E.T. Scharlemann

Generation of XUV light by resonant frequency tripling in a two-wiggler FEL amplifier

Abstract FEL operation at short wavelengths is limited by electron-beam quality, by the availability of mirrors for oscillators and by the availability of input sources for FEL amplifiers. It is possible to use an FEL amplifier as a resonant-frequency tripling device, generating light and strong bunching at the third harmonic of a conventional input source in an initial wiggler section, then using a second wiggler section resonant at the tripled frequency to amplify the short-wavelength light. Neither mirrors nor a short-wavelength input source are required, and some relaxation of the electron-beam quality appears to be possible. We illustrate the scheme with a one-dimensional model and then with NUTMEG simulations of an 80 nm FEL amplifier initiated by a 240 nm input signal, in which an efficiency of the electron-beam power conversion to 80 nm light of nearly 10−4 was obtained.

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.

High-gain free electron lasers using induction linear accelerators

High-power free electron lasers (FELs) can be realized using induction linear accelerators as the source of the electron beam. These accelerators are currently capable of producing intense currents (102-104A) at moderately high energy (1-50 MeV). Experiments using a 500 A, 3.3 MeV beam have produced 80 MW of radiation at 34.6 GHz and are in good agreement with theoretical analysis. Future experiments include a high-gain, high-efficiency FEL operating at 10.6 μm using a 50 MeV beam.

High gain and high extraction efficiency from a free electron laser amplifier operating in the millimeter wave regime

Abstract Experiments at the Electron Laser Facility have generated peak microwave power of 180 MW at 35 GHz. The facility is operated as a single pass amplifier. Gain in excess of 30 dB/m has been observed up to saturation of the amplifier. For the 3.6 MeV, 850 A electron beam, the radiation corresponds to 6% energy extraction from the electron beam. Beyond saturation, the electron beam output power exhibits oscillations corresponding to the synchrotron motion of the trapped electrons in the ponderomotive well. In addition, the TE 21 and TM 21 modes have been studied and have power levels comparable to the fundamental. Third harmonic (105 GHz) radiation has been measured at power levels on the order of a few percent of the peak fundamental power.

Optical guiding in a free electron laser

Abstract The coherent interaction between an optical wave and an electron beam in a free electron laser (FEL) is shown to be capable of optically guiding the light. The effect is analyzed using a two-dimensional approximation for the FEL equations, and using the properties of optical fibers. Results of two-dimensional (cylindrically symmetric) numerical simulations are presented, and found to agree reasonably well with the analytically derived criterion for guiding. Under proper conditions, the effect can be large and has important applications to short wavelength FELs and to directing intense light.

Reducing sensitivity to errors in free electron laser amplifiers

Abstract Random errors that affect the transport of electron beams through long wigglers can degrade the performance of free electron laser amplifiers. We describe the origin and magnitude of the degradation, and test (with simulations) possible ways to improve amplifier performance in the presence of errors.

The SLAC soft X-ray high power FEL

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.

Generation of high power 140 GHz microwaves with an FEL for the MTX experiment

We have used the improved ETA-II linear induction accelerator (ETA-III) and the IMP steady-state wiggler to generate high power (1-2 GW) microwaves at 140 GHz. The FEL was used in an amplifier configuration with a gyrotron driver. Improved control of energy sweep and computerized magnetic alignment in ETA-III resulted in small beam corkscrew motion (<1.5 mm) at 6 Mev, 2.5 kA. Reduction of wiggler errors (<0.2%), improved electron beam matching, and tapered wiggler operation resulted in peak microwave power (single-pulse) of up to 2 GW. These pulses were transported to the MTX tokamak for microwave absorption experiments. In addition, the FEL was run in a burst mode, generating 50-pulse bursts of microwaves; these results are discussed elsewhere.<<ETX>>

Use of a FEL as a buncher for a TBA scheme

Abstract The CERN linear collider (CLIC) [W. Schnell, Proc. Workshop on Physics of Linear Colliders, Capri, 1988, eds. l. Palumbo, S. Tazzari and V.G. Vaccaro (1989) p. 345] is a two-beam-accelerator (TBA) scheme in which the driving beam consists of an intense 3 to 5 GeV electron beam bunched at 30 GHz. One possible way to produce this drive beam is to start with a low-energy (or order 10 MeV), high-current (about 5 kA) beam from an induction linac. In passing through a wiggler, this beam is bunched at 30 GHz into micropulses, each with about 10 12 electrons. To construct the format required for the CLIC drive beam, the bunched beam is subsequently chopped at 350 MHz. It is then accelerated to 3 to 5 GeV in an rf linac driven by conventional, low-frequency klystrons. Rf power is extracted at the bunch frequency of 30 GHz and fed into high-gradient structures to accelerate electron or positron beams to TeV energies. The drive beam is repeatedly reaccelerated in 350 MHz superconducting cavities. This study examines the design trade-offs of the proposed “front end” of the CLIC TBA, the linear induction accelerator and FEL. We examine the relevant figure of merit, the efficiency of bunching, as a function of beam energy, current, and emittance, and we consider the effects of wiggler errors, energy spread and slew, and beam misalignment.

Wiggler taper optimization for free-electron-laser amplifiers with moderate space-charge effects

Abstract The standard synchronous tapering method used to design the wiggler magnetic field for free electron laser (FEL) amplifiers operating in the Compton regime will not work for amplifier systems where space-charge effects are important. The space-charge effects lower the overall gain in the amplifier system and, even more importantly, shift the peak in the gain curve to magnetic field values that are significantly less than the synchronous magnetic field value. As a result, the overall predicted gain using the synchronous tapering method is too low. Moreover, the synchronous magnetic field corresponds to the peak in the gain curve for a frequency below the fundamental frequency. Consequently, shot noise at frequencies below the fundamental frequency can grow to levels that may prevent amplification of the fundamental. We have developed a new tapering strategy that improves the predicted amplifier gain and circumvents the shot-noise growth for systems with moderate space-charge effects. For this new strategy, we hold the wiggler magnetic field constant at a value below the synchronous value but near the peak of the gain curve for the fundamental frequency, for some optimized length at the front end of the wiggler. Beyond this constant wiggler section, the field is tapered using the standard synchronous tapering algorithm. This new tapering scheme results in significant improvement in predicted amplifier gains and limits the growth of shot noise to insignificant levels. We demonstrate the effectiveness of this new tapering algorithm using the tapered wiggler design for the proposed microwave heating experiment (MTX) at the Lawrence Livermore National Laboratory (LLNL).