The ion current collected by a probe in a magnetized plasma is sensitive to the angle between its surface and the flow streamlines. This intuitive concept is the basis of the Gundestrup probe, a polar array of planar collectors mounted around an insulating housing. Probe theory for measuring flows has been developed on two fronts: Recent kinetic and fluid models, reviewed here, give similar predictions for the collected current within the range of applicability of the model assumptions. A comparison with measurements by a rotating Mach probe in the CASTOR tokamak (Czech Academy of Sciences Torus) [J. Stöckel, J. Badalec, I. Ďuran et al., Plasma Phys. Controlled Fusion, 41, 577 (1999)] highlights the role of magnetization in ion collection at grazing angles of incidence between the probe surface and the magnetic field lines.
We show that it is possible to measure the parallel and the perpendicular flow in a strongly magnetized plasma, using a probe with two current collecting surfaces, inclined with respect to the magnetic field. The method is based on a one-dimensional fluid model, which we use to relate the unperturbed velocity components (the quantities we wish to measure) to the velocity and density perturbations caused by the probe. In this way we are able to establish a link between the electric saturation currents collected by the probe surfaces and the unperturbed velocity. The role of the cross-field transport into a pre-sheath of a collecting probe surface and the boundary condition for the flow velocity at the magnetic pre-sheath entrance are also investigated. § Partner in the Trilateral Euregio Cluster (TEC).
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