VU FEL New User Information


Table of Contents:

Scheduling beam time: UP

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.

Mid-IR Tunable Radiation: UP

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
EvW Plot

(Temporal) Pulse Structure: UP

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
Pulse Plot

Laser Spectrum UP

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
Spectrum Plot

Fast Infrared Measurement UP

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."

Laser Stability UP

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.

Interacting with Operators and Staff UP

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:

Glossary UP

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.