CsBr photocathode at 257nm: A rugged high current density electron source

Phys. Rev. ST Accel. Beams

Liu

Dowell

et al. 2008

Self Cite

The linac coherent light source (LCLS), an x-ray free-electron laser project presently under construction at SLAC, uses a 2.856 GHz rf photocathode gun with a copper cathode for its electron source. While the copper cathode is performing well for the LCLS project, a cathode material with higher quantum efficiency would reduce the drive laser requirements and allow a greater range of operating conditions. Therefore a robust CsBr=Cu photocathode with greater than 50 times the quantum yield at 257 nm relative to the present LCLS copper cathode has been investigated. Preliminary experiments using a dedicated electron source development test stand at SLAC/SSRL are encouraging and are presented in this paper.

Section: Resultsmentioning

confidence: 97%

Section: Introductionmentioning

confidence: 99%

RobustCsBr/Cuphotocathodes for the linac coherent light source

Phys. Rev. ST Accel. Beams

Liu

Dowell

et al. 2008

Self Cite

“…Even though it has relatively low quantum efficiency, it is robust-surviving even exposure to air. It is also unusual because photons with energy less than the 7.3 eV band gap can produce emission [4,5]. To do so, UV photons must first generate color centers that serve as intragap states.…”

Section: Introductionmentioning

confidence: 99%

High gradient rf gun studies of CsBr photocathodes

Vecchione

Phys. Rev. ST Accel. Beams

Gierman

et al. 2015

CsBr photocathodes have 10 times higher quantum efficiency with only 3 times larger intrinsic transverse emittance than copper. They are robust and can withstand 80 MV=m fields without breaking down or emitting dark current. They can operate in 2 × 10 −9 torr vacuum and survive exposure to air. They are well suited for generating high pulse charge in rf guns without a photocathode transfer system.

“…CsBr coatings on photocathodes have been extensively studied [13][14][15][16][17][18] and have desirable properties. They greatly enhance the quantum yield of the emitter by reducing the effective work function and are robust and capable of operation under the demanding conditions of ultra bright FEL sources.…”

mentioning

confidence: 99%

“…15,16 For diamondoids, the monolayer acts as an energy filter. The diamondoids exhibit an unusually short mean free path (MFP) and negative electron affinity (NEA).…”

mentioning

confidence: 99%

Photocathode device using diamondoid and cesium bromide films

Clay

Applied Physics Letters

Pianetta

et al. 2012

Self Cite

A photocathode structure is presented that shows promise for use in high brightness electron sources. The structure consists of a metal substrate, a monolayer of a diamondoid derivative, and a thin film of cesium bromide. Diamondoid monolayers reduce the energy spread of electron emitters, while cesium bromide increases the yield and stability of cathodes. We demonstrate that the combined structure retains these properties, producing an emitter with lower energy spread than the corresponding cesium bromide emitter (1.06 eV versus 1.45 eV) and higher yield and stability than un-coated diamondoid emitters. 3 An ideal emitter for these devices would have high intensity, small physical size, good stability under operating conditions, and low energy spread. One method for achieving high yield and small physical size is to use a focused laser beam to create photoelectrons. However, performance of such devices is limited by the quantum yield and the energy spread. In this work, we present a photocathode structure that improves on the quantum yield and reduces the energy spread of laser photoemission devices without sacrificing the other desirable properties. This device consists of a monolayer of diamondoid combined with a thin film of cesium bromide.Diamondoids are hydrocarbon molecules with the same carbon-lattice structure as bulk diamond. They inherit many of the superior properties of diamond 4,5 and possess emergent properties relating to their small size and high surface area ratio. [6][7][8][9][10][11][12] One unique property of these diamondoids is their behavior in photoemission devices. A monolayer of diamondoids on a metal substrate can largely monochromatize the photoemission, with the majority of the electrons emitting in a single peak with a full-width half-max (FWHM) of around 0.3 eV.6,7 This low energy spread is ideal for many cathode applications, but the diamondoid monolayers that have been studied are not stable enough for use in most devices. In order to correct this problem, we have added cesium bromide as a protective over-layer to provide mechanical stability and protection.Cesium bromide provides more than just protection, however. CsBr coatings on photocathodes have been extensively studied [13][14][15][16][17][18] and have desirable properties. They greatly enhance the quantum yield of the emitter by reducing the effective work function and are robust and capable of operation under the demanding conditions of ultra bright FEL sources. 19 Our objective is to combine these attractive properties with the reduced energy spread of diamondoids by creating a device that contains both films using a thiolated diamondoid to create a self-assembled monolayer (SAM).We tested these devices using ultra-violet (UV) laser photoemission, as shown in Figure 1. We measured the quantum yield, the energy spread, and lifetime of devices with and without the diamondoid present with varying thicknesses of CsBr. Through these investigations, we demonstrate that the high quantum yield of the CsBr emitter is preserved ...