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Plasma wakefield acceleration offers acceleration gradients of several GeV/m, ideal for a next-generation linear collider. The beam optics requirements between plasma cells include injection and extraction of drive beams, matching the main beam beta functions into the next cell, canceling dispersion as well as constraining bunch lengthening and chromaticity. To maintain a high effective acceleration gradient, this must be accomplished in the shortest distance possible. A working example is presented, using novel methods to correct chromaticity, as well as scaling laws for a high energy regime.

The superconducting cavities in the CW linacs of LCLS-II will operate at 2 K, where cooling is very expensive. One source of heat is presented by the higher order mode (HOM) power deposited by the beam. Due to the very short bunch length, especially in the L3 region, the LCLS-II beam spectrum extends into the terahertz range. Ceramic absorbers, at 70 K between cryomodules, are meant to absorb much of this power. In this report we perform two kinds of calculations to estimate the effectiveness of the absorbers and the fractional power that remains to be removed at 2 K.

Beam instability caused by the electron cloud has been observed in positron and proton storage rings and it is expected to be a limiting factor in the performance of the positron damping ring (DR) of future linear colliders (LC) such as ILC and CLIC [1,2]. To test a series of promising possible electron cloud mitigation techniques as surface coatings and grooves, in the Positron low-energy ring (LER) of the PEP-II accelerator, we have installed several test vacuum chambers including (i) a special chamber to monitor the variation in the secondary electron yield of technical surface materials and coatings under the effect of ion, electron and photon conditioning in situ in the beam line (ii) chambers with grooves [3] in a straight magnetic-free section and (iii) coated chambers in a dedicated newly installed 4-magnet chicane [4] to study mitigations in a magnetic field region. In this paper, we describe the ongoing R&D effort to mitigate the electron cloud effect for the LC damping ring, focusing on the first experimental area and on results of the reduction in the secondary electron yield due to in situ conditioning.

The first experimental observation of magnetic resonances in electron clouds is reported. The resonance was observed as a modulation in cloud intensity for uncoated as well as TiN-coated aluminum surfaces in the positron storage ring of the PEP-II collider at SLAC. Electron clouds frequently arise in accelerators of positively charged particles, and severely impact the machines performance. The TiN coating was found to be an effective remedy, reducing the cloud intensity by three orders of magnitude.

The effect of surface roughness on the secondary electron emission from a sawtooth and isosceles triangular surface as well as a rectangular surface in a magnetic field under electron bombardment is investigated using a Monte-Carlo method. In all cases, some of the secondary electrons emitted from the surface return to the surface within their first few gyrations, resulting in a low effective secondary electron yield. Either sawtooth or isosceles triangle surface can significantly reduce the secondary emission yield in a magnetic field below the multipacting threshold with a weak dependence on the size of the surface and magnetic field. Rectangular surfaces can also reduce SEY below 1.0 but have a large sensitivity to the size of the grooves and the magnetic field. The impedance as well as other implementations of the grooved surfaces are also discussed.

It is difficult to measure the vertical emittance of the extremely low emittance rings such as third generation synchrotron light sources or damping rings of the future linear colliders due to their small vertical beam size. In such a low emittance ring, the beam lifetime is limited by the Touschek effect and ideally proportional to the bunch volume. Thus the bunch volume, or equivalently the vertical emittance when horizontal and longitudinal beam sizes are known, can be evaluated from the measurement of the Touschek lifetime. We determine the vertical emittance of the beam in KEK Accelerator Test Facility (KEK-ATF) damping ring by the lifetime model. The evaluated vertical emittance was 0.04±0.01 nm and it was consistent with the vertical emittance measured by wire-scanner beam profile monitors.

Design studies of a future TeV e+e− Linear Collider (TLC) are presently being made by five major laboratories within the framework of a world-wide collaboration. A figure of merit is defined which enables an objective comparison of these different designs. This figure of merit is shown to depend only on a small number of parameters. General scaling laws for the main beam parameters and linac parameters are derived and prove to be very effective when used as guidelines to optimize the linear collider design. By adopting appropriate parameters for beam stability, the figure of merit becomes nearly independent of accelerating gradient and RF frequency of the accelerating structures. In spite of the strong dependence of the wake fields with frequency, the single-bunch emittance blow-up during acceleration along the linac is also shown to be independent of the RF frequency when using equivalent trajectory correction schemes. In this situation, beam acceleration using high-frequency structures becomes very advantageous because it enables high accelerating fields to be obtained, which reduces the overall length and consequently the total cost of the linac.

A plasma-based wakefield acceleration experiment E-157 has been approved at SLAC to study acceleration of parts of an SLC bunch by up to 1 GeV/m over a length of 1 m. A single SLC bunch is used to both induce wakefields in the 1 m long plasma and to witness the resulting beam acceleration. The experiment will explore and further develop the techniques that are needed to apply high-gradient plasma wakefield acceleration to large-scale accelerators. The 1 m length of the experiment is about two orders of magnitude larger than for other high gradient plasma wakefield acceleration experiments and the 1 GeV/m accelerating gradient is roughly ten times larger than that achieved with conventional metallic structures. Using existing SLAC facilities, the experiment will study high gradient acceleration at the forefront of advanced accelerator research.

In the final-focus system of a linear collider, the effect of longitudinal space charge can be significant. A diverging or converging ultra-relativistic beam experiences a longitudinal space-charge force which is independent of the beam energy. This force, though weak and often neglected, causes an energy variation across the bunch which depends on the beam size, beam-pipe radius and bunch population and this may affect the chromatic correction of a beam line. Because of the inherent large chromaticity of a typical final-focus system, a very small energy variation, induced in the final focus, can lead to an intolerable increase of the spot size at the interaction point. The space-charge force, thus, gives rise to a limit on bunch intensity beyond which the resulting spot-size increase will degrade the collider performance. In this paper, we evaluate the effect of the longitudinal space charge and derive intensity limits for the three existing or proposed final foci.

This paper describes a method of reducing the transverse emittance dilution in linear colliders due to transverse wakefields arising from misaligned accelerator structures. The technique is a generalization of the Wake-Free [T.O. Raubenheimer, Nucl. Instr. and Meth. A 306 (1991) 61] correction algorithm. The structure errors are measured locally by varying the bunch charge and/or bunch length and measuring the change in the beam trajectory. The structure errors can then be corrected by varying the trajectory or moving the structures. The results of simulations are presented demonstrating the viability of the technique.

Recent technological developments have opened the possibility to construct a device which we call a linac coherent light source (LCLS) (C. Pellegrini et al., Nucl. Instr. and Meth. A 331 (1993) 223; H. Winick et al., Proc. IEEE 1993 Particle Accelerator Conf., Washington, DC, May 1993; C. Pellegrini, Nucl. Instr. and Meth. A 341 (1994) 326; J. Seeman, SPIE Meet. on Electron Beam Sources of High Brightness Radiation, San Diego, CA, July 1993 [1–4]); it would be a fourth-generation light source, with brightness, coherence, and peak power far exceeding other sources. Operating on the principle of the free electron laser (FEL), the LCLS would extend the range of FEL operation to much shorter wavelength than the 240 nm that has so far been reached. We report the results of studies of the use of the SLAC linac to drive an LCLS at wavelengths from about 3 to 100 nm initially and possibly even shorter wavelengths in the future. Lasing would be achieved in a single pass of a low emittance, high peak current, high-energy electron beam through a long undulator. Most present FELs use an optical cavity to build up the intensity of the light to achieve lasing action in a low-gain oscillator configuration. By eliminating the optical cavity, which is difficult to make at short wavelengths, laser action can be extended to shorter wavelengths by self-amplified-spontaneous-emission (SASE), or by harmonic generation from a longer wavelength seed laser. Short wavelength, single pass lasers have been extensively studied at several laboratories and at recent workshops (M. Cornacchia and H. Winick (eds.), SLAC Report 92/02; I. Ben-Zvi and H. Winick (eds.), BNL report 49651 [5,6]). The required low-emittance electron beam can be achieved with recently-developed rf photocathode electron guns (B.E. Carlsten, Nucl. Instr. and Meth. A 285 (1989) 313; J. Rosenzweig and L. Serafini, Proc. IEEE 1993 Particle Accelerator Conf., Washington, DC, 1993 [7,8]). The peak current is increased by about an order of magnitude by compressing the bunch to a lenght of about 0.2 ps (rms). Techniques for beam transport, acceleration, and compression without emittance dilution have been developed at SLAC as part of the linear-collider project (J. Seeman, Advances of Accelerator Physics and Technologies, ed. H. Schopper (World Scientific, Singapore, 1993 [9]). The undulator length required to saturate the laser varies from about 15 m for a 100 nm FEL to about 60 m at 3 nm. Initial experiments, at wavelengths down to about 50 nm are planned using the 25-m long Paladin undulator now located at LLNL. In a proposed future LCLS R&D facility the short wavelength light pulses are distributed to multiple end stations using grazing-incidence mirrors. About 10 14 photons per pulse can be produced at a 120 Hz rate, corresponding to average brightness levels of about 10 21 photons/s/mm 2 /mrad 2 within 0.1% BW and peak brightness levels of about 10 31 photons/s/mm 2 /mrad 2 within 0.1% BW. Peak power levels are several hundred megawatts to several gigawatts. Electron energies required range from about 500 MeV for the 100 nm FEL to about 7 GeV for 3 nm.

We discuss the design and performance of a 2 to 4 nm FEL operating in Self-Amplified Spontaneous Emission (SASE), using a photoinjector to produce the electron beam, and the SLAC linac to accelerate it to an energy of about 7 GeV. Longitudinal bunch compression is used to increase the peak current to 2.5 kA, while reducing the bunch length to about 40 μm. The FEL field gain length is about 6 m, and the saturation length is about 60 m. The saturated output power is about 10 GW, corresponding to about 1014 photons in a single pulse in a bandwidth of about 0.1%, with a pulse duration of 0.16 ps. Length compression, emittance control, phase stability, FEL design criteria, and parameter tolerances are discussed.

The results of calculations and simulations of the vertical emittance in the KEK Accelerator Test Facility damping ring are described. Both systematic and random alignment tolerances and a skew correction scheme are presented which limit the normalized vertical emittance to γϵ y −8 m rad. The effect of intrabeam scattering on the vertical emittance is calculated.

We report the results of preliminary studies of a 2 to 4 nm SASE FEL, using a photoinjector to produce the electron beam, and the SLAC linac to accelerate it to an energy up to 10 GeV. Longitudinal bunch compression is used to increase ten fold the peak current to 2.5 kA, while reducing the bunch length to the subpicosecond range. The saturated output power is in the multi-gigawatt range, producing about 1014 coherent photons within a bandwidth of about 0.2% rms, in a pulse of several millijoules. At 120 Hz repetition rate the average power is about 1 W. The system is optimized for X-ray microscopy in the water window around 2 to 4 nm, and will permit imaging a biological sample in a single subpicosecond pulse.

This article describes a new method of reducing the transverse emittance dilution in linear colliders due to both transverse wakefields and dispersive errors. The technique is a generalization of the dispersion-free [1] correction algorithm; the dilutions are corrected locally by varying the beam trajectory. This technique will complement the BNS damping [2] method which primarily corrects the dilutions resulting from coherent betatron oscillations. Finally, the results of simulations are presented demonstrating the viability of the technique.

In this paper, we describe a new trajectory correction technique for high energy linear accelerators. Current correction techniques force the beam trajectory to follow misalignments of the beam position monitors (BPMs). Since the particle bunch has a finite energy spread and particles with different energies are deflected differently, this causes “chromatic” dilution of the transverse beam emittance. The algorithm which we describe in this paper reduces the chromatic dilution by minimizing the energy dependence of the trajectory. To test the method we compare the effectiveness of our algorithm with a standard correction technique in simulations of the Stanford Linear Collider (SLC) linear accelerator and a design linac for a Next Linear Collider (NLC). While the simulations do not indicate that chromatic dilutions are a serious problem in the SLC linac, they would be debilitating in a future linear collider because of the very small beam sizes required to achieve the necessary luminosity. For example, in simulations of the NLC we have found that with typical alignment tolerances the beam size increased substantially after correcting the trajectory with a standard correction algorithm. In contrast, after correcting with our technique, the dilution was negligible. We feel that this technique will prove essential for future linear colliders.