ERL is a unique machine and bring you to bright future
In order to accelerate the electron beam, it is necessary to store a radiofrequency wave in the accelerating cavity. Basically the conventional accelerating cavity is made of copper which has a high thermal conductivity. However, the cavity will get heated if a high acceleration gradient is required. Therefore, only low-repetition beam operation has been available. While, a superconducting accelerating cavity has been developed to enable continuous-wave (CW) beam operation without the heated problem.
An accelerating cavity made of superconducting niobium (Nb) is cooled with liquid helium at -271 degrees Celsius (2 Kelvin). The compact ERL is operated with the superconducting accelerating cavity with CW mode, and can accelerate the electron beam with a high-average beam current.
We are developing and operating the superconducting accelerator in cooperation with many domestic and international projects such as the KEKB accelerator and the International Linear Collider project.
A machine that accelerates a beam to a high energy in a single pass is called a linear accelerator. Normal linear accelerators are not designed for high average-current operation because the beam is disposed or injected into a storage ring after the acceleration. Therefore, conventional accelerating cavities made of normal-conducting metal (copper) cannot be useed for the high average-current opearation. On an Energy Recovery Linac (ERL), the beam that has been accelerated and used for an experiment is returned to the linear accelerator to recover its energy. The recovered energy is used to accelerate a new beam, thus enabling operation with the high-average current. In particular, the superconducting acceleration cavity significantly reduces the cavity surface loss, enabling 100% energy recovery and high-repetition rate operation with the high-average current. The recovering and reusing the energy enables a significant reduction in power consumption, making it a next-generation accelerator that is also environmentally friendly.
The combination of the ERL and a free-electron laser (FEL) makes an unprecedentedly powerful light source. FEL is a device that draws energy from electrons and converts it into light of a specific wavelength, with conversion efficiencies as high as several percent. A generation of the FEL requires electron beams with small emittance, small energy spread and short beam length. After a lasing of FEL, electrons are greatly disturbed and the energy spread and emittance of electron beams are increased. Such electron beams are difficult to transport, but in ERL, the energy of electron beams is recovered in the acceleration cavity and used to accelerate a new beam. This principle makes it possible to continue the lasing of FEL with the high-average current and high-repetition rate.
This requires a superconducting accelerating cavity that can be operated continuously and a high-current, high-brightness electron gun. At the compact ERL, in addition to demonstrating energy recovery, we are developing new acceleration cavities and electron guns. These high-quality electron beams are also used for irradiation experiments. In addition, beam tuning is important to transport the beam with less beam loss, since the beam operates at a high-average current.
Research has been conducted to extract more intense light from electron beams. Electron beams emit synchrotron radiation when it is bended, and undulators have been developed to obtain more monochromatic and intense light by wiggling the electron beams. A free-electron laser (FEL) is a more intense and coherent light source developed by interacting the light (electromagnetic waves) generated in the undulator with electrons. Most FELs employ an optical resonator type in which light is confined by mirrors. But since there are no highly reflective mirrors for short wavelengths such as extreme ultraviolet (EUV) light and X-rays, the light is emitted using a method called self-amplified spontaneous emission (SASE). Typical SASE FELs include the European X-ray Free Electron Laser Facility in Germany and SACLA in Japan which is located on the same site as SPring-8.
The compact ERL (cERL) has been in operation since 2013 and is currently developing the hardware and beam commissioning using the superconducting acceleration cavity for industrial applications. The high-blightness and high-current electron beam generated by DC electron gun with 500 keV is accelerated to 10-20 MeV by a superconducting acceleration cavity. The accelerated beam returns to the superconducting acceleration cavity again through a circumferential transport path of about 100 meters, and the energy of electron beam is recovered at the cavity. A key feature of this system is that the recovered energy can be reused to accelerate a new electron beam. The energy recovery was demonstrated for the first time in 2014, and in 2016, almost 100% energy recovery was successfully achieved at a high-current CW operation of 1 mA.
In 2020, two undulators were installed in the circumferential transport path under the NEDO project "Development of next-generation laser technology with high brightness and efficiency" to demonstrate the SASE FEL. A machine learning was introduced for beam tuning, and with the cooperation of the National Institute of Advanced Industrial Science and Technology (AIST), infrared light with the designed wavelength was observed.
Meanwhile, in 2019, a beamline and irradiation system for irradiation experiments were built. The irradiation system is tightly shielded and is capable of shooting electron beams of up to 26 MeV with a current of 10 μA. So far, 99Mo production for nuclear medicine test drugs, basic research on asphalt longevity, and electron irradiation of wood for nanocellulose production have been conducted.
| Parameter / Operation mode | Energy-recovery mode | Electron irradiation mode |
|---|---|---|
| Beam injection energy | 3～5 MeV | 3～5 MeV |
| Beam operation energy | ～20 MeV (CW mode) | 3.5～19 MeV (CW mode) |
| ～23 MeV (pulse mode) | ||
| Normalized emittance | 0.15 mm mrad (low charge) | |
| ～5 mm mrad (high charge, compressed) | ||
| Bunch length | 2~3 ps (normal operation) | 2~3 ps (normal operation) |
| Bunch length | ＜ 1 ps (compressed) | |
| Average beam current | ~1 mA | ~10 μA |
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