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, before the realization of this diagnostic, 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, we designed and built an innovative beam diagnostic device, the emittance meter, consisting of a movable emittance measurement system. 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. With this device we obtained the first experimental evidence of the double minimum in the emittance oscillation, which provides the optimized matching to the SPARC linac.
- “Direct measurement of the double emittance minimum in the beam dynamics of the sparc high-brightness photoinjector”, M. Ferrario et al., Phys. Rev. Lett. vol. 99 n. 23 (2007) pag. 234801, doi: 10.1103/PhysRevLett.99.234801
- “High brightness electron beam emittance evolution measurements in a RF photoinjector”, A. Cianchi et al., Phys. Rev. ST Acc. Beams vol. 11 n. 3 (2008) pag. 032801, doi: 10.1103/PhysRevSTAB.11.032801
- “Analysis methodology of movable emittance-meter measurements for low energy electron beams” A. Mostacci et al., Rev. Sci. Instrum. vol. 79 n. 1 (2008) pag. 013303, doi: 10.1063/1.2835715