Charles A. Brau

354‐nm laser action on XeF

This letter reports laser action on the xenon monofluoride 2Σ1/2→2Σ1/2 band at 354 nm. Lasing on discrete vibrational bands has been achieved by pulse excitation of high‐pressure mixtures of F2/Xe/Ar with an electron beam. XeF is a member of a new class of diatmoic molecules, the noble gas monohalides, which all exhibit similar molecular structure and spectra, and laser action should be attainable on the various bands of other members of this class of molecules. The kinetics and loss mechanisms of these laser candidates are briefly discussed.

Results of the Los Alamos free-electron laser experiment

A free-electron laser (FEL) amplifier experiment to test the performance of a tapered wiggler at high optical power has been successfully completed. A well-separated two-component electron energy distribution has been obtained that is characteristic of a tapered wiggler. Energy distribution spectra and extraction efficiencies have been measured as a function of initial electron energy, energy spread, emittance, optical power, and spatial and temporal misalignments of the laser and electron beams. A maximum efficiency of ∼ 4 percent was measured, and good agreement of efficiency with a one-dimensional theory was obtained.

Optical performance of the Los Alamos free-electron laser

During a year of oscillator experiments, the Los Alamos free-electron laser has demonstrated high-power and diffraction-limited output capabilities with a factor-of-4 wavelength tunability in the infrared. A conventional, L -band RF linear accelerator produced a 100 μs long, 2000 pulse train of 35 ps wide electron-beam pulses with peak currents to 50 A and nominal energy of 20 MeV. Small-signal gain in excess of 40 percent was generated in a 1 m, plane-polarized, uniform-period undulator for wavelengths between 9 and 11 μm. Best performance included an electron-energy extraction efficiency of 1 percent, 10 MW peak output power, and a corresponding average power of 6 kW over a 90 μs pulse train. A Strehl ratio of 0.9 characterized the output spatial beam quality. By reducing the electron energy by a factor of 2, the wavelength was tuned continuously from 9 to 35 μm.

Theoretical studies of UV-preionized transverse discharge KrF and ArF lasers

A theoretical model has been developed to examine the behavior of UV preionized, fast transverse discharge, rare-gas halide lasers. The model integrates the time dependent circuit equations for the discharge, the kinetic equations for the important atomic and molecular species and electrons, and the equations describing the growth of the spontaneous and stimulated emission. Results of KrF and ArF are compared with those in a companion experimental paper by Sze and Loree. The effects of various self-absorption mechanisms in the laser media are discussed, along with the possible impact of several uncertainties regarding the fundamental processes occurring in such lasers.

Laser action on the 342‐nm molecular iodine band

A new laser operating on the 342‐nm band of I2 is reported. Electron‐beam‐excited mixtures of argon with CF3I and HI produced this I2 laser. The excited state of this laser is probably formed by ion recombination reactions, and a plausible mechanism is given.

Near-monochromatic X-ray beams produced by the free electron laser and Compton backscatter

The intense photon output of a free electron laser may be made to collide with its own high energy electron beam to create nearly monochromatic x-rays using Compton backscatter techniques. These x-rays can be used for imaging and non-imaging diagnostic and therapeutic experiments. The initial configuration of the Vanderbilt Medical Free Electron Laser (Sierra Laser Systems, Sunnyvale, CA) produces intense x-rays up to 17.9 keV, although higher energies are easily attainable through the use of frequency doubling methods, alteration of the energy of the electron beam and coupling to conventional laser inputs.

The Los Alamos free electron laser oscillator: Optical performance ☆

Abstract During nearly a year of oscillator experiments, the Los Alamos free electron laser has demonstrated high-power and diffraction-limited output capabilities with continuous wavelength tunability in the infrared. A conventional L-hand rf linear accelerator produced a 100-μs-long, 2000-pulse train of 35-ps-wide electron-beam pulses with peak currents to 50 A and nominal energy of 20 MeV. Small-signal gain in excess of 40% was generated in a 1-m, plane-polarized, uniform-period undulator for wavelengths between 9 and 11 μm. Best performance included an electron-energy extraction efficiency of ∼1%, 10-MW peak output power, and a corresponding average power of 6 kW over a 90-μs pulse train. A Strehl ratio of 0.9 characterized the output spatial beam quality.

High-brightness photoelectric field-emission cathodes for free-electron laser applications

Field emission from very fine needles in an electric field is characterized by a very small energy spread, less than 1 eV, and emittance of the order of 10−8π m rad, normalized. In microsecond pulses, total currents in excess of 6 A have been observed, corresponding to normalized brightness of the order of 1015 A/m2 str, which is four orders of magnitude higher than conventional sources. Recently, photoemission has been observed from needles operating in fields too small for significant field emission. Total currents as high as 10 A have been observed in nanosecond pulses. The quantum efficiency is of the order of unity. Using e-beams of such high brightness, single-pass FELs can operate at very long and very short wavelengths, with very low total e-beam current. In the FIR, it is possible to construct Cerenkov FELs with enough gain (e20) to reach saturation in a single pass using only 10 mA total electron-beam current. In the UV, it is possible to build single-pass Compton-backscatter FELs operating at total current less than 300 mA.

The Oxford Free Electron Laser Project

It is proposed to use the Oxford 10 MV Van de Graaff accelerator as an electron beam source for a free electron laser (FEL) operating in the far infra red (FIR). The configuration and layout of the Van de Graaff make it very suitable for conversion, with a potentially high efficiency for electron beam recovery. Using a 2 m long wiggler of 34 mm period, the FEL would operate in the 60-300 /spl mu/m wavelength band, with extension down to 30 /spl mu/m on the 3rd harmonic. When constructed, the FEL will support a programme of FEL research and development, concentrating at first on investigations of behaviour in the moderately high-gain regime (/spl equiv/ 100% per pass) and mechanisms for lasing on higher harmonics. It will also be the basis for a national UK user facility in the FIR.

Uniformity conditioning of diamond field emitter arrays

The authors present recent advances in the uniformity conditioning of diamond field emitter arrays (DFEAs). Postfabrication conditioning procedures consisting of thermal annealing and high field/current operation have been examined. Nonuniformity due to varying contamination states of the emitters can be mitigated by moderate temperature (∼150–300°C) operation. Operating the emitters at elevated current levels was found to enhance the spatial uniformity in a self-limiting manner. The conditioning mechanism is most likely thermal-assisted field evaporation of the diamond nanotips, however, the nature of the dc tests does not definitively exclude back bombardment as a possible contributor. Pulsed testing is underway to remove this ambiguity, provide conditioning for high-density arrays, and demonstrate the operational current density limits of DFEAs.