G. A. Loew

SLAC/CERN high gradient tests of an X-band accelerating section

High frequency linear collider schemes envisage the use of rather high accelerating gradients: 50 to 100 MV/m for X-band and 80 MV/m for CLIC. Because these gradients are well above those commonly used in accelerators, high gradient studies of high frequency structures have been initiated and test facilities have been constructed at KEK, SLAC and CERN. The studies seek to demonstrate that the above mentioned gradients are both achievable and practical. There is no well-defined criterion for the maximum acceptable level of dark current but it must be low enough not to generate unacceptable transverse wakefields, disturb beam position monitor readings or cause RF power losses. Because there are of the order of 10,000 accelerating sections in a high frequency linear collider, the conditioning process should not be too long or difficult. The test facilities have been instrumented to allow investigation of field emission and RF breakdown mechanisms. With an understanding of these effects, the high gradient performance of accelerating sections may be improved through modifications in geometry, fabrication methods and surface finish. These high gradient test facilities also allow the ultimate performance of high frequency/short pulse length accelerating structures to be probed. This report describes the high gradient test at SLAC of an X-band accelerating section built at CERN using technology developed for CLIC.

Computer Calculations of Traveling-Wave Periodic Structure Properties

The versatility and accuracy of programs such as LALA and specially SUPERFISH to calculate the rf properties of standing-wave cavities for linacs and storage rings is by now well established. Such rf properties include the resonant frequency, the phase shift per periodic length, the E- and H-field configurations, the shunt impedance per unit length and Q. While other programs such as TWAP have existed for some time for traveling-wave structures, the wide availability of SUPERFISH makes it desirable to extend the use of this program to traveling-wave structures as well. That is the purpose of this paper. In the process of showing how the conversion from standing waves to traveling waves can be accomplished and how the group velocity can be calculated, the paper also attempts to clear up some of the common ambiguities between the properties of these two types of waves. Good agreement is found between calculated results and experimental values obtained earlier.

RF breakdown studies in copper electron linac structures

The authors present a summary of RF breakdown-limited electric fields observed in experimental linac structures at SLAC (Stanford Linear Accelerator Center) and a discussion of how these experiments can be interpreted against the background of existing, yet incomplete, theories. The motivation of these studies, begun in 1984, is to determine the maximum accelerating field gradients that can be used safely in future e/sup +or-/ colliders, to contribute to the basic understanding of the RF breakdown mechanism, and to discover whether a special surface treatment can make it possible to supersede the field limits presently attainable room-temperature copper structures.<<ETX>>

Recent Progress on Sled, The SLAC Energy Doubler

From the following description, it will be seen that SLED raises the accelerator peak energy by 40% if the present 2.7 1s RF pulse length is used. The energy increase ” is 80% if the pulse length is extended to 5 ps. TO do this, however, changes have to be made in the modulators and trigger system, and the maximum repetition rate has to be halved to maintain the present average power level. In addition, more extensive switchyard modifications are required for~handling the higher energy beams. For these reasons, the SLED system will be initially installed and run at the present 2.7 ns pulse length. Performance at both pulse lengths is discussed in this paper.

Minimizing the Energy Spread within a Single Bunch by Shaping Its Charge Distribution

When electrons or positrons in a bunch pass through the periodic structure of a linear accelerator, they leave behind them energy in the form of longitudinal wake fields. The wakefields thus induced by early particles in a bunch offset the energy of later particles. For a linear collider, the energy spread introduced within the bunches by this beam loading effect must be minimized because it limits the degree to which the particles can be focused to a small spot due to chromatic effects in the final focus system. For example, for the SLC, the maximum allowable energy spread is + or - 0.5%. It has been known for some time that partial compensation of the longitudinal wake field effects can be obtained for any bunch by placing it ahead of the accelerating crest (in space), thereby letting the positive rising sinusoidal field offset the negative beam loading field. The work presented in this paper shows that it is possible to obtain complete compensation, i.e., to reduce the energy spread essentially to zero by properly shaping the longitudinal charge distribution of the bunch and by placing it at the correct position on the wave.

Accelerator and RF system development for NLC

An experimental station for an X-band Next Linear Collider has been constructed at SLAC. This station consists of a klystron and modulator, a low-loss waveguide system for RF power distribution, a SLED II pulse-compression and peak-power multiplication system, acceleration sections and beam-line components (gun, pre-buncher, pre-accelerator, focussing elements and spectrometer). An extensive program of experiments to evaluate the performance of all components is underway. The station is described in detail in this paper, and results to date are presented.<<ETX>>

Elementary principles of linear accelerators

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Single Bunch Beam Measurements for the Proposed SLAC Linear Collider

Single S-band bunches of ~ 109 electrons have been used to study the characteristics of the SLAC linac in anticipation of its operation as a linear collider. Emittance measurements have been made, the longitudinal charge distribution within single bunches has been determined and transverse emittance growth has been produced by deliberately missteering the beam. New equipment is being installed and checked out, and the sensitivity of new traveling-wave beam position monitors has been measured.

Electron Linac Instabilities

This paper attempts to summarize the present state of understanding of electron linac instabilities which result in beam loss along the accelerator. The major emphasis is placed on the type of instability which is caused by cumulative interaction of the beam with a multisection accelerator, as distinguished from the effect observed in short high-current accelerators where the instability is due to local regenerative interaction. The manifestations of the effect are first described and physical models are proposed. The properties of the HEM11 mode which is responsible for the instability are discussed in detail. Following, a short summary of the Panofsky theory is presented to illustrate various scaling laws which have been verified experimentally on the SLAC accelerator. Measured results are also compared with a detailed computer study made by R. Helm and agreement is generally found to be very good. After the discussion of beam break-up gain, a few conjectures are presented on the possible starting mechanisms of the effect. In conclusion, the program presently underway at SLAC to increase the current threshold of the instability is outlined and early results of this program are presented.

Wakefield measurements of SLAC linac structures at the Argonne AATF

Results are presented of measurements of both longitudinal and transverse wakefields performed at the Argonne National Laboratory Advanced Accelerator Test Facility with two SLAC-built X-band disk-loaded waveguides: a conventional 30-cavity long constant-impedance structure and a nonconventional 50-cavity long structure along which the iris and spacer diameters have been varied so as to stagger-tune the HEM/sub 11/ mode frequency by 37%. The results are shown to be in excellent agreement with computations made by KN7C, TRANSVRS, TBCI, and LINACBBU.<<ETX>>