Allen K. Kinkead

High-peak power K/sub a/-band gyrotron oscillator experiments with slotted and unslotted cavities

A K/sub a/-band gyrotron oscillator powered by a compact pulseline accelerator has been operated using oscillator cavities with and without axial slots. The oscillator was operated at high voltage ( approximately 900 keV) and high current ( approximately 500 A) in the approximate frequency range of 20-50 GHz. The use of axial slots has been shown to suppress low-starting-current whispering-gallery modes, in particular, modes of the TE/sub m2/ type, allowing stable operation in a linearly polarized TE/sub 13/ mode. A peak power of 35 MW has been observed at 6% efficiency. >

Initial operation of a high-power frequency-doubling X-band magnicon amplifier

This paper reports the initial high-power operation of a frequency-doubling magnicon amplifier at 11.120 GHz. The deflection cavities operate at 5.560 GHz. The device is operating in a single-pulse mode at 650 kV and /spl sim/225 A, using a 5.5-mm diameter beam from a plasma cathode, at a magnetic field of 6.7-8.2 kG. In order to overcome a gain saturation problem in the deflection cavities caused by plasma loading, the penultimate deflection cavity is operated very close to self-oscillation. The typical output pulselength is 100 ns full width at half maximum (FWHM), and is limited by RF breakdown of the penultimate cavity. Based on the measured far-field antenna pattern and absolute calibration of all microwave components, the measured output power is 14 MW (/spl plusmn/3 dB), corresponding to an efficiency of /spl sim/10%.

High-power RF tests on X-band dielectric-loaded accelerating structures

A joint Argonne National Laboratory (ANL)/Naval Research Laboratory (NRL) program is under way to investigate X-band dielectric-loaded accelerating (DLA) structures, using high-power 11.424-GHz radiation from the NRL Magnicon Facility. DLA structures offer the potential of a simple, inexpensive alternative to copper disk-loaded structures for use in high-gradient radio-frequency (RF) linear accelerators. A central purpose of our high-power test program is to find the RF breakdown limits of these structures. In this paper, we summarize the most recent tests results for two DLA structures loaded with different ceramics: alumina and Mg/sub x/Ca/sub 1-x/TiO/sub 3/ (MCT). No RF breakdown has been observed up to 5 MW of drive power (equivalent to 8 MV/m accelerating gradient), but multipactor was observed to absorb a large fraction of the incident microwave power. The latest experimental results on suppression of multipactor using a TiN coating on the inner surface of the dielectric are reported. Although we did not observe dielectric breakdown in the structure, breakdown did occur at the ceramic joint, where the electric field is greatly enhanced. Lastly, the MCT structure showed significantly less multipactor for the same level RF field.

Material Processing with a High Frequency Millimeter-Wave Source

A millimeter-wave beam based on a 15-kW, continuous-wave, 83-GHz gyrotron with superconducting magnets system is being investigated for use in material processing. The millimeter-wave beam can be focused to a few millimeters and manipulated quasi-optically and has been used in the following experiments: joining of ceramics (both similar and dissimilar materials), brazing of poled piezoelectric ceramics without significant heating and depoling, and coating of metals and polymers. Joining has been done directly and with reactive brazes. In coating, the beams short wavelength and absorption depth permit effective ceramic-coating deposition on lower temperature materials, e.g., polymers and metals, without significant substrate heating, and localized deposition of coatings as well. Finally, the millimeter-wave source has been used in the efficient production of nanophase metal and ceramic powders, via a greatly accelerated modified polyol process producing smaller powders of greater uniformity. The results and implications of the various experiments will be discussed with some theoretical calculations and modeling.

X-band magnicon amplifier for the Next Linear Collider

The magnicon is a scanning-beam microwave amplifier that is being developed as a high power, highly efficient microwave source for use in powering the next generation of high gradient electron linear accelerators. This article first discusses the results from a cold cathode magnicon experiment at 11.12 GHz, driven by a single-shot Marx generator. Following this, a design is presented for a new thermionic magnicon experiment to produce more than 50 MW at 11.4 GHz, using a 210 A, 500 kV beam from an ultrahigh convergence thermionic electron gun driven by a repetition-rated modulator. This new design has a predicted efficiency in excess of 60%.The magnicon is a scanning-beam microwave amplifier that is being developed as a high power, highly efficient microwave source for use in powering the next generation of high gradient electron linear accelerators. This article first discusses the results from a cold cathode magnicon experiment at 11.12 GHz, driven by a single-shot Marx generator. Following this, a design is presented for a new thermionic magnicon experiment to produce more than 50 MW at 11.4 GHz, using a 210 A, 500 kV beam from an ultrahigh convergence thermionic electron gun driven by a repetition-rated modulator. This new design has a predicted efficiency in excess of 60%.

Joining of ceramic tubes using a high-power 83-GHz millimeter-wave beam

High purity, high density alumina tubes have been successfully joined using a high-power millimeter-wave beam. This technique exploits the use of the beam-forming capability of an 83-GHz gyrotron-based system allowing the deposition of energy into a narrow region surrounding the joint area with minimal heating (<100/spl deg/C) of the metal fixturing (a modified microlathe). The power deposition and heating was modeled using a closed form analytical approach that has been compared with experimental results. The modeling results indicated areas of improvement that were implemented to make the process more effective. Conjoined tubes resulting from this technology meet the requirements for the dielectric-loaded accelerator (DLA) being developed by the Argonne National Laboratory.

High-power high-convergence electron gun for an 11.424-GHz pulsed magnicon

This paper describes the design and testing of an ultrahigh-convergence electron gun that was developed for a high-power X-band magnicon amplifier. The gun operates at an area compression ratio of 1400:1 to produce a /spl sim/500-kV /spl sim/200-A beam with a diameter of /spl sim/2 mm.

High Power Testing of ANL X‐Band Dielectric‐Loaded Accelerating Structures

In the second phase of a program to develop a compact accelerator based on a dielectric-loaded accelerating structure, we have conducted high power tests on a traveling-wave and a standing-wave prototype. Indications are that the traveling-wave structure achieved an accelerating gradient of 3-5 MV/m before the input coupling window failed, while the standing wave structure was poorly matched at high power due to contamination of copper residue on its coupling window. To solve both of these problems, a new method for coupling RF into the structures has been developed. The new couplers and the rest of the modular structure are currently under construction and will be tested at the Naval Research Laboratory shortly.

High Power Accelerator R&D at the NRL 11.424‐GHz Magnicon Facility

An 11.424‐GHz magnicon amplifier has been jointly developed by the Naval Research Laboratory and Omega‐P, Inc. as an alternative technology to klystrons for powering a future X‐band linear collider. This paper will discuss its background, operating principles, and results to date, as well its present status as part of a facility for collaborative research on accelerator‐related technologies that require high‐power 11.424‐GHz radiation. Two collaborative research programs are currently under way using the magnicon output. The first, a collaboration with Omega‐P, Inc. and the Institute of Applied Physics, is investigating active microwave pulse compressors using plasma switch tubes. The second, a collaboration with Argonne National Laboratory and SLAC, is investigating dielectric‐loaded accelerating (DLA) structures, with the ultimate goal of developing a compact DLA accelerator.

Progress Toward Externally Powered X-Band Dielectric-Loaded Accelerating Structures

We summarize recent progress in a program to develop externally powered dielectric-loaded accelerating (DLA) structures that can sustain high accelerating gradients. High-power RF tests of earlier structures showed strong multipactor loading. In addition, arcing at dielectric joints between the uniform DLA structure and matching sections at either end limited the achievable gradient. In this paper, we study the onset of multipactor in a DLA structure. We also study the effect of thin-film TiN coatings applied by atomic layer deposition and the effect of a reduction in the inner diameter of the structure. Test results of these structures show significant decreases in multipactor loading. We also test new structure designs that eliminate separate dielectric matching sections and, thus, the requirement for dielectric joints, including a DLA structure using a coaxial coupler and a clamped DLA structure. The clamped structure demonstrated a significantly improved gradient without breakdown.