What is SCSS? Milestone What is SASE? In Vaccum Undulator Low Emittance Beam Injector Beam Line Back to Home


SCSS is an abbreviation for SPring-8 Compact SASE Source. This is Soft X-ray radiation facility based on the SASE-FEL. SCSS is designed "compact" by means of (1) Short-period in-vacuum undulator, and (2) high-gradient C-band accelerator. Whole system including the electron beam injector, C-band accelerator and the undulator fits within 100 m long facility.



The SCSS will be build at SPring-8, Hyogo Japan.



Funding for SCSS has been authorized  April 2001. SCSS aims to start operation at VUV radiation in the end of Year 2003, and eventually down to 3.6 nm wavelength. It is within the "water window" region, which is suitable to study "wet material" such as living cells, or biological samples. By accumulating infomation through construction and operation of SCSS project, we expect to build X-ray FEL, which aims to reach 0.1 nm X-ray region.

SASE is an abbreviation for Self Amplification of Spontaneous Emission. The conventional FEL (Free Electron Laser) uses two-reflection mirrors to confine the laser beam in a optical resonator, which enables to repeat the interaction of electron beam with the optical beam inside the undulator, resulting in generating coherent hihg-power radiation. In contrast with this type of FEL, the SASE type FEL does not use reflection mirror, and there is undulator only. Since the undulator in SASE FEL designed fairly long, the electron beam injected into undulator keeps interaction with undulator field and radiation field long time, the noise signal associated with "population fluctuation" is amplified and finally reaches saturation. Emitted radiation is "quasi-monochromatic", not fully coherent in time domaine, but fairly high-peak power, it reaches higher than a few GW level. SASE FEL does not need reflection mirror, thus it can operate short wavelength such as VUV, Soft X-ray and even X-ray region where no reflection mirror is available.

Learn more >>> G. Ramian's The World Wide Web Virtual Library: Free Electron Laser research and applications

Undulator is an array of permanent magnets, which generates vertical (or horizontal) transverse magnetic field of periodically changing its polarity. Passing electron beam inside undulator experience periodic transverse acceleration from the magnetic field, and generates radiation strongly focused in to beam axis. The radiation electric field pattern is exact copy of the undulator field, and the Doppler effect squeezes longitudinal pattern, thus the radiation wavelength becomes very short, such as, VUV, Soft-X-ray and X-ray according to the electron beam energy. The radiation becomes beautiful sin-wave whose spectrum becomes quasi-monochromatic.

Short-period magnet array is desirable to make FEL-machine compact, i.e., reducing beam energy and shortning the stauration length. To generate enough magnetic field-strength in a short-period undulator, we need to bring two magnetic pole pieces closer. "In-vacuum undulator" design eliminates vacuum pipe from the undulator gap, and enable to close the undulator gap down to a very narrow width. As a result, we can realize short-period as short as 1 cm.
To obtain high peak power from SASE FEL, the electron beam emittance has to be low. Emittance is a measure of paralellisity of group of particles. As seen in the figure, low emittance beam has uniform velocities, thus the radiation becomes pallarel. SASE FEL need very low emittance beam. To generate this beam, generally the rf-gun with photo-cathode combination is commonly used. By irradiating short-pulse laser beam (order of a few psec down to sub-pico-second), the short bunched beam is directly generated.
In SCSS, however, high-voltage pulsed gun with single crystal thermionic cathode is employed. This is because, this system does not requrie the short pulse laser system which is sometimes troublesome, and needs manpower to maintain. For the cahode material, we will use CeB6 cathode. It is a single crystal cathode, which maintains a very flat cathode surface after usage of cathode at high temperature by evaporation of cathode material.






Following figure shows the beam line layout of SCSS project. The high-votlage electron with CeB6 cathode generates low emittance electron beam, whose energy is modulated with 476 MHz rf-field. After drifting, a bunched beam is formed at the entrance of L-band accelerator. The bunch compressor-1 (BC-1) compresses the bunch length from 33 psec down to 4 psec, as a result the peak current reaches 250 A. C-1 through C-4 denote the C-band accelerator unit, whose accelerating gradient is about 40 MV/m, and energy gain per unit is 300 MeV maximum. After four unit, the beam energy reaches 1 GeV nominal.
The undulator consists of five section of in-vacuum type. The undulator period is 15 mm, and the maximum K-parameter is 1.3 at the minimum gap of 3.7 mm. Each undulator section length is 4.5 m long, and 22.5 m in total. which is longer than the expected staturation length of 20 m at the shortest wavelength opeation (3.6 nm).





What is SCSS? Milestone What is SASE? In Vaccum Undulator Low Emittance Beam Injector Beam Line Back to Home