Mach Probe Research Articles

Plasma flow measurements along the presheath of a magnetized plasma

Plasma flow measurements in the presheath have been performed using two types of directional electric ‘‘Mach’’ probes, in the PISCES facility at UCLA [J. Nucl. Mater. 121, 277 (1984)]. A fast scanning versatile probe combination has been developed, which operates simultaneously as a ‘‘magnetized’’ Mach probe, an ‘‘unmagnetized’’ Mach probe (with characteristic probe size greater than and smaller than ion gyroradius, respectively), and an emissive probe. Presheaths have been investigated by inserting a small object at the center of the plasma column. Variations in plasma flow velocity, density, and potential along the presheath have been deduced by fluid and kinetic theories. A comparison is made between Mach numbers obtained from the magnetized probe and the unmagnetized probe. Incorporation of shear viscosity of order ∼0.5nmiD⊥ in the cross-field transport along the presheath seems best to model the results. The cross-field diffusivity (D⊥) is found to scale approximately proportional to B−1/2, with magnitude about 4× larger than Bohm in the PISCES plasma. The effect of an electrical bias applied to the object on the presheath characteristics is discussed.

Presheath profiles in simulated tokamak edge plasmas

Steady state magnetized plasmas produced by the PISCES experiment are used to study plasma-wall interaction phenomena relevant to confinement devices such as tokamaks. An experimental investigation of the presheath region that extends from a wall surface into “simulated tokamak” edge plasmas along magnetic field lines is reported. Diagnostics especially developed for this work include a fast-scanning multiple Langmuir/Emissive/Mach probe system and a CID camera imaging system. Measurements of density, electron temperature, floating potential, space potential, and bulk plasma flow velocities have been obtained in plasmas with densities ranging from 10 12 to 10 13 cm −3, electron temperatures from 5 to 15 eV. and axial magnetic fields of 0.2 to 1.4 kG. Plasma density profiles along the magnetic field typically show a characteristic factor of 2 decrease towards the wall surface. A plasma potential variation in the near presheath zone of order 0.5 T e is measured, consistent with the bulk plasma flow approaching the ion sound speed near the wall surface, as inferred from a simple “free fall” model. A Boltzmann model for the presheath density profile accuracy tracks the density profile measured both by the Mach probe and by spectroscopic means. Flow profiles are used as a consistency check on various magnetized Mach probe theories. Results suggest that cross-field transport of parallel momentum through viscosity is relatively unimportant in PISCES plasmas and thus may be unimportant in tokamak boundary layer plasmas. Discharges with non-thermal electrons display axial profiles of space potential and floating potential which indicate a “hotter” electron distribution function near the wall surface, consistent with “colder” electrons being reflected by the presheath potential drop.

Edge plasma investigation on T-10

The results of studying the plasma scrape-off layer (SOL) structure in the T-10 tokamak are given in this paper. A set of Langmuir probes was used, including single probes located on the limiter and far from it and Mach probes. The presence of plasma flows along the upper and lower plasma column circumference is shown, their velocities and directions depend on plasma parameters and on main magnetic field and current directions. The fluxes correspond to co-current rotation of the plasma column periphery at high edge temperature and a plasma flow from the external circumference of the torus to the internal one at low temperature. A strong plasma asymmetry in the shadow of a circular limiter is shown. The results of a special experiment on propagation of a potential perturbation along the magnetic field lines are also given.

Anomalies in the downstream flux to a Mach number probe

Data derived from a probe inserted into the limiter scrape-off layer of a tokamak do not correspond to unperturbed plasma conditions if the length of the shadow cast by the probe along the magnetic field is greater than the probe-limiter connection length. Measurements have been made in DITE using small sized, Mach number probes located close to the limiter. The magnitude of the downstream flux of ions to the probe corresponds to that expected if the length of the downstream shadow is shorter than the probe-limiter connection length. This short length of shadow cannot be explained on the basis of the observed cross field diffusion in the unperturbed scrape-off layer.

Laminar Heat-Transfer and Pressure Measurements Over Blunt-Nosed Cones at Large Angle of Attack

This result is consistent with the findings of Preston and Hsu. The accuracy of the law of the wall for compressible flow, as expressed by Eq. (2), was investigated experimentally in the Defense Research Laboratory Variable Mach Number Wind Tunnel. The investigation included impact-probe surveys of the boundary layer on the tunnel wall and direct measurements of the local shear stress by means of skin-friction balances. Typical results are shown in Fig. 1, and it can be seen that good agreement between Eq. (2) and the data was obtained in the neighborhood of the wall for all values of Mach Number and Reynolds Number, based on the boundary-layer momentum thickness. Surface- impact-probe measurements and direct shear-stress measure­ ments were made at nine Mach Numbers and with four probe diameters. Several Reynolds Numbers were utilized for each combination of Mach Number and probe diameter. The result­ ing data are compared with Eq. (3) in Fig. 2. It is to be concluded that the surface-impact-probe technique can be used to measure with good accuracy the local skin friction in supersonic flow over thermally insulated surfaces. For the case of isobaric surfaces, the maximum probe size that can be used in a given application has been theoretically calculated and experimentally confirmed in reference 4.