Stephen V. Benson

The Stanford Mark III infrared free electron laser

Abstract We have built and operated a compact infrared free electron laser using a conventional linear accelerator and a broadband optical cavity. The gain was measured to be 28% and saturation was obtained with a 2.5 μs macropulse. The laser was tuned from 3.1 to 2.6 μm by changing the undulator gap. Coherent spontaneous radiation at the fifth and seventh harmonics was seen.

Status report on the Stanford Mark III infrared free electron laser

Abstract We review recent experimental results from the Stanford Mark III infrared FEL. More than 900 h of lasing operation have achieved from 2.0 to 5.5 μm and continuous gap tuning has been achieved over the design range of 1.7:1. Micropulses with peak powers as high as 2 MW and pulse lengths as short as 500 fs have been measured. A multiple output coupler arrangement has been tested which allows variation of the output coupling from 0.1% to 7% in a matter of a few minutes. Accelerator performance will also be discussed and the results of recent improvements to the microwave gun will be presented. The measured brightness of this gun is in excess of 10 12 A/cm 2 which is sufficient for operation in the UV.

First demonstration of a free-electron laser driven by electrons from a laser-irradiated photocathode

We report the results from the first operation of a free electron laser (FEL) driven by an electron beam from a laser-irradiated photocathode. The Rocketdyne/Stanford FEL achieved sustained oscillations, lasting in excess of three hours, driven by photoelectrons accelerated by the Stanford Mark III radiofrequency linac. A LaB6 cathode, irradiated by a tripled Nd: Yag mode-locked drive laser was the source of photoelectrons. The drive laser, operating at 95.2 MHz, was phase-locked to the 30th subharmonic of the S-band linac. Peak currents in excess of 125 A were observed and delivered to the Rocketdyne 2 m undulator which was operated as a stand-alone oscillator. Sustainable small-signal gain of 100% per pass was observed over a 2 h time period with periodic observation of small-signal gain as high as 150% per pass. Preliminary estimates of the electron-beam brightness deliverable to the undulator range from 3.5 × 1011 to 5.0 × 1011 A/(rad m)2.

A high quality permanent-magnet wiggler for the Rocketdyne/Stanford infrared free electron laser

Abstract A high quality, variable gap, variable taper, permanent-magnet wiggler has been built for infrared free electron laser (FEL) experiments to be performed at the Stanford Photon Research Laboratory. The design and characterization procedure used to assemble the wiggler is discussed. A simulated annealing code was used to minimize field errors arising from variations in the individual magnets. The computed electron trajectories associated with the measured magnetic fields are presented for a range of different operating points of the wiggler. These plots indicate a very high quality field over a large range of different wiggler operating regimes. Resultant trajectory wander over the 2 m long wiggler for a 40 MeV electron at a wiggler gap corresponding to 3.3 kG was calculated to be less than 25 μm. The ability to control trajectory wander and optical phase slip using the simulated annealing code suggests future extensions to extremely long wigglers.

Transverse mode frequency pulling in free electron lasers

Abstract We consider the spectral evolution of transverse modes in free electron lasers. While the gain maxima of the various transverse modes occur at different frequencies, coupling between the modes via the electron beam causes the modes to shift frequency such that the final laser frequency differs from what one would expect if only the TEM00 mode were present. This can strongly effect the turn-on behavior in lasers with short electron pulses.

Shot and quantum noise in free electron lasers

Abstract We present an analysis of the effects of both classical and quantum noise on the operation of free electron lasers. Shot noise is considered from both classical and quantum mechanical perspectives. The photon number fluctuations due to shot noise are thermal in nature and should mask quantum effects in most conceivable devices. It is found that shot noise could be reduced in some devices in order to study the effects of quantum fluctuations but there are limits below which the shot noise cannot be reduced due to the Heisenberg uncertainty principle. It is possible to introduce both shot and quantum noise into classical models. The results of some simulations carried out to study the effects of noise on pulse propagation are shown.

First operation of the Rocketdyne/Stanford free electron laser

Abstract A near infrared free electron laser (FEL) has been built and installed by Rocketdyne in the Stanford Photon Research Laboratory. The Rocketdyne/Stanford FEL utilizes a very high quality, 2 m long, permanent magnet planar wiggler whose gap may be continuously tuned, and magnetic field axially tapered by varying the gap at one end relative to the other. The laser is operated with an e-beam supplied by the Stanford Mark-III accelerator. A stable resonator with a broadband, dielectric coated element permits transmissive outcoupling over the 2.7–3.7 μm wavelength range. Results from initial operation of this laser are presented.

Real time processing of picosecond and femtosecond laser pulses: Application to free‐electron lasers

An original and reliable technique, based on a three‐wave interaction, has been designed and successfully tested to analyze the evolution of the modes and the pulse length of a free‐electron laser during the buildup of the radiation. The technique was developed in order to study the effects of the optical guiding in the free‐electron laser built at Stanford and driven by the MARK III linear accelerator. We explicitly mention that this technique can be easily exploited to monitor, in real time, the pulse‐to‐pulse fluctuations of the mode size and the pulse length of the pulses delivered by any laser independently of its pulse length from femtosecond to millisecond.

Design concept for a common rf accelerator driven free electron laser master oscillator/power amplifier

Abstract We present a conceptual design of a near infrared free electron laser master oscillator/power amplifier (FEL-MOPA) driven by a common radiofrequency accelerator. We present techniques for radiofrequency switching that permit exploration of the FEL-MOPA operation on single electron macropulses, in both the small signal and large signal regimes. Feasibility of this concept is shown by using, as an example, the parameters of the Stanford Mark-III oscillator [1] and the Rockwell tapered undulator [2], the latter in the amplifier configuration. Important physics issues of the FEL-MOPA addressed by this suggested experiment are (1) (synchronization and optimization of electron and laser pulses, and (2) gain-guided temporal and spatial evolution of the laser mode in the amplifier.

Initial results of operating the rocketdyne undulator in a tapered configuration

Abstract The near-infrared Rocketdyne/Stanford free electron laser (FEL) uses a very-high-quality precision undulator whose field strength and field taper are adjustable. The Rocketdyne undulator has been operated in both an amplifier configuration, as in the master-oscillator power amplifier (MOPA) experiments, and an oscillator configuration, as in the photocathode and tapered-undulator experiments. The tapered-undulator experiment was performed at the Stanford Photon Research Laboratory (SPRL) using an electron beam supplied by the Mark III rf-linac. During the experiment we observed sustained oscillations as the undulator magnetic-field taper was continuously tuned from 0% to 10%. We observed ∼1.2% extraction efficiency for a magnetic-field taper of 9.6%. During the same experiment we observed sustained oscillations as the undulator gap was continuously varied over 120 mil. Details of the experiment are presented.