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Chapter 11: Plasma physics 55 The diffusion coefFcient D is deFned by means of the ±ux Γ by ± Γ = n ± v di f = - D n . The equation of continuity is t n + ( nv di f )=0 t n = D 2 n . One Fnds that D = 1 3 λ v v . A rough estimate gives τ D = L p /D = L 2 p τ c / λ 2 v . ²or magnetized plasma’s λ v must be replaced with the cyclotron radius. In electrical Felds also holds ± J = neμ ± E = e ( n e μ e + n i μ i ) ± E with μ = e/m ν c the mobility of the particles. The Einstein ratio is: D μ = kT e Because a plasma is electrically neutral electrons and ions are strongly coupled and they don’t diffuse inde- pendent. The coefFcient of ambipolar diffusion D amb is deFned by ± Γ = ± Γ i = ± Γ e = - D amb n e , i . ²rom this follows that D amb = kT e /e - kT i /e 1 e - 1 i kT e μ i e In an external magnetic Feld B 0 particles will move in spiral orbits with cyclotron radius ρ = mv/eB 0 and with cyclotron frequency Ω = B 0 e/m . The helical orbit is perturbed by collisions. A plasma is called
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This note was uploaded on 01/30/2011 for the course PHYSICS 208 taught by Professor Ye during the Spring '10 term at Blinn College.

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