E. M. Choi

Plasma structures observed in gas breakdown using a 1.5 MW, 110 GHz pulsed gyrotron

Regular two-dimensional plasma filamentary arrays have been observed in gas breakdown experiments using a pulsed 1.5 MW, 110 GHz gyrotron. The gyrotron Gaussian output beam is focused to an intensity of up to 4u2002MW/cm2. The plasma filaments develop in an array with a spacing of about one quarter wavelength, elongated in the electric field direction. The array was imaged using photodiodes, a slow camera, which captures the entire breakdown event, and a fast camera with a 6 ns window. These diagnostics demonstrate the sequential development of the array propagating back toward the source. Gases studied included air, nitrogen, SF6, and helium at various pressures. A discrete plasma array structure is observed at high pressure, while a diffuse plasma is observed at lower pressure. The propagation speed of the ionization front for air and nitrogen at atmospheric pressure for 3u2002MW/cm2 was found to be of the order of 10 km/s.

Experimental Observation of the Effect of Aftercavity Interaction in a Depressed Collector Gyrotron Oscillator

This paper presents the experimental observation of the effect of an aftercavity interaction (ACI) in a depressed collector gyrotron oscillator. The gyrotron generates an output power of 1.5MW at 110GHz in 3μs pulses with a 96kV and 40A electron beam and has a single-stage depressed collector. The ACI arises from an unintended cyclotron resonant interaction between the microwave beam traveling out from the cavity and the gyrating electron beam. The interaction occurs in the uptaper of the launcher, immediately downstream from the cavity, where the magnetic field is slightly lower than its value in the cavity region. The ACI results in a reduction in efficiency since the electron beam tends to extract power from the wave. There is also a broadening of the spent beam energy profile, which reduces the effectiveness of the depressed collector and in turn limits the overall efficiency of a gyrotron. Measurements of the maximum depression voltage of the collector vs beam current at 96kV are compared with simula...

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.

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%.

Observation and Study of Low-Frequency Oscillations in a 1.5-MW 110-GHz Gyrotron

We report the observation of low-frequency oscillations (LFOs) in the range 165-180 MHz in a 1.5-MW 110-GHz gyrotron operating in 3-mus pulses. The oscillations have been measured by a capacitive probe located just before the entrance to the cavity. The LFOs are observed only in a narrow region of beam parameter space, at voltages between 45 and 60 kV, where no microwave emission occurs. When the gyrotron operates near 96 kV, with high output power, they are not seen. The variation of the frequency of the oscillations with electron beam voltage and magnetic compression was measured, and the results are reported. Time-domain analysis of the probe signal shows the influence of the beam current and cathode voltage on the time of onset of the oscillations. The amplitude of the time-domain signal indicates that the trapped electron current associated with the LFOs represents a few percent of the total electron current.

Imaging of Atmospheric Air Breakdown Caused by a High-Power 110-GHz Pulsed Gaussian Beam

We present the images of regular filamentary plasma arrays produced upon the breakdown of air at atmospheric pressure at the focal region of a high-power 110-GHz pulsed Gaussian beam. The source of the millimeter wave beam is a gyrotron that can generate up to 1.5-MW output power with 3-mus pulselength. This unique plasma structure exists only at high pressures. With decreasing pressure, the structure changes into layers of curved plasma sheets and into more familiar diffuse plasma. A main cause of the formation of the regular array structure appears to be the reflection from filaments. The successive generation of conductive filaments modifies the incident field pattern and creates local hot spots upstream of the existing filaments with regular spacing of roughly a quarter wavelength.

Efficiency enhancement of A 1.5-MW, 110-GHz gyrotron with a single-stage depressed collector

We report new experimental results from a 1.5-MW, 110-GHz gyrotron with a single-stage depressed collector. The gyrotron was operated in the TE22,6 mode with 3-μs pulse duration. An internal mode converter, which consists of a launcher and four mirrors, has been installed and tested. A highly Gaussian-like output beam was observed. A single-stage depressed collector has been operated for the study of efficiency enhancement using the same cavity V-2005 as was used in a previous experiment in the axial configuration, in which the output microwave beam propagated through a circular waveguide that also served as a collector. Output power of 1.5 MW, corresponding to 50% efficiency, was measured at 97 kV of beam voltage and 42 A of beam current at 25 kV of collector depression voltage. The results are compared between the axial configuration and the internal mode converter configuration.

Validation of 3-D Time Domain Particle-in-Cell Simulations for Cold Testing a W-Band Gyrotron Cavity

This paper evaluates the performance and reliability of a commercially available 3-D conformal finite-difference time-domain particle-in-cell (PIC) code, VSim, for cold test simulations of a gyrotron cavity. An interaction cavity for a 95-GHz gyrotron is simulated and optimized using the VSim PIC code to achieve TE6,2 mode excitation. The optimized cavity design is also studied and compared using the commercially available numerical code, CASCADE, and PIC code, MAGIC. A rigorous analysis of the simulation results obtained through VSim and MAGIC is performed using the CASCADE results as a reference. The performance of VSim is also compared with MAGIC based on its accuracy for calculating the resonant frequency and quality factor. Finally, the optimized cavity is fabricated and experimentally tested to measure the resonant frequency and the quality factor. The experimental results confirm the reliability and accuracy of the VSim cold cavity results.

Experimental study of a 1.5 MW, 110 GHz gyrotron with a single-stage depressed collector

A single-stage depressed collector has been installed on a 1.5 MW, 110 GHz gyrotron and tested in 3 microsecond pulse operation at 96 kV and 40 A. The overall efficiency with the depressed collector was 50 % at 1.5 MW of output power with a voltage depression of 25 kV. Theory predicts up to 35 kV of depressed voltage for these operating conditions. The discrepancy between the experiment and the theory has been explained by the effect of an aftercavity interaction (ACI). In this paper a theoretical study using MAGY code will be compared with the experimental results. Low frequency oscillations due to reflected electrons bouncing in the adiabatic trap are identified on the 3 microsecond pulse, for accelerating voltages in the range of 45 kV to 90 kV, with observed oscillation frequencies between 100 MHz and 170 MHz. A characterization of the oscillations is performed, through the scaling of the oscillation frequency with cathode voltage and beam current. The experimental results are compared with existing theories.

Experimental Study of a High Efficiency 1.5 MW, 110 GHz Gyrotron

Recently, an internal mode converter system has been installed in a 1.5 MW, 110 GHz gyrotron at MIT. We achieved an output power of 1.5 MW, corresponding to an efficiency of 37 % at a beam voltage of 96.6 kV and a beam current of 42 A. A Gaussian output beam was measured and the beam waist was determined. The depressed collector has been also setup. With a depressed collector, the efficiency was 50 % at an output power of 1.5 MW with a depressed voltage of 25 kV