Showing papers by "Takashi Tanaka published in 2012"

A compact X-ray free-electron laser emitting in the sub-ångström region

Abstract: Researchers report sub-angstrom fundamental-wavelength lasing at the SPring-8 Angstrom Compact Free-Electron Laser in Japan. The output has a maximum power of more than 10 GW, a pulse duration of 10−14 s and a lasing wavelength of 0.634 A.

Determination of the pulse duration of an x-ray free electron laser using highly resolved single-shot spectra.

Abstract: We determined the pulse duration of x-ray free electron laser light at 10 keV using highly resolved single-shot spectra, combined with an x-ray free electron laser simulation. Spectral profiles, which were measured with a spectrometer composed of an ultraprecisely figured elliptical mirror and an analyzer flat crystal of silicon (555), changed markedly when we varied the compression strength of the electron bunch. The analysis showed that the pulse durations were reduced from 31 to 4.5 fs for the strongest compression condition. The method, which is readily applicable to evaluate shorter pulse durations, provides a firm basis for the development of femtosecond to attosecond sciences in the x-ray region.

Performance upgrade in the JAEA actinide science beamline BL23SU at SPring-8 with a new twin-helical undulator

Abstract: The soft X-ray beamline BL23SU at SPring-8 has undergone an upgrade with a twin-helical undulator of in-vacuum type to enhance the experimental capabilities of the endstations. The new light source with a fast helicity-switching operation allows not only the data throughput but also the sensitivity in X-ray magnetic circular dichroism (XMCD) to be improved. The operational performance and potential are described by presenting XMCD results of paramagnetic β-US2 measured with a 10 T superconducting magnet.

Pulse energy measurement at the hard x-ray laser in Japan

Abstract: The pulse energies of a free electron laser have accurately been measured in the hard x-ray spectral range. In the photon energy regime from 4.4 keV to 16.8 keV, pulse energies up to 100 μJ were obtained at the hard x-ray laser facility SACLA (SPring-8 Angstrom Compact free-electron LAser). Two independent methods, using a cryogenic radiometer and a gas monitor detector, were applied and agreement within 3.3% was achieved. Based on our validated pulse energy measurement, a SACLA online monitor detector could be calibrated for all future experiments.

X-Rays: Self-seeded FEL emits hard X-rays

Abstract: X-ray free-electron lasers are bright, femtosecond X-ray sources. Researchers have now operated one in a seeding scheme that allows X-ray pulse output approaching the single-mode ideal and produces a remarkable enhancement in monochromatic power.

Pulse-mode measurement of electron beam halo using diamond-based detector

Abstract: Using a diamond-based detector, the electron beam halo in a high-energy accelerator can be measured with a lower detection limit than that using other instruments, such as a core monitor, a dose meter, or an optical fiber. We have successfully measured an electron beam halo using diamond-based detectors operating in the ionization mode, which were installed in the beam duct to measure the intensity of the beam halo directly. Pulse-by-pulse measurements were adopted to suppress the background noise efficiently. Feasibility tests on the diamond-based detector and beam halo monitor were performed in the beam dump area of the 8 GeV SPring-8 synchrotron booster and at the 250 MeV SPring-8 Compact SASE Source test accelerator for the SPring-8 Angstrom Compact free electron LAser (SACLA), respectively. We achieved a lower detection limit of $2\ifmmode\times\else\texttimes\fi{}{10}^{3}\text{ }\text{ }\mathrm{electrons}/\mathrm{pulse}$ for single-shot measurement, which corresponds to a ratio of about ${10}^{\ensuremath{-}6}$ relative to the typical charge of the beam core of 0.3 pC. We also confirmed the feasibility of the electron beam halo monitor for use as an interlock sensor to protect undulator permanent magnets used in SACLA from radiation damage.

Macro and micro physics of magnetic reconnection in a multi-hierarchy open system

Abstract: Based on particle-in-cell (PIC) simulation results of collisionless driven reconnection in a steady state, an effective resistivity model is developed for a magnetohydrodynamic (MHD) simulation in order to bridge the huge gap between macro- and microphysics of magnetic reconnection. The PIC simulation reveals that the reconnection electric field sustained by microscopic physics is found to evolve so as to balance the flux inflow rate, which is determined by global dynamics in a macroscopic system. This effective resistivity model is applied to MHD phenomena controlled by magnetic reconnection in the Earth's magnetosphere. Although this model does not include any adjustable parameters related to kinetic dissipation processes, some global phenomena such as the onset of magnetic substorm, dipolarization and propagation of flux rope, detailed processes of which are longstanding questions, are reproduced well in the MHD simulation and are consistent with the observations.

Laser wavelength tuning by variable-gap undulators in SACLA

Abstract: Wavelength tunability by variable-gap in-vacuum undulators is one of the features of SACLA. To fully utilize this advantage, it is important to suppress gapdependent magnetic field errors down to the tolerance level, namely sub-microradian per undulator segment, which assures high SASE amplification gain enabling XFEL power saturation. For this purpose, we introduced a “feed-forward correction” scheme, which is a well-known technique in third-generation light sources. However, in linac-based XFELs, it was not easy to make sufficiently accurate correction tables of steering magnets to cancel out error fields due to shot-by-shot beam orbit and energy fluctuations propagating from the accelerator. By using a linear accelerator model, we so far succeeded in suppressing the gap-dependent orbit deviation down to a 10-m level over the undulator section. Owing to this effort, experimental users at SACLA can quickly change the laser wavelength in a few seconds according to their demands by setting only the undulator K-value. In this paper, we report the present status of wavelength tuning by the undulator gap in SACLA and problems to be solved towards better accuracy.

A preliminary analysis of dynamic and realistic heliosphere using interplanetary scintillation and photospheric magnetic data

Abstract: A new MHD simulation method for reproducing a realistic and time-varying global heliospheric modelling is developed using solar-wind speed data from interplanetary sintillation (IPS) and photospheric magnetic data from optical observations. The IPS together with the magnetic data provide us inner boundary conditions which include both co-rotating solar-wind speed distribution and magnetic field distributions using a tilted current sheet model. Hence it enables us to reproduce dynamical co-rotating wind-wind interaction and also magnetic field structures such as an oscillating equatorial magnetic neutral sheet in a three-dimensional heliosphere. In our preliminary results of this method, a realistic and oscillating equatorial neutral sheet and wind-wind interactions are shown both in the interplanetary space and the heliosheath.

Enormous Amplification of Full-Coherent Radiation in the Extreme Ultraviolet Region with a Free-Electron Laser

Abstract: High-order harmonic beam was injected as a seeding source to a 250-MeV free-electron-laser amplifier. When the amplification conditions were satisfied, strong enhancement of the radiation intensity by a factor of 650 was observed at a wavelength of 61.5 nm. The random and uncontrollable spikes, which appeared in the spectra of the Self-Amplified Spontaneous Emission based FEL radiation without the seeding pulse, were found to be suppressed drastically to form to a narrow-band, single peak profile at the seeding wavelength. The properties of the seeded FEL radiation were well reproduced by numerical simulations.