Takafumi Kondoh

Osaka University, Suika, Ōsaka, Japan

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Publications (51)37.49 Total impact

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    ABSTRACT: Ultrashort electron bunches are essential for time-resolved 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.
    IPAC2014, Dresden, Germany; 06/2014
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    ABSTRACT: The solvated electron in room temperature ionic liquids (RTILs) has been the subject of several investigations and several reports exist on its nature and absorption spectrum. These studies concluded that the solvated electron exhibits an absorption spectrum peaking in the 1000–1400 nm region; a second absorption band peaking in the UV region has been assigned to the hole or dication radicals simultaneously formed in the system. Here we report on the fate of the excess electron in the ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, P14+/NTf2− using nanosecond pulse radiolysis. Scavenging experiments allowed us to record and disentangle the complex spectrum measured in P14+/NTf2−. We identified a bi-component absorption spectrum, due to the solvated electron, the absorption maxima located at 1080 nm and around 300 nm, as predicted by previous ab-initio molecular dynamics simulations for the dry excess electron. We also measured the spectra using different ionic liquids and confirmed the same feature of two absorption peaks. The present results have important implications for the characterization of solvated electrons in ionic liquids and better understanding of their structure and reactivity.
    Radiation Physics and Chemistry. 01/2014; 100:32–37.
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    ABSTRACT: This paper proposes the use of a solid-state plasma lens in the fs linacs, in which the bunch radii of the beams are much larger than the lengths. Although gas plasmas degrade vacuum, solid-state plasmas do not. Another problem is that a return current flowing in the beam region of plasmas decreases the focusing power of the lens. By orientating anisotropic plasmas, we can constrain the return current, while maintaining the focusing power.
    Proceedings of the 10th Meeting of Particle Accelerator Society of Japan, Nagoya; 08/2013
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    ABSTRACT: 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.
    Applied Physics Letters 06/2013; 102(22). · 3.79 Impact Factor
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    ABSTRACT: The kinetics of geminate ion recombination was studied by the femtosecond pulse radiolysis of n-dodecane with biphenyl solution. The complicated biphenyl concentration-dependence behaviors of the transient absorptions at 800 nm are observed and elucidated with a Monte Carlo simulation of geminate recombination and charge transfer processes in the biphenyl-dodecane solution. The experimental data suggested the existence of n-dodecane excited radical cation in the radiolysis. It plays very important roles especially in the high-concentration solution. The reaction rate constant of the excited radical cation with biphenyl molecule was 3.5×1011 dm3 mol-1 s-1, which is comparable with the reaction rate constant of the electron with biphenyl molecule. The contributions of the geminate ion recombination, the charge transfer reaction and the relaxation of the excited radical cation were obtained as a function of the biphenyl concentration.
    Radiation Physics and Chemistry 03/2013; · 1.38 Impact Factor
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    ABSTRACT: 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.
    The Review of scientific instruments 07/2012; 83(7):073302. · 1.52 Impact Factor
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    ABSTRACT: Pulse radiolysis spectroscopically studies the initial stage of atomic and molecular reactions induced by electron beams. Single-bunched beams with bunch lengths on the order of femtosecond or shorter are fervently requested for this purpose. The laser wake field acceleration is readily applicable now, which is able to realize beams with bunch length of 1 fs, energy of 30 MeV, repetition rate of 1 Hz, bunch charge of 10 pC, energy width of 10%, etc.
    International Journal of Modern Physics B 01/2012; 21(03n04). · 0.46 Impact Factor
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    ABSTRACT: Two ultrashort electron-bunch photocathode RF guns in picosecond (ps) and femtosecond (fs) time region are reviewed. The ps gun was developed and used successfully to generate an ultralow-emittance and high-charge electron beam with normalized emittance of 1.2 mm-mrad at bunch charge of 1 nC. The low-emittance electron beam has been used successfully to produce a 100-fs high-charge electron single bunch with a booster linear accelerator and a magnetic bunch compressor. An ultrafast pulse radiolysis with time resolution of 240 fs was constructed using the fs electron beam for the study of ultrafast reactions in radiation chemistry and biology. The fs gun, developed under the KEK/Osaka University collaboration with a new structure cavity and many improvements, was succeeded in producing directly a near-relativistic 100-fs electron beam with energy of 1-3 MeV. Femtosecond time-resolved electron diffraction has been constructed using the fs RF gun for the study of photon-induced phase transition in materials. Some experimental results of pulse radiolysis and MeV electron diffraction in fs time region were reported and discussed.
    Journal of the Vacuum Society of Japan 01/2012; 55(2):42-49.
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    ABSTRACT: In this paper, Smith-Purcell effect, which utilizes metallic grating and electron bunches, was investigated for a new device of terahertz (THz)-wave generation. Femtosecond electron bunches induced multimode THz-wave at a frequency of
    Infrared, Millimeter, and Terahertz Waves (IRMMW-THz), 2012 37th International Conference on; 01/2012
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    ABSTRACT: 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.
    Applied Physics Letters 12/2011; 99(23):231503-231503-3. · 3.79 Impact Factor
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    ABSTRACT: Femtosecond time-resolved pulse radiolysis was developed using a femtosecond electron bunch and a femtosecond laser light. This development initiated the experimental study of pulse radiolysis in the femtosecond time region. The femtosecond transient absorption kinetics of hydrated electrons in water radiolysis was observed. The initial yield of the hydrated electron in water radiolysis was obtained to be G=4.6±0.3. The experimental data indicates that about 10% of the hydrated electrons undergo the spur reaction with H3O+ cations and OH radicals in the first 10ps. The dependences of the transient absorption signal on the electron bunch length, the electron charge, and the sample thickness were also investigated.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 01/2011; 629(1):6-10. · 1.14 Impact Factor
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    ABSTRACT: In this study, we have analytically shown that if the electron bunch length is in the 100-as range, the energy loss of the bunch is proportional to the square of the number of electrons in the bunch. If the number of electrons is large, the collective loss introduces a high-energy-density state in the target. The results were verified by carrying out 2D PIC simulations.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 01/2011; 637(1). · 1.14 Impact Factor
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    ABSTRACT: A femtosecond pulse radiolysis with a time resolution of 210 fs has been developed by using a femtosecond electron beam and a femtosecond laser light. It has successfully opened the study of ultra-fast reactions or phenomena in materials—the first observation of the femtosecond formation process of the hydrated electron in water pulse radiolysis. The use of a photocathode femtosecond electron gun, which produces a near-relativistic 100 fs electron beam, has been approached to construct femtosecond megavolt electron diffraction. The dependencies of the emittance, bunch length, and energy spread on the radio-frequency (rf) and space-charge effects in the rf gun have been investigated.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 01/2011; 637(1). · 1.14 Impact Factor
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    ABSTRACT: The primary process of radiation chemistry was studied by the femtosecond pulse radiolysis in n-dodecane. The time-dependent behavior of geminate ion recombination in n-dodecane was investigated in neat n-dodecane and the solutions containing electron and cation scavengers. The excited radical cation was found from analyzing the geminate process by the theoretical simulation based on the diffusion theory. The life time of the excited radical cation was 7 ps in n-dodecane. The reaction rate constant of the excited radical cation with the triethylamine (TEA) was 3.3×1011 dm3 mol−1 s−1, which is one order higher than the normal diffusion rate constant. Although 74% of the excited radical cations are relaxed to the radical cation, the residual 26% are geminately recombined with electrons.
    Radiation Physics and Chemistry 01/2011; 80(2):286-290. · 1.38 Impact Factor
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    ABSTRACT: A novel method to generate high-power THz radiation is proposed. If a beam with a bunch length on the order of 100fs is injected into an electron–hole plasma of a semiconductor with a plasma frequency on the order of THz, THz wake fields are coherently generated. If the beam moves on the axis of a hollow tube covered by a metal, the frequency spectrum of the radiation is composed of discrete components. Monochromatic radiation is obtained by making only the lowest frequency component coherent.
    Radiation Physics and Chemistry 01/2011; 80(12):1323-1326. · 1.38 Impact Factor
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    ABSTRACT: Pulse radiolysis, a stroboscopic method involving the use of an ultrashort electron bunch and ultrashort light, is essential for the observation of ultrafast reactions. The time resolution of pulse radiolysis depends on the electron bunch length. At Osaka University, a 98-fs electron bunch was generated using a photocathode RF gun linac for the development of femtosecond pulse radiolysis. In order to improve the time resolution of pulse radiolysis, the feasibility of sub-femtosecond electron bunch generation using a photocathode RF gun linac and a magnetic bunch compressor was studied in a simulation. Increase in bunch length due to higher-order effects and bunch charge was investigated using the simulation codes PARMELA and ELEGANT.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 01/2010; 622(1):35-40. · 1.14 Impact Factor
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    ABSTRACT: In this article, we report the reaction of halophenols with solvated electrons in room-temperature ionic liquids (RTILs) initiated by γ-ray and pulsed electron radiolyses. The decomposition G-values of ortho-chlorophenol (CP) in N-methyl-N-propylpyrrolidinium–bis(trifluoromethanesulfonyl)imide (TFSI), N-butyl-N-methylpyrrolidinium–TFSI and N-methyl-N-propylpiperidinium–TFSI were estimated to be 1.4, 1.6, and 1.7 molecules 10−2 eV−1 under γ-ray irradiation; these values were almost the same as the yield of solvated electron formation. The second-order rate constant for the reaction of CP with solvated electrons in diethylmethyl(2-methoxyethyl)ammonium (DEMMA)–tetrafluoroborate (BF4) was one order of magnitude lower than that in DEMMA–TFSI although the G-values of CP decomposition and phenol formation in DEMMA-BF4 were higher. The decomposition yield of ortho-iodophenol in DEMMA–TFSI was slightly higher than that of the other halophenol (ortho-fluorophenol, CP, and ortho-bromophenol), and the formation yield of phenol for the decomposition of only ortho-fluorophenol was lower.
    Radiation Physics and Chemistry 01/2010; · 1.38 Impact Factor
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    ABSTRACT: The formalism of the stopping power for cluster beams was adapted to the stopping power for short electron bunches using the wake field of a medium characterized by plasma frequency. It was shown that, if the bunch length is in the 100 as range, the energy loss of the bunch is proportional to the square of the number of electrons in the bunch. If the number of electrons is large, the collective loss is able to excite a high-energy density state in the target. The target medium and beam parameters were examined to demonstrate the collective effect, and an accelerator system consisting of an accumulation ring and an inverse free-electron laser (IFEL) was considered to produce attosecond bunches. # 2009 The
    Japanese Journal of Applied Physics 05/2009; 48:056002. · 1.07 Impact Factor
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    ABSTRACT: The single crystal of MgSO4.7H2O is very interesting to study the structure and kinetics of trapped electron in solids, because the hydrated salt has many H2O molecules in the crystal lattice like water. The pulse radiolysis study of the trapped electrons in MgSO4.7H2O single crystal was carried out based on an electron linear accelerator. Two optical absorption peaks of the trapped electron in MgSO4.7H2O single crystal were observed: one is at the wavelength of 600nm with a short lifetime of 81.4ns; another is at 400nm with a long lifetime of 1.06μs. The kinetics of the trapped electron in the crystal was also compared with that in the aqueous solution.
    Radiation Physics and Chemistry 01/2009; 78(12):1153-1156. · 1.38 Impact Factor