A new method to generate ultrahigh-power microwave pulses compatible with mildly relativistic electron sources is proposed. This method involves a novel microwave compressor in the form of a metal helically corrugated waveguide, which can enhance the power of frequency-modulated nanosecond pulses up to the multigigawatt level. The results of the proof-of-principle experiments at kilowatt power levels are in good agreement with theory. DOI: 10.1103/PhysRevLett.92.118301 PACS numbers: 84.40.Ik, 41.20.Jb, 41.60.Cr, 84.40.Fe Multigigawatt pulsed rf power, which has recently become available due to the development of relativistic electronics, opens up many new opportunities both for fundamental studies and future applications [1]. For example, it is important to recall that the power P 0 m 2 c 5 =e 2 8:7 GW (m, e are electron mass and charge, respectively, and c is speed of light) focused on an area of the order of the wavelength squared imparts a relativistic oscillatory velocity to electrons. Power at the multi-GW level was obtained at relatively long wavelengths of 3-10 cm when using electron beams with particle energies higher than 1 MeV and currents of tens of kA that were available at a small number of unique high-current accelerators. In this Letter, an alternative method of producing multi-GW pulses is proposed. It uses the amplification of significantly lower power radiation, which is generated with a certain frequency modulation during the pulse. Compression of this pulse is achieved after propagation in a waveguide with the proper frequency dispersion. If a compression factor K p 10-100 is possible, then the input pulse power required will be of the order of hundreds of MW. Such pulses are produced routinely in Cherenkov relativistic traveling wave amplifiers (TWTs) or backward-wave oscillators (BWOs), driven with modest energy 0.5-1.0 MeV electron beams [2,3]. However, compression of power from these devices requires additionally a sufficiently broadband frequency modulation within the pulse. A novel rf compressor in the form of a helically corrugated cylindrical waveguide can have the necessary dispersion while simultaneously it can sustain high strength rf fields as well as possessing a low level of reflections. Similar helically corrugated waveguides were intensively studied for gyro-TWTs [4].Consider compression of a quasimonochromatic pulse with frequency, monotonously varying in time from ! 1 to ! 2 . If the wave group velocity in the dispersive medium is an increasing function of frequency, v gr ! 2 > v gr ! 1 , then the tail of the pulse will overtake its leading edge, resulting in pulse shortening and a corresponding growth in amplitude if the losses are sufficiently low. To produce ultrahigh powers, the compressor characteristics should be optimized in order to achieve a maximum power compression factor at reasonable energy losses. Among the various methods of compression (see, e.g., [5]), the use of a metal waveguide is attractive because of its capability of handling high power, as well as...