[en] This text contains the following subjects. (1) Introduction, (1-1) Introduction, (1-2) Beam position monitors (BPM), (1-3) Beam profile monitor, (2) Cavity-type (BPM), (2-1) Introduction, (2-2) Principle of the cavity-type BPM, (2-3) Design and fabrication of the cavity, (2-4) Fundamental performance testing, (2-5) Signal detection, (3) Beam-size measurement using lasers, (3-1) Limit of wire scanner, (3-2) Propagation of laser light, (3-3) Compton scattering, (3-4) Single-pass type laser-wire monitor, (3-5) Cavity-storage type laser-wire, and (3-6) Laser interference-pattern monitor. (K.Y.)

[en] Excitation of a cavity mode by a relativistic charged particle beam can be calculated by integrating the field of the mode along the beam trajectory. The method can be applied for calculating radiation in a free space by considering the limit of moving the cavity boundary to infinite distance. We give two examples, transition radiation from a metal target and vortex radiation from a helical undulator. (author)

[en] To obtain high-brightness quasi-monochromatic X-rays via inverse Compton scattering (ICS), an optical cavity for intensifying laser beams was designed and implemented in a new beam line at the KEK Superconducting RF Test Facility (STF) accelerator. The optical cavity adopts a planar configuration consisting of 4 mirrors. This confocal type resonator provides stable laser storage even with a long mirror distance, enabling head-on collision with the electron beams. To overcome the well-known astigmatism problems of the planar-type optical cavity, two forcibly bendable cylindrical mirrors were introduced instead of flat mirrors. With this new function for laser profile adjustment, an almost round laser profile at the waist point in the accelerator environment was successfully achieved. Estimated waist sizes were 43.7 μm for the horizontal and 50.8 μm for the vertical dimensions. The feedback control of this 4-mirror optical cavity worked with a stiff plate supporting all 4 mirrors. 1.7×10³ finesse and 2.8-kW stored power for a 1-ms duration with 5 Hz were achieved

[en] To demonstrate the operation of a mid-infrared SASE FEL system in an energy recovery linac (ERL) layout, we constructed an SASE FEL setup in cERL, a test facility of the superconducting linac at KEK. Despite the adverse circumstance due to the given boundary condition of the facility and the limited budget and time schedule of the project, we successfully established the beam condition at the undulators, and observed FEL emission at a wavelength of 20 µm. We present the background stories that we have experienced in the project. (author)

[en] Various types of accelerators are proposed as a future light source. Especially those design which can produce high peak power at high repetition rate utilizing energy recovery scheme are interesting also for searching a new particle which couples with photons. We estimated sensitivity of axion search with future XFEL-O, EUV-FEL, and LCSS-γ machines. It shows experiment at these machines can extend the parameter space. We also checked the feasibility of FEL output power assumed in this discussion. (author)

[en] Energetic photon beam produced via Laser-Compton scattering is expected to have variety of applications. An optical enhancement cavity with a small spot size at the collision point is a key to realize a high average flux. It is known that astigmatism arising from finite incident angle on the concave mirror limits the smallest spot size in the case of conventional 4-mirror ring cavity. We discuss a design of an optical cavity with astigmatism compensation utilizing additional convex mirrors. It improves the ellipticity of beam profile at the focus point and can realize a smaller spot size. (author)

[en] Accelerator-based THz radiation has been expected to realize a high power broad-band source. With a low emittance and short-bunch electron beam at a high repetition rate, a scheme of coherent-diffraction-radiation in an optical cavity layout is proposed. It can greatly enhance the efficiency of the radiation by the stimulated radiation process between bunches. We give an introduction of the principle of this process. We performed an experiment at cERL in KEK. A sharp resonance signal of THz which indicates broad-band stimulated radiation correlating with beam deceleration was observed while scanning the round-trip length of the cavity. It proves the efficient beam-to-radiation energy conversion due to the stimulated radiation process. (author)

[en] In order to realize a high repetition rate seeded coherent radiation source, it is necessary to develop a seeding system which works in a continuous mode. Utilizing the longitudinal electric field in a higher transverse mode laser stored in an optical cavity, it is possible to introduce an energy modulation in an electron bunch. Through acceleration and dispersion handling, the modulation at laser wavelength can be converted into a finer density structure. It can be used as the seed of coherent radiation. We are developing a laser system to be used in the laser modulator system. (author)

[en] Recently, the compact energy recovery linac has been generating infrared free-electron laser (infrared-FEL) using the process of self-amplified spontaneous emission. This process starts when an electron bunch is injected into undulators. Passing through the undulators, the infrared-FEL is generated by the stimulated emission from the electrons whose longitudinal density is periodically modulated inside the bunch. Therefore, performance of the infrared-FEL strongly depends on the longitudinal distribution of the electrons at the undulator exit. To improve the performance of the infrared-FEL, we plant to measure the longitudinal distribution by deflecting cavities. The deflecting cavity is required to be position resolution of 10 μm in order to resolve the structure inside the electron bunch. To achieve the requirement, we develop a c-band cavity. In this paper, we report the design of the prototype cavity and the cavity production. (author)

[en] Commissioning operation of Compact-ERL injector has started in KEK. It consists of a photo-cathode electron gun, a super conducting accelerator, and a beam diagnostic line. The beam diagnostic line was built for proving the performance of the injector. It has a bunch length monitor with an RF deflection cavity system, an energy spread measurement system at a dispersive section. We did a series of longitudinal beam characteristics measurements at bunch charge up to 7.7 pC/bunch. (author)