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Thousand-Times-Compressed Electron Beam: Proof of Successful Generation of High-Quality Laser - Innovative Idea for Promoting Downsizing of X-Ray Free Electron Laser (XFEL) System - (Press Release)

Release Date
29 Aug, 2009
  • XFEL
Scientists of the "SPring-8 Joint Project for XFEL", a joint project of RIKEN and Japan Synchrotron Radiation Research Institute, have developed a method of efficiently compressing a high-quality electron beam to achieve a few-thousand-times higher density and have clarified its effectiveness by computational simulation. This method will greatly contribute to improving the optical performance and stability of the X-ray free electron laser (XFEL).

RIKEN
Japan Synchrotron Radiation Research Institute

Key research achievements
• Our original idea overcomes the difficulty in downsizing XFEL systems
• Development of method for stably producing ideal electron beam
• Application to overseas XFEL systems is possible

Scientists of the "SPring-8 Joint Project for XFEL" (Nobuo Fujishima, Director), a joint project of RIKEN (Ryoji Noyori, President) and Japan Synchrotron Radiation Research Institute (JASRI; Tetsuhisa Shirakawa, President), have developed a method of efficiently compressing a high-quality electron beam to achieve a few-thousand-times higher density and have clarified its effectiveness by computational simulation.*1 This method will greatly contribute to improving the optical performance and stability of the X-ray free electron laser (XFEL).*2

XFEL is a new light source with excellent characteristics of angstrom-level*3 spatial resolution and femtosecond-level*4 time resolution that will be used for irradiating substances. Scientists worldwide as well as industry have high expectations that the XFEL will trigger innovations in life science and nanotechnology, for example, the development of effective drugs for intractable diseases including cancer and AIDS and breakthroughs in research on new energy systems necessary for the sustainable development of our society.

XFEL generators are being developed in Japan, the United States, and Europe. Those proposed by the United States and Europe utilize conventional technologies and require gigantic equipment with a total length of a few km. In contrast, Japan, led by RIKEN and JASRI, has focused on the development of a compact XFEL generator using a novel concept completely different from those used in the XFEL generators developed by the United States and Europe. In 2005, a small SCSS test accelerator*5 was constructed as a prototype to experimentally demonstrate the principle of the XFEL. In 2006, the lasing of an extreme ultraviolet*6 laser was confirmed for the first time (press release on 22 June 2006 in Japanese). After that, scientists improved the performance of the SCSS accelerator and succeeded in producing a stable extreme ultraviolet laser (wavelength of 50-61 nm*3) with a high power of over 100 MW. As a result of these achievements, scientists now consider it very important to further increase the beam current from 300 A, which has been achieved using the SCSS test accelerator, by one order of magnitude to improve the light characteristics of the actual XFEL system. The method of realizing a beam current higher than a few kA has been experimentally demonstrated in the United States and Europe, but requires a frequency threefold to fourfold higher than the main acceleration frequency.*7 The Japanese compact XFEL generator, however, cannot easily adopt this method because the main acceleration frequency is almost at its high-frequency limit for the current technology. In this joint project, the scientists developed an innovative method that does not require a frequency higher than the main acceleration frequency, and they verified the effect of the method by computational simulation.

These achievements will lead to a dramatic improvement in the light performance and stability of the world's smallest XFEL facility (Fig. 1), now under construction on the campus of SPring-8 (Harima Science Garden City) as the joint project by RIKEN and JASRI whose target completion date is in FY 2010, and will pave the way for the further downsizing of the XFEL system in the future.

The results of this research were published in the American scientific journal, Physical Review Special Topics – Accelerators and Beams, on 26 August 2009 (27 August in Japan).

Publication:
"Electron-bunch compression using a dynamical nonlinearity correction for a compact x-ray free-electron laser"
Kazuaki Togawa, Toru Hara, and Hitoshi Tanaka
Physical Review Special Topics - Accelerators and Beams 12 (8), 080706 (2009), published online 26 August 2009.

<Figure>

Fig. 1 XFEL facility under construction on the campus of SPring-8 Fig. 1 XFEL facility under construction on the campus of SPring-8
A compact, slendar XFEL facility is being constructed, as shown in the top left of the photograph. Electrons are accelerated from left to right. Five beamlines will be installed in the experimental facility (under construction further downstream). The circular facility to the right of the XFEL facility is the SPring-8 storage ring and the surrounding experimental hall. Currently, 50 beamlines are operational in these facilities, which have circumference of about 1.5 km.

Fig. 2 Tunnel interior of SCSS test accelerator Fig. 2 Tunnel interior of SCSS test accelerator
A high-voltage tank of the electron gun is shown in the foreground. An electron beam is emitted from the hot cathode behind the tank towards two undulators downstream.

Fig. 3 Schematic of bunch compression by electromagnet chicane Fig. 3 Schematic of bunch compression by electromagnet chicane
Red, black, and light-blue circles represent the top, center, and tail of an electron bunch, respectively. The three circles indicate how an electron bunch with gradually increasing energy from the top to the tail is compressed while passing through the chicane.

Fig. 4 Principle of linearization of energy chirp (conventional method) Fig. 4 Principle of linearization of energy chirp (conventional method)
The figure shows the accelerating electric field (red dotted line), the decelerating electric field with a frequency threefold higher than that of the accelerating electric field (blue dotted line), and the electric field obtained by superposing them (black dotted line). The superposed electric field becomes partly linear.

Fig. 5 Curve of energy chirp enhanced by compression Fig. 5 Curve of energy chirp enhanced by compression
Compressing the electron beam sharpens the curve of the energy chirp. The threefold-compressed electron beam acts similarly to the third harmonic electric field without changing frequency.

Fig. 6 Two-stage simple bunch compression system used for simulation Fig. 6 Two-stage simple bunch compression system used for simulation

Fig. 7 Enhanced correction effect of first-stage bunch compressor Fig. 7 Enhanced correction effect of first-stage bunch compressor
When the compression coefficient is 6 or smaller, the voltage used for correcting the curve decreases quadratically with increasing compression coefficient. When it is greater than 6, the effect of the compressor gradually decreases and the correction voltage approaches a constant value.

Fig. 8 Distributions of electron beam current and energy at inlet of undulator Fig. 8 Distributions of electron beam current and energy at inlet of undulator
The beam current is confirmed to exceed the 3 kA necessary for laser oscillation.

<Glossary>

*1 Computational simulation
The program used for integrating the motion of the electron beam in three-dimensional electromagnetic fields, where the three-dimensional space-charge effect, coherent synchrotron radiation, and spontaneous emission are taken into consideration.

*2 X-ray free electron laser (XFEL)
World's state-of-the-art research facility that will produce X-rays at least one billion times brighter than current synchrotron radiation and will enable the analysis of atomic-level ultrafine structures and the instantaneous measurement of ultrahigh-speed dynamics and structural changes in chemical reactions. The XFEL was designated a key technology of national importance in the 3rd Science and Technology Basic Plan of Japan, and has been under construction next to SPring-8 in Harima Science Garden City, since FY 2007. The public use of the XFEL facility is planned to start in FY 2011. The XFEL has the aim of opening up new research areas in various fields of science and technology, such as life science, nanotechnology, and materials science, and of achieving breakthroughs ahead of Europe and the US.

*3 Angstrom and nanometer
One angstrom is a ten-billionth of a meter (10-10 m). A nanometer is a billionth of a meter (10-9 m).

*4 Femtosecond
A femtosecond is a quadrillionth of a second (10-15 s). A femtosecond has such a short duration that light (velocity: approximately 300,000 km/s) travels only 0.3 μm in one femtosecond.

*5 SCSS test accelerator and self-amplified spontaneous emission
SCSS is an abbreviation for the SPring-8 Compact SASE Source, where SASE stands for self-amplified spontaneous emission. The SCSS test accelerator is an extreme ultraviolet free electron laser generator operating on the principle of SASE.

*6 Extreme ultraviolet
Electromagnetic waves with a wavelength from 30 to 100 nm.

*7 Main acceleration frequency
The frequency of radio-frequency electromagnetic waves used in the main acceleration systems of the accelerator.

For more information, please contact:

Dr. Kazuaki Togawa (XFEL Project Head Office, RIKEN)
e-mail: mail.