Proceedings of IBIC2012, Tsukuba, Japan

MOCB03

MODELING AND PERFORMANCE EVALUATION OF DCCTS IN SSRF∗
Zhichu Chen† , Yongbin Leng, Yun Xiong, Weimin Zhou, SSRF, SINAP, Shanghai, China

Abstract
Direct Current Current Transformer (DCCT) is the mostcommonly used high precision current monitor in modern
particle accelerators including Shanghai Synchrotron Radiation Facility (SSRF). Three types of noise have been observed in the output signal of the DCCT in the storage ringof SSRF: power line noise, beam current related narrow
band noise and random square wave noise from nowhere.
This article will discuss the noise removal algorithms in
SSRF and the performance of the DCCTs afterwards.

As the biggest science research facility in China, SSRF
has started user run with 7 beam lines since March, 2009.
The whole facility contains a 150 MeV LINAC, a full energy booster and a 3.5 GeV storage ring and has offered
77200 hours’ user time. Since the demand for the beam
quality is being increasing with the growth of the usergroup, the accuracy of the measurement of the direct current (DC) component of the beam current has become particularly important.
According to the design requirements of the SSRF as athird generation light source, a high precision DC beam
current monitor with the ranger greater than 400 mA andthe resolution requirements for booster and storage ring are
50 µA@10 Hz and 10 µA@1 Hz respectively. The refresh
rate of the DC beam current and the corresponding lifetime
should not be lower than 1 Hz in order to keep the machine
to operate normally.
The New Parameter Current Transformer (NPCT) sensor
from the Bergoz Instrumentation has been evaluated√ at theSPEAR3.[1] NPCT175 with a resolution of 1 µA/ Hz waschosen to meet the design requirements. Two DCCTs werepositioned at section 15 and 17 and were named DCCT15and DCCT17 respectively. The sensor is designed to besensitive to the electromagnetic fields so that a shielding
system is needed to isolate the sensor from the outside
fields. The design of the shielding system was borrowed
from SPEAR3 (see the exploded view in Fig. 1) since it
seems rational enough and it is proved to perform competently on that machine.
The PXI bus industrial computer was selected as theinput-output (IO) controller platform for its compatibility,stability, availability for various of IO boards and the CPU∗ Work supported by National Natural Science Foundation of China(No. 11075198)† chenzhichu@sinap.ac.cn

Figure 1: Exploded view of the shielding system designed
by SPEAR3.
computing capability. In order to choose a proper digital voltmeter (DVM) module from the 4070 series of National Instrumentation (NI), a noise test was made between
NI4070 (6.5 bit) and NI4071 (7.5 bit). Another test using
the NPCT175 and NI4070 as a whole system was also processed. The result is shown in Fig. 2 and we can see that:• The performances of NI4070 and NI4071 are quite
close when the sampling rate is higher than 200 Hz;• The noise of the DVMs—both NI4070 and NI4071—
is much less than the NPCT-DVM system when thesampling rate is lower than 40 Hz which means thenoise of the sensor has a great contribution to it;• The performance of the NPCT-DVM system is close
to that of the DVMs when the sampling rate is higher
than 40 Hz.
The sampling rate was chosen to be a relatively highvalue of 10 kHz because showing more details of the beam
current waveform was preferred, especially during thecommisioning. NI4070 DVM was selected as the data acquisition module of the DC beam current monitor systemsince the performance of NI4071 wouldn’t be significantly

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Beam Charge and Current Monitors

1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○INTRODUCTION

Proceedings of IBIC2012, Tsukuba, Japan

NI4070NI4071
NI4070 + NPCT175

1E-3

1E-3

1E-4

1E-4
1E-5

1E-5
1E-6

1E-6
1E-7
100

1k

10k

100k

1MCurrent RMS noise level @ 2A range (A)DVM RMS noise level (Volts)

0.01

Sampling rate (Hz)A further study on the narrowband noise, as shown inFig. 4, gives the fact that the amplitude of narrowband noise
are approximately proportional to the beam current. BothDCCTs behave alike but DCCT17 seems more sensitive tothe noise. This may due to the slight differences during theinstallation.
0.3
measurements STD @ 10kHz / mA

MOCB03

0.2
0.15
0.1
0.05

Figure 2: Noise test result of DVM and NPCT.
better. The PXI input-output controller (IOC) then fetches
the signals from the DVM with the 10 kHz sampling rate.
The 2 Hz data are also provided by averaging the raw signalwaveform.

Despite the elegant shielding design from SPEAR3, theDCCTs still suffers from various kinds of noise including
narrowband noise, square wave noise and zero drift, etc.

Narrowband Noise
A waterfall plot (see Fig. 3) is a good tool to show narrowband signals with highlighted lines. At least 3 typesof narrowband noise can be seen appearing in the DCCTsystem:• power line noise (50 Hz) and its harmonics, which arestable all the time;• other frequency specific noise, e.g., the one whose frequency is 1.147 kHz, but the amplitude varies throughtime;• frequency drifting noise, e.g., the one whose frequency drifted from 4.4 kHz to 2.2 kHz.

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 


160

170
180
beam current / mA

190

200

The square wave noise appears randomly without anywarnings. This noise can be seen at every cycle for days
and disappear for the next few weeks. We can see this noise
approximately once a week, on the average. The noise always presents the shape of square waves with indefinite
periods and fixed amplitudes (−60 µA for DCCT17 and−600 µA for DCCT15). The square wave noise has a great
contribution to the measurement error. It affects the resolution of the system considerably, especially for lifetime
measurements.

Performance Before Optimization
A comprehensive evaluation was made before any
workarounds were applied as shown in Fig. 5. The
DCCT15 has a better performance than DCCT17 does, due
to the installation process. There’s a great potential to enhance the system, although both DCCTs have already met
the primary requirements.

RMS noise / µA 150

Square Wave Noise2 140

Figure 4: Amplitudes of the narrowband noise are approximately proportional to the beam current in both DCCTs.

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c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○NOISE IN DCCT

DCCT15,raw data,80~116µA
DCCT17,raw data,184~269µA

0.25

1

NPCT175 specification
DCCT15, data rate 2Hz
DCCT17, data rate 2Hz
DCCT15, data rate 10kHz
DCCT17, data rate 10kHz

10010  


0.1

1

10100
frequency / Hz1k10k   ¢Þ Ó ×Ï   ÒÙßÜ Ý

Figure 5: Amplitudes of the narrowband noise are approximately proportional to the beam current in both DCCTs.

Figure 3: Waterfall plot of the spectrum of the DCCT.

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Proceedings of IBIC2012, Tsukuba, Japan

MOCB03

DCCT MODELING
After the analysis of the beam current monitor system inSSRF, the DCCT output is considered to contain:• DC component of beam current (main signal),• NPCT175 sensor noise,• AC component of beam current and its harmonics,• square wave noise,• power line noise,• other frequency specific noise,

WORKAROUND AND ACHIEVEMENTS• zero drift,

Since the frequency and the amplitude of the frequency
drifting noise are both changing during the machine operating, an adaptive notch filter has been developed to trace thefrequency and suppress the noise. The other narrowband
noise can benefit from this filter as well. Another evaluation has been made after the notch filter was applied, theresolutions of DCCT15 and DCCT17 have been improved
from about 100 µA and 200 µA to 12.4 µA and 30 µA, respectively, for the 5 kHz data as shown in Fig. 8.

Another experiment was made to try to determine where
the narrowband noise came from. A reasonable guess
would be that this noise should be invoked by the beam
current for the linear relation between its amplitude andthe beam current. A reverse 179 mA current was then provided to DCCT17 when the beam current is about 180 mA
to cancel out the DC component. The spectra of the output
waveform with or without the reverse current are shown inFig. 6 to see whether the average current of the beam takes
part in the noise. There’re no significantly different results
between those situations. This implies that the noise may
be related to the alternative current (AC) component of thebeam current or some kind of wake field.

210
RMS noise / µA

• DVM electrnoic noise.

1

10NPCT175 specification
DCCT15
DCCT17
DCCT15, raw data
DCCT17, raw data
DCCT15, filter applied
DCCT17, filter applied010
41010amplitude / a.u.

0.1

normal measurements
reverse current of179mA was feeded

3210100
frequency / Hz

1k10k

Figure 8: The resolution has a great improvement after using the adaptive notch filter.

beam current is 180mA then

10

1

1

10

0

10

00.5

1

1.5

2

2.5
3frequency / kHz

3.5

4

4.5

5

Figure 6: Comparion of the spectra.
The probability of appearance of the square wave noise
seems current related to us. The DCCT data were archived
for two weeks since June 11, 2012 in order to obtain arough and qualitative conclusion. The absolute values ofthe difference data of the 2 Hz DCCT readings show thatthe square wave does not occur at the current lower than
197 mA (see Fig. 7). The distribution is quite interestingand it seems that the square wave “tends” to appear at somebeam currents than others.

The square wave noise does not seem easy to be removed
with traditional time-domain or frequency-domain filteringmeans, so a discrete Kalman filter[2, 3] has been used tosuppress this noise with its efficient computationial (recursive) means to estimate the state of a process by minimizing
the mean of the squared error. This filter is very powerful for it can do estimations of states at anytime even theprecise nature of the modeled system is unknown like oursituation.
The resulting waveform is more satisfactory than had
been anticipated. The standard deviation of the beam current measurement has been reduced from 53 µA to 6 µA
for the 2 Hz data. The corresponding standard deviation oflifetime measurement has also been decreased from 1.052
hours to 0.168 hours.

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 ○Figure 7: The square wave noise has a threshold and aninteresting distribution.

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○MOCB03

Proceedings of IBIC2012, Tsukuba, Japan

CONCLUSIONSREFERENCES

The combination of NPCT175 from Bergoz Instrumentation and PXI4070 DVM from National Instrumentation
was chosen to meet the design requirements in SSRF. Inorder to screen the sensor from electromagnetic fields outside, the shielding system designed by SPEAR3 was applied. Various kinds of noise, including narrowband noise,
random square wave noise and zero drift, still exist despite
the elegant shielding design. An adaptive notch filter and a
Kalman filter have been developed to minimize the systemmeasurement error and the results are exciting. The narrowband noise has almost be suppressed to the electronics
limit and the square wave noise has been filtered with littleremaining.
Some other appreaches have also been tried, e.g., optimize the installation process to reduce the zero drift or useBPM sum signal as the replacement for the DCCT[4]. Anyway, those methods are merely stopgaps. We’ll keep looking for the source of the noise and try to remove it from theroots.

[1] R. Hettel, et al., “SPEAR3 upgrade project: the final year,”
PAC’03, Portland, May 2003, p. 235.
[2] Rudolph Emil Kalman, “A New Approach to Linear Filtering and Prediction Problems,” Transactions of the ASME–
Journal of Basic Engineering, 1960, Vol. 82 Series D: 35–45[3] Peter S. Maybeck, Stochastic Models, Estimation, and Control, (New York: Academic Press, Inc., 1979), 141.
[4] Z.C. Chen, Y.B. Leng, Y.B. Yan, et al., “BPM Selection for
Beam Current Monitoring in SSRF,” these proceedings.

ISBN 978-3-95450-119-9
4Beam Charge and Current Monitors