Emission-related Activities at PITZ Proposals for the DFG-Networking "Hi 2 PE"

Transcription

1 Emission-related Activities at PITZ Proposals for the DFG-Networking "Hi 2 PE" Ye Chen for the PITZ team , 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, (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, (2014) [8] P. Piot, et al., PRSTAB 16, (2013) [9] M. Zolotorev, SLAC-PUB-5896 (1992)

2 Photoemission (PE) Studies 1. (Cs 2 Te) Experimental & Numerical PE Studies Ongoing activity at PITZ and TU Darmstadt 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) Further modeling requires realistic cathode laser distributions Radially uniform distribution Dynamic emission process from cathode taking into account 3D full EM fields at cathode in a Lienard-Wiechert approach Charge production per simulation time step: Measured Bunch charge Space charge dominated regime Non-uniform distribution generated according to VC2 data OSS signal with measured flattop pulse shape (old data) Quantum efficiency formulism: Measured pulse shape + length Field-induced work function modification: 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 Ye Chen Proposals for Hi2PE Networking: Emission-related Studies at PITZ Page 2/5 + Possible laser intensity and time dependent cathode work function variation, e.g., in [9]

3 Photoemission (PE) Studies (cont.) 2. (Cs 2 Te) Investigations of a µs-scale spiky structure during electron bunch train extraction at FLASH Ongoing activity at PITZ, INFN and 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 Emission issue of fresh cathode 73.3 (and some others) at FLASH, , QE=10% ~8% Experiments at PITZ Fixed BSA SP mm, 43.3 MV/m vs MV/m, LT=10% Set the working point in linear emission regime ~9% Laser pulse heating? Ye Chen Proposals for Hi2PE Networking: Emission-related Studies at PITZ Page 3/5

4 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 DC on Low.Scr.1 β 0 β i uniform E 0 β 2 β 3 β 6 β 1 β 4 Cathode surface DCM at PITZ and XFEL Local field enhancement factor, Simulated DC patterns in [2] with uniform field distribution at cathode surface Microscopic nonuniform local field forming structures Resulting in nonuniform field distribution E n at cathode surface Ye Chen Proposals for Hi2PE Networking: Emission-related Studies at PITZ Page 4/5

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 ) 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) In the limit of cigar aspect ratio, Maximum cathode brightness for Gaussian bunches? Thank you very much! Ye Chen Proposals for Hi2PE Networking: Emission-related Studies at PITZ Page 5/5