A Preliminary Study for a Sub-Micron Electron Bunch Length Diagnostic

by Matthieu George (EPFL, Switzerland / SLAC, USA)

Wednesday 3 Feb 2010, 16:00 → 17:00 Europe/Zurich

WBGB/019 (PSI)

In recent years, the operation of new 4th generation, free-electron laser (FEL) light sources has become a reality, especially with the successful operation of the LCLS (Linac Coherent Light Source) at SLAC. This revolutionary new type of facility operates with high transverse photon coherence, femtosecond pulse durations, and extremely high photon brightness levels, which are already ten orders of magnitude above existing 3rd generation sources.<o:p></o:p>

<o:p> </o:p>

Such a machine requires precise diagnostics, which become quite challenging, especially in the measurement of femtosecond pulse lengths. We propose a new laser-based scheme to obtain the length of the electron bunch, and therefore the approximate length of the photon pulse, within the femtosecond regime.<o:p></o:p>

As in the classical “zero-phasing” measurement method, the working principle of this device relies on a very fast external field variation to deflect the electrons differentially in time, similar to a standard streak camera approach. The main difference here is in the use of optical laser wavelengths (10 micron) rather than radio frequency (RF) wavelengths (10 cm).<o:p></o:p>

<o:p> </o:p>

As the phase of such a laser system is unknown and cannot be easily controlled, the development of a new approach is needed. The interaction of a CO2 laser with the electrons propagating through a wiggler, as already demonstrated in the LCLS laser heater system, leads to an energy deviation, or a simple linear energy chirp over the electron bunch length. After the “modulation”, the electron beam is bent by a dipole magnet dispersing it in the transverse direction onto an intercepting imaging screen. Both the beam’s centroid position and its transverse size are read on each pulse, where each new measurement depends on the uniformly random phase of the laser, which is then reconstructed from this data. The physical parameters of the electron beam, i.e. the bunch length, its energy chirp, and the amplitude of the modulation, are contained in both the size and the centroid position of the beam.<o:p></o:p>

<o:p> </o:p>

The aim of the present study is both to establish an analytical model of this relationship between the data read on the screen and these physical variables and to develop a method allowing to extract such variables. The result of numerical simulations of this diagnostic are presented and discussed in detail.<o:p></o:p>

The dependence of the errors of the measurement on both the device and bunch of electron parameters, such as its energy chirp and length, is also established.<o:p></o:p>

<o:p> </o:p>

Some key words: Diagnostic, sub-micron bunch length, energy chirp, zero-phasing, CO2 laser.<o:p></o:p>

<o:p> </o:p>

Contact: Leonid Rivkin (3214)<o:p></o:p>