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phys documents (dragged) 61 - Chapter 11 Plasma physics 55...

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Chapter 11: Plasma physics 55 The diffusion coefficient D is defined by means of the flux Γ by Γ = nv di ff = - D n . The equation of continuity is t n + ( nv di ff ) = 0 t n = D 2 n . One finds that D = 1 3 λ v v . A rough estimate gives τ D = L p /D = L 2 p τ c / λ 2 v . For magnetized plasma’s λ v must be replaced with the cyclotron radius. In electrical fields also holds J = neμE = 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 coefficient of ambipolar diffusion D amb is defined by Γ = Γ i = Γ e = - D amb n e , i . From this follows that D amb = kT e /e - kT i /e 1 e - 1 i kT e μ i e In an external magnetic field 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 magnetized if λ v > ρ e , i . So the electrons are magnetized if
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