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Description
IShTAR (Ion Sheath Test ARrangement), a facility located at the Max-Planck Institut für Plasmaphysik in Garching (IPP-MPG Germany), is dedicated to the investigation of the interactions between an ICRF antenna and a plasma [1, 2]. It is an international collaboration with three main partners: University of Lorraine (France), Ghent University (Belgium), and IPP-Garching and other universities. In contrast to a tokamak, a dedicated test stand provides more experimental time and freedom to impose the parameters, gives better access for the instrumentation and antennas and can be set up to have a simple geometry more easily amenable to comparison with theory. The main aim of the project is indeed to provide the experimental data on RF sheaths in order to validate and improve theoretical predictions. The operating conditions are representative of the plasma edge of a magnetic confinement fusion machine. The test bed is composed of magnetised (Bmax = 0.24 T) plasma column of about 0.4 m diameter in a cylindrical vacuum vessel (1 m diameter, 1.1 m length). The plasma column is created by the expansion of an external cylindrical magnetized plasma source (0.4 m diameter, 1 m length) generated by a helical antenna that has been designed to excite the m = 1 helicon mode. An optimisation of the plasma source in order to get the highest density and most radially uniform plasma is presented. A single strap RF antenna has been designed, to excite an RF sheath, the plasma-facing surface of which is aligned to the cylindrical plasma to ease the modelling [3]. The plasma parameters it faces will be varied in the ranges relevant for a tokamak edge plasma. The installed and planned diagnostics are designed to measure the plasma wave interactions. With a 2D probe array and probes mounted on a moving manipulator [4], we can measure simultaneously the density and the potential on magnetic field lines in the neighbourhood of the antenna. With this information also directly in front of the RF antenna, a detailed study of the density modifications induced by RF sheaths can be made. Measuring the electric field directly inside the sheath will require the development of specific diagnostic methods, different options are looked into [5, 6, 7].
[1] R. D’Inca et al, AIP Conf. Proc. 1689, 050010 (2015) [2] K. Crombe et al, AIP Conf. Proc. 1689, 030006 (2015) [3] F. Louche et al, AIP Conf. Proc. 1689, 070016 (2015) [4] E. Faudot et al, Review of Scientific Instruments 86, 063502 (2015) [5] L. Chérigier-Kovacic et al, Review of Scientific Instruments 86, 063504 (2015) [6] E. Martin et al, AIP Conf. Proc. 1689, 030011 (2015) [7] U. Czarnetzky et al, Phys. Rev. Lett. 81, 4592 (1998)
