detailed information of the gas processing in each subsystem of the AGHS. As a result of the performance of the AGHS during DTE1 we can state confidently that the AGHS is ready for further Deuterium–Tritium Experiments.

Operational experience with the JET Impurity Processing system during and after DTE1

The hydrogen-containing gases generated during operation and maintenance of a fusion machine need to be detritiated prior their discharge to the environment. The amounts of gaseous impurities with their chemical and isotopic compositions which were decontaminated in the JET impurity processing system during and after Deuterium–Tritium Experiment, DTE1, are presented. Two different techniques for impurities detritiation were tested. The first technique is based on catalytic oxidation of hydrogen-containing species followed by water decomposition in reaction with uranium at elevated temperature. The second technique is based on direct decomposition of impurities with molecular hydrogen liberation in reactions with uranium at elevated temperatures. The results of the impurity detritiation using both techniques are given.

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

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.

The analytical gas chromatographic system of the JET Active Gas Handling System — tritium commissioning and use during DTE1

The analytical gas chromatographic system (AN-GC) was the main tool used to analyse gas mixtures from various subsystems of the Active Gas Handling System (AGHS) and to characterise the functioning of the different processes during the Deuterium–Tritium Experiment (DTE1). Samples were transferred to AN-GC via long 1 4 ′′ lines. Calibration factors, accuracy and minimum detection limits for different gas species were determined for the thermal conductivity detectors, flame ionisation detectors and ionisation chambers. The performance of the AN-GC is discussed by means of various chromatograms and the experience gained with these detectors as well as their limits are mentioned. The AN-GC worked very reliably and about 320 analyses were performed during DTE1 and the subsequent cleanup phase.

Health physics aspects of tritium operation at JET

JET has completed its first stage of active operations, processing more than 100 g of tritium during the deuterium–tritium experiment (DTE1), the neutral beam intervention and remote tile exchange shutdown. The radiological safety of tritium handling operations has been of particular interest. A wide range of tritium operations has been carried out in the period 1995–1998. This paper describes some of the radiological protection measures for work with tritium, and discusses Health Physics operational experience of handling tritium in this period. Descriptions are given of active operations in the gas handling plant; in the torus hall during DTE1; in related interventions, and of the remote exchange of in-vessel divertor modules. Workplace contamination levels over 100 DAC (HTO) have been encountered, tritiated water with activity of 2 TBq/l and tritiated carbon with activities of ∼4 TBq/g has been handled. Control measures involving the use of purge and extract ventilation, and of personal protection using air-fed pressurised suits are described. The project imposes tight limits on radiation exposures. Tritium doses to staff in this period have been very low (individual doses <170 μSv/year, collective doses ≤2.1 mSv/year). Aerial discharges have been <3% of annual authorised limits, and average environmental (HTO) levels have been a few Bq/m 3. Lessons have been learnt concerning exposure control, large-scale permeation effects, and the appearance of residual tritium on exposed surfaces. Tritium operations at JET have been conducted without incident and with very low personnel exposures. The methodology of using containment and ventilation systems and tight radiological control has been successful in limiting doses. JET experience shows that large-scale tritium handling and exposure control can be achieved within stringent dose limits.

Waste management aspects of the DTE1 and RTE campaigns

The waste management activities at JET include not only the collection, processing and dispatch of radioactive and beryllium contaminated waste, but also for the operation of several supporting facilities (the beryllium-controlled areas); the provision of respiratory protection; (including pressurised suit operations) and the procedural support for maintenance of active handling operations. The consequences of carrying out the Deuterium-Tritium experiment (DTE1) were reviewed throughout 1996 and 1997 in order that appropriate preparations could be implemented. Several procedural, regulatory, technical, and managerial functions were identified for modification and the planning and subsequent execution of these functions is described here. In general the regulatory and procedural aspects were accurately anticipated, e.g. radioactive waste authorisations, the operation of the radioactive drainage system and improvements in managerial control were successfully implemented. However, in the case of materials storage the increased tritium off-gassing rates were higher than expected. This required the engineering controls and storage facilities to be enhanced before the DTE1 tile modules could be transferred from the Torus Hall. The predicted consequences of DTE1 and the preparations made are detailed here with discussion of further actions which proved necessary in practice.

Operation of the JET Active Gas Handling System during and after DTE1

During and after the Deuterium–Tritium Experiment (DTE1) the JET Active Gas Handling System fulfilled all expectations and requirements: it pumped, processed and purified the gases from the Torus and connected systems, isotopically separated hydrogen and supplied 100 g tritium to the machine with only 20 g on JET site which means that the tritium was recycled five times. In addition, it supplied ventilation air detritiation services during interventions inside and outside the Active Gas Handling building. This demonstrated for the first time that high tritium quantities can be recycled safely in connection with a large fusion facility. The paper describes the operation of the Active Gas Handling System.

Combined heating experiments in ELM-free H modes in JET

High power combined NBI + ICRF heating experiments have been carried out in the JET Mark IIa divertor configuration in the hot ion ELM-free H mode regime, both in deuterium (DD) and in deuterium-tritium (DT) plasmas. Results are presented from a wide range of additional heating power levels, ICRF up to 9.5 MW tuned to the fundamental hydrogen minority, NBI up to 22 MW, and for plasma currents up to 4.2 MA and toroidal fields up to 3.6 T. Discharges with combinedNBI + ICRF heating show a clear improvement in electron temperature, DD neutron yield andstored energy with respect to NBI only discharges. High energy neutral particle analyser data show that acceleration of the NBI deuterons takes place due to absorption of ICRF power at the second harmonic deuterium resonance. This is confirmed by numerical simulations with the PION code, indicating that up to 40% of the ICRF power is absorbed by bulk and NBI ions. ICRF heating has been an essential ingredient in the DT experiments in the ELM-free hot ion regime, contributing to the achievement of a record fusion power of 16.1 MW and a record stored energy of 17 MJ.

Physics of high performance JET plasmas in DT

The Joint European Torus (JET) has recently operated with deuterium-tritium (DT) mixtures, carried out an International Thermonuclear Experimental Reactor (ITER) physics campaign in hydrogen, deuterium, DT and tritium, installed the Mark IIGB `Gas Box' divertor fully by remote handling and started physics experiments with this more closed divertor. The DT experiments set records for fusion power (16.1 MW), ratio of fusion power to plasma input power (0.62, and 0.95 ± 0.17 if a similar plasma could be obtained in steady state) and fusion duration (4 MW for 4 s). A large scale tritium supply and processing plant, the first of its kind, allowed the repeated use of the 20 g of tritium on-site to supply 99.3 g of tritium to the machine. The H mode threshold power is significantly lower in DT, but the global energy confinement time is practically unchanged (no isotope effect). Dimensionless scaling `wind tunnel' experiments in DT extrapolate to ignition with ITER parameters. The scaling is close to gyro-Bohm, but the mass dependence is not correct. Separating the thermal plasma energy into core and pedestal contributions could resolve this discrepancy (leading to proper gyro-Bohm scaling for the core) and also account for confinement degradation at high density and at high radiated power. Several radiofrequency heating schemes have been tested successfully in DT, showing good agreement with calculations. Alphaparticle heating has been clearly observed and is consistent with classical expectations. Internal transport barriers have been established in optimized magnetic shear discharges in DT and steady state conditions have been approached with simultaneous internal and edge transport barriers. First results with the newly installed Mark IIGB divertor show that the in-out symmetry of the divertor plasma can be modified using differential gas fuelling, that optimized shear discharges can be produced and that krypton gas puffing is effective in restoring L mode edgeconditions and establishing an internal transport barrier in such discharges.

Ion Cyclotron Emission from JET D-T Plasmas

Ion cyclotron emission (ICE) excited by collective instability of fusion $\ensuremath{\alpha}$ particles has been observed during deuterium-tritium experiments with radio-frequency heating and neutral beam injection (NBI) in the Joint European Torus. A model based on classical $\ensuremath{\alpha}$-particle confinement is broadly consistent with this data. ICE spectra from discharges with high-power NBI also show evidence of ion hybrid wave excitation by beam ions, relevant to $\ensuremath{\alpha}$ channeling.

Tritium recycling and retention in JET

JET's 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.

Report on the 5th European Fusion PhysicsWorkshop, Sesimbra, Portugal, 10-12 December1997

The 5th EFPW took place in December 1997 at Sesimbra in Portugal. It was hosted by the Instituto Superior Técnico, Lisbon and sponsored by the European Commission. Within an overall theme of magnetohydrodynamic (MHD) phenomena in magnetically confined plasmas, four topics of importance to the future development of magnetic confinement fusion were discussed in detail: integrated control of fusion plasmas, MHD in advanced scenarios, neoclassical modes and disruptions. In addition, an extended review of the results of the recent JET deuterium-tritium experiments was presented and the physics research opportunities which would be made available by ITER operation were discussed. The main issues discussed in each session and the areas identified as requiring further study are summarized here.

A 14 MeV neutron spectrometer for the Joint European Torus deuterium-tritium experiments

A 14 MeV neutron spectrometer, based on the principle of the proton recoil telescope, has been installed as a deuterium–tritium (DT) plasma diagnostic at the Joint European Torus. The device comprises three identical telescope modules arranged one behind the other, to increase efficiency without worsening resolution. It views the plasma along a horizontal line of sight. The total efficiency is 7.05×10−5 counts per (neutron per cm2), and the calculated response function has a full width at half maximum of 2.2% at an incident neutron energy of 14.1 MeV. The spectrometer was operational throughout the recent DT experiment No. 1, and selected neutron spectra have been analyzed in terms of the contributions from the various neutron production mechanisms.

Ion cyclotron emission from fusion products and beam ions in the Tokamak Fusion Test Reactor

Ion cyclotron emission (ICE) observed during deuterium and deuterium-tritium experiments in TFTR can be driven by both fusion products and injected beam ions. The emission driven by fusion products is a result of the excitation of the fast magnetoacoustic wave through toroidicity affected cyclotron resonance, the magnetoacoustic cyclotron instability. For beam driven ICE, the excitation mechanism, outlined by R.O. Dendy et al. (Phys. Plasmas 1 (1994) 3407) for the cylindrical case, is generalized by including the curvature and ∇B drift in the local resonance condition. The phase velocity of the destabilized, predominantly electrostatic, wave is typically an order of magnitude smaller than the Alfvén velocity. For both fusion and beam driven ICE, it is found that the instability growth rate is greatly increased as compared with the cylindrical case and reaches its maximum for nearly perpendicular propagation. The results of the numerical and analytical stability analysis are consistent with the experimental observations and provide an explanation for the simultaneous excitation of sequential multiple cyclotron harmonics of the fast ions, the sublinear correlation of the growth rate with the fast ion density and the stabilizing effect of the increasing fast ion velocity spread.