SwissFEL Injector Test Facility: Summary of Experimental program achievements until decommissioning

The SwissFEL Injector Test Facility (SITF) at the Paul Scherrer Institute completed its four-year experimental program on October 13, 2014. Since its official inauguration in August 2010, the injector plant has fulfilled its purpose as a development facility for beam dynamics studies and a test bed for diagnostics developments, both in view of the realization of the SwissFEL.

After an initial commissioning phase in 2010 and early 2011 to bring into operation the photocathode RF gun and the ensuing booster linac, the injector facility was equipped with a magnetic bunch compression chicane in July 2011. The following period lasting up to March 2012 was mainly dedicated to the consolidation of the S-Band RF system. During the same time, THz-based longitudinal beam diagnostics was implemented near the bunch compressor and the stability of the photo-cathode laser system was improved.

In April 2012, with all RF cavities in operation, the nominal design energy of 250 MeV could be attained for the first time. Soon afterwards, the transverse beam quality fulfilling the FEL requirements for uncompressed beam could be demonstrated for the nominal bunch charge of 200 pC, thus reaching a first important milestone. At this time, values as small as 0.37 and 0.25 mm mrad could be measured for global (projected) and so-called slice emittance, respectively. The slice emittance is the transverse emittance measured for individual slices along the bunch length, representing a key beam parameter for free-electron lasers.

Further systematic optimizations of the beam setup (coupling correction,dispersion) resulted in a stable working point for uncompressed electron bunches, which ensures beam transport with minimal emittance dilution. Once the influence of the accelerator itself on the beam emittance is kept at a minimum, subtle effects of the laser generating the electrons at the cathode (via the photo-electric effect) become accessible to beam optics measurements. In this way it was possible, for instance, to demonstrate the effect of the laser photon energy (or wavelength) on the beam emittance. During these studies extremely small emittances were measured, in particular for low bunch charges, where the adverse effects of Coulomb repulsion (space charge) are also smallest. But even at the nominal SwissFEL working point of 200 pC charge and with the standard laser wavelength of 260 nm, which ensures high electron yield, record low emittances well below the SwissFEL requirements have been achieved. For these conditions projected emittances below 0.35 mm mrad could be obtained routinely, with best values around 0.30 mm mrad. The more relevant slice emittance was typically below 0.20 mm mrad, with a record value of 0.18 mm mrad. Systematic measurements of the longitudinally compressed beam had to wait for the installation of a harmonic cavity in front of the compression chicane, realized in early 2013. The additional cavity is needed to compensate the electron bunch curvature in longitudinal phase space introduced by the RF non-linearity in the preceding accelerating cavities. Initial compression studies revealed a worrisome emittance growth with increasing compression ratio, which was not reproduced in simulations. After a great number of thorough studies lasting up the very end of injector operation in October 2014, this effect could eventually be mitigated by a careful setup of the beam optics matching in front of the bunch compressor, employing lower quadrupole currents, and adaptations in the beam optics within the compression chicane itself. With these measures the emittance could be preserved under compression at the 10%-level; the effect causing the emittance growth, however, is not fully understood yet.

In early 2014, a 4 m long prototype of the SwissFEL undulator module with an undulator period length of 15 mm (U15) was tested with electron beam at the injector test facility. At beam energies varying between 120 and 200 MeV, FEL radiation in SASE mode was observed in the UV and optical spectral ranges (photon wavelength between 70 and 800 nm). The wavelength was tuned by changing the electron energy as well as the undulator gap. The SASE nature of the radiation was confirmed both by measuring the divergence of the FEL beam and by recording the characteristic fluctuations of the photon pulse energy. The measurements with the undulator prototype also allowed the realistic testing and optimization of the alignment procedures planned for SwissFEL. In addition to the beam dynamics and undulator studies, the injector test facility provided the opportunity to test various systems relevant for SwissFEL under realistic conditions. At the source, we tested the new PSI RF gun at 100 Hz repetition rate, as well as different cathode materials (copper and cesium telluride) and laser profiles (transverse spot size and longitudinal profile). The electron beam was used to test numerous electron diagnostics concepts such as cavity beam-position monitors, integrated current transformers, beam arrival-time monitors, but also a novel screen design minimizing effects from crystal thickness. Different longitudinal diagnostics systems based on electro-optical sampling or THz radiation made use of the compressed bunches for tests and optimizations.

After 4 years operation the accelerator test facility is presently being decommissioned. Components are recovered for the SwissFEL accelerator complex, while the experimental hall will be first partially emptied to host pre-assembly activities for SwissFEL.
Facility: SwissFEL
Reference: Thomas Schietinger; thomas.schietinger@psi.ch; Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland