Michael Read

Methods of Efficiency Enhancement and Scaling for the Gyrotron Oscillator

It is shown that a gyrotron oscillator operating in a slightly tapered magnetic field can attain an efficiency of ~78 percent, approximately 1.7 times higher than that obtainable in a constant magnetic field. Extensive numerical data have been tabulated and a convenient parameter is introduced to generate numerical efficiency scaling relations through which optimum operating conditions are clearly exhibited. Conditions for reaching the high efficiency operating regime are also studied and numerically illustrated.

The electron cyclotron maser as a high-power traveling wave amplifier of millimeter waves

The electron cyclotron maser instability has been exploited as the basis for a new type of traveling wave amplifier which operates at unusually high-power levels at millimeter wavelengths. The first experimental model of this amplifier has been operated at 35 GHz and has demonstrated a stable gain of 17 dB and an output power of 10 kW (unsaturated). The gain was linear over a dynamic range > 30 dB. The absolute value of the gain and its dependence on current and magnetic field were in excellent agreement with theoretical calculations. Bandwidth and saturated power have yet to be measured directly, but no fundamental problems were observed which will prevent successful achievement of the design predictions (viz., bandwidth ≃ 10 percent, power on the order of 105W, efficiency > 10 percent).

Experimental investigation of a 140-GHz coaxial gyrotron oscillator

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.

Design of a gridded gun and PPM-focusing structure for a high-power sheet electron beam

We describe the design of a gridded electron gun and periodic permanent magnet (ppm)-focusing structure to create and guide a 415-kV, 250-A sheet beam with a cross section of 100 mm /spl times/ 8 mm. The gun is intended for use with a 40-MW X- band sheet beam klystron for driving accelerators. Using the three-dimensional (3-D) code suite AMAZE, we designed a ppm structure that guided the beam over 80 cm with 98% transmission.

Microreplicated RF toroidal inductor

This paper reports on the modeling and fabrication of a truly three-dimensional high-quality-factor toroidal inductor using polymer replication processes. The critical dimensions are in the micrometer range, and the applied manufacturing method is based on polymer replication. Electrical measurements show that the inductor with an inductance of 6.0 nH exhibits a peak quality factor of 50 at a frequency of 3 GHz. Model verified by the measurement results shows that further improvement is still possible. Furthermore, the applied manufacturing technique can be extended to become a flexible packaging platform.

Development of Backward Wave Oscillators for Terahertz Applications

Calabazas Creek Research, Inc. is funded by the National Aeronautics and Space Administration to develop advanced backward wave oscillators that incorporate energy recovery, air cooling and improved performance. An improved coupler transforms the generated RF power to a high-purity, Gaussian output mode. The construction of a 600-700 GHz BWO is currently underway with higher frequency sources in development. Simulations predict 6-8 mW of RF power over a 100 GHz bandwidth.

Design of an electron gun using computer optimization

This paper considers an optimization technique in which the objective is attained via alterations to the physical geometry of the system. This optimization framework, to be considered in the context of electron guns, is known as optimal shape design. Optimal shape design has been used in a number of applications including wing design, magnetic tape design, and nozzle design, among others. In this investigation, we use the methods of shape optimization to design the cathode of an electron gun. The dynamical equations modeling the electron particle path as well as the generalized shape optimization problem will be presented. Illustrative examples of the technique on gun designs that were previously limited to spherical cathodes will be given.

Excitation of “monotron” oscillations in klystrons

A general method for calculations of starting currents of modes in klystron cavities is developed. This method allows one to determine the threshold for excitation of so-called monotron oscillations in klystrons having an arbitrary geometry of cavities and an arbitrary cross section of an electron beam. The method is used for evaluating the starting currents of monotron oscillations in a sheet beam, X-band klystron, which is currently in the design state at Calabazas Creek Research Inc.

Design of a 3-MW 140-GHz gyrotron with a coaxial cavity

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.

Computer Optimized Design of Electron Guns

This paper considers the problem of designing electron guns using computer optimization techniques. Several different design parameters are manipulated while considering multiple design criteria, including beam and gun properties. The optimization routines are described. Examples of guns designed using these techniques are presented. Future research is also described.