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Influence of the Tokamak ISTTOK Plasmas on a Liquid Gallium Jet Dynamic Behavior R. B. Gomesª, C. Silvaª, H. Fernandesª, P. Duarteª, I. Nedzelskiy,ª O. Lielausis b , A. Klyukin b and E. Platacis b . ªAssociação Euratom/IST, Instituto de Plasma e Fusão Nuclear – Laboratório Associado, Instituto Superior Técnico, 1049-001 Lisboa, Portugal b Association EURATOM/University of Latvia, Institute of Solid State Physics, 8 Kengaraga Str., LV-1063 Riga, Latvia 1. Introduction Free flowing liquid metals as plasma facing component in fusion devices could be used to protect & exhaust power. Current experiments uses static lithium as testing material [1, 2]. MHD issues are acknowledged as the main concern about the use of liquid metals in fusion devices. Interaction of free flowing gallium jets with the ISTTOK tokamak discharges are studied. The tokamak plasmas influence on the jets dynamic behavior has been investigated: • A deviation from the jet initial trajectory has been detected. • The evaluation of the jet power extraction capability from droplets surface temperatures increase has been assessed. 2. Experimental set-up ISTTOK is a small size tokamak (R = 0.46 m, a = 0.085 m, B T = 0.45 T, n e = 5x10 18 m -3 , T e (0) ~ 150 eV, I P ~6 kA and V loop ~3 V). Jets produced by hydrostatic pressure (1.3 m gallium column on a suitable size nozzle). Jet parameters: 2.3 mm diameter, 2.5 m/s flow velocity and ~13 cm Break-Up-Length. The injector position is moveable within a 13 mm range (59 < r (mm) < 72). ISTTOK Toroidal field may be changed from 0.32 to 0.6 T, while the poloidal field goes from 9 to 15 mT at r=60 mm and a 4.5 kA discharge. 3. Plasma induced jet trajectory modification Under the influence of the plasma the liquid metal trajectory is perturbed along the radial direction. No jet deviation is observed when no plasma discharge occurs in ISTTOK (even though the loop voltage as well as the toroidal and equilibrium magnetic fields are present). Analysis performed using a fast frame CCD camera (Canadian Photonics Labs Inc. MS10K) at an exposure time of 320 s and a frame rate of 520 fps for a 320 320 pixels resolution. Physical phenomena that could be responsible for such radial displacements include: • Mechanical stress on the injector. • MHD forces due to 3D magnetic field gradients [3]. • Plasma kinetic pressure. • Plasma and gallium are conductors: the plasma could induce a current on the jet. The full liquid metal loop used to inject gallium in ISTTOK has been carefully designed to avoid large scale current flow by using electrical isolators at specific positions [4] but it is clearly not feasible to avoid current induction within the jet segment immersed into the plasma. An object in contact with the plasma will generally float electrically at a potential f which is smaller than the plasma potential by ~ 3kT e /e for hydrogen plasmas. The jet will “short-circuit” regions of the plasma with different potentials and since gallium is a good electrical conductor, a current will be generated along the jet with the plasma acting as an electron source. 5. Summary The perturbation of the liquid gallium jets that interact with ISTTOK plasmas has been investigated. It has been shown that the induction of currents within the metal due to the plasma potential variation along the jet trajectory is the most suitable explanation for the observed deviation. The dependence of the shift amplitude and direction on the plasma positioning within the chamber strongly corroborates this conclusion. This is true even though the produced force is not enough to destabilize the jet. The liquid metal temperature increase due to its interaction with the plasma has been measured using absolutely calibrated infrared sensors. From this value the power extracted by the jet has been evaluated showing the ability of liquid gallium to effectively exhaust energy form the plasma. References [1] J. N. Brooks, J. P. Allain, R. Bastasz, R. Doerner et al., Fusion Sci. Technol. 47 (2005) 669, [2] V. Pericoli-Ridolfini, M. L. Apicella, et al., Plasma Phys. Control. Fusion, 49 (2007) S123, [3] H. Hulin and H. Dong, Plasma Sci. Technol. 7 (2005) 3092, [4] R. B. Gomes, H. Fernandes, C. Silva et al., Fus. Eng. Des., 83 (2008) 102. Acknowledgements: This work has been carried out within the framework of the Contract of Association between the European Atomic Energy Community and "Instituto Superior Técnico". Financial support was also received from "Fundação para a Ciência e Tecnologia" and "Programa Operacional Ciência, Tecnologia, Inovação do Quadro Comunitário de Apoio III". The interaction of this current with toroidal magnetic field will provide a force along the radial direction which could explain the observed shift. An average force of 4.1 mN, applied during 30 ms, is enough to produce a 10 mm shift to the jet. The mean value of the current that has to flow through the jet to produce such force is estimated to be around 130 mA (B T = 0.42 T). Asymmetries in the floating potential profile along the jet trajectory (caused, for instance, by a vertical displacement of the plasma) could explain the occurrence of positive or negative displacement. The “short circuit” hypothesis is corroborated by the observed shift dependence on B T amplitude and plasma vertical position. 4. Gallium jet power extraction from the plasma Cryogenically cooled HgCdTe infrared sensors are used in ISTTOK to measure the temperature of the gallium droplets after their interaction with the plasma. Absolute calibration has been done by measuring the sensor response produced by gallium droplets with known temperature, up to 115 ºC. Higher values are extrapolated by fitting data to a parabola. The total power extracted by the jet has been evaluated as 2.4 kW. This was achieved during shot by shot scanning of sensors optics across viewing area to “match” moving droplets with optical FOV. A series of highly reproducible discharge was used for that purpose.
