K. Wada

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.

Fabrication of ultra-clean copper surface to minimize field emission dark currents

Abstract The current from copper electrodes treated by four different types of surface cleaning procedures was measured under DC high field gradient condition. The best results were obtained by using the electrode rinsed with ultra-pure water after diamond turning. A field gradient of 47xa0MV/m was achieved with dark current at the level of 1xa0nA, and the microscopic field enhancement factor was estimated to be a very low value of 56. The dark current from this electrode was dependent only on the field gradient at the cathode and not affected by the total voltage applied to the gap. In this case the surface would be covered by a Cu2O layer, which creates few secondary ions by the electron bombardment. On the other hand, a large total voltage effect and a large vacuum increase were observed for the electro-polished electrode. Cu(OH)2, which would be formed at the copper surface during the electro-polishing process, would emit H2O molecules during the electron bombardment.

Production of polarized electron beam with sub-nanosecond multi-bunch structure from superlattice photocathode

AbstractPolarized electron beam with sub-nanosecond multi-bunch structure has been rst produced from an In-GaAs}AlGaAs strained-layer superlattice photocathode with heavy surface doping. The extracted charge of0.6]1010e~/bunch is determined by the space charge limit of the 50keV gun. The multi-bunch e!ect due to the surfacechargelimitphenomenon,whichispeculiartoanegativeelectrona

Polarized electron source for a linear collider in Japan

nitysurfaceofsemiconductorphotocathode,isnotobserved in the second bunch separated by 2.8ns. We conclude that a superlattice photocathode with a heavily dopedsurface is the best photocathode for the future linear colliders. ( 2000 Elsevier Science B.V. All rights reserved. 1. IntroductionPolarized electron source based on semiconduc-torphotocathodeis expectedto play a very impor-tant role in future linear colliders [1,2]. In order toachieve a high luminosity of &5]1033cm~2s~1,about 80 micro-bunches with a separation time of2.8nsmustbeproducedat150HzrepetitionrateatJapan Linear Collider (JLC). Each bunch mustcontainmorethan2]1010e~(each)()1ns width)at the source [3].The generation of such a high-intensity multi-bunch beam is not easy due to a surface chargelimit (SCL) phenomenon [4,5]. This phenomenonis peculiar to a negative electron a

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

nity (NEA)surface, which is a key technique for the semicon-ductorphotocathodestoextracttheopticallyexcit-edelectronsintheconductionbandtothevacuum.When a photocathode is illuminated with a high-intensity laser light, the electrons which cannotescape to the vacuum are trapped at the surfaceband bending region. The accumulated surfacecharge prevents electrons in the later portion ofthe pulse from escaping to the vacuum.In order to overcome the SCL problem, weinvestigated the multi-bunch generation using the70keVpolarizedelectronsourcesystemat NagoyaUniversity. It was demonstrated that a super-lattice photocathode (for example, GaAs}AlGaAs

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

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.

200 keV Polarized Electron Source at Nagoya University

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 200 keV polarized electron gun

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.

Basic R&D Studies for Lower Emittance Polarized Electron Guns

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.

Reduction of field emission current from stainless steel and copper surface

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.