SURFACE MODIFICATION OF THE PLT LOWER HYBRID WAVEGUIDES TO IMPROVE OPERATIONS.

Arrays of waveguides for use in PLT during lower hybrid wave heating and current drive experiments have been treated to improve resistance to high power rf breakdown. The breakdown has been attributed to gas evolved from waveguide walls, electron multipacting, photoelectron emission and/or arcing. Mechanical polishing of the waveguide, by means of a unique polishing jig, removes surface protrusions of greater than equivalent to 0. 001 in. Electrochemical polishing is then employed to remove imbedded polishing grit and reduce surface roughness, thereby reducing the surface area for adsorbed gases. Ultrasonic cleaning in hot Alconox removes the residue. The interior of the waveguide array is then coated with glyptal or by electrophoretic deposition of carbon lamp black suspended in a glyptal/xylene solution. The resultant 1-5- mu -thick film is vacuum pyrolyzed to carbon. A treated waveguide array has been operated successfully on a test stand and another in PLT.