Technical Problems and Solutions
A two-temperature gas, globally cool but with hot electrons (Te >> Tg) is a key feature for practical MHD converters, because it allows the gas to reach sufficient electrical conductivity while protecting materials from thermal ablation. This idea was first introduced for MHD generators in the early 1960s by Jack L. Kerrebrock and Alexander E. Sheindlin.
But the unexpected large and quick drop of current density due to the electrothermal instability ruined many MHD projects worldwide, while previous calculation envisaged energy conversion efficiencies over 60% with these devices. Whereas some studies were made about the instability by various researchers, no real solution was found at that time. This prevented further developments of non-equilibrium MHD generators and compelled most engaged countries to cancel their MHD power plants programs and to retire completely from this research field in the early 1970s, because this technical problem was considered as an impassable stumbling block in these days.
Nevertheless experimental studies about the growth rate of the electrothermal instability and the critical conditions showed that a stability region still exists for high electron temperatures. The stability is given by a quick transition to "fully ionized" conditions (fast enough to overtake the growth rate of the electrothermal instability) where the Hall parameter decreases cause of the collision frequency rising, below its critical value which is then about 2. Stable operation with several megawatts in power output had been experimentally achieved as from 1967 with high electron temperature. But this electrothermal control does not allow to decrease Tg low enough for long duration conditions (thermal ablation) so such a solution is not practical for any industrial energy conversion.
Another idea to control the instability would be to increase non-thermal ionisation rate thanks to a laser which would act like a guidance system for streamers between electrodes, increasing the electron density and the conductivity, therefore lowering the Hall parameter under its critical value along these paths. But this concept has never been tested experimentally.
In the 1970s and more recently, some researchers tried to master the instability thanks to oscillating fields. Oscillations of the electric field or of an additional RF electromagnetic field locally modify the Hall parameter.
Finally, a solution has been found in the early 1980s to annihilate completely the electrothermal instability within MHD converters, thanks to non-homogeneous magnetic fields. A strong magnetic field implies a high Hall parameter, therefore a low electrical conductivity in the medium. So the idea is to make some "paths" linking an electrode to the other, where the magnetic field is locally attenuated. Then the electric current tends to flow in these low B-field paths as thin plasma cords or streamers, where the electron density and temperature increase. The plasma becomes locally Coulombian, and the local Hall parameter value falls, while its critical threshold is risen. Experiments where streamers do not present any inhomogeneity has been obtained with this method. This effect, strongly nonlinear, was unexpected but led to a very effective system for streamer guidance.
But this last working solution was discovered too late, 10 years after all the international effort about MHD power generation had been abandoned in most nations. Vladimir S. Golubev, coworker of Evgeny Velikhov, who met Jean-Pierre Petit in 1983 at the 9th MHD International conference in Moscow, made the following comment to the inventor of the magnetic stabilization method:
|“||You bring the cure, but the patient already died...||”|
However it should be noted that this electrothermal stabilization by magnetic confinement, if found too late for the development of MHD power plants, might be of interest for future applications of MHD to aerodynamics (magnetoplasma-aerodynamics for hypersonic flight).
Read more about this topic: Electrothermal Instability
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