Plasma Ionization MHD Converters
MAGNETOHYDRODYNAMICS. - A new type of magneto-hydrodynamic converters: induction devices. Note (*) by Jean-Pierre Petit and Maurice Viton, presented by M. André Lichnérowicz. CRAS 1976.
...A new type of disk-shaped induction MHD accelerator with controlled ionization is presented. When coupled with a lightweight and powerful electric generator, this device could become an induction and ionization-controlled MHD aerodyne. Solutions for confining the plasma to the wall are indicated.
*...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 with a rather unusual geometry, utilizing a strong Hall effect, were described. Some of these devices are currently undergoing experimentation.
...In the present note, we consider an MHD accelerator with an alternating magnetic field. Consider a disk made of an insulating material, equipped at its periphery with a solenoid carrying an alternating current (*fig.*1). The changing magnetic field tends to induce circular currents in the fluid adjacent to the disk. The Hall effect is assumed negligible. These induced currents interact with the instantaneous value of the magnetic field B to produce radial forces, alternately centrifugal and centripetal. Due to the symmetry of the system, these forces only cause radial agitation, with the net momentum integral being zero over a cycle. The magnitude of the induced current depends on the peak value of the magnetic field B and its period T. Below a certain threshold of the B/T ratio, these induced currents remain weak.
Accelerators with Ionization Control. - This changes if we equip the disk walls with any system capable of producing ionization (*fig.*2). Thanks to this device, we can now control the intensity of the induced currents at every point near the accelerator's wall. Now, modulate the power supplies to the ionizers according to Figure 3. We operate under conditions where the ionization relaxation time is short compared to the period T of the magnetic field.
...It is easy to see that, when they appear, the radial Lorentz forces are centrifugal near the upper part of the disk and centripetal near the lower part. The ionized air will thus be pulsed, resulting in fluid propulsion according to the scheme shown in Figure 4.
MHD Aerodyne. - If this accelerator has an autonomous electric power source, an induction and ionization-controlled MHD aerodyne will be obtained. In a previous note (1), an MHD engine was mentioned. This was a two-stroke engine where, at the end of the MHD compression phase, fusion reactions occurred within a gaseous mixture. The plasma then expanded, and during this second phase, the device functioned as a Hall-type electric generator.
For reference, mention should be made of a recent article by Ralph Moir (2). He also proposes an alternative MHD engine, with a different configuration, apparently simpler. The key element is a toroidal chamber, essentially a tokamak. Moir estimates that Lawson conditions can be achieved through MHD compression, which for this device is of the theta-pinch type. The expansion of the fusion plasma then compresses the magnetic field lines, directly inducing electric current. The advantage of this dual configuration—both aerodyne and induction-powered engine—is that it avoids the need for large currents passing through electrodes.
Plasma Confinement to the Wall. - The inductive solenoid of the disk-shaped accelerator produces a magnetic field that is maximal at the wall. The magnetic pressure will therefore tend to push the discharge away from the wall. To control the location of the MHD interaction, one must use a geometry that produces a magnetic field maximum not at the wall, but near it. This can be achieved using a multi-solenoid geometry, as shown in Figure 5. Calculations show that the magnetic field intensity is then maximal on a surface approximately conical. It is recommended to choose the aerodyne wall as a trajectory orthogonal to the magnetic field lines, so that the Lorentz force is tangent to the wall. The concept of the MHD aerodyne is due to Jean-Pierre Petit; the multi-solenoid confining geometry is due to Maurice Viton.
(*) 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, UCRL 76096 report from the Lawrence Livermore Laboratory, California, USA.