Electrothermal instability
[1][2][3] "This paper shows that it is possible to assert sufficiently specifically that the ionization instability is the number one problem for the utilization of a plasma with hot electrons.
It arises when a magnetic field powerful enough is applied in such a plasma, reaching a critical Hall parameter βcr.
When the transverse magnetic field is applied on the bulb, some oblique grooves appear in the plasma, typical of the electrothermal instability.
Physically, when the Hall parameter is low, the trajectories of electrons between two encounters with heavy particles (neutral or ion) are almost linear.
The electrothermal instability occurs in a plasma at a (Te > Tg) regime when the Hall parameter is higher than a critical value βcr.
The growth rate of the instability is And the critical Hall parameter is The critical Hall parameter βcr greatly varies according to the degree of ionization α : where ni is the ion density and nn the neutral density (in particles per cubic metre).
[7] But the unexpected large and quick drop of current density due to electrothermal instability ruined many MHD projects worldwide, while previous calculations had envisaged energy conversion efficiencies over 60% with these devices.
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.
[10] The stability is gained by a quick transition to "fully ionized" conditions (fast enough to overtake the growth rate of the electrothermal instability) where the Hall parameter decreases because of the collision frequency rising, below its critical value which is then about 2.
[11][12][13][14][15] But this electrothermal control cannot provide an adequate decrease of Tg over long durations (to avoid thermal ablation), so such a solution is not practical for industrial energy conversion.
[17][18] Finally, a solution has been found in the early 1980s to completely remove the electrothermal instability within MHD converters, by means of non-homogeneous magnetic fields.
A strong magnetic field implies a high Hall parameter, and therefore a low electrical conductivity in the medium.
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.
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[citation needed] to the inventor of the magnetic stabilization method: You bring the cure, but the patient already died...However, this electrothermal stabilization by magnetic confinement, although 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).
