Oblique ion collection in the drift approximation: How magnetized Mach probes really work

The anisotropic fluid equations governing a frictionless obliquely flowing plasma around an essentially arbitrarily shaped three-dimensional ion-absorbing object in a strong magnetic field are solved analytically in the quasineutral drift approximation, neglecting parallel temperature gradients. The effects of transverse displacements traversing the magnetic presheath are also quantified. It is shown that the parallel collection flux density dependence upon the external Mach number is n(infinity)c(s) exp[-1-(M-parallel to infinity-M-perpendicular to cot theta)], where theta is the angle (in the plane of field and drift velocity) of the object-surface to the magnetic-field and M-parallel to infinity is the external parallel flow. The perpendicular drift, M-perpendicular to, appearing here consists of the external EandB drift plus a weighted sum of the ion and electron diamagnetic drifts that depends upon the total angle of the surface to the magnetic field. It is that somewhat counterintuitive combination that an oblique (transverse) Mach probe experiment measures.