Secondary Electron Emission in Photocathode RF Guns Jang-Hui Han 4th October 2006 High QE Workshop at LASA

Secondary Electron Emission Contents Introduction Secondary emission model Detection of secondary electrons Emission phase scan for bunch charge Emission phase scan for bunch momentum Multipacting at photocathode Measurement at PITZ Model simulation Summary jang.hui.han@desy.de Secondary Electron Emission

SE in photocathode RF guns SEE during photoemission? Multipacting? Dark current emission? No direct measurement data for Cs2Te  indirect measurement under RF operation jang.hui.han@desy.de Secondary Electron Emission

Secondary emission (SE)  process When a primary electron strikes a solid material, it may penetrate the surface and generate secondary electrons. From J. Cazaux, JAP89, 8265 (2001) Energy (keV) SEY SE process has a great similarity to photoemission process.  Good photo-emitters are good secondary electron emitters. Secondary electron yield, SEY () From N. Hilleret et.al., EPAC2000 4 3.5 3 2.5 2 1.5 1 0.5 SEY 500 1000 1500 2000 Energy (eV) Aluminum 99.5% Titanium Copper OFHC Stainless steel TiN jang.hui.han@desy.de Secondary Electron Emission

Secondary emission (SE)  modeling A SE model has been implemented into ASTRA jang.hui.han@desy.de Secondary Electron Emission

Measurement setup (PITZ) With changing the relative phase between the RF and the drive-laser, the emission phase of electron bunch is determined. jang.hui.han@desy.de Secondary Electron Emission

Secondary Electron Emission Qbunch & pmean vs. emit Max. Qbunch ~ 5 pC Max. RF field: ~22 MV/m (negligible dark current) Laser profile: [temporal] ~2.3 ps rms Gaussian [transverse] ~0.5 mm rms (b) (a) (e) (c) (d) jang.hui.han@desy.de Secondary Electron Emission

SE dependence on emission phase  measurement points  photoelectrons (simul.)  secondary electrons (simul.) jang.hui.han@desy.de Secondary Electron Emission

SE dependence on emission phase measurement ASTRA simulation  photoelectrons  secondary electrons jang.hui.han@desy.de Secondary Electron Emission

Secondary Electron Emission Multipacting: electron multiple impacting Explosive increase of the number of electrons Multipacting may cause RF power loss, lead to vacuum breakdown, and even damage the surface inside the cavity. secondary electrons jang.hui.han@desy.de Secondary Electron Emission

Observation at PITZ and TTF phase 1 This multipacting has not been observed with Mo cathodes except for the case of very bad vacuum in the gun cavity.  Multipacting at the Cs2Te photocathode From D. Setore et.al., FEL2000 RF forward power to the gun signal from the Faraday cup multipacting peaks multipacting peaks PITZ TTF phase1 jang.hui.han@desy.de Secondary Electron Emission

Secondary Electron Emission DC and multipacting vs. max RF gradient 40 MV/m 33 MV/m Dark current following the Fowler-Nordeim relation Multipacting peak Independent of the gradient jang.hui.han@desy.de Secondary Electron Emission

Secondary Electron Emission Description of multipacting peak RF forward RF stored dark current multipacting peak starting of the multipacting peak tdelay RF stored Emax EMP EMP: RF field when the multipacting starts Emax: maximum field of the RF pulse tdelay : the delay between the RF pulse and the multipacting peak  : fill/decay time of the RF field in the cavity jang.hui.han@desy.de Secondary Electron Emission

Secondary Electron Emission Delay time vs. max RF gradient max RF gradient The shape and height do not depend on the max RF gradeint cathode #60.1 #43.2 Meas. time Mar.04 Sep.04 Apr.05 EMP (MV/m) 2.70 1.04 1.07  (s) 2.80 2.83 RF measurement in the cavity: 2.78 (s) jang.hui.han@desy.de Secondary Electron Emission

Dependence on solenoid field profile main solenoid current Strong dependence on the solenoid field profile  Magnetic mirror configured by the solenoid field plays a crucial role in generation of the multipacting. No multipacting region Multipacting sometimes takes place in the region according to the cathode parameter and vacuum condition jang.hui.han@desy.de Secondary Electron Emission

Secondary Electron Emission ASTRA simulation of multiplication process Number of secondary electrons per one seed electron simulation conditions: max = 20, Emax = 1 keV, s = 2.2 Imain = 400 A & Ibucking = 30.5 A Estarting = 0.6 MV/m,  = 2.78 s (1 RF cycle ~ 0.77 ns) jang.hui.han@desy.de Secondary Electron Emission

Secondary Electron Emission Summary Secondary electron detected in photocathode RF guns at selected machine conditions Multipacting explained by high SEY of the Cs2Te cathode At the nominal operation condition of FLASH (1 nC, ~45 MV/m, ~ 38), no secondary electron generated during photoemission No clear evidence of SE in dark current jang.hui.han@desy.de Secondary Electron Emission