Chad D. Marchewka

Experimental results for a 1.5 MW, 110 GHz gyrotron oscillator with reduced mode competition

A new result from a 110GHz gyrotron at MIT is reported with an output power of 1.67MW and an efficiency of 42% when operated at 97kV and 41A for 3μs pulses in the TE22,6 mode. These results are a major improvement over results obtained with an earlier cavity design, which produced 1.43MW of power at 37% efficiency. These new results were obtained using a cavity with a reduced output taper angle and a lower ohmic loss when compared with the earlier cavity. The improved operation is shown experimentally to be the result of reduced mode competition from the nearby TE19,7 mode. The reduced mode competition agrees well with an analysis of the startup scenario based on starting current simulations. The present results should prove useful in planning long pulse and CW versions of the 110GHz gyrotron.

Generation of chaotic radiation in a driven traveling wave tube amplifier with time-delayed feedback

The application of chaos in communications and radar offers new and interesting possibilities. This article describes investigations on the generation of chaos in a traveling wave tube (TWT) amplifier and the experimental parameters responsible for sustaining stable chaos. Chaos is generated in a TWT amplifier when it is made to operate in a highly nonlinear regime by recirculating a fraction of the TWT output power back to the input in a delayed feedback configuration. A driver wave provides a constant external force to the system making it behave like a forced nonlinear oscillator. The effects of the feedback bandwidth, intensity, and phase are described. The study illuminates the different transitions to chaos and the effect of parameters such as the frequency and intensity of the driver wave. The detuning frequency, i.e., difference frequency between the driver wave and the natural oscillation of the system, has been identified as being an important physical parameter for controlling evolution to chao...

Megawatt Power Level 120 GHz Gyrotrons for ITER Start-Up

We report operation of a 110 GHz gyrotron with 1.67 MW of output power measured in short pulses (3µs) at an efficiency of 42% in the TE22,6 mode. We also present a preliminary design of a 1 MW, 120 GHz gyrotron for ITER start-up with an efficiency greater than 50%.

Experimental results for a 1.5 MW, 110 GHz gyrotron with an improved cavity

Recently, a cavity (named V-2005) has been designed and successfully tested for the 1.5 MW, 110 GHz gyrotron experiment at MIT. The novel feature of the cavity lies in its low diffractive Q factor which corresponds to reducing the Ohmic loss on the cavity wall. The design and mode competition analysis were done with the simulation code, MAGY. In short pulse experiments, we achieved 1.67 MW of output power with an efficiency of 42 % at 97 kV of beam voltage and 41 A of current. Compared to the previous cavity (named V-2003), the new cavity has a smaller output taper angle and the Ohmic loss has been reduced by 25 %. The experimental result shows an increase in efficiency (42 % vs. 37 %) in the new cavity compared to the previous one. A map of the observed modes was generated by varying the cathode and cavity magnetic fields for both cavities. Less competition from the TE/sub 19.7/ mode was observed in the V-2005 cavity.

Observation of Low-Frequency Parasitic Oscillations in a 1.5 MW, 110 GHz Gyrotron

We report the observation of low-frequency parasitic oscillations, near 100 MHz, in a 1.5 MW, 110 GHz gyrotron operating in 3 microsecond pulses at MIT. The oscillations were measured on capacitive probes, which are located near the microwave resonator and are ordinarily used to measure the beam velocity pitch factor. The dependence of the frequency of oscillation on beam current, voltage, and magnetic compression ratio is discussed as are the regions of I-V space, in which the oscillations exist

Nonuniform cathode emission studies of a MIG gun

We present results on the first full 3D modeling of a magnetron injection gun (MIG) to study emission nonuniformity using the MICHELLE 3D code. The results indicate the effects of nonuniform current emission on velocity spread and pitch factor. Nonuniform emission has been shown to be detrimental to the efficiency of gyrotron operation. The model is based on a 96 kV, 40 A MIG used in a 110 GHz, 1.5 MW gyrotron experiment operating in short pulses at MIT and in long pulse/CW mode at CPI. The MIT gyrotron uses capacitive probes to determine the azimuthal variation in the pitch factor to compare with simulation.

Investigation of Synchronization and Transition to Chaos in a Driven TWT Delayed Feedback Oscillator

A chaotic waveform generator based on a TWT amplifier with delayed recirculated feedback driven with an external harmonic signal has been developed. In this paper, a numerical model of the oscillator is described and results of numerical and experimental investigation are presented. Such as oscillator is of great interest for novel communication and radar systems utilizing noise-like waveform signals

Non-Uniform Cathode Emission Studies of a Mig Gun

We present results on the first full 3D modeling of a magnetron injection gun (MIG) to study emission nonuniformity using the MICHELLE 3D code. The results indicate the effects of nonuniform current emission on velocity spread and pitch factor. Nonuniform emission has been shown to be detrimental to the efficiency of gyrotron operation. The model is based on a 96 kV, 40 A MIG used in a 110 GHz, 1.5 MW gyrotron experiment operating in short pulses at MIT and in long pulse/CW mode at CPI. The MIT gyrotron uses capacitive probes to determine the azimuthal variation in the pitch factor to compare with simulation.