Laser Electron Accelerator Facility

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). 

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

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. 

Fig. 6. Scheme of the laser-driven RF electron accelerator of pulse radiolysis facility ELYSE. IP ion vacuum pump, CPC cathode preparation chamber, W vacuum valve, SOL solenoid, D dipole, TRl and 2 triplets, Q quadrupole, WCM wall current monitor, PC Faraday cup, T translator for Cerenkov light emitter and visualization screen LME laser entrance mirror, LMEx laser exit mirror, VC virtual cathode FIS horizontal slit, VS vertical slit. (Reproduced with permission from Ref 28.)... 

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. 

One final example of ultrafast kinetics performed at radiolysis facilities is the study of excited states of radical ions. An accelerator pulse can be used to generate radical species, which can then be excited by a pump laser beam and probed with femtosecond resolution by another laser pulse with variable optical delay. This application does not depend on precise correlation of the electron and laser pulses and can be done at almost all radiolysis facilities. The availability of femtosecond lasers in photocathode facilities places all the necessary components to hand. Effective pump-probe measurements will require significant concentrations of radical ions. This can be accomplished by frequency-quadrupling a 5-9 nanosecond Nd YAG pulse to irradiate the photocathode, thereby creating a macropulse containing several tens of nanocoulombs which will produce a high concentration of radicals for the pump-probe experiment. 

The RF photocathode electron gun is the newest type of accelerator used for pulse radiolysis. Such devices have been under development since the mid-1980s as electron beam sources for experimental physics facilities and free-electron laser development. They are typically used to produce electron beams in the 4-10-MeV range. The unique quality (low emittance and clean position-momentum relationships) of the electron beams they produce makes extremely sophisticated beam manipulation possible. 

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