BNL Chemistry
Department
Photo-
and Radiation Chemistry | Group Members | LEAF/Center
for Radiation Chemistry Research
Brookhaven National Laboratory
Center for Radiation Chemistry Research
 |
Laser-Electron Accelerator Facility
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A detailed description of the LEAF facility can be found in a recent paper (Rev. Sci. Inst. 75, 4359-4366 (2004)) that can be found by following this link.
Accelerator System Components
The LEAF facility layout indicates the
locations of the laser system, the RF components, the electron gun and the beam
lines.
RF System
The modulator cabinet and S-band (2.856 GHz) klystron are located in the laser
room. A copper waveguide carries the 15 MW RF pulse from the klystron to the
electron gun in the accelerator vault. (A klystron is a high-power RF amplifier.
You can visit the ALS MicroWorlds site for more information on klystrons
and the principles
of RF particle acceleration.)
Electron Gun Accelerator and Beam Line
The electron gun (link to picture) is located in the southwest
corner of the accelerator vault. The Northrop
Grumman Advanced Energy Systems Group, now known as Advanced Energy Systems, Inc, built the gun, beam line, modulator
system and control system. The gun is made entirely of copper, except for the
magnesium photocathode. Click here to see the gun
with the back plate removed to show the photocathode.
The design of the beam line allows for several
experimental stations, but only two have been built at present. One
arm of the beam line is equipped with two 45° bending magnets. It is
designed to transport a 5 picosecond, 10 nanocoulomb electron pulse to target
station A, where picosecond-resolution kinetic measurements can be performed,
including the use of additional laser pulses for detection and/or photolysis of
transients. (A profile of the electron beam near target A can be seen by clicking
here.) The straight section of the beam line transports 30 picosecond, 20
nanocoulomb electron pulses to target station B.
There are a total of 35 magnets (a solenoid, many dipoles, and many
quadrupoles) on the beam line. The whole system is controlled by a
Macintosh running National Instruments LabView (screen shot). You can see a picture
of our technician Steve Howell at the control console.
Laser System
(Photo) The purpose of the laser system is to provide a 3-20 picosecond-long pulse of
266 nm light to excite photoelectrons off of the magnesium metal photocathode
inside the radio-frequency electron gun. The temporal and spatial profiles of
the accelerated electron pulse are determined by the characteristics of the
excitation laser pulse. When the electron gun is filled with ~15 MW of 2.856 GHz
microwave power, electrons emitted at the proper point in the RF cycle are
accelerated to 9 MeV. The laser pulse must be synchronized to the RF signal to
an accuracy of 1 picosecond. In order to achieve this accuracy, a Ti:sapphire
laser oscillator (Spectra Physics Tsunami) has been fitted with an active
cavity-length stabilization circuit (Lok-to-Clock) and a piezoelectric actuator
to maintain the frequency of the laser at the 35th sub-harmonic of the RF
frequency (81.60 MHz). The resulting ~50 femtosecond, 7 nanojoule pulses are
passed into the Positive Light TSA-10 regenerative amplifier where they are
stretched with a grating, regeneratively amplified, passed through a double-pass
linear amplification stage, and recompressed. The amplifier is pumped by a 10 Hz
GCR Pro 290 green YAG laser. The amplified 798 nm pulses are converted to the
third harmonic (266 nm), stretched from ~100 fs to 1-3 ps using a grating, and
spacially and temporally shaped to get the best performance out of the
accelerator. The laser system continuously produces 0.4 mJ UV pulses at 10 Hz. A
mechanical shutter is used to select pulses for transmission to the photocathode
electron gun. This method of pulse generation is also useful because it
generates time-correlated laser and electron pulses with a jitter of only 1 ps.
The experimental station on the "A" beam line is equipped for
combined laser/electron pulse experiments.
Experimental Detection Systems
The primary experimental systems for pulse radiolysis are the pulse-probe and digitizer-based transient absorption detection systems. A picosecond streak camera is available for laser- and electron-beam-induced emission measurements. Other experimental techniques are under development, including ultrafast single-shot (UFSS) transient absorption for high-time-resolution studies of samples that can't be used in a flow cell, and pulse-pump-probe detection of transients produced by the photolysis of radiation-induced species.
Pulse-Probe Transient Absorption
The A-target beam line is designed for synchronized electron-laser probe experiments. Probe beams of ~100 fs duration of 798 nm (fundamental), 399 nm (second harmonic), or any OPA-generated wavelength can be delayed up to 10 ns using a 1.5 meter translation stage and a corner-cube retroreflector. The probe beam is split into sample and reference arms for a dual-beam absorbance measurement. The sample beam passes through the sample (up to 1 cm pathlength) co-linearly and in the same direction as the electron beam. Silicon, InGaAs and germanium photodiodes are used for detection over the range of wavelengths from 400 to 1700 nm. Because the experiments usually require thousands of shots, 5-100 mL of sample is typically passed through a 0.4 or 1.0 cm path length flow cell in a closed-loop system. The experiment is controlled using LabVIEW software, which passes the data directly to custom routines running under Igor Pro software for processing and data analysis. An screen shot of pulse-probe data collection at 1400 nm (decay of the solvated electron in acetonitrile) can be seen here. Absorbance measurements are made with the electron beam on (top chart, blue circles) and off (orange circles). The absorbance differences are corrected for the Faraday current averaged at each stage position (bottom chart) to provide the corrected absorbance trace (middle).
Digitizer-Based Transient Absorption
Digitizer-based transient absorption, emission and conductivity experiments are conducted at the B target. For the absorption measurements, detection light is provided by a pulsed xenon arc lamp or laser diodes. The analyzing light is propagated through the sample (up to 2 cm path length) co-linearly but opposite in direction to the electron beam to minimize collection of Cerenkov light. The analyzing light is then transported by lenses and mirrors to a detector in the control room. Narrow (10 nm) and wide (40 nm) bandpass filters are used to select the analyzing wavelength from the UV to the NIR. Several types of photodiode (Si, Ge, InGaAs) and biplanar phototube (Hamamatsu R1328U) detectors are used depending on the desired wavelength and time resolution. Available digitizers include Tek TDS-680B and TDS-694C oscilloscopes, a Tek 7250 scan conversion digitizer, and a LeCroy Wavemaster 8620 oscilloscope, the latter two units having 6 GHz bandwidth. Signal rise times as short as 100 ps have been measured using the phototube/8620 combination. A Faraday cup within the sample holder intercepts the electron pulse after it has passed through the sample so that the absorbance measurements can be normalized for dose. Samples are usually contained in Suprasil spectrophotometer cuvettes or cylindrical cells up to 2 cm in length. Sample temperature control is achieved using a water-jacketed cell holder, or for lower temperatures, a thermostated cell holder cooled by liquid nitrogen boil-off. Experimental hardware is controlled and data is collected using a G4 Macintosh running LabView (screen shot). Customized Igor Pro software is used to process and analyze the data (screen shot).
Van de Graaff
The Brookhaven Center for Radiation Chemistry
Research also includes a 2 MeV electron Van de Graaff (40 nsec minimum
pulsewidth) equipped for UV/Vis/NIR transient absorption and pulse conductivity experiments. The data acquisition
system is based on IBM-compatible computers primarily
interfaced through GPIB. The data analysis software is supported on Windows and Macintosh
platforms and the data is easily exported for analysis using other software if
so desired.
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