Start of MHD7
MHD Propulsion
...A previous drawing showing the flow induced by the action of Laplace forces around a cylinder demonstrates that these forces can be used for the propulsion of flying or navigating machines. Nevertheless, the cylindrical shape does not appear to be the most suitable. It is then easy to move to a spherical form by equipping such an object with a ring of electrodes.
...A rotating switch system allows two diametrically opposed electrodes to be sequentially powered, one acting as an anode and the other as a cathode. The key is then to couple this device with a rotating magnetic field system. There is no need, in this case, to place a magnet mounted on an axle inside the model (although this is what we had done during hydraulic experiments in 1976, by embedding a rotating magnet inside a ping-pong ball). All physics students know that arranging three solenoids at 120° and powering them with appropriately phased currents produces the equivalent of a rotating magnetic dipole. The resulting configuration is:
...If the shockwave annihilation experiment around the lens-shaped profile had succeeded, we had planned to repeat the operation using a multi-electrode model with a rotating field, all powered by synchronized capacitor discharges.
...The experiment in cold gas would also have been interesting. It would have sufficed to use the model as an HF antenna. We had already conducted very interesting experiments on this subject as early as 1978. Once again, ionization would have remained neatly localized near the object.
Lens-shaped MHD Flyers
...However, the most interesting experiment would have focused on the topic of the lens-shaped MHD flyer (publication in the Comptes Rendus de l'Académie des Sciences, 1975, under the title "A New Type of MHD Converter"). This would be a machine devoid of electrodes.
...Consider a solenoid carrying an alternating current. It generates an induced field in the surrounding air, which may be accompanied by circulating currents, associated with a secondary field opposing (Lenz's Law) the variation of the inducing field.
...The induced current (i), forming closed loops, interacts with the inducing field B(t), producing radial Laplace forces that alternately act centrifugally and centripetally. For example, in the figure above, at time t₀, the directions of the B field (exciter) and current density J (induced field, circulating within the gaseous mass) would produce a centripetal radial force.
At time t₁, this force would be centrifugal.
...If the gas adjacent to the disk equipped with its internal solenoid is not ionized, nothing notable will happen. But if we ionize this gas, it will be shaken by a system of alternating centrifugal and centripetal forces, like a shaker.
...One can conceive a propulsion system based on this principle by arranging for time-modulated ionization on the upper and lower faces, such that the gas located above the vehicle becomes conductive when the forces are centrifugal:
and conversely, the gas located below the vehicle becomes conductive when these forces are centripetal:
...This would result in a combined force system tending to strongly circulate air around the vehicle:
...The formula (Comptes Rendus de l'Académie des Sciences, Paris, 1975) is appealing. However, one must find a way to create this pulsed ionization near the wall. The problem is delicate, as the time during which the air is made conductive must be on the order of magnitude smaller than the transit time of the gaseous mass around the object. Considering an object moving at 3,000 meters per second, and a characteristic length of ten meters (the diameter of the craft), this leads to times on the order of a millisecond, which is not unfeasible using pulsed microwave emission at 3 gigahertz. Therefore, the upper and lower walls of the machine should be lined with miniature klystrons, emitting alternately and stripping free electrons from air molecules.
...Another solution appears, a priori, more promising. It is known that if molecules are bombarded with electrons having a precisely tuned energy, electron attachment occurs. Some molecules then acquire an extra electron and become negative ions with a very short lifetime—interesting in our case.
...The peripheral electron cannons would take the form of miniature mousetraps. The principle is simple. A solenoid generates a magnetic field with the configuration shown below:
...This field, perpendicular to the wall, decreases in intensity with distance from the wall. It is associated with a magnetic pressure:
...On the right-hand figure, an electric discharge erupting between a central electrode and an annular electrode will expel electrons toward regions where the magnetic pressure is weaker—thus away from the wall—with an energy depending on the value of B. If B is properly adjusted, these electron jets will trigger the formation of negative ions in the air, efficient carriers of the induced current linked to the variation of the inductor field B created by the annular solenoid (see above). Maximum aerodynamic efficiency requires acting within the gaseous layer immediately adjacent to the wall (known as the "boundary layer"). However, this raises a plasma confinement problem, which was experimentally studied during low-pressure experiments and quickly resolved.
...The magnetic field B generated by an equatorial solenoid is itself associated with a magnetic pressure. This pressure decreases as one moves away from the plane of symmetry. Any electric discharge would then tend to move significantly away from the wall, becoming uncontrollable.
...The solution consisted in using not just one solenoid, but three: two smaller-diameter secondary solenoids acting as confinement solenoids.
...At any given moment, the currents passing through
...- the equatorial solenoid
...- the two confinement solenoids
are in opposite directions. The geometry allows creating, near a concave wall, a magnetic pressure gradient capable of pressing the electric discharge against the wall, keeping it within the boundary layer (in practice, for a machine about ten meters in diameter, within a layer a few centimeters thick).
...These peripheral confinement experiments were among the most spectacular we conducted at the end of the 1970s, using modest means.
...Overall, the craft then had the appearance of two...