AbstractSuccessful gyrotron experiments at FZK employing a conventional hollow cylindrical waveguide cavity, a quasi-optical mode converter with dimple-type launcher, a broadband silicon nitride Brewster window and a single-stage depressed collector (SDC), gave up to 1.6 MW output power at efficiencies between 48 and 60% for all operating mode series in the frequency range from 114 to 166 GHz. Frequency tuning in 3.7 GHz steps has been achieved by slow variation (minutes) of the magnetic field strength in the cavity. A specific hybrid magnet system for fast frequency tuning (1 s) is being manufactured. These experiments confirm the preliminary results achieved with a prototype fused quartz glass Brewster window. Up to now, the pulse duration has been 1–5 ms. However a water-edge-cooled silicon nitride composite Brewster window with its thermal conductivity of k=60 W/mK, permittivity εr=7.85 (θBrewster=70.35°), tan δ=3.5×10−4, excellent mechanical properties and clear window aperture of 100 mm diameter, will allow long-pulse operation (1 s) in the 1 MW power range. By increasing the beam current to 70 A (70% of the limiting current!), a maximum power of 2.14 MW, the highest value ever generated by a weakly relativistic gyrotron, has been obtained at 140 GHz with an efficiency of 34% (53% with SDC). With a TE31,17 coaxial cavity gyrotron, also equipped with a quasi-optical mode converter and a SDC, a maximum RF-output power of 1.7 MW with an efficiency of 26.2% has been achieved at a beam current Ib=69.7 A and a cathode voltage Uc=93.2 kV. Around the nominal beam parameters (Ib=52.2 A, Uc=91.8 kV, Bcav=6.65 T) an RF-output power of 1.3 MW, with an efficiency of 27.2% (41% with SDC), has been measured. In experiments on frequency step tuning, also using the silicon nitride composite Brewster angle window, 19 different modes with ≈2.2 GHz frequency spacing have been excited with ≥1 MW power at frequencies in the range between 134 and 169.5 GHz.