Beam position monitor for superconducting post-linac in RAON

Accepted Manuscript Beam position monitor for superconducting post-linac in RAON J.W. Kwon, H.J. Woo, G.D. Kim, Y.S. Chung, E.-S. Kim

PII: DOI: Reference:

S0168-9002(18)31004-0 https://doi.org/10.1016/j.nima.2018.08.046 NIMA 61087

To appear in:

Nuclear Inst. and Methods in Physics Research, A

Received date : 19 June 2018; Revised date : Accepted date : 15 August 2018

15 August 2018;

Please cite this article as:, Beam position monitor for superconducting post-linac in RAON, Nuclear Inst. and Methods in Physics Research, A (2018), https://doi.org/10.1016/j.nima.2018.08.046 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Beam position monitor for superconducting post-linac in RAON J.W. Kwona,b , H. J. Wooa , G. D. Kima , Y. S. Chunga , E.-S. Kimb,∗ a Rare

Isotope Science Project, Institute for Basic Science (IBS), Daejeon 34047, Korea of Accelerator Science, Graduate School, Korea University, Sejong, Korea

b Department

Abstract The Rare-isotope Accelerator complex for ON-line experiments (RAON) is an accelerator for heavy ions, such as uranium and oxygen. To correct the beam trajectory in the post-linac with low-β, we developed a beam position monitor. In the post-linac, the beam is accelerated from 0.5 MeV/u to 18.5 MeV/u, and each bunch has an electric charge of less than 10 pC. To achieve higher signal strength and better linearity, we investigated stripline- and button-type beam position monitor (BPM) and designed the BPMs with CST Particle Studio. We have fabricated a button-type BPM. We tested 10 BPMs by using a wire test stand to achieve the characterization of each BPM, including the calibration factor with single-pass H electronics of I-Tech. The developed wire test stand has a stretched wire and is movable in the 2D plane with servomotors. In this paper, we present the results of the design, fabrication, and off-line test of a button-type BPM for a low-β, heavy-ion beam. Keywords: linear accelerator, beam diagnostics, beam position monitor

1. INTRODUCTION The layout of the rare-isotope accelerator complex for on-line experiments (RAON) is shown as Fig. 1 [1]. ∗ Corresponding

author Email address: [email protected] (E.-S. Kim)

Preprint submitted to Journal of LATEX Templates

August 19, 2018

Figure 1: Layout of RAON

Raon is a superconducting linear accelerator that consists of SCL1, SCL2 5

and SCL3 sections [2]. Post-linacc(SCL3 section) is a superconducting linear accelerator that consists of quarter-wave resonator (QWR) and half-wave resonator (HWR) sections. In the post-linac, the initial uranium beam has an energy of 0.5 MeV/u maximum, pulse current of 340 µA, and a bunch length of approximately 1 ns. At the end of post-linac section, particle energy becomes

10

18.5 MeV/u with β = 0.19. RAON has periodic sections with a warm section for beam focusing and beam diagnostics in each unit part of linear accelerator. The beam position monitor (BPM) will be set in between quadrupole magnets in the warm section [3]. The inner diameter of the beam pipe in post-linac is 40 mm, and BPM is required to measure the beam position and phase. The

15

required transverse-position resolution is below 100 µm and phase resolution is below 0.5

◦

at 81.25 MHz.

In the low-β section, a single induced voltage signal from the low-β bunch is longer than the real single bunch length. Considering this effect, we investigated 2

several types of BPMs with CST Particle Studio using the parameters of a 20

uranium case [4]. Usually, BPM electrodes pick up signals from the bunched beam by inductive coupling or capacitive coupling, referred to as ”stripline” or ”button” respectively [5]. The electrical characteristic is different between the inductive and capacitive coupling cases. It can be easily explained by the signalformation process in the time domain. Thus, the signal strength, phase, and

25

position linearity were considered for the optimum design of BPMs. When we examine the BPM after fabrication, the reproducibility and signal transmission of the wire test stand setup are considered. In this paper, we present the considerations of design optimization and the off-line test results of the BPM.

2. ELECTRONIC SIGNAL

Figure 2: Illustration of low-β effect by non-relativistic beam for MEBT output

30

In the case of a high-β bunch beam, a single induced signal length in the time domain is similar to the bunch length in the time domain because the electric field width is similar to that of a moving charged-particle bunch length by the relativistic effect. However in case of low-β beam, electric field is broader than that of charged particle bunch length. The position sensitivity of high frequency

35

BPMs may be affected at low beta beams. The position sensitivity depends on signal processing frequency, beam velocity and BPM aperture [6]. It means that electric field becomes broad for longitudinal axis by non-relativistic effect. 3

Accordingly for the heavy ion beam having low-β, we should consider the low beta effect. In our case, the output beam from the medium-energy beam 40

transport (MEBT) has 0.5 MeV/u, β = 0.033, and an approximate bunch length of 0.83 ns (6 σ). A single induced signal from the image current is about 10.5 ns, which is much longer than that of bunch length, as illustrated in Fig. 2. At the end of the post-linac, the output beam has 18.5 MeV/u, β = 0.19, and a bunch length of 0.24 ns, and the single-voltage signal length is around 1.7 ns. To

45

prevent from overlapping the signal of the next bunch, all the signals by single bunch should be below 12.3 ns, which is same as the bunch repetition of RAON. The output signals could be small by overlapping with the signal of the next bunch, because total signal at each electrode for single bunch is bipolar shape. The bipolar shape is depend on electrode type, as discussed in below. In the

50

start of post-linac in RAON, a single induced signal length is close to the bunch repetition of 12.3 ns. Based on this, we should consider which type of BPM is suitable for a low-β beam to achieve greater signal strength because the signalformation processes of stripline- and button-type BPMs are different depending on the electrode shape. Fig. 3 shows the processes of signal occurrence for the

55

two types of BPM.

Figure 3: Signal formation concept of stripline-type electrode and button-type electrode (red is induced signal and green is reflected signal)

4

Short stripline BPM consists of a single feedthrough and a single electrode that is attached to the housing (ground) to form an inductive coupling shape. The signal at each electrode is formed by the addition of an induced signal and reflected signal. First, the induced signal is generated at a gap between 60

the electrode and housing. Then, the signal is separated along two paths of the pick-up port and the electrode. The signal that travels to the pick-up port arrives earlier than the other. The signal that goes through the electrode is reflected at a connected part of the electrode and housing which has connected to ground. The reflected signal is the opposite polarity of the induced one at

65

a gap [7] because the end of the electrode is connected to the ground. There is a time delay between the signal that arrives earlier and the reflected signal that arrives later and the delay corresponds to the length of the electrode. The signal transmission speed is equal to the speed of light in a vacuum, so we can the express reflection time delay as 2 l/c. Here l is the electrode length and c

70

is the speed of light. Typically, the length of an electrode is shorter than 30 cm, so the time delay is much shorter (≤ 2 ns) than the induced signal length, which is 10.5 ns for the MEBT output beam. The signal strength is expected to be very small by overlapping the induced signal and reflected signal. This can be understood by the simple signal-formation process equation in the time

75

domain as Equation. 1. In this equation, we assumed that the single induced signal has the Gaussian distribution shape. Here, σt is the standard deviation of the induced signal distribution, which depends on the charge distribution and beam velocity. l is the length of a stripline electrode.

Ustrip (t) = Zgeo (induced + ref lected)I0

Ustrip (t) = Zgeo (e−t

2

/2σt2

2

− e−(t−2l/c)

/2σt2

)I0

(1)

(2)

Usually, button-type BPMs consist of circular electrodes that look like a 80

button. However we consider a rectangular electrode welded with a single feedthrough to produce capacitive coupling. The rectangular-button BPM sig-

5

nal is formed by the addition of 4 signals: 2 induced and 2 reflected signals occurred at either side of the electrode. An induced signal occurs at the first gap where beam reaches. Then the signal is separated to the pick-up port and 85

electrode same as in the stripline-type. The signal that travels through the electrode is reflected at the second gap which is not connected to anything, and is also called ”open”. In that case, the reflected signal polarity is the same as the incoming signal through the pick-up port [7]. Typically, the longitudinal length of the button is shorter than 3

90

cm. Therefore the reflection time delay (≤ 0.1 ns) is much shorter than the low-β single induced signal length. This means that the signal induced from the first gap can be transmitted to the pick-up port with its own strength without signal canceling due to overlapping. When the ion beam passes through the second gap, another induced signal occurs, which has the opposite shape to

95

the first induced signal. At the first gap, the charged particle goes into the electrode; however, at the second gap, the particle goes out from electrode. Signal transmission to the pick-up port is the same as that for the first gap. However, the time interval between two signals caused by particle flight (l/βc) is 3 ns for the MEBT output beam and 3 cm electrode length. 3 cm is long enough

100

to avoid excessive signal canceling between the signals occurring at the first and second gaps. Therefore the signal strength of the button-type is greater than that of the stripline-type and we conclude that button-type BPM is suitable to measure the transverse beam position in the low-β section. The signal-formation process for a rectangular button-type BPM is described as Eq. 3.

Ubutton (t) = Zgeo (induced1 + ref lected1 + induced2 + ref lected2 )I0

Ubutton (t) = Zgeo (e−t

2

/2σt2

−e−(t−l/c)

2

/2σt2

2

+e−(t−l/βc)

/2σt2

(3)

2

−e−(t−l/βc−l/c)

/2σt2

)I0

(4) 105

Two types of BPM are understood in terms of the signal formation equations in the time domain. The stripline-type BPM can be suitable for high-β, however 6

it requires detailed design study. In post-linac, the beam has low-β from 0.033 to 0.19, and button-type BPM is suitable for the entire post-linac section.

3. SIMULATION AND DESIGN

Figure 4: Simulation results of stripline vs button type BPMs for MEBT output and post-linac output beams with current of 1 mA

110

We have simulated BPM designs with CST Particle Studio to confirm the signal-formation processes of stripline and button-type BPMs for both the MEBT output and the post-linac output. The design of BPM and its simulation results are shown in Fig. 4. In the simulation the stripline electrode has a length of 7 cm and width of 1.4 cm. And the rectangular button has a length of 20 mm

115

and width of 20 mm. Each gap between the electrode and the housing is set to be 2 mm. The simulation results are based on the beam pulse current of 1 mA and the bunch length and velocity are obtained from beam dynamics studies which are different for MEBT and post-linac output. The simulated results of button-type BPM show that the peak voltage is 30 mV for the MEBT output

7

120

beam and is 150 mV for the post-linac output beam. However, the outputs of stripline-type BPM show that the peak voltages is about 3 mV for the MEBT output beam and is about 60 mV for the post-linac output beam. The CST simulation also shows that the button-type BPM is suitable to measure the transverse beam position in the post-linac as in terms of signal strength.

125

We have a geometrical constraint of BPM installation, as it must be inserted in quadrupole magnets of the warm section which consists of quadrupole magnet doublet, a BPM, and a beam diagnostics chamber between two cryomodules. Hence, the volume of the BPM is restricted. We chose a rectangular electrode considering the volume constraint and optimal fabrication process. The elec-

130

trode width is related to the longitudinal impedance and a broader electrode yields higher signal strength. This means that the broader electrode has a higher induced signal by covering wide angle of the electric field from the charged particle. The length of electrode affects the signal formation as we discussed earlier. We chose a button width of 20 mm to have maximum coverage angle. We then

135

investigated the shape of the electrode (flat or curved) and the button length (20 mm or 30 mm) for the MEBT output beam. Until now, we only focused on the signal strength, the position linearity also needs to be considered because the BPM is a device to measure position. For this, we simulated various beam position offsets of 0-10 mm with 2-mm

140

intervals for each X and Y axis. Fig. 5 shows the calculated beam position(X,Y ) with peak voltages using Eq. 5.

X0 =

Right − Lef t U p − Down , Y0 = Right + Lef t U p + Down

(5)

Both of the flat and curved electrodes, the signal strength for the 30-mm electrode length is 15% higher strength than that of the 20-mm length. Also the signal strength of the curved type is 20% higher than that of the flat type. 145

The length of electrode is related to the signal process and the button longer than a half of the single induced length is good to avoid the invasion of peak values with each other. However the electrode is designed to be welded to the

8

Figure 5: Signal strength of a beam passing the center of the BPM and linearity in area of 1 cm2 (black square) for curved and flat type button BPMs with electrode length of 20 mm and 30 mm. Uranium beam has energy of 0.5 MeV/u and current of 1 mA.

center conductor of the feedthrough, and the weight of electrode is limited as the feedthrough must support it. The curved type BPM shows better linearity 150

than the flat type as shown in Fig. 5. This is simply because curved electrode can have a wider coverage angle than the flat type. We calculated the signal strength of 81.25 MHz using Matlab code as shown in Fig. 6. For the electrode with a width of 20 mm and length of 30 mm, the CST Particle Studio simulation shows that beams passing through the center

155

of the BPM with a pulse current of 100 µA produces signal of -45 dBm for the MEBT output beam and -55 dBm for the post-linac output beam in a 50Ω system.

9

Figure 6: Estimation of signal strengths by Matlab code using simulation result of a width of 20 mm and length of 30 mm button type BPM. Uranium beam has energy of 0.5 MeV/u and current of 100 µA. All blue lines denote raw signal and all red lines denote filtered signal of 81.25 MHz. Bule lines in center figure show second and third harmonics of 81.25 MHz.

4. FABRICATION AND CHARACTERIZATION

Figure 7: Drawing and fabricated BPM.

Post-linac will be equipped with button-type BPMs for monitoring and the 160

stabilization of the particle-beam trajectories through feedback. The button with an rectangular electrode (20 mm × 30 mm) was welded concentrically to the center conductor pin (φ 1.84 mm) of the Kyocera SMA feedthrough by using a specific welding template. Each electrode has angular coverage of 58.2 ◦ . Four button-feedthrough assemblies and BPM housing were welded together

165

according to the specific procedure using a split jig rod not to allow more than the geometrical tolerance of about 50 µm. Because the BPM is mounted in the middle of the quadrupole magnet in the warm section, the housing, electrode, and flange were fabricated from 316L stainless steel, that is a non-magnetic 10

material. The deviation of the capacitance values of the electrodes at a BPM 170

block was measured to be about 0.14 pF, and this small deviation corresponds to 2.5% compared to average capacitance of 5.5 pF. Thus it seems to be acceptable based on the positional accuracy requirement of the BPM. The capacitances of 300 kyocera SMA feedthroughs were measured to be 1.81±0.016 pF, and the four feedthroughs were matched by less than 0.01 pF capacitance deviation to

175

form a single BPM.

5. WIRE TEST

Figure 8: Wire test bench(Left) and BPM test setup(Right)

The wire test method is a simple and effective way to check the BPM performance without a real beam. Using the induced signal from a stretched wire, we can check the signal-transmission characteristic of the BPM. The developed 180

wire test stand has a stretched wire that is set through the BPM and a movable bench for the 2D planes. The movable bench consists of X-Y stages with 2 servomotors controlled by serial communication. Each servomotor can be controlled with an accuracy of one micrometer. A coaxial cable from the signal generator is connected to the wire of diameter 0.5 mm. The induced signal at

185

the BPM pick-up from the wire is transmitted to the read-out electronics. The wire is covered with metal shielding to minimize the electromagnetic noise.

11

Table 1: The position resolution versus input powers(electronics only)

Input signal

Corresponding current

RMS of position [measured]

-25 dBm

1 mA

0.41 µm

-35 dBm

330 µA

1.33 µm

-45 dBm

100 µA

4.72 µm

-55 dBm

33 µA

14.4 µm

-65 dBm

10 µA

46.1 µm

-75 dBm

3 µA

103.8 µm

The Libera Single Pass H(LSPH) of I-Tech is used to calculate the beam positions for the X axis and Y axis, and phase to the reference signal by using the IQ method [8]. The reference signal frequency of 81.25 MHz which is 190

RAON bunch repetition frequency is used. The processed frequencies to measure position and phase are 81.25 MHz and 162.5 MHz using 105 MHz sampling frequency. The LSPH requires one reference input and four pick-up signal inputs for each Rf module. Before the overall BPM performance check, we tested the electronics itself

195

to determine the static calculation value solely from electronics. The input reference-signal power was 7 dBm and we used a 4-way splitter to input the same signal strength to 4 input ports of the electronics. The measured position resolutions in terms of the various input power are shown in Table 1. Here, the input signal is the signal strength to 4 input port of

200

the electronics. The root-mean-square (RMS) of the position is the deviation of 1000 measured position values. As the signal strength decreases, the measured RMS position variation increases as expected. The corresponding current is the beam current value estimated to the signal strength at 81.25 MHz as described in Fig. 6.

X, Yposition = Kx,y × X0 , Y0 + of f setx,y 205

(6)

Electronics calculate the position for each axis independently and each axis

12

requires 2 signals from either side of the BPM port. As shown in Eq.

5,

we can determine X0 , Y0 with 4 input signals. Signal strength of each pick up port which has positive value is used for calculation of X0 ,Y0 . The X0 is calculated value by delta over sum calculation method, which is unitless and is 210

supposed to have a value between -1 to 1 [5]. The calibration factor to give the position meaning with respect to the real position is shown in Eq. 6. The calibration factor mainly comes from BPM geometrical parameters which are the electrode shape, pipe size, and electrode type, etc. There are several sources for the presence of the offset. One of them is the BPM fabrication error which

215

is typically an order of few micrometers. The transmission characteristic is not perfectly identical for all 4 electrodes by capacitance difference. Therefor the beam travels through the center of the BPM, 4 electrodes cannot pickup the same signal strength with allowed manufacturing tolerance. Other sources are the cable characteristic difference, Analog to Digital Converter(ADC) offset and

220

installation alignment error , etc. Therefore, the offset value to each axis should be considered to correct the measured position.

Figure 9: Calibration result between real position and measured position and 2D X,Y mapping

We obtain a calibration factor and offset value with the wire test for each BPM. We measure X0 and Y0 by moving the BPM by a range of ± 10 mm for each axis. The calibration factor and offset value can be calculated easily within 225

the range of ± 4 mm which is considered as the linear region. Fig. 9 shows the calibration result and 2D mapping results considering calibration factor. Here 13

Table 2: Wire test results of 10 pre-production BPMs 10 BPM result of 2D mapping Accuracy

Deviation

Kx

offsetx

Ky

offsety

[um]

[um]

[nm/A.U]

[nm]

[nm/A.U]

[nm]

14436

BPM list

5 mm

12 mm

12 mm

BBPM40 - 005

48.75

902.26

61.21

10851656

-30350

10889296

BBPM40 - 006

48.99

904.84

72.89

10848730

110847

10859304

81685

BBPM40 - 007

50.14

901.08

70.32

10869313

-34318

10865093

-79230

BBPM40 - 008

52.72

904.26

61.37

10864993

-65884

10883246

-4225

BBPM40 - 009

46.91

899.51

54.75

10873025

-34906

10887750

-39339

BBPM40 - 010

54.98

905.39

56.11

10861474

37746

10889416

19926

BBPM40 - 011

49.49

901.88

54.48

10869284

64998

10885832

27631

BBPM40 - 012

49.43

901.47

55.27

10856529

18971

10885328

-23104

BBPM40 - 013

47.46

900.54

55.28

10873856

49476

10883401

26076

BBPM40 - 014

63.02

905.78

56.05

10873463

5085

10888971

34806

the results are corresponding to a 100µA beam current. We measured 1000 times at each position, and scanned with 1-mm interval up to a radius of 12 mm circular region. We then calculated the average position 230

and standard deviation of 1000 measurements for each position. Table 2 shows 2D mapping results of 10 pre-production BPMs for RAON post-linac. Accuracy is the average value of difference between measured value and real wire position for each 5 mm and 12 mm circular region. Deviation is the average value of standard deviation for every position in a 12-mm radius. Kx,y and offsetx,y are

235

the calibration factor and offset value for each X,Y . Results show that the accuracy is around 900 µm within 12 mm, and around 50 µm within 5 mm and position deviation within 12 mm is always under 100 µm.

6. CONCLUSIONS 240

Most of ion beam accelerated in post-linac of RAON, will have less than β=0.2, and the single induced signal to BPM by a charged particle is longer than the bunch length. Based on the CST simulation and focused on the higher signal strength, we conclude that button-type BPM is more suitable than the

14

stripline-type BPM for low-β region. We designed a button-type BPM with 4 245

rectangular electrodes. Each electrode is 20 mm long and 30 mm width with SMA feedthrough and a curved inner surface. We fabricated 10 BPMs as preproduction version for RAON post-linac and tested them at a wire test bench with LSPH. The input signal strength which is corresponding to the 100µA beam current at the post-linac input beam is used for the test. The calibration

250

factor and offset value of the X and Y axies is obtained with the linear region in each axis. The difference of the calibration factor K is obtained under 0.3%. The difference of the offset values between the maximum and minimum value is obtained less than 200 µm for X and Y. Correcting with the calibration factor and offset value, we also obtain the 2D(X,Y ) mapping. 2D mapping shows that

255

all the BPMs have an accuracy value of around 50 µm in the 5 mm radius range and a deviation value of less than 100 µm. The accuracy and deviation are within our BPM fabrication requirements.

7. ACKNOWLEDGEMENT This work was supported by the Rare Isotope Science Project of Institute 260

for Basic Science funded by Ministry of Science and ICT and NRF of Korea (2013M7A1A1075764) References [1] S. K. Kim, Baseline design summary, http://risp.ibs.re.kr/. [2] E.-S. Kim, et al., Start-to-end simulations for beam dynamics in the risp

265

heavy-ion accelerator, Nucl. Instrum. Meth. A 794 (2015) 215–223. doi: 10.1016/j.nima.2015.05.044. [3] J. G. Hwang, et al., Beam dynamics for high-power superconductinf heavyion linear accelerator of raon, IEEE Trans. Nucl. Sci. 63 (2016) 992–1000. doi:10.1109/TNS.2015.2500909.

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[4] Computer simulation technology, https://www.cst.com/. 15

[5] P. Forck, Lecture Notes on Beam Instrumentation and Diagnostics, Joint Univ. Accelerator School, 2009. [6] R. E. Shafer, Beam position monitor sensivity for low-beta beams, AIP Conference Proceeding 319 (1994) 303–308. 275

[7] P. Strehl, Beam Instrumentation and Diagnostics, Springer, Berlin, Heidelberg, 2006. [8] Instrumentation technologies, https://www.i-tech.si.

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