Reviews surface flashover of insulator, primarily in vacuum, although some comments are made about the effect of ambient gases on surface flashover. It presents theoretical mechanisms of surface flashover and pertinent experimental results. The holdoff voltage of insulators depends upon many insulator parameters, such as material, geometry, surface finish, and attachments to electrodes, but also on the applied voltage waveform (duration, single pulse or repetitive), the process history of the insulator operating environment, and previous applications of voltage. Several suggestions are made regarding choice of the material, geometry, and processing when selecting an insulator for a particular application. Some specific techniques for improving the holdoff voltage of insulators are recommended. >

Flashover of insulators in vacuum: review of the phenomena and techniques to improved holdoff voltage

This review discusses the present state-of-the-art of passive high-power microwave components for applications in microwave systems for RF plasma generation and heating, plasma diagnostics, plasma and microwave materials processing, spectroscopy, communication, radar ranging and imaging, and for drivers of next generation high-field-gradient electron-positron linear colliders. The paper reports on high-power components for overmoded high-power transmission systems such as smooth-wall waveguides, HE/sub 11/ hybrid mode waveguides and quasi-optical TEM/sub 00/ beam waveguides. These include various types of mode converters, polarizers, cross-section tapers, bends, mode selective filters, pulse compressors, DC-breaks, directional couplers, beam combiners and dividers, vacuum windows, and instruments for mode analysis. Problems of ohmic attenuation and unwanted conversion to parasitic modes are discussed in detail and rules for alignment requirements are given. In the case of waveguide transmission, this review mainly concentrates on circular waveguide components but also deals with rectangular waveguide.

Passive high-power microwave components

The purpose of this article is to serve as a tutorial review on the subject of field emission and rf breakdown in high-gradient room-temperature accelerator structures and associated devices. The need to understand and control these two phenomena has become increasingly important because of the prospect of using high-gradient structures in future linear colliders. Electron field emission creates so-called dark current which parasitically absorbs rf energy, causes radiation, backgrounds, and possibly wakefields; it seems to be the precursor of rf breakdown, possibly in combination with local outgassing and plasma formation. In turn, rf breakdown limits the operation of accelerators and can cause irreversible damage to their physical structures. Research on these topics is interesting and challenging because it involves a mixture of disciplines such as surface physics, metallurgy, fabrication technologies, microwaves, beam dynamics and plasmas. This review consists of four parts: (1) field emission under dc, enhanced and rf conditions; (2) experimental set-ups; (3) prebreakdown stage--dark current and radiation; (4) experimental observations and analysis of rf breakdown. The review ends with conclusions and an outline of work that remains to be done.

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Field emission and RF breakdown in high gradient room temperature linac structures

The feasibility of future electron-positron colliders operating at energies >1 TeV will depend on both operating efficiency and cost of the microwave amplifiers that can be developed to drive the collider. To zeroth order, the required number of amplifiers depends inversely on the parameter A=P/sub p/T/sub p///spl lambda//sup 2/, where /spl lambda/ is the wavelength, and P/sub p/ and T/sub p/ are, respectively, the power and duration of the amplifier output pulses. Thus, one goal of amplifier research is to maximize A while keeping other parameter values within bounds so as not to excessively increase the cost of either the individual amplifier system or the collider structure (e.g., amplifier voltage V/spl lsim/500 kV, wavelength /spl lambda//spl gsim/1 cm). Operating within these bounds, gyro-amplifiers have demonstrated values of A=11/spl times/10/sup 4/ W s/m/sup 2/, which compares favorably with the best values of A achieved by klystrons. The gyro-amplifier program which led to this accomplishment is reviewed. Some 20 different gyro-amplifier configurations have been examined on our 450 kV gyro-amplifier test facility during the past several years. These tubes fall into five major categories: first-, second-, and third-harmonic gyroklystrons, as well as first- and second-harmonic gyrotwystrons. Peak powers in excess of 30 MW with pulse duration of 0.8 /spl mu/s have been achieved at 19.76 GHz in the TE/sub 02/ mode via a two-cavity second-harmonic gyroklystron with a first-harmonic drive cavity. The peak efficiency and gain were 28% and 27 dB, respectively. At present, there is ongoing construction of a new three-cavity second-harmonic coaxial gyroklystron, driven by a 500-kV 720-A beam, which is predicted to have an output power well above 100 MW at 17.136 GHz with an intrinsic efficiency in excess of 40%. With the use of a depressed collector, achievable overall amplifier efficiency, which is of very great importance in the collider application, could be in the range of 50-65%.

Gyro-amplifiers as candidate RF drivers for TeV linear colliders

A simple model of vacuum/dielectric/vacuum interface breakdown initiation caused by high power microwave has been developed. In contrast to already existing models, a spatially varying electron density normal to the interface surface has been introduced. Geometry and parameter ranges have been chosen close to the conditions of previously carried out experiments. Hence, physical mechanisms have become identifiable through a comparison with the already known experimental results. It is revealed that the magnetic field component of the microwave plays an important role. The directional dependence introduced by the magnetic field leads to a 25% higher positive surface charge buildup for breakdown at the interface downstream side as compared to the upstream side. This and the fact that electrons are, in the underlying geometry, generally pulled downstream favors the development of a saturated secondary electron avalanche or a saturated multipactor at the upstream side of the dielectric interface. The previously observed emission of low energy x-ray radiation from the interface is explained by bremsstrahlung generated by impacting electrons having initially a higher energy than the average emission energy. Final breakdown is believed to be triggered by electron induced outgassing or evaporation, generating a considerable gas density above the dielectricsurface and eventually leading to a gaseous breakdown.

Initiation of high power microwave dielectric interface breakdown

We have used relativistic klystron technology to extract 290 MW of peak power at 11.4 GHz from an induction linac beam, and to power a short 11.4-GHz high-gradient accelerator. We have measured rf phase stability, field emission, and the momentum spectrum of an accelerated electron beam. An average accelerating gradient of 84 MV/m has been achieved with 80 MW of relativistic klystron power.

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High-gradient electron accelerator powered by a relativisitic klystron.

In the course of developing new high-peak-power klystrons, high electric fields in several regions of these devices have become an important source of vacuum breakdown. In addition, a renewed interest in breakdown phenomena for nanosecond-pulse, megavolt-per-centimeter fields has been sparked by recent work in the area of gigawatt RF sources. The most important regions of electrical breakdown are in the output cavity gap area, the RF ceramic windows, and the gun ceramic insulator. The experiments and results on the breakdown in these regions are discussed, as well as the solutions to alleviate this breakdown problem. >

Breakdown phenomena in high power klystrons

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Breakdown phenomena in high-power klystrons

It is important to minimize power loss in the waveguide system connecting klystron, pulse-compressor, and accelerator in an X-Band NLC. However, existing designs of klystron output cavity circuits and accelerator input couplers utilize rectangular waveguide which has relatively high transmission loss. It is therefore necessary to convert to and from the low-loss mode in circular waveguide at each end of the system. A description is given of development work on high-power, high-vacuum 'flower-petal' transducers, which convert the TE/sub 10/ mode in rectangular guide to the TE/sub 01/ mode in circular guide. A three-port modification of the flower petal device, which can be used as either a power combiner at the klystron or a power divider at the accelerator is also described. >

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Flower-petal mode converter for NLC

SLAC is committed to developing an X-band source capable of producing 100 Megawatt, 1 microsecond pulses to power the next linear collider. The first experience encountered at SLAC in the X-Band Regime above a few Megawatts was in the relativistic klystron program in cooperation with LLNL and LBL. About 280 MW had been transmitted through a variety of waveguide components but at very short pulse widths ({approximately}40 nanseconds) and very low pulse repetition rates. The likelihood of high peak power rf breakdown in most X-band components and especially rf windows increases as the rf pulse length becomes longer. Testing components at peak power levels above that at which they are expected to reliably perform is essential in a development program.

R.S. Callin

Papers

High-gradient electron accelerator powered by a relativisitic klystron.

Breakdown phenomena in high power klystrons

Breakdown phenomena in high-power klystrons

Flower-petal mode converter for NLC

High power rf window and waveguide component development and testing above 100 mw at x-band