K.W. Berryman

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.

Optical modes in a partially waveguided cavity

Abstract The behavior of the optical beam in an FEL cavity is well understood in both the long- and short-wavelength limits. At long wavelengths the optical mode interacts strongly with the walls, which act as a waveguide. At short wavelengths the optical beam in an FEL is undisturbed by the walls; it propagates as a simple Gaussian. We are interested in the intermediate regime, where, as the wavelength increases, the optical mode grows and begins to feel the influence of the walls, causing the mode shape to distort and losses to occur. Using a Fox-Li technique to propagate in free space and a decomposition of the Gaussian beam into waveguide modes inside the undulator, we calculate the changes in mode shape and cavity losses as the wavelength increases. The resulting theory and numerical calculations are an important consideration for the design of mid- to far-infrared FEL systems.

First lasing, capabilities, and flexibility of FIREFLY

Abstract FIREFLY is a free electron laser (FEL) designed to produce picosecond pulses of intense light between 10 and 100 μm. It uses an inexpensive electromagnetic wiggler and variable outcoupling to provide maximum flexibility for user experiments. FIREFLY first lased on November 23, 1994, and has now operated successfully from 15 to 65 μm. It has lased in both a traditional undulator configuration and as an optical klystron, and has also operated in the undulator configuration on the third harmonic. During initial tests FIREFLY reached theoretical extraction efficiency for fundamental, third harmonic, and optical klystron operation, and demonstrated wavelength switching between adjacent peaks in the gain spectrum of an optical klystron for the first time.

FEL center user diagnostics and control

Abstract In the past year, the Stanford Picosecond FEL Center has produced more than two thousand hours of beam time dedicated to user experiments. To assure reliable beam delivery and to maximize productivity of our users we have developed a sophisticated system of diagnostics and control. An integrated display is now available in all experimental areas which provides continuously updated measurements of beam spectrum, micropulse duration, power, position, and pointing — all of which may be saved to document beam conditions during an experiment. The beam is actively wavelength and amplitude stabilized to better than 0.01% and 2%, respectively. Direct wavelength control is available to users in every experimental area, allowing changes of wavelength as large as a few percent. Larger wavelength shifts, and adjustments in macropulse or micropulse width or timing, are readily available with operator assistance.

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.

Facilities at the Stanford Picosecond FEL Center

The Stanford Picosecond Free Electron Laser (FEL) Center has been established to support a broad range of biomedical and materials science research. In addition to the FEL, the Center has several conventional lasers available for experiments with high power, picosecond pulses of light. The FEL and the conventional lasers provide wavelength coverage from the visible to 100 microns. A wide variety of equipment, instruments, and standardized optical setups provide users with a flexible and powerful environment for research. This paper lists the facilities available at the FEL Center, and discusses the challenges encountered in developing a new user facility.

Applications of wavelength agility in an FEL facility

Abstract In utilizing the optical beam produced by an FEL the ability to change the wavelength rapidly in a controlled manner is highly desirable. A wavelength-agile FEL can be used as a voltage-controlled optical oscillator (VCOO) in which the oscillator frequency (or wavelength) tracks a control voltage at modulation frequencies that range from 10 Hz to 1 kHz. Applications of this operational feature include macropulse wavelength switching and fast wavelength scanning. Preliminary demonstrations of both of these features will be presented.

FEL cavity length measurement with an external laser

Abstract The nature of linac-driven free electron laser (FEL) operation requires that the length of the laser optical resonator be matched to the repetition rate of the accelerator in order for the returning optical pulse to overlap the newly arriving electron bunch. The range of cavity lengths over which oscillation will occur is limited in typical FEL designs to a few tens of microns, while the total resonator length is often many meters. The task of correctly setting the length of a new FEL resonator is therefore quite difficult, particularly if the cavity geometry contains more than a single line segment. We have demonstrated that such an absolute length measurement with micron accuracy over many meters in any cavity configuration is possible by establishing resonances in the cavity with an external laser of known repetition frequency. This technique was successfully applied in the development of two new FELs at the Stanford FEL Center. We will discuss the advantages of this method over conventional measurement schemes.

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.