Abstract
The new generation of linac injectors driving free electron lasers in the self-amplified stimulated emission (SASE-FEL) regime requires high brightness electron beams to generate radiation in the wavelength range from UV to x rays. The choice of the injector working point and its matching to the linac structure are the key factors to meet this requirement. An emittance compensation scheme presently applied in several photoinjectors worldwide is known as the “Ferrario” working point. In spite of its great importance there was, so far, no direct measurement of the beam parameters, such as emittance, transverse envelope, and energy spread, in the region downstream the rf gun and the solenoid of a photoinjector to validate the effectiveness of this approach. In order to fully characterize the beam dynamics with this scheme, an innovative beam diagnostic device, the emittance meter, consisting of a movable emittance measurement system, has been designed and built. With the emittance meter, measurements of the main beam parameters in both transverse phase spaces can be performed in a wide range of positions downstream the photoinjector. These measurements help in tuning the injector to optimize the working point and provide an important benchmark for the validation of simulation codes. We report the results of these measurements in the SPARC photoinjector and, in particular, the first experimental evidence of the double minimum in the emittance oscillation, which provides the optimized matching to the SPARC linac.14 MoreReceived 24 July 2007DOI:https://doi.org/10.1103/PhysRevSTAB.11.032801This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Highlights
The SPARC [1] project is a research and development photoinjector facility for the generation of high brightness electron beams to drive a SASE-FEL experiment in the visible
We report the results of these measurements in the SPARC photoinjector and, in particular, the first experimental evidence of the double minimum in the emittance oscillation, which provides the optimized matching to the SPARC linac
A detailed theoretical study of the emittance compensation process in a photoinjector [4] has demonstrated that its best optimization is achieved by accelerating and propagating the space charge dominated beam through the device following as close as possible two relevant beam equilibrium conditions: the laminar Brillouin flow in drifts and the so-called invariant envelope in accelerating sections, which is a generalization of the Brillouin flow for an accelerated beam
Summary
The SPARC [1] project is a research and development photoinjector facility for the generation of high brightness electron beams to drive a SASE-FEL experiment in the visible. A detailed theoretical study of the emittance compensation process in a photoinjector [4] has demonstrated that its best optimization is achieved by accelerating and propagating the space charge dominated beam through the device following as close as possible two relevant beam equilibrium conditions: the laminar Brillouin flow in drifts and the so-called invariant envelope in accelerating sections, which is a generalization of the Brillouin flow for an accelerated beam. In this regime the beam exhibits emittance oscillations produced by space charge collective forces, called plasma oscillations. The basic point in the design of a photoinjector is to properly match the beam from the injector into any accelerating section, by means of a laminar waist at injection and a
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