Photocathode development for accelerator in Japan
Laser - aided Accelerator Association (LAAA)This report is a summery of discussion in LAAA
(Oct. - Nov. 2006’) about photocathodedevelopment for an accelerator in Japan.

Hiromitsu Tomizawa (JASRI/SPring8),
Tomohiro Nishitani (JAEA),
Harue Sugiyama (Saga Univ.),
Masahiro Yamamoto, Naoto Yamamoto (Nagoya Univ./ SP lab. ),Nobuhiro Terunuma (KEK),
Jun Sasabe (Hamamatsu Photonics)
LAAA November report 2006

About Cesium-Telluride
Cesium-Telluride
1, Wavelength of excitation light2, Transparent type photo-cathodephoto-cathode3, Deterioration of QE
1. Wavelength of excitation light・Differential of QE spectrum around theleading edge ~ 0.5eV(Te=120nm)

・Width of leading edge depends on
crystalline or formation (●, ●)・ Behavior below the gap depends on
thickness <= diffusion effect ?
[Ref] R. A. Powell, W. E. Spicer, G. B. Fisher and P. Gregory: Phys. Rev.B.8 (1973)
H. Sugiyama et al., Proceeding of The 30th Linear Accelerator Meeting in Japan ,pp.99-101(2005)
LAAA November report 2006

2. Transparent type1.

Backward Illuminationwith 3D-shaped laser
pulse
2. Monochromatic energy
by thermalization
process
From Sasabe (Hamatsu Photonics) : GaAs+NEA cathodeThis type of cathode might be realizedwith CsTe (much as PEA)

[ref] J. Sasabe et al., NIM A, 528, pp.382-386, 2004
LAAA November report 2006

3. Deterioration of QE• Q.E. dropping was measured
at all wavelength.
• Peak of Q.E. seems to be
shifted to the shorter wave
length. It suggests theelectron affinity is changing

Effect of Oxidation ?
RF field is impressed@ KEKKEK-ATF[ref] N. Terunuma, http://wwwlasa.mi.infn.it/WSPhotocathodes/proceedings

NEA-GaAs photocathode
NEA-GaAs: A ＧａAs semiconductor with an negative electron affinity surface•High Quantum Efficiency(QE)
Bulk-GaAs :1-4% (870nm @band-gap), 10% (shorter wavelength than 650nm)
Thin-GaAs type polarized electron source : 0.6%(770nm), 1%(740nm)(thickness<500nm)

GaAs-GaAsP

GaAs-AlGaAs•Small Energy Spread
An energy spread of excited electrons by band-gap energy is small(<0.1eV?).
(vacuum level is near the bottom of a conduction band. Excited electrons by band-gap energy
can be extracted to a vacuum through the bottom of a conduction band. )•High Polarization
Possible to generate polarized electrons using circular polarized laser.
•Surface lifetime
An NEA surface is fragile. Extreme high vacuum is essential.

LAAA November report 2006

NEA-GaAs for a polarized electron source
History of photocathode development at Nagoya Univ.
(High QE and High Polarization)¾Straind GaAs (1990~)
Polarization > 50% breakthrough.
[ref] T. Nakanishi, et al., Phys. Lett. A158, 345 (1991)[ref] T.Saka, et al., J. Crystal Growth 124 (1992) p346-351¾GaAs-AlGaAs superlattice (SL) (1990~)
The first SL photocathode with high polarization.
[ref] T. Omori, et al., Phys. Rev. Lett. 67, 3294 (1991)¾InGaAs-GaAs strained layer SL (1994~)
The first SL photocathode with a strained layer
structure.
[ref] T. Omori, et al., Jpn. J. Appl. Phys. 33, 5676 (1994)¾InGaAs-AlGaAs strained layer SL (1996~)
Higher Q.E. than that of InGaAs-GaAs SL.
[ref] T. Nakanishi, et al., AIP Conf. Proc., 421, 300 (1998)

Polarization and QE Performance*T.Nakanishi et al., NIM A 455 (2000) 109.
T.Nishitani, et al., J. Appl. Phys. 97 (2005)¾GaAs-GaAsP strained-layer SL (2000~ )The highest performance photocathode of
all test samples.
[ref] T. Nakanishi, et al., NIM A455(2000)109[ref] T.Saka, et al., Surface Science 454-456(2000)
T.Nishitani, et al., J. Appl. Phys. 97 (2005)

LAAA November report 2006

NEA-GaAs for a polarized electron source
The relationship between QE, polarization and excitation wavelength¾The excitation laser wavelength tuned tothe band-gap energy is suitable for a highpolarization.
¾High polarization (>80%) can achieved byphotocathodes with various band-gap energy.
(1.28-1.65eV / 740-920nm)：InGaAs-GaAs, InGaAs-AlGaAs, GaAs-GaAsP
strained-layer superlattice*T.Nakanishi et al., NIM A 455 (2000) 109.

¾Larger band-gap photocathode is moresuitable for higher QE photocathode.
*T. Nakanishi, et al., AIP Conference Proceedings 421(1998) p. 300-310

The dependence of QE and polarization on excitation wavelength(GaAs-GaAsP strained-layer photocathode)T.Nishitani, et al., J. Appl. Phys. 97 (2005)

LAAA November report 2006

NEA-GaAs for a polarized electron source
Requirements for ILC

Polarized electron beam structure*http://www.linearcollider.org/wiki/¾When QE at the excitation wavelength of 780nm is 0.1%, pulse laser energy is needed
10mJ/micropulse (~30mJ/macropulse) for 6.4nC/bunch .
¾2ns bunch width is compressed less than 50ps by sub harmonic bunching system.
(Timing jitter of several tens ps may be acceptable.)¾Energy jitter should be less than 1% (rms).
(Bunch charge can induce many problems. (energy spread, beam instability,...))LAAA November report 2006

NEA-GaAs for a high brightness electron source
The strategy of higher brightness photocathode than the exist GaAs (JAEA)(High QE-large current- , Small Energy Spread and Long Lifetime)•Higher QE•Longer Surface Lifetime
Larger band-gap energy and smaller electron affinity
JAEA PHOTOCATHODE DCDC-GUN FOR AN ERL INJECTOR
T. Nishitani, R. Hajima,Hajima, H. Iijima,Iijima, R. Nagai, M. Sawamura,Sawamura, N. Kikuzawa,Kikuzawa, N. Nishimori,
Nishimori, E. Minehara
Proceedings of the 28th International FEL Conference, (2006) to be published•Smaller Energy Spread
A superlattice structure- quantum confinement of electron energy state Photocathodes for the energy recovery linacs
T. Rao,Poelker, E. Seddon,Seddon, F.E.
Rao, A. Burrill,Burrill, X.Y. Chang, J. Smedley,Smedley, T. Nishitani,
Nishitani, C. Hernandez Garcia, M. Poelker,Hannon, C.K. Sinclair, J. Lewellen,Lewellen, D. FeldmanNuclear Instruments and Methods in Physics Research A 557, pp.124
pp.124–130, (2006)

LAAA November report 2006

NEA-GaAs for a high brightness electron source
Development of a bulk-AlGaAs (JAEA & Takeda Lab.-Nagoya Univ.-)

High QELong Surface Lifetime*QE spectrum of a bulk-GaAs and a bulk-AlGaAs sample.
*These samples were fabricated by
Molecular Beam Epitaxy.
(Takeda lab. and Venture BuissinessLabolaty, Nagoya University.)

The QE of bulkbulkAlGaAs is 2~8%.
(Two times higher thanthat of the bulkbulk-GaAs.)*the band-gap energy to 0.2eVabove the band-gap energy

JAEA PHOTOCATHODE DCDC-GUN FOR AN ERL INJECTOR
T. Nishitani, R. Hajima,Hajima, H. Iijima,Iijima, R. Nagai, M. Sawamura,Sawamura, N. Kikuzawa,
Kikuzawa, N. Nishimori,
Nishimori, E. Minehara
Proceedings of the 28th International FEL Conference, (2006) to be published*Lifetime of bulk-GaAs and bulk-AlGaAs sample,
where laser photon energy was chosen at 0.1 eV above
the band-gap for each sample

The bulkbulk-AlGaAs has a longer
lifetime of NEA surface than thebulkbulk-GaAs.
*These results agree with ourprediction that smaller electron affinity
is suitable for longer NEA life.

LAAA November report 2006

NEA-GaAs for a high brightness electron source
Excitation laser ?

¾The excitation laser wavelength tuned tothe band-gap energy is suitable for a smallelectron beam energy spread.
¾Higher QE than that of the exist GaAs canachieved by bulk-AlGaAs with various bandgap energy. (1.4-1.8eV / 690-870nm)

Possible bandband-gap range of Ⅲ-Ⅴ
semiconductor is 0.40.4-3.5eV.
(several μm - 350nm )(JAEA & Takeda Lab.-Nagoya Univ.-)¾Larger band-gap photocathode is moresuitable for higher QE photocathode.

Possible large current dependon tradetrade-off between QE andlaser power.

LAAA November report 2006