INTRODUCTION. IPFN (Instituto de Plasmas e Fusão Nuclear) has organized in October 2007 the Host Laboratory. Experiment on the tokamak ISTTOK.
ISSN 1063780X, Plasma Physics Reports, 2009, Vol. 35, No. 10, pp. 841–846. © Pleiades Publishing, Ltd., 2009. Published in Russian in Fizika Plazmy, 2009, Vol. 35, No. 10, pp. 914–920.
SMALL FUSION DEVICES
The International Joint Experiment ISTTOK1 H. Fernandesa, C. Silvaa, R. B. Gomesa, A. Netoa, P. Duartea, G. Van Oostb, M. Gryaznevichc, and A. Malaquiasd a
Associação Euratom/IST, Instituto de Plasmas e Fusão Nuclear, Lisbon, Portugal b Ghent University, Belgium c Euratom/UKAEA, Fusion Association, Culham Science Centre, Abingdon, United Kingdom d International Atomic Energy Agency, Vienna, Austria Received December 12, 2008; in final form, February 4, 2009
Abstract—The 3rd International Joint Experiment has been carried out on ISTTOK following the last RUSFD (technical meeting on Research Using Small Fusion Devices) conference, held at Lisbon, in the scope of the IAEA Coordinated Research Project. This program has allowed a great knowledge interchange among the “small” fusion devices community and, in particular, in the development of common workpro grams. This communication gives an overview of the impact of such activities. The JE have proved to be a very useful applied forum to share knowledge and to initiate young scientists in some fields of tokamak operation and diagnostics. Many of the ISTTOK 2008 relevant results were obtained under this scope. PACS numbers: 52.55.Fa DOI: 10.1134/S1063780X09100067 1
1. INTRODUCTION
meeting regularly organized to plan experiments and discuss results.
IPFN (Instituto de Plasmas e Fusão Nuclear) has organized in October 2007 the Host Laboratory Experiment on the tokamak ISTTOK. The Joint Experiment was planned in cooperation with the IAEA in the framework of the IAEA Coordinated Research Project (CRP) on “Joint Research Using Small Tokamaks” with the participation of 24 scien tists from 13 different laboratories. Taking into account the ISTTOK scientific pro gramme and the feedback from the preregistered par ticipants the following areas were explored during the ISTTOK JE: (i) Study of the poloidal structure of the edge fluctuations; (ii) Tokamak operation in alternat ing current regimes; and (iii) Testing of the liquid metal limiter concept. These activities were success fully carried out being distributed in 5 experimental sessions. Other areas related with plasma engineering and diagnostics were also investigated, although with no dedicated experimental sessions attributed. These areas, organized in small workshops, included: (i) plasma diagnostics; (ii) plasma control; (iii) data acquisition; and (iv) remote data access. Remote experimental sessions in the following weeks were also planned to complement the experi ments performed during the JE in ISTTOK. The par ticipants could run experiments using the ISTTOK remote participation tool FireSignal [1]. Working groups have been formed for data analysis and remote 1 The article is published in the original.
Transverse areas to ISTTOKJE were data acquisi tion, signal processing and remote access tools where IPFN has a long experience. These systems have been successfully used for data management on several tokamaks and they may serve as a platform for a uni fied environment for data exchange and processing in the framework of small tokamak activity (IAEA CRP). This program has allowed a great knowledge inter change among the “small” fusion devices community and, in particular in the development of common workprograms. This communication gives an over view of the impact of such activities, with the main emphasis on the following four main working areas: (i) Liquid gallium limiter; (ii) Control and data acqui sition; (iii) Tokamak AC operation; and (iv) Edge plasma physics studies. 2. LIQUID METAL LIMITER Presently one of the main challenges for nuclear fusion technology is related to plasmawall interac tion. In large size devices (including ITER), plasma facing components (PFC) are submitted to high power loads under steady state operation that could even reach the GW/m2 range during offnormal events in the divertor region. One possible solution for this issue is the use of liquid metal flows as they may provide an efficient mean to exhaust heat produced in the core plasma.
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The interaction of a liquid gallium jet with plasma has been investigated on ISTTOK [2]. A stable, free flying, fully formed liquid metal jet has been produced using hydrostatic pressure (1.6 mGa head) as driving mechanism. The developed stream of gallium has 2.5 mm diameter, 2.5 m/s flow velocity and a 13 cm breakuplength (continuous part of the jet before spontaneously decomposing into droplets due to Ray leigh instability). The aim of the experiment is to study the relevance of liquid metals as PFC focusing on their power extraction capability and dynamic behavior under the influence of plasmas and magnetic field gradients. A comparison of the tokamak discharges with and without the plasmaliquid gallium jet interaction has been performed [3]. It is possible to conclude that the presence of the jet does not change significantly the discharge performance and that radiation losses do not increase. The analysis of data clearly shows that the gallium jet only influences locally the ISTTOK plasma as no noteworthy gallium radiation has been observed toroidally away from the jet. Furthermore, no gallium emission is detected in discharges without the gallium jet, evidencing that there is no meaningful machine contamination. The experiments on ISTTOK proved that gallium jets are compatible with tokamak operation. A more detri mental effect on the plasma performance had been detected on a previous experiment, on T3M that used a galistan (gallium alloy) droplet curtain as a limiter [4]. The reported heat fluxes, on that device, have twice the values available for the experiment (up to
5 MW/m2) described in this work. Other major differ ences for ISTTOK are: (i) the use of high purity gal lium (99.9999%) with a lower content of higher atomic mass impurities (gallistan has 1/3 indium + tin com position) and (ii) a strong effort has been made to avoid the penetration of gallium droplets, backscat tered from the liquid metal collector, into the plasma core [3]. These are believed to provide a probable explanation for the difference in the observed results. Recent activities in this field have been concen trated on the study of the liquid gallium jet power removal capabilities. The response of an IR sensor, intended to perform the measurements of the gallium jet surface temperature, was investigated. When abso lutely calibrated, this diagnostic will provide an esti mate on the power extraction capability of such liquid metal jets in tokamaks. A MCT (Mercury—Cad mium–Tellurium) sensor operated at cryogenic tem perature (78 K) and specially designed to measure low temperature (>150°C), low emissivity materials like this kind of liquid metals has been tested. Since this sensor is sensitive in the infrared region of the spectra (peak efficiency from 6 to 7 µm) the chosen optics (viewing window and collection lens) needs to be transparent in this range. From the analyses of the obtained data, it has been possible to identify the heat ing of each individual droplet (the radiation detection is done 0.5 m away from the main tokamak chamber where the jet in droplets form) and also a shift of the jet due to electromagnetic forces (Fig. 1). It is possible to observe a wavy formation corresponding to the droplet signal passing in front of the detector. The disappear PLASMA PHYSICS REPORTS
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ance of this signal (at about 160 ms) for the jet at r = 68 mm can be related to the motion of the droplets that partially leave the sensor field of view (FOV), as shown by the arrow in Fig. 1. The signal for the jet at r = 58 mm appears only in the detector FOV when the discharge occurs due to the influence of these forces. Measurements performed at that viewing port using a fast frame CCD camera have confirmed the displace ment of the droplets. This was only noticeable when a discharge occurred (no motion when no gas was injected in the chamber). The increase on the MCT sensor response, above the preshot level shown in the spike amplitude for the r = 68 mm jet, clearly evi dences an increase on the droplet temperature due to plasma interaction. 3. CONTROL AND DATA ACQUISITION The activities in this area were based on a workshop with two main proposes: (i) the training of personnel for remote data access, were the methods for data retrieval and mining were teaches and (ii) a discussion about the actual methods for converging in a single approach that could be used among the community. Work allocation about future developments of data connectors among the CRP community has been pre PLASMA PHYSICS REPORTS
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pared foreseeing a unique approach of the type learn once/usemuch. The ISTTOK control and data acquisition have serve as a testbench of this ideas and proved to be suc cessful. Moreover this system has been tested with several platforms like IDL, Mathematica and MatLab, as a great diversity of users were present with different skills in highlevel languages for computational physics. 4. AC OPERATION One handicap of iron core tokamaks such as IST TOK is the limitation in the discharge duration due to its saturation. Typically, 40 ms discharges can be achieved in DC ISTTOK operation. With the use of two new power supplies and their controllers, the plasma position could be controlled and long duration AC discharges achieved. A 250ms discharge with ten 25ms halfcycles has been achieved (Fig. 2). As shown in Fig. 2, the plasma current is reduced in the later part of the discharge as the particle confinement degraded. Comparing the temporal evolution of the plasma density and Hα radi ation it can be concluded that recycling increases over
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the discharge time, possibly limiting the duration of the discharge. The temperature increase of the first wall components due to the longer discharge duration can contribute significantly to an increase of this hydrogen wall influx. A realtime gas puffing system has been recently implemented that could help in the recycling control. Other physics studies are being carried out compar ing the positive and negative plasma current half cycles, such as edge fluctuations and asymmetries in the plasma current reversal. Fluctuations in the edge quantities have been found to be higher in negative cycles in spite of the fact that plasma position tends to be slightly different in positive vs. negative cycles. 5. EDGE PLASMA PHYSICS STUDIES Recently, the plasma physics studies on ISTTOK were concentrated mainly on the characterization of the edge turbulence. In particular, a significant effort has been dedicated to the identification and character ization of zonalflow–like structures. Zonal flows are characterized by a symmetric structure potential in the poloidal and toroidal directions (m = n = 0) and a rap idly varying structure with finite radial wavelength [3]. Several experimental observations demonstrating the
existence of zonal flows have been obtained in the last few years (see [3, 5] and references therein). In partic ular, experimental measurements of geodesic acoustic mode GAM have shown characteristics consistent with the theoretical predictions. ISTTOK is equipped with two probe systems that allow the investigation of the edge fluctuations: (i) a 8pin radially movable poloidal array of Langmuir probes with a resolution of 2 mm, installed in an equa torial port; and (ii) a 8pin radial array of Langmuir probes with a spatial resolution down to 3 mm toroi dally located at about 120° from the poloidal array and installed near the top of the poloidal crosssection. Such an experimental arrangement allows the investi gation of the threedimensional characteristics of the edge fluctuations. It has been found that the ISTTOK fluctuations have distinct characteristics for r > a (SOL) and r ≤ a (edge plasma). Figure 3 shows the Vf crosscorrelation for pins poloidally separated by 4 and 8 mm, measured in the SOL and in the edge plasma regions, as well as the Vf autocorrelation. In the SOL, fluctuations are characterized by short correlations both in space and time (poloidal correlation length, λc ~ 10 mm and autocorrelation time, τc ~ 3–4 µs). However, in the plasma edge, the autocorrelation time is significantly larger, τc ~ 10 µs, and the poloidal crosscorrelation only shows a very small reduction across the 14 mm extension of the poloidal array (λc 14 mm). Poloidal wavenumbers in the range of kθ < 3 cm–1 and a broad frequency spectrum are observed in the SOL. Assuming a poloidally uniform structure, these wavenumbers correspond to poloidal mode numbers up to m = 25. In the edge plasma the wavenumbers are smaller, kθ < 1.0 cm–1, and the spectrum is dominated by low frequency components (10–25 kHz). Further more, kθ is close to zero for frequencies below 50 kHz consistent with a poloidally symmetric structure. The Isat fluctuations have also been investigated and comparable results obtained. However, contrary to the observed with Vf, the Isat fluctuations are not dominated by the low frequency fluctuations. The rel ative magnitude of the low frequency fluctuations (ratio of the spectrum amplitude in the range 10– 25 kHz to the total spectrum amplitude) is signifi cantly smaller for Isat than for Vf. As a consequence, the Isat fluctuations in the edge plasma show evidence of both turbulent and low frequency scales, clearly vis ible in the crosscorrelation. Results indicate that the characteristics of the potential fluctuations in the SOL are consistent with the typical broad band turbulent fluctuations, while in the edge plasma they are dominated by low frequency oscillations consistent with a symmetric structure in the poloidal direction, characteristic of the geodesic acoustic mode (GAM), which for the ISTTOK edge plasma is expected to have a frequency of ~20 kHz PLASMA PHYSICS REPORTS
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(Ti = Te = 20 eV). Furthermore, the amplitude of the density fluctuations in the 10–25 kHz range is signifi cantly smaller than that of the potential as expected from the GAM theoretical predictions [3]. Results suggest therefore the existence of GAMlike modes in the edge plasma region of the ISTTOK tokamak. PLASMA PHYSICS REPORTS
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GAMs have been shown to exist in most of the ISTTOK discharges recently performed (Ip = 3–5 kA, n = (3–4) × 1018 m–3). Potential signals measured simultaneously at dif ferent toroidal locations show a striking similarity, par ticularly at low frequencies. To quantify the similarity
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between potential fluctuation probe signals the toroi dal crosscorrelation has been computed. Raw data for potential signals evolution, frequency spectra as well as toroidal crosscorrelation (between potential sig nals measured by both probe arrays) are shown in Fig. 4. As illustrated in Fig. 4a, a clear similarity is observed between floating potential signals measured in the edge plasma by the two probe systems toroidally apart. Signals are dominated by a ~20kHz oscillation with time varying amplitude (Fig. 4b). As shown in Fig. 4c, a high toroidal crosscorrelation (up to 0.9) is found. As no significant phase shift is observed between signals measured by the probe systems poloi dally and toroidally separated, results suggest that the potential has an (m = 0, n = 0) structure compatible with GAM. Figure 4c also shows the amplitude of the Vf power spectrum in the range 15–25 kHz. Both long range correlations and lowfrequency components show a significant degree of intermittency and a good correlation between them is found. The lifetime of the GAM is estimated to be ≤100 µs, which is compatible with the spectral width of the GAM peak, ~10 kHz. These findings show direct evidence of (intermittent and low frequency) longrange correlations for poten tial fluctuations in the plasma edge region. The phase and coherence between toroidally sepa rated probe signals have been evaluated from the crossspectrum. Spectral analysis reveal that the coherence at large distances is significant (0.7–0.9) for frequencies around 10–25 kHz and that the cross phase is close to zero only for this frequency range confirming the symmetric structure of the potential oscillations. 6. SUMMARY Joint Experiments in the framework of the IAEA Coordinated Research Project (CRP) on “Joint Research Using Small Tokamaks” have also been car ried out on ISTTOK in October 2007 with the partici pation of 24 scientists from 13 countries. The ISTTOK achievements demonstrate that small tokamaks can play an important role in the fusion plasma physics community as a result of their flexibility, high avail ability and good opportunity for the development of sophisticated diagnostics and technology tools. This paper reviews the work recently developed on ISTTOK. The highlights of this work can be summa rized as follows:
(i) Study of fusion relevant materials: An important research area on ISTTOK has been the study of the liquid metals as plasma facing components. A gallium jet limiter has been developed and installed at IST TOK. Successful plasma operation with a jet interact ing with the plasma demonstrated that gallium is com patible with fusion plasmas. (ii) Edge plasma physics: The physics program has been based mainly in the characterization of the edge fluctuations at different scales (local versus longrange correlations). It has been found that the floating potential fluctuations, dominated by low frequency oscillations, exhibit a significant toroidal correlation at large distances that can be attributed to the geodesic acoustic mode. The level of longdistance correlations is strongly bursty showing a significant degree of cou pling with the local electrostatic turbulent transport. (iii) Plasma position control has been implemented that allowed the achievement long AC discharges. Detailed studies are being carried out in edge recycling and plasma position control to accomplish better quality and longer AC discharges. ACKNOWLEDGMENTS This work, supported by the European Communi ties and “Instituto Superior Tecnico,” has been car ried out within the Contract of Association between EURATOM and IST. Financial support was also received from “Fundação para a Ciência e Tecnolo gia” in the frame of the Contract of Associated Labo ratory. This work has partial support through IAEA Contract no. 12938. REFERENCES 1. A. Neto, H. Fernandes, A. Duarte, et al. Fusion Eng. Design 82, 1359 (2007). 2. R. B. Gomes, H. Fernandes, C. Silva, et al., Fusion Eng. Design 83, 102 (2008). 3. P. H. Diamond, S.I. Itoh, K. Itoh, and T. S. Hahm, Plasma Phys. Controlled Fusion 47, R35 (2005). 4. S. V. Mirnov and V. A. Evtikhin, Fusion Eng. Design 81, 113 (2006). 5. A. Fujisawa, T. Ido, A. Shimizu, et al., Nucl. Fusion 47, S718 (2007).
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