Atsushi Mano

Super-High Brightness and High-Spin-Polarization Photocathode

Using a newly developed transmission-type photocathode, an electron beam of super-high brightness [(1.3±0.5)×107 Acm-2sr-1] was achieved. Moreover, the spin-polarization was as high as 90%. We fabricated a transmission-type photocathode based on a GaAs–GaAsP strained superlattice on a GaP substrate in order to enhance the brightness and polarization greatly. In this system, a laser beam is introduced through the transparent GaP substrate. The beam is focused on the superlattice active layer with a short focal length lens. Excited electrons are generated in a small area and extracted from the surface. The shrinkage of the electron generation area improved the brightness. In addition, a GaAs layer was inserted between the GaP substrate and the GaAsP buffer layer to control the strain relaxation process in the GaAsP buffer layer. This design for strain control was key in achieving high polarization (90%) in the transmission-type photocathode.

High brightness and high polarization electron source using transmission photocathode with GaAs-GaAsP superlattice layers

In order to produce a high brightness and high spin polarization electron beam, a pointlike emission mechanism is required for the photocathode of a GaAs polarized electron source. For this purpose, the laser spot size on the photocathode must be minimized, which is realized by changing the direction of the injection laser light from the front side to the back side of the photocathode. Based on this concept, a 20kV gun was constructed with a transmission photocathode including an active layer of a GaAs–GaAsP superlattice layer. This system produces a laser spot diameter as small as 1.3μm for 760–810nm laser wavelength. The brightness of the polarized electron beam was ∼2.0×107Acm−2sr−1, which corresponds to a reduced brightness of ∼1.0×107Am−2sr−1V−1. The peak polarization of 77% was achieved up to now. A charge density lifetime of 1.8×108Ccm−2 was observed for an extracted current of 3μA.

Real Time Magnetic Imaging by Spin-Polarized Low Energy Electron Microscopy with Highly Spin-Polarized and High Brightness Electron Gun

We developed a spin-polarized low energy electron microscopy (SPLEEM) with a highly polarized and high brightness spin electron gun in the present study. Magnetic structures of Co/W(110) were observed with an acquisition time of 0.02 s with a field of view of 6 µm. We carried out a dynamic observation of magnetic structures with the SPLEEM during the growth of Co on W(110).

Thermal emittance measurements for electron beams produced from bulk and superlattice negative electron affinity photocathodes

Extremely low emittance electron beams are required for next generation accelerators. GaAs semiconductor photocathodes with negative electron affinity (NEA) surfaces have an intrinsic advantage for generating such low emittance beams and the thermal emittance as low as 0.1 π mm mrad is expected in ideal case. The thermal emittance of photoelectrons was measured for two different NEA photocathodes: a bulk-GaAs photocathode and a GaAs-GaAsP superlattice strained photocathode. The normalized root-mean-sqare emittances for the beam radius of 1.0 mm were as low as 0.20−0.29±0.02 and 0.15±0.02 π mm mrad, respectively. A comparison of these results shows that the superlattice photocathode minimizes the thermal emittance for photon excitation energies higher than the band gap energy.

Effect of crystal quality on performance of spin-polarized photocathode

GaAs/GaAsP strain-compensated superlattices (SLs) with thickness up to 90-pair were fabricated. Transmission electron microscopy revealed the SLs are of high crystal quality and the introduced strain in SLs layers are fixed in the whole SL layers. With increasing SL pair number, the strain-compensated SLs show a less depolarization than the conventional strained SLs. In spite of the high crystal quality, the strain-compensated SLs also remain slightly depolarized with increasing SL pairs and the decrease in spin-polarization contributes to the spin relaxation time. 24-pair of GaAs/GaAsP strain-compensated SL demonstrates a maximum spin-polarization of 92% with a high quantum efficiency of 1.6%.

High-Performance Spin-Polarized Photocathodes Using a GaAs/GaAsP Strain-Compensated Superlattice

Optimized transmission-type photocathodes with a GaAs/GaAsP strain-compensated superlattice were developed. The strain-compensated superlattice structures were of high crystal quality, and electron beams from the photocathodes had a maximum spin polarization of 92% and a quantum efficiency of 0.4% without an antireflection coating. The strain-compensated superlattice structure effectively prevented strain relaxation, and the high spin polarization was maintained up to a superlattice layer thickness of 300 nm. Increasing the superlattice layer thickness effectively improved the quantum efficiency while keeping the super high-brightness.

Status of the high brightness polarized electron source using transmission photocathode

Recently, we have developed a transmission-type polarized electron sources (PES)s for the generation of a high brightness beam. The developed PES can applied the extraction voltage of 20 kV at 4 mm electrode-gap and enables to realize the source beam radius of a few micro meter. As the results of prototype gun experiments, the brightness of ~2 ? 107 A.cm?2.sr?1 and a charge density lifetime of 1.8 ? 108 C.cm?2 were obtained. A maximum polarization of ~90 % and quantum efficiency of 0.09 % was achieved simultaneously using the transmission photocathode with GaAs-GaAsP strained superlattice layers. Up to now, another two electron gun was manufactured, and those mechanical designs are almost same as that of the prototype. The prototype gun and the second gun have been already operated for the photocathode R&D and for the Spin-LEEM application, respectively. In the near future, further experiments are prepared by using the third gun.

Initial Emittance Measurements for Polarized Electron Gun with NEA‐GaAs Type Photocathode

Extremely low emittance electron beams are necesarry for new generation accelerators. The value of the required emittances is as low as 0.1 π.mm.mrad. NEA‐type photocathodes have an intrinsic advantage for generating such a low emittance beam. In this paper, emittance measuremets of photelectrons extracted from two different NEA phtocahtodes are described. The measurements were carried out using Nagoya University 200kV polarized electron source. The normalized RMS emittances of bulk‐GaAs and GaAs‐GaAsP superlattice strained photocathodes are as low as 0.12–0.17 ± 0.02 π.mm.mrad and 0.09 ± 0.01 π.mm.mrad with very low charge density, respectively.

Production of High Density Polarized Electron Beam from GaAs‐GaAsP Superlattice Photocathode

Future high energy accelerators require polarized electron sources that can generate high bunch charge and/or high density electron beam with low emittance. Various superlattice photocathodes and a high voltage load‐lock DC gun have been developed at Nagoya University for this purpose. Using a GaAs‐GaAsP strained superlattice photocathode, the bunch charge of 8nC was extracted in a 1.6ns bunch with a 20 mm diameter, and that of 3.3pC in ∼30ps bunch with 1.2mm diameter.

Present Status of Accelerators in Aichi Synchrotron Radiation Center

Aichi Synchrotron Radiation Center is a synchrotron radiation facility in operation since 2013. The electron energy of the storage ring is 1.2GeV and the circumference is 72m. In spite of the compact size of the storage ring, synchrotron radiation up to hard X-ray region (∼20 keV) is available from the 5 T superconducting bending magnets. Presently (Apr. 2017), 8 beamlines (5 hard X-ray and 3 soft X-ray) are in operation. In the daily operation, 300mA top-up mode has been performed to realize the beam current stability of better than 0.3%. In addition to the delivery of the synchrotron radiation to the beamlines, R&Ds for accelerator components have been continuously performed; fabrication of a model permanent bending magnet and turn-by-turn beam profile measurement at the beam injection by a pulse sextupole magnet.