C. Pellegrini

COLLECTIVE INSTABILITIES AND HIGH-GAIN REGIME IN A FREE ELECTRON LASER

Abstract We study the behavior of a free electron laser in the high gain regime, and the conditions for the emergence of a collective instability in the electron beam-undulator-field system. Our equations, in the appropriate limit, yield the traditional small gain formula. In the nonlinear regime, numerical solutions of the coupled equations of motion support the correctness of our proposed empirical estimator for the build-up time of the pulses, and indicate the existence of optimum parameters for the production of high peak-power radiation.

Free electron lasers for the XUV spectral region

Abstract The generation of high intensity coherent radiation in the soft X-ray region from a free electron laser will require the FEL to operate in the high gain or collective instability regime. In this mode of operation, which does not require a cavity resonator, the radiation field grows exponentially along the undulator until nonlinear effects bring on saturation. We discuss the conditions that the electron beam and the undulator must satisfy for the collective instability to develop. We present an example of an electron storage ring with an undulator in a bypass section which satisfies these conditions. We present estimates of the output power that one can expect from such systems.

Research and development toward a 4.5−1.5 Å linac coherent light source (LCLS) at SLAC

Abstract In recent years significant studies have been initiated on the feasibility of utilizing a portion of the 3 km S-band accelerator at SLAC to drive a short wavelength (4.5−1.5 A) Linac Coherent Light Source (LCLS), a Free-Electron Laser (FEL) operating in the Self-Amplified Spontaneous Emission (SASE) regime. Electron beam requirements for single-pass saturation in a minimal time include: 1) a peak current in the 7 kA range, 2) a relative energy spread of e = λ 4π , where λ[m] is the output wavelength. Requirements on the insertion device include field error levels of 0.02% for keeping the electron bunch centered on and in phase with the amplified photons, and a focusing beta of 8 m/rad for inhibiting the dilution of its transverse density. Although much progress has been made in developing individual components and beam-processing techniques necessary for LCLS operation down to ∼20 A, a substantial amount of research and development is still required in a number of theoretical and experimental areas leading to the construction and operation of a 4.5−1.5 A LCLS. In this paper we report on a research and development program underway and in planning at SLAC for addressing critical questions in these areas. These include the construction and operation of a linac test stand for developing laser-driven photocathode rf guns with normalized emittances approaching 1 mm-mrad; development of advanced beam compression, stability, and emittance control techniques at multi-GeV energies; the construction and operation of a FEL Amplifier Test Experiment (FATE) for theoretical and experimental studies of SASE at IR wavelengths; an undulator development program to investigate superconducting, hybrid/permanent magnet (hybrid/PM), and pulsed-Cu technologies; theoretical and computational studies of high-gain FEL physics and LCLS component designs; development of X-ray optics and instrumentation for extracting, modulating, and delivering photons to experimental users; and the study and development of scientific experiments made possible by the source properties of the LCLS.

Collective instability of a free electron laser including space charge and harmonics

Abstract The effects of harmonics, space charge and electron energy spread on the collective instability regime of an electron beam coupled to a planar undulator are analyzed. Both analytical and numerical results are presented.

Progress toward a soft X-ray FEL☆

Abstract We review the FEL physics and obtain scaling laws for the extension of its operation to the soft X-ray region. We also discuss the properties of an electron beam needed to drive such an FEL, and the present state of the art for the beam production.

Generation of high-intensity coherent radiation in the soft-x-ray and vacuum-ultraviolet region

An electron beam can be made to interact with an undulator magnet so that a collective unstable mode is excited. In this mode, the beam generates coherent radiation whose wavelength is determined by the undulator period and the electron energy. By proper choice of the electron-beam energy, energy dispersion, and density, one can obtain coherent radiation in the soft-x-ray region with peak and average power of the order of hundreds of megawatts and hundreds of milliwatts, respectively. Larger peak powers, of the order of a gigawatt, can be expected for UV radiation with λ in the range of 500–2000 A. We discuss the physical principles of these systems and give examples of how they might be built.

Initial measurements of the UCLA rf photoinjector

The 1.5 cell standing wave rf photoinjector has been operated for the past several months using a copper cathode. The photoinjector drive laser produces sub 2 ps pulses of UV (A = 266 nm) light with up to 200 p~J/pulse which generates up to 3 nC of charge. The emittance of the photoinjector was measured as a function of charge, rf launching phase, and peak accelerating field. Also, the quantum efficiency and pulse lengths of the laser beam and the electron beam were measured.

Hamiltonian model of a free electron laser

Abstract Both the Compton and the Raman regimes of a free electron laser are described by a relative hamiltonian which originates the evolution equations for 2 N + 2 canonically conjugate electron and field variables, with the space coordinate as the independent variable. Space charge and field contribution to electron transverse velocity are included. Scaled variables are introduced which allow for a description of the behaviour of the system in terms of a single electron-beam parameter.

A 2 to 4 nm high power FEL on the SLAC linac

Abstract 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.

High power femtosecond pulses from an X-ray SASE-FEL

Abstract We discuss how to use the large-gain bandwidth of an X-ray SASE-FEL to produce femtosecond long pulses by chirping and compressing the output FEL radiation. We consider the power level, spectral width, and intensity fluctuations of the compressed X-ray pulses, compared to the case with no compression.