F. Furuta

Surface charge limit in NEA superlattice photocathodes of polarized electron source

The “surface charge limit (SCL)” phenomenon in negative electron affinity (NEA) photocathodes with GaAs–AlGaAs superlattice and InGaAs–AlGaAs strained-layer superlattice structures has been investigated systematically using a 70 keV polarized electron gun and a nanosecond multi-bunch laser. The space-charge-limited beam with multi-bunch structure (1.6 A peak current, 12 ns bunch width and 15 or 25 ns bunch separation) could be produced from the superlattice photocathodes without suffering the SCL phenomenon. From the experimental results, it has been confirmed that the SCL phenomenon is governed by two physical mechanisms at the NEA surface region, the tunneling of conduction electrons against the surface potential barrier (escaping process) and that of valence holes against the surface band bending barrier (recombination process); these effects can be enhanced using the superlattice structure and heavy p-doping at the surface, respectively. We conclude that a superlattice with heavily p-doped surface is the best photocathode for producing the multi-bunch electron beam required for future linear colliders.

Highly polarized electrons from GaAs-GaAsP and InGaAs-AlGaAs strained-layer superlattice photocathodes

GaAs–GaAsP and InGaAs–AlGaAs strained-layer superlattice photocathodes are presented as emission sources for highly polarized electron beams. The GaAs–GaAsP cathode achieved a maximum polarization of 92(±6)% with a quantum efficiency of 0.5%, while the InGaAs–AlGaAs cathode provides a higher quantum efficiency (0.7%) but a lower polarization [77(±5)%]. Criteria for achieving high polarization using superlattice photocathodes are discussed based on experimental spin-resolved quantum efficiency spectra.

Polarized electron source for a linear collider in Japan

Abstract The research to develop the polarized electron source required by future linear colliders has been conducted by our collaboration. Recent advances in settling three difficult problems to realize the high polarization and high quantum efficiency, the long cathode lifetime and the multi-bunch beam generation are briefly described in this article.

Atomic hydrogen cleaning of GaAs photocathode with a load-lock system

We are constructing a new polarized electron source for Japan Linear Collider. It is designed to operate the gun at 200 kV. The “load-lock” mechanism is employed to avoid the dark current due to the Cs accumulation on the electrodes and to exchange the activated NEA photocathode quickly. We have developed superlattice photocathode which has advantages of high spin polarization, high quantum efficiency and high resistance against surface charge limit phenomenon. However, it seems difficult to clean the surface of such a thin layer photocathode by the normal etching procedure without destruction of its delicate structure if it has no As caplayer. Atomic hydrogen is expected to clean the surface of superlattice effectively. We have introduced these techniques to the new source design.

Development of spin polarized electron photocathodes: GaAs-GaAsP superlattice and GaAs-AlGaAs superlattice with DBR

We have tested two kinds of spin-polarized electron photocathodes, the GaAs-GaAsP strained layer superlattice and the GaAs-AlGaAs superlattice with distributed Bragg reflector. The experimental results of these photocathodes are briefly reported.

HIGH FIELD GRADIENT POLARIZED ELECTRON GUN FOR ILC

A 200-keV gun has been developed for generation of bunch charge of 2 3.2 nC, bunch length of 5 2 ns multi-bunch polarized electron beam that is required for International Linear Collider. In this paper, the beam simulations using General Particle Tracer (GPT) code for such space-charge dominated regions, and a fabrication method of electrode for higher electric field (2 3 MV/m) at the photocathode surface by using a titanium anode and a molybdenum cathode are described.

200 keV Polarized Electron Source at Nagoya University

200 keV polarized electron source with load‐lock system has been constructed to produce a beam with high peak current and low emittance that are required by a future linear collider. GaAs photocathode was cleaned by atomic hydrogen and dark currents between the electrodes of the gun that degrade an NEA (Negative Electron Affinity) surface of photocathode could be reduced to less than 1 nA at 200 kV. Recent data on photocathode preparation and dark current measurement are reported in this paper.

Polarized electron source for Japan Linear Collider

Our collaboration group has been conducting the R&D works on the polarized electron source for Japan Linear Collider. The experimental results to produce the high-intensity multi-bunch beam from the superlattice photocathodes are summarized in this report.

Development of 200 keV polarized electron gun

A 200 keV polarized electron gun system with a load lock mechanism has been developed to produce the high-intensity and low-emittance beam required by Japan Linear Collider. The construction of this system has been completed and the performance tests are in progress. Up to now, DC high voltage of 150 kV could be successfully applied to the accelerating electrodes with an extremely low dark current of <0.1 nA.

RECENT PES PHOTOCATHODE R&D AT NAGOYA UNIVERSITY

The strained-layer superlattice structures have been exhibiting the most promising performance as a photocathode for the polarized electron source (PES). In our experiments, the GaAs-GaAsP photocathode achieved maximum polarization of 92f6% with quantum efficiency of 0.5%, while the InGaAsAlGaAs photocathode provided higher quantum efficiency (0.7%) with lower polarization (77f5%). Criteria for achieving high spin polarization and high quantum efficiency using superlattice photocathodes were clarified by employing the spin-resolved quantum efficiency spectra. However, it seems that major problems still remained for the PES R&D are to improve (1) beam emittance and (2) NEA lifetime under gun operations for high peak current and high average current, respectively. In order to overcome these problems simultaneously, we started a development of a new type photocathode using field emission mechanism. First, we tried to use a pyramidal shape GaAs (tipGaAs). Using the tip-GaAs, electrons can be emitted from a small area at the top of pyramid, and thus the beam emittance is expected to be small. This emission mechanism also enables to extract electrons from the poor NEA or small PEA surface into vacuum, and it helps to relax the NEA lifetime problem. Preliminary results were already obtained.