E. Jongewaard

W-band Sheet Beam Klystron Research at SLAC

SLAC is developing sheet beam klystron technology for narrow bandwidth, high peak and average power applications from L-band to W-band. Sheet beam devices are advantageous for several reasons. The primary advantage is the increased surface area in the RF circuit which significantly increases the dissipated heat that can be transferred through the circuit. The reduced charge density in the beam decreases the magnetic field required for beam transport and increases the achievable efficiency compared to a pencil-beam tube with the same beam voltage and current. Finally, both the RF circuit and the magnetic focusing system are simpler and less expensive to fabricate. The combination of features provided by a sheet beam klystron make it an ideal source for linear accelerator and high average power applications

X-band klystron development at the Stanford Linear Accelerator Center

X-band klystrons capable of 75 MW and utilizing either solenoidal or Periodic Permanent Magnet (PPM) focusing are undergoing design, fabrication and testing at the Stanford Linear Accelerator Center (SLAC). The klystron development is part of an effort to realize components necessary for the construction of the Next Linear Collider (NLC). SLAC has completed a solenoidal-focused X-band klystron development effort to study the design and operation of tubes with beam microperveances of 1.2. As of early 2000, nine 1.2 (mu) K klystrons have been tested to 50 MW at 1.5 microsecond(s) . The first 50 MW PPM klystron, constructed in 1996, was designed with a 0.6 (mu) K beam at 465 kV and uses a 5-cell traveling-wave output structure. Recent testing of this tube at wider pulsewidths has reached 50 MW at 55% efficiency, 2.4 microsecond(s) and 60 Hz. A 75 MW PPM klystron prototype was constructed in 1998 and has reached the NLC design target of 75 MW at 1.5 microsecond(s) . A new 75 MW PPM klystron design, which is aimed at reducing the cost and increasing the reliability of multi- megawatt PPM klystrons, is under investigation. The tube is scheduled for testing during early 2001.

THE NEXT LINEAR COLLIDER KLYSTRON DEVELOPMENT PROGRAM

Klystrons capable of 75 MW output power at 11.4 GHz have been under development at SLAC for the last decade. The work has been part of the program to realize all the components necessary for the construction of the Next Linear Collider (NLC). The effort has produced a family of solenoid-focused 50 MW klystrons, which are currently powering a 0.5 GeV test accelerator at SLAC and several test stands, where high power components are evaluated and fundamental research is performed studying rf breakdown and dark current production. Continuing development has resulted in a Periodic Permanent Magnet (PPM) focused 50 MW klystron, tested at SLAC and subsequently contracted for manufacture by industry in England and Japan. A 75 MW version of that PPM klystron was built at SLAC and reached 75 MW, with 2.8 microsecond pulses. Based on this design, a prototype 75 MW klystron, designed for low-cost manufacture, is currently under development at SLAC, and will eventually be procured from industry in modest quantities for advanced NLC tests. Beyond these developments, the design of Multiple Beam Klystrons (MBKs) is under study at SLAC. MBKs offer the possibility of considerably lower modulator costs by producing comparable power to the klystrons nowmorexa0» available, at much lower voltages.«xa0less

Periodic permanent magnet development for linear collider X-band klystrons

The Stanford Linear Accelerator Center (SLAC) klystron group is currently designing, fabricating and testing 11.424 GHz klystrons with peak output powers from 50 to 75 MW at 1 to 2 μs rf pulsewidths as part of an effort to realize components necessary for the construction of the Next Linear Collider (NLC). In order to eliminate the projected operational-year energy bill for klystron solenoids, Periodic Permanent Magnet (PPM) focusing has been employed on our latest X-band klystron designs. A PPM beam tester has operated at the same repetition rate, voltage and average beam power required for a 75-MW NLC klystron. Prototype 50 and 75-MW PPM klystrons were built and tested during 1996 and 1997 which operate from 50 to 70 MW at efficiencies greater than 55%. Construction and testing of 75-MW research klystrons will continue while the design and reliability is perfected. This paper will discuss the design of these PPM klystrons and the results of testing to date along with future plans for the development of ...

Recent advances in high power rf windows at X-band

The peak rf power levels produced by advanced high frequency power sources is beyond the level that can safely be transmitted through a single rf window of conventional design. New approaches are required to keep the rf electric fields at a managable level in the vicinity of the rf window. This paper describes some of the recent rf window design considerations and test results. Included are rf windows using one or more of the following features: over-moded dimensions, multiple modes, rf corona shields, and forced electric field reduction.

W-band sheet beam klystron design

Sheet beam devices provide important advantages for very high power, narrow bandwidth RF sources like accelerator klystrons. Reduced current density and increased surface area result in increased power capability, reduced magnetic fields for focusing and reduced cathode loading. These advantages are offset by increased complexity, beam formation and transport issues and potential for mode competition in the overmoded cavities and drift tube. This paper describes the design issues encountered in developing a 100 kW peak and 2 kW average power sheet beam klystron at W-band including beam formation, beam transport, circuit design, circuit fabrication and mode competition.

W-band micro-fabricated modular klystrons

Conventional CW millimeter amplifiers (coupled-cavity TWTs or gyro-klystrons) are limited in power by the maximum current hat can be accommodated in a single beam. Cathode current density, beam optics, and the magnetic field necessary to confine the beam, combine to limit beam current and add cost and bulk to the device. If the microwave source is designed as a pulsed klystron operating at a high voltage, larger lateral as well as axial dimensions can be employed. Beam optics become easier and permanent magnet periodic focusing is possible. A higher efficiency also results, because of the low perveance. A number of klystrons can then be fabricated on single substrate, using a deep- etch lithography technique. They can be water-cooled individually, and operated in parallel. Several such modules can be stacked to form a klystron `brick, requiring a relatively low voltage for the peak and average power produced. The `brick can be provided with a single output, or with individual, spatially-combined radiators. The design of a 4 X 10 X 1.5-inch module producing 500 kW peak, 500 W average at 91 GHz, and operating at 120 kV, 10 A, will be described.

Fabrication and Testing of a W-band Sheet Beam Klystron

SLAC has developed a 50 kW peak power, 2.5 kW average power, W-band, sheet beam klystron (WSBK). The magnetic circuit was modified to provide increased field at the start of the penultimate cavity to account for RF bunching and defocusing. The output section of the magnetic uses higher energy product magnets and eliminates the polepiece offsets used to provide focusing in the wide dimension of the beam.

W-band Sheet Beam Klystron PCM Focusing Design

A sheet-beam klystron has been designed, built and tested in the klystron department at SLAC. The first prototype, WSBK-1A, is a 95 GHz W-band device with a 74 kV, 3.6 A electron gun. Electrostatic focusing in the gun produces a sheet beam of approximately elliptical cross section with a height to width ratio of 1 to 12. Beam transmission over 90% was measured

High power W-band klystrons

The development of W-band klystrons is discussed. Modeling of the klystron performance predicts 100 kW output power from a single klystron. The permanent magnet focusing and small size of the circuit permit combination of multiple klystrons in a module. A six-klystron module in a single vacuum envelope is expected to produce 500 kW peak power and up to 5 kW average power. The critical issues in the W-band klystron development are the electron beam transport and the fabrication of the klystron circuit. Two microfabrication techniques, EDM and LIGA, are being evaluated to produce the W-band circuit.