Abstract
We propose a design for a minimally perturbing diagnostic minichicane, which utilizes optical synchrotron radiation (OSR) generated from magnetic bends in the chicane, to measure the rms horizontal and vertical beam sizes, divergences, emittances, Twiss parameters and energy spread of a relativistic electron beam. The beam is externally focused to a waist at the first bend and the OSR generated there can be used to measure the rms beam size. Subsequent pairs of bends produce far field OSR interferences (OSRI) whose visibility depends on the beam energy spread and the angular divergence. Under proper conditions, one of these two effects will dominate the OSRI visibility from a particular pair of bends and can be used to diagnose the dominant effect. The properties of different configuration of bends in the chicane have been analyzed to provide an optimum diagnostic design for a given set of beam parameters to: (1) provide a sufficient number of OSR interferences to allow a measurement of the fringe visibility; (2) minimize the effect of coherent synchrotron radiation and space charge forces on the particles motion; and (3) minimize the effect of compression on the bunch length as the beam passes through the chicane. A design for the chicane has been produced for application to the FERMI free electron laser facility and by extension to similar high brightness linear accelerators. Such a diagnostic promises to greatly improve control of the electron beam optics with a noninvasive measurement of beam parameters and allow on-line optics matching and feedback.
Highlights
Optical synchrotron radiation (OSR) and optical edge radiation interferences from preexisting magnetic bends and field transitions in storage rings and linacs have previously been studied and utilized to measure beam parameters [1,2,3]
Our primary goal in this paper is to present a benchmark design for a compact diagnostic chicane based on observations of OSR that (1) can measure both horizontal and vertical transverse normalized emittances ∼1 micron, which are typically produced by state-of-the-art photoinjectors; (2) is noninvasive, i.e., does not significantly alter the beam’s fundamental properties, in particular the transverse emittance and bunch length, during the course of the measurements; (3) is optimized for a given set of operating beam conditions typically used at free electron lasers (FELs); (4) can measure energy spread; and (5) can serve as the basis of a slow feedback system to adjust the accelerator to transport variations and maintain the beam brightness
The noninvasive energy spread and emittance diagnostic chicane (NIEM) chicane has the physical configuration of a magnetic bunch length compressor and, in principle, is subject to the same potential threats to the electron beam brightness, i.e., (1) emission of coherent synchrotron radiation (CSR) and the associated increased projected emittance in the bending plane and increased energy spread; (2) enhancement of microbunching instability driven by longitudinal space charge (LSC); and (3) direct space charge effects at energies ≤100 MeV
Summary
Optical synchrotron radiation (OSR) and optical edge radiation interferences from preexisting magnetic bends and field transitions in storage rings and linacs have previously been studied and utilized to measure beam parameters [1,2,3]. In contrast to previous papers we propose an approach that can distinguish between the beam’s spatial and angular properties This is accomplished by simultaneous observations of the source distribution of OSR and the far field OSR interference (OSRI) patterns from two sources, to independently obtain the beam size and divergence, respectively [4]. ELEGANT [9] and WARP [10] simulation codes as well as a special OSR code that we have developed [4] to calculate the far field angular distribution of synchrotron radiation from an electron moving along an arbitrary trajectory This code has been extensively checked against data and output from other synchrotron codes and is able to calculate the effect of beam energy spread and angular divergence on OSRI generated from any pair of magnetic dipoles. Two Appendices are included: Appendix A provides details on how the beam parameters vary as they pass through the NIEM chicane and Appendix B describes a simplified, approximate analysis of the effect of beam parameters on OSR interferences, based on the analogy to OTR interferences, which in lieu of employing a full OSRI simulation code, can be utilized to evaluate a NIEM for other accelerators
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