( ) U-probe for the COMPASS Tokamak. Introduction

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1 WDS'11 Proceedings of Contributed Papers, Part II, , ISBN MATFYZPRESS U-probe for the COMPASS Tokamak K. Kovařík, 1,2 I. Ďuran, 1 J. Stöckel, 1 J. Seidl, 1,2 D. Šesták, 1 J. Brotánková, 3 M. Spolaore, 4 E. Martines, 4 N. Vianello, 4 C. Hidalgo, 5 M. A. Pedrosa 5 1 Institute of Plasma Physics AS CR v.v.i., Association EURATOM/IPP.CR, Prague, Czech Republic. 2 Faculty of Mathematics and Physics, Charles University in Prague, Czech Republic. 3 Institute for Plasma Research, Bhat, Gandhinagar, Gujarat, India Consorzio RFX, Associazione EURATOM-ENEA sulla fusione, Padova, Italy. 5 Laboratorio National de Fusión, Asociación EURATOM-Ciemat, Madrid, Spain. Abstract. A complex electrostatic-magnetic probe diagnostics, named as U-probe, is being prepared for measurement of properties of the filamentary structures in the edge plasmas of COMPASS tokamak. Probe will be composed of two identical towers. Each tower will house 3 sets of 3D coils one balanced triple probe and six single Langmuir probes. The design of the probe is based on U-probe used on reversed field pinch RFX-mod [1] and takes advantage of our experience accumulated during design, manufacturing and exploitation phase of our combined electromagnetic probe at TJ-II stellarator. This new COMPASS diagnostic will measure vorticity and longitudial electric current of the plasma filaments. Analysis of the properties and, particularly, the correlation between filament vorticity and its current is expected to shed light into physics of filamentary structures in the tokamak plasma edge. The simulation of the probe surface temperature evolution as a function of expected incident heat loads within the COMPASS edge plasmas is presented. Introduction Due to their characteristic shape, plasma structures elongated along the magnetic field lines are called plasma filaments. Filaments have increased density and temperature in comparison to the rest of the edge plasma [1]. They are conducting electric current along them as was recently observed on RFX-mod [2, 3]. Therefore, the filaments carry out energy from plasma and deposit it unequally on the walls of the vacuum vessel. Particularly, large groups of filaments are representing severe danger for the first wall and divertor components as well as inserted diagnostics and instrumentation. This work describes design of the new probe diagnostics, the so-called U-probe, see Fig. 1, for the COMPASS tokamak. It will be part of complex diagnostics system for studies of the particle and energy transport as well as appearance, sustaining and disintegration of transport barrier in the edge plasma and scrape-off layer. Acquired knowledge will be used on analysis and mitigation of large and potentially dangerous structures mentioned above. Plasma filaments Important parameters of the plasma filament are transversal profiles of electron density n e, electron and ion temperature T e and T i, plasma potential Φ, vorticity ω, and parallel electric current density j par. We are interested in measurement of vorticity and parallel electric current density. Neither vorticity nor parallel electric current density can be measured directly. Vorticity is derived from plasma potential profile, see equation 1, measured by radial array of Langmuir probes. Locally, we can neglect toroidal shaping of the whole plasma d Φ d Φ d Φ ω = ΔΦ = + + (1) B B dr dy dz 1 1 B y ( ) B B z Bz Br Br y jr, j y, jz = j = rot B =,, (2) μ0 μ0 z y r z y r 227

2 Figure 1. Schematic drawing of the U-probe for COMPASS tokamak. Component of plasma potential second derivative parallel to the magnetic field (z-direction) is assumed to be zero because the properties of the filaments along the magnetic field lines, i. e. along the filament, are assumed to be constant. Radial component (r-direction) of the Laplace operator will be calculated directly from radial array of Langmuir probes and poloidal component (y-direction) will be calculated from temporal evolution of the signal of the Langmuir probes using poloidal velocity of the filament movement. The poloidal velocity will be measured from time delay of the signal between two radial arrays with fixed distance. Similarly, the parallel electric current density will be calculated from magnetic field according to equation 2. We neglect the toroidal components of the derivatives as well as in vorticity computation. Magnetic field will be measured by a set of 3D coils distributed around the filament. Electric current density calculation needs magnetic field vector measured in at minimum 3 different positions placed within a plane perpendicular to the magnetic field lines but not placed on one axis. We want to measure the electron density and electron temperature at the top of the probe head to get global information about the general plasma parameters during the probe operation. Requirements for dislocation of measuring elements mentioned above imply necessity of two identical parts of the probe head towers fixed in parallel to each other with a given distance between them. Each tower has to house 3 sets of 3-axial coils (3D coils), radial array of Langmuir probes at the poloidal side and a triple probe at the top of the probe (radially oriented). Usage of two identical towers will also give us information about disturbing of the filament during passage over the first tower and the poloidal component of the filaments velocity. Shape of the two towers connected at the bottom with support structure is similar to the letter U, therefore, the whole probe was baptized as U-probe. U-probe design As previously mentioned, the U-probe consists of two identical towers. The probe will be inserted in the tokamak scrape-off layer near separatrix. Therefore, it has to sustain relatively high thermal loads in the order of a few hundreds of kw/m 2. Exposure to the plasma will be usually for about 0.5 s each minutes. As a result, we need material that will allow heating up to several hundreds of degrees Celsius with a good thermal conductivity and high specific heat. Moreover, the material has to be an electrical insulator not to short-circuit the electric fields and currents in the filaments. Boron nitride ceramics with specifications given in Table 1 has been chosen as the best material according to these criteria. Material for Langmuir probes was chosen according to the similar requirements as the material for the probe head. The only difference is the request on high electrical conductivity. We also require easy 228

3 replaceability of those tips damaged due to the heat or mechanical loads, over the U-probe lifetime. Two materials fulfilled these requirements graphite and tungsten. We have chosen the graphite, because there is no risk of welding with other metals at high temperatures and tungsten is very hard to be machined by usual methods and too brittle when turned (particularly, as we want to use 2 mm thick tips). As the typical dimensions of the filaments are in order of a few cm and velocities in the order of km/s, see Table 2 [4], the temporal and spatial resolution of the U-probe diagnostic has to be in the order of microseconds and millimeters respectively. Spatial resolution is given by the distance and the thickness of the tips. The compromise between the heat load resistance of the tips and the spatial resolution was found in the tips diameter of 2 mm. Axis distance of the tips will be 4 mm, i. e. free distance between walls of the tips remain 2 mm. This is also the minimum distance allowing mechanical works with safe electric separation of the tips. As the best solution of the tip fixation we found a system of a female screw tip simply screwed on a male screw that is fixed to heat conducting substrate hidden inside the probe head. Signal wires are fixed to the screws at the opposite side of the substrate, see Fig. 3. The 3D coil system is fixed in a rift with the same dimensions as the 3D coil blocks. As the coils are wound from a copper wire of diameter of 0.1 mm insulated with enamel, the signals will be reconnected directly in the probe head to more massive wires that will better sustain any mechanical manipulation with no risk of breaking. Each tower will consist of a boron nitride coffin and a covering lid with wall thickness of 5 mm. Space inside each tower is divided into two parts. One houses radial array of the Langmuir probes and the second covers 3D coil sets, see Fig. 4. Position of the array of the Langmuir probes is as near as Table 1. Main parameters of used boron nitride. density ρ 2.0 g/cm 3 thermal conductivity λ W/m/K specific heat c 1600 J/kg/K electric resistivity Ω cm Table 2. Typical dimensions and velocities of filaments in tokamaks [4]. Direction dimension velocity Toroidal a few meters 30 km/s poloidal < 5 cm 2-3 km/s radial < 5 cm < 1 km/s Figure 2. Schematic drawing of mutual position of poloidaly drifting current filament and the two radial arrays of Langmuir probes (filled circles) at both towers in the four consecutive time instances. 229

4 Figure 3. Schematic view of Langmuir probe fixation (1 signal outlead, 2 screw, 3 support plate of alumina, 4 female screw for fixation of the screw in the support plate, 5 boron nitride wall of the probe, 6 graphite tip of Langmuir probe). Figure 4. Design drawing of one tower of the U-probe. possible to the toroidaly oriented wall to minimize shadow effect of the second tower or another disturbance of the filament approaching because they will have the key role for search and observation of the filaments. Coil systems are located at the rear side because the electric current is more stable than electrostatic structures. We have performed simple 1D simulation of the U-probe surface temperature as a function of plasma parameters i.e. density and temperature. We have used the simple equation of temperature diffusion for material with density ρ, thermal conductivity λ and specific heat c, equation 3. Boundary condition was the energy flow q 0 given by the flow of the particles, equations 4 and 5. Assumption of losing of all energy by impact was used. dt dt λ = ΔT ρc (3) q 0 = ΓE kin (4) 1 2kT Γ = ne (5) 4 m According to this simulation, the surface temperature of the U-probe will not exceed 850 C for plasmas up to n e = m -3 and T e = 35 ev that is assumed in the scrape-off layer, see Fig. 5. This temperature is well below the temperature limit for melting of the boron nitride of 3000 C. Probe manipulator Probe will be located at the outer wall of the COMPASS tokamak above the midplane. Diameter of the access port is 95 mm, see Fig. 6. Probe manipulator doesn t have to only fix the probe in a given 230

5 position but also it has to allow in-situ radial movement of the probe from separatrix to fully-hidden position in the access port. The manipulator has to allow change of inclination of the probe head and its rotation around axis to align the U-probe according to the magnetic field, i. e. axis of the towers perpendicularly to the separatrix (about 15 degrees) and the U-probe plane perpendicularly to the magnetic field lines (about 10 degrees). This rotation and inclination will be set off-site in the mechanical workshop for the whole experimental campaign. Required precision of the alignment of the U-probe is less than 3 degrees. Figure 5. Final surface temperature of the boron nitride after 0.5s discharge with given constant plasma parameters. Figure 6. Schematic drawing of the U-probe positioning inside the COMPASS tokamak. 231

6 Summary The U-probe has been designed for scrape-off layer observations of the electrostatic and magnetic properties of the plasma filaments in COMPASS. Optimal location and orientation of the measuring elements were analyzed in detail. The way of plasma filament properties evaluation from measured data was explained in detail, particularly, for the electrostatic part that is important for the filament search and analysis. Heat loads on the boron nitride U-probe surface due to plasma flow were calculated to establish maximal surface temperature after each shot. It was found that the surface temperature is well below the acceptable limit. Results obtained with the U-probe will significantly improve our knowledge about processes observed in the scrape-off layer and anomalous transport and transport barriers in the edge plasma. Construction and commissioning of the U-probe will be finished before end of Acknowledgment. The work was supported by project of MPO 2A-TP1/101, CSF 202/08/H057, AS CR #AV0Z , MSMT #7G09042 and #7G10072 and Euratom. The views and opinions expressed herein do not necessarily reflect those of the European Commission. References [1] M. Spolaore et al., Direct measurements of current filament structures in magnetic-confinement fusion device, Physical Review Letters 102, (2009). [2] M. Spolaore et al., Magnetic and electrostatic structures measured in the edge region of the RFX-mod experiment, Journal of Nuclear Materials (2009) [3] E. Martines et al., Current filaments in turbulent magnetized plasmas, Plasma Phys. Control. Fusion 51 (2009) [4] A. Schmid et al., Experimental observation of the radial propagation of ELM induced filaments on ASDEX Upgrade, Plasma Phys. Control. Fusion 50 (2008)