Summary of recent photocathode studies - DESY - FLASH · PDF fileTransport Box • UHV Vacuum System - base pressure 10-10 ... DESY FLASH seminar, ... • fitted by sin² to evaluate

Summary of recent photocathode studies

S. Lederer, S. SchreiberDESY

L. Monaco, D. SertoreINFN Milano – LASA

FLASH seminar November 17 th, 2009

S. Lederer, DESY FLASH seminar, November 17th, 2009

Outlook

• Cs2Te photocathodes• Pulsed QE measurements

– laser transmission– measurements

• Cathode life time• Dark current investigations• Summary and conclusion


Cs2Te photocathodes

Cs2Te photocathodes for FLASH prepared at INFN-Milano, LASA, Italy

Preparation Chamber @ LASA

Transport Box • UHV Vacuum System - base pressure 10 -10

mbar• 6 sources slot available• Te sources out of 99.9999 % pure element• Cs sources from SAES ®

• High pressure Hg lamp and interference filter for online monitoring of QE during production

• Masking system• 5 x UHV transport box

After preparation transport to FLASH or PITZ under UHV conditions


Cs2Te photocathodes

band gap EG

CB

VB

vacuum level

electron affinityEA

Cs2Te photocathodes• semiconductor• band gap EG = 3.3 eV• positive electron affinity EA = 0.2 eV

+ ability to release high peak current electron bunches+ high QE+ stabe under RF operation+ response time ~ 1 ps

- need UHV (<10-9 mbar)- UV laser needed

For DESY:active area 5 mmthickness controlled by Te amount (no co-evaporation), standard 10 nm Te


12000 14000 16000 18000 20000 22000 240000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

12000 14000 16000 18000 20000 22000 2400036

38

40

42

12000 14000 16000 18000 20000 22000 240000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

data sin 2 fit

laser data

Ela

ser

( µµ µµJ)

attenuator

calculated transmissionmean: 37.3568 %

trans

mis

sion

attenuator

data sin 2 fit

tunnel data iris 3 mm

E (

µµ µµJ)

attenuator

QE measurements

47.1 %2009-07-133 mm

33.5 %2009-07-132 mm

16.0 %2009-07-131 mm

48.35 %2009-04-273 mm

37.7 %2009-04-272 mm

18.04 %2009-04-271 mm

37.6 %2009-03-153 mm

26.4 %2009-03-152 mm

12.0 %2009-03-151 mm

21.7 %2009-01-173 mm

12.39 %2009-01-172 mm

5.4 %2009-01-171mm

transmissiondateiris

For the pulsed QE measurements the laser energy at the cathode has to be determined• laser energy measured with joulemeter (Molectron J-5)• laser energy measured on laser table and in laser h ut as function of the attenuator• transmission of view port (92 %) taken into account• reflectivity of vacuum mirror (90 %) accounted for• fitted by sin² to evaluate transmission (half-wave plate/polarizer attenuator)

iris 3 mm2009-03-15


0 0.05 0.1 0.15 0.2 0.25 0.30

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

Laser Energy (uJ)

Cha

rge

(nC

)

QE measurements

example of QE measurement at FLASHcathode #77.2 2009-04-27

QE = 9.2 %

Pfor = 3.8 MW phase 38 deg w.r.t. zero crossing , iris = 2 mm

[ ] [ ] [ ][ ]

[ ] [ ][ ]JE

nCQQE

JE

eVECQQE

n

nQE

cath

cath

ph

ph

el

µ5.0%

100%

≈

=

=

@ 262 nm space chargeeffect

charge trend at low charge fitted

QE measurement• charge measured with toroid T1• laser energy measured in laser hut• automated procedure for measuring charge vs. laser energy dependence


QE measurements

Charge versus laser energy obtained for different laser diameters at the cathode,accelerating field constant.

The slope of the linear part in the low energy region is independent from the spot size since here the emission is not effected by space charge.

In the space charge affected regime the extracted charge depends on the laser spot size. cathode #77.2

2009-03-15


QE – vs. field

( ) ( )m

eeAG

EqqEEhAQE

⋅⋅⋅⋅⋅⋅++−⋅=επ

φβν4

sin

From the fit of QE versus electric field at the cathode one gets information about the work function and the geometric enhancement factor.

EG+EA = 3.5 eVββββ = 4.7QE @ zero field = 11.2 %

RF data analysis – QE enhancement– QE @ given acc. gradient E and phase φφφφ– with a given laser energy without space charge

where E is the accelerating field, φφφφ is the phase RF/laser, ββββ is the geometric enhancing factor

cathode #77.22008-01-17


QE – vs. field

cathode #77.2 2009-01-17QE @ zero gradient = 11.2 %

W = EG+EA = 3.5 eVββββ = 4.7

cathode #77.2 2009-03-15QE @ zero gradient = 4.5 %

W = EG+EA = 3.8 eVβ= 12.7

56 days of operation

→ QE decreased→ EG+EA increased→ field enhancement increased

measurements impossible


Cathode life time

0 20 40 60 80 100 120 140 160 1800

4

8

12

16

20

24 cathode #13.4 cathode #77.2

QE

(%

)

days of operation

For the actual operation mode of FLASH, there are NO life time problems.Cathode changes have not been motivated by low QE!Since summer 2008 only 3 cathodes have been used

(#13.4, #77.2, and #53.2)!

open question: life time for long pulse train operation


QE maps

Laser with smallest possible spot size (0.26 mm) is moved over the cathode and the extracted charge is measured with the toroid T1. The aim of this studies is to get an idea of how homogeneously the charge is extracted from the cathode.In addition the QE map is used to center the laser on the cathode for operation.

QE maps before and after realignment of the laser to the cathode. Red circle represents the 5 mm photo emissive Cs2Te film.

Cathode #13.4


QE maps

Cathode #77.2 2009-01-17, P for = 3.4 MW Cathode #77.2 2009-03-13, P for = 3.85 MW

Cathode #77.2 2009-03-15, P for = 1.4 MW Cathode #77.2 2009-03-15, P for = 0.9 MW

No significant differences for lower gradients


Optical inspection

before usage (2009-01-17) after usage (2009-06-08)

cathode #77.2

2009-07-13


Pfor=3.94 MWrf length 400 µµµµssolenoid 305 Abucking -22.8 A

dc level 500 µµµµA







cathode #77.2

cathode #53.2

In general dc hard to measure!! The hot spots for #53.2 go from left to right while increasing the solenoid field

Dark current


Dark current

• Dark current constantly high and only acceptable if no problem with the cathode

• In addition caused by ageing of the gun full scans have been impossible (vacuum interlocks)

2.0 2.5 3.0 3.5 4.00.0

0.1

0.2

0.3

0.4

0.5

Mo: #90.2 2009-01-16

Cs2Te:

#13.4 2008-10-02 #13.4 2009-01-15 #77.2 2009-01-17 #77.2 2009-03-13

dark

cur

rent

(m

A)

RF power (MW)

200 220 240 260 280 300 320 340 360 3800.14

0.16

0.18

0.20

0.22

0.24

0.26

0.28

0.30

0.32

0.34

dark

curr

ent (

mA

)

main solenoid current (A)

Dark current vs. main solenoid currentMax. dark current from solenoid

scan vs. RF power


Dark current

2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.00.0

0.1

0.2

0.3

0.4

0.5

dark

cur

rent

(m

A)

RF power (MW)

FLASH gun#2

PITZ gun #4.2typical dark current with Cs2Te cathode (#128.1)

Gun 4.2 will be mounted at FLASH January 2010


Summary and Conclusion

• Results from last beam times presented– Laser beam line transmission (from January until Ju ly 2009)– pulsed QE

• Measurements and analysis• Evolution over time• Life time

– QE maps

• More studies needed for further understanding the Q E behaviour under influence of RF field

• More studies needed after long pulse train operatio n

• Dark current issues– dc images– dc vs. main solenoid scans– dc for different RF power

Documents

Summary of recent photocathode studies - DESY - FLASH · PDF fileTransport Box • UHV Vacuum System - base pressure 10-10 ... DESY FLASH seminar, ... • fitted by sin² to evaluate