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We have developed a soft x-ray laser (SXRL) beamline equipped with an intensity monitor dedicated to the ablation study such as surface processing and damage formation. To get the correct irradiation energy/fluence, an intensity monitor composed of a Mo/Si multilayer beam splitter and an x-ray charge-coupled device camera has been installed in the beamline. Mo/Si multilayer beam splitter has a large polarization dependence in the reflectivity around the incident angle of 45 deg. However, by evaluating the relationship between reflectivity and transmittance of the beam splitter appropriately, the irradiation energy onto the sample surface can be derived from the energy acquired by the intensity monitor. This SXRL beamline is available to not only the ablation phenomena but also the performance evaluation of soft x-ray optics and resists.

Flat beam generation from angular momentum dominated beam with a phase-space rotation technique is an unique method to manipulate the phase-space distribution of beam. As an application, the asymmetric emittance beam generation for linear colliders is considered to compensate the Beamstrahlung effect at Interaction point. By using this technique, the asymmetric beam can be generated directly with the injector, instead of radiation damping with a huge damping ring. We present the result of a proof-of-principle experiment at KEK-STF., Proceedings of the 13th International Particle Accelerator Conference, IPAC2022, Bangkok, Thailand

Semiconductor photocathodes such as Cs-Te and Cs-K-Sb are used as electron sources in accelerators to generate high brightness beams using radio frequency (rf) electron guns. Alkali antimonide photocathodes have a high quantum efficiency (Q.E.) of ~10%, and their excitation wavelength is in the visible light region (532 nm), so that they are expected to reduce the requirements on the optical system and increase the amount of charge compared to Cs-Te. However, alkali antimonide photocathodes have a short lifetime and degrade under poor vacuum conditions, so it is essential to improve durability by protective film coatings. Therefore, we are currently working on the fabrication of high Q.E. alkali antimonide photocathodes that can withstand the Q.E. reduction during coating. In this presentation, we will report the results of comparison between the fabricated alkali antimonide photocathode and Cs-Te photocathode, and future prospects., Proceedings of the 13th International Particle Accelerator Conference, IPAC2022, Bangkok, Thailand

We are trying to elucidate the initial process of radiation chemical reactions, which is considered to be an unknown area. One of the methods to elucidate this initial process is pulse radiolysis. In pulse radiolysis, a substance is irradiated with ionizing radiation and at the same time irradiated with analytical light, and the absorption spectrum of the light can be traced back in time to the active species. However, the radiation chemical reaction starts in a very short time, and the pulse radiolysis system needs to have the same time resolution. Therefore, the analytical light should be ultra-short pulses. In addition, the absorption wavelength of the substance is not always known. Hence, the wavelength range of the analytical light should be broad. Since the absorption wavelengths of important active species are in the visible light region, it is desirable to cover the visible light region as well. We believe that supercontinuum light generated from the second harmonic of the Er fiber laser is the best analytical light to meet these requirements. In this presentation, we describe the current status of the development of the supercontinuum light generation and future prospects., Proceedings of the 13th International Particle Accelerator Conference, IPAC2022, Bangkok, Thailand

We have studied the generation of the broadband THz pulse using a compact linear accelerator. The THz pulse is generated by control of an electron beam angle to Cherenkov radiation angle. In addition, we have succeeded in producing a quasi-monochromatic THz pulse by the spatially modulated electron beam by passing through a slit. This work aims to develop a detection system to elucidate the spectrum of the quasi-monochromatic THz pulse. To detect it stably in a noisy radiation environment, the stability of probe laser system for Electro Optic sampling and timing synchronization system are important. In this conference, the generation method of each THz pulses, the results of development of detection system, and future prospect will be reported., Proceedings of the 13th International Particle Accelerator Conference, IPAC2022, Bangkok, Thailand

We have been conducting basic and applied research for generating high quality electron beams, using 1.6 cell laser photocathode RF-gun. In our laboratory, Cesium Telluride (Cs-Te), one of the semiconductor photocathodes, is used as an electron source for accelerator experiments. This semiconductor photocathode is known for high quantum efficiency (Q.E.) about 5~10% and 3-month 1/e lifetime. High Q.E. photocathodes can reduce the power requirement of the laser system, and long lifetime photocathodes can decrease the maintenance frequency, contributing to an efficient experimental environment. For these reasons, high Q.E. and long lifetime photocathodes are necessary in accelerator experiments. In order to produce robust photocathodes and extend the lifetime, we have conducted covering Cs-Te photocathodes with CsBr and CsI protective films. In this conference, we report the thickness dependency on the lifetime of Cs-Te photocathodes when we intentionally exposed oxygen gas to coated and non-coated Cs-Te photocathodes., Proceedings of the 12th International Particle Accelerator Conference, IPAC2021, Campinas, SP, Brazil

We have been studying terahertz (THz) generation via Cherenkov radiation with high-quality electron beams from a photocathode rf (radio frequency) gun. In our early studies, we have succeeded in the generation of coherent Cherenkov radiation by controlling the tilt of the electron beam using an rf-deflector. For further enhancement, we are planning to stack the THz pulses in an optical cavity. Multi-bunch operation of the rf-gun will generate electron beams with a repetition rate of 119 MHz, and THz pulses as well. These pulses will be accumulated in the cavity for up to 150 pulses. In this conference, we report the design study of the enhancement cavity and discuss the performance of the THz source., Proceedings of the 12th International Particle Accelerator Conference, IPAC2021, Campinas, SP, Brazil

THz waves, located between microwaves and light waves, have transparency, directionality and fingerprint spectrum of specific materials. Therefore, they are expected to be useful for various applications. We have been studying THz waves generation via Cherenkov radiation with electron beams from a photocathode rf-gun. In our early studies, we have succeeded in the generation of coherent Cherenkov radiation by tilted electron beams using an rf-deflector. Furthermore, we have generated quasi-monochromatic THz waves by spatially modulated electron beams and have succeeded in its measurement by bandpass filters. This study aims to obtain the THz wave form in time domain by electro-optic (EO) sampling, which is an useful detection system for obtaining the information of the electric field and the phase simultaneously with high S/N. In this conference, we report about our probe laser system, results of the time-domain spectroscopy measurement of THz waves by EO sampling, and future prospects., Proceedings of the 12th International Particle Accelerator Conference, IPAC2021, Campinas, SP, Brazil

Pulse radiolysis is one of the absorption measurement methods for investigating the fundamental, ultrafast process of radiation chemical reactions. Analytical light is transmitted simultaneously with the timing of electron beam irradiation, and its absorption by reactive species is detected. Since the target reactions arise in pico second time scale or even shorter, analytical light is required to have such duration. Besides, so as not to be buried in noise of the radiation source, the optical power of the analytical light must be high enough. Furthermore, it is desirable that the analytical light covers visible region because important absorptions caused by irradiation products such as hydrated electron, hydroxyl radical, or so exist in the region. We considered that the supercontinuum light generated from an ultrashort pulse laser is suitable as an analytical light because it has all these characteristics. In this study, we generate the second harmonic (775 nm) of an erbium fiber laser (1550 nm) as a seed laser for supercontinuum generation. In this presentation, we report the current situation of our laser system and prospects., Proceedings of the 12th International Particle Accelerator Conference, IPAC2021, Campinas, SP, Brazil