Unit of Diffusion Coefficients Not Clear

Hello all,

I have been using Magboltz (interfaced with Garfield++) to generate some gas files to study the transport properties of some gas mixtures. I have noticed that while retrieving the transport parameters of the gas, the unit of Diffusion Coefficients (both longitudinal and tranverse) is in cm^(0.5) rather than in the unit that is generally used - (cm^2)/s. I tried to look at the Doxygen documentation - Garfield++: Garfield::Medium Class Reference
and also at the Garfield++ User Guide - https://garfieldpp.web.cern.ch/garfieldpp/documentation/UserGuide.pdf,
but I can’t see the mention of any conversion factor.

Hence, I would really appreciate it if someone would let me know the conversion factor that is associated with these units because otherwise I will not be able to make any inferences from the values of the Diffusion coefficients that I get.

Thank you.

I’m sure @hschindl will be able to help you

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the (transverse) diffusion coefficient used in Garfield and Magboltz (let’s call it D’) is defined such that electrons drifting in a uniform electric field over a distance z have a spread

σ = D’ \sqrt{z}

in the directions transverse to the electric field. D’ (unit: \sqrt{cm}) is related to the usual diffusion coefficient (unit cm^2 / s) by

D’ = \sqrt{2 D / v}

where v is the drift velocity.


I agree that it’s a bit confusing; I should add an explanation to the user guide…

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Thank you for such a prompt reply. So just to be sure, the same relation also works for longitudinal diffusion coefficient, or is there a different conversion relation for it?

yes, the conversion works the same way for the longitudinal diffusion coefficient.

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Awesome! Thank you.

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