to the larger beam area reducing the initial penetration rate and the maximum frequency is limited to about 300 ghz by present megawatt gyrotron technology calculated transmission curves for two nonconducting glassy ceramic wall borehole sizes and two gyrotron frequencies with an assumed wall dielectric constant of 2 are shown in fig2 a and 2 b the transmission factor for 6 inch 15 cm and 8 inch 20 cm diameter boreholes as a function of distance into the borehole for the 170 ghz gyrotron frequency is shown in fig2 a and for 280 ghz in fig2 b transmission at 170 ghz would decrease to 50 after about 1600 and 3600 ft 0 5 and 1 1 km in 6 and 8 inch 15 and 20 cm boreholes respectively at 280 ghz the 50 transmission distances would be 4300 and 9800 ft 1 3 and 3 0 km respectively for these same boreholes extremely deep boreholes can be achieved by inserting into the borehole a more efficient waveguide metallic waveguides are more efficient than dielectric waveguides and for a given diameter larger than two wavelengths a metallic waveguide with an internally corrugated surface has significantly better transmission efficiency than a smooth walled waveguide 6 for 4 inch 10 cm diameter aluminum tubing the transmission efficiency would be 90 for 25 miles 40 km at 170 ghz therefore inserting into the borehole an internally corrugated aluminum waveguide would make possible extremely deep penetration that could be used beyond present limits of rotary drilling technology the basic elements of a borehole with an.