Emission-related Activities at PITZ Proposals for the DFG-Networking - - PowerPoint PPT Presentation

Emission-related Activities at PITZ

— Proposals for the DFG-Networking "Hi2PE"

Ye Chen for the PITZ team

22.09.2017, Helmholtz-Zentrum Berlin, Germany

Contents

 Photoemission (PE)  Dark Current (DC)  Other (Long-Term) Topics

Literature

[1] S. Schreiber, S. Lederer, FEL 2015 [2] L. Monaco, P. Michelato, et al., EPAC 2006 [3] D. Filippetto, P. Musumeci, et al., PRSTAB 17, 024201 (2014) [4] D. Lipka, et al., DIPAC 2011 [5] K. L. Jensen, Advances in Imaging and Electron Physics, Electron Emission Physics Vol.149, 2007 [6] E. Gjonaj, ICAP 2012 [7] R. Xiang, A. Arnold, T. Kamps, et al., PRSTAB 17, 043401 (2014) [8] P. Piot, et al., PRSTAB 16, 010102 (2013) [9] M. Zolotorev, SLAC-PUB-5896 (1992)

Ye Chen | Proposals for Hi2PE Networking: Emission-related Studies at PITZ | 22.09.2017 | Page 2/5

Photoemission (PE) Studies

• 1. (Cs2Te) Experimental & Numerical PE Studies
• Impacts of full EM field (RF + space charge)@cathode on PE (Cont.)
• Modeling of PE in space charge dominated regime
• Charge production using various RF gun and measured PC laser

parameters (simulation vs. measurement)

• Dedicated studies on bunch length, slice energy spread, etc., to explain

discrepancies between simulation and measurement (e.g., Bunch length in Astra systematically longer)

Ongoing activity at PITZ and TU Darmstadt Charge production per simulation time step: Quantum efficiency formulism: Field-induced work function modification: Dynamic emission process from cathode taking into account 3D full EM fields at cathode in a Lienard-Wiechert approach OSS signal with measured flattop pulse shape (old data)

realistic cathode laser distributions + Possible laser intensity and time dependent cathode work function variation, e.g., in [9] Further modeling requires

Radially uniform distribution Non-uniform distribution generated according to VC2 data Measured pulse shape + length

 More precise treatments needed for the modeling in space charge dominated regime  Remaining deviations w.r.t. simulations probably due to the initial ideal beam distributions plugged in the simulations Measured Bunch charge Space charge dominated regime

Ye Chen | Proposals for Hi2PE Networking: Emission-related Studies at PITZ | 22.09.2017 | Page 3/5

Photoemission (PE) Studies (cont.)

• 2. (Cs2Te) Investigations of a µs-scale spiky structure during electron bunch train

extraction at FLASH

• Flat UV laser energy pulse train producing µs-scale spiky structure along electron bunch train
• Toward modeling of RF pulse and/or laser pulse heating effect

Ongoing activity at PITZ, INFN and FLASH

Emission issue of fresh cathode 73.3 (and some others) at FLASH, 04.02.2015, QE=10% Experiments at PITZ

Fixed BSA SP ≈ 0.799 mm, 43.3 MV/m vs. 28.97 MV/m, LT=10% Set the working point in linear emission regime

Laser pulse heating?

~8% ~9%

Ye Chen | Proposals for Hi2PE Networking: Emission-related Studies at PITZ | 22.09.2017 | Page 4/5

Dark Current (DC) Characterization and Simulation

• 1. Detailed experimental DC characterization
• DC vs. Solenoid current, RF power, pulse length, etc.
• DC spectrum measurement
• DC transportation
• 2. DC monitoring method
• Using Faraday Cups (e.g., ~10 pC resolution)
• Using non-destructive resonator prototype (e.g., 0.5mV for 1µA)
• 3. DC modeling and simulation based on

Fowler-Nordheim theory

• "Realistic" cathode geometry model
• Surface condition determination (scanning electron

microscopy?)

• 3D dark current generator
• Dark current transportation studies
• Identify critical areas for the source of dark currents by

particle tracking simulations Cathode surface Local field enhancement factor, Microscopic non- uniform local field forming structures β0 βi β2 β3 β4 β6 β1

uniform E0

Resulting in non- uniform field distribution En at cathode surface

Simulated DC patterns in [2] with uniform field distribution at cathode surface DCM at PITZ and XFEL DC on Low.Scr.1 (Z~0.85m, @6.15MW)

Ye Chen | Proposals for Hi2PE Networking: Emission-related Studies at PITZ | 22.09.2017 | Page 5/5

Other (Long-Term) Topics

> Charge production at theoretical space charge limitation for different PC laser pulse shapes and parameters (modeling + simulation) > Cathode brightness studies with variable beam aspect ratios > "Green photocathode" studies and tests (PITZ + INFN) > Studies of slice emittance formation for various PC laser pulse shapes and for various gun field gradients (towards SC and CW regimes) > Formation of ellipsoidal electron bunches by space charge driven expansion using PITZ Pharos laser (variable PL down to ~200 fs )

Thank you very much!

 In the limit of cigar aspect ratio,  Semi-analytical solution for flattop-shaped bunch in cigar regime (in [3]),  The theory may be less effective for Gaussian bunches (even more prevalent in many facilities)

Maximum cathode brightness for Gaussian bunches?