WDS'08 Proceedings of Contributed Papers, Part II, 100 104, 2008. ISBN 978-80-7378-066-1 MATFYZPRESS Design of New Optical System for Visible Plasma Radiation Measurements at COMPASS Tokamak D. I. Naydenkova 1,2,*, V. Weinzettl 2, J. Stockel 2, D. Šesták 2, M. Aftanas 1,2 1 Charles University in Prague, Faculty of Mathematics and Physics, Ke Karlovu 3, Prague 2, 18000 2 Institute of Plasma Physics AS CR, v.v.i, Za Slovankou 3, Prague 8, 18200 Abstract. The paper outlines main parameters and characteristics of the currently designed multichannel optical system for visible plasma radiation measurements on the COMPASS tokamak. It based on registration of visible light in large observation angle (~70-80º) for 35 lines of sights by means of detectors (photodiode or photomultiplier array) and spectrometer. System has resolution about 1cm in vertical direction. The aim of the introduced system is to get information about parameters of tokamak plasma (impurity content and their distribution) and fast prosesses which take place in it. Introduction The COMPASS tokamak, a divertor device with a clear H-mode and ITER- relevant geometry (R=0.56 m, a=0.23 x 0.38 m, Ip=200-400 ka, B T =1.2-2.1 T and pulse length up to 1 s), is being reinstalled to IPP Prague from Culham in U.K. [The COMPASS project [1983]]. New diagnostic tools will be built to address aims of the COMPASS scientific program focussed on H-mode physics and pedestal investigations. Among spectroscopic diagnostics, an optical system for visible plasma radiation measurements is being designed to be used to obtain information on hydrogen and impurity emission and its evolution during tokamak discharges. Monitoring of integral plasma radiation in the visible range (400-800 nm) is planned for the first stage of the COMPASS operation. Complementary, this system can be used for measurement of specific spectral lines, like hydrogen or the most intense impurity lines (see Table 1). Consequently, information on neutral atoms density, impurity inflow, recycling processes and rough estimation of particle confinement can be derived. Also an MHD activity can be studied using such observations. The effective ionic charge Z eff will be evaluated from measured bremsstrahlung radiation in the line free region slightly above 520 nm using known plasma density and temperature [K.Ida et.al]. In future, a modified system will be used for plasma rotation estimation. Similar diagnostic system are designed and used in TCV tokamak [H. Weisen at. al], for example, and in many other devises. Optical system design The tomographic systems [see for example M. Anton et. al.], including a new optical system for visible plasma radiation measurement, will be installed in the sector 6/7 of the COMPASS tokamak (Figure 1). The first and essential part of the optical system will use the angular upper diagnostic port (flange NW100) together with soft X-Ray and bolometric detectors. Other parts will be designed in the same manner and install in the port located in the same poloidal cross-section below midplane (see Figure 1). In the frame of investigations, a measurement of spectrally integrated visible radiation will be carried out both from the plasma core and from the plasma edge. To reach both regions, i.e. an upper half of the plasma column and approximately 50 mm under horizontal midplane, an observation of plasma at as large angles as about 70-80 is necessary, limited mainly by engineering reasons. A design of the optical system is quite complex, as it is shown at Fig. 2, because of the requirements mentioned in the next paragraphs. The optical system is based on a pinhole camera equipped by the focusing lens giving a higher throughput, both ensuring a spatial resolution in the vertical direction. The slit option will use 2 slits, rotated by 90, to control the vertical and toroidal * e-mail: naydenkova@ipp.cas.cz 100
resolutions separately ensuring a minimal light intensity necessary for the registration. Because of a large observation angle of the system the focusing lenses are planed to be inserted into the diagnostic port. It creates a lot of problems connected with its design. Visible light detectors will be located far away from the tokamak because they are sensitive to X-ray radiation from plasma. Twenty meters optical cables will be used to realise it. A shutter located in front of the slit protects the optical system from an impurity deposition on optical components during vessel cleaning procedures. Similar system observing point Figure 1. Observation angle of optical diagnostic in the poloidal cross-section of COMPASS tokamak in sector 6/7. Table 1. Spectral lines planned to be monitored. state λ, nm state λ, nm state λ, nm state λ, nm O V 412.4 He II 468.6 C II 514.5 C III 569.6 N V 433.3 He I 471.3 Be II 527.0 C II 588.9 H I (Hg) 434.0 H I (Hβ) 486.1 N V 527.5 O V 633.0 O V 454.9 Dβ 486.0 C VI 529.0 N V 634.3 N V 460.4 He I 492.2 C II 503.2 O V 646.6 N V 462.0 C V 494.5 O V 511.4 H I (Hα) 656.3 C III 464.7 HeI 501.6 C II 534.2 Dα 656.1 C III 465.0 C II 503.2 He I 541.1 C II 658.1 C III 465.1 He I 504.8 O V 559.8 He I 667.8 C IV 465.8 N V 506.8 N II 567.9 He I 706.5 He I 728.1 Transmitted light through the slit is collimated into a parallel beam using a set of lenses. A narrow wavelength range can be detected by interference filters. Afterward the light is collected by a linear set of optical fibres. Each fibre corresponds to one spatial channel. Designed system is quite flexible and output of the fibres is connected to a multi-channel detector if we are going to monitor the integral plasma radiation or to spectrometer (measure hydrogen and impurity spectral lines) [J. Figueiredo et.al]. It is possible to use also interference filters (bremsstrahlung radiation measured for Z eff estimation). Therefore, system changes its configuration depending on a goal of the experiment (see introduction). 101
Figure 2. Assembly view of Optical System for Visible Plasma Radiation Measurements:1 shutter, 2 slit, 3 focusing system, 4 vacuum window, 5 optical fibres, 6 detectors. Set-up details The scheme of the diagnostic port enclosure is shown in Fig. 3. The front part containing soft X- ray detectors, AXUV-based bolometers and a free space for a slit and lenses of the optical system is installed into the diagnostic port. The back flange is the atmosphere side, where the quartz vacuum window (NW35), a hole for shutter (NW16) and two 41-pins electrical feedthroughs will be installed. Figure 3. Scheme of the diagnostic port enclosure. Plasma side. 1- Soft X-ray detector, 2 - bolometers, 3- focusing system space, 4- hole for cooling, 5- vacuum window flange. All elements of the focusing system and the vacuum window will be made from UF fused silica. This typically used optical material fully meets tokamak requirements, such as high vacuum need (~10-6 Pa). It also survives high temperature during baking (110ºC) of the vacuum vessel. It is resistant against neutron fluxes and highly transparent for measured wavelengths radiation. All used materials were chosen to be made from non-magnetic materials. Elements sensitive to high temperatures (soft X-Ray and bolometric detectors) have to be protected during a baking of the vacuum vessel leading to the flange heating by designed cooling system. It gives in the result a sandwich structure of the diagnostic port enclosure. 102
Light is transferred from the tokamak to the diagnostic room by means of optical cables of length of approximately 20 meters. For such distance, the transmission of the optical fibres plays a key role in a selection of the fibres material. Each cable will contain 36 fibres. Majority of them will be used as detectors channels (see Table 2) and one will be exploited for spectrometric measurements. Because of asymmetry of our focusing system, a fibre will be set asymmetrically against the vacuum window in line row, each fiber will be placed above each other at the tokamak side, it means the line of sights will be placed above each other in line in vertical direction. From the detector side the fibres position is resulted from the detector elements size and numerical aperture of the optical fibres. To make this system more flexible, SMA (SubMiniature version A) connectors will be used. More details about the tokamak and detector sides connection are seen in figure 4 and 5. Table 2. Detectors parameters. photomultiplier photodiode Channels (line of sights) 32 35 Spectral responce 185-880nm 190-1000nm Rise time 0.6ns 0.1μs Max. sensitivity 60mA/W (without amplifier) 0.5A/W Fused silica with high OH (core and cladding) seems to be the best material for a wide wavelength range as 300-800 nm and compatible with neutron fluxes anticipated on COMPASS after the neutral beam injector installation. Typical transmittance is 80% for 20 m long fibre. The numerical aperture (dimensionless value proportional to the maximal angle over which the system can accept or emit light) of this optical fibre is 0.22. A selection of the fibres core diameter is given by following factors: core/cladding ratio, i.e. coverage of the focused light area, bend curvature, and finally the price. For about 32-5 spatial channels giving a spatial resolution in vertical direction of 1 cm, fibres with 200 µm core, 40 (60) µm cladding and 160 (140) µm buffer are a good compromise. A spatial resolution in toroidal direction is not important for the goals of our experiments. The cable will be consists of unjacketed wires gathered together and covered by a nonmagnetic material. The designed system is multichannel, therefore it is cheaper to use a multichannel detectors array than to buy many single detectors separately. The two detector arrays, H7260A (photomultiplier) and S4114 (photodiode) developed by the HAMAMATSU company, fulfil the system requirements in number of channels (given by spatial resolution), their spacing (limited by optical fibres), a spectral response range, dark current and rise time. The main disadvantage of photodiodes is a strong dependence of their photosensitivity on the wavelength. A significantly higher sensitivity of photomultipliers is balanced by a low price of photodiodes. Approximately the same width of the detectors (effective area 4.4x0.9 for photodiode and 7x0.8mm for photomultiplier) will allow changing cheaper photodiodes to better but more expensive photomultipliers in future. Figure 4. Tokamak side view of fibers connection. 1 - air side of diagnostic port enclosure, 2 - vacuum window, 3- row of fibres. 103
Moreover, the minispectrometer USB2000+ from Ocean Optics can be used to get the whole visible spectrum at once connecting its entrance slit to the output of the chosen optical fibres. Its main parameters are the sensitivity of 200 V/lx*s, the spectral resolution of 0.3-10 nm, and the integration time higher than 1 ms corresponding to the experimental program requirement in the first stage of the tokamak operation. It can be replaced by a spectrometer with better spectral resolution in future. Figure 5. Detector side view of fibers connection. 1 SMA connector, 2 detectors elements, 3 fibres, 4- fibres holder. Conclusion The new optical system for visible plasma radiation measurements at COMPASS tokamak is being designed. Arrays of SXR detectors and bolometers are integrated in the same diagnostic port as the visible plasma radiation system. Such configuration will allow observing the same plasma volume in several spectral ranges. Similar arrangement of detectors located at the same poloidal cross-section will allow fast tomography and a correlation analysis of turbulent plasma events. A spatial resolution at a centimetre scale will be introduced using collimating optics. The light emission of each spatial point will be collected by the optical fibre and led to the detector. The design meets some technical constraints like observation angle limitations, a spatial channels separation (as chords in a plasma, beams of the focused light, and finally as different detector channels), and a throughput of the whole system. In future, the parameters of the optical system will be slightly modified according to the first observations of plasma in the COMPASS tokamak and correspondingly to emergent physical needs. The diagnostic system has to be operational until end of 2008. Acknowledgment. the work was performed in the frame of the grant 202/08/H057 and supported by the INTAS project :05-100000088046 and the EFTS project EODI 042884. References M. Anton et al., X-ray tomography on the TCV tokamak, Plasma Physics Control Fusion 38 (1996) 1849-1878. The COMPASS project documents. Part II. UKAEA, Culham Laboratory. December 1983. J. Figueiredo et al., Plasma Spectroscopy in ISTTOK, in Plasma and Vision Science: 17th IAEA Technical Meeting on Research Using Small Fusion Devices. AIP Conference Proceedings, Vol.996(2008), pp. 213-218. K. Ida et al., Z eff measurements and low-z impurity transport for NBI and ICRF heated plasmas in JIPP T-IIU, Nuclear fusion, Vol.30, 4(1990), 665-674 H. Weisen et al., Multi-chord diagnostics on the TCV tokamak, in The 2 nd German-Polish Conference on Plasma Diagnostics for Fusion and Applications (GPPD-2004). Cracow, Poland, Sept. 8-10, 2004. 104