Nuclear Engineering Research Laboratory Research Articles

SYNCHRONIZATION BETWEEN THE LASER AND ELECTRON BEAM IN A PHOTOCATHODE RF GUN

The chemical reactions of hot or room temperature and/or critical water in a time-range of picosecond and sub-picoseconds have been carried out by the 18 MeV S -band linac and a Mg photocathode RF gun with the irradiation of third harmonic Ti : Sapphire laser, at Nuclear Engineering Research Laboratory (NERL), the University of Tokyo. Although this short bunch and 100 fs laser light are enough to perform the experiment of radiation chemistry in the time-range of sub-picoseconds, the total time-resolution become worse by the instability of synchronization between laser and radio frequency of linac. We found that the fluctuation of room temperature causes the instability, particularly the cycle of turning on/off of the air-conditioner. When we decrease the fraction of the room temperature within 0.1 degrees, the timing drift in an hour reaches 600 fs, closed to the timing jitter of 340 fs than previous results.

Experimental Verification of Velocity Bunching via Shot-by-Shot Measurement at S-Band Photoinjector and Linac

We present an experimental verification of a bunch compression method named “velocity bunching”. Velocity bunching based on the rectilinear compression uses a traveling wave accelerating tube as a compressor. The experiment was performed using an S-band photoinjector and a linac at the Nuclear Engineering Research Laboratory, University of Tokyo. The shot-by-shot bunch length was measured to be 0.5±0.1 ps (rms) on average using a femtosecond streak camera at a bunch charge of 1 nC. The experimental result is in good agreement with the PARMELA simulation result.

Emittance and Energy Measurements of Low-Energy Electron Beam Using Optical Transition Radiation Techniques

Emittance and energy of an electron beam in the range of 8 to 22 MeV were measured via optical transition radiation (OTR) techniques. The beam divergence effect on observations of the far-field OTR image at low energies was studied by means of numerical analysis. The numerical analysis indicates that if the beam divergence is under 1.5 mrad, a simultaneous single-shot measurement of emittance and energy is possible. The results of the single-shot experiment agree with independent measurements conducted using the quadrupole scan method and an electron spectrometer. The experiments were performed with an S-band linac at the Nuclear Engineering Research Laboratory, The University of Tokyo (UTNL).

Current status of the ultra-fast pulse radiolysis system at NERL, the University of Tokyo

In order to investigate the early events in radiation physics and chemistry, a new pulse radiolysis system with higher time resolution based on the pump-and-probe method has been developed at Nuclear Engineering Research Laboratory (NERL), The University of Tokyo. Electron pulses with duration of a few picoseconds are generated from an 18 MeV S-band linear accelerator (linac) by using a combination of a laser photocathode rf-gun and a chicane-type magnetic compressor. The pulses are synchronized with a femtosecond laser pulse as the analyzing light. The precision of synchronization between the pump and the probe has attained within 1.6 ps (rms). By converting the fundamental laser into white light continuum, it allows to measure in the wavelength range from 400 to 1100 nm. With a sample cell having an optical path of 1 mm, a time resolution of 5 ps has been achieved.

Measurement and Numerical Analysis of Ultrashort Electron Bunch Using Fluctuation in Incoherent Cherenkov Radiation

Measurement of ultrashort electron bunches using the fluctuation of incoherent radiation has been pursued at Nuclear Engineering Research Laboratory (NERL), University of Tokyo. The fluctuations of Cherenkov radiation have been measured for 20- and 35-MeV electron bunches and the bunch lengths were obtained from one-dimensional evaluation as 4.5- and 30-ps FWHM respectively, while the femtosecond streak camera showed 1.0- and 1.5-ps FWHM for the respective bunches. The discrepancy comes from the finite transverse emittance. The dependence of the transverse beam emittance upon the fluctuation of Cherenkov radiation is investigated by three-dimensional numerical analysis and the new formula is deduced where the fluctuation of time-integrated intensity of Cherenkov radiation is inversely proportional to the transverse beam emittance. The model for quantitative analysis of the longitudinal electron bunch length including the effects of transverse coherent modes is presented. The possibility of applying the method to laser accelerated bunches is discussed.

Diagnostics of sub-picosecond electron pulse using the fluctuation method

Numerical analysis and measurement of longitudinal bunch length of subpico- and picosecond electron beams by the fluctuation method has been carried out at the S-band twin linear accelerators at Nuclear Engineering Research Laboratory (NERL), University of Tokyo. The diagnostics method utilizes a statistical analysis of shot-noise-driven fluctuations in incoherent radiation. Measurements were performed on Cherenkov radiation emitted by ∼ 1.0 ps (FWHM), 20 MeV electron bunch. As a result, the fluctuation method indicated 5 times longer than that by the streak camera. The discrepancy was caused by neglecting the influence of the transverse emittance. Then, the influence of the transverse profile upon the measurement was analyzed and discussed.

Time-resolved X-ray diffraction for visualization of atomic motion

For ultrafast material analyses, we constructed the time-resolved X-ray diffraction system utilizing ultrashort X-rays from laser produced plasma generated by the 12TW-50fs laser at Nuclear Engineering Research Laboratory. Ultrafast transient changes in laser-irradiated GaAs crystals were observed as X-ray diffraction patterns. A subpeak appears in X-ray diffraction profiles and shifts its position gradually as time goes in the case of 1.3 mJ/mm laser fluence. Experimental results were compared with numerical analyses to infer the atomic arrangement during the transition.

Performance study of high current photo-cathode RF-gun with new laser system

For a sub-picosecond pump-and-probe-type radiation chemistry work, a new synchronized electron linac and laser system was installed in Nuclear Engineering Research Laboratory (NERL) of University of Tokyo. The new 0.3 TW laser system with a Ti:Sapphire oscillator (795 nm) and amplifiers generate 300 ps pulses at 10 Hz at first. The laser is splitted into the two beams. The first is compressed to 4 ∼ 11 ps so as to drive the RF-Gun and generate a pump-electron-beam. The second is compressed to 100 fs and used for the probe-light. The drive laser is converted to the third harmonics (265 nm, < 250 µJ). Moreover, a new 15 MW klystron was installed for the linac RF system. The RF power is divided to the gun and the accelerating section. Finally the time jitter between the pump-electron-beam and the probe-laser is designed to be 320 fs (rms). By using the new drive laser system, the charge of 7 nC per bunch was observed at the exit of the gun. Improvement of the vacuum in the gun (< 10 −9 Torr) is most effective to obtain such a high charge beam. After two and a half years operation of the gun, no degradation of QE has been found for the Cu photo-cathode.

Ultra-fast pulse radiolysis system combined with a laser photocathode RF gun and a femtosecond laser

In order to study the early events in radiation physics and chemistry, two kinds of new pulse radiolysis systems with higher time resolution based on pump-and-probe method have been developed at the Nuclear Engineering Research Laboratory, the University of Tokyo. The first one, a few picosecond (2 ps at FWHM) electron beam (pump) from an 18 MeV S-band Linac using a laser photocathode RF gun (BNL/KEK/SHI type: GUN IV) was operated with a femtosecond laser pulse (100 fs at FWHM), which also acted as the analyzing light (probe). The synchronization precision between the pump and the probe was 1.7 ps (rms). In a 1.0 cm sample cell, a time resolution of 12 ps was achieved. The second one, a picosecond (4 ps at FWHM) electron pulse from a 35 MeV S-band Linac employing a conventional thermionic gun with a sub-harmonic buncher, was synchronized with the femtosecond laser pulse, with a synchronization jitter of 2.8 ps (rms). A time resolution of 22 ps was obtained with 2 cm cell. This makes it possible to do the pulse radiolysis experiments in the time range from picosecond to sub-microsecond.

Overall comparison of subpicosecond electron beam diagnostics by the polychromator, the interferometer and the femtosecond streak camera

Measurements of longitudinal bunch length of subpicosecond and picosecond electron beams have been performed by three methods with three radiation sources at the 35 MeV S-band twin liner accelerators at Nuclear Engineering Research Laboratory, University of Tokyo. The methods we adopt are the femtosecond streak camera with a nondispersive reflective optics, the coherent transition radiation (CTR) Michelson interferometer and the 10 ch polychromator that detects the spectrum of CTR and coherent diffraction radiation (CDR). The measurements by the two CTR methods were independently done with the streak camera and their results were consistent with one another. As a result, the reliability of the polychromator for the diagnostics of less than picosecond electron bunch and the usefulness of the diagnostics for the single shot measurement were verified. Furthermore, perfect nondestructive diagnostics for subpicosecond bunches was performed utilizing CDR interferometry. Then the good agreement between CDR interferometry and the streak camera was obtained.

High-Charge S-Band Photocathode RF-Gun and Linac System for Radiation Research.

For sub-picosecond pump-and-probe-type radiation chemistry work, a new synchronized electron linac and laser system was installed in the Nuclear Engineering Research Laboratory (NERL) of University of Tokyo. The new laser system, with a Ti:Sapphire oscillator (795 nm) and amplifiers, generates 300 ps pulses at 10 Hz. The laser is transported through the vacuum chamber and then split into two beams. The first is compressed and converted to the third harmonics (265 nm, <250 μJ, 4–11 ps) so as to drive the photocathode RF-gun and generate a pump-electron beam. The second is compressed to 100 fs and used for the probe light. The high-power RF, which is provided by a new 15 MW klystron, is divided into the gun and the accelerating section. Finally, a time jitter of 330 fs (rms) was achieved between the pump-electron beam and the probe laser, which is equivalent to the design value of 320 fs. A charge of 7 nC/bunch was observed at the exit of the gun from this new laser system. Improvement of the vacuum in the gun (<10-9 Torr) is the most effective way to obtain such a high-charge beam. After about three years of operation, the Cu photocathode has shown no degradation of quantum efficiency.

High-Charge S-Band Photocathode RF-Gun and Linac System for Radiation Research

For sub-picosecond pump-and-probe-type radiation chemistry work, a new synchronized electron linac and laser system was installed in the Nuclear Engineering Research Laboratory (NERL) of University of Tokyo. The new laser system, with a Ti:Sapphire oscillator (795 nm) and amplifiers, generates 300 ps pulses at 10 Hz. The laser is transported through the vacuum chamber and then split into two beams. The first is compressed and converted to the third harmonics (265 nm, <250 μJ, 4–11 ps) so as to drive the photocathode RF-gun and generate a pump-electron beam. The second is compressed to 100 fs and used for the probe light. The high-power RF, which is provided by a new 15 MW klystron, is divided into the gun and the accelerating section. Finally, a time jitter of 330 fs (rms) was achieved between the pump-electron beam and the probe laser, which is equivalent to the design value of 320 fs. A charge of 7 nC/bunch was observed at the exit of the gun from this new laser system. Improvement of the vacuum in the gun (<10-9 Torr) is the most effective way to obtain such a high-charge beam. After about three years of operation, the Cu photocathode has shown no degradation of quantum efficiency.

Intense Optical Field Science. Generation and Application of Ultrashort X-rays and Electrons by Intense Laser.

Femto- and picosecond X-rays and electrons can be generated and used at the Femtosecond Ultrafast Quantum Phenomena Research Facility in Nuclear Engineering Research Laboratory, University of Tokyo. The beams are mainly applied to pump-and-probe analysis. The analysis utilizes two synchronized ultrashort beams. Especially, the time-resolved X-ray diffraction, which is one of the analysis, enables the dynamic visualization of atomic motions. Visualized atomic motions in coherent phonon, phase relaxation, phase transition, thermal expansion and shock wave in GaAs monocrystal irradiated by a femtosecond laser are described in this paper.

Design and development of a sub-picosecond pulse radiolysis system

In order to reveal the radiation-induced phenomena in the time scale of picosecond, or even sub-picosecond, a new sub-picosecond pulse radiolysis system has been designed and developed at Nuclear Engineering Research Laboratory (NERL), the University of Tokyo. The system is composed of the 18 MeV S-band linac with a laser driven photocathode rf-gun and a chicane-type magnetic compressor, a femtosecond Ti:Sapphire laser, and the synchronization system. At a preliminary experiment the timing jitter of the ps electron pulse and the fs laser pulse was determined to be 2.1 ps (rms) and the total time resolution of the pulse radiolysis was evaluated to be 30 ps.

Hundreds- and tens-femtosecond time-resolved pump-and-probe analysis system

Two-hundred and forty femtosecond, 17 MeV electron beam generation and 330 fs (rms) resolved synchronization with 100 fs Ti : Sapphire laser are available for radiation chemistry research at the S-band twin linacs of Nuclear Engineering Research Laboratory, University of Tokyo. Further, numerical and experimental analyses on the laser plasma linac of ∼10 fs ∼20 MeV electron beam using 12 TW 50 fs laser and laser plasma wake-wave breaking are under way. It is synchronized by a 33 fs step with 50 fs probe laser. These works are done using the Femtosecond Quantum Phenomena Research Facility installed in 1999. Updated experimental results on the linac–laser synchronization and laser plasma linac are presented.

Time-resolved X-ray diffraction at NERL

For ultrafast material analyses, we constructed the time-resolved X-ray diffraction system utilizing ultrashort X-rays from laser-produced plasma generated by the 12-TW–50-fs laser at the Nuclear Engineering Research Laboratory. Ultrafast transient changes in laser-irradiated GaAs crystals were observed as X-ray diffraction patterns. Experimental results were compared with numerical analyses.

Numerical analysis of 10's femtosecond relativistic electron beam generation using single 12 TW50 fs laser pulse

Particle-in-cell two-dimensional (PIC-2D) simulations have been performed to verify the trapping of plasma electrons in the wake of ultrashort, multi-terawatt laser pulse as it propagates through underdense uniform plasma. It is supposed that the wake wavebreaking is the reason for the beam generation. This mechanism may lead to a compact ultrashort-pulse relativistic electron accelerator/injector (we call it laser–plasma linac). For a laser pulse with a duration of 50 fs and a peak power of 12 TW propagating through plasma, the simulation results show the generation of electron beam from the self-trapped plasma electrons with maximum energy >25 MeV and pulse duration of about 12 fs at FWHM. The two-dimensional effect of the plasma wave and the phase space of accelerated electrons are discussed. The present numerical results are relevant to the laser–plasma acceleration experiments that use the 12 TW 50 fs laser system at the Nuclear Engineering Research Laboratory (NERL), University of Tokyo.

Generation and application of femtosecond X-ray pulse

Generation and application of femtosecond X-ray pulses using the linac-laser synchronization system at Nuclear Engineering Research Laboratory, University of Tokyo, were performed. The time resolution of the synchronization between 240 fs (FWHM) 17 MeV electron pulse and 100 fs (FWHM) 3 TW Ti:Sapphire laser pulse is 3.5 ps (rms). Femtosecond X-ray pulses were generated via orthogonal and head-on Thomson scatterings between the electron and laser pulses. Then, the time-resolved X-ray diffraction was tried to visualize atomic motion in nonequilibrium thermal expansion in several mono-crystals. Here we generated picosecond X-ray pulse by colliding 10 ps electron pulse to 100 μm∅ Cu wire for simplicity. X-ray diffraction patterns of Cu K α 1,2 from the static lattices were obtained. Visualization of the atomic motion is under way.

Ultrashort X-ray pulse generation using subpicosecond electron linac

As a promising tool for ultrafast material analyses, we propose to utilize the X-ray pulse which may be generated in a quite simple manner using subpicosecond electron linacs. The properties of the X-ray were numerically studied with the EGS4 code. Verification of the X-ray generation was also conducted at the Nuclear Engineering Research Laboratory (NERL) linac and clear diffraction patterns of characteristic X-ray were obtained for typical single crystals.

フェムト秒ビームフラッシュ法による原子の動画像化

Dynamic visualization of atomic motion is going to be available by the femtosecond beam flash method. The method utilizes two synchronized femtosecond beams such as electron, laser, X-ray, ion and neutron. This can be done at Nuclear Engineering Research Laboratory, University of Tokyo. Especially, the time-resolved X-ray diffraction, which is one of the femtosecond beam flash methods, enables the dynamic visualization of atomic motions. Visualized atomic motions in coherent phonon, phase relaxation, phase transition, thermal expansion and shock wave in GaAs monocrystal irradiated by the femtosecond laser are described in this paper.