Laser Electron Accelerator Laboratory

Pulse-probe studies using the Laser Electron Accelerator Facility (LEAF) at Brookhaven National Laboratory have revealed changes in optical absorption occurring on the picosecond time scale in rare gas fluids. In xenon, excimers are formed which absorb in the visible and near infra-red as shown in Fig. la. The absorption grows in during the first 50 picoseconds [see Fig. 1(b)].This growth is concomitant with ion recombination that leads first to excited atoms, reaction 1(a), which immediately form excimers, Xe, because of the high density of xenon. 

James R Wishart received a B.S. in Chemistry from the Massachusetts Institute of Technology in 1979 and a Ph.D. in Inorganic Chemistry from Stanford University in 1985 under the direction of Prof Henry Taube. After a postdoctoral appointment at Rutgers University, in 1987 he joined the Brookhaven National Laboratory Chemistry Department as a Staff Scientist in the Radiation Chemistry Group. He founded and presently supervises the BNL Laser-Electron Accelerator Facility for picosecond electron pulse radiolysis. His research interests include ionic liquids, radiation chemistry, electron transfer, and new technology and techniques for pulse radiolysis. He has authored over 90 papers and chapters, and is the co-editor of Advances in Chemistry Series o. 254, Photoehemistry and Radiation Chemistry. 

Figure 1 Schematic representation of the Laser-Electron Accelerator Facility at Brookhaven National Laboratory. The laser beam is split to generate both the electron pulse and the probe light (scheme courtesy of Dr. J. Wishart, Brookhaven National Laboratory).

To exploit the capabilities of fast lasers, a new picosecond Laser-Electron Accelerator Facility (LEAF) has been recently developed at Brookhaven National Laboratory. In this facility, schematically shown in Figure 1, laser light impinging on a photocathode inside a resonant cavity gun merely 30 cm in length produces the electron pulse. The emitted electrons are accelerated to energies of 9.2 MeV within that gun by a 15 MW pulse of RF power from a 2.9 GHz klystron. The laser pulse is synchronized with the RF power to produce the electron pulse near the peak field gradient (about 1 MeV/cm). Thus the pulse length and intensity are a function of the laser pulse properties, and electron... 

As mentioned above, a 3.5-cell RF photocathode gun is in operation as the accelerator for the Brookhaven National Laboratory Laser-Electron Accelerator Facility. Recently, 1.6-cell RF photocathode guns have replaced thermionic cathode systems as injectors for 30 MeV linear accelerators at Osaka University and the Nuclear Engineering Research Laboratory in Tokai-mura, Japan [6]. Another RF photocathode gun accelerator is under construction at the ELYSE facility at the Universite de Paris-Sud at Orsay, France. A magnesium cathode is in use at LEAF, copper is used at NERL, while the Orsay accelerator will use Cs Te. 

Brookhaven National Laboratory s Laser Electron Accelerator Facility (LEAF) uses a titanium-sapphire laser Ahv = 798 nm fwhm = 100 fs and energy per pulse is 0.01 J. 

This laser is a component of Brookhaven National Laboratory s Laser Electron Accelerator Facility (LEAF). 

Although this work is restricted to electron acceleration, it has to be mentioned here for completeness that an increasing amount of work is currently devoted (often in the same laboratories) to the study of proton and light ion acceleration with laser pulses, also in view of future medical applications. Among a number of recent works on this subject, see for example [9] and [10]. 

Laser-plasma accelerators represent a unique tool for investigating matter properties at a reduced laboratory scale, compared to large-scale facilities. Nowadays, efficient electron acceleration can be provided by such table-top accelerators and can be used to perform experiments in a variety of fields. 

The new accelerator at Brookhaven is based on an RF photocathode gun with one or more resonant cavities in which microwaves create transient electric fields up to 1 MeV cm [104], A pulse of laser light is used for generating photoelectrons which are accelerated to 9 MeV in a distance of 30 cm. The laser pulse can also be used as the analyzing light source this means it is closely synchronized with the electron pulse. The time resolution of the electron pulse is therefore that of the laser pulse, so that subpicosecond pulse radiolysis is possible. A similar system is planned at Argonne National Laboratory [146],... 

Time Scales. The time scales measurable by the two techniques are illustrated in Figure 3. The time resolution of pulsed lasers is far superior, reaching to as short as 20 fs, with 200-fs measurements becoming routine in many laboratories. Pulse radiolysis measurements achieve rise times of 20-30 ps in only a few laboratories, while 1-10 ns is more common. Pulse radiolysis is slower because the accelerated particles, usually electrons, repel each other, making it difficult to bunch many of them into a very short pulse. Pulses as short as 5 ps have been reported and new accelerators may achieve 1 ps, but it is likely that pulsed lasers will remain the leader in very high time resolution. 

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