Summary: LPC Workshop
January 11-12, 2000
As the FEL program enters its tenth year, lean resources
still limit the pace. Nonetheless, all the original goals are being achieved
and more. Since the last consortium meeting in June 1999:
The FEL exceeded 170% of its design power and delivered more
than 200 hours of 3 - 6 micron beam to the user labs in the sum of two
month-long running periods. New capabilities beyond the original version
emerged: useful power at wavelengths much shorter than the primary output
via harmonic lasing and picosecond pulsed x-rays by scattering the electron
beam off the laser output.
The start-up and commissioning are encountering surprisingly
few problems considering the novelty of the undertaking, surely a tribute
to those responsible for the FEL's design, construction and operation.
No issues have emerged that limit attainment of any of the original goals.
The program and technology path is in place for a ten-fold
power upgrade with wavelength expansion from the present 3 - 6 micron range
to 2 - 20 microns. A follow-on will add 1 kW of UV down to 250 nm.
Key experiments demonstrated the FEL's capability to do scientifically
important and technologically useful work.
The FEL effort has been greatly strengthened by the addition
of Gwyn Williams, most recently the progenitor of IR microscopy at NSLS,
as the scientific program coordinator/leader.
Clear procedures are now in place for seeking FEL beamtime,
arranging for use of facilities, getting required safety training for personnel
and safety approval for experiments. They will be put on the JLab website.
Improvements in clarity and user-friendliness will be in place before the
summer running period. Efficiently, fairly and safely serving our growing
community depends on all of us faithfully following the procedures.
Main characteristics of the present FEL operation are:
High average power (100's of watts) is available from 3 -
6 mm, with individual micropulse energy up to
15 mJ. Highest pulse energy increases the spectral
width of pulses to 2.5%, from 0.4% at 3.1 mm,
which may be significant for some experiments. Minimum micropulse length
(FWHM) is about 0.7 ps, with a maximum separation of 55 ns. In addition
to the primary wavelength, harmonic lasing can deliver watts at 1 micron,
though stability issues require resolution before that light can be usefully
exploited. Straightforward improvements to the optical cavity and transport
system could raise the harmonic power several-fold. Available pulse formats
range from CW to macropulses as short as 10 ms
at any reasonable macropulse repetition rate. The beam is highly polarized
(6000:1). Since laser operation may not be drift-free on the time scale
of hours with presently installed equipment. Experimenters need to identify
and monitor critical parameters, e.g., wavelength, micropulse energy, beam
Most operating regimes do not permit users to be in the lab
while beam is being delivered. However, they may be present during "alignment
mode" conditions: 18.75 MHz, 250 ms (or less)
macropulse length, 2 Hz or less macropulse repetition rate. Many users
chose alignment mode for either set-up or experiments. Burst mode delivers
macropulse having a chosen length, repetition rate and total number.
Generally-available facilities in the user labs remain spartan.
Since most experimenters are already working with lasers, they are able
to bring what they need. Researchers doing so need to be aware that they
need safety approval for the equipment and the experiments before operations
The future will arrive in phases spread over the next few
FEL users need to take care of several procedural matters
before their work can begin. Required safety training is available through
the JLab User Liaison Office and includes: general safety, radiation safety,
laser safety and oxygen deficiency hazards. The experiments to be carried
out at the FEL also require both safety approval and instructions to the
FEL operator as to what beam is to be provided. The Experimental Safety
Approval and Laser Test Plan forms are available in MS Word from George
Neil (firstname.lastname@example.org) and will be on
the JLab website.
Existing IR Demo: Envisioned performance improvements include:
beam-shaping optics to give other than Gaussian profiles, e.g., top-hat
for micromachining; a pulse-picker to deliver micropulses individually
or at a chosen rate; a pulse-stacker to combine and deliver the energy
from a train of FEL micropulses as much larger micropulses at a correspondingly
reduced repetition rate; a stretcher/compressor combined with chirping
the FEL to extend the pulse length range to about 0.1 - 10 ps; an optical
cavity to accumulate the energy of successive pulses for in-cavity experiments
that need high peak power but consume little net energy.
IR Upgrade (3 cryomodules): wavelength range becomes to 2
- 20 mm, micropulse energy increases 10 X over
IR demo and average power exceeds 10 kW. If a wiggler like the OK-4 can
be used, the, wavelength can be extended below 1 mm.
Reaching the future in timely fashion depends on the same
combination of solid science, bold technology and heroic effort that got
us where we are today.
UV Follow-On: maximum pulse spacing increases to 440 ns;
add UV down to below 250 nm (possible emittance limit); peak micropulse
energy of 25 mJ at 350 nm. Improvements to the mirror coatings and the
accelerator performance could bring the lower wavelength limit below 190
FEL users are already doing important experiments:
Spectroscopy of H in Si (Luepke) - despite its importance
and ubiquity, the behavior of H in Si has not been adequately characterized
because suitable spectroscopic tools have been lacking, a situation the
JLab FEL high average power uniquely overcomes. This fall's experiments
yielded the first measurement of vibrational lifetime of isolated H in
crystalline Si, and contributed to the understanding of annealing by showing
strong coupling between H-associated modes and optical-mode lattice phonons.
Pulsed Laser Deposition of Metals (Reilly) - the combination
of high peak power, picosecond pulse length and high average power make
the FEL an attractive tool for PLD, ultimately as a way to go beyond the
limits of the existing excimer laser based technology. Though the present
55 ns maximum pulse to pulse spacing prevents fully accessing what the
FEL can do, it was possible to make metal films with particulate counts
apparently lower than excimer laser technology despite the latter's long
Micromachining of Polyimde (Kelley) - precisely drilled polyimide
films are widely used as printed wiring board dielectrics, ink jet printer
heads and potentially as space sails. The FEL offers a unique opportunity
to tune to strong absorbances and to use pulse lengths (10 ms)
near the thermal diffusion transition. Percussion drilling experiments
near to (5.85 mm) and away from (3.12 mm)
the strongest absorption resonance attained partial or full penetration
in a single macropulse, yielding a diameter less than about 30 mm
at the smallest.
While these and other efforts are expected to continue,
further exciting opportunities are now opening:
Energy Flow in Proteins (Austin) - the exact position of
the amide absorption and out-flow of vibrational excitation energy deposited
into it, both depend on the detailed local structure of the protein. The
protein's biological function depends on related energy flows. Based on
previous experiments at FELIX, the JLab FEL's high-average power and excellent
stability will combine to offer the signal-to-noise performance needed
for high sensitivity at the needed wavelengths near 6 mm.
"Spintronics" (Tolk et al.) - a novel semiconductor electronics
based on spin rather than charge appears possible and capable of important
advances, e.g., sharply increased memory density at reduced power input.
The FEL's high average power and tunability enable the first round of experiments:
band offset measurement at semiconductor interfaces, lifetime measurement
of photo-induced, spin-polarized carriers, and (more broadly) study of
UV Photobiology (Sutherland) - assessing the nature and extent
of the human health risk arising from increased UV light reaching the earth
requires uniform exposure of a statistically significant number of target
organisms (e.g., animals) at levels relevant to chronic human exposures
in the critical 290 nm - 400 nm wavelength range. There is simply no source
in prospect other than the JLab UV FEL.
Laser/Materials Interaction Fundamentals (Pate) - combining
FEL and table-top laser based experiments gives a way to study events at
a molecular level. The theme is to selectively deposit energy in a chose
bond and then follow its fate with a probe laser or sequential multiphoton
excitation. The most immediate practical benefit will be to indicate the
range of opportunity to use multiphoton high peak power IR to achieve photochemistry
requiring, e.g., UV if driven by single photons.
Pulsed IR/X-RayExperiments (Boyce/Happek) - The IR beam can
act as a pump to induce structural transformations in materials (e.g.,
amorphization, melting) that can be characterized after a controlled brief
time interval by a x-ray pulse. Accordingly, the time course of rapid thermal
processing in the picosecond regime can be studied directly. Initial hardware
construction and demonstration experiments are planned.
Laser Processing Applications for Automotive Fuel Components
(Hamann) - Increasingly stringent automotive emission requirements drive
fuel injector technology toward smaller, more precise orifices than can
be made by established punching, EDM or drilling technology. Desirable
next-generation orifices would be 50 - 200 mm
diameter in 75 - 350 mm thick stainless steel,
have straight walls with zero taper, and sharp entry/exit edges. To displace
other laser technology, the FEL will need to deliver high power at 1 mm
or preferably shorter, with a micropulse rate between 10 and 100 kHz.