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

Outline Introduction Design and Consideration Beam Dynamics Study Summary

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.

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

Bunch Length Measurement Methods

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

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

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

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

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

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: -173.6° Displacement: 0.61 mm Divergence angle: 0 mrad *Bunch length calculated of both cases are almost the same.

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

Parasitic modes TM110 849.6 MHz TM120 1363 MHz TM220 1692 MHz

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

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

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)

Beam transverse distribution without slit Before deflector After deflector

Beam transverse distribution with slit Before deflector After deflector

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

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

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%

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

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.

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