Start of MHD6
...With the size of a kitchen bucket, it is a chamber containing an anode and a cathode, the latter taking the form of a pool of mercury. Between the anode and cathode: a vacuum. That is, a space filled with saturated mercury vapor corresponding to ambient temperature, having an electrical conductivity too low to allow current flow, with the electrodes under high voltage (5 kV). A "trigger" is a small electrode located near the surface of the mercury. When a discharge is initiated between this electrode and the mercury-cathode, the mercury is vaporized, and this vapor then fills the chamber, enabling an electric arc to pass. In effect, a lightning bolt in a closed vessel. Once the discharge is initiated, it sustains itself until the energy stored in the capacitors has been dissipated by Joule heating in the copper conductors. Then the mercury vapor condenses, and the ignitron is ready for the next attempt. A second ignitron, the size of a beer can, was sufficient to trigger the current flow at the right moment in the electrodes equipped on the test model.
...Below is the diagram of the operation control:
...In 1965, the main cost of such experiments concerned electronics and data recording. Of course, at that time, microcomputers did not exist. The bandwidth of the most advanced oscilloscopes of the era (American Tektronix models, vacuum tube-based) would seem laughable today: 1 megahertz. But in the 1960s, their unit price reached 40,000 francs. Today, this cost could be divided by ten at equivalent performance.
The traces appearing on oscilloscope screens were photographed on Polaroid film. Today, the entire acquisition of these experimental parameters could be managed by a low-end microcomputer equipped with a dedicated card.
...Recording the parameters of the wind tunnel was extremely simple. It sufficed to place, in the wall, small pairs of needles under low voltage. The distance between the needles was one millimeter, and the voltage was low enough that current could not pass through the rarefied argon atmosphere. But when the shock wave passed, the mere fact that these electrodes were immediately downstream of the wave in argon at 10,000°C was enough to produce a signal. By recording, using a dual-trace oscilloscope, the signals emitted by two such "ionization probes" spaced ten or twenty centimeters apart and located upstream of the nozzle, one could measure the shock wave velocity, and thus calculate all the gasdynamic parameters: temperature, pressure, degree of ionization, electrical conductivity. Additional oscilloscopes were required for supplementary measurements. To protect these oscilloscopes from strong electromagnetic interference generated by the spark gaps in the high-pressure chamber and generally by all electrical switching components, they were connected to the probes via shielded coaxial cables and enclosed in a Faraday cage, where the experimenters themselves also took place.
...Thus, this is the description of the experimental setup that would have allowed us to verify the validity of the theory we developed between 1975 and 1980 regarding the feasibility of an object moving at supersonic speed through a gas without generating a shock wave. It remains to describe how to demonstrate the annihilation of these waves. One can then use a classical and proven method: creating a system of horizontal lines by making two light beams interfere, one passing through the test jet and the other passing outside. A shock wave represents an abrupt jump in gas density, which manifests as a change in the refractive index. Thus, shock waves are traditionally revealed by this method. Below, on the left, the typical appearance of a "fringe jump" due to the presence of an oblique shock wave attached to the leading edge of an airfoil. On the right, the same image with shock waves annihilated.
...The argon plasma at 10,000°C is quite luminous, so the light source to use would be a small helium-neon laser, delivering a brighter light than that of the plasma.
...In the late 1980s, Lebrun and I calculated all the parameters for such an experiment, within the framework of his doctoral thesis funded by the CNRS. I am convinced this experiment would have worked on the first try, just like all the MHD experiments I had previously attempted in the laboratory using shock tubes. I particularly recall an experiment from 1966 (which I will discuss in a future document), where the goal was to operate an MHD generator in "bitemperature" mode, meaning the electron temperature (10,000°C) was significantly higher than that of the test gas (6,000°C). The obstacle was then "Vélikhov instability" (which nullified all MHD efforts in many countries). A clever trick allowed us to circumvent this obstacle, and the experiment worked on the first try. I presented this work at the international conference in Warsaw in 1967. But the terrible atmosphere prevailing in that laboratory forced me to leave and switch disciplines, becoming an astrophysicist. My student, Jean-Paul Caressa, took over the entire research theme, which he made the subject of his thesis (although he clearly did not grasp the subtleties of Vélikhov's ionization instability, whose annihilation was the key to the experiment), earning him the Worthington Prize and later enabling him to become director of the Meudon aero-thermodynamics laboratory, and then regional director of the CNRS for the Provence-Alpes-Côte d'Azur region.
What became of such a project.
...In the mid-1980s, I managed to interest the Director General of the CNRS, Pierre Papon, in this research theme. He supported us, relayed by his deputy Michel Combarnous, director of the Department of Physical Sciences for Engineers. At the time, I was already based at the Marseille Observatory, a location ill-suited for such experiments. Combarnous then found us a host laboratory, that of Professor Valentin in Rouen. The CNRS was to finance part of the operation, with the army expected to provide a supplement. But quickly, the military demanded that I be completely excluded from these activities, for reasons unrelated to science. With a change in CNRS leadership, I lost the support of Papon and Combarnous. Since Lebrun’s grant was exhausted, nothing was done to allow him to continue his work.
...The Rouen team, completely inexperienced in MHD (though possessing an old shock tube), accumulated errors. The funds were ultimately squandered without results (the MHD nozzles and high-power electrical installations built by these amateurs exploded one after another).
...This is truly regrettable. In the near future, I will place on a CD-ROM all theoretical and experimental elements capable of enabling an interested laboratory to successfully carry out this relatively simple type of experiment. Although this description is brief, it still allows one to realize that, given the significant reduction in electronic equipment costs, this kind of research is within reach of an engineering school or the physics department of a second-tier university in North America. But I strongly doubt such activities could develop in France, where civilian research is often (at least in these fields) under military control.
...One might think they wish to maintain exclusive control. Not even so. After investigation, it appears that fourteen years later (after my departure in 1986) "military MHD" remained completely nonexistent.
...If this experiment had succeeded, we would then have considered experiments in cold gas (atmospheric air). An interesting experiment (which was completely failed in 1979 by a team from Toulouse, the "GEPAN," under conditions one might describe as "unpleasantly human") concerned suppressing the turbulent wake behind a cylinder, which we had successfully achieved in 1975 using hydraulics.
...Returning to the diagram of the cylindrical MHD machine mentioned earlier.
...We previously described how we used such a setup to eliminate the bow wave in front of the object. But if we limit the interaction parameters to lower levels, we can then, in a still fluid, create a rather interesting induced flow.
...At the time, the flow could be visualized using colored threads (for the record: in the kitchen of my colleague and friend Maurice Viton, an astronomer at the laboratory of space astronomy, who happened to shoot a superb 16 mm film).
...Placed in a moderate-speed fluid flow, this model could completely eliminate the highly turbulent wake that normally forms downstream of a cylinder whose generators are perpendicular to the flow. Thus, my idea from 1979 was to attempt to detect, using a simple microphone placed in the wall, the disappearance of this turbulent (noisy) flow during subsonic experiments in atmospheric pressure air. In principle, the setup was simple. Two lateral solenoids could provide several thousand gauss continuously, more than sufficient. The remaining challenge was to solve the problem of ionization near the model.
...In a report I submitted to GEPAN in 1979, titled "Perspectives in Magnetohydrodynamics," the principles of this experiment were described. I had suggested using microwaves at 3 gigahertz to create the required ionization. These people therefore built, without my knowledge, the following experiment, using a very powerful HF source (pulsed at 500 hertz, peak power: 1 MW).
...The microwaves were brought laterally into the nozzle via a large 10 cm by 10 cm waveguide, terminating at a Teflon window.
...The engineer in charge of the project, Bernard Zappoli, directly under the then head of GEPAN, Alain Esterle, imagined he could create ionization throughout the entire flow near the model by injecting microwaves transversely. Ignorant of the phenomenon of HF ionization, he obtained a result that greatly puzzled him. Ionization did occur, but was limited to just a few millimeters of gas adjacent to the Teflon window.
...Ionization means plasma. It is well known that plasmas are excellent shields against electromagnetic waves; otherwise, we could freely communicate by radio with astronauts during atmospheric re-entry.
...It is unfortunate that this earnest young man did not consult me at that time. I would have easily saved him from his predicament. Indeed, where should ionization occur? Around the model. His solution should have been to route the HF through the interior of a hollow model (a simple PVC tube, such as used by plumbers). Two iron straws purchased from the local pharmacy would then have ensured excellent microwave diffusion, which, acting on the air immediately adjacent to the model, would have created a well-homogeneous envelope of ionized gas around it.
...The experiment would very likely have worked on the first try, just like all those I attempted during my research career.