Micro-striction 1 and 2 (1961)

A certain quantity of gas is enclosed between two co-axial glass tubes (diameters 67 and 87 mm). In this annular space thus formed exists a permanent magnetic configuration, whose lines of force are perpendicular to the surface of the tubes (0-500 gauss). An electric field is then produced by induction such that its lines of force are circles whose axis coincides with that of the tubes (200 V/cm). The electric field lasts for 0.5 μs. This field ionises the gas and causes an electric current to flow perpendicularly to the magnetic field. The corresponding Laplacian force accelerates the plasma along the axis of the tubes. In a first series of measurements, the discharge characteristics in the absence of a permanent field were studied. It is, in particular, convenient to group together all the irreversible phenomena which occur by assuming the existence of an electrical discharge resistance. The plasma movement in the permanent magnetic field induces in this latter a back-e.m.f. By taking into account the discharge resistance, the electric current and the forces tending to accelerate the plasma can be evaluated. The efficiency of the acceleration process has been measured. It passes through a maximum for a certain value of the permanent magnetic field. The experimental results have been compared with the simple model described above. (authors)