How can I put myself in the best position to get a postdoc in a different field?
I'm in the same boat as you, but then for my thesis I added a few elements of the thing that I want to do later in my career, so I can use it as background. If you can incorporate something related to experimental physics in your plasma physics research, that'd help. Otherwise, you may join some research lab that works in the field of exp. physics, that'd look good on your resume.