A plasma source for processing or imaging a substrate, for ion source for proton therapy, for ion thrusters, or for high energy particle accelerators includes a coolant circuit passing adjacent to a plasma ion reactor chamber and RF antenna coils. In a method for operating the plasma ion source having an induction coil adjacent to a reaction chamber for inductively coupling power into the plasma from a radio frequency power source, the method comprises pumping a dielectric fluid into contact with induction coils of the plasma ion source along the coolant circuit. Use of the dielectric fluid both electrically insulates the plasma chamber, so that it can be biased to 30 kV and up, and efficiently transfers heat away from the plasma chamber.