[en] The Plasma Separation Process now employs a 28 GHz cw gyrotron (VGA-8000) for electron cyclotron heating. Frequency increases to 56 GHz in FY 84 and approx. 140 GHz in FY '87 are planned. The tripple miter bend waveguide system in use with the VGA-8000 will be upgraded to provide mode control. The mode output of the tube is being measured; cippled wall converters will transform the tube output (TE₀₁, TE₀₂, TE₀₃) to a single mode. Transmission will proceed through reduced diameter corrugated waveguide bends or bends will be eliminated by use of a horizontally mounted tube. Directional couplers will measure forward power in the dominant mode and multi-mode reflected power