Dark current measurements
1.
Main participants
2.
Measurement set-up
3.
Field enhancement factor and field emission area
4.
Results for Cs2Te photo-cathodes, copper plugs without and
with etching cleaning
5.
Strange phenomena during conditioning process
6.
Conclusion
G. Suberlucq AB-ATB
CLIC Meeting 8/07/03
Main participants
Ä
PS Operators
Ä
Klystron Team
Ä
Eric Chevallay
Ä
Thibaut Lefèvre
Ä
Guy Suberlucq
G. Suberlucq AB-ATB
CLIC Meeting 8/07/03
Faraday cup
C = 0.11 nF
Measurement set-up
e-beam
Cup
ZOUT = 50Ω
G = 0.9
1.4 nF
1 MΩ
Cyl.
2m
UFC
+ 125 V
Ion
pump
Ion
pump
Valve
Intensified
Camera
I (A) = UFC(V)/4.93
Ion
pump
MDK-98
e-beam
Faraday cup
C = 0.11 nF
Transport carrier
BPM
103 DVT
SNF
DHZ SNF
100
105 110
Q (nC) = 17xUFC (V)
FC
Cup
Cyl.
6.8Ω
WCM
DB RF gun
Pumping
Tank
Cs-Te cath.
ZIN = 1 MΩ
Probe ÷ 10
10 MΩ / 3 pF
1 MΩ
UFC
+ 125 V
G. Suberlucq AB-ATB
CLIC Meeting 8/07/03
Field enhancement factor and field emission area
Ä
For RF gun the field emission current could be approximated by :
I FE =
Ä
5.7 × 10
−12
4.52×φ − 0.5
× 10
φ 1.75
Ae( βE )
2.5
The modified Fowler-Nordheim plot :
×e
(1)
− kφ 1.5
βE
I FE
1
vs
give :
log( 2.5 )
E
E
2.84 × 109 φ 1.5
the field enhancement factor
β=
slope of the plot
I
−12
4.52φ
2.5
1.75
log( Ae) = log( FE
)
−
log(
5
.
7
×
10
×
10
β
)
+
log(
φ
)
E
→
∞
2.5
E
− 0.5
Ae = area of a single dominant emitter or
area of a collection of emitters of similar strengths
(1) J.W. Wang, PhD Thesis, Stanford University (1989)
G. Suberlucq AB-ATB
CLIC Meeting 8/07/03
Dark current measurement on Cs2Te photocathode
Fowler-Nordheim curve for RF gun : photocathode No 162 - 19/06/03
-22
0.00E+00
2.00E-09
4.00E-09
6.00E-09
8.00E-09
1.00E-08
1.20E-08
1.40E-08
1.60E-08
-22.2
FC 5 Ω
-22.4
-22.6
-22.8
FC high Z
-23
y = -2.8637E+08x - 1.9275E+01
R2 = 9.9935E-01
-23.2
-23.4
1/E (m/V)
Dark current : photocathode No 162 (fresh)
19/06/03
0.008
0.007
0.006
Mean dark current (A)
log(I/(E^2.5))
y = -2.6077E+08x - 1.9560E+01
R2 = 9.9867E-01
From Q meas.
mean duration of the
dark current ? 1.3 µs
0.005
From Q meas.
Icomp with
βHZ = 66.3
0.004
(after Q measurement)
From I meas.
Icomp with
βR = 72.8
Ae = 9.52*10-15 m2
(R ? 55 nm )
0.003
I meas.
Ae = 3.91*10-15 m2
(R ? 35 nm )
0.002
0.001
0
75
80
85
90
95
100
105
E (MV/m)
G. Suberlucq AB-ATB
CLIC Meeting 8/07/03
Main results from dark current measurements (1)
Ä
Standard conditioning process:
Slow increase of the klystron output power by minimizing break-downs, until
18MW nominal power, corresponding to 100 MV / m. After more than 10 minutes
without breakdown, the cathode is considered as conditioned.
Fresh Cs2Te
photo-cath.
Used Cs2Te
photo-cath
Chemically
cleaned
copper plug
ICE cleaned
copper plug
ICE cleaned
used Cs2Te
photo-cath.
φ (eV)
3.55
3.55
4.6
4.6
4.6
β
From - to
73 - 66
77 - 53
104 - 70
94 - 49
102 - 100
Eq.Radius
(nm)
35 - 55
27 - 165
38 - 269
55 - 2616
31 - 37
Imean (mA)
7.3 – 6.9
6.9 – 6.5
5.2 – 4.8
4.3 – 3.8
3.2
at 100MV/m
ICE : Argon ion bombardment at 5x10-2 mbar eq. N2
G. Suberlucq AB-ATB
CLIC Meeting 8/07/03
Main results from dark current measurements (2)
Conditionning process of the Cs2Te photocathode No 162 from 100 MV/m to 105 MV/m
105
100.3
100.3
101.8
95
4.1
4
85
3.5
3.5
75
3.2
3.2
3
65
Power
off
2.5
Power off
photocathode stored in the MPC
@ 5x10-11 mbar
55
2
45
-9
-9
2 - 5 x 10 mbar
2 - 5 x 10 mbar
1.5
35
1
25
0.5
15
0
Electric field (MV/m)
Charge produced by dark current (nC)
5
100.2
4.5
5
0
20
40
60
80
100
120
140
Time (hours)
G. Suberlucq AB-ATB
CLIC Meeting 8/07/03
Strange phenomena
charge produced by dark current
100µs/div
Electric field
1.7 nC/div
5µs/div
1µs/div
Long tail shape sometime during conditioning process
often just before an RF breakdown. Confirmed by light measurements
Nominal shape
Evolution of the dark current shape during the conditioning process : Copper plug 4A-17 after ICE
0
100
100
E (100 %) = 95.9 MV/m
U_GL1 (100%) = 0.494 V
I (100%) = 5.4 mA
Light (100%) = A.U.
-0.5
E
delay = -88 ns
-1
80
80
U_GL1
Light
delay = -88 integration
ns
time 200 ns
Dark curent (mA)
I
60
E (MV/m)
E
4.5 h
later
40
RF pulse width = 3 µs FWHM
-1.0E-06
0
0.0E+00
Time (sec.)
G. Suberlucq AB-ATB
1.0E-06
-3
I
60
40
-4
-4.5
1rst meas.
After standard
conditioning
-2.0E-06
-2.5
-3.5
20
-3.0E-06
-2
% max value
-1.5
20
-5
2.0E-06
3.0E-06
4.0E-06
-5.5
5.0E-06
0
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
Time (µs)
CLIC Meeting 8/07/03
5
Conclusion
Ä
No major difference between Cs2Te photo-cathodes and
copper plugs in term of dark current up to 100 MV/m
Ä
Cleaning by argon ion bombardment (ICE) helps to reduce
dark current
Ä
Conditioning process of the copper plug, with or without ICE,
before Cs2Te cathode preparation, helps for the photo-cathode
conditioning.
Ä
The long tail of the charge produced by dark current during
the conditioning process should come from ions which can
potentially destroy the photo-cathode.
More investigations should be done.
G. Suberlucq AB-ATB
CLIC Meeting 8/07/03