MAGNETOHYDRODYNAMICS. - Magnetohydrodynamic converters of a new kind: induction devices. Note (*) by Jean-Pierre Petit and Maurice Viton, presented by Mr. André Lichnérowicz. CRAS 1976.
...A new type of disk-shaped MHD accelerator, with induction and ionization control, is presented. Provided with a self-contained electricity generator, this device could give rise to an MHD aerodyne. A solution allowing the plasma to be confined at the wall is mentioned.
...A new induction MHD accelerator, disk shaped, with controlled ionization, is presented. Associated with a light and powerful electric generator it could become a MHD aerodyne. Solutions for confining the plasma to the wall are indicated.
Introduction. In a previous note (1), MHD converters of a rather particular geometry, using a strong Hall effect, had been described. Some of these devices are currently the subject of experimentation.
...In the present Note it is this time a matter of an MHD accelerator with an alternating magnetic field. Let us consider a disk made of an insulating material, provided at its periphery with a solenoid traversed by an alternating current (*fig.*1). The variation of the magnetic field tends to create circular induced currents in the fluid surrounding the disk. The Hall effect is assumed negligible. These induced currents combine with the instantaneous value of field B to give radial forces, alternately centrifugal and centripetal. The system being symmetrical, these forces have only an effect of radial agitation, the integral of the momentum being zero over one cycle. The intensity of the induced current depends on the peak value of field B and on its period T. Below a certain threshold of the ratio B/T these induced currents remain weak.
Accelerators with ionization control. - The same is no longer true if the walls of the disk are provided with any system capable of creating ionization (fig.2). Thanks to this device one can now control the intensity of the induced currents at any point near the wall of the accelerator. Let us now modulate the supplies of the ionizers in accordance with figure 3. One places oneself in conditions where the ionization relaxation time is short compared with the period T of field B.
...It is easy to see that, when they appear, the radial Laplace forces are centrifugal in the upper vicinity of the disk, and centripetal in the lower vicinity. The ionized air will be acted upon in a pulsed manner and a fluid entrainment will occur according to the scheme of figure 4.
MHD aerodyne. - If this accelerator possesses a self-contained source of electrical energy, an induction MHD aerodyne with ionization control will be obtained. In a previous note (1) an MHD engine had been mentioned. A two-stroke engine where, at the end of the MHD compression phase, fusion reactions occurred within a gaseous mixture. The plasma then entered into expansion, the device behaving in this second phase as an electrical generator of the Hall type.
Let us mention for the record a recent article by Ralph Moir (2). The latter also proposes an alternating MHD engine, of a different formula, and apparently simpler. The essential element is a toroidal chamber which is none other than a tokamak. Moir estimates that the Lawson conditions can be reached in an MHD compression, which for this type of device is of the theta pinch type. The expansion of the fusion plasma then compresses the lines of force of the magnetic field and there is direct production of electric current by induction. The interest of this double formula: aerodyne and engine operating by induction, is to free oneself from the passage of strong currents through electrodes.
Confinement of the plasma at the wall. - The inducing solenoid of the discoid accelerator provides a magnetic field which is maximal at the wall. The magnetic pressure will therefore tend to make the discharge migrate away from it. If one wishes to control the localization of the MHD interaction one must resort to a geometry giving a maximum of field, not at the wall, but in the vicinity of the latter. This can be obtained by a geometry with several solenoids as indicated in figure 5. The calculation shows that the intensity of the magnetic field is then maximal on an approximately conical surface. It is advisable to choose for the wall of the aerodyne a trajectory orthogonal to the lines of field, so that the Laplace force is tangent to it. The idea of the MHD aerodyne is Jean-Pierre Petit's, that of the confining geometry with several solenoids is Maurice Viton's.
(*) Session of December 8, 1976
(1) J.P.Petit, Comptes rendus, 281, series B, 1975, p.157
(2) R.Moir, Direct Conversion of Energy from Fusion, report UCRL 76096 of the Lawrence Livermore Laboratory, California, USA.