T. Nishitani

Chapter 7: Diagnostics

In order to support the operation of ITER and the planned experimental programme an extensive set of plasma and first wall measurements will be required. The number and type of required measurements will be similar to those made on the present-day large tokamaks while the specification of the measurements—time and spatial resolutions, etc—will in some cases be more stringent. Many of the measurements will be used in the real time control of the plasma driving a requirement for very high reliability in the systems (diagnostics) that provide the measurements. The implementation of diagnostic systems on ITER is a substantial challenge. Because of the harsh environment (high levels of neutron and gamma fluxes, neutron heating, particle bombardment) diagnostic system selection and design has to cope with a range of phenomena not previously encountered in diagnostic design. Extensive design and R&D is needed to prepare the systems. In some cases the environmental difficulties are so severe that new diagnostic techniques are required. a Author to whom any correspondence should be addressed.

Alfvén eigenmode and energetic particle research in JT-60U

Recent results on investigations of Alfven eigenmodes, fast ion confinement and fast ion diagnostics in JT-60U are presented. It was found that toroidicity induced Alfven eigenmodes (TAEs) were stable in negative shear discharges with a large density gradient at the internal transport barrier (ITB). If the density gradient was small at the ITB, multiple TAEs appeared around the q = 2 surface (pitch minimum) and showed a large frequency chirping (Δf ≈ 80 kHz). In low-q positive shear discharges, the location of the TAEs changed from outside to inside the q = 1 surface, owing to a temporal change of the q profile. A significant depression of the megaelectronvolt ion population was observed only with high-n (n up to 14) multiple TAEs inside the q = 1 surface. Non-circular triangularity induced Alfven eigenmodes were observed for the first time. Considerable depression of the triton burnup was observed in negative shear discharges. Orbit following Monte Carlo simulations indicated that ripple loss was responsible for the enhanced triton losses. The fast ion stored energies in ICRF heated negative shear discharges were comparable to those of positive shear plasmas. Tail ion temperatures in high (second to fourth) harmonic ICRF heating experiments were first analysed with an MeV neutral particle analyser. The behaviour of MeV ions produced by ICRF heating was studied with gamma ray diagnostics. A scintillating fibre detector system for detecting the 14MeV neutron emission was developed for the triton burnup studies. Ion cyclotron emission measurements discriminating between parallel and perpendicular components of the electric field were carried out for the first time.

Impact of irradiation effects on design solutions for ITER diagnostics

An overview of the results of the irradiation tests on diagnostic components under the ITER technology R&D tasks and the solutions for the present diagnostic design are given in the light of these results. A comprehensive irradiation database of diagnostic components has been accumulated and permits conclusions to be drawn on the application of these components in ITER. Under the ITER technology R&D tasks, not only has work been shared among four home teams, but also several bilateral collaborations and round-robin experiments have been performed to enhance the R&D activities. 2000 Elsevier Science B.V. All rights reserved.

Absolute calibration of the JT‐60U neutron monitors using a 252Cf neutron sourcea)

Absolutely calibrated measurements of the neutron yield are important for the evaluation of plasma performance such as the fusion gain Q in D–D operating tokamaks. The time‐resolved neutron yield is measured with 235U and 238U fission chambers and 3He proportional counters in the JT‐60U tokamak. The in situ calibration was performed by moving the 252Cf neutron source toroidally through the JT‐60 vacuum vessel. Detection efficiencies of three 235U and two 3He detectors were measured for 92 locations of the neutron point source in toroidal scans at two different major radii. The total detection efficiency for the torus neutron source was obtained by averaging the point efficiencies over the whole toroidal angle. The uncertainty of the resulting detection efficiency for the plasma neutrons is estimated to be ±11%.

Alfvén eigenmodes driven by Alfvénic beam ions in JT-60U

Instabilities with frequency chirping in the frequency range of Alfven eigenmodes have been found in the domain 0.1% < βh < 1% and vb||/vA ~1 with high energy neutral beam injection in JT-60U. One instability with a frequency inside the Alfven continuum spectrum appears and its frequency increases slowly to the toroidicity induced Alfven eigenmode (TAE) gap on the timescale of an equilibrium change ( ≈ 200 ms). Other instabilities appear with a frequency inside the TAE gap and their frequencies change very quickly by 10-20 kHz in 1-5 ms. During the period when these fast frequency sweeping (fast FS) modes occur, abrupt large amplitude events (ALEs) often appear with a drop of neutron emission rate and an increase in fast neutral particle fluxes. The loss of energetic ions increases with a peak fluctuation amplitude of θ/Bθ. An energy dependence of the loss ions is observed and suggests a resonant interaction between energetic ions and the mode.

Ripple induced fast ion loss and related effects in JT-60U

Experiments have been carried out in JT-60U to verify the modelling of fast ion ripple transport. The ripple induced loss was estimated from the neutron decay following neutral beam pulse injection and the loss related heat load on the first wall. Comparison of the lost fraction and the hot spot positions between measurements and orbit following Monte Carlo calculations exhibited good agreement, indicating that the ripple transport governing fast ion losses is explained within the framework of existing theory. Neutral beam heating experiments in JT-60U also indicate that H modes free of ELMs are still obtainable for ripple amplitudes of up to 2.2%

Irradiation test of diagnostic components for ITER application in the Japan Materials Testing Reactor

Radiation effects in components and materials will be one of the most serious technological issues in nuclear fusion systems realizing burning-plasmas. Especially, diagnostic components, which should play a crucial role in controlling plasmas and understanding the physics of burning-plasmas, will be exposed to high-flux neutrons and gamma rays. Dynamic radiation effects will affect the performance of components substantially from the beginning of exposure to radiation environments, and accumulated radiation effects will gradually degrade their functioning abilities in the course of their service. High-power-density fission reactors will be the only realistic tools to simulate the radiation environments expected to occur in burning-plasma fusion machines such as the International Thermonuclear Experimental Reactor (ITER), at present. Some key diagnostic components, namely magnetic coils, bolometers, and optical fibres, were irradiation-tested in a fission reactor, to evaluate their performances in heavy radiation environments. Results indicate that ITER-relevant radiation-resistant diagnostic components could be developed in time, although there are still some technological problems to be overcome.

Status of ITER neutron diagnostic development

Due to the high neutron yield and the large plasma size many ITER plasma parameters such as fusion power, power density, ion temperature, fast ion energy and their spatial distributions in the plasma core can be measured well by various neutron diagnostics. Neutron diagnostic systems under consideration and development for ITER include radial and vertical neutron cameras (RNC and VNC), internal and external neutron flux monitors (NFMs), neutron activation systems and neutron spectrometers. The two-dimensional neutron source strength and spectral measurements can be provided by the combined RNC and VNC. The NFMs need to meet the ITER requirement of time-resolved measurements of the neutron source strength and can provide the signals necessary for real-time control of the ITER fusion power. Compact and high throughput neutron spectrometers are under development. A concept for the absolute calibration of neutron diagnostic systems is proposed. The development, testing in existing experiments and the engineering integration of all neutron diagnostic systems into ITER are in progress and the main results are presented.

Round-robin irradiation test of radiation resistant optical fibers for ITER diagnostic application

Fused silica core optical fibers are expected to play crucial roles especially in the size-reduced International Thermonuclear Experimental Reactor (ITER-FEAT). Several radiation resistant optical fibers have been developed in Japan and the Russian Federation. The task force on radiation effects in diagnostic components in the ITER-EDA (engineering and design activity) promoted international round-robin irradiation experiments on the developed optical fibers. Ten different optical fibers were tested in a cobalt-60 gamma-ray irradiation facility and in the Japan Materials Testing Reactor. The paper reports results obtained on five different optical fibers, which include purified, hydrogen loaded, and fluorine doped ones. Results show that the developed optical fibers could be deployed in remote handling and out-of-vessel applications. But, for the in-vessel diagnostics in the visible range optical spectroscopy, further improvement of the radiation resistance of optical fibers will be needed.

Fast ion losses due to toroidal field ripple in JT-60U

A previous experiment in JT-60U supported an orbit following Monte Carlo (OFMC) calculation regarding ripple trapped loss, and the present experiment, furthermore, suggests that the OFMC also predicts banana drift loss fairly well. In the experiment presented, the total fast ion losses due to toroidal field ripple were estimated from the decay in neutron emission following a short neutral beam injection (90 keV, D). The neutron decay for co-passing beam injection showed a diffusivity of about 0 m2/s, which indicates no fast ion loss. In contrast, the neutron decay for trapped particle injection exhibited characteristic enhancement of fast ion losses due to toroidal field ripple: the fast ion losses consisted of ripple trapped convection and ripple banana diffusion in the low collisionality regime. The OFMC calculation reconstructed completely the experimental neutron decay irrespective of the total ripple losses and the fraction of banana drift loss. Considering the previous work on ripple trapped loss and this result, it can be concluded that the OFMC code gives a good quantitative estimation of banana drift loss as well as ripple trapped loss