General Lab Information

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Laser-Electron Accelerator Facility System Components

Accelerator System Components

A detailed description of the LEAF facility is given in Rev. Sci. Inst. 75, 4359-4366 (2004), which can be found by following this link.

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.

beam layout

The design of the beam line allows for several experimental stations; three 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 former 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. Optical fiber single-shot (OFSS) transient absorption has been developed for high-time-resolution studies of samples that can't be used in a flow cell. Pulse-pump-probe detection of transients produced by the photolysis of radiation-induced species is also available.
Optical Fiber Single-Shot Transient Absorption (OFSS)

A detailed description of the OFSS system is given in Rev. Sci. Inst. 80, 073106 (2009), which can be found by following this link.

A new type of single-shot transient absorption apparatus based on a bundle of optical fibers has been constructed. The bundle contains 142 fibers of different lengths, covering a total of 5 ns delay with sampling intervals as short as 5 ps near time zero. Data are collected by imaging light from the exit of the bundle into a sample where it is overlapped with an electron pulse or laser excitation pulse, followed by imaging onto a charge coupled device CCD detector where the intensity of light from each fiber is measured simultaneously. Application to both ultrafast pump-probe spectroscopy and pulse radiolysis is demonstrated. For pulse radiolysis, the prototype bundle provides the ability to collect data with a time resolution limited only by the electron pulse width of 7-10 ps. Tunable probe light is obtained from a titanium-sapphire laser and a TOPAS optical parametric amplifier. Corrections are made to remove the fiber-to-fiber variations in signal magnitude due to the spatial overlap of the electron beam and probe image. High quality data can be collected over most of the sensitivity range of the CCD camera detectors. The single-shot instrument is valuable for measurement of samples that are only available in very limited quantities, are too viscous to flow, or are rigid. It is therefore excellent in applications, such as picosecond pulse radiolysis, where the thousands of pulses per kinetic trace typical in classical pump-probe experiments can damage the sample before useful results could be obtained.

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, LeCroy 8620A (6 GHz), LeCroy WaveRunner 640Zi (4 GHz) or Teledyne LeCroy HRO 66Zi (600 MHz, 12-bit) oscilloscopes. 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 LabView (screen shot). Customized Igor Pro software is used to process and analyze the data (screen shot).

Mid-Infrared Nanosecond Transient Absorption

Vibrational spectroscopy offers highly specific molecular and structural characterization that until now has received only limited attention with pulse radiolysis. Thanks to the recent commercialization of compact, high-power, tunable, mid-IR external-cavity quantum cascade lasers (EC-QCLs), we have implemented routine time-resolved infrared TRIR detection at LEAF for detailed mechanistic studies with 20 ns time resolution. This has opened up many new opportunities for future scientific research directions that were previously difficult or impossible with only UV-VIS-NIR detection. We have a beam line dedicated to the TRIR experiment and a suite of eleven EC-QCLs with extensive coverage throughout a large region of the mid-IR (2317 - 1051 cm-1). 

A paper describing the new TRIR system is in preparation. A paper describing our earlier proof-of-concept studies using two EC-QCLs on our optical TA beamline B can be found at Appl. Spectrosc. 64, 563-570 (2010), which can be found byfollowing this link.

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).

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 platforms and the data is easily exported for analysis using other software if so desired.