Don't sign up for 15 year commitment with anyone! Who knows if the DoE will be doing work that interests you in 2035. If you don't like the work, you won't be able to leave.

You should be able to make enough between tuition waivers, being a teaching assistant, and summer internships to live cheaply through grad school. Then when you're done you'll be free, with your new PhD, to pick the awesomest job you can find.