Operation of Cs Te photocathodes at FLASH and PITZ, DESY

Transcription

Operation of Cs2Te photocathodes at
FLASH and PITZ, DESY preparation
system.
Sven Lederer, Klaus Floettmann, Siegfried Schreiber
Deutsches Elektronen-Synchrotron
01.03.2011
EUROFEL Workshop on
Photocathodes for RF guns
Lecce, Italy
Involved persons – thanks to
> DESY
Ingo Hansen
Hans-Hinrich Sahling
> INFN – Milano, LASA
Paolo Michelato
Laura Monaco
Daniele Sertore
Sven Lederer | EUROFEL Workshop on Photocathodes for RF guns | 01-Mar-2011, Lecce | Slide 2
Outline
> DESY requirements on photocathodes
> General remarks on Cs2Te
photocathodes
> Operation of Cs2Te cathodes
QE measurements
QE-maps
Dark current
> DESY preparation System
> Off-line analysis tools
XPS
EDX
SAFEM
Machine requirements on cathodes
FLASH requires
• Design:
7200 bunches in a bunch train
with 10 Hz
(9 MHz@1 nC per bunch = 9 mA)
PITZ requires
•
•
•
•
1 nC bunches at (1 MHz)
flat top laser pulses 2/20\2 ps
RF: 60 MV/m max gradient at cathode
Stronger usage of cathodes than FLASH
• Operated:
up to 2400 bunches (3 MHz/3 nC/9 mA)
• FEL operation:
up to 800 bunches (1 MHz and lower/0.11 nC (<1 mA)
• Rep.rate 5 or 10 Hz
→ up to 72 μC/sec
→ cathode should be able to deliver
about 1000 C in 6 months
→ currently about 1 C in 6 month during
user operation
European XFEL requires
•
•
•
•
•
Bunch charge up to 1 nC
4.5 MHz, 2900 Bunches
10 Hz repetition rate
Flat top laser pulse ~3/20\3 ps
RF: ~50 MV/m gradient at cathode
• RF gun:
46 MV/m max gradient at cathode
Cs2Te photocathodes
+ Large band gap semiconductor
+ High quantum efficiency
+ Short response time, no tails in the emission (positive affinity
cathode → ps time scale)
+ Capability to produce high current density
+ Survives in high electric field area
+ Long lifetime
− requires UV laser
− requires UHV vacuum conditions
− sensitive to contamination
Up to now no limitations in machine operation
caused by photocathodes issues
Operation of Cs2Te cathodes: QE - cw measurements
> Measurements of the spectral response of the Cs2Te cathodes in the transport box
> set-up (comparable to preparation systems)
Hg lamp
band pass filter for wavelength/photon energy selection
photodiode to determine light power
some optics for focusing
Pico ampere meter to measure current of emitted electrons
Operation of Cs2Te cathodes: QE - cw measurements
QE = A1 (hν − W1 ) 1 + A2 (hν − W2 )
m
m2
with Wi = EGi + E Ai
W: work function, A proportional constant, m: parameter giving some
insights into the emission process
10
10
cathode #91.1, unused
W1 = 1.8 eV
W2 = 3.7 eV
QE @ 254 nm = 4.6 %
1
QE [%]
QE [%]
1
cathode #123.1, unused
0.1
0.01
W1 = 2.4 eV
W2 = 3.7 eV
QE @ 254 nm = 3.3 %
0.1
0.01
1E-3
2.5
3.0
3.5
4.0
photon energy [eV]
4.5
5.0
1E-3
2.5
3.0
3.5
4.0
photon energy [eV]
4.5
5.0
Operation of Cs2Te cathodes: QE measurements
> QE measurement
> Charge measured downstream RF
Gun with a calibrated toroid
> Laser energy measured (FLASH in
laser hut, PITZ in tunnel) with
calibrated transmission
> Automated procedure scanning laser
energy and measure charge
QE [% ] = 100
Standard RF Gun setings:
Pfor = 3.8 MW, phase 38 dg w.r.t. zero
crossing, laser spot diam. = 2 mm
2
QE = 9.2 %
1.8
1.6
1.4
Q[C ]E ph [eV ]
Ecath [J ]
Charge (nC)
n
QE = el
n ph
example of QE measurement at FLASH
cathode #77.2 2009-04-27
1.2
1
0.8
0.6
space charge
effect
0.4
charge trend at low
charge fitted
0.2
0
0
0.05
0.1
0.15
Laser Energy (uJ)
0.2
0.25
0.3
Operation of Cs2Te cathodes: QE measurements
Charge versus laser energy obtained for different laser diameters at the cathode,
accelerating field constant.
In the space charge affected regime
the extracted charge depends on the
laser spot size.
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.
cathode #77.2
2009-03-15
Operation of Cs2Te cathodes: QE – vs. field
RF data analysis – QE enhancement
– QE @ given acc. gradient E and phase φ
– with a given laser energy without space charge
⎡
qe ⋅ β ⋅ E ⋅ sin (φ ) ⎤
QE = A ⋅ ⎢hν − (EG + E A ) + qe ⋅
⎥
4
π
ε
⋅
⋅
0
⎦
⎣
m
where E is the accelerating field, φ is the phase RF/laser, β is factor covering
all effects by the electrical field
From the fit of QE versus electric field at the
cathode one gets information about the work
function and the geometric enhancement
factor (covering all possible effect of the high
electric field onto the emission).
EG+EA = 3.5 eV
β = 4.7
QE @ zero field = 11.2 %
cathode #77.2
2008-01-17
Example from FLASH
cathode #77.2
QE @ zero gradient = 11.2 %
W = EG+EA = 3.5 eV
β = 4.7
56 days of operation
cathode #77.2
W = EG+EA = 3.8 eV
β= 12.7
→ QE decreased
→ EG+EA increased
→ field enhancement increased
Example from PITZ
10.00
cathode #128.1
W = EG+EA = 3.9 eV
β = 1.9
QE [%]
8.00
6.00
42 days of operation
4.00
cathode #128.1
W = EG+EA = 4.1 eV
β = 2.6
Jan 2009 data
Jan 2009 analysis
Mar 2009 data
Mar 2009 analysis
2.00
0.00
0
10
20
30
40
→ QE decreased
→ EG+EA increased
50 field enhancement increased
→
Eacc [MV/m]
Operation of Cs2Te cathodes: Cathode lifetime
24
cathode #77.2
fit
cathode #13.4
20
QE [%]
16
12
8
4
0
0
20
40
60
80
100
120
140
160
180
days of operation
Exponential fit yields decay time of 144 days for cathode #77.2
Operation of Cs2Te cathodes: Cathode lifetime
2005
180
2007
2009
2008
initial cw QE
final cw QE
160
20
18
16
140
14
120
100
10
53
.2
77
.2
0
13
.4
0
91
.1
2
21
.3
20
92
.1
10
8.
1
4
94
.1
40
95
.1
6
13
.3
60
78
.1
8
77
.1
80
QE [%]
12
73
.1
days of operation
2006
cathode
Statistics for cathode usage and cw QE before and after usage for FLASH
Operation of Cs2Te cathodes: QE maps
Cathode #77.2 2009-01-17, Pfor = 3.4 MW
Cathode #77.2 2009-03-15, Pfor = 1.4 MW
Cathode #77.2 2009-03-13, Pfor = 3.85 MW
Cathode #77.2 2009-03-15, Pfor = 0.9 MW
No significant differences for lower gradients
Operation of Cs2Te cathodes: QE maps
FLASH, cathode #13.4, Pfor 3.8 MW
PITZ, cathode #128.1, Pfor 7 MW
1.8
1
12
3
1.6
0
2
10
1
8
0
6
−1
4
y position [mm]
1.2
1
−2
0.8
−3
y position [mm]
1.4
−1
0.6
−4
0.4
−5
0.2
−2
−1
0
1
2
x position [mm]
3
4
0
−2
2
−3
0
−3
−2
−1
0
1
2
3
x position [mm]
At FLASH the cathode was operated for more than 150 days, the one at PITZ only 42 days.
Nevertheless the PITZ cathode is more inhomogeneous.
This behavior is still not understood.
Operation of Cs2Te cathodes
FLASH
> For the operation modes of FLASH 2008/2009, there were NO life time
problems
Cathode changes have not been motivated by low QE!
> Between summer 2008 and end of 2009 only 5 cathodes have been used
#91.1, #13.4,#90.2, #77.2, and #53.2
> Because of problems with RF-gun in 2010 there was no time for dedicated
measurements
A ‘felt’ stronger QE decrease
Worse vacuum situation (outgassing RF-window)
> open question: lifetime for extended long pulse train operation
PITZ
> Stronger cathode usage
Higher field
MUCH more phase scans in comparison to FLASH
MUCH more solenoid scans / changes
> Stronger change in uniformity
More severe than at FLASH because laser spot size is changed more often (daily –
weekly)
> But for the same time period as FLASH also only 6 cathodes used.
For European XFEL
> Lifetime for long pulse train and high gradient operation?!
Operation of Cs2Te cathodes: Dark current
>Critical issue because high
gradients and long RFpulses
>Can damage components if
transported through the
machine
>Can prohibit operation of
machine
> Routinely measured
> Measured with faraday cup
new development of MDI-DESY => dark current monitor (non destructive)
> Maximum dark current obtained from solenoid scan
Example from PITZ
500
gun 2, Mo:
450
#90.2 2009-01-16
400 gun 2, Cs Te:
2
#77.2 2009-01-16
#77.2 2009-03-13
#13.4 2009-01-15
#13.4 2008-10-02
dark current [μA]
350
300
250
200
150
100
50
0
0
1
2
3
4
5
6
7
Pfor [MW]
At FLASH with DESY gun2 dark current independent from cathode
=> major amount seems to originate from gun body
500
gun 2, Mo:
450
#90.2 2009-01-16
gun 4.2, PITZ, Cs2Te:
#128.1 2009-03-26
#127.1 2009-04-14
400 gun 2, Cs Te:
2
#77.2 2009-01-16
#77.2 2009-03-13
#13.4 2009-01-15
#13.4 2008-10-02
dark current [μA]
350
300
250
gun 4.2, Cs2Te:
200
#76.3 2010-05-09
150
100
50
0
0
1
2
3
4
5
6
Pfor [MW]
New CO2 cleaned RF-gun:
dark current reduced by a factor of ~10!
7
DESY preparation system
Built together with INFN Milano LASA
Copy of the LASA system
Now in commissioning phase
First cathode box scheduled for March 2011
First cathodes produced cw QE > 10 %
DESY preparation system
dispenser
mask (5 mm diameter)
Coupled manipulators:
> Outer one: cathode movement
> Inner one: lamp + thermocouple
baking of Mo plug (450 dgC)
constant temperature during film
deposition
micro balance
Off-line tools: XPS, chemical composition
>Collaboration with HZB
Ruslan Ovsyannikov
Mike Sperling
Antje Vollmer
>Set-up for handling
cathodes in Mustang
chamber
>Up to now three beam
times
Off-line tools: XPS, chemical composition
> #90.1 unused, XPS measurement 2007
relatively high O 1s peak
>
#92.1 used in 2007, XPS measurement 2007
high contamination with fluorine and carbon
most probable source: Teflon washers
> #23.3 used in 2008, XPS measurement 2008
after removal of Teflon washers
only small amount of oxygen
> Already the first measurement period was great
success: identification of fluorine
> Very powerful tool, but we are limited to one
experimental chamber, because of big transport
box and complicated cathode handling
Off-line tools: EDX / SEM, particle investigation
In winter 2007 and spring 2008 several cathodes exhibited an enormous dark current, up
to 1.8 mA. With these cathodes no operation was possible!!
• main solenoid current fixed to 300 A
• cathode #21.3 was in the gun before
cathode exchange
2008-02-08
EDX measurements at University of Hamburg, to identify particles and their origin.
Therefore the box was opened - under clean room conditions.
Fe
Origin can be everything in the
load-lock-system.
Ag
most probable source:
rf contact spring
Identification of problems is important
EDX very useful tool for characterization of particles / scratches!
We reanimated an old set-up for analysis of used cathodes.
Off-line tools: SAFEM, field emitter
Future tool for cathode quality control in terms of field emission:
> Scanning Anode Field Emission Microscope
> dc field of up to 100 MV/m possible
> 3d scans of anode over photocathode
Off-line tools: SAFEM, field emitter
Gun back plane
3d movement of anode
via UHV motors
Cathode
RF-contact spring

Operation of Cs Te photocathodes at FLASH and PITZ, DESY

Transcription

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