Electron guns photocathode

Another alternative involves focusing the probe pulse onto the silver photocathode of an electron gun, providing a well-defined pulse of electrons through the photoelectric effect so that time-dependent electron diffraction data for shortlived intermediates may be obtained. 

In the nanosecond (ns) time-scale the use of kinetic detection (one absorption or emission wavelength at all times) is much more convenient than spectrographic detection, but the opposite is true for ps flash photolysis because of the response time of electronic detectors. Luminescence kinetics can however be measured by means of a special device known as the streak camera (Figure 8.2). This is somewhat similar to the cathode ray tube of an oscilloscope, but the electron gun is replaced by a transparent photocathode. The electron beam emitted by this photocathode depends on the incident light intensity I(hv). It is accelerated and deflected by the plates d which provide the time-base. The electron beam falls on the phosphor screen where the trace appears like an oscillogram in one dimension, since there is no jy deflection. The thickness of the trace is the measurement of light intensity. 

Fig. 3. Scheme of a laser photocathode electron gun, showing the relationship between laser and microwave synchronization. The example resembles the BNL LEAF laciUty. The length of the electron gun is approximately 30 cm. 

Table 1. Comparison of specifications of photocathode electron gun accelerators for picosecond pulse radiolysis.

In photocathode electron guns, the timing between the microwaves used for acceleration and the photocurrent-generating laser pulse is of critical importance. Precise synchronization between the laser and electron beams is obtained by using a MHz quartz master oscillator to control the cathode pump laser repetition rate and the microwave amplifier system seed frequency (Fig. 3). 

Figure 4 Schematic representation of the BNL LEAF photocathode electron gun accelerator showing the laser-microwave synchronization relationship.

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. 

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. 

Figure 5. A radio-frequency photocathode electron gun. The overall length is about 30.5 cm (12 in.).

One key feature of the photocathode electron gun is the fact that the temporal and spatial characteristics of the electron pulse are to some extent determined by the incident laser pulse. A 5-ps laser pulse and a well-designed beam transport system will maintain the 5-ps pulse width at the target. Electron gun systems are capable of studying extremely fast reactions. 

A particular feature of the RF photocathode electron gun is the picosecond-synchronized laser system needed to generate the photoelectrons. Part of the laser s output can be diverted to generate a visible light continuum for time-resolved kinetics or used for combined photolysis-radiolysis experiments. For example, the photochemistry of radiation-induced transients can be studied. 

James F. Wishart is a Chemist in the Chemistry Department of Brookhaven National Laboratory (BNL), Upton, New York. He received his Ph.D. degree in inorganic chemistry from Stanford University under Professor Henry Taube, and his S.B. in chemistry from the Massachusetts Institute of Technology. He served as project manager for the construction of the new pulse radiolysis facihty at BNL, which is based on a 10 MeV radio-frequency photocathode electron gun, the first of its Idnd in the world to be dedicated to pulse radiolysis. 

Figure 2. Schematic representation of an S-band RF Photocathode electron gun injector system. In actuality, the accelerator is about 30 cm long.

Picosecond accelerators for ultrafast radiolysis studies are undergoing a period of renewal as the RF photocathode electron gun technology becomes... 

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],... 

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. 

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