Calorimetric loads for high power transmission lines at millimeter wavelengths

For the electron cyclotron resonance heating (ECRH) system on the frascati tokamak upgrade (FTU), composed of four 0.5 MW gyrotrons pulsed for 0.5 s, two different calorimetric loads were designed to measure the mm-wave power at 140 GHz propagating either as a gaussian beam or as an HE11 mode carried by an oversized corrugated waveguide. The first one, designed for low reflectivity and 1 MW power handling capability, is now routinely used for gyrotron testing, conditioning and optimization on the generator side of the transmission line at 500 kW, 0.5 s. The basic design of the load, with a substantial upgrading of the cooling system for adequate heat removal, has full CW operation capability. The other one, whose cylindrical shape allows positioning as a waveguide replacement, is occasionally used when tests on the transmitted power along the 40 m of waveguide are needed. The loads are described together with low power measurements of reflected radiation.