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Development of a 750-mm-long stripline kicker for HEPS

  • H. Shi
  • J. H. Chen
  • L. Wang
  • N. Wang
  • L. H. Huo
  • G. W. Wang
  • P. Liu
  • X. L. Shi
Original Paper
  • 155 Downloads

Abstract

Purpose

The superfast kickers are required for the HEPS storage ring on-axis injection system due to its very small dynamic aperture. A 750-mm-long stripline pair type of kicker prototype was researched and developed to demonstrate achieved performance of bandwidth, impedance, kicker strength, field uniformity and beam power loss.

Methods

The cross sections of the kicker main body and the end of taper part are optimized for a good impedance matching and field uniformity. 3D simulation further optimizes the taper part to minimize the beam power loss and maintain a lower reflection. The high-voltage feedthrough is also designed and optimized by 3D CST. RF and high-voltage measurements are taken to verify the design of kicker assembly.

Results

The testing transmission odd-mode impedance is 50 ± 0.5 Ω, the even-mode impedance is 60 ± 0.5 Ω, and return loss is less than − 13 dB. The peak voltage and the rise time of pulse width inserting kicker assembly just decrease 3% of 20 kV and slow down 80 ps, respectively.

Conclusions

RF testing results agree well with the simulation ones, which meet the design specification. The kicker assembly works well at ± 20 kV pulse.

Keywords

Stripline kicker Feedthrough On-axis swap-out injection Odd-mode (even-mode) impedance Storage ring 

Introduction

The high-energy photon source (HEPS) adopts a full energy 6 GeV injector and a storage ring [1, 2]. The on-axis swap-out injection is the baseline design for the storage ring due to its very small dynamic aperture (DA) [3]. Superfast kicker and pulser are required to minimize the perturbation to stored beam during the injection [4, 5].

The specification of the swap-out injection system of the storage ring (SR) is shown in Table 1 [6]. We adopt the stripline pair type of kicker [7] which works as a counter traveling wave kicker [8]. The stripline injection kickers for the ILC damping ring have been developed well and tested in the accelerator test facility (ATF) [9] and the Frascati Φ factory (DAΦNE) [10]. But more kick strength and higher field uniformity are needed for the next-generation light source based on ultra-low emittance storage rings [11, 12]. Based on the kicker design of the advanced photon source upgrade (APS-U), the structure of kicker for HEPS is optimized [13, 14], and the lower even-mode impedance and the lower beam coupling impedance [15, 16] are achieved.
Table 1

Specification of the SR swap-out injection system

Injection scheme

Swap-out

Kick direction

Vertical

Length of injection straight section (m)

6

Kick angle (mrad)

3

Rise time of electrical pulse (10–90%) (ns)

4

Flat top of electrical pulse (90%) (ns)

5

Fall time of electrical pulse (10–90%) (ns)

4

Repetition rate (Hz)

50

Good field region (mm)

± 2.3 (x)

 

± 1.0 (y)

Field uniformity

2%

Degree of vacuum (Torr)

10−9

In the research and development stage of HEPS (HEPS-TF), a prototype stripline kicker has been developed. The test bench has been built with the prototype kicker and four sets of commercial feedthrough. The RF characteristics of the final kicker assembly have been measured with network analyzer. The high-voltage pulse measurement has also been taken in vacuum condition. We have also performed the physics design of the feedthrough.

Design of the kicker assembly

Stripline kicker design

For HEPS on-axis swap-out injection, four sets of 750-mm-long stripline kicker are adopted to satisfy the pulse time structure and kick angle [4]. If the gap between two blades is 10 mm, the operation pulse voltage should be ± 15 kV. The odd-mode impedance (Zodd), i.e., the transmission impedance, should be close to 50 Ω for good matching with the pulser, cables and attenuators. Although the ideal even-mode impedance (Zeven) should also be matching to minimize reflection signals induced by the stored beams, this value is much higher than characteristic impedance 50 Ω because of strong coupling of narrow gap between two blades [17]. The kicker parameters are given in Table 2. Zodd and Zeven are required to be optimized to 50 ± 0.5 Ω and 60 ± 0.5 Ω, respectively.
Table 2

Parameters of the stripline kicker

Length of blades (mm)

750

Number of kicker

4

Gap between two blades (mm)

10

Operation pulse voltage (kV)

± 15

Odd-mode impedance (Zodd) (Ω)

50 ± 0.5

Even-mode impedance (Zeven) (Ω)

60 ± 0.5

The 2D model of the stripline kicker is shown in Fig. 1. The two D-shape blades are placed vertically. Three equidistance ellipses define the location and shape of blades, here length of long and short axes of the center ellipse is a, b, and the thickness is blade. The half outer shell is composed of two half ellipses, whose center point, long axis and short axis are (Xc, a0, b0) and (Xc, a00, b0) separately. The vane is introduced to decouple the electromagnetic (EM) field between 2 blades to lower Zeven [13]. Tapered ends are also needed to lower the longitudinal beam impedance [10, 13].
Fig. 1

Parametric geometry of the stripline kicker cross section

If we choose vane = b (shown in left figure of Fig. 2a), Zeven can be optimized to about 65 Ω as well as considering Zodd = 50 ± 0.5 Ω, lower maximum electric field and field uniformity, which agrees with the experience of APS-U [18]. Then three optimization methods are introduced step by step with 3D CST [19], as shown in Fig. 2.
Fig. 2

Optimization of the kicker: (1) decrease the vane value, (2) widen the vane at the taper end, (3) add 2 end covers at the ends of kicker model

In step (1), the vane value is reduced to decrease the field coupling of two blades (shown in the middle figure of Fig. 2a), i.e., decrease in Zeven. The geometry size of main body is identified in step (1). Taper parts are optimized in steps (2) and (3), but gap and vane values are same as those in main body for the continuity of beam pipe. In step (2), we widen the vane width at the taper end (shown in right figure of Fig. 2a) to reduce the coupling further. But the vane width couldn’t be too wide because that will result in the worse field uniformity. In step (3), two end covers are placed at the two ends of kicker to narrow the distance between blades and vacuum tank in the transverse section (shown in Fig. 2b) to 3 mm which is safe in 20 kV, because less beam wake field deposits in smaller volume of kicker.

The simulation return loss S11 and longitudinal wake potential W(z) at σz = 3 mm beam length are shown in Fig. 3. Although S11 is almost unchangeable during the optimization (Fig. 3a), the beam loss factor (η) decreases gradually (Fig. 3b). η decreases about 19% (1.042 V/pC → 0.839 V/pC) after step (1) and further decreases about 17% (0.839 V/pC → 0.698 V/pC) after steps (2) and (3).
Fig. 3

a S11 and b longitudinal wake potential at σz = 3 mm comparison before and after each optimization

The final geometry sizes of two transverse sections of main body and taper end are shown in Table 3. Three-mm-thick blade was chosen, considering the balance of the blade rigidity and bandwidth of the stripline kicker. The length of the main body and two taper parts is 650 mm and 50 mm, respectively. The elliptic vacuum tubes defined by (vane, gap/2) are connected with two taper parts.
Table 3

Optimized geometry sizes of two transverse sections

 

Main body

Taper end

a (mm)

8.3

7.5

b (mm)

7.85

7.85

Gap (mm)

10

10

Blade (mm)

3

3

Vane (mm)

6.13

6.13

Xc (mm)

3.18

3.8

a0 (mm)

3.11

2.8

a00 (mm)

15.6

11.2

b0 (mm)

14.4

13.86

Length (mm)

650

50

The optimized RF characteristics of two transverse sections of main body and taper end are given in Table 4. Although the even-mode impedance of taper end section is lower, the field uniformity becomes worse in both directions. However, the uniformity of integrated field is little affected for the 50 mm short length of taper part. In the condition of the pulse voltage ± 15 kV between the two blades, the maximum electric field Emax in the two sections is below 7 MV/m (shown in Fig. 4), which is less than Kilpatrick limit 13 MV/m.
Table 4

Optimized results of two transverse sections

 

Main body

Taper end

Zodd (Ω)

49.81

50.00

Zeven (Ω)

60.39

58.23

Emax (MV/m)

6.42

6.63

Field uniformity @ y (± 1 mm) (%)

0.20

0.63

Field uniformity @ y (± 5 mm) (%)

1.70

5.76

Field uniformity @ x (± 2.3 mm) (%)

1.45

4.39

Fig. 4

The maximum electric field distribution in a main body and b taper end

Feedthrough design

Although four sets of the commercial feedthrough from FID GmbH have been assembled in the prototype kicker and the feature can satisfy the requirement, the short inner conductor at the vacuum side makes the assembling very difficult. According to the specification of feedthrough, operation voltage above 20 kV, bandwidth above 1 GHz, vacuum degree better than 1 × 10−9 Torr, the design of the feedthrough has been performed.

The cross section of the feedthrough is shown in Fig. 5. The machinable glass ceramic of low permittivity approximate 6 is adopted for better impedance matching. The insulator material at air side is poly tetrafluoroethylene (PTFE) of permittivity ~ 2.1. The feedthrough assembly without welding has been fabricated and tested with Keysight E5071C Network Analyzer (NA). The voltage standing wave ratio (VSWR) in 300 kHz–3 GHz is shown in Fig. 6a, and the maximum testing value is just ~ 1.5 till 2.5 GHz, so the bandwidth is widen enough. The testing time-domain reflectometer (TDR) impedance agrees with simulation one (shown in Fig. 6b).
Fig. 5

Cross section of the feedthrough

Fig. 6

a VSWR and b TDR impedance comparison of the testing and simulation results of the feedthrough

The final mechanical drawing of kicker and designed feedthrough is shown in Fig. 7. The blades are sustained only by feedthrough inner conductors at the ends, so the deflection is a little large in the middle of the copper blades. Several improvements have been considered, such as stainless steel blades with copper coating, ceramic supports and thicker blades, and the first one is adopted [20].
Fig. 7

Mechanical drawing of kicker assembly with designed feedthrough

Figure 8a shows the end part of the kicker prototype with four sets of FID feedthrough. The assembly of vacuum pipe and kicker is shown in Fig. 8b.
Fig. 8

Photographs of a end part of kicker and b assembly of vacuum pump (left) and kicker (right)

RF measurement of the assembly

Before the high-voltage test, the RF test with E5071C NA is performed and the test setup is shown in Fig. 9. At one end of kicker, the vacuum pump is used to achieve < 1.6 × 10−9 Torr vacuum degree, which is lower than the requirement because the volume of vacuum tank and pipe are too small. Additional vacuum pump is being considered to be installed at the other end of the kicker. Four sets of commercial FID feedthrough are assembled with the prototype kicker, and four adapters to SMA are designed to connect them to the NA.
Fig. 9

RF test setup

As shown in Fig. 10a, the deterioration of S11 is mainly caused by feedthrough from the CST simulation results. But from DC to 1 GHz, the maximum simulation S11 is less than − 17 dB, and the maximum testing one is less than − 13 dB, i.e., the power reflection loss coefficient is approximate 5%. The testing TDR impedance is shown in Fig. 10b, which agrees with the simulation one. The testing impedance distortion in the middle part of kicker has been improved because we have chosen stainless steel blades with copper coating [20]. Testing odd-mode impedance of kicker is 50 ± 0.5 Ω and even-mode one is 60 ± 0.5 Ω, which meet the specification very well.
Fig. 10

a S11 and b TDR impedance comparison between simulation and measurement of the kicker

High-voltage pulse measurement

The commercial pulsed power supply (PPS) is used to provide bipolar HV pulse of ± 20 kV peak and 4 ns pulse width simultaneously. The PPS output voltage waveforms on the attenuator of 50 Ω are shown in Fig. 11. The black curve and red curve are the positive pulse (PP) and negative pulse (NP), respectively. After inserting the kicker assembly and another 3-m-long coaxial-line cables (RG217), the output pulses, which are shown in blue and in magenta, become slower and weaker a little. It is about less than 80 ps of rise time and 3% of peak. So the performance of prototype kicker is good enough for our application.
Fig. 11

The voltage waveforms of positive and negative pulse with and without kicker

Summary

Three improvement methods have been adopted to achieve the required impedance of kicker, i.e., odd-mode and even-mode impedance being 50 ± 0.5 Ω and 60 ± 0.5 Ω, respectively. Accordingly, the beam depositing loss decreases about 36%, whereas the return loss is almost constant. The scheme of stainless steel blades with copper coating is adopted to decrease the deflection.

The physical and mechanical design of high-voltage feedthrough has been finished. The cold model has been tested after assembling the inner, outer conductor, machinable glass ceramic and PTFE. The maximum VSWR in 300 kHz–2.5 GHz is ~ 1.5, i.e., the reflection power coefficient is ~ 4%. The welding between ceramic and inner and outer conductor is being considered.

The prototype of 750-mm-long stripline kicker has been fabricated and been tested in the laboratory. The testing odd-mode and even-mode impedance satisfies the requirement, and the maximum testing S11 in 300 kHz ~ 1 GHz is less than − 13 dB. The performance of kicker in high-voltage pulse test shows very low mismatching, which just slows the rise time less than 80 ps and decreases the peak voltage less than 3%.

Notes

Funding

Funding was provided by National Natural Science Foundation of China (Grant Nos. 11475200 and 11675194).

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Copyright information

© Institute of High Energy Physics, Chinese Academy of Sciences; Nuclear Electronics and Nuclear Detection Society and Springer Nature Singapore Pte Ltd. 2018

Authors and Affiliations

  1. 1.Key Laboratory of Particle Acceleration Physics and Technology, Institute of High Energy PhysicsChinese Academy of SciencesBeijingChina
  2. 2.University of Chinese Academy of SciencesBeijingChina

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