Dark current measurement at KEK
M. Gerbaux, S. Döbert, T. Higo, S. Matsumoto, K. Yokoyama
Experimental setup
Magnet
Accelerating structure (T18)
Lounine
IP H load
IP
Varian IP
GV FC
GV
KX03
VAC
Varian
IP
Q-mass
VAC DC
E
DC
FC-Mid
75cm
FC
FC-DN
FC-Mid
FC-UP
WC
IP H Otsuka
load
Load
FC-UP
75cm
Spectrometer
FC
AM
1 87
PM
Slit
VAC
230
WC
Insul
425
PM
FC
DC
E
H IP
FC
AM
195cm
FC-DN
Field enhancement factor measurement
10
 upstream = 33
 downstream = 29
 end of line = 35
-19
I/E2.5
10
-18
10
10
-20
-21
3
3.5
4
1/E
4.5
5
x 10
-9
pA fluctuations
Data manually recorded during a very stable running period at (64.0  0.3) MW
16
14
FC-current (pA)
12
10
8
6
4
Input power fluctuation < 0.5 %
Sampling rate ≈ 1.2 Hz
2
0
1
21
41
61
81
101
121
141
161
181
201
221
241
261
281
301
Dark current spectra measurement
Pulse length : 252 ns
Horizontal error bars : spectrometer resolution (Δslit/ρ)
opening fluctuations
: 33 mm
Vertical error barsSlit
: pA-meter
20
Faraday cup current (pA)
18
108.7 MV/m (2/03)
16
95.5 MV/m (2/03)
14
85.4 MV/m (3/03)
12
108.7 MV/m (3/03)
10
8
6
4
2
0
0
2
4
6
8
10
Energy (MeV)
12
14
16
Dark current spectra measurement
Pulse length : 252 ns
Slit opening : 17 mm
Faraday cup current (pA)
9
8
107.9 MV/m (3/03)
7
107.9 MV/m (3/03)
100 MV/m (4/03)
6
5
4
3
• Always the same low-energy cut off (≈3.5 MeV)  low energy e- lost in the pipe ?
2
• Spectrum maximum around 5 MeV
1
• Often exhibits a double peak structure that tends to smooth while decreasing the
0 ( dark current originating mainly from 2 cells at high power ?).
gradient
0
2
4
6
8
Energy (MeV)
10
12
14
T18_VG2.4_Disk_#2
Dark current spectra measured 18 June 2009
Dependence on power
Dependence on width
090618
T18_VG2.4_Disk #2
Spectrum_vs_power at 252ns
T18_VG2.4_Disk #2
Spectrum_vs_Width at 64MW
1.2
1.4
I_slit_peak [microA] 64MW, 252ns
I_slit_peak [microA]
64MW, 252ns
I_slit_peak [microA]
55MW, 252ns
I_slit_peak [microA]
70MW, 252ns
I_slit_peak [microA] 113ns, 64MW
1
I_slit_peak [microA] 64MW, 331ns
I_slit peak [microA]
I_slit peak [microA]
1.2
1
0.8
0.6
0.4
0.8
0.6
0.4
0.2
0.2
0
0
0
5
10
pc (MeV/c)
15
0
5
10
pc (MeV/c)
64 MW  108 MV/m
55 MW  100 MV/m
70 MW  113 MV/m
Higo 090703
15
Conclusion
• Nice test bench, very stable running which makes the
measurements easy.
• An automation of the pA-meter read-out would make the
measurement even easier and more precise.
• Change in the spectra shape since March to be investigated.
• Possible measurement of the dark current spectrum upstream ?
Dark current spectra measurement
Pulse length : 252 ns
Slit opening : 33 mm
Faraday cup current (pA)
20
18
108.7 MV/m (2/03)
16
95.5 MV/m (2/03)
14
85.4 MV/m (3/03)
12
108.7 MV/m (3/03)
10
?????????
8
???
6
4
2
0
0
2
4
6
8
10
Energy (MeV)
12
14
16
Experimental setup
Magnet
Accelerating structure (T18)
Lounine
IP H load
IP
Varian IP
GV FC
GV
KX03
VAC
FC-UP
Varian
IP
Q-mass
Acoustic
sensors
(1 / cell)
VAC DC
E
DC
FC
FC-DN
FC-Mid
FC-Mid
75cm
WC
IP H Otsuka
load
Load
FC-UP
75cm
Spectrometer
FC
AM
1 87
PM
Slit
VAC
230
WC
Insul
425
PM
FC
DC
E
H IP
FC
AM
195cm
FC-DN