1. BUNCH LENGTH MEASUREMENT SYSTEM FOR 500 KV PHOTOCATHODE DC GUN AT IHEP
Ouzheng Xiao
Institute of High Energy Physics
SAP2017, August 30, 2017
Jishou, Hunan, China

2. Outline
Introduction
Design and Consideration
Beam Dynamics Study
Summary

3. Background
High brightness and low emittance electron source are demand for future accelerator based facilities such as X-ray free electron light (XFEL), ultrafast electron microscopy (UEM) and ultrafast electron diffraction (UED). High voltage photocathode DC gun is one of the excellent electron source.
In 2009, a FEL-ERL two purpose facility was proposed at IHEP. As one of the key technologies, a 500 kV photocathode DC gun was supported in 2012.
In the end of 2016, a preliminary high voltage conditioning has been carried out. The maximum voltage is up to 440 kV.
To measure the bunch length and longitudinal profile at exit of DC gun, a bunch measurement system is essential.

4. 500 kV Photocathode DC Electron Gun
Parameters
Value
Voltage
500 kV
Cathode
GaAs QE
5-7%(initial),1%
Driven laser
2.3W，530nm
Repetition rate
100 MHz/1.3 GHz
Normalized emittance
<1 mm.mrad
Bunch length
20 ps (flat top)
Beam current
(1~10) mA
Xiaoping Li, A Beam Test Facility for High Current Photoinjector and its Key Technologies Development at IHEP,ERL2017,CERN
*Two operation modes:
1). 100MHz-7.7mA-77pC, 2)1300MHz-10mA-7.7pC

5. Bunch Length Measurement Methods

6. Pros and Cons for deflector system
High resolution(fs level).
Versatile.
Stable and Reliable
Intuitive.
Cons
Destructive
Demands relatively long space

7. Principle Introduction
David Alesini，RF deflector-based sub-ps beam diagnostics: application to FELs and advanced accelerators

8. Design Consideration
Input power.
Reduce the beam initial size.
Bunch length stretched due to Space charge effect.
Bunch length

9. Parameters of Bunch Measurement System
Value
Beam Energy(MeV)
0.5
Beam Normalized Emittance (mm.mrad)
0.3
Bunch Total Length(ps) 30
Bunch RMS Length(ps) 6
Beam Size without Deflecting Cavity(mm)
Resolution length(ps) 1
Drift Length（m）
1.4
Deflecting Cavity Frequency（GHz）
1.3
Deflecting Voltage(kV) 23
Input Power(W)
250
Shunt impendence (MΩ)
0.88
Solenoid Magnetic Field（Gs）
310
Operating point
Relation of Resolution length and input power

10. Tsinghua 3-cell SW deflector
Deflector Types
TW and SW deflector
IHEP TW deflector
IHEP SC deflector
IHEP NC deflector
Tsinghua 3-cell SW deflector
KEK NC deflector
Cornel NC deflector

11. Deflector Design Consideration
A single-cell rectangular cavity operating at TM210 mode was selected.
Transverse displacement of bunch center due to transit time.
Suppression of parasitic mode especially degenerate mode
Phase: -173°
Displacement: 0 mm
Divergence angle: -0.4 mrad
Phase: °
Displacement: 0.61 mm
Divergence angle: 0 mrad
*Bunch length calculated of both cases are almost the same.

12. Deflector Design
3D simulation include the input coupler, pick-up and tuner.
Electromagnetic field pattern of TM210

13. Parasitic modes TM110 849.6 MHz TM120 1363 MHz TM220 1692 MHz

14. Deflector Measurement
The parameters measured are in agreement with that simulated.
Parameters
Simulation
Test
f (GHz)
1.3 Q0
23323
20964
Z⊥(MΩ)
0.88 - β
1.07
1.01
Vacuum leak detection
S11
S21
Smith chart

15. Electromagnetic field along the longitudinal position without slit
Dynamics Simulation
The beam dynamics of the whole bunch measurement system is simulated in two different cases using ASTRA. One case is low input power of 250 W with slit, and the other case is high input power of 1000 W without slit.
Electromagnetic field along the longitudinal position without slit

16. Beam size optimization on screen
Solenoid magnetic field strength scan
Solenoid magnetic field scan without slit
(366Gs,0.54 mm)
Solenoid magnetic field scan with slit
(310Gs,0.29 mm)

17. Beam transverse distribution without slit
Before deflector
After deflector

18. Beam transverse distribution with slit
Before deflector
After deflector

19. Bunch length calculation
Bunch length calculation(6.15 ps before deflecting cavity).
Deflecting voltage calibration(Two methods).
Parameters
Without slit
With slit
Vdef(kV) 42 21
σx0(mm)
1.08
0.58
σx(mm)
5.96
3.02
σt(ps)
6.1
6.18
Bunch length calculation results
Calibrate voltage :41.2 kV

20. Beam Longitudinal Profile
The horizontal distribution on the screen is in agreement with the longitudinal shape before deflector.
Longitudinal distribution before deflector
Transverse distribution on screen
Distribution comparison

21. Bunch Length Measurement Error Source
The error sources of bunch measurement mainly include:
Resolution length
Deflecting voltage calibration
Beam energy spread
Beam energy spread (HV system fluctuation less than 0.02%)
The error due to resolution length can be defined as:
If k is 6, the transverse error due to resolution length is 1.4%

22. Bunch measurement system layout
Gun
Beam line and measurement system
HV power supply
500 kV photocathode DC gun
Beam line and measurement system

23. Summary
The diagnostic of ultra-short electron bunch length becomes a key technique in many accelerator. The bunch measurement system based on deflecting cavity is promising.
A bunch length and longitudinal profile measurement system based on deflector for 500 kV photocathode dc gun at IHEP is presented.
A 1.3 GHz rectangular deflecting cavity operating at TM210 mode has been designed , fabricated and measured.
The beam dynamics of the bunch length measurement system has been simulated. The slit before the deflecting cavity can be used to reduce the input power requirement.
So far, all components used in the beam length measurement system have been installed.

24. Thanks for your attention!
Thanks to Jingru Zhang, Xiaoping Li,
Xiangjian Wang, Darui Sun.
Thanks for your attention!