1. Recent studies on BEPCII longitudinal impedance
Na Wang, Qing Qin, Wen Kang Institute of High Energy Physics, Beijing, China

2. Introduction on BEPCII
BEPCII — Beijing Electron Positron Collider Upgrade
— Serves for high energy physics experiments and synchrotron radiation applications
— Multi-bunch collision with double rings
Linac
BESIII Spectrometer
Accumulator
储存环

3. Main parameters achieved in this run (collision mode)
Design
Achieved
BER
BPR
Energy (GeV)
1.89
Beam curr. (mA)
910
~800
Bunch curr. (mA)
9.8
~9.0
Bunch number 93
80 – 88
RF voltage
1.5
1.5 – 1.7
Circumference
240
y* (m)
0.015
0.014 – 0.015
Lifetime (hrs)
B – B parameter
0.04
0.0327
Lum. (1032cm-2s-1) 10
6.49
Progresses are needed to achieve higher beam current and higher luminosity!

4. Hardware failure during this HEP run
Downstream injection kicker

5. What brings the damages?
Low uniformity of the coating (metallic mixture)
Beam or photon hitting on the plate
Power loss heating due to the impedance
(less impossible as the upstream kickers work fine) …
What can we do about it?
Change coating material
Use metal slot instead of ceramic
Beam orbit optimization
The impedance of different kicker structures are calculated and compared

6. Impedance calculation with MAIFA code
Simulation model
Longitudinal wake obtained by MAFIA time domain solver
7mm gap

7. Longitudinal Impedance
f, GHz
Im Z// , 
Re Z// , 
f, GHz
3.7 GHz
L = 11 nH
kloss = V/pC
P = 177 W (nb=93,Ib=9.8mA)
7 mm gap in current bus

8. Optimization of the shielding structure
Ceramic with metal coating, w=34mm
Slotted pipe
Multi-slots
w=15mm
w=10mm
6mm×5
10mm×3
kloss (V/pC)
0.025
0.041
0.059
0.019
0.022
P (W)
nb=93, Ib=9.8mA
177
290
418
134
156
L (nH) 11 25 39 4 6
The structure is chosen to compromise between high field uniformity and low beam impedance.

9. Impedance spectrum one slot 10mm multi slots 10mm3 Re Z// , 
Im Z// , 
one slot 10mm
f, GHz
f, GHz
multi slots 10mm3

10. Problems due to the gap in the slots
Multi-slots 10mm3
In order to keep high uniformity, small gaps of 1~2mm wide in the side slots are needed.
L = 38 nH
kloss = V/pC
P = 304 W (nb=93,Ib=9.8mA)

11. Discussions & summary
The reasons for the kicker damage are not quite clear, and further studies need to be done to avoid more damages.
The multi-slot structure shows lower impedance and are less vulnerable to incident beam, and it might be used to replace the ceramic structure in the future.
It seems that we can’t benefit much from the multi-slot structure if there is gaps in the side slots.

12. Thanks for your attention!