Selected Beam Studies at PITZ in 2017

Transcription

1 Selected Beam Studies at PITZ in 7 Ye Chen and Mikhail Krasilnikov for the PITZ Team TEMF-DESY Collaboration Meeting..7, TEMF, Darmstadt, Germany Contents Photoemission modeling in the gun Updates on beam asymmetry studies Summary & Discussion

2 Review of Three-Step Photoemission (PE) model. Optical excitation of electrons Reflection Transmission Energy distribution DOS. Migration of electrons to solid surface e - -phonon scattering (momentum change) e - -defect scattering (momentum change) e - -e - scattering (energy change, metal) Random Walk (Monte Carlo). Escape to vacuum Overcome work function Eg(band gap) + Ea(electron affinity) for semiconductor Eg variation, Ea variation Surface potential reduction due to field effect For thorough descriptions, see: W. E. Spicer, Phys. Rev., (958) M. Cardona and L.Ley: Photoemission in Solids, (Springer-Verlag, 978) W. E. Spicer & A. Herrera-Gomez, SLAC-PUB-66 (99) D. H. Dowell et al., Appl. Phys. Lett., 6, 5 (99) J. Smedley, P workshop 6 K. L. Jensen, P workshop 6 L. Cultrera, EWPAA 7 J. Smedley, EWPAA 7 Ye Chen and Mikhail Krasilnikov Selected Beam Studies at PITZ in 7 (st ½) 6..7 Page

3 Motivation of further PE modeling at PITZ > Explain PE associated measurement-simulation discrepancies in the gun Charge production Slice energy spread Bunch length Beam asymmetries, etc. > Assist semiconductor photocathode R&D Background: + Photocathode R&D usually focuses on "single electron" emission mechanism + Operation condition of PITZ for optimized transition regime between QE and space charge limited emission regimes + Classical electrodynamics seems not sufficient explaining transient PE process + Intrinsic emittance modeling not yet thorough Our Challenge: Improved modeling of photoemission process At the semiconductor-vacuum interface in the gun how to model quantum mechanics with the presence of strong electromagnetic fields (RF + SPCH = collective effects) Ye Chen and Mikhail Krasilnikov Selected Beam Studies at PITZ in 7 (st ½) 6..7 Page

4 Space charge dominated PE modeling.driving (UV) laser Realistic transverse (Virtual-Cathode-based Core+Halo model*) distributions Realistic temporal distributions initializing transient emission process.photocathode QE map and QE characterization intrinsic emission homogeneity Cathode UV laser spot Laser temporal profile.em fields in close cathode vicinity RF, image charge & space charge time and space dependent cathode work function modulation.quantum mechanics with the presence of strong EM fields at Semiconductor-Vacuum interface Surface states Band bending time and space dependent electron affinity variation kinetic energy variation QE homogeneity 5.Others Temperature Surface charge limit** Secondary emission, etc. * C. Hernandez-Garcia et al., NIM A 87 (7) 97 **M. Zolotorev, SLAC-PUB-5896,99 Ye Chen and Mikhail Krasilnikov Selected Beam Studies at PITZ in 7 (st ½) 6..7 Page

5 Charge [pc] Status: Core + Halo Model applied to ASTRA simulations If a uniform distribution is used instead, the charge saturates Laser radial distribution image 5 5 Extracted charge with core + halo for.8 mm beam diameter with.5 ps rms Gaussian temporal at maximum cathode field (f =9 o ) E = 58MV/m E = MV/m.68 mm Generated ASTRA input distribution core + halo Transverse radial profile core + halo 5 E = 9MV/m.8 mm Input charge [pc] Nominal ASTRA input uniform distribution Nominal transverse uniform radial profile Ye Chen and Mikhail Krasilnikov Selected Beam Studies at PITZ in 7 (st ½) 6..7 Page 5

6 x' (mrad) y' (mrad) x' (mrad) y (mm) y (mm) ASTRA simulations for Gaussian pulses using Core+Halo > BUT for flattop photocathode laser pulses? X-Y f gun =MMMG Q=.5nC e x =.8 mm mrad e x =.8 mm mrad X-X Y-Y.9.9 X-X X-Y Parameters plugged from measurements: f gun =MMMG Q=.5nC e x =.5 mm mrad y (mm) MaxB[T]=-(A+B*.98*Imain[A]) Ye Chen and Mikhail Krasilnikov Selected Beam Studies at PITZ in 7 (st ½) 6..7 Page 6

7 Status: ASTRA simulations for case using Core+Halo > BUT for flattop photocathode laser pulses X-Y f gun =MMMG+6 Q=nC* e x =.7 mm mrad e x =.6 mm mrad X-X Y-Y X-X X-Y Parameters plugged from measurements: f gun =MMMG+6 Q=.97nC e x =.5 mm mrad Ye Chen and Mikhail Krasilnikov Selected Beam Studies at PITZ in 7 (st ½) 6..7 Page 7

8 Status: PE modeling using a D full EM Lienard-Wiechert (LW) approach* LW Approach with D emission process LW solution for the electromagnetic field of a charged particle in arbitrary motion Full particle trajectory stored and used for field computation Field-induced work function modification: *E. Gjonaj, TEMF, TU Darmstadt Charge production per simulation time step: R w + hυ Φ w E QE = a (p + )( + E K. Jensen, 7 a ) hυ Φ w Status Dynamic generation of emitted particle distribution at cathode according to time-dependent emission models, taking into account full electromagnetic fields (RF + space-charge) during emission Charge production in QE limited regime agrees with measurements In space charge dominated regime, remaining deviations w.r.t. simulations probably due to: Ideal beam distributions initially plugged in the simulations or/and time dependent work function variation resulting from quantum mechanics Ye Chen and Mikhail Krasilnikov Selected Beam Studies at PITZ in 7 (st ½) 6..7 Page 8

9 Further PE modeling: band bending space charge layer > Surface states Due to lattice translational symmetry surface Surface band lying within bandgap of the bulk (fewer bonds) Possessing charging character > Surface states band bending Charge carriers falling into surface states "surface charged Matching fermi level at bulk and surface band bent Band bending space charge layer formed (from surface into the bulk) > Characteristics Band bends quadratically Local curvature proportional to local space charge density Bending amount and width depending also on material properties bulk vacuum E bb ρ sc r, t d ε Ec Ev Further modeling approach needed Ye Chen and Mikhail Krasilnikov Selected Beam Studies at PITZ in 7 (st ½) 6..7 Page 9

10 PE modeling: interpretation of surface space charge layer > For Cs Te Donor-like surface, acceptor-like bulk Band bends downwards at surface > Electrons may be extracted from Valence band (VB) Surface band (SB) ratio of VB and SB (emitted) electrons changes E kin and ε th > Surface space charge layer may affect electron affinity time and space dependent E a ρ sc r, t UV@57nm E ph.8 ev Intrinsic cathode work function Φ w = E g + E a Work function due to presence of strong space charge surface Φ w Φ w_sb = E g E SB + E a E bb [ρ sc r, t ] Φ f r, t Φ w Φ w_vb = E g + E a E bb [ρ sc r, t ] Φ f r, t Surface band Band bending Field effect p-typed bulk vacuum z n-typed surface.65 ev E SB Φ f r, t = q πε E rf r, t + E sc r, t R w + hυ Φ w E QE = a E (p + )( + a ) hυ Φ w QE: K. Jensen Kinetic energy E kin varied accordingly Intrinsic emittance If φ φ max = arccos E a E kin ε n,rms = r E kin m c + cos φ max cosφ max cosφ max Ye Chen and Mikhail Krasilnikov Selected Beam Studies at PITZ in 7 (st ½) 6..7 Page optimization w.r.t. space charge

11 Updates on beam asymmetry studies (RF coupler kick simulations & Gun quadrupole for compensating beam asymmetries) + Igor Isaev Ye Chen and Mikhail Krasilnikov Selected Beam Studies at PITZ in 7 (st ½) 6..7 Page

12 Y' (mm) X' (mm) Motivation of beam asymmetry studies Asymmetry kick? E-beam x-y asymmetry X-X Space charge ε n,x ε n,y M. Krasilnikov et al., PRSTAB 5, 7,. Possible sources of the beam asymmetry: Vacuum mirror Stray magnetic fields Related to the laser polarization Particular cathode RF coupler field asymmetry Solenoid imperfections (anomalous quadrupole fields) Ongoing activities X (mm) Y-Y Y (mm) ε n,x ; ε n,y? coupler kick simulations solenoid field simulations simulations with rotational quads model for fitting measurements gun quadrupole designs and simulations gun quads compensation Ye Chen and Mikhail Krasilnikov Selected Beam Studies at PITZ in 7 (st ½) 6..7 Page

13 Updates on coupler RF kick studies (no solenoids) > Kick characterization Beam centroid tracking using field map Field calculation done under optimum operation condition of the gun (mini.s by adjusting inner conductor length) Vertical displacement at z =. m and kick strength as a function of the gun phase D field map used for later particle tracking simulations RF dynamics no solenoids, no space charge Y. Chen et al.,fel7 proceedings, WEP5 Ye Chen and Mikhail Krasilnikov Selected Beam Studies at PITZ in 7 (st ½) 6..7 Page

14 Updates on coupler RF kick studies (no solenoids) > Kick quantification Particle tracking simulation results for multipole expansion based quantification of the integral kick Beam centroid positions on cathode plane Using field map tracking a set of particles on the cathode plane though kick region till doorknob transition region Fitting multipole expansion form of the integral kick using simulation results Multipole expansion of the integral kick Vertical dipole kick~.576 kev/c, time dependent X, Y: particle offsets from the axis at the location of the integral kick P X, P Y : particle transverse momenta in the horizontal and vertical direction P ox, P oy : horizontal and vertical dipole kicks K RF : RF focusing strength of cylindrical symmetric mode K N and K s : normal and skew quadrupole kick strength Quadrupole kick strength estimation Normal quadrupole component ~.e-5 kev/c/μm Skew quadrupole component~5.e-6 kev/c/μm (-ps) Bunch tail sees higher kick strength than the head by.5 6.5MW Ye Chen and Mikhail Krasilnikov Selected Beam Studies at PITZ in 7 (st ½) 6..7 Page

15 y (mm) y (mm) x' (mrad) y' (mrad) No gun quads With gun quads y (mm) y (mm) x' (mrad) y' (mrad) Electron beam X-Y asymmetry compensation with gun quads (.5nC, Gaussian photocathode laser pulse) Electron beam measurements without gun quadrupoles.5.5 X-Y at EMSY X-Y at H.Scr X-X Y-Y / -. A / -.9 A / -. A (I Gun quad Q/Q scan at High.Scr (Binx); Imain = 8 A Gun.Quad ; I Gun.Quad ) scan at EMSY -. /. A -. /.7 A y (mm).... Normal Gun.Quad Skew Gun.Quad -.8 / -. A -. / -. A -.8 / -.9 A -. / -.9 A -.8 / -. A -. / -. A -.8 /. A -. /. A -.8 /.7 A -. /.7 A Gun quads copy installed at EXFEL and prepared for FLASH. / -. A. / -.9 A. / -. A. /. A. /.7 A.8 / -. A.8 / -.9 A.8 / -. A.8 /. A.8 /.7 A Electron beam measurements with gun quadrupoles (I Gun.Quad =-.6A; I Gun.Quad =-.5A) X-Y at EMSY X-Y at H.Scr X-X Y-Y M. Krasilnikov et al.,fel7 proceedings, WEP y (mm) I main (A) 86 8 I gun.quad (A) -.5 I gun.quad (A) -.6 σ (mm).5.8 σ (mm).5. ε x,n (mm mrad)..8 ε y,n (mm mrad).7.8 ε x,n ε y,n (mm mrad).9.8 Ye Chen and Mikhail Krasilnikov Selected Beam Studies at PITZ in 7 (st ½) 6..7 Page

16 Summary > Further photoemission modeling towards quantum mechanics with the presence of strong space charge densities at cathode surface current status further modeling approaches > Coax coupler RF kick characterized and quantified under optimized operation conditions of the gun Time dependent vertical dipole kick, ~.65 mrad (MMMG phase, 6.5MW) Small quadrupole kick estimated > Beam asymmetry compensation with gun quadrupoles optimization Promising results "round beam, round emittance" Thank you very much! Ye Chen and Mikhail Krasilnikov Selected Beam Studies at PITZ in 7 (st ½) 6..7 Page 6

17 Intensity Backup: Updates on "Pz modulation" studies > (?)Cathode laser temporal profile Long Gaussian (-.5ps FWHM, Lyot filter in) Pz modulation observed Short Gaussian (~ps FWHM, Lyot filter out) not yet observed Lyot filter, the source of modulation? See: M. Krasilnikov, DESY- TEMF-Meeting,.7 Pz modulation at LEDA > (?)Emission mechanism Emitted charge fields on surface that affects subsequent emissions "oscillations induced by a sudden influx of charge can persist". Demonstration for Cu and Cs Sb using MICHELLE J.J. Petillo et al., IEE trans. Electron Devices 5, 7 (5) K.L. Jensen et al., J.Vac.Sci. Technol. 6 (), 8 (8) Lyot filter in the regenerative amplifier Ye Chen and Mikhail Krasilnikov Selected Beam Studies at PITZ in 7 (st ½) 6..7 Page 7