E.R. Crosson

Coherent spontaneous radiation from highly bunched electron beams

Abstract Coherent spontaneous undulator radiation has now been observed in several FELs, and is a subject of special importance to the design of self-amplified spontaneous emission (SASE) devices. We report observations of coherent spontaneous radiation at the Stanford Picosecond FEL Center at wavelengths as short as 5 microns. Enhancement of spontaneous radiation over predicted incoherent levels by as much as a factor of 6 × 10 4 has been observed at longer wavelengths when the electron bunches are compressed after off-peak acceleration. We discuss the possible structure responsible for these enhancements and present direct measurements of the electron distributions using transition radiation techniques.

T2 selective scanning vibrational echo spectroscopy

Abstract Experiments are presented on a mixture of tungsten hexacarbonyl (W(CO) 6 ) and (acetylacetonato)dicarbonylrhodium(I) (Rh(CO) 2 acac) in liquid dibutylphthalate (DBP) in which the spectrum of the asymmetric CO stretching modes (∼2000 cm −1 ) is taken by continuously scanning the frequency of a ps vibrational echo pulse sequence and observing the vibrational echo intensity. While the absorption spectrum shows a large solvent background with DBP vibrational peaks, the vibrational echo spectrum is background free and contains only the W(CO) 6 and Rh(CO) 2 acac peaks. When the delay between the vibrational echo pulses is changed, the ratio of the W(CO) 6 and Rh(CO) 2 acac peaks changes because of differences in their homogeneous dephasing times.

THE DETERMINATION OF AN ELECTRON BEAM'S LONGITUDINAL PHASE SPACE DISTRIBUTION THROUGH THE USE OF PHASE-ENERGY MEASUREMENTS

Abstract We have been able to measure the longitudinal phase space distribution of the Stanford Superconducting Accelerators (SCA) electron beam by applying tomographic techniques to energy spectra taken as a function of the relative phase between the beam and the accelerating field. The temporal profile of the beam obtained by projecting the distribution onto the time axis is compared with that obtained from interferometric transition radiation measurements.

Sub-picosecond FEL micropulse length and electron bunch measurements

Abstract Recently, transform limited optical micropulses with lengths of less than 600 fs FWHM have been produced at the Stanford Picosecond Free Electron Laser (FEL) Center. These sub-picosecond FEL optical pulses are important for many types of experiments, especially those investigating fast kinematic processes. In an effort to understand the details of short optical micropulse production, we have made measurements of the electron beams micropulse structure with sub-picosecond resolution using a newly constructed electron beam diagnostic which uses transition radiation.

Multi-color, multi-user operation at the Stanford free electron laser center

Abstract Requests for beam time at the Stanford Picosecond Free Electron Laser Center far exceed the amount available. We describe our efforts to increase the available beam time by interleaving pulse trains of different wavelengths and delivering these to different users, and present some of our results.

MULTI-USER OPERATION AT AN FEL FACILITY

Abstract We have implemented a control system for the free electron laser (FEL) and its linear accelerator which allows us to produce independently controlled FEL beams multiplexed on a macropulse time scale. In this paper, we discuss the generation of multiple beams at Stanford. We go on to describe FEL beam diagnostics, the separation of multiple optical beams when generated using a single undulator, and finally optical beam distribution.

A technique for measuring an electron beam’s longitudinal phase space with sub‐picosecond resolution

We have developed a technique for measuring the longitudinal phase space distribution of the Stanford Superconducting Accelerator’s (SCA) electron beam which involves applying tomographic techniques to energy spectra taken as a function of the relative phase between the beam and the accelerating field, and optionally, as a function of the strength of a variable dispersion section in the system. The temporal profile of the beam obtained by projecting the inferred distribution onto the time axis is compared with that obtained from interferometric transition radiation measurements.

Sub-picosecond electron bunch profile measurement using magnetic longitudinal dispersion and off-phase RF acceleration

Using a longitudinally dispersive bending-magnet transport system followed by off-phase acceleration in a radio frequency electric field, it is possible to transform a highly relativistic electron bunch such that the relative time coordinate within the initial bunch is uniquely mapped to the relative energy coordinate of the final bunch. An energy spectrometer can be used to analyze the final energy distribution, revealing the longitudinal density profile of the initial electron bunch. We discuss factors which limit the resolution of this technique, and we present bunch profile measurements from the Stanford Superconducting Accelerator with a resolution better than 300 femtoseconds.

Enhanced coherent undulator radiation from bunched electron beams

When energetic bunches of electrons traverse an undulator field, they can spontaneously emit radiation both coherently and incoherently. Although it has generally been assumed that undulator radiation is incoherent at wavelengths short compared to the longitudinal size of the electron bunch, several recent observations have proved this assumption false. Furthermore, the appearance of coherent radiation is often accompanied by a significant increase in radiated power. Here we report observations of strongly enhanced coherent spontaneous radiation together with direct measurements, using transition radiation techniques, of the electron distributions responsible for the coherent emission. We also report demonstrated enhancements in the predicted spontaneous radiated power by as much as 6×104 using electron bunch compression.

Multiuser operation at the Stanford Free Electron Laser Center

The Stanford Picosecond Free Electron Laser Center has been extremely productive, and requests for beam time far exceed the amount available. In this paper we review the unique characteristics of Stanfords lasers that make them so popular. We then describe our efforts to increase the available beam time by interleaving pulse trains of different wavelengths and delivering these to different users, and present some of our results.