positive column 2d

Hi,

I am trying to create a plasma simulation using a gas mixture of argon and oxygen and nitrogen. To start I took the positive column 2d model and changed the cross section from Argon to Oxygen. Then i simply tried to run it again with the new gas. It fails to converge and fails at approximately 3.5E-7S. I have tried lowering the voltage with no success. I am at a lost as to what the problem would be.

Any suggestions would be appreciated.
Andrew