Electron beam instrumentation for FELs
Frontier fast dynamics science is strongly founded on acceleratorbased pulsed photon sources. The longitudinal electron bunch properties play a crucial role to guarantee high performance of the most advanced acceleratorbased facilities. Advanced instrumentation have been designed to control picosecond and sub picosecond electron bunch duration. The simplest and most robust bunch length diagnostics are based on the measurement of coherent radiation power and they are used in existing accelerators to measure the relative variation of the bunch length nondestructively and shot by shot. The resulting information is used in bunch length control feedback loops. The drawback is that for an absolute estimation of the bunch length, external instrumentation like a transverse RF deflecting cavity is usually needed.
We developed a novel experimental methodology to selfcalibrate a simple device based on diffraction radiation from a ceramic gap and demonstrated its capability to provide the second order moment of the electron bunch longitudinal distribution (i.e. the bunch length). The method is best suited for measuring bunches with durations from picoseconds to sub picoseconds. Moreover it does not depende upon other external instruments and the required equipment is extremely simple.
In general incoherent radiations have been used for a variety of electron diagnostics but they do not provide any information on the bunch as a whole since each electron in the bunch emits radiation with a different phase. Electrons in a very short bunch emit radiation, with wavelength of the order of the bunch length, with the same phase. This is called coherent radiation. Since all these electromagnetic waves have the same phase the sum of the amplitudes adds as N^{2}, where N is of the order of 10 billion electrons per bunch. Consequently the coherent emission is very intense, increasing inversely with the bunch length. The spectral content of the coherent radiation of a picosecond – sub picoseconds electron bunch is enhanced in the millimeter waves and THz spectrum range.
In order to provide an absolute bunch length measurement by using the coherent emission of the bunch we need other important ingredients. The first one is finding an analytical formalism for describing the physics of the system. In our device we detect the diffracted coherent radiation emitted by the electron bunch passing through a ceramic gap. This Maxwell problem has a formal analytical solution which is very complex and hardly usable for practical applications. For this reason we developed an analytical approximation that can be used to calculate the bunch length. The second ingredient is related to the spectral content. This depends on the Fourier transform of the longitudinal bunch distribution. However, by choosing a properly low detection frequency and using a relatively small bandwidth we can measure only the bunch length dependence without being affected by the bunch shape details. The third ingredient is a distinctive feature of the coherent emission from a gap in a waveguide: as the bunch gets shorter the emitted signal intensity initially increases but for very short bunches it reaches an asymptotic value.

Figure 1. Bunch length measurement validation: data acquired with the CBLM (red) and with the deflecting cavity (blue) vs L01 rf phase.



Experimentally, the selfcalibration method consists in varying the bunch length while detecting the diffraction signal until reaching the asymptotic value. We fit the obtained curve with the theory, where we demonstrated that the only free parameter is a scaling factor. At this point the device is calibrated and available to provide absolute bunch length measurements. We validated the method at the FERMI linac, by changing the bunch length and detecting the coherent signal from the gap by means of a 30 GHz Schottky diode installed in the Coherent Bunch Length Monitor (CBLM). The bunch is shortened by increasing the rf phase of the L01 linac, located upstream of the magnetic chicane used as bunch compressor. The results are reported in Figure 1 for 200 pC and 350 pC bunches. Bunch lengths as measured by CBLM (red dots) are compared with bunch lengths measured with the RF deflecting cavity (blue curve). Figure 1 shows a very good agreement and proves the independence of the proposed method from the bunch charge.
The advantage of this method is its unprecedented simplicity in providing a shot by shot, nondestructive, absolute bunch length measurement and it can easily be applied to different accelerators.
This research was conducted by the following research team:

Marco Veronese, Roberto Appio^{*}, Paolo Craievich^{#}, Giuseppe Penco, Elettra  Sincrotrone Trieste S.C.p.A., Trieste, Italy
^{* }now at Lund University, Max IV Laboratory, Lund, Sweden.
^{#}on leave at Paul Scherrer Institute, 5232 Villigen PSI, Switzerland
Contact person:
Marco Veronese, email: marco.veronese@elettra.eu
Reference
Marco Veronese, Roberto Appio, Paolo Craievich, Giuseppe Penco,”Absolute Bunch Length Measurement Using Coherent Diffraction Radiation”, Phys. Rev. Lett. 110, 074802 (2013), DOI: 10.1103/PhysRevLett.110.074802
Last Updated on Monday, 09 December 2013 09:52