Room temperature accelerator structures for linear colliders

Conference Record of the 1991 IEEE Particle Accelerator Conference, 2000

Damped and detuned linac structures designed to minimize the effects of wakefields excited bye* bunch trains in future linear colliders are presently under investigation at SLAC. This paper describes the results of measurements of both longitudinal and transverse wakefields performed at the ANL Advanced Accelerator Test Facility with two SLAC-built X-Band diskloaded waveguides: a conventional 30-cavity long constant-impedance structure and a non-conventional 50-cavity long structure along which the iris and spacer diameters have been varied so as to stagger-tune the HEM, t mode frequency by 37%. The results arc shown to be in excellent agreement with computations made by KN7C [l], TRANSVRS [2], TBCI [33, and LINACBBU [4].

AIP Conference Proceedings, 1995

Proceedings of the 2003 Bipolar/BiCMOS Circuits and Technology Meeting (IEEE Cat. No.03CH37440), 2003

The linacs proposed for the Next Linear Collider (NLC) and Japanese Linear Collider (JLC) would contain several thousand X-Band accelerator structures that would operate at a loaded gradient of 50 MV/m. An extensive experimental and theoretical program is underway at SLAC, FNAL and KEK to develop structures that reliably operate at this gradient. The development of standing wave structures is a part of this program. The properties of standing wave structures allow them to operate at the loaded gradient in contrast to traveling wave structures that need conditioning to the unloaded gradient (65 MV/m for NLC/JLC). The gradients in the standing structures tested thus far have been limited by input coupler breakdowns. The behavior of these breakdowns is consistent with a model of pulsed heating due to high magnetic fields. New input couplers have been designed to reduce maximum magnetic fields. This paper discusses design considerations related to high power performance, wakefield suppression and results of high power tests of prototype standing wave structures.

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment

The energy upgrade of the SPARC_LAB photo-injector at LNF-INFN (Frascati, Italy) has been originally conceived replacing one low gradient (13 MV/m) 3 m long SLAC type S-band traveling wave (TW) section with two 1.4 m long C-band accelerating sections. Due to the higher gradients reached by such structures, a higher energy beam can be obtained within the same accelerator footprint length. The use of C-band structures for electron acceleration has been adopted in a few FEL linacs in the world, among others, the Japanese Free Electron Laser at SPring-8 and the SwissFEL at Paul Scherrer Institute (PSI). The C-band sections are traveling wave, constant impedance structures with symmetric input and output axial couplers. Their design has been optimized for the operation with a SLED RF pulse compressor. In this paper we briefly review their design criteria and we focus on the construction, tuning, low and high-power RF tests. We also illustrate the design and realization of the dedicated low level RF system that has been done in collaboration with PSI in the framework of the EU TIARA project. Preliminary experimental results appear to confirm the operation of such structures with accelerating gradients larger than 35 MV/m.

1994

This paper describes the current SLAC R&D program to develop room tem~rature accelerator structures for the Next Linear Collider (NLC). me s~ctures are designd to operate at 11.4 GHz at an accelerating gradient in the range of 50 to 100 MV/m. In the past year a 26 cm constant-impedance traveling-wave section, a 75 cm constant-im~dmce traveling- wave section, and a 1.8 m traveling-wave section with detuned deflecting modes have been high-power tested. The paper presents a brief description of the RF test setup, the design and manufacturing details of the structures, and a discussion of test results includlng field emission, RF processing, dark current spwtrum and RF breakdown. Experimental Setup ., Accelerator structures descriti in this paper were dl testi in the Accelerator Structure Test Area (ASTA) facility located

2003

In the NLC project multiple bunches of electrons and positrons will be accelerated initially to a centre of mass of 500 GeV and later to 1 TeV or more. In the process of accelerating 192 bunches within a pulse train, wakefields are excited which kick the trailing bunches off axis and can cause luminosity dilution and BBU (Beam Break Up). Several structures to damp the wakefield have been designed and tested at SLAC and KEK and these have been found to successfully damp the wakefield [1]. However, these 2π/3 structures suffered from electrical breakdown and this has prompted us to explore lower group velocity structures operating at higher fundamental mode phase advances. The wakefield partitioning amongst the bands has been found to change markedly with increased phase advance. Here we report on general trends in the kick factor and associated wakefield band partitioning in dipole bands as a function of phase advance of the synchronous mode in linacs. These results are applicable to both TW (travelling wave) and SW (standing wave) structures

2004

Designs for a future TeV scale electron-positron Xband linear collider (NLC/GLC) require main linac units which produce and deliver 450 MW of rf power at 11.424 GHz to eight 60 cm accelerator structures. The design of this rf unit includes a SLED-II pulse compression system with a gain of approximately three at a compression ratio of four, followed by an over-moded transmission and distribution system. We have designed, constructed, and operated such a system as part of the 8-Pack project at SLAC. Four 50 MW X-band klystrons, running off a common 400 kV solid-state modulator, drive a dualmoded SLED-II pulse compression system. The compressed power is delivered to structures in the NLCTA beamline. Four 60 cm accelerator structures are currently installed and powered, with four additional structures and associated high power components available for installation late in 2004. We describe the layout of our system and the various high-power components which comprise it. We also present preliminary data on the processing and initial high-power operation of this system.

2000

During the initial phase of operation, the linacs of the Next Linear Collider (NLC) will contain roughly 5000 X-Band accelerator structures that will accelerate beams of electrons and positrons to 250 GeV. These structures will nominally operate at an unloaded gradient of 72 MV/m. As part of the NLC R&D program, several prototype structures have been built and operated at the Next Linear Collider Test Accelerator (NLCTA) at SLAC. Here, the effect of high gradient operation on the structure performance has been studied. Significant progress was made during the past year after the NLCTA power sources were upgraded to reliably produce the required NLC power levels and beyond. This paper describes the structures, the processing methodology and the observed effects of high gradient operation.

2017

The FERMI seeded free-electron laser (FEL), located at the Elettra laboratory in Trieste, is driven by a 200 meter long, S-band linac routinely operated at nearly 1.5 GeV and 10 Hz repetition rate [1]. The high energy part of the Linac is equipped with seven, 6 meter long Backward Traveling Wave (BTW) structures: those structures have small iris radius and a nose cone geometry which allows for high gradient operation [2]. Nonetheless a possible development of high-gradient, S-band accelerating structures for the replacement of the actual BTW structures is under consideration. This paper investigates a possible solution for RF couplers that could be suitable for linac driven FEL where reduced wakefields effects, high operating gradient and very high reliability are required.

Ultra-short electron pulses suffer from transverse wake fields resulting in a degradation of the beam quality. Since transverse emittance is a crucial parameter for FEL drivers, a careful characterization of wakefields is necessary in the design and commissioning phase of a high-brightness linear accelerator. In this paper we investigate the effect of transverse wakefields in the MAX IV linac. Estimations of the wakepotentials have been done with 2D modeling of the accelerating structures as well as with analytical models. In addition, electron beam effects caused by transverse wakes were studied at FERMI@Elettra [1] to verify the accuracy of the corresponding wakefield model in elegant [2].