Picosecond Laser-Electron Accelerator Facility

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... 

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. 

Table 2. Comparison between time-resolved spectrophotometric detection set-ups of several laser-photocathode electron accelerator facilities for picosecond pulse radiolysis. 

Photocathode-based picosecond electron accelerators are conceptually simpler than pre-bunched thermionic systems, although they require reasonably powerful, multicomponent femtosecond or picosecond laser systems to drive the photocathode. In addition, the availability of synchronized laser pulses allows the development of advanced detection capabilities with unprecedented time resolution. The combination of ease of use and powerful detection methods has stimulated strong interest in photocathode gun systems. Since the installation ofthe first photocathode electron gun pulse radiolysis system at BNL [5,13], four additional photocathode-based facilities have become operational and two more are in progress. The operational centers include the ELYSE facility at the Universite de Paris-Sud XI in Orsay, France [7,8], NERL in Tokai-Mura, Japan [9,10], Osaka University [11,12], and Waseda University in Tokyo [13]. Facilities under development are located at the Technical University of Delft, the Netherlands, and the BARC in Mumbai, India. 

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