A linear particle accelerator (Linac) is used to accelerate charged particles (electrons, protons or ions) in a straight line by generating a high frequency alternating electric field along a series of cavities, forming an accelerator structure. In a free-electron laser (FEL), the accelerated electron beam is passed through a periodic magnet array, called undulator, causing transverse acceleration of the electrons which results in the release of photons (see Figure 1 ). The SwissFEL project at the Paul Scherrer Institut will develop a free-electron laser which provides a source of very bright and short X-ray pulses enabling scientific discoveries in a wide range of disciplines, from fundamental research to applied science. The SwissFEL injector and linac RF drives operate in a pulsed mode at the rate of 100 Hz, using normal conducting RF accelerating structures. The input RF pulse length is relatively short (in the order of 1-3 us) and there is no RF feedback loop running within a pulse. Beam measurements can therefore only influence subsequent pulses.
Fig. 1 : The SwissFEL (phase 1) schematic.
The SwissFEL has a number of unique features which distinguish it from other X-ray FEL projects worldwide. The SwissFEL will deliver hard X-ray pulses, down to 1 A (= 0.1 nanometer) wavelength, as will only three other projects: LCLS (Stanford), SCSS (Harima, Japan) and the European X-FEL (Hamburg) (see Table 1). In comparison to the LCLS in Stanford and the European X-FEL in Hamburg, the SwissFEL will be more compact and cost-effective, thanks to a low electron beam emittance, low charge-per-pulse, compact undulators, and optimization of the FEL process. Whereas the European X-FEL will produce bursts of high repetition rate X-rays, the SwissFEL will be operated at a constant pulse rate.
Table 1. A comparison of the SwissFEL with approved hard X-ray FEL projects.