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We report experimental results on a megawatt power level, 140-GHz coaxial gyrotron oscillator. The gyrotron has an inverted magnetron injection gun (IMIG) designed for operation at up to 95 kV and 88 A. The IMIG has an inner grounded anode which extends from the center of the gun down through the entire length of the tube including the cavity and collector. The IMIG was tested at up to 105 kV and 93 A in 3 /spl mu/s pulses, achieving an electron beam power of 10 MW. The output power from the coaxial gyrotron cavity was transported to an internal mode converter and a single mirror that coupled the power out transversely from the tube axis. A maximum output power of up to 1 MW was obtained in the TE/sub 27,11/ mode at 142 GHz at an efficiency of 16%, about one half of the design efficiency. The reduced efficiency was attributed to nonuniformity of the cathode emission and the sensitivity to the relative alignment of the electron gun, coaxial insert, and cavity. The cathode emission over the azimuthal angle was measured for two cathodes and was shown to be nonuniform due to both temperature and emitter work function nonuniformity. The gyrotron was also tested in two alternate configurations: 1) with the internal mode converter removed (axial output), and 2) with both the internal converter and the coaxial insert removed (empty cavity). In operation in the empty cavity configuration, which is equivalent to a conventional gyrotron oscillator, output power of up to 0.9 MW was observed.

A design for a 3-MW 140-GHz gyrotron based on the use of a coaxial cavity is given. The cavity mode is TE/sub 21,13/, chosen so that the ohmic heating on both the inner and outer conductors would be low enough for CW operation. The mode selection process, nonlinear, multimode and time-dependent modeling of the beam wave interaction, and gun design are discussed in detail. An inverted magnetron injection gun (MIG) is used to accommodate the inner conductor. The radiation is coupled out via a quasi-optical mode converter, consisting of an irregular cylindrical waveguide section followed by a step-cut launching aperture and a single near-parabolic mirror. The design of these components is also described.

The development of high power gyrotrons (118 GHz, 140 GHz) in continuous wave operation (CW) has been in progress for several years in a joint collaboration between different European research institutes and industrial partners. In this frame, two 140 GHz prototype gyrotrons for CW operation had been constructed and tested at the Forschungszentrum Karlsruhe. According to the results of the prototypes, seven 140 GHz CW gyrotrons were ordered. The first tube was operated at the Forschungszentrum Karlsruhe. A power of 950 kW at efficiency of 43 % (with energy recovery) could be obtained for pulse lengths of 180 s (limited by the available high-voltage power supply). A 30 minute pulse was performed with an output power of 540 kW. During this pulse almost no decrease in performance was found, especially the tube pressure only increased in the range of 10/sup -9/ mbar.

We report the first operation of a 17 GHz RF photocathode electron gun. This is the first photocathode electron gun to operate at a frequency above 2.856 GHz. Such electron guns have the potential for achieving record high values of electron beam quality. The 112 cell, π-mode, copper cavity was tested with 5–10 MW, 100 ns, 17.145 GHz pulses from a 24 MW Haimson Research Corp. klystron amplifier. Klystron power is stable to within ±5% up to 15 MW. Conditioning resulted in a maximum surface field of 250 MV/m, corresponding to an on-axis gradient of 150 MV/m. Dark current of 0.5 mA was observed at 175 MV/m, consistent with Fowler-Nordheim field emission theory if a field enhancement factor of about 100 is assumed. Electron bunches were generated by a regenerative laser amplifier that produces 2 ps, 1.9 mJ pulses at 800 nm with ±10% energy stability. These pulses were frequency tripled to 46 μJ of UV, and then focused on the wall of the cavity. Preliminary beam measurements indicate 0.12 nC bunches were produ...

Summary form only given. A gyrotron designed to produce 3 MW at 140 GHz has been realized and tested. The cavity is coaxial with a desired operating mode of TE/sub 21, 13/. The coaxial cavity was designed to have peak ohmic heating of 2.9 kW/cm/sup 2/ with low mode competition, and thus a higher electronic efficiency (45%) than the 37% predicted for a hollow cavity with the same mode number. The efficiency calculations were done by a time dependent, multimode simulation.

A high power radiation source in the THz range with long pulse and narrow line width is required for diagnosing fusion type plasmas by collective Thomson scattering. Gyrotrons currently meet the requirements concerning power, pulse length and line width when operating in the 50-150 GHz range. They may have the potential also to do this in the THz range. In this paper two methods based on harmonic operation are investigated. Both look promising, if high conversion efficiency can be attained. This, however, still has to be demonstrated. (C) 1999 Published by Elsevier Science B.V. All rights reserved.

We investigate experimental dependence of the third harmonic generation efficiency in the n-type Si crystals on the geometrical dimensions of the sample, polarization and power of the fundamental wave. The efficiency increases monotonically with the rise of the sample thickness up to a threshold value, and decreases dramatically above the threshold. At shorter propagation distances the generation efficiency could be correctly simulated using the layered medium approximation and the numerically calculated electron drift velocity response to the pumping wave electric field to describe the change of the semiconductor properties under high-power microwave irradiation.