34 GHZ, 45 MW MAGNICON: FIRST EXPERIMENTAL RESULTS*

A high efficiency, high power magnicon at 34.272 GHz has been designed and built as a microwave source to develop RF technology for a future multi-TeV electron- positron linear collider. To develop this technology, this new RF source is being perfected for necessary tests of accelerating structures, RF pulse compressors, RF components, and to determine limits of breakdown and metal fatigue. After preliminary RF conditioning of only about 2×10 5 pulses, the magnicon produced an output power of 10.5 MW in 0.25 µs pulses, with a gain of 54 dB. Slotted line measurements confirmed that the output was monochromatic to within a margin of at least 30 dB. In order to develop RF technology in the millimeter wavelength domain for a future multi-TeV electron- positron linear collider, it is necessary to test in realistic regimes accelerating structures, RF pulse compressors, RF components, and to determine limits of breakdown and metal fatigue. A key element of a test facility required for these kind of experiments is a high-power (tens of MW), 0.5-1 µsec pulse microwave amplifier. The most attractive candidate for this role is the magnicon, a microwave amplifier employing circular deflection of an electron beam (1). Magnicons have shown great potential with both high efficiency and high power. A first magnicon to have demonstrated these qualities was built and tested in the 80's in Novosibirsk. A power of 2.6 MW was obtained at 915 MHz with a pulse width of 30 µsec and an electronic efficiency of 85% (2). In experimental tests also at Budker INP (3), a second harmonic magnicon amplifier operating at 7.0 GHz achieved an output power of 55 MW in a 1.1 µsec pulse, with a gain of 72 dB and efficiency of 56%. Another frequency-doubling magnicon amplifier at the NLC frequency of 11.424 GHz has been designed and built in a collaboration between Omega-P, Inc and Naval Research Laboratory (NRL). The tube is designed to produce ~60 MW at 60% efficiency and 59 dB gain, using a 470 kV, 220 A, 2 mm-diameter beam. At present, the tube is conditioned up to power level of 25 MW for a 0.2 µsec pulse width (4). In scaling magnicon amplifiers from cm to mm wavelengths (consequently, to smaller physical dimensions), a few design problems arise at high power due to the limitations imposed by cathode loading, breakdown field, and pulsed heating of the cavity walls.