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12th Int. Beam Instrum. Conf.
ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3

IBIC2023, Saskatoon, Canada
JACoW Publishing
ISSN: 2 6 7 3 - 5 3 5 0
d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - T U 3 C 0 5

LOW INTENSITY BEAM CURRENT MEASUREMENT
OF THE ASSOCIATED PROTON BEAM LINE AT CSNS
R.Y. Qiu, W.L. Huang, F. Li, L. Zeng, Muhammad Abdul Rehman, M.Y. Liu, Q.R. Liu, R.J. Yang,
T. Yang, Z.H. Xu, Z.X. Tan
Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, China
Spallation Neutron Source Science Center, Dongguan, China
Abstract
The Associated Proton beam Experiment Platform(APEP) beamline is the first proton irradiation facility touse naturally-stripped protons which come from H- beams
interacting with the residual gas in the linac beampipe at
CSNS. The stripped beam current, which is in the order ofthen injected into the Rapid Cycling Synchrotron (RCS) by
charge-exchange stripping. The proton beam is then accelerated to 1.6 GeV and finally extracted to bombard thetungsten target for neutron production with a repetition rateof 25 Hz [3]. In the linac of CSNS, a portion of H- beaminteracts with the residual gas. As a result, in the beampipe

Figure 1: Layout of the APEP beam line. [2].
0.1% of the original H- beam and approximately 10 microamperes, should be measured precisely to provide the proton number for irradiation experiments. Therefore, a lowintensity beam current measurement system was developedwith considerations to eliminate the external interferences.
An anti-interference design is adopted in this system with
an elaboration of probes, cables and electronic low-noisetechnology to minimize the impact of environmental noise
and interferences. This improves the signal-to-noise ratio
and enables a more precise measurement of the microampere-level pulsed beam current. The system was installed
and tested during the summer maintenance in 2021 and2022. It shows a good agreement with the measurement of
the Faraday cup.

INTRODUCTION
The linear accelerator of China Spallation Neutron
Source (CSNS) consists of a negative hydrogen ion source,
a 3 MeV radio frequency quadrupole accelerator (RFQ),
and a 4-tank drift tube linear accelerator (DTL) [1]. The
negative hydrogen ion beam is accelerated to 80 MeV andthere are three types of particles with different charge
states: H- (~15 mA, 100~500 μs), hydrogen atom (H0), andproton (H+). The associated proton beam current is about
0.1% of the original H- beam. Therefore, the APEP beamline is established from the bending magnet at the end oflinac. The number of protons in the beam is a crucial parameter for irradiation experiments.
Figure 1 shows the layout diagram of the APEP area.
There are three devices used for measuring beam intensity.
The first one is the current transformer measurement system located at the entrance of APEP. In order to precisely
measure this microampere-level beam intensity, a low-intensity beam current measurement system was developedin 2021. It achieved stable measurements by addressing external interferences in September 2022.The second device,
located behind collimator3, is the beam intensity measurement system based on the secondary electron emission
method. The third device is the Faraday cup beam intensity
measurement device located in front of the DUMP. The
two devices behind are currently undergoing debugging.

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12th Int. Beam Instrum. Conf.
ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3

This article primarily focuses on the design of the microampere-level beam intensity measurement system for the
beam current transformer.
Table 1: Parameters of the Associated Proton Beam
ParameterProton Beam Energy
Repetition Frequency
Number of Particles
Macro Bunch Length
Proton Beam Intensity

Value
80 MeV
25 Hz~5.0*1010 ppp@100 kW
100 μs ~ 500 μs
20 μA@400 μs,100 kW

SYSTEM DESIGN*$" +,-()/,)+#$" ()+,)

Design of Beam Current Detector
Before developing a current transformer, several soft
magnetic alloy materials were tested and compared. Finally, an iron-based soft magnetic alloy core was selected
due to its high magnetic permeability (µr), low coercivity(Hc), high saturation magnetic induction (Bs) and low cost.
In the design of the transformer, it is necessary to consider
that the droop of the 500 μs macro pulse signal should beless than 1% within the pulse width, that is, τdroop > 0.05
s or flow-cutoff = 7 Hz. According to the measured noise levelof the experiment, the turns of the secondary coil Ns was
selected as 50. An additional coil of 1 turn was wound forthe online calibration. Since the inner diameter of the vacuum pipeline is Φ110 mm, a soft magnetic core of Febased nanocrystalline alloy (nominal relative magnetic permeability µr≈90000@10 Hz) with an inner diameter
Di = 130 mm, outer diameter Do = 160 mm and longitudinallength h = 25 mm was selected for winding the current
transformer. The equivalent inductance Ls of the sensor was
calculated according to Eq. (1) as 0.175 H. It was measured
by an LCR meter (HIOKI IM3536) to be 0.176 H@10 Hz,
which is close to the theoretical calculation.

𝐿! = 𝑁" #$! $" %#$%%#&'ℎ𝑙𝑛 '&-7$

drical magnetic shield layer, and µr is the relevant permeability of magnetic material [4]. The magnetic shielding efficiency of a double-layered DT4 shield is calculated as a
product of that of each layer with a radial thickness of
1 mm, that is, 37.8×38.7=1463, which is better than thatof a single layer permalloy shield (S of which is 1220) witha radial thickness of 2.0 mm and a same outer diameter ofΦ162 mm. The comparison of different magnetic shield isshowed in Table 2. What’s more, the material of DT4 iseasy to process and costs much less. It can be seen thatcompared with a single-layer permalloy shield, this scheme
has low cost and can meet the requirements of current
transformers for magnetic shielding effect.
($" )
𝑆= '
(2)

(1)

Here in Eq. (1): μ0 = 4π×10 H/s is the vacuum magnetic
permeability and ffill = 0.75 is the filling factor of the ribbons forming the soft magnetic core. The input impedanceof electronics r = 2.5 Ω, leads to the droop time constant of
the pulse signal τdroop = Ls/r = 0.062 > 0.05 s, which meetsthe design requirements.
As shown in Fig. 1,the APEP current transformer is installed at the downstream of a bending magnet, which isused to guide the associated proton beam to the L-Dump or
the APEP beam line. The stray magnetic field may cause
the current transformer of APEP-CT to be magneticallysaturated and fail to accurately measure the beam current
strength of associated protons, so it is necessary to design
a magnetic shield semi-surrounding the outside of the current transformer, as the inner shielding shown in Fig. 2.
Considering the environmental stray magnetic field, a double-layered DT4 magnetic shield is designed according to
Eq. (2). In this formula, p=b2/a2, where a and b are respectively the outer diameter and inner diameter of the cylin-

Table 2: Calculation of Different Magnetic Shields as the
Inner Shielding of APEP-CT
Layer
Material
DT4

b mm

a mm

p

μr

S

162.0

160.0

1.0252

6000

37.8

DT4
Permalloy

158.0
162.0

156.0
158.0

1.02581.0513

6000
106

38.7
1220

A fully-enclosing outer shield of the APEP-CT sensor
acts as a low-impedance path of the wall current and an
RF shielding [5].

Figure 2: APEP CT sensor and magnetic shields.

Figure 3: Proton beam intensity detector installed at the
APEP tunnel.

Design of Electronic Circuit
As a current transformer, when the sensor detects the
proton beam current, the induced output signal is a very
weak current (here Ns=50). In the design of electronics, weuse a transimpedance amplifier as the first-stage IV conversion chip. For instance, a 10 μA beam current passes
through the sensor core with the 50-turn secondary coil, theinput current of the electronics will be 200 nA. The input
TU3C0506 Beam Charges and Current Monitors

175

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ISSN: 2 6 7 3 - 5 3 5 0
d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - T U 3 C 0 5

This is Content
a preprint— thepublished
with
IOP of the CC-BY-4.0 licence (© 2023). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI
fromthis final
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may be isusedunder theterms

12th Int. Beam Instrum. Conf.
ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3

IBIC2023, Saskatoon, Canada
JACoW Publishing
ISSN: 2 6 7 3 - 5 3 5 0
d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - T U 3 C 0 5

and feedback resistances of the electronics are r = 2.5 Ω
and Rf = 10 kΩ * 20 separately. When selecting a low-noise
operational amplifier, the input bias current Ib cannot be ignored. The bandwidth of the electronics should be designed
to minimize high frequency noise introduction while meeting the rise time requirement. The circuit diagram is shownin Fig. 4. The electronic baseline is automatically adjusted
by a technique of baseline dynamic feedback [6].

Figure 4: Diagram of CT electronics.

achieved < 10 μs, the droop time is < 1%/ms, and the relevant error of the calibration data is within 1% FS, as shownin Figs. 6 and 7. The parameters of this low intensity measurement system meet the design requirements.

Figure 6: The blue signal is the electronics output with
1 µA calibrated input current.

Experiments on Cable Selection and Shielding
We tested two kinds of cables to transport the signal fromthe sensor output to the electronics input. A comparison
was made among the noise measurements of four-core cables (two-core twisted pair with braided outer shield) andtriaxial cables under different grounding methods with thesame sensor and electronics. It showed that the best signalto-noise ratio was obtained when the triaxial cables weregrounded at both ends. After the multi-point grounding
test, it was found that outermost layer of triaxial cable was
grounded to the vacuum pipeline at the sensor end, and theelectronic end was grounded to cabinet ground to achieve
the best effect, as shown in Fig. 5.

Figure 5: Diagram of the triax cable grounding to minimize
noises.

SYSTEM CALIBRATION AND TESTING
System Calibration
We used a high precision pulsed current source Keithley
6221 to generate a current of square wave with a 500 μs
pulse width in a range of 2~10 μA, which passed throughthe calibration coil of the APEP-CT sensor to simulate thebeam current. The induced current in the secondary coil is
input to the electronics via a 25 m triaxial cable and theelectronics output voltage is measured by an oscilloscope
or JYTEK DAQ for the data acquisition and waveform display. The calibration formula is obtained by a linear fittingof the electronic output amplitude with the simulated input
beam current intensity. Through this formula, the undertest
beam intensity can be calculated along with the background de-noising at a homo-metric beam-off zone during
operation. Thereby we realized a precise low intensity
measurement of the associated pulsed proton beam. Afterthe system commissioning, the rise time of the electronics

Figure 7: CT Electronics calibration data.

Troubleshooting and Experimentation for Interference Investigation
The low beam intensity measurement system was installed in the APEP tunnel during the summer maintenancein 2021. In the following accelerator operation, we performed several beam tests with this system, which needs
guiding the beam to the APEP beamline. It performed abnormal due to significant interferences. The interference
was large and stable, characterized by a fixed frequency
relating to the beam current. A Faraday cup at the end of
the APEP beam line detects the same interference signal,
but with a relatively smaller impact, shown in Fig. 8.

Figure 8: Measurement of the APEP-CT and Faraday cup
electronic output waveforms. Blue trace is the signal of
Faraday Cup. Yellow trace is that of APEP-CT. Purple
trace is the timing trigger signal.
We made a thorough examination of the CT interference
sources along the APEP beamline and RCS. The magnetic
elements such as Linac RCS Switch Magnet, Painting
Bump Magnet at the RCS injection zone, and Extraction
Magnets at the extraction zone were sequentially shut

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IBIC2023, Saskatoon, Canada
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down. The position of the magnet is shown in Fig. 1. The
BH (Horizontal painting bump magnets) and BV (vertical
painting bump magnets) are used for the horizontal andvertical painting respectively in the phase space of the injected beam. The main contribution of interference is from
BH, followed by BV, which has a high similarity to the current curve of the power supply during the beam injectionin the phase space painting mode. The correlation indicatedthat the interference was transmitted from the orbit bumper
to the APEP-CT sensor along the accelerator vacuum pipe.
The distance is about 55 meters. While other interference
effects to the APEP-CT are small enough to be ignored.
Due to the consistent shape and amplitude of the interference signal, the DAQ system is triggered by the Timing
system, allowing it to collect the interference signal once
without a beam and save it as the background interference
signal. Subsequently, for each subsequent signal collection
with a beam, the interference signal is subtracted in orderto restore the beam signal.

Figure 9: Output of APEP-CT electronics under different
accelerator operation conditions. Pic (a) without BH and
BV magnets operation. Pic (b) with the BH & BV magnet
enabled. Pic (c) with only the BH magnet enabled. Pic (d)with only the BV magnet enabled.
To keep the interferences and high harmonics of thebeam out of the sensor cavity, a bypass capacitor was designed (as shown in Fig. 2), and installed during the summer maintenance in 2022. The design principle of the bypass capacitor is described in Ref. [7]. The wall current
splits in two: the high frequencies pass through the capacitance of the ceramic gap, which cancel the high frequency
part of the beam “seen” by the sensor, and the low frequencies follow the wall current bypass, therefore do not passthrough the sensor hole. Here, we built a capacitor over theceramic gap with layers of copper foil separated by a layerof 100 µm-thick Kapton foil. The cavity impedance wasdominated by the nanocrystalline core, which is measured
to be 63.8 Ω@1 kHz and will be much bigger in higher
frequencies. The bypass capacitance is designed as 5 nF,
allowing the main part (<50 kHz) of the beam passing
through the sensor hole. To obtain the desired capacitancevalue, the overlapping area is obtained by [8]:𝑆 = 𝐶 𝑑'𝜀! 𝜀"(3)

where: C is the capacitance [F],S = 1.61×10-2 is the overlapping area [m2],d =1×10-4 is the dielectric thickness [m],εr is the relative dielectric constant, 3.5 for Kapton polyimid,
ε0 is the dielectric constant 8.86 x 10-1.

Figure 10: A bypass capacitor over the ceramic gap was
built in the APEP-CT sensor.

Test Results
After the bypass capacitor was installed in the APEP-CT
sensor, we successfully measured the associated proton
beam in low intensity, shown in Fig.11. It was observedthat the interference from the BH and BV magnets diminished significantly, and in fact, became barely noticeable.
The signal-to-noise ratio of the beam signal was improved,
and the amplitude of the pulsed associated proton beam intensity is clearly measured as 16.8 μA. The blue waveform
represents the signal from the APEP-CT system, while thered waveform represents the current intensity signal measured by the Faraday cup after the beam passes through theenergy degrader.

Figure 11: Output of the electronics of the APEP-CT and
Faraday cup.

CONCLUSIONSIn the beam line of the Associated Proton beam Experiment Platform at CSNS, the proton beam intensity is a crucial parameter for irradiation experiments. The APEP-CT
system achieves a signal-to-noise ratio improvement andmeets the design specification for microampere-level current intensity measurement by implementing methods suchas, the bypass capacitance design, the cable shielding andgrounding methods, a low-noise electronic with the correct
LP-filter design, and background noise subtraction algorithms in DAQ.

TU3C0506 Beam Charges and Current Monitors

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12th Int. Beam Instrum. Conf.
ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3

12th Int. Beam Instrum. Conf.
ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3

IBIC2023, Saskatoon, Canada
JACoW Publishing
ISSN: 2 6 7 3 - 5 3 5 0
d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - T U 3 C 0 5

This is Content
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