Editorial – The New Radiation Science

Prompted by our Founding Editor, Kai Siegbahn, the 500th volume of Nuclear Instruments and Methods-A featured a special section that recounted a vision for the development of instrumentation for synchrotron radiation science. In recognition of Kai’s keen interest in this field Elsevier will be awarding, later this year, the Kai Siegbahn Prize for achievement in synchrotron radiation instrumentation and science. While each of the types of synchrotron facilities described in that issue promise significant improvements to their users, one approach – the free electron laser – offers the possibility to investigate phenomena that are not open to scrutiny in any other way. This frontier of research into ultra-fast phenomena or requiring ultra-fast probes can rightly be called the New Radiation science.

Diligent efforts over the past four years have yielded spectacular results at the FLASH FEL facility at DESY (Hamburg). Using Self-Amplified Spontaneous Emission (SASE), FLASH has generated 10 - 50 femtosecond pulses of X-rays at ~7 nm powerful enough to make a nanoscale image of living cells – or perhaps more correctly the ghosts of cells before they exploded due to the absorbed radiation. These results have also relied on strong advances in algorithms to reconstruct spatial images from diffraction patterns. At this time FLASH has a vigorous user program on a single FEL photon line with multiple end stations. The FLASH upgrades over the next year will produce ultra-short X-ray pulses in the water window and will include an experiment on direct FEL seeding in the 20-30nm wavelength range, as well as a pilot pump-probe experiment based on this.

On the science front two new Max Planck Institutes have been founded in Hamburg, one for ultra-fast imaging and one for ultra-fast dynamics.  This vigorous commitment to femtosecond X-ray science is also preparation for the XFEL at Hamburg to be commissioned in 2014.  XFEL will produce ultra-bright hard X-rays at an unprecedented repetition rate.

In April of this year the Linac Coherent Light Source (LCLS) at SLAC (Stanford) has lased at 0.15 nm, making it the world’s first hard X-ray FEL.  The remarkable speed at which lasing was achieved is a tribute to the meticulous care and dedication of the LCLS design, construction and commissioning teams. We extend to them our hearty congratulations and look forward to important scientific results.  Their success must also bring encouragement to the teams working on FEL projects in Europe and Asia.

One the user science front, the development of the scientific cases for facilities such as FLASH, FERMI@elettra (Italy) and the SSCS (Japan) (to name just two) have elucidated the wealth of scientific opportunity to be harvested at soft X-ray FEL facilities. That richness is compounded as the New X-ray Science approaches the sub-femtosecond frontier – a time scale that characterizes the electron motion in atoms. A recent workshop¹ at UCLA brought together FEL physicists, and experts in lasers, X-ray optics, and synchrotron radiation science to assess how rapidly and along what path an FEL could be realized at the attosecond frontier to unlock the door to breakthrough science. Thanks to advances in accelerator physics over the last few years such a dramatic step could be remarkably close and of modest cost.

One of the earliest results at the LCLS was the continued lasing as the charge in the electron bunch was reduced – a first observation of the potential benefits of low charge operation of an FEL. These measurements were stimulated by simulations of low charge architectures performed at UCLA (and elsewhere), that predicted that as the charge in the electron bunch is reduced, the beam quality increases sufficiently rapidly that lasing continues or even improves, while the electron bunch duration can be reduced to femtoseconds or less.

Combining a low charge photo-injector with the technique of “velocity bunching”, increases the peak beam current at low energy without spoiling the beam quality. This prediction has been confirmed in recent experiments at the SPARC FEL in Frascati. Thus, one can keep the FEL gain high despite the low charge and at the same time naturally produce electron bunches a few femtoseconds or less in duration with very small temporal jitter. Moreover, the X-ray optics and laser technology communities have now had sufficiently experience with sub-femtosecond pulses in the EUV that experiments in the time domain of 0.5 – 5 fs are now practical.  Consequently, from an accelerator technology, laser, and X-ray optics perspective a first generation, attosecond frontier FEL could be realized in the immediate future.

The history of science shows clearly the truth of what Kai Siegbahn’s also strongly believed, that breakthroughs in instruments and experimental methods lead to breakthroughs in science at a fundamental level. This journal was founded and is dedicated to reporting those advances in instrumentation. We look forward to receiving many manuscripts describing the keys that unlock the doors to the New Radiation Science.

William Barletta
Fulvio Parmigiani

Editors for NIM-A

¹ The authors were members of the organizing committee. Talks at this workshop may be found at http://home.physics.ucla.edu/calendar/Workshops/CFC_FEL_2009/Talks/index_talks.html