JLab FEL User Meeting and
Laser Processing Consortium Workshop
January 18 Ė 19, 2001
We completed the first year of "routine" operation with all systems in place and functioning for three month-long runs; operation in 2001will be the same. Funding for both phases of the IR upgrade is in hand and funding for the first phase of the UV upgrade is expected in time for both upgrades to proceed together. The importance of users getting underway now with the design of equipment and experiments to use this marvelous light cannot be overstated.
A major step toward DOE funding of FEL operation was the development of the scientific case at a workshop last October, "Scientific Frontiers with Accelerator-Based Lasers". The workshop report is available at www.jlab.org/~gwyn/wkshp_1.pdf.
The routine operation established in 2000 is expected to carry forward with the upgraded machine.
- Submission of proposals for beamtime, experiment approval and beamtime allocation, and user liaison arrangements are all in place and functioning on the website.
- The base-level FEL performance also described there was delivered in fact for more than 1000 hours last year, more readily for conditions that have been run extensively. There is a steady stream of enhancements, e.g. shorter macropulses, higher pulse macropulse repetition rates in alignment mode, increasing micropulse energy, etc.
- Such problems as have been encountered may be traced to the need for more operating time; the entire total to date is not yet a year.
Light source development continues at JLab and among our partners
mm primary wavelength range, 100 mJ micropulse energy, PRF down to 4.7 MHz. Assuming a 160 MeV electron beam energy, the UV upgrade will extend the wavelength range to below 300 nm with more than 1 kW average power. The enormously greater (often 100-fold) absorbance of many materials in the UV vs. the IR plus the opportunity to drive electronic transitions directly make this difference far more significant than the numbers alone might suggest.
Progress continues to be made in characterizing the ps-pulse x-rays generated by Compton scattering when the optical and electron pulses collide in the FELís optical cavity.
The Helios synchrotron and the associated system components are now on site. The output is similar to bending magnet lines on the VUV ring at NSLS. A proposal has been submitted for an addition to the FEL building to house Helios. A second proposal has been submitted jointly with Virginia Commonwealth University to add an x-ray lithography beamline. A third proposal is in development with U. Virginia to put a 9 Tesla wiggler in one of the straight sections to obtain a hard x-ray output similar to the bending magnet lines on the NSLS x-ray ring, aiming at diffraction studies of proteins.
Copious amounts of terahertz radiation will inherently result from normal operation after the IR upgrade, six orders of magnitude beyond any existing source. User access needs to be developed.
- The Deep UV FEL project at Brookhaven has equipment substantially in place to use a seed laser to drive high gain harmonic generation which is then amplified in a single pass to yield a deep UV output
- The Advanced Light Source at LBL successfully operates IR spectromicroscopy beamlines on the current ring and plans a dedicated IR ring for the future.
- The present JLab IR FEL was made to lase directly at its 3rd harmonic so as to produce 300 watts of 1 micron light, a record for harmonic lasing. This wavelength in turn gave access to still shorter wavelengths through use the same harmonic generation crystals already employed with Nd:YAG lasers.
- Activities for the IR upgrade are well underway. The major specified output enhancements include 10 kW average power, 1.5 Ė 15
Though the FEL has great flexibility with respect to some parameters (e.g., wavelength), it is rather limited with respect to others. Accordingly, beam parameters desired for micromachining and PLD are best sought by conditioning the FELís native beam. A proposal has been submitted to NSF for beam conditioning facilities consisting of:
- A pulse picker to isolate individual micropulses from a steady stream. One is already operating at lower average power at Vanderbilt.
- A pulse stacker, an optical storage cavity having a switchable internal mirror to accumulate and deliver as a single pulse the energy of more than 50 micropulses. The stacker will operate only in the IR and deliver output pulses at the PRF of the switching laser, initially intended to be 1 kHz. Stackers are in operation at Stanford and FELIX.
- A stretcher/compressor based on a grating pair combined with chirped pulse FEL operation to permit micropulse length to be varied from about 0.1 ps to 10 ps. A demonstration has already been made at Duke.
- Pulse shaping optics to transform the present Gaussian profile into a top hat, while preserving the short pulse length.
Users continue to make progress not possible at any other light source:
- H defects in Si play a major role technologically, and it is critical to try to understand the associated dynamics. Measuring the time constant for recovery after bleaching leads to understanding of both local structure and energy transfer (Luepke).
- Photoinduced chemistry in a new regime of very high electric fields has been utilized to produce carbon nanotube structures. The FEL offers opportunities in understanding the fundamental non-linear processes that will ultimately allow significant quantities to be made for properties studies (Holloway). Additional photochemistry of metal-nitrogen interactions was studied and utilized to make surface nitride coatings on several metals (Schaaf).
- Tunable IR lasers can put energy into specific molecular vibrations. Whether selective chemistry or nothing more than general heating results depends critically on intramolecular vibrational energy redistribution (IVR). Recovery from photobleaching in the gas or solution phase provides a measure of harmonicity of potentials and processes that critically define the dynamics, such as de-phasing (DeWitt).
- Proteins are characterized by many intramolecular vibrational modes which are expected to play a large role in their function. In the first series of experiments the energy relaxation processes of several amide vibrational modes in myoglobin was examined. A major finding was that major, quantitative relaxation times were found for vibrational modes that were almost energetically degenerate (Austin).
- Ablation and PLD continue to be attractive applications
- Less evidence of target melting and enormously less evidence of particulates is seen for metals (Shinn)
- The reduced thermal transport properties of polymers (compared to metals) offer opportunity to explore both the thermal confinement and deterministic/stochastic transitions in ablation at the present FEL performance levels. Results are consistent with "ultrafast" studies on optical materials (Kelley)
- Pulsed laser deposition of polymers on-resonance at the Vanderbilt FEL yielded deposits with IR spectra closely similar to that of the target, indicating substantially faithful transport of molecular structure (Bubb).
Researchers in other fields described issues where the FEL could prospectively contribute:
- In dermatology, removal of tatoos in a way that the constituent dye chemicals do not enter the wearerís bloodstream is needed. Conceptually, an FEL tuned to the dyes could achieve it (Anderson)
- Laser ablation is already successful in art conservation, especially cleaning (cf. laser ocular lens sculpting) and analysis by laser-induced breakdown spectroscopy. A high-powered tunable laser offers could further opportunity for selectivity (Miller).
- The intense plasma resulting from "ultrafast" ablation results in isotope mass separation as a function of angle away from the plume axis, more so for higher masses. The FELís high average power could produce enough material to constitute a useful process; economics still to be assessed (Pronko).
- Geoscientists wishing to better understand deep-earth processes seek experimental access to ever-higher pressure and temperature, presently using diamond anvils. The FEL might micromachine superior anvil shapes or perhaps transmit a beam through materials under high pressure (Mao).
- MALDI driven by an IR FEL offers advantages over UV-driven MALDI in terms of gentleness to massive, fragile biomolecules in that photochemistry can be avoided and more "natural" matrices (e.g., water) can be used (Baltz-Knorr).
- Special glasses (e.g. "Foturan") can be activated for etching by suitable UV exposure, enabling micromachining without ablation. Such micro/nanostructures will be valuable for the coming generation of small satellites (Helvajian).