Tak Sum Chu

Long-pulse and CW tests of a 110-GHz gyrotron with an internal, quasi-optical converter

A high-power gyrotron, employing an internal converter that produces a Gaussian-like output mode, has been designed and tested. The tube employed a TE/sub 22.6.1/-mode interaction cavity that was designed for operation at a frequency of 110 GHz. An internal converter, consisting of an advanced launcher design and four mirrors, produced a Gaussian mode that had a relatively uniform profile at the tube output window to minimize the peak power density. Tests on the tube resulted in output power levels of 680, 530, and 350 kW for pulse durations of 0.5, 2.0, and 10.0 s, respectively. Measurements of the temperature of the output window were made during the long-pulse tests. Output power levels of 1 MW were achieved under short-pulse (1 ms) operation and the tube was operated at CW power levels in excess of 100 kW.

Development and testing of a high-average power, 94-GHz gyroklystron

The development of a 10-kW average power, 94-GHz gyroklystron amplifier is described. This average power was obtained with 11% radio frequency (RF) duty factor and 92-kW peak power in the TE/sub 01/ circular cavity mode. The instantaneous bandwidth was 420 MHz, and the efficiency was 33.5%. Low-duty-factor testing also yielded a peak power of as much as 115 kW with 600-MHz instantaneous bandwidth. This development effort was carried out over the past three years and represents record average power performance in an amplifier at this frequency.

Demonstration of a high-power long-pulse 140-GHz gyrotron oscillator

A high-power long-pulse 140-GHz gyrotron oscillator has been designed, constructed, and tested. Key features of the gyrotron include an internal converter to transform the TE/sub 28,7,1/ operating mode to a Gaussian output beam, a single-stage-depressed collector, and a chemical-vapor-deposition diamond output window. Peak output powers up to 930 kW at 34% efficiency have been demonstrated at 5-ms pulse lengths. Due to power supply limitations, long-pulse operation was not possible for beam currents above 25 A. At 25-A beam current and 500-kW output power, pulse lengths up to 700 s in duration were achieved. The gyrotron has been shipped to the Wendelstein 7-X facility in Greifswald, Germany, where long-pulse demonstrations up to 180s will be carried out at the 930-kW power level.

Gyrotron internal mode converter reflector shaping from measured field intensity

We present the formulation and experimental results of a new approach to designing internal mode converter reflectors for high-power gyrotrons. The method employs a numerical phase retrieval algorithm that reconstructs the field in the mode converter from intensity measurements, thus accounting for the true field structure in shaping the beam-forming reflectors. An outline for designing a four-reflector mode converter is presented and generalized to the case of an offset-fed shaped reflector antenna. The requisite phase retrieval and reflector shaping algorithms are also developed without reference to specific mode converter geometry. The design approach is applied to a 110 GHz internal mode converter that transforms the TE/sub 22,6/ gyrotron cavity mode into a Gaussian beam at the gyrotron window. Cold test experiment results of the mode converter show that a Gaussian beam with the desired amplitude and phase is formed at the window aperture. Subsequent high-power tests in a 1 MW gyrotron confirm the Gaussian beam observed in cold tests. The general development of the approach and its validation in a quasi-optical mode converter indicate that it is also applicable to other quasi-optical, microwave applications such as radio astronomy, free-space transmission lines, and mitre bends for overmoded waveguides.

Development and demonstration of high-average power W-band gyro-amplifiers for radar applications

The results of a focused program to develop high-power W-band gyro-amplifiers, which culminated in the demonstration of record average output powers from amplifiers in this band, are described. Following an experimental and theoretical study of low-duty prototype amplifiers, two high-average power devices were designed, built, and demonstrated. The first high-average power amplifier achieved 10.1-kW average output power at 33% efficiency in the TE/sub 0.1/ mode at 93.8 GHz. The instantaneous bandwidth was 420 MHz and the saturated gain at the 10.1-kW point was 32 dB. The second high-average power gyroklystron, designed for improved bandwidth, demonstrated 10.2-kW average power at 31% efficiency with 700-MHz instantaneous bandwidth and 33-dB saturated gain. The measured results of the low-duty prototype amplifiers and the high-average power gyroklystrons are described in detail. In addition, theoretically predicted results for a high-average-power W-band gyrotwystron amplifier, which is currently in construction, are presented.

Long‐pulse millimeter‐wave free‐electron laser and cyclotron autoresonance maser experiments

Experimental results on high‐power long‐pulse free‐electron laser (FEL) and cyclotron autoresonance maser (CARM) experiments are summarized. Single‐mode operation of a free‐electron laser oscillator at 27.4 GHz with a Bragg resonator has been obtained, with an output power of 990 kW for a beam energy of 320 keV and transmitted current of 30 A, corresponding to an efficiency of 10.3%. Free‐electron maser (FEM) amplifier operation at 35 GHz has yielded a gain of 26 dB with an output power of 800 kW, corresponding to an efficiency of 8.6%. CARM oscillator experiments at 32 GHz with a different electron gun have yielded lower powers because of poor beam quality; planned CARM experiments are discussed.

Design of correcting mirrors for a gyrotron used at Large Helical Device

Abstract We report the design of a mirror system used at the Large Helical Device (LHD) to convert the output radiation of the CPI 84-GHz gyrotron into the HE 11 mode of a corrugated waveguide transmission line. The radiation pattern of the gyrotron is measured using an IR camera at LHD. The measured data is analyzed at Massachusetts Institute of Technology (MIT) to retrieve the phase distribution of the radiation. The moments of the measured amplitude arrays are calculated to improve the reliability of the IR image data. The retrieved radiation structure is treated to synthesize the correcting mirrors.

High-power millimeter-wave Bragg free-electron maser oscillator experiments

Abstract Single-mode, high-power operation of a free-electron maser (FEM) oscillator with a Bragg cavity is reported. The beam is produced by a 0.27 μperv thermionic electron gun which was successfully operated up to 580 kV and 120 A. compressed to a nominal radius of 4 mm with a measured axial energy spread of Δγ 1 /γ 11 , transported through the interaction region by a 2.35 kG axial magnetic field, and pumped by a 500 G amplitude. 30 mm period permanent-magnet helical wiggler. A 10-period-long adiabatic introduction ensures high-quality group-I operation. The Bragg resonator is designed to provide feedback in the TE 11 mode at frequencies considerably higher than cut-off ( ω/2π 2 7.5 GHz and ω c /2π=11 GHz , respectively) to avoid beam interception and allow high-power operation. The FEM oscillator generated 990 kW (±0.5 dB ) of microwave power in a single axial mode at 27.47 GHz with a beam voltage of 320 kV and a transmitted current of 30 A, yielding an efficiency of 10.3%, in good agreement with nonlinear simulations. The starting current of this mode was measured to be 4 A, and a peak efficiency of 12.5% was observed for I t = 23 A and P μ w = 920 kW . The system was operated with three of the four axial modes of the Bragg resonator with an observed nonlinear frequency pulling of about 70 MHz. Far-field radiation pattern measurements confirm the TE 11 operation.

Long pulse high gain 35 GHz free-electron maser amplifier experiments

Results of the design and operation of a 35 GHz high gain, long pulse free-electron maser (FEM) amplifier are presented and discussed. The electron beam used in this experiment is produced by a thermionic electron gun which was successfully operated up to 580 kV and 120 A. with a measured perveance of 0.27 μperv, in excellent agreement with the design value. The beam is compressed to a nominal radius of 4 mm and guided by a 2.35 kG axial magnetic field. A 500 G amplitude, 30 mm period SmCo permanent magnet helical wiggler with a 10 period long adiabatic introduction provides the perpendicular momentum to the electrons and ensures high quality group 1 orbits in the interaction region. The measured energy spread before injection in the wiggler is Δγ∥/γ∥ < 0.5%; the spread in the interaction region inferred from the FEM performance is Δγ∥/γ∥ ≅ 1.0% The amplifier operates in the TE11 mode, with a cylindrical waveguide radius a = 8 m. and a corresponding cutoff frequency ωc/2π = 10.98 GHz. The amplifier was voltage-tuned from 18 GHz to 40 GHz in excellent agreement with TE11 linear theory. An instantaneous low gain bandwidth of more than 12 GHz was observed. In the high gain regime, a total output power of 800 kW was observed for an input power of 2 kW, yielding 26 dB of gain. The FEM was operating at an energy of 310 keV and a transmitted current of 30 A with a corresponding electronic efficiency of 8.6%.

Design and tests of a gyrotron with a radially-extracted electron beam

A high-power gyrotron, employing an output coupling concept where the spent electron beam was extracted radially though a slot in the output waveguide, has been designed and tested. The separation of the collection area for the electron beam from the outgoing microwave power is required to achieve MW-level average power in gyrotrons. The tube employed a TE/sub 22,2,1/-mode interaction cavity that was designed for operation at a frequency of 110 GHz. Tests on the tube resulted in an output power level of 470 kW for pulse durations of 2.5 seconds in the desired TE/sub 22,2/ mode at 110 GHz. Output power levels of 1 MW were achieved under short-pulse (1 ms) operation,. >