Takafumi Kondoh

Experimental Studies of Transverse and Longitudinal Beam Dynamics in Photoinjector

Both the transverse and longitudinal beam dynamics in a photoinjector were investigated experimentally for high-brightness electron beam generation. The transverse emittance growth and the energy spread, both due to the rf and the space-charge effects in the rf gun, were investigated with the laser injection phase. A 31.7 MeV electron beam at 1.0 nC with a normalized rms transverse emittance of 3.2±0.2 mmmrad, an rms bunch length of 1.8±0.2 ps, and an rms relative energy spread of 0.04±0.01% was obtained in the photoinjector with a 5 ps (full width at half maximum; FWHM) laser. The phase compression in the bunch length was observed in the rf gun. A sub-picosecond electron bunch was observed experimentally in the rf gun at the low injection phase, such as 0.7±0.2 ps at 15° and 0.5±0.1 ps at 10°. The dependences of the transverse emittance, the bunch length and the energy spread on the rf phase of a booster linear accelerator (linac) downstream of the rf gun were investigated experimentally.

Double-decker femtosecond electron beam accelerator for pulse radiolysis

A new concept of double-decker electron beam accelerator based on a photocathode radio-frequency (rf) gun was proposed for studying chemical kinetics and primary processes or reactions of radiation chemistry. The synchronized double-decker electron beams with time interval of 1.4ns were generated in the rf gun by injecting two laser beams on the photocathode. The double-decker electron beams were accelerated by a booster linear accelerator (linac) up to 31.8MeV with energy-phase correlation and compressed into femtosecond by rotating the bunch in the energy-phase distribution in magnetic fields. The normalized transverse emittance of both beams downstream of the linac was obtained to be 2.5±0.6mmmrad for the up beam with bunch charge of 0.47nC and 3.6±0.7mmmrad for the down beam with bunch charge of 0.65nC. The minimum relative energy spread was (0.14±0.03)% for the two beams. The compressed bunch length was obtained to be 430±25fs for the up bunch and 510±20fs for the down bunch.

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.

Collective Energy Loss of Attosecond Electron Bunches

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.

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.

Dynamic optical modulation of the electron beam for the high performance intensity modulated radiation therapy

The radiation therapy of cancer is developing to non-uniform irradiation as intensity modulated radiation therapy (IMRT), reducing the dose to normal tissue and concentrating the dose to cancer tissue. A photocathode RF gun is able to generate a low emittance electron beam pulse using laser light. We thought that a photocathode RF gun can generate an intensity-modulated electron beam by optical modulation at the incident optics dynamically. Because of a low emittance, the modulated electron beam pulse was able to accelerate and keep its shape. The human body is always moving, for example during breathing, thus the cancer is also always moving. For radiation therapy, the electron beam must be synchronized to the breathing. Toward high performance (high speed modulation and high spatial resolution, etc.) IMRT, dynamic optical modulation of the electron beam pulse were studied using a photocathode RF gun. Images on photo masks were transported to a photocathode surface by optical relay imaging. Dynamic optical control of the electron beam was carried out by a remote mirror. The modulated electron beam had fine spatial resolution, good enough to use in radiation therapy. Spatial separation spot-to-spot is about 0.3 mm. The moving electron beam images like an electron beam movie were measured, something that was impossible by former methods. Modulated and moving electron beams are reported here.

Selective Polymerization of 5,7-Dodecadiyne-1,12-Diol Bis(Phenylcarbamate) (TCDU) Crystals and Thermochromism of the Polymerized Crystals

The crystals of 5,7-dodecadiyne-1,12-diol bis[phenylcarbamate] (TCDU) with the two different structures, TCDU-1 and TCDU-2, were grown from TCDU-acetone solution by controlling the rate of evaporation of solvent. The monomer crystals with the two different structures polymerize selectively into different phases of polymer crystals: TCDU-1 into B-phase, and TCDU-2 into A-phase. Studies of thermally-induced phase transition of polymerized crystals have revealed a reversible A-B phase transition at 393 K in A-phase crystals polymerized from TCDU-2. However, it has also been shown that the phase transition is not induced thermally in B-phase crystals polymerized from TCDU-1 at any temperatures from 7 to 450 K. These results are discussed on the basis of the current structural model of A and B phases of polydiacetylene crystals.

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.