ERL2013 - List of Authors (Scheibler, H.E.)

Paper

Title

Page

Studies of NEA-photocathodes

22

A.S. Terekhov, V.V. Bakin, D.V. Gorshkov, S.N. Kosolobov, H.E. Scheibler
ISP, Novosibirsk, Russia

Photoemission from p-GaAs(Cs,O) and p-GaN(Cs,O) photocathodes with NEA will be discussed. Photoelectron energy analyzers for determination of longitudinal and transverse energy distributions will be compared. By use of these analyzers, the following results were obtained. Domains of validity for actual models of the (Cs,O) - activation layer of GaAs - photocathode were determined. It was revealed, that the maximal value of QE of p-GaAs(Cs,O) - photocathode is achievable with (Cs,O) – activation layer, which fit with dipole layer model. On the other hand, the maximal value of NEA for this photocathode is achievable with (Cs,O) – layer, which correspond to heterojunction model. Two – step photoelectron escape from NEA-photocathode with their transitional capture within near – surface band bending region is proved. Subsequent escape of photoelectron to the vacuum occurs in the ballistic mode, or is accompanied by the elastic and inelastic scattering processes. Elastic scattering is dominated by the random electric field at the semiconductor – vacuum interface and by the surface roughness. Dominant mechanisms of inelastic scattering of photoelectrons include their interaction with surface optical phonons and with surface plasmons. Both processes of inelastic scattering are accompanied by the scattering of photoelectron's direction.

Slides WG104 [2.346 MB]

WG107

First Measurements of Photoelectron Transverse Energy Distribution Curve using TESS

H.E. Scheibler, D.V. Gorshkov, A.S. Terekhov
ISP, Novosibirsk, Russia
R.J. Cash, B.D. Fell, L.B. Jones, K.J. Middleman, B.L. Militsyn, T.C.Q. Noakes
STFC/DL/ASTeC, Daresbury, Warrington, Cheshire, United Kingdom

Electron injector brightness is limited by the transverse energy spread of the electrons emitted from a photocathode. Knowledge and understanding of the physical mechanisms underpinning this energy spread allows one to predict changes in the beam parameters. Ultimately, this information may help to optimize photocathode design, delivering improvements in photoinjector performance. In order to measure transverse photoelectron energy distribution curves (TEDC) ASTeC, in collaboration with ISP, have developed a Transverse Energy Spread Spectrometer (TESS). This equipment supports photocathode performance measurements both at room and LN2-temperature, under illumination from a range of fixed- and variable-wavelength light sources. TESS also includes a piezo-electric leak valve to allow controllable degradation of the photocathode whilst monitoring the energy spread of emitted electrons. In this work we discuss the principles of TEDC measurements on TESS, and present the first results for a GaAs-photocathode operated at room temperature. The measured TEDCs have exponential shape, and the mean transverse energies were equal to 45 meV for an illumination wavelength of 635 nm, and 10² meV when illuminated at 532 nm.

Slides WG107 [1.536 MB]

Paper	Title	Page
WG104	Studies of NEA-photocathodes	22
	A.S. Terekhov, V.V. Bakin, D.V. Gorshkov, S.N. Kosolobov, H.E. Scheibler ISP, Novosibirsk, Russia
	Photoemission from p-GaAs(Cs,O) and p-GaN(Cs,O) photocathodes with NEA will be discussed. Photoelectron energy analyzers for determination of longitudinal and transverse energy distributions will be compared. By use of these analyzers, the following results were obtained. Domains of validity for actual models of the (Cs,O) - activation layer of GaAs - photocathode were determined. It was revealed, that the maximal value of QE of p-GaAs(Cs,O) - photocathode is achievable with (Cs,O) – activation layer, which fit with dipole layer model. On the other hand, the maximal value of NEA for this photocathode is achievable with (Cs,O) – layer, which correspond to heterojunction model. Two – step photoelectron escape from NEA-photocathode with their transitional capture within near – surface band bending region is proved. Subsequent escape of photoelectron to the vacuum occurs in the ballistic mode, or is accompanied by the elastic and inelastic scattering processes. Elastic scattering is dominated by the random electric field at the semiconductor – vacuum interface and by the surface roughness. Dominant mechanisms of inelastic scattering of photoelectrons include their interaction with surface optical phonons and with surface plasmons. Both processes of inelastic scattering are accompanied by the scattering of photoelectron's direction.
	Slides WG104 [2.346 MB]

WG107	First Measurements of Photoelectron Transverse Energy Distribution Curve using TESS
	H.E. Scheibler, D.V. Gorshkov, A.S. Terekhov ISP, Novosibirsk, Russia R.J. Cash, B.D. Fell, L.B. Jones, K.J. Middleman, B.L. Militsyn, T.C.Q. Noakes STFC/DL/ASTeC, Daresbury, Warrington, Cheshire, United Kingdom
	Electron injector brightness is limited by the transverse energy spread of the electrons emitted from a photocathode. Knowledge and understanding of the physical mechanisms underpinning this energy spread allows one to predict changes in the beam parameters. Ultimately, this information may help to optimize photocathode design, delivering improvements in photoinjector performance. In order to measure transverse photoelectron energy distribution curves (TEDC) ASTeC, in collaboration with ISP, have developed a Transverse Energy Spread Spectrometer (TESS). This equipment supports photocathode performance measurements both at room and LN2-temperature, under illumination from a range of fixed- and variable-wavelength light sources. TESS also includes a piezo-electric leak valve to allow controllable degradation of the photocathode whilst monitoring the energy spread of emitted electrons. In this work we discuss the principles of TEDC measurements on TESS, and present the first results for a GaAs-photocathode operated at room temperature. The measured TEDCs have exponential shape, and the mean transverse energies were equal to 45 meV for an illumination wavelength of 635 nm, and 10² meV when illuminated at 532 nm.
	Slides WG107 [1.536 MB]