Proceedings of IBIC2012, Tsukuba, JapanTUPA07

BPM SELECTION FOR BEAM CURRENT MONITORING IN SSRF∗
Zhichu Chen† , Yongbin Leng‡ , Yingbing Yan, SSRF, SINAP, Shanghai, China

INTRODUCTION
Beam current is one of the fundamental parameters tobe measured in any particle accelerators and its direct
current (DC) component is especially important in synchrotrons and storage rings. Thus, DCCT is almost themost widely used DC monitor in modern light sources
around the world for its ﬁne resolution less than 1 μA
r.m.s. [1] and long-term stability.
An NPCT175 from Bergoz Instrumentation have been
positioned on the storage ring as the beam current monitor and another one in addition as its backup since thebeginning of the commisioning in SSRF [2, 3]. Both ofthem have been suffering from various noise from time totime [4]:• power line noise,• narrow band noise which is strongly related to thebeam current,• random square wave noise from nowhere.
Figure 1 shows a typical performance of the DCCT without any noise mentioned above and the BPM sum signal.
The quasi-constant resolution of DCCT reading is less than
2 μA in all circumstances and that can be regarded as thelimitation of the electronics. The situation of the BPM is alittle more complex. The resolution of a single BPM is better than the DCCT’s for a really low current (weaker than10 mA) but gets worse as the beam current rises. Averaging the whole BPM system, on the other hand, can improve
the performance signiﬁcantly. The resolution of the BPM
system is better than that of the DCCT when the current isweaker than 60 mA and it seems still under control even thecurrent is stronger. It all seems that the BPM alternative is∗ Work supported by National Natural Science Foundation of China(No. 11075198)† chenzhichu@sinap.ac.cn‡ Corresponding author, leng@sinap.ac.cn

Libera current measurement test @ 2009.02.18

101

10010-1

NPCT175 + 6.5 digits DVM
Single libera readings
Average of 140 libera readings
0102030405060Beam current (mA)

7080

90

100

Figure 1: A performance evaluation of DCCT and BPM
sum signal as the beam current monitor.
There’re 140 BPMs on the storage ring [2] and some ofthe probes can be considered stable enough to accomplish
the task of been current monitors. Using the BPM sum
signal to relatively measure the DC beam current has already been a handful means during the commisioning of
SSRF [2]. This idea is being urged by all the beneﬁts it canoffer: faster data rate, dead time free and sensitive even in
low current situations. Hence, a performance evaluation isneeded to pick out the qualiﬁed BPMs.

PRINCIPLES
The sum signal on the pick-ups of a BPM does not position insensitive. For such a resolution requirement, thenonlinearity problem must be taken into account. There’repositions that the transverse motion of the beam is ﬁerce
and the BPMs at these locations are less desirable. SomeBPMs might suffer from some kind of local noise like theDCCT does, or just encounter some machining, installation, even connection problems. So the algorithm we need
will only choose good BPMs at good positions.
Some algorithms have been tried and compared, but itturns out that the one inspired by the theory of Principal
Component Analysis (PCA) is better than others. One example is that we used to rate the BPMs by the r.m.s. of thedifference the each BPM “waveform.” The noisy or unstable BPMs could be picked out without problem. The onesmooth but wrongly decayed or slowing drifting can getaway. But PCA can be helpful to list all of them.

Overview of PCA
PCA is a useful mathematical technique for ﬁnding patterns in data of high dimension which has been introduced
to the particle accelerator physics [5]. Only a statisticalanalysis of the BPM data matrix is needed to study theISBN 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 ○Although New Parametric Current Transformers (NPCT), commonly called Direct Current Current
Transformer (DCCT), is the general solution of beam
current monitor, Beam Position Monitor (BPM) sum
signals may still surpass it in some aspects such as thefaster data rate and higher resolution in low current
situations. Nevertheless, an additional monitor should beharmless. Meanwhile, the DCCTs in the storage ring ofShanghai Synchrotron Radiation Facility (SSRF) havebeen suffering from various noise and the signals from theBPMs could be an aid to provide the beam current moreaccurately. There’re 140 BPMs in the storage ring in SSRF
but not all of them are suitable for this particular usage.
This article focuses on the methods used here to dynamicly
choose the BPMs that meet the criteria.

especially suitable for the low current mode. It is not unreasonable to assume that some BPM may act worse thanthe others and the new BPM set will work even better. The
purpose of our beam experiment is to ﬁnd an algorithm todynamicly maintain such a list in which each BPM is relatively stable.
Current reading error (uA)AbstractProceedings of IBIC2012, Tsukuba, Japan

beam dynamics by using PCA without the knowledge ofthe machine model. PCA uses singular value decomposition (SVD) to convert the P -by-M BPM data matrix B intoa product of three matrices [5]:†BP ×M = UP ×P SP ×M VM
×M ,(1)S = diag(λ1 , λ2 , . . . , λk )

(2)

BPM 1
BPM 470.95

0.90.85is the singular matrix and λi the singular values.
Each singular value has its mode. A global mode,
which is usually related to an aspect of the beam dynamics physics, tends to correspond to a signiﬁcantly large singular value and should be shared by all BPMs. The thermal noise modes could be divided and mixed in an unpredictable combination of almost negligible singular values.
A local mode will only imply the malfunction of a speciﬁcBPM. Thus, only the thermal noise modes and the local
modes are needed to evaluate the performance of BPMs.

The Result of PCA

Selection Algorithm

After a simple SVD of the BPM data, unrelated modes
are separated:†BP ×M = UP ×P SP ×M VM
×M ,where(3)0.75

S = diag(0, λ2 , . . . , λk ).

(4)

The r.m.s. value of each column denotes for the standard
deviation of each BPM.

BEAM EXPERIMENTS
The data of the BPMs and the DCCT are being fetchedduring the daily operation and machine study time is notreally required. The normal operation mode in SSRF is toinject beam into the storage ring with the energy of 3.5 GeV
until the current reaches 210 mA or so every 12 hours before it starts to decay for the rest of the time. The timeseries data from 10:20 a.m. to 4:20 p.m. on May 2, 2012were used here as a demonstration. Figure 2 shows a typical BPM sum signal (BPM 1) and a noisy one (BPM 47)
which this algorithm aims to highlight.

1

23Time / hour

45

6Figure 2: Typical BPM sum signals.

B = (u1 , u2 , . . . ) × S × (v1 , v2 , . . . )† .

(5)

The principal mode u1 is the decay mode which matches
the DCCT readings (see in Fig. 3) as expected. The slightdifference between them implies the nonlinearity of theADCs.
Principal Mode
0.078

2000.076

1950.074

1900.072

1850.071800.068

1750.066

1700.064

165

0.062

160

0.06

155

0.058



00

123

Time / hour

456

DCCT reading / mA

Normally, PCA extracts the beam transverse motion
modes along with the longitudinal ones. In our case, theBPM sum signal matrix would be decomposited to get thespacial vectors as well as the temporal ones. The principalcomponent will be the decay mode with little doubt. Theremight also be some singular values much greater than thenoise ﬂoor in practical cases. They may due to the nonlinearity of the BPM probes or something alike, but whatshould be concerned is always the principal decay mode.
The other modes should be considered as some kind ofnoise in spite of their internal physical mechanics because
we only use the sum signal to monitor the beam current
without optimization.
Once the SVD is done, the deviation matrix can be easily derived by setting the ﬁrst element of the singular value
matrix, say λ1 , to zero without touch the others and multiplying them back:

0.8

0.7

Temporal component

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○where1

Normalized BPM sum signal

TUPA07150

Figure 3: The principal mode of the BPM data (blue) alongwith the DCCT data (red).
The peak in the spacial component v2 of the secondarymode u2 (see in Fig. 4) indicates this mode is a local modethat concerns just one single BPM. The mode appeared atthe 47th component exclusively which is why the waveform is so messy in Fig. 2.
To obtain the performance of the overall system, the estimating process as mentioned in Eq. (3) is made. Figure 5
shows that some BPMs may behave poorly but we can still
get a couple of BPMs that are both steady and correlated tothe beam current.
Sorting the standard deviations should offer the conﬁdence list. There is no human intervention needed, so theISBN 978-3-95450-119-9
2Beam Charge and Current Monitors

Proceedings of IBIC2012, Tsukuba, JapanTUPA070−0.2−0.401213Time / hour4560.01
0.009
0.008
0.007
0.006
0.005
0.004
0.003
0.002
0.001
0← 47

020406080

BPM location

100120140

Figure 6: Result of the performance of the overall BPM
system by using the difference method.

0.50204060

80

100

BPM location

120140Figure 4: The secondary mode of the BPM data (upper)
and its corresponding spacial component (down).
−3x 107654
3210020406080

BPM location

100120140

Figure 5: The performance of the overall BPM system inthe PCA process. The BPM sum signals are normalized
before the process and the absolute value of the standard
deviation of each BPM is irrelevant so that an arbitrary unit
is adopted here.
list could use the live BPM data and refresh itself automatically.

The Result of Difference Method
As a comparison, the difference method we’ve talked
about is also applied and the result is show in Fig. 6. BPM
47 is quite distinguishable while other BPMs behave justalike. It is hard to tell whether one BPM is more suitable
for the current measurement than another since the result
suggests many of them have reached the electronic noiselimits. It’ll be very difﬁcult to decide the threshold to ﬁlter the “improper BPMs.” Thus, it is not safe to run thismethod without the manual monitoring.
This method seems to be qualiﬁed to ﬁlter those obviously noisy BPMs. The stable BPMs can also be entangled
among low frequency ﬂuctuations, which means they are
not totally usable, or they can have unfortunately intolerable nonlinear problems, which means each BPM should becalibrated separatedly. So even if a threshold is chosen, theselected BPMs still need a careful check individually.

CONCLUSIONSThe BPM sum signal has been used as an alternative tothe DCCT in SSRF. The BPM probes still have readings
even there’s no beam in the ring, so the zero drift is notneglectable. The ADCs at the electronics front-end do nothave perfect linear responses. Therefore, a careful calibration is needed when the sum signal method becomes a practical application.
In spite of the above problems, the BPM has a better performance than DCCT sometimes, e.g., under the conditionof low current. BPMs with ﬁne resolutions, little noise andluck locations that the transverse movement of the beam isof small amplitude are needed to be picked out to aid theDCCT. A new BPM selection algorithm based on PCA wasthen proposed to choose such a set of BPMs.
The new algorithm for the beam current measurement
has several advantages over other ones tried in SSRF. The
main reason why it’s chosen is that PCA can extract thecorrelationship between BPMs. This feature assures thatthe selected BPMs have the max likelihood of the beam
current. Besides, the PCA idea is user friendly. The total
process is neat and obvious. That makes the algorithm easy
to be implemented and work online.

REFERENCES[1] D.J. Martin, S. Allison, and B. Scott, et al., “Evaluation ofBergoz Instrumentation NPCT,” Proceedings of PAC09, Vancouver, BC, Canada, 2009: 4021–4023.
[2] Y.B. Leng, K.R. Ye, and W.M. Zhou, et al., “SSRF Beam
Diagnostics System Commissioning,” DIPAC2009, Basel,Switzerland, 2009: 24–26.
[3] Y.B. Leng, Y.B. Yan, and W.M. Zhou, et al., “Precise Beam
Current Measurement for Storage Ring Using Beam Position Monitor,” High Power Laser and Particle Beams, 2012,22(12): 2973–2978.
[4] Z.C. Chen, Y.B. Leng, and Y. Xiong, et al., “Modelling andPerformance Evaluation of DCCTs in SSRF,” these proceedings.
[5] C.X. Wang, “Measurement and Application of Betatron
Modes with MIA,” Proceedings of PAC03, Portland, Oregon,
U.S.A., 2003, 5: 3407–3409.

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 ○0

Standard Deviation / arb. unit

Standard Deviation / arb. unit

0.2

Spacial component

Temporal component

Secondary Mode0.4