K.E. Kreischer

Characteristics and applications of fast-wave gyrodevices

Gyrodevice oscillators and amplifiers (or gyro-oscillators and gyro-amplifiers) are being utilized in a variety of applications where high power levels are required at millimeter-wave frequencies. Gyro-oscillators, developed primarily for magnetic fusion research applications, have achieved power levels near 1 MW for pulse durations in excess of 1 s at frequencies above 100 GHz. Continued work on these devices should enable them to achieve continuous-wave operation at multimegawatt power levels at frequencies in the 100-GHz to 200-GHz range, thereby meeting the requirements of planned magnetic fusion experiments. The development of gyro-oscillators for fusion experiments has led to the utilization of the devices in several industrial applications, such as ceramic sintering and metal joining. Activities in this area involve adapting the oscillators to the industrial environment where reliability, efficiency, and ease of operation are paramount. Gyro-amplifiers are being developed for applications requiring phase coherence and instantaneous bandwidth, such as in linear accelerators and millimeter-wave radar. Impressive results from X-band to W-band already suggest the promise of these devices. Potential new applications and novel gyrodevice design approaches continue to attract the attention of researchers around the world.

Second harmonic operation at 460 GHz and broadband continuous frequency tuning of a gyrotron oscillator

We report the short-pulse operation of a 460 GHz gyrotron oscillator both at the fundamental (near 230 GHz) and second harmonic (near 460 GHz) of electron cyclotron resonance. During operation in a microsecond pulse length regime with 13-kV beam voltage and 110-mA beam current, the instrument generates several watts of power in two second harmonic modes, the TE/sub 2,6,1/ at 456.15 GHz and the TE/sub 0,6,1/ at 458.56 GHz. Operation in the fundamental modes, including the TE/sub 0,3,1/ mode at 237.91 GHz and the TE/sub 2,3,1/ at 233.15 GHz, is observed at output powers up to 70 W. Further, we demonstrate broadband continuous frequency tuning of the fundamental modes of the oscillator over a range of more than 2 GHz through variation of the magnetic field alone. We interpret these results in terms of smooth transitions between higher order axial modes of the resonator. The 460 GHz gyrotron is currently being processed for continuous duty operation, where it will serve as a microwave source for sensitivity-enhanced nuclear magnetic resonance (dynamic nuclear polarization) studies at 16 T (700 MHz /sup 1/H), a field strength which is two-fold higher than has been accessible with previous technology.

Accurate parametric modeling of folded waveguide circuits for millimeter-wave traveling wave tubes

In this paper, results of different models are compared for calculating effective, cold-circuit (beam-free) phase velocities and interaction impedances of folded waveguide (FW) slow wave circuits for use in millimeter-wave traveling wave tubes (TWT). These parameters are needed for one-dimensional (1-D) parametric model simulations of FW traveling wave tubes (FWTWTs). The models investigated include approximate analytic expressions, equivalent circuit, three-dimensional (3-D) finite difference, and 3-D finite element. The phase velocity predictions are compared with experimental measurements of a representative FW circuit. The various model results are incorporated into the CHRISTINE1D code to obtain predictions of small signal gain in a 40-55 GHz FWTWT. Comparing simulated and measured frequency-dependent gain provides a sensitive, confirming assessment of the accuracy of the simulation tools. It is determined that the use of parametric 1-D TWT models for accurate, full band predictions of small signal gain in FWTWTs requires knowledge of phase velocity and impedance functions that are accurate to <0.5% and <10%, respectively. Saturated gain predictions, being approximately half as sensitive to these parameters, appear to require correct specification of phase velocity and interaction impedance to within /spl sim/1% and 20%, respectively. Although all models generate sufficiently accurate predictions of the interaction impedance, not all generate sufficiently accurate predictions of the effective axial phase velocity.

Dynamic nuclear polarization at 9T using a novel 250GHz gyrotron microwave source.

In the 1990s we initiated development of high frequency gyrotron microwave sources with the goal of performing dynamic nuclear polarization at magnetic fields (∼5-23 T) used in contemporary NMR experiments. This article describes the motivation for these efforts and the developments that led to the operation of a gyrotron source for DNP operating at 250 GHz. We also mention results obtained with this instrument that would have been otherwise impossible absent the increased sensitivity. Finally, we describe recent efforts that have extended DNP to 460 GHz and 700 MHz (1)H frequencies.

The Design of Megawatt Gyrotrons

The design parameters of a 120-GHz gyromonotron capable of output powers in excess of 1 MW are determined. A nonlinear model of the interaction between the beam and RF field is used in which the efficiency is a function of only three normalized variables. By expressing the technological constraints in terms of these variables, permissible design parameters yielding high-efficiency operation can be calculated. Constraints that are considered include ohmic heating of the walls, voltage depression of the beam, reduced coupling between the beam and RF field due to beam thickness, and efficiency degradation due to space-charge forces within the beam. An analysis of the trade-offs between current and voltage at the 1-MW level indicates that lower-order modes can be utilized at lower voltages, but the constraints based on current limitations are difficult to satisfy. An 80-kV 29-A design is presented that achieves a total efficiency of 44 percent. The primary uncertainty of these designs is the severity of competition due to parasitic modes. However, a number of isolated asymmetric modes appear capable of single-mode emission at 1 MW based on present experimental results. Multimegawatt operation is also considered. It is shown that powers exceeding 20 MW are possible if single-mode operation can be achieved in very-high-order modes. The methodology presented in this paper is general and can be easily adapted to other frequencies and output powers.

Submillimeter-wave harmonic gyrotron experiment

A theoretical and experimental investigation of the operation of a harmonic gyrotron at submillimeter wavelengths is reported. Using a waveguide cavity with an iris at the output end of the straight section, 14 different second-harmonic modes were observed with frequencies of 301-503 GHz, output powers of 1-22 kW, and a 12-MHz emission frequency bandwidth. The highest output power was 22 kW, with a total efficiency of 3.5% at 467 GHz, and an output power of 15 kW with a 6% efficiency was obtained at 417 GHz. Research was conducted using a 65-75 kV up to 10-A electron gun with a 1/1.5- mu s pulse length and a 4-Hz repetition rate, which produced a helical electron beam in magnetic fields of up to 14 T. These results represent the first operation of a high-power harmonic gyrotron in the submillimeter region. >

Theory of gyrotrons with coaxial resonators

In the development of powerful gyrotrons capable of long-pulse or continuous wave operation the cardinal problem is to provide stable, single-mode highly efficient operation at the desired mode with an acceptable level of ohmic losses in the resonator walls. In the present paper the selective properties of various coaxial resonators as well as ohmic losses of the microwave power in outer and inner conductors are considered. The start-up scenario in powerful gyrotrons is discussed. The results of numerical simulations of a 1 MW, 280-GHz coaxial gyrotron are presented which demonstrate that successive excitation of a number of modes during the voltage rise may not hinder the stable operation of the desired mode on the top of the voltage pulse. An electronic efficiency of 48% is predicted. >

Experimental demonstration of an emission-gated traveling-wave tube amplifier

This paper reports the results of the development of a traveling-wave tube (TWT) amplifier designed and operated using a high-frequency emission-gated field emitter array (FEA) cold cathode. The TWT was conservatively designed to operate with only 1% cathode current modulation but results show that 30% modulation of the current was achieved in the C-Band frequency range. The emission-gated TWT prototype was operated up to a current of 5 mA and RF output power of 280 mW using a 300-/spl mu/m diameter FEA cathode having 10 000 emitter tips with testing performed in single-pulse mode using 100-/spl mu/s pulses. Excellent beam control was demonstrated under all experimental conditions tested. Simulation shows that, with the same TWT circuit and demonstrated cathode modulation level, a 1-mm diameter cathode would generate /spl sim/60 W of output power in the same frequency band and /spl sim/80 W if the circuit were optimized for the measured level of modulation. Measurements also show that performance of the device does not degrade with frequency up to at least 7.0 GHz, which is the maximum operating frequency of the TWT. Cold measurements of the FEA electron gun alone indicate operation of the cathode up through 20 GHz might be possible. These results represent the first operation of an emission-gated cathode in a TWT and the highest power operation ever recorded in a microwave vacuum device using an emission-gated electron source.

Operational characteristics of a 14-W 140-GHz gyrotron for dynamic nuclear polarization

The operating characteristics of a 140-GHz 14-W long pulse gyrotron are presented. The device is being used in dynamic nuclear polarization enhanced nuclear magnetic resonance (DNP/NMR) spectroscopy experiments. The gyrotron yields 14 W peak power at 139.65 GHz from the TE(0,3) operating mode using a 12.3-kV 25-mA electron beam. Additionally, up to 12 W peak has been observed in the TE(2,3) mode at 136.90 GHz. A series of mode converters transform the TE(0,3) operating mode to the TE(1,1) mode. Experimental results are compared with nonlinear simulations and show reasonable agreement. The millimeter-wave output beam was imaged in a single shot using a pyroelectric camera. The mode patterns matched reasonably well to theory for both the TE(0,1) mode and the TE(1,1) mode. Repeatable mode patterns were obtained at intervals ranging from 0.8 s apart to 11 min apart at the output of the final mode converter

Theoretical and experimental investigation of a quasi-optical mode converter for a 110-GHz gyrotron

A quasi-optical mode converter has been designed to transform the TE/sub 22,6/ mode at 110 GHz to a Gaussian beam in free space. The converter consists of a rippled-wall waveguide launcher and two toroidal focusing reflectors. A full vector diffraction theory was developed to simulate the converter operation and predict the characteristics of the output beam. The simulation results were used to modify and improve the reflector design. The converter was built and tested on a 3-/spl mu/s pulsed gyrotron operating in the TE/sub 22,6/ mode at 110 GHz. Beam expansion and calorimetric efficiency measurements agreed well with diffraction theory predictions. Greater than 95% of the TE/sub 22,6/ power generated by the gyrotron was converted to a fundamental Gaussian beam and coupled into a corrugated waveguide. Four additional reflectors were built to transform the fundamental Gaussian beam into two similar Gaussian-like beams of approximately equal power level. The vector diffraction theory analysis suggested that simple sinusoidal and toroidal shaping mirrors can achieve high-efficiency beam splitting. Experiments showed that the beam splitting mirror relay successfully converted the fundamental Gaussian beam, produced by the launcher and two mirror relay, to two Gaussian-like beams. Calorimetric measurements indicated that 94% of the total power leaving the gyrotron was converted to the dual beam output with 52% of the power in the upper beam and 42% in the lower beam. The measured beam patterns and expansions were in good agreement with predictions of vector diffraction theory.