Kenji Fukami

Extreme ultraviolet free electron laser seeded with high-order harmonic of Ti:sapphire laser

The 13th harmonic of a Ti:sapphire (Ti:S) laser in the plateau region was injected as a seeding source to a 250-MeV free-electron-laser (FEL) amplifier. When the amplification conditions were fulfilled, strong enhancement of the radiation intensity by a factor of 650 was observed. The random and uncontrollable spikes, which appeared in the spectra of the Self-Amplified Spontaneous Emission (SASE) based FEL radiation without the seeding source, were found to be suppressed drastically to form to a narrow-band, single peak profile at 61.2 nm. The properties of the seeded FEL radiation were well reproduced by numerical simulations. We discuss the future precept of the seeded FEL scheme to the shorter wavelength region.

Pulse-by-Pulse Multi-XFEL Beamline Operation with Ultra-Short Laser Pulses

The parallel operation of multiple beamlines is an important issue to expand the opportunity of user experiments for linear accelerator based FELs. At SACLA, the parallel operation of three beamlines, BL1~3, has been open to user experiments since September 2017. BL1 is a soft x-ray beamline driven by a dedicated accelerator, which is a former SCSS test accelerator, and BL2 and BL3 are XFEL beamlines sharing the electron beam from the SACLA main accelerator. In the parallel operation, a kicker magnet with 10 ppm stability (peak-to-peak) switches the two XFEL beamlines pulse by pulse at 60 Hz. To ensure wide spectral tunability and optimize the laser performance, the beam energy and the electron bunch length are independently adjusted for the two XFEL beamlines according to user experiments. Since the electron bunch of SACLA has typically 15 fs (FWHM) in length and its peak current exceeds 10 kA, CSR effect at a dogleg beam transport to BL2 is quite significant. In order to suppress the CSR effect, an isochronous and achromatic beam optics based on two DBA structures was introduced. The parallel operation of the three FEL beamlines substantially increases user time at SACLA.

Magnet Development for SPring-8 Upgrade

Major upgrade of SPring-8 is being planned, aiming at next generation light source [1, 2]. One of features for newly designed magnets for the upgrade is a permanent magnet based dipole magnet for substantial energy saving [3, 4]. The new dipole magnets have been designed to be equipped with (i) a field variable function by mechanically controlling magnetic flux into a beam axis, (ii) a nose structure on iron poles for smooth magnetic field transition in its longitudinal gradient field, and (iii) a nearly zero temperature coefficient by specifically designed magnetic circuits. Demagnetization due to radiation is also a critical issue and has been studied. Although electromagnet based multi-pole magnets are rather conventional technologies, yet the new magnets need to be designed to fit in a high packing factor lattice. Magnet alignment is as well a key issue in order to secure adequate dynamic apertures. Current designs and recent progress in the magnet developments are presented. PERMANENT MAGNET BASED DIPOLE MAGNETS One of underlying concepts for the SPring-8 upgrade is to achieve significantly higher brilliance with lower energy consumption [2]. For that, permanent magnet (PM) based dipole magnets have been developed. It would be expected to save a fraction of Megawatt power, or even more. PMs have been occasionally used for accelerators for a variety of purposes, but still several concerns have been pointed out when one discusses reliability, stability, or usability of such magnets [3-5]: 1. Magnetic field adjustability 2. Temperature dependence of magnet materials 3. Demagnetization Other details, such as field quality, fringe field, specific field distributions like so-called longitudinal gradient field, and manufacturing cost, should also be verified in order to build a robust and reliable accelerator. At SPring-8, we have investigated above issues through research-and-development (R&D) to make sure of a feasibility of PM for SPring-8-II. In the paper, we summarize the progress in the R&D. Magnetic Field Adjustability Remotely controlled magnetic field adjustability may not be necessary, especially for dipole magnets. However, some amount of field adjustability is still demanded, because it could help adjust the magnetic field in an initial tuning of magnets, and compensate for demagnetization in a long time range. We have proposed and demonstrated a field adjustable magnetic circuit, in which a portion of magnetic flux generated by PM was intentionally leaked out of a closed loop for beam axis. Hence a flux density on a beam axis can be adjusted by changing the portion [3]. In the case, the dynamic range of tuning can be tens of per cent. Temperature Dependence Magnetic flux generated by PM, ΦPM , changes as temperature is drifted by ΔT . The ratio, kPM , is in general negative, i.e., the magnetic flux density electrons experience decreases as temperature increases. When some of flux, Φshunt , is shunted in a magnetic circuit, the flux on a magnetic gap, Φgap , is expressed as

Magnetic-field Variable Permanent Dipole Magnet for Future Light Sources

A permanent dipole magnet with variable magnetic field has been designed, fabricated, and tested at SPring-8. Permanent magnet can be advantageous over electromagnet in terms of power consumption, stability and reliability etc. One of critical issues to apply permanent magnets to future light sources and other accelerators is that the magnetic field should be somehow tuned. In designing future light sources, combined-function or longitudinal gradient magnet may play a key role in achieving extremely small emittance. Therefore, it may not be appropriate to change a gap for changing the field. We have proposed an alternative way to tune the magnetic field of permanent magnet by using outer plates, and the performance has been investigated.