LASER-AIDED PLASMA DIAGNOSTICS FOR ITER. Euregio Cluster, P.O. Box 1207, 3430 BE Nieuwegein, The Netherlands;

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1 LASER-AIDED PLASMA DIAGNOSTICS FOR ITER A.J.H. DONNÉ, 1 O. BUZHINSKIJ, 2 A.E. COSTLEY, 3 K. EBISAWA, 3 S. KASAI, 4 A. KOCH, 5 T. KONDOH, 4 I. MOSKALENKO, 6 P. NIELSEN, 7 G. RAZDOBARIN, 8 G. VAYAKIS, 3 C. WALKER, 9 V. ZAVERIAEV 6 1 FOM-Institute for Plasma Physics Rijnhuizen, Associatie EURATOM-FOM, Trilateral Euregio Cluster, P.O. Box 1207, 3430 BE Nieuwegein, The Netherlands; 2 TRINITY, Troitsk, Russia; 3 ITER JCT, Naka Joint Work Site, Japan; 4 JAERI, Naka, Japan; 5 Technical University, Munich, Germany; 6 Kurchatov Institute, Moscow, Russia; 7 Consorzio RFX, Padova, Italy; 8 Ioffe Institute, St. Petersburg, Russia; 9 ITER JCT, Garching Joint Work Site, Germany. Diagnostics form an integral part of the International Thermonuclear Experimental Reactor (ITER) project and must ensure proper operation of the device for a range of different operating scenarios. First, a brief introduction of ITER will be given with the emphasis on the specific physical environment in which diagnostics have to operate. Then the paper will focus on those plasma parameters that can be measured by means of laser-aided plasma diagnostics. A brief description of a number of these systems is included. 1 Introduction The Engineering Design Activity of the International Thermonuclear Experimental Reactor (ITER) was completed in July The main parameters of the machine are now fixed and the design of the main machine components has been developed to an advanced level (see Fig. 1). In the next phase of the project site specific topics will be developed. The design of ITER-FEAT is flexible in the sense that the machine will be capable of operating in a range of scenarios: standard inductive ELMy H-mode, hybrid (extended pulse) and ultimately steady state. 1 An extensive range of plasma measurements is required to support the various operation scenarios. The exact measurement requirements vary according to the scenario and the principal goals of the experimental programme. 2,3 An extensive list of diagnostic systems has been designed for ITER-FEAT to meet these requir e- ments and to withstand the hostile environment. 4,5 First a brief description of the ITER device and the specific impact of the ITER environment on diagnostic design in general will be given. Then a number of laser-aided diagnostics will be presented. Some laser diagnostics with potential for application on ITER are not yet in the list of selected systems, at least partly because specific feasibility studies and/or a conceptual designs have not been carried out. A number of these diagnostics are also outlined. 2 ITER-FEAT The technical requirements of ITER-FEAT are: 1

2 To achieve extended burn in inductively driven plasmas at Q > 10 (Q is the ratio of generated fusion power over the input power); To aim at demonstrating steady-state operation by non-inductive current drive at Q > 5; Not to preclude controlled ignition. On the engineering side, ITER-FEAT should be able to: Demonstrate and integrate essential fusion technologies; Test components for future reactors including tritium breeding blankets with an average neutron flux >0.5 MW m -2, and an average neutron fluence >0.3 MW a m -2. Fig. 1. ITER tokamak cut-away. The upper, equatorial and divertor ports can be clearly seen. 3 ITER-FEAT: a harsh environment for diagnostics The implementation of diagnostics on ITER is one of the most challenging tasks ever encountered in fusion research. This is because the measurement requirements are, in general, the same or more stringent than on present day devices while the environment in which the diagnostics have to be implemented is much harsher. Moreover, in view of the key role many of the measurements will play in real-time control of the plasma (e.g. in the stabilisation of neo-classical tearing and resistive wall modes, and active control of the presence and properties of internal transport barriers), a very high level of reliability is required. 2

3 Compared to large-scale tokamak devices like JET, JT-60U and TFTR, the conditions for diagnostics on ITER will be much harsher: 6 neutral particle fluxes are about 5 times higher, neutron flux levels up to 5 times higher, the neutron fluence is about 10,000 times higher, pulse lengths are about 100 times longer. In practice this implies that diagnostic components in ITER will have to cope with a much more challenging environment compared to those in present-day devices. Components installed inside the vacuum vessel and close to the first wall will be in the harshest environment, since here will be the highest neutron and gamma fluxes, the highest neutral particle fluxes from charge exchange processes at the plasma edge, and the most substantial heat loads from plasma radiation. Although the poloidal field (PF) inside the machine is similar to present devices (~ 1 T), and its time variation is relatively slow (~ 1 s), the scale of the ITER PF set and buildings means that, at the locations of sources and instrumentation the magnetic field is rather high (extremes at cryostat: 0.2; in pit ~0.1; in diagnostic hall ~0.05 T). An additional hazard for components mounted closely behind the first wall is the re-deposition of material that has been evaporated from the divertor and first wall. In the light of this, the most critical design issue for the optical diagnostics is the survivability of the first mirrors: these must maintain a good optical quality in the presence of all the above mentioned effects. Study of the first mirror aspects and development of candidate mirror materials are being pursued in a co-ordinated diagnostic R&D programme. 7 The neutron/gamma radiation (up to 8 Wcm -3 ) in combination with plasma radiation (up to ~50 kwm -2 ) will lead to a substantial heating of all plasma facing components implying that all materials have to be carefully chosen and components appropriately designed. Furthermore, radiation-induced effects can alter the electrical properties of insulators as well as the optical properties of lens and window materials. A particular notorious effect is radiation induced absorption that prevents the use of refractive optical components to directly view the plasma. Instead actively cooled mirrors need to be employed as the first optical element. The material and coating of these mirrors must be carefully chosen to minimise potential changes in the reflectivity and possible physical damage by any of the above mentioned effects. The measurements required for ITER are grouped into three categories: those required for machine protection and basic control, for advanced control, and for physics evaluation. An extensive list of all control measurements and the potential diagnostics to yield these measurements has been developed. 2,3 In Table 1 only those parameters that can be potentially measured by laser-aided diagnostics are listed. Of the full ITER complement of ~ 40 diagnostic systems, the ones able to measure these parameters are included in the table, whether laser-aided or not. As far as possible the designs of these systems are based on existing techniques, but in some cases novel approaches have to be adopted. A number of the laser-aided ITER diagnostics will be described in more detail in the following section. Given the large number of laser-aided diagnostics along with the limited number of pages left for this review, it is unfortunately not possible to describe the systems to a large level of detail. 3

4 4 Laser-Aided Diagnostics for ITER-FEAT In broad terms we can divide the laser-aided diagnostics for ITER into a number of different types of systems: incoherent Thomson scattering (including LIDAR and conventional Thomson scattering), coherent Thomson scattering (to diagnose α-particles, fast ions and density fluctuations), interferometry/polarimetry, reflectometry (which is strictly speaking not a laser-aided diagnostic, but is taken into account here, since it is always explicitly on the scientific programme of the LAPD meetings). laser-induced fluorescence, speckle interferometry and laser ablation. In the following sub-sections various planned or proposed laser-aided plasma diagnostics will be briefly presented. Table 1: Overview of the plasma parameters in ITER which can be possibly diagnosed by means of laseraided techniques. For completion also the non laser-aided diagnostics are indicated. The systems in italics have not yet been selected for ITER but are under consideration. Parameter Possible laser diagnostic Other diagnostics Machine protection and basic control: Line-averaged density Interferometer/polarimeter Edge Localized Modes (type and occurrence) Reflectometry ECE, H α -spectroscopy Gap between separatrix and wall Reflectometry (long pulses) Magnetics Advanced control MHD activity Reflectometry Magnetics, ECE Electron temperature profile (core) LIDAR ECE Electron density profile (core) LIDAR, Reflectometry, Interferometer/polarimeter Electron density profile (edge) Reflectometry Current density profile Polarimetry Motional Stark Effect Electron density and temperature (divertor) Reflectometry, ECA Thomson scattering Erosion of divertor tiles Speckle interferometry Impurity monitors Physics evaluation Fishbones, TAE modes Reflectometry Magnetics, ECE Confined α particles Collective scattering Knock-on tail neutron spectroscopy, Neutral Particle Analysis (NPA) n T, n D, n H (edge) Laser Induced Fluorescence NPA, H α -spectr. Electron temperature profile (edge) Edge Thomson scattering Electron density and temperature (X- X-point LIDAR point) Electron density fluctuations Collective scattering, Reflectometry, microwave scattering 4

5 4.1 Incoherent Thomson scattering The main emphasis of incoherent Thomson scattering is to measure the electron temperature and density profile in various parts of the plasma including the core, edge, X-point and divertor. The specific measurement requirements for parameters that are accessible by means incoherent Tho m- son scattering are listed in Table 2. The spatial resolution required for the measurement of the T e and n e profiles in the plasma core and across the X-point region is achievable by means of LIDAR systems, which has obvious advantages above conventional Thomson scattering systems because less port space is needed for access. For the measurement of the parameters in the plasma edge region (see Fig. 2), and along the length of the leg in the divertor, a high spatial resolution is required and two 'conventional' Thomson scattering systems are envisaged. The various incoherent Thomson scattering systems are described elsewhere in these proceedings. 8 Table 2: Measurement requirements for the electron temperature profile in the various regions of the plasma Measurement Parameter Condition Range or Coverage D T D X Accuracy Electron Temp erature Core Te r/a < kev a/30 10 ms Profile Edge Te r/a > kev 0.5 cm 10 % Electron Density Profile Edge ne r/a > m ms Core ne r/a < m -3 a/ cm 5 % Divertor electron parameters Te ev 3 mm across leg ne m cm along leg, 1 ms 20 % Fig. 2. Cut-away diagram of one of the upper ports, showing the laser beam and the collection optics of the edge Thomson scattering. 4.2 Coherent Thomson scattering Coherent or collective Thomson scattering is a possible diagnostic to measure the parameters of the confined α-particle population (and in principle also that of the fast ions; see Table 3). Further, in principle the technique can also be used for the measurement of the electron density fluctuations but thus far no detailed study has been done on this possible application on ITER. Apart from some conceptual studies carried out earlier in the EDA, the same is true for the fast ion collective Thomson scattering system. The fact that no detailed study has been done for fast ion collective Thomson scattering on ITER can be attributed partly to the difficulties that have been experienced with experimental sys- 5

6 tems on present-day devices and the limited results that have been obtained thus far. Two wavelengths can be considered for implementing fast ion Thomson scattering: the 10 µm-region (employing CO 2 -lasers as source), and the mm-range (using gyrotrons as a source). Another option to use a wavelength near 100 µm has been considered but was discarded at an early stage because potential sources in this wavelength range (e.g. Free Electron Lasers employing super-conducting linear accelerators) are bulky, expensive and still in the development phase. The CO 2 -type of fast ion collective scattering system is being explored by a US-Japanese collaboration on JT-60U (see Fig. 3). 9 At the time of writing this paper, the JT-60U system is in the commissioning phase. Fast ion collective scattering systems in the mm- and sub-mm wavelength region have been implemented on a number of devices, employing either D 2 O lasers (TCA and W7-AS) or gyrotrons (JET) as a source. Although first results have been reported from all these devices, the diagnostics did not reach the status of becoming a routine diagnostic, until an important step forward was recently made with a gyrotron-based system on TEXTOR With this system fast ion scattering spectra are routinely gathered with a time resolution of 4 ms during the 200 ms long gyrotron pulse. Table 3: Measurement requirements for the fast alpha population Measurement Parameter Condition Range or Coverage D T D X Accuracy Confined alphas Energy spectrum Energy resolution TBD MeV Density profile m ms a/10 20 % Fig. 3. Set-up of the fast ion collective Thomson scattering on JT-60U. 9 The experts in the field of collective Thomson scattering should make detailed feasibility studies to determine the optimum wavelength and geometry for ITER. The system is included in the diagnostics set for ITER-FEAT. However, no port has been allocated to it, because a conceptual design is not available. 6

7 4.3 Interferometry/polarimetry Interferometry and polarimetry will be used to measure a number of plasma parameters (see Table 4). Although the density profile is listed in this table, it is not envisaged that interferometry and/or polarimetry will meet the spatial resolution of a/30 that is for the detailed profile measurement. Table 4: Measurement requirements for parameters that are accessible by means of interferometry/polarimetry. Measurement Parameter Condition Range or Coverage D T D X Accuracy Line averaged electron density Default m -3 1 % ne dl / dl After killer pellet m -3 1 ms integral 100% Electron Density Core ne r/a < m -3 a/30 Profile Edge ne r/a > m ms 5 % 0.5 cm Current density profile % q(r) Physics study a/20 10 ms 5 - TBD 0.5 r(q=1.5,2)/a NTM feedback cm / a r(q min)/a Reverse shear control s - 5 cm / a The line-averaged density, and with low spatial resolution the density profile, will be measured by means of a vibration-compensated tangential polarimeter system. 11 The advantage of a polarimeter over an interferometer is that, provided the rotation can be kept below 2π (met for 10.6 µm), the system is not vulnerable to fringe jumps. The system features a fan of six tangential viewing chords and operates at wavelengths of 10.6 and 5.3 µm. Small retro-reflectors viewing the beams will be mounted in the gaps between blanket modules and in a number of diagnostic ports at the low-field side of the tokamak. The main function of the tangential system is to measure the lineaveraged density, albeit that to a limited extent some profile information (up to 6 six points) is also obtained. One of the main concerns for the tangential polarimeter is whether or not the operation of the retro-reflectors will be deteriorated due to erosion and deposition effects. The current density profile is one of the most difficult parameters to measure in ITER. The standard techniques to diagnose this quantity at present-day devices are polarimetry and the Motional Stark Effect. Both techniques have substantial difficulties when it comes to implementation on ITER. The proposed polarimeter system that has been proposed has a poloidal fan of viewing chords and is operating at a wavelength of 118 µm (see Fig. 4). 12 The 'pivot point' of the fans is located at the front end of the equatorial port plug. Small 37 mm diameter retro-reflectors are embedded in remote handling slots in the center of the blanket modules at the high-field side. An important issue in the design of the polarimeter system is how to maintain the alignment of the laser beams onto the small retro-reflectors over such a large distance. An automated scanning and alignment system is envisaged to ensure optimal alignment during the discharge. 7

8 4.4 Reflectometry Reflectometry is a diagnostic with a potential to measure many different plasma parameters (see Table 5). Three reflectometer systems are presently planned with as main tasks the measurement of the electron density profile in the main plasma (with emphasis on the edge) and in the divertor plasma, as well measuring the plasma position (in particular the gap between first wall and separatix) in long pulse discharges. The reflectometer for the main plasma consists of three sub-systems: a system operating in the extra-ordinary (X) mode at the upper cutoff from various poloidal directions to provide measurements of the density profile in the scrape-off layer and to determine the distance between the separatrix and the first wall in long-pulse discharges, a system operating in the ordinary (O) mode from both the low- and high-field sides to yield the edge density profiles at both sides of the plasma, an X-mode system operating on the lower cutoff from the high-field side to provide the core electron density profile with a reasonable spatial resolution. Although the measurements of the electron density profile as well as the distance from separatrix to first wall are the prime objectives of the above three systems, it is envisaged that information on many of the parameters mentioned in Table 5 is also accessible by means of the same systems. The divertor reflectometer system should provide the density profile along the divertor legs. For this purpose multiple sight lines are employed. Profile information must be synthesized from a range of frequency bands because of the extremely wide density range. Fig. 4. Possible viewing geometry for the poloidal polarimeter system for ITER. The viewing lines through the upper port are not in the current system design, but are under consideration since they could enhance the accuracy of the measurements in the region of negative magnetic shear in the case of advanced tokamak operation. The key engineering issue for reflectometry is the installation of the waveguides and antennas in particular on the high-field side. It is planned to locate the antennas in the gaps between adjacent blanket modules. The waveguides will run behind the blanket modules to one of the upper ports. On the physics side there are also a number of question marks. For the gap measurements it should be studied whether the position of the separatrix can indeed be deduced from a reflectometric measurements of the position of the cutoff layer. In recent experiments at ASDEX-UG it was demonstrated that a reflectometer system is able to track changes in the plasma position. 13 It has not yet 8

9 been tested though to use the reflectometry signal for real-time feedback of the plasma position. Another issue is that it is not yet clear how the rotation of microscopic density fluctuations that can be measured by means of reflectometry is related to the fluid rotation of the plasma. In all experiments that have been done thus far, the two rotations have been found to agree within the errors of measurement (- one recent example can be found in ref.14). However, the mechanism behind this is not clear and also it is questioned whether this will hold under all circumstances. Table 5: Measurement requirements for parameters that are diagnosed by means of reflectometry. The requirements for the measurement of the density profile at the edge and in the divertor are already given in Table 2 and are not repeated here. Measurement Parameter Condition Range or Coverage D T D X Accuracy Ip > 2 MA, Plasma position Main plasma 1 cm full bore - 10 ms - and shape gaps, sep Ip Quench 2 cm Edge Localized Modes ELM density transient r/a > 0.9 TBD TBD TBD TBD Plasma rotation Divertor operational parameters High frequency macroinstabilities V TOR V POL Position of ionisation front Fishbone instabilities TAE modes km/s 10 ms a/30 30% 1-50 km/s 0 - TBD m 1 ms 10 cm - TBD khz (m,n) = (1,1) khz n = Laser Induced Fluorescence LIF can potentially provide measurements of the He density, the density of extrinsic impurities (e.g., Ne, Ar, Kr), and the ion temperature in the divertor (see Table 6). It is proposed to use a diverging laser beam to increase the reliability of maintaining sufficient overlap between the input and the collection beams. The predicted s/n is typically 25. It should be also possible to measure the ion temperature. Recent experiments of T I (Ar II) on the PNX-U device demonstrated that it should be possible to reach a relative accuracy of <20%. Table 6: Requirements for measurements that are accessible with laser-induced fluorescence. Measurement Parameter Condition Impurity concentration & in influx Divertor helium density Extrinsic (Ne,Ar,Kr) rel. conc. Extrinsic (Ne,Ar,Kr) influx Range or Cove r- age D T D X Accuracy s ms integral 10% relative n He m -3 1 ms 20% Ion temperature in divertor Ti ev 1 ms 10 cm along leg, 3 mm across leg 20% 9

10 4.6 Speckle interferometry and laser ablation Although most diagnostics envisaged for ITER are focused on the measurement of actual plasma parameters, a number of diagnostic systems need to be implemented for measuring the condition of the divertor and first wall surfaces (see Table 7). An example is the real-time measurement of the erosion of the divertor target plates. A method that has been proposed for this purpose is speckle interferometry. 15 In this technique the erosion rate can be deduced from the speckle images of the footprint of a laser beam on the divertor tiles. In a proof-of-principle experiment it was demonstrated that the system can measure height changes (due to erosion) of 100 nm with a spatial resolution of about 0.5 mm. The time resolution is essentially determined by the repetition rate of the laser. Although the system has no difficulty at all to meet the measurement requirements, the basic difficulty lies in the fact that measurements are obtained at a single location only in the divertor. An alternative technique that has been proposed for the same purpose is a copper-vapour laser viewing system for real-time monitoring of the divertor and first wall (see Fig. 5). 16 The given technique is based on the intensity amplification effect of the surveyed surface image with a simultaneous spectral and time filtration of noise. The high repetition rate of the laser makes it possible to monitor object changes in real time (typically 0.1 ms). A panoramic scanning of the laser beam makes it possible to inspect a large area of divertor and first wall surfaces, in spite of the presence of plasma background radiation. A spatial resolution at the object plane about 1 mm was achieved in laboratory tests. The system can measure thickness changes of about 0.5 mm in a time interval of 1 ms. The design and test of the laser beam scanning system, capable to operate with the required accuracy under ITER conditions is the first step in the further system development. Furthermore, it will be necessary to study the effect of vibrations, temperature and magnetic fields on the measurement. Fig. 5. Block scheme of the copper vapor laser viewing system. 1 Copper vapor laser generator, 2 Copper vapor laser amplifier, 3 Pumping pulse source, 4 Pumping pulse source, 5 Object under investigation, 6 Plasma volume, 7 Wall aperture, 8 Receiving objective, 9 Optical path, 10 Matching optics, 11 Lens, 12 CCD matrix, 13 Computer, 14 Synchronizer, 15 Tunable time delay. 10

11 Table 7: Requirements for some machine related parameters that can be measured by laser diagnostics. Measurement Parameter Condition Range or Coverage D T D X Accuracy Divertor operational parameters Presence and quantity of dust Real-time net erosion 0 3 mm 1 s 1 cm 10 % Dust concentration TBD TBD TBD TBD TBD Another important parameter that has to be determined is the concentration and distribution of dust in the machine. From a diagnostic point of view, it is important to know the minimum specific measurement requirements so that an appropriate measurement system can be designed. The main difficulty is that the safety limits for dust are set in terms of global quantities (total amount of dust on hot and cold surfaces), whereas only local measurements of dust can be envisaged. Hence a model or method will be needed to extrapolate from the local measurements to the global quantities. The requirements for dust measurements can not be developed in isolation but have to be developed in the context of the overall strategy of dealing with dust including the clean-up techniques. An active laser ablation technique has been proposed to measure the quantity and composition of dust in the divertor and blanket modules viewing from equatorial port by scanning the laser beam. 17 By scanning the laser beam and collection optics a limited survey can be made (see Fig. 6). A successful proof of principle has been conducted. Key issues are the calibration and the use of local measurements in the determination of total dust quantities. It may be possible to extend the technique to measure codeposited hydrogenic species (especially T). This could be very important since these deposits can potentially trap significant quantities of tritium. Fig. 6. Poloidal view of the divertor region in ITER-FEAT showing the probe/viewing diagnostic beams. The inset shows the components in the diagnostic equatorial port plug. 11

12 5 Summary Several laser aided diagnostic systems are planned for ITER and are expected to provide the measurement of key core and divertor plasma parameters (such as electron density, electron temperature, etc.). The designs and integration of the systems into the tokamak are well advanced and a performance meeting, or in some cases exceeding, the target measurement specifications can be expected. Systems for measuring the fast ion population (by collective scattering), the density of impurities including He (LIF), and the condition of the first wall and divertor plates (speckle interferometry, copper vapour laser viewing) are also in principle possible and may be incorporated if subsequent design work shows this to be beneficial and feasible. Acknowledgements This report is an account of work undertaken within the framework of the ITER EDA Agreement. The views and opinions expressed herein do not necessarily reflect those of the Parties to the ITER Agreement, the IAEA or any agency thereof. Dissemination of the information in this paper is go v- erned by the applicable terms of the ITER EDA Agreement. References 1 Y. Shimomura, et al., Proc. 18th IAEA Fusion Energy Conf. (2000) Sorrento, Italy in press. 2 A.J.H. Donné, et al., Proc. 18 th IAEA Fusion Energy Conf. (2000) Sorrento, Italy, in press. 3 ITER Physics Expert Group on Diagnostics, et al., Nucl. Fusion 39 (1999) K. Ebisawa, et al., Rev. Sci. Instrum. 72 (2001) A.E. Costley, et al., Proc. 28 th EPS Conf. on Controlled Fusion and Plasma Physics (2001), Funchal, Portugal, in press. 6 A.E. Costley, et al., Proc. 9 th Int. Symp. On Laser-Aided Plasma Diagnostics (1999) Lake Tahoe, USA, p. 7 V. Voitsenya, et al., Rev. Sci. Instrum. 72 (2001) P. Nielsen, These Proceedings. 9 T. Kondoh et al., These Proceedings and T. Kondoh et al., Rev. Sci. Instrum. 72 (2001) H. Bindslev, et al., These Proceedings. 11 T.N. Carlstrom, et al., in Diagnostics for Experimental Thermonuclear Fusion Reactors 2 (Plenum Press, New York, 1998), p A.J.H. Donné, et al., Rev. Sci. Instrum. 70 (1999) M.E. Manso, in Proc. 5 th Int. Workshop on Reflectometry, Toki (2001), NIFS-PROC-49, p M. Hirsch et al., Proc. 5 th Int. Workshop on Reflectrometry, Toki (2001), NIFS-PROC-49, p E. Berger, et al., Appl. Opt. 38 (1999) O.I.Buzhinsky et al., submitted for publication in Fusion Eng. Des. 17 G.T. Razdobarin et.al., submitted for publication in Fusion Sci. Techn. 12