Proceedings of IBIC2012, Tsukuba, Japan

TUPA05

THE CALIBRATION FACTOR DETERMINED AND ANALYSIS FOR HLS
BUNCH CURRENT MEASUREMENT SYSTEM*Y.L. Yang#, T.J. Ma, B.G. Sun, P. Lu, J.G. Wang, J.Y. Zou, C.C. Cheng
NSRL&SNST, USTC, Hefei 230029, P. R. China
For bunch current measurement system, button
electrode or strip-line electrode can be selected as its
signal pickup, peak value or integral of signal frompickup can be used to indicate the related bunch currentvalue. To obtain the absolute value of bunch current, thecalibration factor should be determined with the help ofDCCT. So, the calibration factor is a key parameter forbunch current measurement system. At HLS, the stretch
effect of bunch length was observed when bunch current
decay over time and this will affect the performance ofbunch current detection for different pickups andcalculate methods, which mean the calibration factor willbe different. In this paper, theoretical analysis andexperimental validation results are performed to find out
an acceptable solution about pickup and signal processing
method for bunch current measurement system at HLS.
The results show that, the best performance can obtainedby adopting strip-line pickup.

INTRODUCTION
Bunch current measurement is necessary elements formost accelerators, especially for synchrotron radiation
light source. A lot of various types of measurement
systems are developed by now [1-4]. Current transformers,
pickup-electrodes and wall current monitors are mostwidespread among devices for bunch current
measurement.
Hefei Light Source (HLS) is a synchrotron light source,
many button pickups and strip-line pickups have been
mounted on the vacuum pipe for the measurement ofelectron beam position called as BPM. Typically, a BPM
have four electrodes to calculate both vertical andhorizontal beam position. Additionally, the sum signal
from the four electrodes carry the bunch charge
information and its change rate is less than 0.005 whenbeam position alternated within 4mm [5]. So the sum
signal can be used to calculate the bunch current.
HLS storage ring operates in 800MeV with
204.016MHz RF and 45 bunches, the bunch separated
from each other only 5 ns, and the bunch length is about
300ps. So, two type of four-electrode pickup can beselected as the pickup of HLS bunch current measurement
system [6]: button electrode and strip-line electrode. Peakvalue or integral of bunch signal from pickup can be used
to calculate the related bunch current value. To obtain theabsolute value of bunch current, the calibration factor
should be determined with the help of DCCT. At HLS, thestretch effect of bunch length was observed [7] whenbunch current decay over time and this will affect theperformance of bunch current detection for different
pickups and calculate methods, which mean thecalibration factor will be much different. So, theoreticalanalysis and experimental validation results are
performed to find out an ideal solution for bunch current
measurement at HLS.

BUNCH CURRENT CALCULATE
The electrons in a bunch from storage ring are usually
expressed with a Gaussian distribution. When the total
charge in bunch is Q0 and bunch length is V W , formula 1
show the expression of a bunch in time domain.

2SV W

exp(t2)
2V W2

(1)

For button pickup, the sum signal of four electrodes canbe expressed as follow:

V6 (t )

dI b (t )dtQ0t2
k
exp()
2V W2
2SV W2k(2)k is the scale factor of electronics. The chart wasshown in Figure 1(a) [8].
Peak value and integral value of each bunch sum signal
carry bunch charge information, which can be obtained
from formula 2:

Vpeak
Vint egral

KP0Q0

VW

³ V (t )dt
t1

62v

Q0

(3)

VW 2KIQ0VWv

Q0VW

(4)

Where K p is the calibration factor for using peak valueof sum signal and K I is the calibration factor for using
integral of sum signal. The above equation shows thestrong influence of the bunch length V W , both on V peakand Vint egral .
Alike, for strip-line electrode, the chart was shown in
Figure 1(b) and the sum signal expression is:___________________________________________*Work supported by National Natural Science Project (11105141) andChinese Universities Scientific Fund.
#ylyang@ustc.edu.cn

Q0

I b (t )V6 (t )

MZ ª§ 2l · ºIb t  Ib ¨ t  ¸»«4S ¬c ¹¼©

(5)ISBN 978-3-95450-119-9
Beam Charge and Current Monitors

1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○AbstractProceedings of IBIC2012, Tsukuba, Japan

Vpeak and Vint egral are approximated as formula 6 andformula 7:

VpeakVint egral

KP

Q0

VW

t2

³ V (t )dtt1

6vQ0(6)VWK I Q0 v Q0(7)The equation 7 shows that the integral of sum signal ofstrip-line electrodes is proportional to bunch charge Q0and have no influence of the bunch length V W, WE0t1

320
bunch length
bunch length of fit

300
280260240200180160140120100Total 45 Bunch Current value I (mA)

80

Figure 3: Bunch length changed with bunch current.
The bunch length stretch will affect the performance ofbunch current detection for different pickup type andcalculate method. With beam current decay from 200mA
to 80mA, strip-line BPM and button BPM are used as
pickup to detect the bunch current, integral value andpeak value for each bunch of the sum signal from BPM
are used to calculate the calibration factor.

a) Sum signal from button electrodes.

96 W

Calibration Factor for Bunch Current
When beam current decayed from 198mA to 86mA,
sum signal from button and strip-line pickup have beenrecorded by oscilloscope. Figure 4 show the sum signal offour-strip-line pickup, include all 45 bunches.

t2

OF

43b) Sum signal from Stripline electrodes
Figure 1: Sum signal of BPM.

2EXPERIMENT DATA ANALYSIS

Beam Current Value Decay Over Time
Figure 2 shows the DCCT beam current value decay
over time in HLS storage ring. Beam current decay from200mA to 80mA within about 10 hours.
DCCT current value vs time
200150

100

50

00.5

11.5

22.5Time (second)

33.5

4
4Signal (V)1To evaluate how bunch length will influence thecalibration factor of bunch current measurement, Beamcurrent, bunch length and bunch current were monitored
at the same time at HLS storage ring.

Current value (mA)

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○In this case, the stretch effect of bunch length wasobserved by Streak Camera [9]. Figure 3 shows that thebunch length changed from 303ps to 244ps, and thechange rate is about 19%.

t296 Wt1

Bunch Length Stretch Effect at HLS

Average Bunch Length (ps)

TUPA050-1-2-3-4

0

0.5

1
samples (points)

1.5

24x 10Figure 4: Sum signal of four-strip-line pickup.
From the sum signal waveform, peak value andintegrate value of every bunch can be extracted and thenthe relative bunch by bunch current value can be obtained.
Combined the DCCT beam current value, we cancalculate the calibrate factor which changed following thebeam current. The normalized calculating results were
showed in Figure 5. With strip-line pickup and integral(Fig.5 a), the normalized calibration factor changed only
1.4% when bunch length have changed 19.3%, themeasurement RMS is 0.003. The factor change rate is
16.1% with button pickup and integral (Fig.5 b), 15.5%
with strip-line pickup and peak value, and 27.1% withbutton pickup and peak value.

x 10

Figure 2: DCCT beam current value.

ISBN 978-3-95450-119-92

Beam Charge and Current Monitors

Proceedings of IBIC2012, Tsukuba, Japan

TUPA05SUMMARY
1.1
10.90.8factor
factor of fit

0.7
200

180160140120Total 45 Bunch DCCT Current Value (mA)

80

100

Nom. Factor (A.U.)b)button and integral, change rate =16.1%,rms=0.025
1.1
10.90.8

ACKNOWLEDGMENT

factorfactor of fit

0.7
200

180160140120Total 45 Bunch DCCT Current Value (mA)

100

80

Nom. Factor (A.U.)c)stripline and peak, change rate =15.5%, rms=0.023
1.1
10.90.8

factorfactor of fit

0.7200180160140120Total 45 Bunch DCCT Current Value (mA)

100

80

Nom. Factor (A.U.)d)button and peak, change rate =27.1%, rms=0.041
1.1
10.90.8

factorfactor of fit

0.7200180160140120Total 45 Bunch DCCT Current Value (mA)

10080

Figure 5: The calibration factor has changed with bunch
current value.
So, strip-line pickup and integral method were finallyselected to calculate the calibration factor and get theabsolute bunch by bunch current value at HLS storage
ring. Figure 6 show the bunch current measured by HLSbunch current measurement system.
No.1 bunch current value vs time

Current value (mA)

54

3

2

1

00.5

Theoretical and experiment data analysis show that,using strip-line as signal pickup and processing the sum
signal waveform by integral, the bunch length stretch
effective on calibration factor can be omitted, the best
choice for bunch current detection for electron storage
ring. The experiment data analysis show that the factor
will change about 27% when bunch length changed about
19% if using button pickup and processing by peak valueof bunch signal.

1

1.5

22.5
Time (second)3

3.5

Figure 6: Bunch current decay over time.

44x 10

The authors would like to present their thanks to Prof.
Weiming Li, Prof. Hongliang Xu, Prof. Lin Wang of HLS
for direct contributions of technical expertise. We also
thank Prof. Li Ma, Prof. Jianshe Cao and Dr. Jun-hui Yue(IHEP), and Prof. Yongbin Leng and Dr. Yingbing Yan(SSRF) for helpful discussion.

REFERENCES[1] Zhang L, Ma H. Z., Yue J. H., Design of BEPCII bunch
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ISBN 978-3-95450-119-9
Beam Charge and Current Monitors

3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Nom. Factor (A.U.)a)stripline and integral, change rate =1.4%, rms=0.003