I've never used GTAW to join silver but would probably try DCEN first, then AC.  I'd write a weld procedure.  Probably argon gas.  I'd contact Harris (or another filler metal manufacture) and ask them directly what to use.

However, my first process choice would be to torch braze.

Good Luck,

Keith

I have used AC and DCSP GTA with Argon for the thinner pieces (Wire), and AC with a heating platten for a thick Plate (2") with internal cooling.  My customer had several more complex coolers made with Electron Beam welding.