VU FEL New User Information
Table of Contents:
The FEL is currently operating 16 hrs/day, 5 days/week. Midnight to
8am shifts are available if requested one week in advance. Shift times
are from 0800-1200, 1200-1600, 1600-2000, 2000-2400, and
0000-0800. Please allow around 30 minutes at the beginning of the
shift to be used for machine tuning; the actual time required will
depend on the parameters requested by the user. For best results,
please inform the control room of the wavelength and repetition rate
you will need at least 24 hours before the scheduled shift time.
To request beam time, contact
John Kozub
(john.kozub@vanderbilt.edu)
by e-mail.
The output coupling mirrors of the laser cavity have nominal
bandwidths of 6-9 microns, 4-6, 2.8-4.2, and 2-3 microns. All mirrors
are currently available; please note that changing mirrors may add a
few minutes to the time required to tune the laser.
New users are quite welcome; a variety of experimental tools are available,
as well as experienced staff. For users not approved through the peer review
board, we request that a one page letter briefly explaining the
experiment be submitted. This allows us to "log" hours in an official
manner for our funding agency.
The FEL center wavelength can be tuned from 2.1 to 9.4 microns reliably.
It has lased from 1.9 to 10.4 microns, but the extremes may be difficult,
i.e. it may take a lot of time to tune the machine, or may not be achievable
due to a finicky cathode, etc.
Four dielectric coated mirrors are used to cover the entire wavelength range.
Their nominal ranges are: 6-9 microns, 4-6 microns, 2.8-4.2 microns and
2-3 microns. If a mirror change is required during a shift (all four mirrors
are in the cavity on a carrousel) you should allow additional time
for machine tuning.
The physics of the FEL and the laser cavity sets certain restrictions on
the energy per macropulse (see below) available at each wavelength.
Figure 1 shows the energy per macropulse at different wavelengths. It
shows all the logged experiments---some users request very low energy
while others request the highest. It is reasonable to expect the machine
to deliver around 80% of the highest energy.
Figure 1-Macropulse Energy vs Wavelength
 |
|---|
The timing of the FEL pulse is perhaps a bit unusual for lasers,
though typical of FELs that use a pulsed electron beam as the gain
medium. It replicates in its general features the structure of the electron
beam produced by the microwave electron gun (U.S. Patent No. 4,641,103,
John Madey et. al.) which uses a thermionic cathode to produce a 20-40 ampere,
picosecond bunch every RF cycle (350 ps) for up to 8 microseconds.
The electron beam is pulsed at 30 Hz, and the
electrons are ON for 8 microseconds. Because of settling times in the
linac and other effects, the laser is ON for 3-6 microseconds,
depending on the wavelength and the "tune". This "long" pulse of several
microseconds is referred to as the macropulse. Since the macropulse
of electrons is made up of a train of very short micropulses,
less than 1 picosecond
long (measured with an interferometer and coherent microwave
transition radiation), the laser micropulses are also about 1 picosecond long.
In fact, using a two-photon absorption autocorrelator, the laser
micropulses have been measured from 0.75 to 1 picoseconds. These are
spaced by the timing of the RF oscillations, in our case, 350 picoseconds.
See Figure 2 for a representative schematic of the pulse train.
Figure 2-Temporal Pulse Structure
 |
|---|
The spectral width from FEL physics considerations is about 1% FWHM,
in general. However, narrower line widths have been observed, and
wider widths are typical. For
some wavelengths which interact with water (OH stretch), especially
6.45 microns, the line width is wider and the spectrum "uglier".
The electron beam emits enhanced spontaneous radiation at the odd harmonics
of the lasing wavelength. Filters are available to remove the harmonics.
Figure 3-Laser Spectrum at 3.9 microns
 |
|---|
One of the standard diagnostics in the FEL Control Room is the
"fast-IR", or the measurement of the IR macropulse. This measures
the evolution of infrared over the 3-6 microseconds the laser is "on."
The stability of the FEL is pretty good, but not as good as the best
lasers. This consideration must be built into any experiment, normalizing
to a picked off reference for instance. Typical energy per macropulse
stability is about 10% FWHM, sometimes better. Due to FEL physics, the
higher the energy per macropulse the worse the stability. So, if you do
not need very high energy be sure to inform your operator.
Almost always between shifts the FEL will have been re-steered for
another lab. Do not plan on the laser coming out at the same point
between shifts. Over the intercom, the user can call out directions
and the operator can steer the beam.
The staff of the FEL is friendly, despite outward appearances:) Our
operators have long experience tuning the FEL and sometimes teasing
almost impossible beam parameters out of the machine at the same time.
Our "engineers" (all PhD physicists) have been FEL users and are
experienced in experiment design and are knowledgeable in the
equipment available. For efficiency, here are several suggestions for
users:
-
When you sign up for beam time, give the wavelength, repetition rate,
and energy you will need. Even though you may change your mind, it
gives something to tune up for if the previous shift ends early.
Something like, "I will need 6.45 microns, 30 Hz, and lots of power
(AKA energy)," or "I will need 3.9 microns, any rep rate, and a very
pretty stable spectrum."
-
Call 30 minutes before your shift for machine status. The machine is
very reliable for this class of laser, however parts can break
requiring replacement, delaying or even cancelling shifts. If you
have an early morning shift you might check in the night before and
inquire about any problems.
-
If you need to cancel, or if your equipment breaks, please inform the
staff as soon as possible so that other users can be found and machine
time not wasted.
-
Interact with your operator and learn about the Control Room
diagnostics: realtime spectrum, energy, and IR macropulse (fast-IR).
There are several issues involving these parameters, for example more
energy almost always means wider spectrum (not always though). The
operators have a feel for these rules of thumb.
-
The lab and users have some equipment available, especially energy
meters, harmonic filters, oscilloscopes, etc. When you set up your
experiment you should check on the availability of equipment.
-
Using a digital delay generator, a TTL trigger can be sent to the lab
with almost any offset from the laser pulse.
| Beam |
This has too many meanings or no meaning at all. Depending on the
context it can mean the electron beam (sometimes ebeam), the FEL
infrared laser beam, the HeNe alignment laser (nominally co-aligned
with the FEL laser), or some structural component in the building!
|
| Burn paper |
Thermal fax paper which is convenient for aligning the invisible
FEL beam.
|
| Core (Corps?) |
The (FEL) Core staff that runs, maintains, and schedules the FEL.
Currently three "engineers" (all PhD physicists) and four operators
supposedly supervised by a physicist. The operators generally man
the Control Room while the engineers work on projects or repairs.
|
| Fast-IR |
The infrared macropulse measured on a less than 1% pickoff IR beam
using a PEM (photoelectromagnetic detector).
|
| Macropulse |
Reference to either the IR or ebeam collection of micropulses, or packets
of light or electrons. Usually measured in microseconds. Macropulses
repeat at the repetition rate of the machine, usually 30 Hz, but submultiples
are possible.
|
| Micropulse |
The smallest duration packet of light or electrons. Both are about
1 picosecond long.
|
| Molectron |
Usually one of several Molectron energy meters and heads used to
measure the energy per macropulse.
|
| MXI |
Pronounced Mix-ee, for Monochromatic X-ray Imaging, the latest incarnation,
along with MXISystems, Inc., of Dr. Frank Carroll's tunable, monochromatic
xray experiment.
|
| Power |
Unfortunately, this usually means the energy per macropulse given in
mJ. Oops.
|
| Users |
The (FEL) Users which are why the Core staff exist. Researchers that
use the laser beam, or electron beam, for experiments. Currently research
runs the gamut from semiconductor devices to clinical ablation of tumors.
|
Written by Bill Gabella (
b.gabella@vanderbilt.edu ) on April 1, 2005.
