Regarding solving drift-diffusion equation in Comsol

Dear Mr Gritter

I have come across your post in the Comsol forum about implementing a drift-diffusion equation using the PDE interface in Comsol. You mentioned that the drift-diffusion equations need to be solved in a time dependent manner to obtain a solution (www.comsol.com/community/forums/general/thread/41356/).

I am working on a similar problem of modeling the plasma in a discharge tube and it would help me if you can elaborate on your comment about the drift diffusion equation. Are you suggesting that a steady state drift diffusion (coefficient of transient term is '0') equation also needs to be solved as a transient problem?

I am trying to model the plasma in a discharge tube as a coupled system of two convection-diffusion equations and one Poisson equation. I am using the equations from the mathematics(Classical PDE) interface of comsol 4.3. Another question I have about the transient convection diffusion equation is about the initial values Comsol requires.I was not able to understand why Comsol requires two initial values (value and the derivative of dependent variable) for solving the transient convection diffusion equation. I would think either one of them will be sufficient as the equation is 1st order in time.

Thanks in advance for your time.

Sincerely
Pavan Poosarla
Graduate Assistant
Iowa State University