An overview of particle and photon beam bunch length measurements is presented in the context of free-electron laser (FEL) challenges. Particle-beam peak current is a critical factor in obtaining adequate FEL gain for both oscillators and self-amplified spontaneous emission (SASE) devices. Since measurement of charge is a standard measurement, the bunch length becomes the key issue for ultrashort bunches. Both time-domain and frequency-domain techniques are presented in the context of using electromagnetic radiation over eight orders of magnitude in wavelength. In addition, the measurement of microbunching in a micropulse is addressed.
Construction of a single-pass free-electron laser (FEL) based on the self-amplified spontaneous emission (SASE) mode of operation is nearing completion at the Advanced Photon Source (APS) with initial experiments imminent. The APS SASE FEL is a proof-of-principle fourth-generation light source. As of January 1999 the undulator hall, end-station building, necessary transfer lines, electron and optical diagnostics, injectors, and initial undulators have been constructed and, with the exception of the undulators, installed. All preliminary code development and simulations have also been completed. The undulator hall is now ready to accept first beam for characterization of the output radiation. It is the project goal to push towards full FEL saturation, initially in the visible, but ultimately to UV and VUV, wavelengths.
The OK-4/Duke storage ring free electron laser (FEL) was commissioned in November, 1996 and demonstrated lasing in the near UV and visible ranges. During one month of operation we performed preliminary measurements of the main parameters of the OK-4 FEL: its gain, lasing power and temporal structure. In addition to lasing, the OK-4/Duke FEL generated a nearly monochromatic 12.2 MeV (gamma) -ray beam. In this paper we describe the status of the main subsystems including the injector system and the ring itself, and discus future and in-progress upgrades to these systems. We also describe the parameters measured to date of the injector, the storage ring, the generated optical laser beams, and the backscattered (gamma) -ray beam.
The OK-4/Duke storage ring FEL was commissioned in November 1996 and demonstrated lasing in the near UV and visible ranges (345 - 413 nm). The OK-4 is the first storage ring FEL with the shortest wavelength and highest power for UV FELs operating in the United States. During one month of operation we have performed preliminary measurements of the main parameters of the OK-4 FEL: its gain, lasing power and temporal structure. In addition to lasing, the OK-4/Duke FEL generated a nearly monochromatic (1% FWHM) 12.2 MeV gamma-ray beam. In this paper we describe the design and initial performance of the OK-4/Duke storage ring FEL. We compare our predictions with lasing results. Our attempt to lase in the deep UV range (around 193 nm) is discussed. The OK-4 diagnostic systems and performance of its optical cavity are briefly described.
Designs are being developed to produce diffraction-limited sources based on storage-ring free-electron lasers (FELs) for the VUV and soft x-ray regime and linac-driven FELs in the few angstrom regime. The requirements on the beam quality in transverse emittance (rms, normalized) of 1 - 2 pi mm mrad, bunch length (1 ps to 100 fs), and peak current (1 to 5 kA) result in new demands on the diagnostics. The diagnostics challenges include spatial resolution (1 - 10 micrometer), temporal resolution (less than 100 fs), and single-pulse position measurements (approximately 1 micrometer). Examples of recent submicropulse (slice) work are cited as well as concepts based on spontaneous emission radiation (SER). The nonintercepting aspects of some of these diagnostics should also be applicable to high-power FELs.
The Advanced Photon Source will be a third-generation synchrotron radiation user facility in the hard x-ray regime (10 - 100 keV). The design objectives for the 7 GeV storage ring include a positron beam natural emittance of 8 X 10<SUP>-9</SUP> m-rad at an average current of 100 mA. Proposed methods for measuring the transverse and longitudinal profiles will be described. Additionally, a research and development effort using an rf gun as a low- emittance source of electrons for injection into the 200- to 650-MeV linac subsystem is underway. This latter system is projected to produce electron beams with a normalized, rms emittance of approximately 2 (pi) mm-mrad at peak currents of near one hundred amps. This interesting characterization problem will also be briefly discussed. The combination of both source types within one laboratory facility will stimulate the development of diagnostic techniques in these parameter spaces.
The optical cavity of the Boeing free-electron laser (FEL) was reconfigured as a semiconfocal ring resonator with two glancing incidence hyperboloid-paraboloid telescopes. The challenge for this experiment was the complexity of the ring resonator compared to the simplicity of a concentric cavity. The ring resonator's nonspherical mirror surfaces, its multiple elements, and the size of the components contributed to the problems of keeping the optical mode of the resonator matched to the electron beam in the wiggler. Several new optical diagnostics were developed to determine when the optical mode in the FEL was spatially and temporally matched to the electron beam through the wiggler. These included measurements of the focus position and Rayleigh range of the ring resonator optics to determine the spatial match of the optical mode through the wiggler, and a measurement of the position of the optical axis for multiple passes around the ring resonatorto determine the stability of the resonator alignment. This paper also describes the optical measurements that were necessary to achieve reliable lasing. The techniques for measuring ring resonator Rayleigh range and focus position, multiple pass alignment, cavity length, optical energy per micropulse, peak power, optical extraction, small signal gain, ringdown loss, lasing wavelength, electron bunch pulse width, and energy slew are discussed.
The optical cavity of the Boeing visible free electron laser was reconfigured from a concentric cavity to a glancing incidence ring resonator in late 1989 and was operated until December 1990. the crucial requirement for the optical cavity of an FEL is to provide an optical mode which is spatially and temporally matched to the electron beam as it moves through the wiggler. Several new optical diagnostics were developed to determine when the above requirement was satisfied. This paper will discuss those diagnostics which achieved and maintained the alignment of the ring resonator within tolerance to lase and measured the quality of lasing. The new diagnostics included measurements of the focus position and Rayleigh range of the ring resonator optics to determine the spatial match of the optical mode through the wiggler, and a measurement of the position of the optical axis for multiple passes around the ring resonator to determine the stability of the resonator alignment. Accelerator performance was determined by measuring the electron beam pulse width and charge, which indicated electron beam brightness, and by measuring the width of the spontaneous emission spectrum, which gave an indication of the alignment between the electron beam and the optical axis. Temporal overlap of electron and optical pulses was assured by measuring the optical cavity length. In addition, several other diagnostics which indicated FEL performance will be described: optical energy per micropulse, small signal gain, ringdown loss, laser pulse width, laser wavelength, and time resolved spectroscopy.
The results of several types of time-resolved experiments on rf-linac driven free-electron lasers (FELs) using streak-camera techniques are presented. In the past these techniques generally traded off time resolution, time span, and timing jitter to address either submicropulse or submacropulse effects. More recently, we have taken advantage of synchroscan streak cameras that were phase-locked to the reference 108.3 MHz rf signal combined with an orthogonal slow ramp deflection. One can then obtain submicropulse, submacropulse, and phase information during a single 100-microsecond(s) long macropulse. Samples of results include electron beam bunch lengths, cavity length tuning, phase slew/jitter, drive- laser phase stability, and visible FEL output temporal effects. Several of these demonstrations are the first of their kind on a FEL system (to our knowledge).