SPring-8, the large synchrotron radiation facility

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Customized Acceleration of Electron Beams (Press Release)

Release Date
16 Aug, 2013
  • SACLA
- New technology for multibeamline operation of X-ray free-electron laser (XFEL) facility -

RIKEN
Japan Synchrotron Radiation Research Institute (JASRI)

Key points
• Development of a method of providing electron beams having the optimum energy for generating laser pulses at all beamlines
• Realization of bunch-to-bunch energy control of electron beams without degrading the stability of a linear accelerator or the performance of the electron beams
• Achievement of essential technology for the SPring-8 Upgrade Plan aimed at increasing the brightness of synchrotron radiation by 100 times

RIKEN (President, Ryoji Noyori) and JASRI (President, Yoshiharu Doi) have developed a method of accelerating electron beams from a linear accelerator*1 to target beam energies that differ from bunch to bunch (where a bunch is a collection of electrons) without degrading the stability of the accelerator or the performance of electron beams, and demonstrated the effectiveness of this method using the XFEL facility SPring-8 Angstrom Compact Free Electron LAser (SACLA*2). This was achieved by a joint research group consisting of Toru Hara (Team Leader) of the Beam Dynamics Team, Accelerator Research and Development Group, Hitoshi Tanaka (Division Director), and Kenji Tamasaku (Senior Research Scientist) of the Beam Line Development Team, Beam Line Research and Development Group, XFEL Research and Development Division at RIKEN SPring-8 Center (Director, Tetsuya Ishikawa).

Various experiments such as structural analyses and measurements of physical properties are carried out using SACLA. Because each experiment requires laser beams of an appropriate wavelength, the facility provides laser beams of different wavelengths by changing the energy of electron beams. In order to deal with the increasing number of experiments carried out using SACLA, the expansion of beamlines is being planned. However, a problem here is determining the beamline for which the energy of electron beams should be optimized. The experiments carried out at different beamlines are independent of each other and require laser beams of different wavelengths. The wavelength can be adjusted to some extent using undulators*3. However, because the intensity of laser beams significantly decreases when the energy of electron beams deviates from the optimum conditions, there seemed to be many limitations in operating multiple beamlines concurrently while maximizing the performance of each beamline.

In order to solve the above problem, the joint research group has developed a method of generating laser oscillation under optimum conditions at all beamlines. In the method, the energy of electron beams provided to multiple beamlines is controlled bunch to bunch by changing the operating frequency of some of the accelerating units of the linear accelerator. In a demonstration experiment using SACLA, electron bunches of 10 Hz were successfully accelerated to multiple target beam energies to generate laser pulses without degrading the stability or performance of the electron beams.

The method developed in this study will improve the laser performance in multibeamline operation using SACLA. Also, it is an essential technique for supporting both the user operation of SACLA and the top-up operation (the amount of stored current is maintained at the uppermost level for normal operation by the top-up injection of electrons) of the storage ring of SPring-8*5, as planned in the SPring-8 Upgrade Plan*4 currently being discussed.

The achievements were published online in the American scientific journal Physical Review Special Topics-Accelerators and Beams on 15 August 2013 prior to the publication of the printed version.

Publication:
Toru Hara, Kenji Tamasaku, Takao Asaka, Takahiro Inagaki, Yuichi Inubushi, Tetsuo Katayama, Chikara Kondo, Hirokazu Maesaka, Shinichi Matsubara, Takashi Ohshima, Yuji Otake, Tatsuyuki Sakurai, Takahiro Sato, Hitoshi Tanaka, Tadashi Togashi, Kazuaki Togawa, Kensuke Tono, Makina Yabashi, Tetsuya Ishikawa
"Time-interleaved multi-energy acceleration for an XFEL facility"
Physical Review Special Topics-Accelerator and Beams, 16 080701 2013

<<Figures>>

Fig. 1 Design of XFEL multibeamline facility
Fig. 1 Design of XFEL multibeamline facility

(a) Conventional design. The operating repetition rate of the linear accelerator is kept constant. The low-energy beam branches off from the middle of the accelerator.
(b) Design based on the new method developed in this study. The energy of electron beams is controlled bunch to bunch by changing the operating repetition rate of some of the RF units of the linear accelerator.

Fig. 2 Example of customized acceleration of electrons by the new method developed in this study
Fig. 2 Example of customized acceleration of electrons
by the new method developed in this study

Using this method, electron bunches with an optimum energy are provided to the three beamlines. When electron beams of 8 GeV (green) are required, the electron bunches are accelerated by all 52 accelerating units (red, blue, and green). When electron beams of 5.4 GeV (red) are required, the electron bunches are accelerated by the 32 accelerating units (red) upstream of the accelerator. When electron beams of 6.7 GeV (blue) are required, the electron bunches are accelerated by 42 accelerating units (red and blue).

Fig. 3 Measured beam energies of electron bunches, which are 10 Hz and alternately accelerated to two target beam energies
Fig. 3 Measured beam energies of electron bunches,
which are 10 Hz and alternately accelerated to two target beam energies

Each dot indicates the energy of electron bunches at a repetition rate of 10 Hz.

Fig. 4
Fig. 4

Spectra when electron bunches are accelerated to three different beam energies to generate laser pulses with the wavelength corresponding to each beam energy.

<<Glossary>>
*1 Linear accelerator
An accelerator used to accelerate electron beams emitted from an electron gun using radio frequency (RF) electromagnetic fields. Because a linear accelerator uses sine-wave RF electromagnetic fields, electron beams are accelerated not continuously but in a pulsed manner (intermittently) for each collection of electrons, called an electron bunch.

*2 SACLA
Japan’s first XFEL facility constructed jointly by RIKEN and JASRI. As one of the five national critical technologies in the Basic Program for Science and Technology in Japan, the facility was constructed and developed in a five-year project starting from fiscal 2006. It was completed in March 2011 and named SACLA after the initial letters of SPring-8 Angstrom Compact Free Electron LAser. The first successful generationof an X-ray laser was in June 2011. Shared operation started in March 2012. Since then, SACLA has been used in various experiments. Although the facility is smaller than those in other countries, SACLA can produce lasers with the world’s shortest wavelength of 0.1 nm or shorter.

*3 Undulators
Devices in which permanent magnet arrays with opposite poles (N and S) are aligned alternately in upper and lower series. Synchrotron radiation beams are generated from electron beams that meander between the upper and lower series of magnets owing to the periodic magnetic field of the undulator. Also, in XFEL, the synchrotron radiation is amplified to produce X-ray laser beams by the energy exchange between electrons and photons caused by the interaction between the radiation and electron beams inside the undulator. In addition, the wavelength of the laser beams can be adjusted by changing the distance between the upper and lower series of magnets.

*4 SPring-8 Upgrade Plan
A plan to upgrade the SPring-8 storage ring with the aim of increasing the brightness of synchrotron radiation by approximately 100 times. According to this plan, the electron beam size in the storage ring will be reduced to as small as possible. Because high-quality narrow electron beams are required to achieve efficient injection, some of the electron beams accelerated using SACLA will be injected into the storage ring. The beam energy will be set to 6 GeV in the SPring-8 Upgrade Plan. Bunch-to-bunch energy control is therefore essential for supporting both the user operation of SACLA and the injection of electron beams into SPring-8.

*5 SPring-8
A large synchrotron radiation facility that generates the highest-quality synchrotron radiation, located in Hyogo prefecture, Japan. Owned by Riken, and operated by JASRI. The nickname SPring-8 is short for Super Photon ring-8 GeV.
Synchrotron radiation refers to the strong and highly oriented electron magnetic waves generated when the orbit of electrons, accelerated to a near-light speed, is bent by magnetic field. Applications of the synchrotron radiation produced by SPring-8 includes nanotechnology, biotechnology and industrial use.


For more information, please contact:
  D.Sci. Toru Hara (RIKEN)
    E-mail : mail1