Summer Research with Advisor or at National Lab?
One unfortunate by-product of the increasingly specialized nature of doctoral research is the tendency to put on "blinders" when it comes time to work on the dissertation: PhD students tend to forget that there are things to study and learn about outside the field of the dissertation. (For example, I always have a few students who complain in the course evaluation "I'm studying X; I shouldn't have to take this mandatory course!")

However, in spite of this, research is becoming inherently more interdisciplinary and interconnected. So having additional experience—and additional contacts—outside your main field can only help you in the long run, if you're planning to pursue a "knowledge-based" career. 

That said, this is definitely a matter you should bring up with your advisor. This early in your career, it's probably not such a big deal to do something else for the summer. But you should still check if there are some important counterarguments (related to funding or project sponsorship).