Image credit: NSRRC.
With a view to sustaining a large-scale facility at a time of worldwide economic crisis and soaring energy costs and to provide efficient use of beam time, the National Synchrotron Radiation Research Center (NSRRC) in Hsinchu has been exploring ways to make the most of its facilities. One possibility is an ultrafast X-ray source. This is being considered through a feasibility study and technological investigation aimed at gaining additional leverage for NSRRC’s second accelerator, the Taiwan Photon Source (TPS), which is currently under construction. To this end, NSRRC held a mini-workshop on “Storage-ring ultrafast X-ray sources and their applications” on 16–17 January. Nearly 40 participants attended, including speakers invited to join discussions with NSRRC staff and the ultrafast-science research groups from neighbouring universities, including National Tsing Hua University and National Chiao Tung University.
On the first day, Shaukat Khan of the Technical University, Dortmund, offered a comprehensive overview of ring-based, ultrafast and coherent light-sources, including topics such as laser slicing, low-alpha lattice, coherent harmonic generation and echo-enabled harmonic generation. Gerhard Ingold of PSI introduced several topics: the operational performance of the FEMTO source at the Swiss Light Source at PSI; the proposed upgrade of beamline optics and the laser repetition rate from 2 kHz to 10 kHz (or even 20 kHz); and the study of the ultrafast structural dynamics in condensed matter. Karsten Holldack of the Helmholtz-Zentrum Berlin described the laser system of the femtoslicing facility at the BESSY II synchrotron, known as FEMTOSPEX, which was upgraded to a 6 kHz repetition rate in 2010. It has been over-booked by a factor of three, indicating the growing demand in this domain. At Brookhaven’s National Synchrotron Light Source (NSLS) a feasibility study has been carried out on the NSLS II laser-slicing source with a 4.8 m modulator wiggler in response to users’ requests, as Li Hua Yu of Brookhaven explained.
Image credit: NSRRC.
On the second day, Andreas Streun of PSI discussed in-depth the beam-dynamics issues involved in a high-repetition-rate laser-slicing source, saying that noise caused by beam halo is a critical issue. In general, the side effects of laser slicing on storage-ring performance are mainly contributed by the modulator wiggler and chicane. In addition, a series of presentations by NSRRC team members covered the design requirements and considerations of a proposed beamline for the NSRRC TPS laser-slicing source and its potential applications. Yu also chaired a discussion about how to improve the performance of laser-slicing sources. Methods such as maximizing radiator length, reducing the loss of photon flux in the photon-beamline design, multiple slicing, increasing laser repetition rate and single-bunch current are considered essential for a state-of-the-art laser-slicing source.
Based on input generated at the mini-workshop, the preliminary design of the TPS laser-slicing source with a modulator wiggler, one hard-X-ray radiator and one soft-X-ray radiator in three separate, 7-m straight sections, appears to be a feasible scheme and will provide 10 times more flux than the SLS FEMTO source. However, Ingold suggested a different scenario that requires only two straight sections, with a modulator wiggler and a hard-X-ray radiator in one 12-m-long straight section plus a soft-X-ray radiator in one 7-m-long straight section – and this is regarded as an attractive alternative.
