Abstract
We developed a three-dimensional electron bunch charge distribution (3D-BCD) monitor with single-shot detection, and a spectral decoding based electro-optic (EO) sampling technique for a nondestructive monitor enables real-time reconstruction of the three-dimensional distribution of a bunch charge. We realized three goals by simultaneously probing a number of Pockels EO crystals that surround the electron beam axis with hollow and radial polarized laser pulses. First, we performed a feasibility test as a simple case of a 3D-BCD monitor probing two ZnTe crystals as EO detectors installed on the opposite angle to the electron beam axis and confirmed that we simultaneously obtained both EO signals. Since the adopted hollow probe laser pulse is not only radially polarized but also temporally shifted azimuthally, some disorders in the radial polarization distribution of such a laser pulse were numerically analyzed with a plane-wave expansion method. Based on the above investigations, the 3D-BCD monitor is feasible both in experimental and numerical estimations. Furthermore, we previously developed a femtosecond response organic crystal as a Pockels EO detector and a broadband probe laser ($\ensuremath{\ge}350\text{ }\text{ }\mathrm{nm}$ in FWHM); the 3D-BCD monitor realizes 30- to 40-fs (FWHM) temporal resolution. Eventually, the monitor is expected to be equipped in such advanced accelerators as XFEL to measure and adjust the electron bunch charge distribution in real time. The 3D-BCD measurement works as a critical tool to provide feedback to seeded FELs.
Highlights
In x-ray free electron laser (XFEL) accelerators, we must measure a few tens of femtoseconds (FWHM) of electron bunches with a nondestructive method [1,2,3]
The temporal resolution of spectral decoding is mainly limited by the following four factors: (1) restriction from Fourier transform limitation and a chirped pulse width of a realistic probe laser; (2) dispersion of the EO crystal depends on crystal thickness and frequency domain; (3) distance from the electron beam axis and electron bunch energy; and (4) velocity mismatch between the probe laser pulse in the EO crystal and the electron bunch in a vacuum
During EO sampling with a single Pockels EO crystal, Tres % 30 fs (FWHM) is achieved with a c % 300–400 fs (FWHM) linear-chirped probe laser pulse based on Eq (1)
Summary
In x-ray free electron laser (XFEL) accelerators, we must measure a few tens of femtoseconds (FWHM) of electron bunches with a nondestructive method [1,2,3]. The temporal resolution of spectral decoding is mainly limited by the following four factors: (1) restriction from Fourier transform limitation and a chirped pulse width of a realistic probe laser; (2) dispersion (refractive indices) of the EO crystal depends on crystal thickness and frequency domain (both optical and THz regions); (3) distance from the electron beam axis and electron bunch energy; and (4) velocity mismatch between the probe laser pulse in the EO crystal and the electron bunch in a vacuum. The electron beam halo is suppressed to avoid radiation damage to the permanent magnets, especially of the in-vacuum undulators; this is a favorable condition for our EO sampling system Such a 3D-BCD monitor has two main advantages for which spectral decoding is a unique technique rather than temporal or spatial decoding. Based on these above issues, we are developing our novel 3D-BCD monitor with capable 30-fs (FWHM) temporal resolution, as proposed by Tomizawa at SPring-8 in 2006 [10,11]
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