On 20 June, the SPring-8 Compact SASE Source (SCSS) prototype accelerator generated its first pulses at a wavelength of 49 nm in the vacuum ultraviolet (VUV) region. This was achieved using an ultra-low emittance beam provided by an electron gun with a newly developed single-crystal CeB6 thermionic cathode.
The SCSS prototype accelerator is a self-amplified spontaneous emission (SASE) free-electron laser (FEL), similar to the FLASH laser at the TESLA Test Facility. Built during 2004–2005 at the SPring-8 synchrotron radiation facility in Japan, the SCSS has recently been commissioned. Its main purpose is to test components developed at the RIKEN/SPring-8 centre in R&D for an X-ray FEL to generate wavelengths of 0.1 nm (1 ångstrom) using an 8 GeV electron beam. Funded by the Japanese Ministry of Education, Culture, Sports, Science and Technology, construction of this X-ray FEL is scheduled for 2006–2010.
One of the most challenging features of the SCSS is the use of the CeB6 single-crystal cathode to generate an ultra-low emittance beam. The 500 kV electron gun produces a beam current of 1 A, which feeds an injector system of RF cavities and magnetic lenses that have been carefully designed to perform velocity bunching without allowing the emittance to deteriorate. Here the bunch length is compressed several hundred times to produce a beam of a few hundred amps. Then after four C-band accelerating stages, the beam energy reaches 250 MeV.
During beam commissioning, an emittance of 2.9 π mm mrad normalized was measured in the injector, for a bunch charge of 0.25 nC and a bunch length of 1 ps at 50 MeV. Then the SCSS team closed the upstream undulator, and after an hour of tuning observed a narrow spectrum peaked at 49 nm in the VUV. This was totally different to the natural undulator radiation (spontaneous mode). After further careful measurements, first lasing was announced on 20 June.
