Koichi Kan

Radially polarized terahertz waves from a photoconductive antenna with microstructures

The generation of radially polarized terahertz (THz) waves from a photoconductive antenna (PCA) with radial microstructures was investigated. Microstructured electrodes with 10 μm lines and spaces were fabricated on a semi-insulating InP wafer using photolithography. The PCA with three layers was driven by an amplified femtosecond laser and DC power supply. Radially polarized THz waves from the PCA were measured by a Michelson interferometer with a 4.2 K silicon bolometer. The polarization components of THz waves were analyzed by a wire grid polarizer.

Multimode terahertz-wave generation using coherent Cherenkov radiation

Multimode terahertz(THz)-wave generation using coherent Cherenkov radiation (CCR) was investigated. The frequency spectra of CCR, which utilized a metal-wrapped hollow dielectric tube of 7 mm outer radius and a picosecond electron bunch of 27 MeV beam energy, were measured by a Michelson interferometer with a 4.2 K silicon bolometer. In this study, discrete spectral components at frequencies of 0.09, 0.14, and 0.36 THz were observed experimentally and explained as transverse magnetic (TM) modes of TM03, TM04, and TM09, respectively, according to a theoretical calculation for the tube.

Development of double-decker pulse radiolysis

Double-decker pulse radiolysis (DDPR), which utilizes double-decker electron beams, was investigated to develop a new pulse radiolysis with a high time resolution. The double-decker electron beams were generated by injecting two UV pulses into a photocathode radio-frequency gun. In the pulse radiolysis, one electron beam was used as a pump beam, and the other was converted to a probe pulse. Finally, as its first application, the DDPR was successfully used for observing solvated electrons in water, with a 10%-90% rise time of 8.6 ps.

Femtosecond electron beam dynamics in photocathode accelator

Ultrashort-bunch, low-emittance electron beams are essential to reveal the hidden dynamics of intricate molecular and atomic processes in materials through experimentation such as time-resolved pulse radiolysis or time-resolved electron diffraction. The transverse and longitudinal dynamics of ultrashort electron beam in a photocathode linear accelerator were studied for femtosecond electron beam generation. The growths of the emittance, bunch length and energy spread due to the rf and the space charge effects in the rf gun were investigated by changing the laser injection phase. The dependences of the emittance, bunch length and energy spread on the booster linac rf phase were measured. Finally, a 100 fs electron source based on the photocathode rf gun with a femtosecond laser injection is proposed for time-resolved pulse radiolysis and time- resolved electron diffraction. The femtosecond beam dynamics in the rf gun was investigated by simulation.

Beam Parameter Measurement After Relocation of S-Band Linear Accelerator

Ultrashort electron bunches have been applied in many scientific fields including accelerator physics and radiation chemistry. Pulse radiolysis is one of the most powerful tools in radiation chemistry, which is a pump-probe measurement using an electron bunch and a laser pulse. Our laboratory aims to generate electron bunches with durations of less-than femtoseconds using an S-band linear accelerator (linac) at Osaka University in order to improve the time resolution of the pulse radiolysis system. Recently, the linac system has been relocated for expanding application using ultrashort electron bunches. In this study, the parameters of electron bunches generated from the relocated linac system were simulated using General Particle Tracer (GPT) code.

Frequency and Time Domain Measurement of Coherent Transition Radiation

Ultrashort electron beams are essential for light sources and time-resolved measurements. Electron beams can emit terahertz (THz) pulses using coherent transition radiation (CTR). Michelson interferometer is one of candidates for analyzing the pulse width of an electron beam based on frequency-domain analysis. Recently, electron beam measurement using a photoconductive antenna (PCA) based on time-domain analysis has been investigated. In this paper, to improve beam diagnostics of ultrashort electron beam, investigation of characteristics of a PCA for generation and frequency and time-domain measurement of THz pulses was conducted.

Generation and Diagnosis of Ultrashort Electron Bunches from a Photocathode RF Gun Linac

Ultrashort electron bunches are essential for timeresolved measurement methods such as pulse radiolysis from the viewpoint of time resolutions. On the other hand, generation of electro-magnetic wave in the THz range using short electron bunches has been investigated. Frequency spectra of coherent transition radiation (CTR) emitted by an electron bunch depend on bunch form factor (BFF), which is expressed by Fourier coefficients of longitudinal distribution in the electron bunch. In this study, the bunch length measurement was demonstrated by analyzing THz-waves generated by CTR. Femtosecond electron bunches were generated by a laser photocathode RF gun linac and magnetic bunch compressor. THz-waves generated by CTR, which was emitted on an interface of an aluminum mirror along the beam trajectory, were transported to a Michelson interferometer. The bunch length was measured by analyzing interferogram, which was an infrared detector output as a function of a moving mirror position. Finally, the bunch length was measured according to fitting curves for the interferogram near the centerburst. Minimum bunch length of 1.3 fs was obtained at a bunch charge of ~1 pC. INTRODUCTION Ultrashort electron bunches whose durations are picoseconds and femtoseconds have been applied to the accelerator physics applications including free electron lasers and laser Compton X-rays. Such electron bunches are also the key elements in time resolved measurements such as pulse radiolysis [1]. Pulse radiolysis is a powerful and useful tool for investigating ultrafast phenomena induced by the electron bunches. Recently, 100-fs electron bunches were generated using a laser photocathode RF gun linac and a time resolution of ~240 fs was achieved in pulse radiolysis [2]. The time resolution of pulse radiolysis strongly depends on the bunch length of the electron bunches. Therefore, the ultrashort electron bunches are demanded to improve the time resolution of pulse radiolysis and observe the initial process of the radiation chemistry. On the other hand, it is also important to develop a measurement system to diagnose the bunch length of such ultrashort electron bunches. Femtosecond streak camera is generally used as a measurement technique of the bunch length, however, its time resolution is limited to ~100 fs in rms. Therefore, alternative methods to obtain information of the longitudinal profile of the ultrashort electron bunches have been investigated. One of the promising techniques is a method to observe CTR using an interferometer [3]. CTR is a phenomenon that an electron bunch emits intense radiation at a wavelength longer than the bunch length when crossing a boundary between different media. The CTR spectrum depends on the bunch form factor, which is a square modulus of Fourier transform (FT) of the longitudinal bunch distribution. In other words, information of the bunch length can be obtained from the observation of CTR. In the present study, to improve the time resolution of the pulse radiolysis technique, generation and bunch length measurement of the femtosecond electron bunches were investigated by monitoring CTR using a Michelson interferometer. The ultrashort electron bunches were generated by a photocathode-based linac with a magnetic bunch compressor. Bunch length measurement is also important for characterizing electron bunches. Enabling analysis of broadband electromagnetic (EM) waves up to 50 THz, the technique using a Michelson interferometer with two detectors could be a useful and effective method for characterizing a wide range of bunch lengths. EXPERIMENTAL ARRANGEMENT Generation of Femtosecond Electron Bunches using a Photocathode-based Linac In this study, ultrashort electron bunches were generated using an S-band (2.856 GHz) photocathode-based RF gun linac and an arc-type magnetic bunch compressor [4,5,6]. The linac is composed of a 1.6-cell S-band (2.856 GHz) RF electron gun with a copper cathode, a 2-m-long S-band acceleration tube, and a magnetic bunch compressor. Irradiating the cathode by the third harmonic of Ti:Sapphire femtosecond laser, femtosecond electron bunches were generated at the RF gun. The incident angle of the laser on the cathode was ~2° with respect to the electron beam axis. The beam energy was 4 MeV at the exit of the gun and 32 MeV at the exit of the accelerating tube and the charge of the electron bunches were estimated using an integrating current transformer. The bunch charges were suppressed to the pico coulomb order to reduce the bunch-length growth due to the space charge effect. The femtosecond electron bunch was compressed by rotating the phase-space distribution in the magnetic bunch compressor. The compressor was composed of a pair of 45° bending magnets, two pairs of quadrupole magnets, ___________________________________________ #nozawa81@sanken.osaka-u.ac.jp 5th International Particle Accelerator Conference IPAC2014, Dresden, Germany JACoW Publishing ISBN: 978-3-95450-132-8 doi:10.18429/JACoW-IPAC2014-THPME132 06 Instrumentation, Controls, Feedback & Operational Aspects T03 Beam Diagnostics and Instrumentation THPME132 3553 Co nt en tf ro m th is w or k m ay be us ed un de rt he te rm so ft he CC BY 3. 0 lic en ce (© 20 14 ). A ny di str ib ut io n of th is w or k m us tm ai nt ai n at tri bu tio n to th e au th or (s ), tit le of th e w or k, pu bl ish er ,a nd D O I.

Pulse Radiolysis Using Terahertz Probe Pulses

Pulse radiolysis, which utilizes a pump electron beam and a probe pulse, is a powerful tool that can be used for the time-resolved observation of ultrafast radiationinduced phenomena. Recently, double-decker pulse radiolysis using visible probe pulses were demonstrated based on a photocathode RF gun driven by two UV pulses, which enabled synchronized pump electron beam and visible probe pulses. In this study, pulse radiolysis using terahertz (THz) probe pulses which were realized by the “double-decker” electron beams and dynamics of transient quasi-free electrons in semiconductors are presented.

Simulation study on attosecond electro bunch generation

Pulse radiolysis, a stroboscopic method with an ultrashort electron bunch and an ultrashort light, is essential for the observation of ultrafast reactions. The time resolution of pulse radiolysis depends on the electron bunch length. In Osaka University, a 98-fs electron bunch was generated by using a photocathode electron linac for a development of femtosecond pulse radiolysis. Furthermore, a subfemtosecond/attosecond pulse radiolysis is proposed to study the ionization and thermalization processes in subfemtosecond/attosecond time region. In order to realize such a high time resolution, the possibility of attosecond electron bunch generation based on the photocathode RF gun linac and a magnetic bunch compressor was studied. In the simulation, the bunch length growths due to charge, transverse and longitudinal emittance were investigated.