Paul Emma

Generating femtosecond X-ray pulses using an emittance-spoiling foil in free-electron lasers

Generation of femtosecond to sub-femtosecond pulses is attracting much attention in X-ray free-electron laser user community. One method is to use a slotted, emittance-spoiling foil which was proposed before (P. Emma et al., Phys. Rev. Lett. 92, 074801 (2004)) and has been widely used at the Linac Coherent Light Source. Direct experimental characterization of the slotted-foil performance was previously unfeasible due to a lack of appropriate diagnostics. With a recently installed X-band radio-frequency transverse deflector, we are able to characterize the electron bunch spoiling effect and X-ray pulse when using the slotted foil. We show that few-femtosecond X-ray pulses are generated with flexible control of the single-pulse duration or double-pulse separation with comparison to the theoretical model.

Optical klystron enhancement to self-amplified spontaneous emission free electron lasers

The optical klystron enhancement to self-amplified spontaneous emission (SASE) free electron lasers (FELs) is studied in theory and in simulations. In contrast to a seeded FEL, the optical klystron gain in a SASE FEL is not sensitive to any phase mismatch between the radiation and the microbunched electron beam. The FEL performance with the addition of four optical klystrons located at the undulator long breaks in the Linac Coherent Light Source (LCLS) shows significant improvement if the uncorrelated energy spread at the undulator entrance can be controlled to a very small level. In addition, FEL saturation at shorter x-ray wavelengths (around 1.0 \AA) within the LCLS undulator length becomes possible. We also discuss the application of the optical klystron in a compact x-ray FEL design that employs relatively low electron beam energy together with a short-period undulator.The optical klystron enhancement to self-amplified spontaneous emission (SASE) free electron lasers (FELs) is studied in theory and in simulations. In contrast to a seeded FEL, the optical klystron gain in a SASE FEL is not sensitive to any phase mismatch between the radiation and the microbunched electron beam. The FEL performance with the addition of four optical klystrons located at the undulator long breaks in the Linac Coherent Light Source (LCLS) shows significant improvement if the uncorrelated energy spread at the undulator entrance can be controlled to a very small level. In addition, FEL saturation at shorter x-ray wavelengths (around 1.0 \AA) within the LCLS undulator length becomes possible. We also discuss the application of the optical klystron in a compact x-ray FEL design that employs relatively low electron beam energy together with a short-period undulator.

Experimental demonstration of a single-spike hard-X-ray free-electron laser starting from noise

In this letter, we report the experimental demonstration of single-spike hard-X-ray free-electron laser pulses starting from noise with multi-eV bandwidth. This is accomplished by shaping a low-charge electron beam with a slotted emittance spoiler and by adjusting the transport optics to optimize the beam-shaping accuracy. Based on elementary free-electron laser scaling laws, we estimate the pulse duration to be less than 1 fs full-width at half-maximum.

R&D for a Soft X-Ray Free Electron Laser Facility

R&D for a Soft X-Ray Free Electron Laser Facility A White Paper Report prepared by LBNL and SLAC with contributions from LBNL: David Attwood, John Byrd, John Corlett, Peter Denes, Roger Falcone, Phil Heimann, Wim Leemans, Howard Padmore, Soren Prestemon, Fernando Sannibale, Ross Schlueter, Carl Schroeder, John Staples, Marco Venturini, Tony Warwick, Russell Wells, Russell Wilcox, and Alexander Zholents SLAC: Chris Adolphsen, John Arthur, Uwe Bergmann, Yunhai Cai, Eric Colby, David Dowell, Paul Emma, John Fox, Josef Frisch, John Galayda, Robert Hettel, Zhirong Huang, Nan Phinney, Tom Rabedeau, Tor Raubenheimer, David Reis, John Schmerge, Joachim Stohr, Gennady Stupakov, Bill White, and Dao Xiang Lawrence Berkeley National Laboratory SLAC National Accelerator Laboratory June 2009

Design Optimization of Compensation Chicanes in the LCLS-II Transport Lines

LCLS-II is a 4th-generation high-repetition rate Free Electron Laser (FEL) based x-ray light source to be built at the SLAC National Accelerator Laboratory. To mitigate the microbunching instability, the transport lines from the exit of the linac to the undulators will include a number of weak compensation chicanes with the purpose of cancelling the momentum compaction generated by the main bend magnets of the transport lines. In this paper, we will report on our design optimization study of these compensation chicanes in the presence of both longitudinal and transverse space-charge effects.

Optical Klystron Enhancement to SASE X-Ray FELs

The optical klystron enhancement to self-amplified spontaneous emission (SASE) free electron lasers (FELs) is studied in theory and in simulations. In contrast to a seeded FEL, the optical klystron gain in a SASE FEL is not sensitive to any phase mismatch between the radiation and the microbunched electron beam. The FEL performance with the addition of four optical klystrons located at the undulator long breaks in the Linac Coherent Light Source (LCLS) shows significant improvement if the uncorrelated energy spread at the undulator entrance can be controlled to a very small level. In addition, FEL saturation at shorter x-ray wavelengths (around 1.0 A) within the LCLS undulator length becomes possible. We also discuss the application of the optical klystron in a compact x-ray FEL design that employs relatively low electron beam energy together with a shorter-period undulator.

Determination of Longitudinal Phase Space in SLAC Main Accelerator Beams

In the E164 Experiment at the Stanford Linear Accelerator Center (SLAC), we drive plasma wakes for electron acceleration using 28.5 GeV bunches from the main accelerator. These bunches can now be made with an RMS length of 12 microns, and accurate direct measurement of their lengths is not feasible shot by shot. Instead, we use an indirect technique, measuring the energy spectrum at the end of the linac and comparing with detailed simulations of the entire machine. We simulate with LiTrack, a 2D particle tracking code developed at SLAC. Understanding the longitudinal profile allows a better understanding of acceleration in the plasma wake, as well as investigation of related effects. We discuss the method and validation of our phase space determinations.

Plasma Light diagnostic for PWFA at SLAC

Summary form only given. A highly relativistic electron beam passes through an oven filled with the particular gas used in the experiment creating a plasma and a large amplitude wake field which causes the beam to lose and gain energy. The energy dumped into the plasma is dissipated through recombination and thermalization. Intensity of the plasma light is proportional to the wakefield amplitude. As the only non-beam diagnostic, study of plasma light can be used to characterize the plasma beam interaction to get the highest acceleration gradient. Moreover the unique spectrum of the gas can be used to as a reliable tool to measure the density vial the theory of Stark Broadening as an alternative to the other plasma density diagnostic tools which may not be available at the higher densities. Application of Plasma Light diagnostic to the past and future plasma experiments will be presented.

Plasma wakefield acceleration experiments with 28.5 GeV electron and positron beams

Summary form only given, as follows. Large gradient accelerators are necessary to reach the very high energies required at the collision point of future electron/positron colliders In the plasma wakefield accelerator (PWFA), a short electron or positron bunch drives a large amplitude plasma wave or wake. The transverse component of the wake leads to focusing of the particle bunch, while longitudinal components of the wake lead to energy loss and energy gain by particles. The PWFA is an energy transformer in which the energy is transferred from the particles in the core of the bunch in a single bunch scheme, or from a driver bunch in a two bunch scheme, to the particles in the back of the same bunch, or to a trailing witness bunch In the experiments described here, the 28.5 GeV electron or positron beam of the Stanford Linear Accelerator Center Final Focus Test Beam line is sent in a long lithium plasma. The bunch charge density is density is larger than the plasma density and the plasma wake is driven in the non-linear regime. In the case of an electron bunch, the bunch space charge field expels all the plasma electrons from the beam volume. The pure plasma ion column left behind the bunch head acts as an aberration-free plasma lens on the bunch core.

The NLC L-band bunch compressor

The first stage bunch compressor in the NLC injector complex compresses the e+/e- beams from a bunch length of 5 mm rms to 0.5 mm rms at the beam energy of 2 GeV. To obtain this compression ratio, the compressor rf section operates with an rf frequency of 1.4 GHz and a voltage of about 140 MV while a magnetic wiggler is used to generate an R/sub 56/ = 0-5 m. The bunch compressor is designed to operate with a beam from the damping ring that has a bunch spacing slew of 20 ps across the bunch train due to the transient loading in the damping rings. The compressor RF section is required to produce a specific energy profile along the bunch train so that the bunch spacing can be corrected in the compressor bending section. Further, the 1-amp beam heavily loads the compressor linac and beam loading compensation is essential to prevent a phase variation along the bunch train in the downstream linacs. In this paper, we will present simulation results of the beam loading compensation using a AT scheme assuming various initial bunch spacing arrangements. We will study the impact of the different compressor energy profiles on the beam energy, energy spread, and bunch length at the IP.