O.N. Jarvis

The JET neutron emission profile monitor

Abstract This paper provides a technical description of the neutron emission profile monitor as used routinely at the Joint European Torus (JET), and includes representative examples of its operational capabilities. The primary function of this instrument is to measure the neutron emission as a function of both position and time in a poloidal (vertical along major radius) section through the torus. For the first time the spatially localised effects of sawteeth (magnetic relaxation phenomena) have been observed using a neutron diagnostic. The total (global) neutron emission can be obtained from the profile monitor data by performing a volume integral over the plasma; the absolute neutron emission rates agree with those obtained from the JET time-resolved neutron monitor to within ±15%. This was the first such instrument routinely in use on any tokamak. It provides unique data which are independent of all other diagnostic measurements.

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.

Experiments on the release of tritium from the first wall of JET

During the JET Preliminary Tritium Experiment (PTE), an estimated 2 × 1012 (1.1 × 1021 atoms) of tritium were injected into the JET vacuum vessel. A series of experiments was performed whose purpose was to deplete the torus of tritium, to compare the effectiveness of different methods of tritium removal and to obtain a quantitative understanding of the processes involved. The effectiveness of the cleaning procedures was such that the normal tokamak programme was resumed one week after the PTE and the routing of exhaust gases to the atmosphere after two weeks. The release of tritium from the vessel was found to scale with the deuterium release from the vessel, suggesting that dilution and mixing of the hydrogen isotopes in the vessel walls is important. High density, disruptive tokamak discharges were found to be the most successful plasma pulses for tritium removal. Purges with deuterium gas were also effective and have the advantage of operational simplicity. Helium discharges, on the other hand, resulted in low tritium release from the vessel walls. It was demonstrated that the tritium release rate could be predicted using data from hydrogen to deuterium changeover experiments. Using the superior quality of data available from the tritium cleanup experiment, the physical mechanisms necessary to describe the hydrogenic uptake and release from the JET torus were identified. Tile release of tritium is reproduced using a model that incorporates implantation into a thin surface layer as well as diffusion of tritium into and out of the bulk material

Investigations of fast-particle behavior in joint European torus plasmas using nuclear techniques

In this paper an overview of experimental observations of fast particle behavior in Joint European Torus (JET) plasmas is presented. The material is drawn directly from the results of measurements based on nuclear detection techniques. The earliest observations concern escaping 15-MeV protons from the D- {sup 3}He reaction; they are detected in the form of spikes at the time of sawtooth crashes. Subsequent observations with a neutron multicollimator show that sawteeth expel neutral beam injected 80-keV deuterons from the central region of the plasma (but not necessarily out of the plasma). Extensive use has been made of the detection of gamma rays created when ion cyclotron resonance frequency (ICRF)-driven fast ions react with plasma fuel ions and with the main plasma impurity ions carbon, oxygen, and beryllium. Threshold reactions show that ICRF-driven ions can exceed energies of 7.5 MeV. Using ratios of gamma-ray intensities, tail temperatures in the mega-electron-volt range have been diagnosed.

Bulk ion heating with ICRH in JET DT plasmas

Reactor relevant ICRH scenarios have been assessed during DT experiments on the JET tokamak using H mode divertor discharges with ITER-like shapes and safety factors. Deuterium minority heating in tritium plasmas was demonstrated for the first time. For 9% deuterium, an ICRH power of 6 MW gave 1.66 MW of fusion power from reactions between suprathermal deuterons and thermal tritons. The Q value of the steady state discharge reached 0.22 for the length of the RF flat-top (2.7 s), corresponding to three plasma energy replacement times. The Doppler broadened neutron spectrum showed a deuteron energy of 125 keV, which was optimum for fusion and close to the critical energy. Thus, strong bulk ion heating was obtained at the same time as high fusion efficiency. Deuterium fractions around 20% produced the strongest ion heating together with a strong reduction of the suprathermal deuteron tail. The ELMs had low amplitude and high frequency and each ELM transported less plasma energy content than the 1% required by ITER. The energy confinement time, on the ITERH97-P scale, was 0.90, which is sufficient for ignition in ITER. 3He minority heating, in approximately 50:50 D:T plasmas with up to 10% 3He, also demonstrated strong bulk ion heating. Central ion temperatures up to 13 keV were achieved, together with central electron temperatures up to 12 keV. The normalized H mode confinement time was 0.95. Second harmonic tritium heating produced energetic tritons above the critical energy. This scheme heats the electrons in JET, unlike in ITER where the lower power density will allow mainly ion heating. The inverted scenario of tritium minority ICRH in a deuterium plasma was demonstrated as a successful heating method producing both suprathermal neutrons and bulk ion heating. Theoretical calculations of the DT reactivity mostly give excellent agreement with the measured reaction rates.

Time resolved measurements of triton burnup in JET plasmas

Triton production from one branch of the deuteron-deuteron fusion reaction is routinely measured at 6 ms time intervals in JET plasma discharges by recording the 2.5 MeV neutrons produced in the other branch using a set of calibrated fission chambers. The burnup of the tritons is measured by detecting the 14 MeV t-d neutrons with a 0.2 cm3 Si(Li) diode. The 2.5 MeV neutron flux can be used in a simple time dependent calculation based on classical slowing-down theory to predict the 14 MeV neutron flux. The measured flux and the triton slowing-down time are systematically lower than the values estimated from the key plasma parameters but the differences are within the experimental errors.

In‐vessel calibration of the JET neutron monitors using a 252Cf neutron source: Difficulties experienced

The most direct method of obtaining an absolute calibration of the set of fission chambers used at JET for the measurement of the time‐resolved neutron yield is by means of a standardized 252Cf radio‐isotope neutron source which is moved around the vacuum vessel to map out the volume occupied by the plasma. The accuracy of this procedure can approach 10%. However, its applicability depends on the geometry of the tokamak and the location of the fission chambers. At JET, most of the neutrons reach the detectors by passing through the main diagnostic ports. Now that these ports have become surrounded by massive items of hardware, it is found that this equipment moderates and absorbs the 252Cf neutrons more strongly than 2.5 MeV neutrons. This paper examines the magnitude of the errors on the absolute calibration of the fission chambers introduced by this effect.

Runaway electron measurements in the JET tokamak

The perpendicular x-ray emission up a to few MeV of runaway electrons has been measured in JET low-density ohmic discharges by means of the fast electron bremsstrahlung profile monitor. A diffusion model simulating the temporal evolution of the line-integrated x-ray signals is used to determine the runaway radial transport coefficient in the central region of the plasma ( for r/a<0.5); a comparison is made with the predictions of magnetic and electrostatic turbulent transport theories and limits on the level of radial magnetic field fluctuations are found.

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.

Gamma ray emission profile measurements from JET ICRF-heated discharges

Helium 3 minority fuel ions have been observed to be accelerated to megaelectron-volt (MeV) energies by ion cyclotron radiofrequency (ICRF) heating. These energetic ions undergo nuclear reactions with impurity 9Be and 12C ions in the plasma, and characteristic gamma radiation is emitted. For special circumstances, this gamma ray emission has been detected with a 19 channel profile monitor constructed primarily for neutron measurements. Two-dimensional (2-D) profiles of the gamma radiation emission have been derived by tomographic reconstruction; these profiles correspond to a cross-section weighted density of 3He ions having energies of 2 or more MeV. As expected for ICRF heating, the observed spatial profile of the gamma radiation shows that access by the accelerated ions to the plasma volume on the high field side of the RF resonance layer is effectively inhibited. In addition, the profile appears to demand the presence of a group of passing particles that are localized in poloidal extent and are circulating some 15 cm to the low field side of the resonance layer. Apart from the 15 cm displacement, these features are well reproduced by a model that combines a 2-D bounce averaged Fokker-Planck calculation with an orbit-following code. A unique example of a gamma ray spatial profile captured at the time of a sawtooth crash shows that the profile is relatively unaffected apart from the sudden obliteration of the passing group, for which the ions are found to possess relatively high energy, presumably gained from the RF waves through the Doppler shift effect. On the basis of these observations, an explanation is proposed for the selective expulsion of RF heated fast ions from the central region of the plasma to the outer regions