W. Gabella

Free electron laser based biophysical and biomedical instrumentation

A survey of biophysical and biomedical applications of free-electron lasers(FELs) is presented. FELs are pulsed light sources, collectively operating from the microwave through the x-ray range. This accelerator-based technology spans gaps in wavelength, pulse structure, and optical power left by conventional sources.FELs are continuously tunable and can produce high-average and high-peak power. Collectively, FEL pulses range from quasicontinuous to subpicosecond, in some cases with complex superpulse structures. Any given FEL, however, has a more restricted set of operational parameters. FELs with high-peak and high-average power are enabling biophysical and biomedical investigations of infrared tissue ablation. A midinfrared FEL has been upgraded to meet the standards of a medical laser and is serving as a surgical tool in ophthalmology and human neurosurgery. The ultrashort pulses produced by infrared or ultraviolet FELs are useful for biophysical investigations, both one-color time-resolved spectroscopy and when coupled with other light sources, for two-color time-resolved spectroscopy.FELs are being used to drive soft ionization processes in mass spectrometry. Certain FELs have high repetition rates that are beneficial for some biophysical and biomedical applications, but confound research for other applications. Infrared FELs have been used as sources for inverse Compton scattering to produce a pulsed, tunable, monochromatic x-raysource for medical imaging and structural biology. FEL research and FEL applications research have allowed the specification of spin-off technologies. On the horizon is the next generation of FELs, which is aimed at producing ultrashort, tunable x rays by self-amplified spontaneous emission with potential applications in biology.

Free-electron lasers: reliability, performance, and beam delivery

The Vanderbilt free-electron laser (FEL) is a continuously tunable source of pulsed, mid-infrared radiation. FEL applications research has been underway for a decade. Recent experimental advances in FEL ablation of soft tissue indicate the potential for FEL-based protocols in surgery and medicine. In anticipation of these medical applications, the Vanderbilt FEL is being upgraded to meet the reliability and performance standards for a medical laser. Facilities for laser surgery have been constructed and equipped and medical delivery systems are being developed for pre-clinical and clinical research.

First results from bent crystal extraction at the Fermilab Tevatron

Abstract First results from Fermilab Experiment 853 are presented. E853 is an experiment to test the feasibility and efficiency of extracting a low-intensity beam from the halo of the Tevatron using channeling in a bent silicon crystal. The motivation of the experiment is to apply crystal extraction to trans-TeV accelerators like the LHC. Extensive simulation work has been carried out. Two accelerator operating modes have been developed for crystal studies, “kick” mode and diffusion mode. Results from the first successful extraction in kick mode are presented.

A sealed-off strontium vapor laser

A sealed-off strontium-vapor laser for medical applications is examined. This is an integrated system that accommodates an excitation circuit, a laser cavity, and an active element. The active medium is excited by means of a modified Blumlein circuit. An unstable resonator of the telescopic type allows a near-diffraction-limited laser beam to be generated. Lasing is obtained in atomic strontium lines at λ=2.06, 2.2, 2.69, 2.92, 3.011, and 6.45 μm and in ionic strontium lines at λ=1.033 and 1.091 μm. We have studied experimentally the behavior of spectral distribution of the output power at varying power delivered to the discharge. It is found that 95% of laser radiation is concentrated in the line at λ=6.456 μm, which corresponds to a lasing power of ~ 2.5 W. Moreover, the time characteristics of lasing pulses are investigated. The radial inhomogeneity of the laser beam is examined. We have conducted lifetime testing of Sr-vapor active elements. The average output power exhibits a modest decrease (5%) within 300 h of a continuous operation. Notably, the pumping characteristics remain unchanged.

Operation of an ungated diamond field-emission array cathode in a L-band radiofrequency electron source

We report on the operation of a field-emitter-array cathode in a conventional L-band radio-frequency electron source. The cathode consisted of an array of ∼106 diamond tips on pyramids. Maximum current on the order of 15 mA was reached and the cathode did not show appreciable signs of fatigue after weeks of operation. The measured Fowler-Nordheim characteristics, transverse beam density, and current stability are discussed.

Measurements of the spectral and temporal evolution of FEL macropulses

The Vanderbilt Mark III FEL is a tunable source of coherent mid-infrared radiation occurring as a train of high- intensity (picosecond) micropulses with a repetition rate of 3GHz that continues for 3-5 microseconds (the macropulse). We have measured the spectral output of the Vanderbilt FEL as a function of time during the macropulse with ~10nm resolution in wavelength and ~20ns resolution in time. The measurement takes about one minute and gives a representation of the micropulse spectral width average over many macropulses. Data collected thus far indicates a surprising amount of structure produced by overlapping periods of growth, saturation, and decay within the macropulse. It is found that the central wavelength of the FEL slips over the course of the macropulse, and that the instantaneous output typically has a much smaller spectral bandwidth than the macropulse bandwidth. Thus, a user slicing portion fo the macropulse with a Pockels Cell can obtain different central wavelengths by slicing at different times during the macropulse. The evolution of the macropulse spectrum as a function of cavity de-tuning and electron beam parameters is studied with the goal of improving the stability and spectral density of the FEL output.

Attenuation of midinfrared free electron laser energy with eyewear

Purpose: To determine the attenuation of free electron laser (FEL) energy at several wavelengths through microscope objective and eyeglass lenses. Materials and Methods: The FEL at wavelengths of 2.3 um, 2.5 um, 3.0 um, 3.5 um, 4.0 um, 4.5 um, 5.0 um, 6.45 um, 7.0 um, 7.5 um, and 8.0 um was telescoped using a 500 mm nominal focal length lens and a 200 mm focal length lens. The beam had a final spot of about 3 mm and was passed through a 3 mm aperture and onto the 8 mm active area of a J9LP Molectron detector. The eyeglass sample was placed 3 cm in front of the detector. Energy readings were averaged over multiple pulses. Results: Attenuation varied greatly with wavelength and sample from a low attenuation of 0.46 dB, 90% transmission, for short wavelengths through common glass to greater than 60 dB attenuation (transmission at the detector noise level) for IR safe glass by Aura, Inc. Conclusion: Only the designated laser safety goggles effectively attenuate free electron laser energy at 2.3 um and 2.5 um. A microscope objective lens, polycarbonate, and silica glass eyewear is capable of effectively attenuating FEL energy at wavelengths greater than 4.5 um, but the polycarbonate lenses demonstrated material damage.

Mid-infrared FEL absorption spectra

The Vanderbilt Mark III FEL is a tunable source of high- intensity coherent mid-infrared radiation occurring as a train of picosecond pulses spaced 350ps apart. The laser beam is transported to each laboratory under vacuum, but is typically transmitted through some distance of atmosphere before reaching the target. Losses due to absorption by water vapor and CO2 can be large, and since the bandwidth of the FEL is several percent of the wavelength, the spectrum can be altered by atmospheric absorptions. In order to provide an accurate representation of the laser spectrum delivered to the target, and to investigate any non-linear effects associated with transport of the FEL beam, we have recorded the spectrum of the FEL output using a vacuum spectrometer positioned after measured lengths of atmosphere. The spectrometer is equipped with a linear pyroelectric array which provides the laser spectrum for each pulse. Absorption coefficients are being measured for laboratory air, averaged over the bandwidth of the FEL. The high peak powers of this Fel have induced damage in common infrared-transparent materials; we are also measuring damage thresholds for several materials at various wavelengths.

R&D Toward a Compact High-Brilliance X-Ray Source Based on Channeling Radiation

X-rays have been valuable to a large number of fields including Science, Medicine, and Security. Yet, the availability of a compact high-spectral brilliance X-ray sources is limited. A technique to produce X-rays with spectral brilliance B ∼ 1012 photons.(mm-mrd)−2. (0.1% BW)−1.s−1 is discussed. The method is based on the generation and acceleration of a low-emittance field-emitted electron bunches. The bunches are then focused on a diamond crystal thereby producing channeling radiation. In this paper, after presenting the overarching concept, we discuss the generation, acceleration and transport of the low-emittance bunches with parameters consistent with the production of high-brilliance X-rays through channeling radiation. We especially consider the example of the Advanced Superconducting Test Accelerator (ASTA) currently in construction at Fermilab where a proof-of-principle experiment is in preparation.

W.M. Keck-Vanderbilt Free-Electron Laser Center facilities

The W.M. Keck-Vanderbilt Free-electron Laser Center operates a reliable free-electron laser (FEL) that is used in human surgical trials, as well as in basic and applied sciences. The wavelength of the FEL is tunable from 2.1 micrometers to 9.6 micrometers , delivering above 50 mJ per macropulse with a repetition rate of 30 Hz. For soft tissue surgery, especially neurosurgery and surgery on the optic nerve, a wavelength of 6.45 micrometers has been found to ablate with little collateral damage. The free-electron laser beam is delivered to experiments approximately 2000 hours each year. The Center also supports several other tools useful for biomedical experiments: an optical parametric generator laser system with tunable wavelength similar to the free- electron laser except it has much lower average power; a Fourier transform infrared spectrometer to characterize samples; several devices for in vivo imaging including an optical coherence tomography setup, a two-photon fluorescent confocal microscope, and a cooled, integrating camera capable of imaging luciferin-luciferase reactions within the body of a mouse. The Center also houses a tunable, monochromatic x-ray source based on Compton backscattering of a laser off of a relativistic electron beam.