B. Hogan

Experimental studies of stability and amplification in a two-cavity second harmonic gyroklystron

We report the operating characteristics of a sequence of two-cavity second harmonic gyroklystrons which are derived in part from a previous fundamental tube and utilize output cavities which resonate at twice the drive frequency. We present results from the design simulations as well as details of the stable range of operating parameters. While the harmonic tube is somewhat more susceptible to spurious oscillations and more sensitive to parameter variations than the fundamental device, there is still considerable parameter space available for amplifier operation. Peak powers above 30 MW are obtained with efficiencies greater than 28% and large signal gains of 27 dB. These results depend critically on the magnetic field profile which has a slight up-taper at the optimum operating point. The nominal beam parameters include a pulse length of 1 /spl mu/s, a voltage near 450 kV, a current in the range 235-245 A, and a perpendicular to parallel velocity ratio (/spl alpha/=v/sub /spl perp///v/sub z/) near one. >

Experimental studies of a high power, X-band, coaxial gyroklystron

Gyroklystrons are possible candidates for the RF sources in future linear colliders. A two-cavity and a three-cavity X-band gyroklystrons have been designed, built, and tested on the new 100 MW test bed at the University of Maryland. The theoretical design of the three-cavity X-band gyroklystron called for a peak power of 96 MW at an efficiency near 40% at a frequency of 8.568 GHz. In experiments, peak output power in the range of 75-85 MW at a frequency of 8.6 GHz has been measured with the three-cavity gyroklystron tube. The three-cavity tube was operated at electron beam voltage and current of 470 kV and 500 A, respectively. The efficiency is approximately 32% and the gain is approximately 30 dB. The pulse width of the output power signal is 1.7 /spl mu/s (full width at half maximum), while the pulse repetition rate was limited to 2 Hz in the proof-of-principle study. The paper details the design, performance, and diagnostics of these two X-band gyroklystrons operating at the fundamental cyclotron resonance.

High power operation of first and second harmonic gyrotwystrons

We report the first experimental operation of overmoded first and second harmonic gyrotwystron amplifier configurations. Both devices utilize a single cavity which is driven near 9.87 GHz in the TE011 mode, heavily attenuated drift tubes, and long tapered output waveguide sections. A magnetron injection gun produces a 460 kV, 245 A beam with a maximum average perpendicular‐to‐parallel velocity ratio approximately equal to one. The axial magnetic field profile is sharply tapered in the output section. Peak powers above 21 MW are achieved in 1 μs pulses with an efficiency exceeding 22% and a large signal gain near 24 dB in the first harmonic tube. The second harmonic tube achieves nearly 12 MW of the peak power with an efficiency of 11% and a gain above 21 dB. First harmonic amplifier performance is limited principally by competition from a fundamental mode output waveguide interaction; the second harmonic tube is limited by both travelling wave output modes and by a down‐taper oscillation.

The design of serpentine-mode converters for high-power microwave applications

In this paper, we report the design methodology and numerical results for two mode converters that are suitable for high-power microwave applications. Both converters are designed to operate at 11.424 GHz and utilize periodic serpentine structures to convert between modes with different azimuthal-mode indexes. The first converts about 98.5% of an incident linearly polarized TE/sub 12/ mode to the TE/sub 01/ mode when oriented as an H-plane bend, but has just 1% mode conversion to all modes when oriented as an E-plane bend. The second device converts a linearly polarized TE/sub 11/ mode to a TM/sub 01/ mode with over 99.99% effectiveness. The performance of both devices with respect to parametric variations is detailed. Experimental measurements of the radiation patterns from the TE/sub 12/-to-TE/sub 01/-mode converter are consistent with the theoretical predictions.

Experimental studies of stability and amplification in four overmoded, two-cavity gyroklystrons operating at 9.87 GHz

The performance of four overmoded (TE011 circular), two‐cavity gyroklystron amplifier tubes operating in X band is reported. A summary of the theoretical design procedure is presented, followed by a description of the experimental setup. The stability and amplification properties of the four gyroklystron tubes are subsequently described, each being modified in sequence to improve performance. The fourth tube produced 2.0–2.7 MW pulses at 9.87 GHz for 0.5–1.0 μs, and exhibited gains of 17–19 dB and efficiencies of 5%. The results were obtained using electron beams with voltages, currents, and computed beam alphas (v⊥/vz) of 407–425 kV, 115–135 A, and 0.8–1.0, respectively. The presence of instabilities in the gun downtaper and drift tube prevented operation at the design point (500 kV, 160 A, and alpha 1.5). The effects of varying input frequency, input power, and beam parameters (voltage, current, magnetic compression) on the amplifier operation are reported. Detailed descriptions of the instabilities obs...

Measurements of velocity ratio in a 90 MW gyroklystron electron beam

Measurements of the ratio of perpendicular to parallel velocity (/spl alpha/=/spl upsi//sub /spl perp////spl upsi/,) in a 390-420 kV, 160-240 A, small orbit gyroklystron electron beam produced by a magnetron injection gun have been performed using a capacitive probe. The probe employed guard rings to reduce uncertainties in measurement and calibration due to fringing electric fields. These studies represent the first measurements of a in such a powerful rotating beam. Since the current levels in this experiment are two to three times higher than those in previous velocity ratio studies of beams with similar voltage. The effects of changes in voltage, current, and magnetic compression on the measured velocity ratio are investigated. Values of /spl alpha/ in the range of 0.8-1.3 are typically observed, which are considerably higher than the values predicted by theoretical simulations. Errors in the modeling of space charge effects are ruled out as the cause of the discrepancy by the agreement between theoretical and experimental space charge current limits. Instead, a significant portion of the discrepancy is attributed to a reduction in cathode magnetic field by the self-axial magnetic fields of the electron beam. >

Current Status of Gyroklystron Research at the University of Maryland

At the University of Maryland, we have been developing high‐power coaxial gyroklystrons. Our present work is focused on the development of a 17.136 GHz four‐cavity frequency‐doubling gyroklystron amplifier. This device will then be used to drive a high gradient linear accelerator structure recently developed by the Haimson Corporation. Our work has been afflicted by many technical challenges, most arising from thermal imperfections in the custom‐made high current emitter of our electron gun. In our latest experimental run, instabilities were detected in the input cavity of our amplifier tube. These instabilities appear when the beam pitch ratio (α) is approximately 1, thus impeding our search of domains with higher α values (note that the circuit was designed to operate at α=1.4). In order to remedy this problem, we have radically redesigned the input cavity, changing both its geometry and Q factor. The new input cavity has been fabricated and cold‐tested. It will soon undergo hot‐test in the next run of ...

Development of Ku-band frequency-doubling coaxial gyroklystrons for accelerator applications

Summary form only given. At the University of Maryland, we have been exploring the possibility of using gyroklystrons to drive advanced linear colliders above X-band. We have designed and tested a high-gain four-cavity frequency-doubling system to enable us to use a phase-stable TWT driver. We have also designed an output waveguide system which transforms the power from the output waveguide and equally divides it into two standard WR62 rectangular waveguides. The scheme involves a rippled wall converter to convert the TE/sub 02/ mode to the TE/sub 01/ mode, then a custom converter to produce the TE/sub 20/ mode in the rectangular guide, followed by linear tapers and a standard bifurcation. We hope to use this transport system to energize and test Ku-band accelerator structures. In this paper, we present the experimental results of our four-cavity frequency doubling coaxial gyroklystron. The beam voltage and current are 460 kV and 540 A, respectively, and the applied magnetic field is about 5 kG. The expected interaction efficiency and gain at our design point are 34% and 61 dB, respectively. We also present design and test results for a number of other subsystems. We compare the theoretical calculations with the cold test results for each of the components used in the output waveguide system and present amplitude and phase balance measurements for the two output rectangular waveguides. Finally, we describe efforts to improve beam quality via modifications in emitter design and manufacturing.

Design and cold testing of a compact TE/sub 01//spl deg/ to TE/sup /spl square///sub 20/ mode converter

In this paper, we discuss a new compact converter, which converts the TE/sub 01/ circular mode into the TE/sub 20/ rectangular mode. Compared to the Marie converter, the new device has a significantly shorter length and is, therefore, more suitable for use in evacuated waveguide systems. Both the design efforts and the experimental testing of the prototype are described. Based on computer simulations, this converter has a center frequency of 17.15 GHz, and the predicted power transfer between the desired modes is 99.9%. Results from the low-power testing of the prototype indicate that when the TE/sub 20/ output mode is split into two equal parts (each in the TE/sub 10/ rectangular mode), the power transferred to each arm of the split differs by less than 0.15 dB over a 200-MHz bandwidth. This is the first experimental study of a converter based on this design. A high-power version of this device is now being fabricated for use under high vacuum conditions.

Design and operation of a two‐cavity third harmonic Ka‐band gyroklystron

We present the operating characteristics of a two‐cavity third harmonic gyroklystron experiment. The input cavity utilizes a 9.854 GHz TE011 mode which is driven by a 100 kW magnetron. The TE031 output cavity has a resonant frequency of 29.57 GHz. The nominal beam voltage and current are 435 kV and 210 A, respectively. The pulse length is about 1 μs and the average ratio of parallel to perpendicular velocity is near one. Peak powers above 1.8 MW are achieved with an efficiency of about 2% and a gain of 14 dB. The theoretical simulations are in good agreement with the experimental results.