L.H. Yu

A transient analysis of a bunched beam free electron laser

Abstract We studied the problem of the bunched beam operation of a free electron laser. Assuming the electron beam to be initially monoenergetic, the Maxwell-Vlasov equations describing the system reduce to a third order partial differential equation for the envelope of the emitted light. The Green function corresponding to an arbitrary shape of the electron bunch, which describes the transient behavior of the system, is obtained. We use Greens function to discuss the start-up problem as well as the power output and the power spectrum of self-amplified spontaneous emission.

High-gain harmonic generation of soft X-rays with the “fresh bunch” technique

We report numerical simulations (using the TDA code) and analytic verification of the generation of 64 {Angstrom} high power soft X- rays from an exponential regime single pass seeded FEL. The seed is generated in the FEL using the High Gain Harmonic Generation (HGHG) technique combined with the `Fresh bunch` technique. A seed pulse at 2944 {Angstrom} is generated by conventional laser techniques. The seed pulse produces an intense energy modulation of the rear part of a I GeV, 1245 {Angstrom} electron beam in a `modulator` wiggler. In the `radiator` wiggler, (resonant to 64 {Angstrom}), the energy modulation creates beam density modulation followed by radiation of the 46{sup th} harmonic of the seed. We use a magnetic delay to position the 64 A{Angstrom} radiation at the undisturbed front of the bunch to serve as a seed for a single pass, exponential growth FEL. After a 9 m long exponential section followed by a 7 m long tapered section the radiation power reaches 3.3 GW.

The Fresh-Bunch' technique in FELs

Abstract The “fresh bunch” technique is being proposed as a method of increasing the gain and power of FEL amplifiers in which the length of the optical radiation pulse is shorter than the length of the electron bunch. In a multi-stage FEL, electron beam energy spread is increased by the FEL interaction in the early stages. In the “fresh bunch” technique, the low energy spread of the electron beam is recovered by shifting the radiation pulse to an undisturbed part of the electron bunch, thus improving the gain and trapping fraction in later stages. A test case for the application of the “fresh bunch” method is demonstrated by numerical simulation. In this particular example we examine a subharmonically seeded VUV free-electron laser. We begin with the generation of harmonic radiation, which takes place over one part of the electron bunch. Then the radiation is shifted by means of a strong dispersive section to a fresh part of the bunch for exponential amplification and tapered wiggler amplification. By starting over with a new ensemble of electrons, the energy spread introduced by the bunching in the fundamental is removed, leading to an increased gain. Furthermore, it is possible to use a much stronger seed in the fundamental without incurring the penalty of a large energy spread later on. We note that more than a single application of the “fresh bunch” method may be done in a single FEL multiplier-amplifier. Thus X-ray wavelengths may be reached by successive multiplication in a chain of FEL amplifiers starting from a tunable seed laser.

Design of a harmonic generation FEL experiment at BNL

We present design parameters of a harmonic generation FEL experiment to be carried out at the Accelerator Test Facility (ATF) at BNL. This experiment out as a proof-of-principle for the proposed UV-FEL Users Facility at BNL. In the experiment we plan to triple the frequency of a CO{sub 2} seed laser by utilizing two superconducting wigglers and a dispersive section. The first wiggler will be used in conjunction with the CO{sub 2} seed laser to generate a ponderomotive force that will bunch the electron beam. The bunching will then be enhanced by the dispersion section. The second wiggler, tuned to the third harmonic of the seed laser will follow. In the beginning of the second wiggler the bunched beam will produce super-radiant emission (characterized by a quadratic growth of the radiated power), then the radiation will be amplified exponentially. The last part of the wiggler will be tapered. We plan to study the evolution of the various radiation growth mechanisms as well as the coherence of the tripled and exponentially amplified radiation. 12 refs.

The performance of a superconducting micro-undulator prototype

Abstract We report on the performance of a prototype of a superferric micro-undulator. The micro-undulator consists of a continuous winding of niobium-titanium wire wound on a low-carbon-steel yoke. It is about three periods long with a period of 8.8 mm and a gap of 4.4 mm. The undulator achieves a peak magnetic field on axis of over0.5 T. Asymmetry of the field pattern due to a dipole component was identified, analyzed and a correction has been applied to the undulator ends to produce a symmetric field distribution. Within the precision of the measurement the field pattern produced by the superferric undulator needed no correction.

Automatic beam steering in the NSLS storage rings using closed orbit feedback

Abstract This paper describes recent work on beam stabilization systems for the NSLS electron storage rings. It also reviews several types of detectors presently available for monitoring beam motion and describes possible schemes for stabilizing photon beams in a storage ring. Finally, it briefly discusses plans for simultaneous stabilization of many beam lines in the storage rings.

Amplified spontaneous emission in a single pass free electron laser

Abstract We discuss the relationship of the effective start-up noise in a single pass free electron laser to the spontaneous radiation emitted in the initial gain length of the wiggler magnet. Also, it is noted that the number of modes in the output is related to the phase space volume occupied by the spontaneous radiation emitted in the first gain length.

Real time harmonic closed orbit correction

Abstract We discuss several schemes for improving the stability of the closed orbit, by implementing a feedback system based upon harmonic analysis of both the orbit movements and the correction magnetic fields. The harmonic feedback system corrects the Fourier components of the orbit nearest to the betatron tune. Such a system may provide a significant improvement in orbit stability for all beamlines, using simpler electronics than required for an array of local bump feedback systems. Experiments based upon these schemes are in progress at the NSLS.

Ultrashort electron bunch length measurements at DUVFEL

The DUVFEL electron linac is designed to produce sub-picosecond, high brightness electron bunches for driving a short wavelength FEL. Four experiments have been commissioned to address the challenge of accurately measuring bunch lengths on this timescale. In the frequency domain, a short 12 period undulator is used to produce both off-axis coherent emission and on-axis incoherent single-shot spectra. The total coherent infrared power scales inversely with the bunch length and the spectral cutoff is an indication of bunch length. The density of the power spikes in the single-shot visible spectrum may also be used to estimate the bunch length. In the time domain, the linac accelerating sections and a bending magnet are used to implement the RF-zero phasing method, and a subpicosecond streak camera is also installed. The beam measurements with comparisons of these methods are reported.

Storage ring development at the National Synchrotron Light Source

Beam monitoring systems and other electronic systems at the NSLS storage rings are described in detail. Both the VUV and X‐ray rings are discussed and applications, such as lithography, are proposed. (AIP)