Secondary emission in an rf photocathode gun - LASA

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Secondary emission in an rf photocathode gun - LASA Powered By Docstoc
					Secondary Electron Emission
in Photocathode RF Guns

    Jang-Hui Han
    4th October 2006
    High QE Workshop at LASA

   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      Secondary Electron Emission   2
SE in photocathode RF guns

   SEE during photoemission?
   Multipacting?
   Dark current emission?

   No direct measurement data for Cs2Te
     indirect measurement under RF operation   Secondary Electron Emission   3
      Secondary emission (SE)  process
      When a primary electron strikes a solid material, it may
      penetrate the surface and generate secondary electrons.
                                                                    SE process has a great similarity
        Secondary electron yield, SEY ()
                                                                    to photoemission process.
                 # of emitted electrons                              Good photo-emitters are good
                # of irradiated electrons                           secondary electron emitters.
                                                Aluminum 99.5%
      3.5                                       Titanium
        3                                       Copper OFHC
                                                Stainless steel




            0        500          1000         1500         2000
                             Energy (eV)                                          Energy (keV)
                    From N. Hilleret, EPAC2000                     From J. Cazaux, JAP89, 8265 (2001)                       Secondary Electron Emission                               4
Secondary emission (SE)  modeling

A SE model has been                       Ep              s
                        ( E p )   max        
implemented into ASTRA                                    
                                         E p,max s    E p E p,max   
                                                                       s        Secondary Electron Emission                    5
Measurement setup (PITZ)

                                                        With changing the relative phase
 rf strength

                                                        between the RF and the drive-laser,
                                                        the emission phase of electron bunch
                                                        is determined.

               0   30   60     90    120   150    180
                         emission phase                       Secondary Electron Emission                    6
   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

       (a)                               (e)


                                          (d)        Secondary Electron Emission   7
SE dependence on emission phase

                                         measurement points
                                         photoelectrons (simul.)
                                         secondary electrons (simul.)   Secondary Electron Emission                       8
SE dependence on emission phase

                       measurement         ASTRA simulation
                                            photoelectrons
                                            secondary electrons   Secondary Electron Emission                 9
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        Secondary Electron Emission                10
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, FEL2000

       RF forward power to the gun

               signal from
               the Faraday cup

                 multipacting peaks
                                                               multipacting peaks

                       PITZ                                    TTF phase1             Secondary Electron Emission                          11
DC and multipacting vs. max RF gradient

                           40 MV/m                                    33 MV/m

     Dark current following                    Multipacting peak
     the Fowler-Nordeim relation               Independent of the gradient        Secondary Electron Emission                      12
Description of multipacting peak

                                   RF forward

                                                                           E  Emax exp( t /  )
                                    RF stored                 RF stored

                                E  Emax exp( t /  )

                                                          starting of the multipacting peak
EMP  Emax exp( tdelay /  )
tdelay   ln Emax   ln EMP
                                                         dark current

EMP: RF field when the multipacting starts
Emax: maximum field of the RF pulse                                       tdelay
tdelay : the delay between the RF pulse and                                        multipacting
          the multipacting peak                                                    peak
 : fill/decay time of the RF field in the cavity            Secondary Electron Emission                                13
Delay time vs. max RF gradient
max RF gradient
                                             tdelay   ln Emax   ln EMP

                       The shape and
                       height do not       cathode           #60.1             #43.2
                       depend on the
                       max RF gradeint     Meas. time        Mar.04    Sep.04      Apr.05
                                           EMP (MV/m)         2.70      1.04           1.07
                                            (s)             2.80      2.83           2.83
                                             RF measurement in the cavity: 2.78 (s)           Secondary Electron Emission                              14
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
                          region                              of the multipacting.

Multipacting sometimes takes place in
the region according to the cathode
parameter and vacuum condition            Secondary Electron Emission                         15
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)       Secondary Electron Emission                        16

   Secondary electron detected in photocathode RF
    guns at selected machine conditions
   Multipacting explained by high SEY of the Cs2Te
   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   Secondary Electron Emission       17

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