User:Tokamac/Electrothermal instability: Difference between revisions
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[[Image:electrothermal_instability_evolution.gif|thumb|490px|Evolution of the electrothermal instability in a Faraday MHD converter. Electric current lines.]] |
[[Image:electrothermal_instability_evolution.gif|thumb|490px|Evolution of the electrothermal instability in a Faraday MHD converter. Electric current lines.]] |
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This instability is a [[turbulence]] of the [[Free electron model|electron gas]] in a [[Non-equilibrium thermodynamics|non-equilibrium]] plasma (i.e. where the [[electron temperature]] T<sub>e</sub> is greatly higher than the overall gas temperature T<sub>g</sub>). It arises when a [[magnetic field]] powerful enough is applied in such a plasma, reaching a critical Hall parameter β<sub>cr</sub>. |
This instability is a [[turbulence]] of the [[Free electron model|electron gas]] in a [[Non-equilibrium thermodynamics|non-equilibrium]] plasma (i.e. where the [[electron temperature]] T<sub>e</sub> is greatly higher than the overall gas temperature T<sub>g</sub>). It arises when a [[magnetic field]] powerful enough is applied in such a plasma, reaching a critical Hall parameter β<sub>cr</sub>. |
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A two-temperature gas, globally cool but with hot electrons, is a key feature for practical MHD converters, allowing the gas to reach sufficient [[electrical conductivity]] while protecting materials from [[heat|thermal]] [[ablation]]. |
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Locally, the number of [[electron|electrons]] and their temperature fluctuate ([[electron density]] and [[thermal velocity]]) as the [[electric current]] and the [[electric field]]. The physical explanation of this instability comes from a [[heat transfer]] rate increase to free electrons through [[Joule effect]], which overtakes the time needed to evacuate this heat surplus through [[Kinetic theory|collisional, rotational, vibrational]], [[radiative transfer|radiative]] or [[Heat conduction|conductive]] [[energy transfer|energy transfers]]. |
Locally, the number of [[electron|electrons]] and their temperature fluctuate ([[electron density]] and [[thermal velocity]]) as the [[electric current]] and the [[electric field]]. The physical explanation of this instability comes from a [[heat transfer]] rate increase to free electrons through [[Joule effect]], which overtakes the time needed to evacuate this heat surplus through [[Kinetic theory|collisional, rotational, vibrational]], [[radiative transfer|radiative]] or [[Heat conduction|conductive]] [[energy transfer|energy transfers]]. |
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NB: The term "fully ionized gas" has been imposed by [[Lyman Spitzer]] but a "fully" ionized plasma can have a degree of ionization as low as 0.01%. |
NB: The term "fully ionized gas" has been imposed by [[Lyman Spitzer]] but a "fully" ionized plasma can have a degree of ionization as low as 0.01%. |
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== Technical problems and solutions == |
== Technical problems and solutions == |
Revision as of 09:26, 18 August 2008
The electrothermal instability (also known as the ionization instability or Velikhov instability in the literature) is a magnetohydrodynamic (MHD) instability appearing in magnetized non-thermal plasmas used in MHD converters. It was first theoretically discovered in 1962 and experimentally measured into a MHD generator in 1964 by Evgeny Velikhov.[1][2][3]
Physical explanation and characteristics
This instability is a turbulence of the electron gas in a non-equilibrium plasma (i.e. where the electron temperature Te is greatly higher than the overall gas temperature Tg). It arises when a magnetic field powerful enough is applied in such a plasma, reaching a critical Hall parameter βcr.
Locally, the number of electrons and their temperature fluctuate (electron density and thermal velocity) as the electric current and the electric field. The physical explanation of this instability comes from a heat transfer rate increase to free electrons through Joule effect, which overtakes the time needed to evacuate this heat surplus through collisional, rotational, vibrational, radiative or conductive energy transfers.
The electrothermal instability occurs extremely quickly, in a few microseconds. The plasma becomes non-homogeneous, transformed into alternating layers of high free electron and poor free electron densities. Visually the plasma appears stratified, as a "pile of plates".
Hall effect in plasmas
The Hall effect in ionized gases has nothing to do with the Hall effect in solids (where the Hall parameter is always very inferior to unity). In a plasma, the Hall parameter can take any value.
The Hall parameter β in a plasma is the ratio between the electron gyrofrequency Ωe and the electron-heavy particles collision frequency ν:
where
- e is the electron charge (1.6 × 10-19 coulomb)
- B is the magnetic field (in teslas)
- me is the electron mass (0.9 × 10-30 kg)
The Hall parameter value increases with the magnetic field strength.
Physically, when the Hall parameter is low, the trajectories of electrons between two encounters with heavy particles (neutral or ion) are almost linear. But if the Hall parameter is high, the electron movements are highly curved. The current density vector J is no more colinear with the electric field vector E. The two vectors J and E make the Hall angle θ which also gives the Hall parameter:
Plasma conductivity and magnetic fields
In a non-equilibrium ionized gas with high Hall parameter, Ohm's law,
where σ is the electrical conductivity (in siemens per metre),
is a matrix, because the electrical conductivity σ is a matrix:
σS is the scalar electrical conductivity:
where ne is the electron density (number of electrons per cubic meter).
The current density J has two components:
Therefore
The Hall effect makes electrons "crabwalk".
When the magnetic field B is high, the Hall parameter β is also high.
So when β is high,
Thus both conductivities σ⊥ and σ// become weak, therefore the electric current cannot flow in these areas. This explains why the electron current density is weak where the magnetic field is the stongest.
Critical Hall parameter
The electrothermal instability occurs in a plasma at a (Te > Tg) regime when the Hall parameter is higher that a critical value βcr.
We have
where μ is the electron mobility (in m2/(V·s))
and
where Ei is the ionization energy (in electron volts) and k the Boltzmann constant.
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).
The electron-ion collision cross section Qei is much bigger than the electron-neutral collision cross section Qen:
Therefore with only α = 10-4, the electron-ion collision frequency νei equals the electron-neutral collision frequency νen.
- For a partially or weakly ionized gas (non-Coulombian plasma, when νei < νen ):
- For a fully ionized gas (Coulombian plasma, when νei > νen ):
NB: The term "fully ionized gas" has been imposed by Lyman Spitzer but a "fully" ionized plasma can have a degree of ionization as low as 0.01%.
Technical problems and solutions
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% within these devices. 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 1970's, because this technical problem was considered as an impassable stumbling block in these days.
However experimental studies about the growth rate of the electrothermal instability and the critical conditions showed a stability region exists for high electron temperatures.[4][5][6] The stabiliy is given by Coulombian oscillations that bring the Hall parameter down, under the critical value. This shows an MHD generator can be used at (Te > Tg) regime when the gas is "fully ionized", with a critical Hall parameter βcr < 2.
This data would imply only Faraday generators could be used with that solution, while the Hall generators would be condemned because in these systems only a high Hall parameter gives a good efficiency. Studies were made with the two configurations in non-equilibrium regimes and confirmed this problem.[7][8]
Coulombian regime can be achieved through a preionization and a special gas compound with low ionization potential to reach a very high ionization rate, greater than the growth rate of the instability. The plasma is then stabilized by the ionization mechanism itself. A solution with a helium-argon mix and 2% addition of carbon dioxide enables a quick absorbtion of electron energy thanks to the large CO2 cross section. The stability range occurs for Te = 10,000 kelvins and Tg = 4,000 K.[9][10][11] But such an electrothermal control do not allow to go below Tg = 3,000 K and the converter regime is near short circuit, so such a solution is not practical for any industrial energy conversion.
&&&&&
However some discoveries hinted at some possible technological mastering of the instability:
- Special gas compound with very low ionization potential seed. Cools down electrons and overtake the electrothermal instability, with a ionization rate fast enough to decrease the Hall parameter under the critical threshold. For example, 2% addition of carbon dioxide enables a quick absorbtion of elecron energy thanks to the large CO2 cross section. Such a method allows to control the electrothermal instability in a non-equilibrium MHD generator along a stability range of Te = 10,000 Kelvin and Ti = 4,000 K (until 3,000 K, limit where the instability hands over the reins again).[12][13][14]
- Dynamic stabilization by oscillating fields. Oscillation of the electric field or of an additional RF electromagnetic field modify the Hall parameter.[15][16]
- Non-homogeneous magnetic fields with magnetic pressure gradients. Lowest magnetic pressure areas are distributed in order to create magnetic confinement potential wells between opposite electrodes and canalize the current as several fine streamers without turbulence.[17][18] Some fluctuation of the B-field over space creates high B and low B areas. In the high B areas the Hall parameter is high which increases the local resistivity of the plasma, so the electrons will tend to flow along the minimum B pathes. The local current density J is increased along these pathes. There the electron density and temperature are subsequently rised, such as some Coulomb regime is locally reached, that decreases the local Hall parameter value. Where the value of this non-uniform Hall parameter plasma field is lower than the critical value, the streamers are stable in the minimum B pathes, surrounded by unstable high Hall parameter area. This method allows a Faraday MHD generator with segmented electrodes to work with high Hall parameter.
- Laser guidance. Another alternative confinement method, similar to the magnetic gradient effect, would be to force each streamer to follow a direct path between a pair of electrodes thanks to a laser, which would locally increase the ionization rate. But this is still a theoretical idea.
See also
External links
- M. Mitchner, C.H. Kruger Jr., Two-temperature ionization instability: Chapter 4 (MHD) - Section 10, pp. 230–241. From the plasma physics course book Partially Ionized Gases, John Wiley & Sons, 1973 (reprint 1992), Mechanical Engineering Department, Stanford University, CA, USA. ISBN 0-471-61172-7
References
- ^
E.P. Velikhov (1962). "Hall instability of current-carrying slightly-ionized plasmas". Newcastle-upon-Tyne, England. 1st International Conference on MHD Electrical Power Generation, Paper 47.
{{cite conference}}
: Unknown parameter|booktitle=
ignored (|book-title=
suggested) (help) - ^
E.P. Velikhov (1962). "Plasma turbulence due to the ionization instability in a strong magnetic field". Paris, France. 6th International Conference on Ionization Phenomena in Gases.
{{cite conference}}
: Unknown parameter|booktitle=
ignored (|book-title=
suggested) (help); Unknown parameter|coauthors=
ignored (|author=
suggested) (help) - ^
E.P. Velikhov (1965). "Ionization instability of a plasma with hot electrons". Belgrade, Yugoslavia. 7th International Conference on Ionization Phenomena in Gases.
{{cite conference}}
: Unknown parameter|booktitle=
ignored (|book-title=
suggested) (help); Unknown parameter|coauthors=
ignored (|author=
suggested) (help) - ^
J.P. Petit (24–30 July 1968). "Theoretical and experimental study in shock tube of non-equilibrium phenomena in a closed-cycle MHD generator". In International Atomic Energy Agency, Warsaw, Poland (ed.). Proceedings. 8th International Conference on MHD Electrical Power Generation. Vol. 2. pp. 745–750.
{{cite conference}}
: Unknown parameter|booktitle=
ignored (|book-title=
suggested) (help); Unknown parameter|coauthor=
ignored (|author=
suggested) (help)CS1 maint: multiple names: editors list (link) - ^ J.P. Petit (April 14, 1969). "Theoretical performances of a Faraday generator with non-equilibrium ionization". Comptes rendus de l'Académie des sciences. 268 (A). Paris: French Academy of Sciences: 245–247.
{{cite journal}}
: Unknown parameter|coauthor=
ignored (|author=
suggested) (help) - ^ J.P. Petit (September 1, 1969). "Growth rate of electrothermal instability and critical Hall parameter in closed-cycle MHD generators when the electron mobility is variable". Comptes rendus de l'Académie des sciences (269). Paris: French Academy of Sciences: 365–367.
{{cite journal}}
: Unknown parameter|coauthor=
ignored (|author=
suggested) (help) - ^
J.P. Petit (April 14, 1969). "Theoretical performances of a Faraday generator with non-equilibrium ionization". Comptes rendus de l'Académie des sciences. 268. Paris: French Academy of Sciences: 835–838.
{{cite journal}}
: Unknown parameter|coauthor=
ignored (|author=
suggested) (help) - ^ J.P. Petit (April 21, 1969). "Running instability in a Hall generator with non-equilibrium ionization". Comptes rendus de l'Académie des sciences. 268. Paris: French Academy of Sciences: 906–909.
- ^
J.P. Petit (24–30 July 1968). "Electrical characteristics of a converter using as a conversion fluid a binary mix of rare gases with non-equilibrium ionization". In International Atomic Energy Agency, Warsaw, Poland (ed.). Proceedings. 8th International Conference on MHD Electrical Power Generation. Vol. 3.
{{cite conference}}
: Unknown parameter|booktitle=
ignored (|book-title=
suggested) (help); Unknown parameter|coauthor=
ignored (|author=
suggested) (help)CS1 maint: multiple names: editors list (link) - ^
J.P. Petit (January 27, 1969). "Electrical characteristics of a Faraday linear generator using a binary mix of rare gases, with non-equilibrium ionization". Comptes rendus de l'Académie des sciences. 268 (A). Paris: French Academy of Sciences: 245–247.
{{cite journal}}
: Unknown parameter|coauthor=
ignored (|author=
suggested) (help) - ^
S. Hatori (1974). "Stabilization of Ionization Instability in an MHD Generator". Journal of the Physical Society of Japan. 36 (3). Tokyo Institute of Technology, Yokohama, Japan: 920–920. doi:10.1143/JPSJ.36.920.
{{cite journal}}
: Unknown parameter|coauthors=
ignored (|author=
suggested) (help); Unknown parameter|month=
ignored (help) - ^ J.P. Petit; J. Valensi, J.P. Caressa (24–30 July 1968). "Theoretical and experimental study in shock tube of non-equilibrium phenomena in a closed-cycle MHD generator" Proceedings 2: 745–750; "Electrical characteristics of a converter using as a conversion fluid a binary mix of rare gases with non-equilibrium ionization", Proceedings 3 in International Symposium on MHD Electrical Power Generation. International Atomic Energy Agency, Warsaw, Poland.
- ^
J.P. Petit (September 1, 1969). "Growth rate of electrothermal instability and critical Hall parameter in closed-cycle MHD generators when the electron mobility is variable". Comptes rendus de l'Académie des sciences (269). Paris: French Academy of Sciences: 365–367.
{{cite journal}}
: Unknown parameter|coauthor=
ignored (|author=
suggested) (help) - ^
S. Hatori (1974). "Stabilization of Ionization Instability in an MHD Generator". Journal of the Physical Society of Japan. 36 (3). Tokyo Institute of Technology, Yokohama, Japan: 920–920. doi:10.1143/JPSJ.36.920.
{{cite journal}}
: Unknown parameter|coauthors=
ignored (|author=
suggested) (help); Unknown parameter|month=
ignored (help) - ^
G.I. Shapiro (April 12, 1978). "Stabilization of ionization instability in a variable electric field". Pis'ma v Zhurnal Tekhnicheskoi Fiziki. 4 (12). Akademiia Nauk SSSR, Institut Problem Mekhaniki, Moscow, USSR: 393–396.
{{cite journal}}
: Unknown parameter|coauthors=
ignored (|author=
suggested) (help) - ^
T. Murakami (2005). "Dynamic stabilization of the electrothermal instability". Applied Physics Letters. 86 (19). Tokyo Institute of Technology, Yokohama, Japan: 191502–191502.3. doi:10.1063/1.1926410.
{{cite journal}}
: Unknown parameter|coauthors=
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ignored (help) - ^
J.P. Petit (April 27, 1981). "Method for eliminating the Velikhov instability". Comptes-rendus de l'Académie des Sciences. Paris: French Academy of Sciences: 158–161.
{{cite journal}}
: Unknown parameter|coauthors=
ignored (|author=
suggested) (help) - ^
J.P. Petit (1983). "Cancellation of the Velikhov instability by magnetic confinment" (PDF). Moscow, Russia. 8th International Conference on MHD Electrical Power Generation.
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