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We are developing an accelerator-based terahertz source that can produce arbitrary polarization states from linearly polarized coherent undulator radiation (CUR). The polarization manipulation of the CUR can be realized using the Martin’Puplett interferometer employed as an optical phase shifter. This study also demonstrates a variable polarization manipulator by using the terahertz CUR (THz-CUR) source based on an extremely short electron bunch at Research Center for Electron Photon Science (ELPH), Tohoku University. The horizontally polarized CUR with a frequency of 1.9 THz was manipulated into variable polarization state, and Stokes parameters were measured to derive the degree of polarization. Beam experimental results will be presented in this conference., Proceedings of the 31st International Linear Accelerator Conference, LINAC2022, Liverpool, UK

We are currently conducting basic research on the development of a non-destructive real-time bunch length monitor using coherent Smith-Purcell radiation at the t-ACTS test accelerator at the Center for Electron Photon Science, Tohoku University. The angular distribution of coherent Smith-Purcell radiation reflects the longitudinal shape of the electron bunch. Using this, we came up with a method to measure the bunch length from the peak angle of the angular distribution. In this presentation, we mainly report the results of an experiment to determine the bunch length from the peak angle of the angular distribution of coherent Smith-Purcell radiation using a 100 fs electron beam of t-ACTS., Proceedings of the 13th International Particle Accelerator Conference, IPAC2022, Bangkok, Thailand

Cherenkov diffraction radiation (ChDR) has been paid attention to non-beam-destructive diagnostics in these years. However, the physical understanding of ChDR is not well satisfied yet because of precise experimental observation is not much easier than one expects. Although we do not deny the Cherenkov radiation and ChDR are fully explained by the classical electromagnetics, we encounter a couple of difficulties in actual applications. For instance, the theory is usually established for the far-field observation, in spite of that the radiation is often observed near-field in the realistic beam diagnostic tools employing photon measurements. In addition, the theory, as a matter of course, includes some assumptions which is sometimes not valid for the specific experiments. We have carried out experiments for observation of coherent ChDR in THz frequency region by a using 100 femtosecond electron beam supplied by the t-ACTS accelerator at Tohoku University. In a flame work of this study an FDTD simulation in the large space has been developed as well. In this presentation, we will show the experimental results comparing with both the theory and the simulation., Proceedings of the 13th International Particle Accelerator Conference, IPAC2022, Bangkok, Thailand

A novel method to fabricate undulator magnets of a-few-millimetre-period length is being explored. Plate-type magnets, 100 mm-long with 4 mm-period length, have been successfully fabricated. They produce an undulator field of approximately 3 kG at a gap of 1.6 mm. Prototype undulators based on this technology have been constructed. Field measurements and characterization show that the quality of the undulator field of these plate magnets is sufficient for an undulator light source, and the calculated spectrum shows that the fundamental radiation emitted from this field is quite satisfactory. Test experiments for light generation using a real electron beam have been carried out at a test accelerator at the Research Center for Electron Photon Science (ELPH), Tohoku University, Japan, which is able to realize optics conditions to accept a very short gap of 1.6 mm. First observation and characterization of blue light was successfully accomplished.

The coherent Smith-Purcell radiation (SPR) emitted as a short electron bunch passes over a periodic metal surface is expected to be applied as a non-destructive beam diagnostic tool. The longitudinal profile of the electron bunch can be deduced by the measured spectrum of the coherent SPR, which is compared with the theoretical one for single electron. There are several theoretical models that explain the SPR mechanism, such as the surface current (SC) model and the van den Berg model. But the difference of estimation in radiation intensity between different models is not trivial, and also the experimental data to evaluate those validity is not enough. At test accelerator, t-ACTS, in Tohoku University we are conducting experimental research on coherent SPR in the terahertz frequency region using an ultra-short electron bunch of about 100 fs. The status and results of the experiment will be presented., Proceedings of the 12th International Particle Accelerator Conference, IPAC2021, Campinas, SP, Brazil

We have developed crossed-undulator configuration to control the polarization of coherent THz radiation at the femto-second electron beam facility, t-ACTS [1], that has been established at Research Center for Electron Photon Science, Tohoku University. The t-ACTS linac equips a thermionic RF gun, a 3 m accelerating structure and a 50 MW klystron modulator. Ultra-short electron bunch (~80 fs) train can be supplied via velocity bunching scheme. The crossed-undulator system is consisted with two identical transverse undulators intersected by a chicane type phase shifter. Deflecting planes of two undulators are at right angles each other, and the phase shifter makes path length difference between the electrons and the radiation. Target radiation frequency is around 2 THz employing a beam energy of 22 MeV. Since electron bunch trails behind the radiation by the slippage-effect and the nonrelativistic-effect that the electron speed is a bit slower than the speed of light, the radiation from 1st undulator has to be much delayed rather than the electrons. The paper will report the characteristics of polarized radiation and designing work of the phase shifter., Proceedings of the 39th Free Electron Laser Conference, FEL2019, Hamburg, Germany

A variable polarized THz light source using a crossed-undulator configuration has been developed at Research Center for Electron Photon Science (ELPH), Tohoku University. It consists of two planar undulators of which deflecting planes cross at right angles and a phase shifter for phase adjustment. Polarization of the crossed-undulator has observation angle dependence due to that of radiation wavelength and optical path length difference between two radiations. That limits an angular range maintaining the identical polarization state. Assuming undulator parameters for our experiment (a fundamental frequency 1.9 THz and a number of periods seven) degree of circular polarization larger than 0.9 can be obtained only in the range of 2.2 mrad, i. e. 13% of the radiation angular spread., Proceedings of the 10th Int. Particle Accelerator Conf., IPAC2019, Melbourne, Australia

A test-Accelerator as Coherent Terahertz Source (t-ACTS) has been under development at the Research Center for Electron Photon Science (ELPH), Tohoku University, in which an intense coherent terahertz radiation is generated from the femtosecond electron pulses. Velocity bunching scheme in a traveling accelerating structure is employed to generate femtosecond electron pulses, and the generation of femtosecond electron pulses was confirmed by spectrum analysis of coherent transition radiation using Michelson interferometer. Coherent transition radiation and coherent undulator radiation in the terahertz (THz) region from the short electron pulses has been demonstrated, and their characteristics such as frequency spectrum, spatial distribution and polarization were measured and compared with theoretical calculations. We have succeeded to generate the coherent transition radiation up to approximately 5 THz and the coherent undulator radiation with narrow bandwidth from 2.6 to 3.4 THz. At present, development of a variable polarized THz light source using a crossed-undulator system is being carried out. In addition, we are developing a nondestructive beam monitor using Cherenkov radiation emitted from the electron pulses. The status of t-ACTS will be presented in this conference., Proceedings of the 10th Int. Particle Accelerator Conf., IPAC2019, Melbourne, Australia

Cherenkov Diffraction Radiation (ChDR) can be considered as a tool of non-destructive beam diagnostics. It also has the feature that the photon flux of ChDR is proportional to the thickness of the dielectric used as the radiator and can be much larger than ordinary diffraction radiation. An experimental set-up for the measurement of coherent ChDR from short electron bunches of about 100 fs is being prepared at t-ACTS, Tohoku University. Results of a basic experimental study about coherent ChDR will be presented., Proceedings of the 10th Int. Particle Accelerator Conf., IPAC2019, Melbourne, Australia

Cherenkov radiation (CR) emitted from low-refractive index material such as silica-aerogel is a useful tool for electron beam diagnostics because the opening angle (Cherenkov angle) is small and the CR can be transported onto a detector located far from the radiator. We have prepared a thin (1-mm thick) hydrophobic silica-aerogel with a refractive index of 1.05 that has been developed at Chiba University.* Since the intensity of CR is much stronger than that of optical transition radiation, the CR is a better light source for low-intensity beam diagnostics. In order to apply the CR to measurements of a bunch length of electron beams, we have investigated sources of finite time resolution by a numerical simulation study using the Liénard-Wiechert potentials. We will report results of simulations such as pulse duration of CR and discuss what deteriorates the time resolution., Proceedings of the 38th Int. Free Electron Laser Conf., FEL2017, Santa Fe, NM, USA

A variable polarized THz radiation source using a crossed undulator system has been developed at Tohoku University. In this scheme, two coherent undulator radiations from an extremely short electron bunch are used to control the polarization. They are linearly polarized radiations orthogonal to each other. Polarization of superimposed radiation is controlled by adjusting a relative phase between them. A compact planar undulator with seven periods has been designed for an experiment at our facility. The radiation frequency is 2.06 THz for electron beam energy of 22 MeV. The opening angle of the crossed undulator radiation was estimated to be 34 mrad (FWHM). Since the polarization state of the crossed undulator depends on observation angle, its angular dependence was evaluated. It was found that ideal polarization control is realized only in the angle range of 2.5 mrad, which is quite smaller than that of the radiation itself., Proceedings of the 29\textsuperscript{th} Linear Accelerator Conf., LINAC2018, Beijing, China

A test accelerator as a coherent terahertz source (t-ACTS) has been under development at Tohoku University, in which an intense coherent terahertz (THz) radiation generated by an extremely short electron bunch. Velocity bunching scheme in a traveling accelerating structure is employed to generate femtosecond electron bunches. Spatial and temporal coherent radiation in THz region can be produced by the electron bunches with small transverse emittance. A long-period undulator, which has 25 periods with a period length of 10 cm and a peak magnetic field of 0.41 T, has been also developed and installed to provide intense coherent THz undulator radiation. By optimizing the bunch length, we found that it is possible to generate a coherent undulator radiation that contain only the fundamental wave from numerical studies. We are planning an experiment with 30 MeV beam to generate a coherent undulator radiation of 2.5THz. In the conference, we will report the preliminary experimental results., Proceedings of the 28th Linear Accelerator Conf., LINAC2016, East Lansing, MI, USA

The reconstruction work of accelerator complex for the Great East Japan Earthquake in March 2011 had been conducted in Research Center of Electron Photon Science, Tohoku University. Since restoration of the user machine time in 2013, the approved beam time have been regularly implemented as scheduled. Currently, the 1.3 GeV Booster STorage (BST) ring has been well utilized to generate the high energy gamma-rays as before the disaster. The high energy gamma-rays were produced via Bremsstrahlung by inserting an internal target wire to the beam orbit after the beam acceleration. Since the user machine time was recovered, there were some improvements so far, i.e. realignment of synchrotron magnets, orbit correction in energy ramping process by updating the control of power supplies for steering magnets etc., which brought an increase of the beam current in the maximum energy. Present operational status and recent progress of beam performance in the BST ring are reported., Proceedings of the 7th Int. Particle Accelerator Conf., IPAC2016, Busan, Korea

Linear focal Cherenkov ring camera (LFC-Camera) has been developed for single shot measurement of longitudinal phase space distribution of quasi-relativistic electron beam, where the electron's velocity still depends on its energy. The LFC-camera employs velocity dependence of opening angle of Cherenkov light produced by electron beam to observe its energy (momentum) distribution. Since the Cherenkov light contains the time information if the radiator medium is thin enough, we can get the longitudinal phase space distribution measuring both time and energy spectra simultaneously using a streak camera. We employ a thin silica aerogel with water-free hydrophobic treatment as Cherenkov radiator. We have evaluated characteristics of the silica aerogel radiator, and demonstration of the LFC-camera at a beam diagnosis section of t-ATCS is being proceeded., Proceedings of the 7th Int. Particle Accelerator Conf., IPAC2016, Busan, Korea

An accelerator test facility, t-ACTS, was established at Research Center for Electron Photon Science, Tohoku University, in which an intense coherent terahertz (THz) radiation is generated from an extremely short electron bunch. Velocity bunching scheme in a traveling-wave accelerating structure is employed to produce the short electron bunch, and a production of sub-picosecond electron bunch was demonstrated. A long-period linear undulator, which has 25 periods with a period length of 10 cm and a peak magnetic field of 0.41 T, has been developed to produce intense coherent THz radiation. Properties of the radiation from the THz undulator such as radiation fields, spectrum and angular distribution were numerically investigated based on the parameters of short electron bunch and THz undulator. By optimization of bunch compression, it is possible to extract a coherent radiation of fundamental mode excluding higher-order mode. The detail of the numerical studies for the coherent undulator radiation will be reported in the conference., Proceedings of the 7th Int. Particle Accelerator Conf., IPAC2016, Busan, Korea

A test-Accelerator as Coherent Terahertz Source (t-ACTS) project has been under development at Research Center for Electron Photon Science, Tohoku University. In order to generate a coherent radiation in terahertz (THz) region, it is necessary to produce sub-picosecond electron pulses. Velocity bunching scheme is employed for the short electron pulse production in t-ACTS. We experimentally confirmed the production of short electron pulse under 500 fs by measuring the bunch length using a streak camera. Coherent transition radiation in THz region was produced by which the short electron pulses pass through a vacuum-metal interface. Several radiation properties including spatial distribution, polarization and spectrum were measured and compared with theoretical calculations. The details of the beam experiment at t-ACTS are described., Proceedings of the 7th Int. Particle Accelerator Conf., IPAC2016, Busan, Korea

A thermionic RF gun has been developed to generate very short electron bunch for a THz light source at Tohoku University. Bunch compression scheme requires, in general, linear momentum distribution of the particles with respect to the longitudinal position, so that measurement of longitudinal phase space is significant for better bunch compression. However, such measurement for the low energy electrons is difficult because space charge effect is so strong that longer drift space should not be included. Consequently, we have performed deliberation for employing energy dependent angular distribution of Cherenkov radiation. Though the energy dependence of emission angle of Cherenkov radiation is rapidly getting small as increasing the beam energy, it is still 25 deg/MeV at an energy around 2.0 MeV when we use radiator having refractive index of 1.035. Thus the beam energy distribution can be measured if we observe Cherenkov ring with sufficient angular resolution. Since this method needs only thin radiator, the drift space length will be minimized. We will discuss limitation for resolutions of both the time and the energy as well., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan

A test accelerator for the coherent terahertz source (t-ACTS) has been under development at Laboratory of Nuclear Science, Tohoku University*. Intense coherent terahertz radiation will be generated by the very short electron bunch less than 100 fs using a thermionic RF gun (ITC RF-gun). ITC RF-gun is designed to have two cells uncoupled with each other, so that it can be operated at various combinations of different rf-power level and phase difference so as to optimize the longitudinal phase space distribution of the electron beam for bunch compression**. The gun employs single-crystal LaB6 cathode with small diameter of 1.8 mm to obtain a very small initial emittance with sufficiently high current density. The RF gun has been already manufactured and the measurement of RF characteristics is now in progress. We will present the results of low-power measurement and also discuss the effect of the cathode misalignment on the beam parameters such as transverse emittance and longitudinal phase space distribution., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan

Design of an intense but tightly focused ultrashort electron beam for production of sub-hundred femtosecond x-ray pulses that based on head-on inverse Compton scattering (ICS) has been studied. The three dimensional (3D) space charge dynamics has been tracked and optimized throughout the whole beamline. It is found that the focusing ultrashort electron pulses as short as 67 fs can be produced by compressing the energy-chirped beam from a thermionic cathode rf gun with an alpha magnet and linac operating at injection phase near zero crossing. This multi-bunch electron beam has an intensity of 30 pC per bunch and is accelerated to 27 MeV with an S-band linac structure. The compressed electron beam is focused to 64 μm for scattering with an 800 nm, 3.75 mJ laser in the laser-beam interaction chamber. With this method, total peak flux of back-scattered x-ray photons exceeds 10¹⁸ photons/sec is achievable with the shortest wavelength of 0.7 Å., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan

In compact linac system, alpha magnet seems to be a useful device to manipulate the longitudinal phase space. Particularly combined use with thermionic RF gun has been regarded as a convenient system for bunch compression. The alpha magnet simply acts to rotate the longitudinal phase space of the beam, besides energy selection by an aperture in it. However, by using the alpha magnet, if we like to produce high brilliant electron beam with considerable charge, space charge force has to be carefully taken into account to evaluate the beam property for not only the longitudinal but also the transverse. Since the both transverse motions and the longitudinal one are coupled with each other in the alpha magnet, it is mostly impossible to evaluate the space charge effect analytically. Meanwhile, because energies of the electrons from the thermionic RF gun are ranging from zero to the maximum, a conventional way to count Coulomb force in the rest frame may be not satisfactorily valid in numerical simulations. We will discuss space charge dominated phase spaces derived from 3-D tracking simulations* for the alpha magnet. *GPT (General Particle Tracer) and an FDTD code developed ourselves., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan