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ITER R&D: Auxiliary Systems: Plasma Diagnostics
Fusion Engineering and Design 55 (2001) 331– 346 www.elsevier.com/locate/fusengdes
ITER R&D: Auxiliary Systems: Plasma Diagnostics A.E. Costley a,*, D.J. Campbell b, S. Kasai c, K.E. Young d, V. Zaveriaev e a
ITER Naka Joint Work Site, 801 -1 Mukouyama, Naka-machi, Naka-gun, Ibaraki-ken, 311 -0193 Japan b EDFA, c/o Max-Plank Institut fu¨r Plasmaphysik, Boltzmannstrasse 2, D-85748, Garching, Germany c Department of Fusion Engineering Research, Japan Atomic Energy Research Institute, 2 -4 Shirakata, Tokai-mura, Naka-gun, Ibaraki-ken, 319 -1195, Japan d MS-37, Princeton Plasma Physics Laboratory, PO Box 451, Princeton, NJ 08543, USA e Russian Research Center, ‘Kurchato6 Institute’, 123182 Moscow, pl. Kurchato6a, Russia
1. Introduction In order to control and evaluate ITER performance it will be necessary to measure a wide range of plasma parameters and some key first wall parameters. The measurements will be made with an extensive diagnostic system comprising about 40 individual measurement systems. Many of the required measurements can be made with measurement systems (diagnostics) designed on the basis of systems which are in use on today’s large existing magnetic fusion experimental facilities especially tokamaks [1]. However, irradiation effects, which are not important in the present facilities, can be a critical issue in applications on ITER. Moreover, the performance and/or anticipated lifetime of existing diagnostic components may not be adequate. In a few cases, the measurement capability of existing techniques does not meet ITER requirements and new approaches are required. R&D * Corresponding author. Tel.: + 81-29-2707751; fax: +8129-2707507. E-mail address: [email protected] (A.E. Costley).
are therefore necessary in several areas and an extensive, coordinated, activity involving all four ITER partners has been undertaken during the ITER-EDA. In this chapter we review the developments undertaken which were performed under a written agreement with the ITER Joint Central Team. These tasks were aimed specifically at solving key, urgent, measurement challenges. Other ITER motivated developments have occurred in the parallel coordinated physics programme. Those relating to diagnostics have been reviewed in reference [1]. 2. Objectives The ITER diagnostic R&D programme has three major components: (a) irradiation tests on candidate materials and on prototype diagnostic components; (b) development of new diagnostic components; and (c) development of new or improved diagnostic techniques. The overall goal is to provide the data necessary for the selection, design, and construction of the individual diagnostic systems that will be used on ITER.
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3. Irradiation tests on materials and components for diagnostic construction
EDA are summarised below and specific results are given in Table 2.
Diagnostic components will be installed in many regions of the machine—inside the vacuum vessel, in the diagnostic ports, in the inter-space between the vacuum vessel and the cryostat, outside the biological shield and in the remote diagnostic areas—but it is those installed in the vacuum vessel close to the first wall that will be in the harshest environment. Here there are high neutron and gamma radiation fluxes, substantial heat loads from plasma radiation, and high neutral particle fluxes from charge exchange processes in the edge regions of the plasma. The center solenoid and poloidal field coil system produces a large, timevarying, poloidal magnetic field which is about 1 T at the first wall, 0.2 T at the cryostat wall and 0.01 T 40 m from the machine centre. An additional hazard for components in the vacuum vessel is material evaporated from the divertor and first wall which will be re-deposited on plasma facing surfaces. This could be particularly severe during disruptions. It is not just the intensity of these hazardous conditions that is significant but also the longevity. ITER pulses are typically several hundred seconds in duration with steady state operation (] 3600 s) as an ultimate goal. Relative to the harshest conditions experienced on existing machines the neutral particle fluxes are about 5 times higher, the neutron flux levels are about 10 times higher, the neutron fluence is about 10,000 times higher, and the pulse lengths are about 100 times longer. The radiation environment is summarised in Table 1. The principal materials and components examined in the irradiation effects programme are ceramic insulators, wires and cables, mirrors, windows, optical fibres, and bolometers. The properties of concern include electrical resistivity, reflectivity, optical absorption and fluorescence, as well as mechanical and thermal properties [1– 8]. The property of interest varies with the diagnostic component and application. The location of some representative diagnostic components, and the principal physical effects of interest, are shown in Fig. 1. Key investigations undertaken during the
3.1. Ceramic insulators, and wires/cables Several diagnostics, for example magnetics, will have components mounted in the vacuum vessel and these components will use ceramic insulators and wires. The influence of irradiation on the electrical properties of candidate ceramics and insulator materials (different grades of Al2O3, magnesia or Vitox) is therefore important and has been extensively investigated [9–21]. The important physical effects are radiation-induced conductivity (RIC) and radiation induced electrical degradation (RIED). In the case of in-vessel cables for low signal levels, the additional problem of radiationinduced electromotive force (RIEMF) must also be considered. RIC has been studied for many years and an extensive database exists together with a sound theoretical understanding. The results for several bulk ceramics and mineral insulated cables are shown in Fig. 2. Also shown in the figure is the permissible limit for three of the diagnostic sensors. The data shows that the effects of RIC can be rendered negligible by careful choice of materials. On the other hand, the mechanism of radiation-induced electrical degradation (RIED) is still not understood, but an extensive database has been established. The effect has been found to occur only with electric fields \ 50 kV/m applied when the temperature of the ceramic is between about 150°C and 650°C and so can be avoided by design. RIEMF is also under investigation [22–24]. It has been observed to occur in experiments in which mineral insulated (MI) cable and prototype magnetic coils have been irradiated in test reactors. In general the observed RIEMF is current driven, and the generated current is of the order of a microampere or less. In the experiments with magnetic coils the asymmetric component of the induced electromotive force is of the order of microvolts, which could lead to serious long-term integration drifts in the measurement of magnetic flux. Since the magnitude of the effect is small it is difficult to ensure that other effects—for example, thermoelectric effects and grounding problems—are not causing
Location
Neutrons
Typical diag component
\0.1 MeV n/m2s
14 MeV n/m2s
First walla Near blanket gap (on vacuum vessel) Mag. Coils; Bolometers; Retroreflectors Vacuum vessel (behind blanket) Mag. loops Diagnostic block First mirrors Labyrinth second mirrors, windows Vacuum vessel (inboard TFC side) Mag. loops Divertor cassette First mirrors Divertor port Second mirrors
3×1018 0.2–1×1017
a
Dose rate Gy/s
Fluence (\0.1 MeV) n/m2
Particle flux atoms /m2s
Plasma radiation (peak) kW/m2
8×1017 0.8–4×1016
2×103 20–100
3×1025 0.4–2.0×1024
5×1019 1018
500 10
2×1016
3×1014
020
2×1023
0
0
1×1016
9×1015
20
1×1023
1017
1.5
2×1013
3×1013
10−2
2×1020
0
0
1×1014
1×1012
0.1
1021
0
0
1×1018
3×1017
1×10-3
1025
1017 −1019
1−100
1013–1015
1012–1014
10−2 –1
1019–1021
TBD
TBD
Corresponding to a possible maximum fusion power of 700 MW.
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Table 1 Radiation Environment for the Diagnostic Components
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systematic errors in the measurements and further tests are planned.
3.2. Mirrors and reflectors For optical diagnostic systems the plasma facing optical element will be a mirror. The lifetime of these first mirrors is therefore a key parameter. The mirrors will be subject to intense neutron, gamma and ultra violet radiation, neutron heating, particle fluxes arising from charge exchange atoms (CXA) (typically 2×1019 particles/m2/s with energies up to several keV), and will be subjected to the deposition of material eroded from the divertor, first wall and shield structure. Extensive tests in which candidate mirror materi-
als have been subject to different types and levels of radiation have been carried out [25–32]. The tests have shown that neutron and gamma radiation is not a threat to diagnostic mirrors made from bulk metals although care must be taken in the design to deal with any nuclear heating. However, dielectric coated mirrors have been found to be damaged by neutron irradiation: flaking and blistering of the coatings can occur. Similarly, the properties of multi-layer mirrors used in x-ray systems can be affected by neutron irradiation, while the properties of inorganic x-ray crystals (for example graphite) are not affected. Therefore, careful choice of mirror material is essential and guidelines have been developed. For diagnostic first mirrors, probably the most
Fig. 1. Location of some representative diagnostic components and the principal, radiation induced, physical effects of interest.
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Table 2 Results of Irradiation Tests on Candidate Materials for Diagnostic Components Diagnostic components
Materials tested
Accumulated effects
Dynamic effects
Ceramics (electrical insulators)
Single crystal sapphire and polycrystal alumina (Al2O3)
3 dpa in helium gas (RIED: No catastrophic degradation)
104 Gy/s (RIC: B10−6 S/m)
Wires/Cables
MI-cables: SUS, Inconel (sheath)/MgO, Al2O3 (insulator)/Cu, Ni (centre conductor)
1.8 dpa (RIED: No catastrophic degradation)
104 Gy/s (RIC: B10−6 S/m) 103 Gy/s (RIEMF: B10V inner conductor/outer sheath)
Windows
Fused Silica/Quartz (400–1200 nm) Sapphire (800–5000nm)
10−3 dpa (Transmission; 5% degradation: 8 mmt) 0.4 dpa (Transmission; No degradation: 1 mmt)
Radioluminescence: 107 photons / Gy.A, .steradian.cm3 at 410 nm Radioluminescence: 1010 photons/Gy.A, .steradian.cm3 at 410 nm
Optical fibres
Pure silica (core)/F doped (clad)/Al jacket (RF KS-4V) (RF KU1 fused silica)
107 Gy (under gamma) (Transmission: 2–2.5 dB/m) 1-2×1016 n/cm2 (En\0.1MeV) Induced Loss B2.5–4.5 dB/m, respectively 1×10−2 dpa
Radioluminescence:
(Visible region)
(Visible region) (IR region)
Pure silica (core)/F doped (clad)/Al jacket (JA F-doped) Pure silica (core)/F doped (clad)/Al jacket
4.9×1012 n/cm2/s: 10−8 J/s/A, /cm2/steradian (at 500 nm) Radioluminescence
(Transmission: 20 dB/m) 1 dpa (Transmission: 10 dB/m)
Mirrors/Reflectors First mirrors: Metal (Cu, W, Mo, 40 dpa (Cu)a (Reflectivity: No St.St., Al) degradation) 0.1dpa (Mo) (Reflectivity: No degradation) First mirrors for LIDAR: Single coated (Rh/V, St.St) Dielectric mirrors: (HfO2/SiO2, B10−2 dpab (Flaking, Blistering) TiO2/SiO2) LSMsc: (Mo/Si, W/B4C and B10−2 dpa (the shift of the peak W/C) reflectivity to shorter wavelength) 10−2 dpa X-ray crystals: (Ge, Si, SiO2, Graphite) a
Imitation experiment using Cu+ ions of 1 or 3 MeV. Partially damaged, Dielectric mirrors are used as second mirrors. c LSM (Layered Synthetic Microstructures): in well-shielded location and temperature control. b
important effects are the CXA fluxes, which can lead to erosion and/or deposition [25]. Mirrors of several metals (Be, Cu, SS, Mo, Ta, W) with different microstructure (polycrystal, single crystal, film) have been bombarded for long periods (515 hrs) by deuterium ions of energy 0.07 to 1.5 keV and the optical properties of the mirrors (specular and diffuse reflectivity and planarity) have been measured [32].
Due to different sputtering rates of grains with different crystallographic plane orientations, the polycrystal mirrors develop a step structure soon after bombardment starts. They can also develop small-scale microrelief inside separate grains. Both effects lead to a degradation of the mirror properties. One exception to this has been observed with stainless steel mirrors where it appears that the grains continue to be very smooth plateaus even
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Fig. 2. Measured Radiation-Induced Conductivity in bulk single and polycrystalline oxides, and MI cables as a function of ionising dose rate combined with magnetic diagnostic requirements at selected locations. The vertical bars represent the range of design values of RIC that can be tolerated for each coil; the horizontal bars represent the uncertainty on the flux.
after the erosion of a layer of thickness 4.5 mm, irrespective of the energy of the bombarding ions. However, so far measurements have been limited to normal incidence. Long-term bombardment of single crystal mir-
rors (Mo, W) does not demonstrate the step structure or the small-scale microrelief: their surfaces have a high mirror quality after erosion by sputtering of a layer several mm’s thick (Fig. 3 (a)). Further, suitably chosen metal film mirrors mounted on a metal substrate can have a good resistance to the CXA flux. For example, rhodium film mirrors of thickness 10 mm mounted on Cu can be used in locations where the CXA flux onto the mirror surface will not exceed 2·1018atom/m2s ( 1/10 of the CXA flux to the first wall) (Fig. 3 (b)). On the other hand, even very thin layers (h] 10 nm) of a contaminating film can seriously reduce the reflectivity. Mitigating methods (baffles and shutters) as well as potential cleaning methods (e.g. low energy discharge cleaning, laser cleaning) are therefore being investigated in current work.
3.3. Windows The principal properties of concern for diagnostic windows are the radiation induced absorption, which has an instantaneous and a permanent component, and radioluminescence. Mechanical damage is not expected to arise because the windows will be placed in radiation fields well below
Fig. 3. (a) Dependance of reflectance of W mirrors (polycrystal, block monocrystal and real monocrystals with two planes of orientation) on the depth of the eroded layer at l=650 nm. (b) Dependance of reflectance and resolving power of Rh film on copper substrate mirrors on depth of the eroded layer at l = 650 nm [32].
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Fig. 4. Measured transparency of KU1 fused silica (8 mm thick): (a) as a function of wavelength for different neutron and gamma fluxes after irradiation in a nuclear reactor at T = 180°C and with a Co60 gamma source at room temperature, and (b) as a function of gamma dose at three wavelengths after irradiation with a Co60 gamma source at room temperature [38].
the values at which mechanical deformations or strength degradation will occur. The impact of radiation on the optical properties of several materials, including sapphire, and crystalline and amorphous quartz, has been investigated [33–39]. The results show that suitable window materials are available for passive diagnostic systems operating in the wavelength ranges 400 nm to 5 mm; for example, the radiation-induced absorption and radioluminescence are very low in KU1 fused silica. A typical result is shown in Fig. 4 [38]. Moreover it has been found that it is possible to reverse the radiation-induced degradation in the transparency of KU1 fused silica by thermo-annealing (Fig. 5) [39]. Suitable methods exist for bonding these materials to metal and so the window problem for these diagnostics is essentially solved. At shorter wavelengths (B 400 nm) both the absorption and luminescence are enhanced and further work is required to find the optimum window material. At very short wavelengths (B200 nm) the absorption is very high even in the absence of irradiation and direct coupling is necessary. Windows for use in active diagnostics that employ high power lasers have a much lower tolerance to absorption since in this case even a very small absorption (B5%) can lead to unacceptably high power deposition. Further work is required to optimise the windows for such systems. At longer wavelengths (\ 5 mm) there
are two possibilities, ZnSe and diamond. ZnSe is a relatively soft material and a special method of making metal/window seals with a large diameter of 100 mm is being developed. In this respect diamond, which has already been tested for the electron cyclotron resonance heating programme, would be preferred. For microwave diagnostics, several suitable window materials exist (for example diamond, quartz, and fused silica) and no problems are anticipated.
Fig. 5. Annealing effects on transparency of KU-1 fused silica irradiated up to a neutron fluence 6 × 1018 n/cm2 and absorbed dose of 0.3 GGy(Si). The sample dimension is 16 mm in diameter and 8 mm in thickness [39].
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Fig. 6. Dose dependence of absorption of pure silica core fibre under fission neutron irradiations [47].
3.4. Optical fibres Because the optical path in the material is much longer, radiation induced absorption and radioluminescence are even more significant in optical fibres. At high levels of irradiation mechanical damage (embrittlement) can also occur. A substantial programme has been performed in which the optical properties of candidate fibres have been measured at relevant radiation levels [40– 49]. The results show that substantial radiation induced absorption and luminescence occurs espe-
Fig. 7. Cerenkov radiation from optical fibre under fission neutron irradiations [48, 49].
cially at short wavelengths (B800 nm). Typical results are shown in Figs. 6 and 7. In general, this means that optical fibres cannot be used inside the vacuum vessel. However, the effects are sufficiently low at the radiation levels expected outside the bioshield that it may be possible to use fibres at visible and longer wavelengths. In the intermediate region (the cryostat), it is highly desirable to use fibres and this may be possible at infrared wavelengths. More work is required to determine the optimum material and the precise magnitude of the optical properties. A round robin experiment is in progress to attempt to resolve some discrepancies observed in the different experiments. In order to pass the fibres through the boundaries of the ITER device (vacuum vessel, cryostat etc) a low loss, multi-core, optical fibre feedthrough is needed. A multi core (57 fibres) device capable of being handled with remote handling equipment has been developed.
3.5. Bolometers Bolometers are used extensively in existing machines including those which have operated in D/T although the lifetime under ITER conditions is not known [50,51]. The radiation hardness of existing bolometers and candidate bolometer substrate materials is therefore under investigation. The JT60 type bolometer which utilises a gold absorber on a polyimide substrate was found to be insensitive to gamma radiation but the resistivity is expected to change significantly under neutron irradiation. JET type bolometers use mica as the substrate material and this is expected to be relatively radiation hard. Tests on mica have been performed in the JMTR and no significant changes in the physical properties have been observed up to a dose of 10 − 2 dpa. Bolometers based on mica may therefore be suitable in ITER for at least the initial years of operation. In a joint EU/JA investigation, an in-situ irradiation test of a ‘JET’ type bolometer employing a mica substrate is in progress. Alternative potentially radiation hard substrate materials, Al2O3, Aluminum
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Fig. 8. Principle of operation and prototype implementation of the Load Cell Type Steady State Magnetic Sensor [53].
nitride (AIN), CVD diamond, KU1 fused silica and Si3N4, are also under investigation. A new type of bolometer based on the ferroelectric effect has also been proposed but is not yet under development.
4. Development of new diagnostic components The principal components developed in this programme are a steady-state magnetic sensor, enhanced neutral particle analyser (NPA), bubble chamber detector, diamond neutron detectors, and an active optical alignment system.
4.1. Steady-state magnetic sensor For very long pulses, the classical inductive method for measuring the poloidal magnetic field will become subject to unacceptable systematic errors arising from integrator drift. A method of measuring static magnetic fields is therefore re-
quired and a promising technique is being developed [52,53]. A jxB device measures the field by observing the torque on a current loop (Fig. 8). The torque is measured by a balanced load cell (four strain gauges in a Wheatstone bridge arrangement). The initial work has shown the basic viability of the technique. Current work is concentrating on establishing the linearity and sensitivity. Radiation tests are in progress.
4.2. Enhanced NPA detector Measurement and control of the fuel mix ratio, nT /nD, in ITER will be necessary to achieve optimum plasma performance. The NPA will be the main diagnostic for measuring this parameter. In order to make measurements in the plasma core, it is necessary to measure particles emitted with energies up to 100 keV (Fig. 9), but particle detectors presently employed in NPAs cannot be used at high energies because of high sensitivity of the detectors to neutron and gamma radiation.
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An upgraded NPA detector employing an accelerator unit is therefore being developed [54]. The accelerator has been designed, constructed and tested with satisfactory results. It is expected that the improved NPA will be able to make measurements into the core region for plasmas with peaked temperature profiles. For plasmas where the temperature profile is flat (for example following a sawtooth crash), the measurements are restricted to the gradient region. In planned future work the analyser dispersion system will be devel-
oped and its compatibility with the accelerator established.
4.3. Bubble chamber detector The measurement of the confined alpha particle population will be important on ITER and in principle it can be measured by analysing the ‘knock-on’ tail on the neutron spectrum [55,56]. The technique requires a neutron detector with a very sharp energy threshold and bubble chambers are a good candidate [57]. An initial feasibility assessment has been carried out with promising results. Water and liquid CO2 are proposed as the prospective working liquids. In a device under current investigation flowing liquid is used to obtain the required time resolution.
4.4. Diamond detectors for neutron measurements Natural diamond detectors (NDD) are a possible device for measuring the DT neutron spectrum [58–61]. Several NDDs were successfully used for DT neutron spectrometry during tritium experiments at TFTR and JET. They have the advantages of high energy resolution coupled with small size and high radiation resistance. In principle both natural and synthetic diamonds can be used. The technologies of producing diamond by chemical vapor deposition (CVD), and by high pressure, are under development [62–66], but due to a large number of charge capture centers, the performance of synthetic detectors is not yet adequate for spectrometer detectors. Radiation induced defects in the diamond could degrade the performance and current work is concentrating on investigating the change of NDD charge collection efficiency under neutron irradiation. Possible restoration techniques (eg thermal annealing) are also being examined.
4.5. Microfission chamber
Fig. 9. Calculated emissivity functions for D and T under typical ITER-FEAT conditions (peaked Ti) showing that with the upgraded NPA detector measurements of nT /nD should be possible in the plasma core region.
Measurements of the neutron flux will be used to determine the fusion performance of ITER. The most accurate measurements can be obtained from flux monitors placed close to the plasma thereby avoiding the influence of surrounding ma-
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Fig. 10. Schematic diagram of typical microfission chamber. Fissile material, such as 235U, is coated on the cylindrical electrode. The ionizing gas between the electrodes is Ar + 5% N2 (14.6 atm) [68].
terials. Microfission chambers containing 235U or other isotopes are being developed (Fig. 10) [67,68]. They are miniature fission detectors of the type commonly used for in-core neutron flux measurements in fission reactors but are capable of being used in the high gamma background present in a fusion reactor. Their small size means that they can be deployed at several poloidal locations where they provide a measurement of the fusion power which is largely independent of the plasma position and shape.
4.6. Optical alignment system The optical paths of ITER diagnostics run through many relay mirrors, lenses and other optical components which are mounted on different supports. The supports are will be subjected to differential movements. A prototype active alignment system suitable for application on ITER is therefore being developed. The system is being develop specifically for the divertor impurity monitoring system but the techniques and hardware could potentially be applied to other optical diagnostics.
5. Development of new diagnostic techniques While most of the ITER measurement requirements can be met using established tokamak diagnostic techniques, this is not always the case and some new approaches and/or techniques are re-
quired. Two specific cases are the measurement of the plasma shape and position for very long pulse lengths and ultimately steady state, and the measurement of the confined alpha particles. In the former case, inductive magnetic diagnostics are currently used and will have a pulse length limitation. In the latter case, the development of measurement techniques and hardware are at a very early stage. In addition, it can be beneficial to make significant improvements and changes to the method of implementing existing techniques. Development of diagnostic techniques has therefore been part of the ITER R&D diagnostic programme.
5.1. Plasma position reflectometry An alternative approach to magnetics for providing plasma position control of long pulses is plasma position reflectometry [69,70]. In this technique the position of fixed density layers relative to the first wall would be measured at a few poloidal locations using micowave reflectometry. Developments are required in several areas before this technique can be chosen for ITER. The reliability of reflectometer density measurements needs to be enhanced; a real-time method of data-analysis is required; and a method to reference the reflectometry plasma position information to the magnetics position information is required. It is anticipated that the magnetics measurements would continue to be used for plasma start up and termination.
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The concept has been investigated and found to be feasible in principle. Tests on DIIID have shown highly reliable density measurements which exceed the accuracy requirement for measurements on ITER. Measurements were made with a dual, broad band frequency swept system operating in the ranges 33– 50 GHz, and 50– 70 GHz, corresponding to densities of approximately 0– 1.0× 1019 m − 3 and 1.0– 3.0 ×1019 m − 3 when utilising X-mode polarisation at 1.7 T. Typical results are shown in Fig. 11.
5.2. CO2 collecti6e scattering diagnostic A technique which in principle can measure fast ions, including the confined alpha particle population, is collective scattering. In this technique, the plasma is radiated with a laser and the frequency shifted scattered radiation is detected and analysed. In principle, the number density and energy of the fast ions can be determined. A diagnostic system has been developed for JT60-U [71]. The laser is a high power pulsed CO2 laser (15 J, 1 ms) and irradiates the plasma along a vertical line of sight. Radiation scattered at small angles (50.5°) is collected and detected. The detector is a Quantum Well Infrared Photodector (QWIP) with a very wide bandwidth (10 GHz). The system is installed on JT60-U and results are expected in 2001.
5.3. Neutron acti6ation with fluid flow The neutron activation technique with encapsulated foils is an established method for measuring the time-integrated global neutron source strength. By using a flowing fluid it is in principle possible to make time resolved measurements. In this case water in a flowing loop is exposed to the neutron source in one location and gamma rays from the decay of 16N are measured remotely (Fig. 12) [74,75]. Since 16N is produced only by fast neutrons (\ 10 MeV), the technique selectively measures the DT fusion neutrons. Moreover, it has the advantage of requiring very simple hardware near the tokamak. Measurements with an experimental loop of similar size to that which would be needed on ITER have shown that a time resolution of 0.05 s can achieved. The delay in the measurement is about 0.9 s.
5.4. Fast wa6e reflectometry As stated above, the NPA can make measurements of nT /nD into the core region for plasmas with peaked temperature profiles. The spatial resolution will be moderate. Fast Wave Reflectometry is a new technique which offers the promise of making measurements in the centre of the plasma with good spatial resolution. It uses relatively
Fig. 11. Measured density profiles in the edge region of DIIID. Density contour data are shown for densities in the ranges 0 – 7 × 1018 m − 3 as a function of time and radius through two Edge Localised Modes (ELMs). From the radial scale it is seen that the density contours move outward by 4–5 cm at the onset of each ELM [69].
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Fig. 12. Schematic of the experimental test of the Fluid Flow Activation technique and some initial results [75].
simple hardware but requires development before an application on ITER can be planned [76]. The technique utilizes the property that the phase velocity of the fast wave propagating across the magnetic field depends on the ion mass density. It can be simply shown that measurement of the phase velocity in an interferometer arrangement gives the line integral of the ion mass density. Moreover, if the plasma consists of two or more ion species, the fast wave has ion-ion hybrid resonance(s). For a given probing frequency, the resonance and cut-off frequencies are uniquely determined by the fuel mix ratio, nT /nD. In a non-uniform magnetic field, as in a tokamak, it is possible to determine the spatial dependence of nT /nD by measuring the position of the cut-off with a frequency swept reflectometer. For typical conditions the probing frequency is 200 MHz which gives a wavelength in the plasma of 3 cm. Hardware at this frequency is simple to install and use. An initial test of the technique has been carried out on DIIID [77]. The transmitter antenna was a 30 cm long loop installed on outboard side at the midplane. The signal detectors were single-turn B-dot loops having diameter of 2.5 cm. Measurements of the line integral of the mass density show good agreement with the line integral of the electron density measured with a CO2 interferometer (Fig. 13). Further development is necessary to develop the capability of measuring the fuel mix ratio but the initial results are encouraging.
5.5. Intense diagnostic neutral beam The use of a short pulse, intense, ion beam for CXRS on ITER, rather than a conventional steady-state beam, has several advantages. The beam would increase the signal-to-noise ratio in the measurement because the relatively high-intensity, coupled with very short gating times on the detectors, reduces the bremsstrahlung background. Gains in signal/noise of more than an order of magnitude would be possible. Also, the beam would be significantly smaller and would have a lower average power requirement (500
Fig. 13. The electron density measured with a conventional interferometer (noisier curve) and the ion mass density measured from the Alfve´ n speed curve (quieter curve) on DIIID [77].
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kW). However, such beams are at an early stage of development. Initial developments have been carried out at the Los Alamos National Laboratory [78–80]. A diode and accelerator, which would be a suitable source for a beam is under development. It is known as the Continuous High-Average power Microsecond Pulser (CHAMP) and is designed to produce a 15 kA, 250 kV, 1 ms ion beam with a technology capable of being rep-rated at up to 30 Hz, although initial tests were single shot. Initial results are encouraging but show that it may be difficult to meet the requirements on a key parameter, beam divergence. Because the beam would have to be set back from the plasma on ITER a divergence of 50.5° is required. The best values achieved so far are about 0.8° but in these tests no particular effort was made to optimise this parameter. Further work should examine methods of actively focusing or guiding the beam before it is neutralized to reduce the constraint on beam divergence.
6. Concluding remarks During the EDA, the ITER diagnostic R&D programme has investigated the effects of neutron and gamma radiation on candidate materials and prototype diagnostic components that could be used in diagnostic construction. An extensive data base of the results of the tests has been established and is available for diagnostic system designers. In addition, specific studies have been carried on critical components subject to particular hazards; for example diagnostic first mirrors which will be exposed to CX sputtering leading to erosion. The application of some diagnostic techniques depends on the successful development of new components. The critical components have been identified and the necessary developments initiated. For a few of the required measurements, new measurement techniques and approaches are needed. Promising techniques have been identified and are being developed in current work. Further developments are required in some areas and these are currently being addressed. In total, the activity has established a firm base on which
techniques can be selected for ITER, and systems and components can be designed.
Acknowledgements The authors would like to thank Dr E Hodgson (CIEMAT, Spain), Dr Krasilnikov (TRINITI Institute, Russia), Prof. T Shikama (Tohoku University, Japan) and Dr S Yamamoto (ITER Joint Central Team, Germany) for useful comments; Dr G Vayakis (ITER Joint Central Team, Japan) for help in compiling Fig. 2, and Dr A Kislyakov (Ioffe Institute, Russia) for preparing Fig. 9.
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ITER R&D: Auxiliary Systems: Plasma Diagnostics A.E. Costley a,*, D.J. Campbell b, S. Kasai c, K.E. Young d, V. Zaveriaev e a
ITER Naka Joint Work Site, 801 -1 Mukouyama, Naka-machi, Naka-gun, Ibaraki-ken, 311 -0193 Japan b EDFA, c/o Max-Plank Institut fu¨r Plasmaphysik, Boltzmannstrasse 2, D-85748, Garching, Germany c Department of Fusion Engineering Research, Japan Atomic Energy Research Institute, 2 -4 Shirakata, Tokai-mura, Naka-gun, Ibaraki-ken, 319 -1195, Japan d MS-37, Princeton Plasma Physics Laboratory, PO Box 451, Princeton, NJ 08543, USA e Russian Research Center, ‘Kurchato6 Institute’, 123182 Moscow, pl. Kurchato6a, Russia
1. Introduction In order to control and evaluate ITER performance it will be necessary to measure a wide range of plasma parameters and some key first wall parameters. The measurements will be made with an extensive diagnostic system comprising about 40 individual measurement systems. Many of the required measurements can be made with measurement systems (diagnostics) designed on the basis of systems which are in use on today’s large existing magnetic fusion experimental facilities especially tokamaks [1]. However, irradiation effects, which are not important in the present facilities, can be a critical issue in applications on ITER. Moreover, the performance and/or anticipated lifetime of existing diagnostic components may not be adequate. In a few cases, the measurement capability of existing techniques does not meet ITER requirements and new approaches are required. R&D * Corresponding author. Tel.: + 81-29-2707751; fax: +8129-2707507. E-mail address: [email protected] (A.E. Costley).
are therefore necessary in several areas and an extensive, coordinated, activity involving all four ITER partners has been undertaken during the ITER-EDA. In this chapter we review the developments undertaken which were performed under a written agreement with the ITER Joint Central Team. These tasks were aimed specifically at solving key, urgent, measurement challenges. Other ITER motivated developments have occurred in the parallel coordinated physics programme. Those relating to diagnostics have been reviewed in reference [1]. 2. Objectives The ITER diagnostic R&D programme has three major components: (a) irradiation tests on candidate materials and on prototype diagnostic components; (b) development of new diagnostic components; and (c) development of new or improved diagnostic techniques. The overall goal is to provide the data necessary for the selection, design, and construction of the individual diagnostic systems that will be used on ITER.
0920-3796/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 0 - 3 7 9 6 ( 0 1 ) 0 0 2 0 0 - 9
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3. Irradiation tests on materials and components for diagnostic construction
EDA are summarised below and specific results are given in Table 2.
Diagnostic components will be installed in many regions of the machine—inside the vacuum vessel, in the diagnostic ports, in the inter-space between the vacuum vessel and the cryostat, outside the biological shield and in the remote diagnostic areas—but it is those installed in the vacuum vessel close to the first wall that will be in the harshest environment. Here there are high neutron and gamma radiation fluxes, substantial heat loads from plasma radiation, and high neutral particle fluxes from charge exchange processes in the edge regions of the plasma. The center solenoid and poloidal field coil system produces a large, timevarying, poloidal magnetic field which is about 1 T at the first wall, 0.2 T at the cryostat wall and 0.01 T 40 m from the machine centre. An additional hazard for components in the vacuum vessel is material evaporated from the divertor and first wall which will be re-deposited on plasma facing surfaces. This could be particularly severe during disruptions. It is not just the intensity of these hazardous conditions that is significant but also the longevity. ITER pulses are typically several hundred seconds in duration with steady state operation (] 3600 s) as an ultimate goal. Relative to the harshest conditions experienced on existing machines the neutral particle fluxes are about 5 times higher, the neutron flux levels are about 10 times higher, the neutron fluence is about 10,000 times higher, and the pulse lengths are about 100 times longer. The radiation environment is summarised in Table 1. The principal materials and components examined in the irradiation effects programme are ceramic insulators, wires and cables, mirrors, windows, optical fibres, and bolometers. The properties of concern include electrical resistivity, reflectivity, optical absorption and fluorescence, as well as mechanical and thermal properties [1– 8]. The property of interest varies with the diagnostic component and application. The location of some representative diagnostic components, and the principal physical effects of interest, are shown in Fig. 1. Key investigations undertaken during the
3.1. Ceramic insulators, and wires/cables Several diagnostics, for example magnetics, will have components mounted in the vacuum vessel and these components will use ceramic insulators and wires. The influence of irradiation on the electrical properties of candidate ceramics and insulator materials (different grades of Al2O3, magnesia or Vitox) is therefore important and has been extensively investigated [9–21]. The important physical effects are radiation-induced conductivity (RIC) and radiation induced electrical degradation (RIED). In the case of in-vessel cables for low signal levels, the additional problem of radiationinduced electromotive force (RIEMF) must also be considered. RIC has been studied for many years and an extensive database exists together with a sound theoretical understanding. The results for several bulk ceramics and mineral insulated cables are shown in Fig. 2. Also shown in the figure is the permissible limit for three of the diagnostic sensors. The data shows that the effects of RIC can be rendered negligible by careful choice of materials. On the other hand, the mechanism of radiation-induced electrical degradation (RIED) is still not understood, but an extensive database has been established. The effect has been found to occur only with electric fields \ 50 kV/m applied when the temperature of the ceramic is between about 150°C and 650°C and so can be avoided by design. RIEMF is also under investigation [22–24]. It has been observed to occur in experiments in which mineral insulated (MI) cable and prototype magnetic coils have been irradiated in test reactors. In general the observed RIEMF is current driven, and the generated current is of the order of a microampere or less. In the experiments with magnetic coils the asymmetric component of the induced electromotive force is of the order of microvolts, which could lead to serious long-term integration drifts in the measurement of magnetic flux. Since the magnitude of the effect is small it is difficult to ensure that other effects—for example, thermoelectric effects and grounding problems—are not causing
Location
Neutrons
Typical diag component
\0.1 MeV n/m2s
14 MeV n/m2s
First walla Near blanket gap (on vacuum vessel) Mag. Coils; Bolometers; Retroreflectors Vacuum vessel (behind blanket) Mag. loops Diagnostic block First mirrors Labyrinth second mirrors, windows Vacuum vessel (inboard TFC side) Mag. loops Divertor cassette First mirrors Divertor port Second mirrors
3×1018 0.2–1×1017
a
Dose rate Gy/s
Fluence (\0.1 MeV) n/m2
Particle flux atoms /m2s
Plasma radiation (peak) kW/m2
8×1017 0.8–4×1016
2×103 20–100
3×1025 0.4–2.0×1024
5×1019 1018
500 10
2×1016
3×1014
020
2×1023
0
0
1×1016
9×1015
20
1×1023
1017
1.5
2×1013
3×1013
10−2
2×1020
0
0
1×1014
1×1012
0.1
1021
0
0
1×1018
3×1017
1×10-3
1025
1017 −1019
1−100
1013–1015
1012–1014
10−2 –1
1019–1021
TBD
TBD
Corresponding to a possible maximum fusion power of 700 MW.
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Table 1 Radiation Environment for the Diagnostic Components
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systematic errors in the measurements and further tests are planned.
3.2. Mirrors and reflectors For optical diagnostic systems the plasma facing optical element will be a mirror. The lifetime of these first mirrors is therefore a key parameter. The mirrors will be subject to intense neutron, gamma and ultra violet radiation, neutron heating, particle fluxes arising from charge exchange atoms (CXA) (typically 2×1019 particles/m2/s with energies up to several keV), and will be subjected to the deposition of material eroded from the divertor, first wall and shield structure. Extensive tests in which candidate mirror materi-
als have been subject to different types and levels of radiation have been carried out [25–32]. The tests have shown that neutron and gamma radiation is not a threat to diagnostic mirrors made from bulk metals although care must be taken in the design to deal with any nuclear heating. However, dielectric coated mirrors have been found to be damaged by neutron irradiation: flaking and blistering of the coatings can occur. Similarly, the properties of multi-layer mirrors used in x-ray systems can be affected by neutron irradiation, while the properties of inorganic x-ray crystals (for example graphite) are not affected. Therefore, careful choice of mirror material is essential and guidelines have been developed. For diagnostic first mirrors, probably the most
Fig. 1. Location of some representative diagnostic components and the principal, radiation induced, physical effects of interest.
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Table 2 Results of Irradiation Tests on Candidate Materials for Diagnostic Components Diagnostic components
Materials tested
Accumulated effects
Dynamic effects
Ceramics (electrical insulators)
Single crystal sapphire and polycrystal alumina (Al2O3)
3 dpa in helium gas (RIED: No catastrophic degradation)
104 Gy/s (RIC: B10−6 S/m)
Wires/Cables
MI-cables: SUS, Inconel (sheath)/MgO, Al2O3 (insulator)/Cu, Ni (centre conductor)
1.8 dpa (RIED: No catastrophic degradation)
104 Gy/s (RIC: B10−6 S/m) 103 Gy/s (RIEMF: B10V inner conductor/outer sheath)
Windows
Fused Silica/Quartz (400–1200 nm) Sapphire (800–5000nm)
10−3 dpa (Transmission; 5% degradation: 8 mmt) 0.4 dpa (Transmission; No degradation: 1 mmt)
Radioluminescence: 107 photons / Gy.A, .steradian.cm3 at 410 nm Radioluminescence: 1010 photons/Gy.A, .steradian.cm3 at 410 nm
Optical fibres
Pure silica (core)/F doped (clad)/Al jacket (RF KS-4V) (RF KU1 fused silica)
107 Gy (under gamma) (Transmission: 2–2.5 dB/m) 1-2×1016 n/cm2 (En\0.1MeV) Induced Loss B2.5–4.5 dB/m, respectively 1×10−2 dpa
Radioluminescence:
(Visible region)
(Visible region) (IR region)
Pure silica (core)/F doped (clad)/Al jacket (JA F-doped) Pure silica (core)/F doped (clad)/Al jacket
4.9×1012 n/cm2/s: 10−8 J/s/A, /cm2/steradian (at 500 nm) Radioluminescence
(Transmission: 20 dB/m) 1 dpa (Transmission: 10 dB/m)
Mirrors/Reflectors First mirrors: Metal (Cu, W, Mo, 40 dpa (Cu)a (Reflectivity: No St.St., Al) degradation) 0.1dpa (Mo) (Reflectivity: No degradation) First mirrors for LIDAR: Single coated (Rh/V, St.St) Dielectric mirrors: (HfO2/SiO2, B10−2 dpab (Flaking, Blistering) TiO2/SiO2) LSMsc: (Mo/Si, W/B4C and B10−2 dpa (the shift of the peak W/C) reflectivity to shorter wavelength) 10−2 dpa X-ray crystals: (Ge, Si, SiO2, Graphite) a
Imitation experiment using Cu+ ions of 1 or 3 MeV. Partially damaged, Dielectric mirrors are used as second mirrors. c LSM (Layered Synthetic Microstructures): in well-shielded location and temperature control. b
important effects are the CXA fluxes, which can lead to erosion and/or deposition [25]. Mirrors of several metals (Be, Cu, SS, Mo, Ta, W) with different microstructure (polycrystal, single crystal, film) have been bombarded for long periods (515 hrs) by deuterium ions of energy 0.07 to 1.5 keV and the optical properties of the mirrors (specular and diffuse reflectivity and planarity) have been measured [32].
Due to different sputtering rates of grains with different crystallographic plane orientations, the polycrystal mirrors develop a step structure soon after bombardment starts. They can also develop small-scale microrelief inside separate grains. Both effects lead to a degradation of the mirror properties. One exception to this has been observed with stainless steel mirrors where it appears that the grains continue to be very smooth plateaus even
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Fig. 2. Measured Radiation-Induced Conductivity in bulk single and polycrystalline oxides, and MI cables as a function of ionising dose rate combined with magnetic diagnostic requirements at selected locations. The vertical bars represent the range of design values of RIC that can be tolerated for each coil; the horizontal bars represent the uncertainty on the flux.
after the erosion of a layer of thickness 4.5 mm, irrespective of the energy of the bombarding ions. However, so far measurements have been limited to normal incidence. Long-term bombardment of single crystal mir-
rors (Mo, W) does not demonstrate the step structure or the small-scale microrelief: their surfaces have a high mirror quality after erosion by sputtering of a layer several mm’s thick (Fig. 3 (a)). Further, suitably chosen metal film mirrors mounted on a metal substrate can have a good resistance to the CXA flux. For example, rhodium film mirrors of thickness 10 mm mounted on Cu can be used in locations where the CXA flux onto the mirror surface will not exceed 2·1018atom/m2s ( 1/10 of the CXA flux to the first wall) (Fig. 3 (b)). On the other hand, even very thin layers (h] 10 nm) of a contaminating film can seriously reduce the reflectivity. Mitigating methods (baffles and shutters) as well as potential cleaning methods (e.g. low energy discharge cleaning, laser cleaning) are therefore being investigated in current work.
3.3. Windows The principal properties of concern for diagnostic windows are the radiation induced absorption, which has an instantaneous and a permanent component, and radioluminescence. Mechanical damage is not expected to arise because the windows will be placed in radiation fields well below
Fig. 3. (a) Dependance of reflectance of W mirrors (polycrystal, block monocrystal and real monocrystals with two planes of orientation) on the depth of the eroded layer at l=650 nm. (b) Dependance of reflectance and resolving power of Rh film on copper substrate mirrors on depth of the eroded layer at l = 650 nm [32].
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Fig. 4. Measured transparency of KU1 fused silica (8 mm thick): (a) as a function of wavelength for different neutron and gamma fluxes after irradiation in a nuclear reactor at T = 180°C and with a Co60 gamma source at room temperature, and (b) as a function of gamma dose at three wavelengths after irradiation with a Co60 gamma source at room temperature [38].
the values at which mechanical deformations or strength degradation will occur. The impact of radiation on the optical properties of several materials, including sapphire, and crystalline and amorphous quartz, has been investigated [33–39]. The results show that suitable window materials are available for passive diagnostic systems operating in the wavelength ranges 400 nm to 5 mm; for example, the radiation-induced absorption and radioluminescence are very low in KU1 fused silica. A typical result is shown in Fig. 4 [38]. Moreover it has been found that it is possible to reverse the radiation-induced degradation in the transparency of KU1 fused silica by thermo-annealing (Fig. 5) [39]. Suitable methods exist for bonding these materials to metal and so the window problem for these diagnostics is essentially solved. At shorter wavelengths (B 400 nm) both the absorption and luminescence are enhanced and further work is required to find the optimum window material. At very short wavelengths (B200 nm) the absorption is very high even in the absence of irradiation and direct coupling is necessary. Windows for use in active diagnostics that employ high power lasers have a much lower tolerance to absorption since in this case even a very small absorption (B5%) can lead to unacceptably high power deposition. Further work is required to optimise the windows for such systems. At longer wavelengths (\ 5 mm) there
are two possibilities, ZnSe and diamond. ZnSe is a relatively soft material and a special method of making metal/window seals with a large diameter of 100 mm is being developed. In this respect diamond, which has already been tested for the electron cyclotron resonance heating programme, would be preferred. For microwave diagnostics, several suitable window materials exist (for example diamond, quartz, and fused silica) and no problems are anticipated.
Fig. 5. Annealing effects on transparency of KU-1 fused silica irradiated up to a neutron fluence 6 × 1018 n/cm2 and absorbed dose of 0.3 GGy(Si). The sample dimension is 16 mm in diameter and 8 mm in thickness [39].
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Fig. 6. Dose dependence of absorption of pure silica core fibre under fission neutron irradiations [47].
3.4. Optical fibres Because the optical path in the material is much longer, radiation induced absorption and radioluminescence are even more significant in optical fibres. At high levels of irradiation mechanical damage (embrittlement) can also occur. A substantial programme has been performed in which the optical properties of candidate fibres have been measured at relevant radiation levels [40– 49]. The results show that substantial radiation induced absorption and luminescence occurs espe-
Fig. 7. Cerenkov radiation from optical fibre under fission neutron irradiations [48, 49].
cially at short wavelengths (B800 nm). Typical results are shown in Figs. 6 and 7. In general, this means that optical fibres cannot be used inside the vacuum vessel. However, the effects are sufficiently low at the radiation levels expected outside the bioshield that it may be possible to use fibres at visible and longer wavelengths. In the intermediate region (the cryostat), it is highly desirable to use fibres and this may be possible at infrared wavelengths. More work is required to determine the optimum material and the precise magnitude of the optical properties. A round robin experiment is in progress to attempt to resolve some discrepancies observed in the different experiments. In order to pass the fibres through the boundaries of the ITER device (vacuum vessel, cryostat etc) a low loss, multi-core, optical fibre feedthrough is needed. A multi core (57 fibres) device capable of being handled with remote handling equipment has been developed.
3.5. Bolometers Bolometers are used extensively in existing machines including those which have operated in D/T although the lifetime under ITER conditions is not known [50,51]. The radiation hardness of existing bolometers and candidate bolometer substrate materials is therefore under investigation. The JT60 type bolometer which utilises a gold absorber on a polyimide substrate was found to be insensitive to gamma radiation but the resistivity is expected to change significantly under neutron irradiation. JET type bolometers use mica as the substrate material and this is expected to be relatively radiation hard. Tests on mica have been performed in the JMTR and no significant changes in the physical properties have been observed up to a dose of 10 − 2 dpa. Bolometers based on mica may therefore be suitable in ITER for at least the initial years of operation. In a joint EU/JA investigation, an in-situ irradiation test of a ‘JET’ type bolometer employing a mica substrate is in progress. Alternative potentially radiation hard substrate materials, Al2O3, Aluminum
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Fig. 8. Principle of operation and prototype implementation of the Load Cell Type Steady State Magnetic Sensor [53].
nitride (AIN), CVD diamond, KU1 fused silica and Si3N4, are also under investigation. A new type of bolometer based on the ferroelectric effect has also been proposed but is not yet under development.
4. Development of new diagnostic components The principal components developed in this programme are a steady-state magnetic sensor, enhanced neutral particle analyser (NPA), bubble chamber detector, diamond neutron detectors, and an active optical alignment system.
4.1. Steady-state magnetic sensor For very long pulses, the classical inductive method for measuring the poloidal magnetic field will become subject to unacceptable systematic errors arising from integrator drift. A method of measuring static magnetic fields is therefore re-
quired and a promising technique is being developed [52,53]. A jxB device measures the field by observing the torque on a current loop (Fig. 8). The torque is measured by a balanced load cell (four strain gauges in a Wheatstone bridge arrangement). The initial work has shown the basic viability of the technique. Current work is concentrating on establishing the linearity and sensitivity. Radiation tests are in progress.
4.2. Enhanced NPA detector Measurement and control of the fuel mix ratio, nT /nD, in ITER will be necessary to achieve optimum plasma performance. The NPA will be the main diagnostic for measuring this parameter. In order to make measurements in the plasma core, it is necessary to measure particles emitted with energies up to 100 keV (Fig. 9), but particle detectors presently employed in NPAs cannot be used at high energies because of high sensitivity of the detectors to neutron and gamma radiation.
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An upgraded NPA detector employing an accelerator unit is therefore being developed [54]. The accelerator has been designed, constructed and tested with satisfactory results. It is expected that the improved NPA will be able to make measurements into the core region for plasmas with peaked temperature profiles. For plasmas where the temperature profile is flat (for example following a sawtooth crash), the measurements are restricted to the gradient region. In planned future work the analyser dispersion system will be devel-
oped and its compatibility with the accelerator established.
4.3. Bubble chamber detector The measurement of the confined alpha particle population will be important on ITER and in principle it can be measured by analysing the ‘knock-on’ tail on the neutron spectrum [55,56]. The technique requires a neutron detector with a very sharp energy threshold and bubble chambers are a good candidate [57]. An initial feasibility assessment has been carried out with promising results. Water and liquid CO2 are proposed as the prospective working liquids. In a device under current investigation flowing liquid is used to obtain the required time resolution.
4.4. Diamond detectors for neutron measurements Natural diamond detectors (NDD) are a possible device for measuring the DT neutron spectrum [58–61]. Several NDDs were successfully used for DT neutron spectrometry during tritium experiments at TFTR and JET. They have the advantages of high energy resolution coupled with small size and high radiation resistance. In principle both natural and synthetic diamonds can be used. The technologies of producing diamond by chemical vapor deposition (CVD), and by high pressure, are under development [62–66], but due to a large number of charge capture centers, the performance of synthetic detectors is not yet adequate for spectrometer detectors. Radiation induced defects in the diamond could degrade the performance and current work is concentrating on investigating the change of NDD charge collection efficiency under neutron irradiation. Possible restoration techniques (eg thermal annealing) are also being examined.
4.5. Microfission chamber
Fig. 9. Calculated emissivity functions for D and T under typical ITER-FEAT conditions (peaked Ti) showing that with the upgraded NPA detector measurements of nT /nD should be possible in the plasma core region.
Measurements of the neutron flux will be used to determine the fusion performance of ITER. The most accurate measurements can be obtained from flux monitors placed close to the plasma thereby avoiding the influence of surrounding ma-
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Fig. 10. Schematic diagram of typical microfission chamber. Fissile material, such as 235U, is coated on the cylindrical electrode. The ionizing gas between the electrodes is Ar + 5% N2 (14.6 atm) [68].
terials. Microfission chambers containing 235U or other isotopes are being developed (Fig. 10) [67,68]. They are miniature fission detectors of the type commonly used for in-core neutron flux measurements in fission reactors but are capable of being used in the high gamma background present in a fusion reactor. Their small size means that they can be deployed at several poloidal locations where they provide a measurement of the fusion power which is largely independent of the plasma position and shape.
4.6. Optical alignment system The optical paths of ITER diagnostics run through many relay mirrors, lenses and other optical components which are mounted on different supports. The supports are will be subjected to differential movements. A prototype active alignment system suitable for application on ITER is therefore being developed. The system is being develop specifically for the divertor impurity monitoring system but the techniques and hardware could potentially be applied to other optical diagnostics.
5. Development of new diagnostic techniques While most of the ITER measurement requirements can be met using established tokamak diagnostic techniques, this is not always the case and some new approaches and/or techniques are re-
quired. Two specific cases are the measurement of the plasma shape and position for very long pulse lengths and ultimately steady state, and the measurement of the confined alpha particles. In the former case, inductive magnetic diagnostics are currently used and will have a pulse length limitation. In the latter case, the development of measurement techniques and hardware are at a very early stage. In addition, it can be beneficial to make significant improvements and changes to the method of implementing existing techniques. Development of diagnostic techniques has therefore been part of the ITER R&D diagnostic programme.
5.1. Plasma position reflectometry An alternative approach to magnetics for providing plasma position control of long pulses is plasma position reflectometry [69,70]. In this technique the position of fixed density layers relative to the first wall would be measured at a few poloidal locations using micowave reflectometry. Developments are required in several areas before this technique can be chosen for ITER. The reliability of reflectometer density measurements needs to be enhanced; a real-time method of data-analysis is required; and a method to reference the reflectometry plasma position information to the magnetics position information is required. It is anticipated that the magnetics measurements would continue to be used for plasma start up and termination.
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The concept has been investigated and found to be feasible in principle. Tests on DIIID have shown highly reliable density measurements which exceed the accuracy requirement for measurements on ITER. Measurements were made with a dual, broad band frequency swept system operating in the ranges 33– 50 GHz, and 50– 70 GHz, corresponding to densities of approximately 0– 1.0× 1019 m − 3 and 1.0– 3.0 ×1019 m − 3 when utilising X-mode polarisation at 1.7 T. Typical results are shown in Fig. 11.
5.2. CO2 collecti6e scattering diagnostic A technique which in principle can measure fast ions, including the confined alpha particle population, is collective scattering. In this technique, the plasma is radiated with a laser and the frequency shifted scattered radiation is detected and analysed. In principle, the number density and energy of the fast ions can be determined. A diagnostic system has been developed for JT60-U [71]. The laser is a high power pulsed CO2 laser (15 J, 1 ms) and irradiates the plasma along a vertical line of sight. Radiation scattered at small angles (50.5°) is collected and detected. The detector is a Quantum Well Infrared Photodector (QWIP) with a very wide bandwidth (10 GHz). The system is installed on JT60-U and results are expected in 2001.
5.3. Neutron acti6ation with fluid flow The neutron activation technique with encapsulated foils is an established method for measuring the time-integrated global neutron source strength. By using a flowing fluid it is in principle possible to make time resolved measurements. In this case water in a flowing loop is exposed to the neutron source in one location and gamma rays from the decay of 16N are measured remotely (Fig. 12) [74,75]. Since 16N is produced only by fast neutrons (\ 10 MeV), the technique selectively measures the DT fusion neutrons. Moreover, it has the advantage of requiring very simple hardware near the tokamak. Measurements with an experimental loop of similar size to that which would be needed on ITER have shown that a time resolution of 0.05 s can achieved. The delay in the measurement is about 0.9 s.
5.4. Fast wa6e reflectometry As stated above, the NPA can make measurements of nT /nD into the core region for plasmas with peaked temperature profiles. The spatial resolution will be moderate. Fast Wave Reflectometry is a new technique which offers the promise of making measurements in the centre of the plasma with good spatial resolution. It uses relatively
Fig. 11. Measured density profiles in the edge region of DIIID. Density contour data are shown for densities in the ranges 0 – 7 × 1018 m − 3 as a function of time and radius through two Edge Localised Modes (ELMs). From the radial scale it is seen that the density contours move outward by 4–5 cm at the onset of each ELM [69].
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Fig. 12. Schematic of the experimental test of the Fluid Flow Activation technique and some initial results [75].
simple hardware but requires development before an application on ITER can be planned [76]. The technique utilizes the property that the phase velocity of the fast wave propagating across the magnetic field depends on the ion mass density. It can be simply shown that measurement of the phase velocity in an interferometer arrangement gives the line integral of the ion mass density. Moreover, if the plasma consists of two or more ion species, the fast wave has ion-ion hybrid resonance(s). For a given probing frequency, the resonance and cut-off frequencies are uniquely determined by the fuel mix ratio, nT /nD. In a non-uniform magnetic field, as in a tokamak, it is possible to determine the spatial dependence of nT /nD by measuring the position of the cut-off with a frequency swept reflectometer. For typical conditions the probing frequency is 200 MHz which gives a wavelength in the plasma of 3 cm. Hardware at this frequency is simple to install and use. An initial test of the technique has been carried out on DIIID [77]. The transmitter antenna was a 30 cm long loop installed on outboard side at the midplane. The signal detectors were single-turn B-dot loops having diameter of 2.5 cm. Measurements of the line integral of the mass density show good agreement with the line integral of the electron density measured with a CO2 interferometer (Fig. 13). Further development is necessary to develop the capability of measuring the fuel mix ratio but the initial results are encouraging.
5.5. Intense diagnostic neutral beam The use of a short pulse, intense, ion beam for CXRS on ITER, rather than a conventional steady-state beam, has several advantages. The beam would increase the signal-to-noise ratio in the measurement because the relatively high-intensity, coupled with very short gating times on the detectors, reduces the bremsstrahlung background. Gains in signal/noise of more than an order of magnitude would be possible. Also, the beam would be significantly smaller and would have a lower average power requirement (500
Fig. 13. The electron density measured with a conventional interferometer (noisier curve) and the ion mass density measured from the Alfve´ n speed curve (quieter curve) on DIIID [77].
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kW). However, such beams are at an early stage of development. Initial developments have been carried out at the Los Alamos National Laboratory [78–80]. A diode and accelerator, which would be a suitable source for a beam is under development. It is known as the Continuous High-Average power Microsecond Pulser (CHAMP) and is designed to produce a 15 kA, 250 kV, 1 ms ion beam with a technology capable of being rep-rated at up to 30 Hz, although initial tests were single shot. Initial results are encouraging but show that it may be difficult to meet the requirements on a key parameter, beam divergence. Because the beam would have to be set back from the plasma on ITER a divergence of 50.5° is required. The best values achieved so far are about 0.8° but in these tests no particular effort was made to optimise this parameter. Further work should examine methods of actively focusing or guiding the beam before it is neutralized to reduce the constraint on beam divergence.
6. Concluding remarks During the EDA, the ITER diagnostic R&D programme has investigated the effects of neutron and gamma radiation on candidate materials and prototype diagnostic components that could be used in diagnostic construction. An extensive data base of the results of the tests has been established and is available for diagnostic system designers. In addition, specific studies have been carried on critical components subject to particular hazards; for example diagnostic first mirrors which will be exposed to CX sputtering leading to erosion. The application of some diagnostic techniques depends on the successful development of new components. The critical components have been identified and the necessary developments initiated. For a few of the required measurements, new measurement techniques and approaches are needed. Promising techniques have been identified and are being developed in current work. Further developments are required in some areas and these are currently being addressed. In total, the activity has established a firm base on which
techniques can be selected for ITER, and systems and components can be designed.
Acknowledgements The authors would like to thank Dr E Hodgson (CIEMAT, Spain), Dr Krasilnikov (TRINITI Institute, Russia), Prof. T Shikama (Tohoku University, Japan) and Dr S Yamamoto (ITER Joint Central Team, Germany) for useful comments; Dr G Vayakis (ITER Joint Central Team, Japan) for help in compiling Fig. 2, and Dr A Kislyakov (Ioffe Institute, Russia) for preparing Fig. 9.
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