lectur10 - 16.522, Space Propulsion Prof. Manuel...

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16.522, Space Propulsion Prof. Manuel Martinez-Sanchez Lecture 10: Electric Propulsion - Some Generalities on Plasma (and Arcjet Engines) Ionization and Conduction in a High-pressure Plasma A normal gas at T <3000K is a good electrical insulator, because there are almost no free electrons in it. For pressure > 0.1 atm, collision among molecule and other particles are frequent enough that we can assume local Thermodynamic Equilibrium, and in particular, ionization-recombination reactions are governed by the Law of Mass Action. Consider neutral atoms (n) which ionize singly to ions (i) and electrons (e): n R e +i (1) One form of the Law of Mass Action (in terms of number densities n= P j kT , where T j is the same for all species) is nn ei () (2) =S T n n Where the “Saha function” S is given (according to Statistical Mechanics) as 3 () ST i q n = 2 q π e h 2 2m kT eV i e kT - (3) q i = Ground state degeneracy of ion (= 1 for H + ) q n = Ground state degeneracy of neutral (= 2 for H) m e = mass of electron = 0.91 × 10 -30 Kg k = Boltzmann constant = 1.38 ×10 -23 J/K (Note: k = R/Avogadro’s number) h = Plank’s constant = 6.62x10 -34 J.s. V i = Ionization potential of the atom (volts) (V i = 13.6 V for H) Except for very narrow “sheaths” near walls, plasmas are quasi-neutral: e n= n i (4) So that e n n n () =S T (3’) can be used. 2 16.522, Space Propulsion Lecture 10 Prof. Manuel Martinez-Sanchez Page 1 of 12
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e to n n . A second relation is needed and very often it is a specification of the overall pressure P = ( n + n i + n ) kT = ( 2n + n ) kT (5) e n e n Combining (3’) and (5), 2 () P -2n e = ST n = ST kT () ( n-2n e ) e P Where n= is the total member density of all particles. kT We then have 2 n+ 2S n -S n = 0 e e S 2 (6) n= - S + + S n = n e 1+ 1 + n ST () Since S increases very rapidly with T, the limits of (6) are S n ( 0 ) (Weak ionization) n ⎯⎯⎯⎯⎯ e T0 T0 ⎯⎯⎯⎯⎯ n n e T →∞ 2 (Full ionization) G Once an electron population exists, an electric field Ewill drive a current density G j through the plasma. To understand this quantitatively, consider the momentum balance of a “typical” electron. It sees an electrostatic force G G F= - e E (7) E It also sees a “frictional” force due to transfer of momentum each time it collides with some other particle (neutral or ion). Collisions with other electrons are not counted, because the momentum transfer is in that case internal to the electron species. The ions and neutrals are almost at rest compared to the fast-moving electrons, and we define an effective collision as one in which the electron’s directed momentum is fully given up. Suppose there are ν e of these collisions per second ( ν e =collision frequency per electron). The electron loses momentum at a rate
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This note was uploaded on 11/07/2011 for the course AERO 16.512 taught by Professor Manuelmartinez-sanchez during the Fall '05 term at MIT.

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lectur10 - 16.522, Space Propulsion Prof. Manuel...

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