Photocathode Qualification Quantum Efficiency, Thermal Emittance, Response Time, Lifetime
Outline • Photocathode fundamental parameters • Thermal emittance • Phase space ellipse area of “as emitted electrons” • Quantum efficiency • Emission curves, spectral response, uniformity • Response time • Measurements in photoinjectors • Lifetime
Thermal emittance measurements • Phase Space Reconstruction • direct measure of x and px • Single slit • Double slit • Linear Optics (matrix formalism) • Solenoid scan • Quadrupole scan • Tomography • Electron Energy Analyzer • ARPES
sx’j x’j xj Single slit and beam profiles Cu @ SPARC
Variable By Variable By x’j sx’j xj Double slit and beam current GaAs @ Cornell University
Twiss parameters for gaussian beam • Single particle motion can be described (as in optics for gaussian beams) using a matrix formalism • And Twiss parameter (characterizing the rms values of the particles distribution) Liouville Theorem: The phase space density for a Hamiltonian system is an invariant of the motion. Or equivalently, the phase space volume occupied by the system is conserved.
e-Gun Magnetic Lens e p L Profile monitor l Quadrupole and Solenoid Scan • The simplest beamline is made by a focusing element followed by a drift up to the screen
CORNELL, JLAB PITZ, FLASH BNL,SLAC,UCLA, INFN,ELETTRA, PSI U. TOKYO, BEIJING Quadrupole and Solenoid Scan • 2nd order relationship exists between s2 and (1-L/f) Solenoid add also a rotation of the spatial coordinates x and y by an angle defined as follows:
R E S Tomography • Technique used to reconstruct phase space (2D) • large number of 1D profiles collected for different phase space induced rotations; • very useful with beams no gaussian-like; Detailed evaluation of the beam line transport matrix has to be performed to ensure an accurate retrieval of the beam profiles due only to the phase space rotation. Xiang et al.(PRSTAB, 12, 022801 2009) calculated the transfer matrix due the solenoid slicing the magnetic field in 50 parts.
ASTRA simulation Reconstructed phase space Tomography Xiang et al.,PRSTAB, 12, 022801 (2009) GaAs @ Cornell University Laser beam shape
Energy analyzer Trajectory of an electron (blue) in a longitudinal magnetic field (red) ADIABATIC INVARIANCE Increasing B Plans to built a similar device at Cornell University D. A. Orlov et al. Appl.Phys.Lett., 78, 2001
Need to neglect additional terms Very small charge (less than picoCoulomb) Short electron bunch (fs) and/or DC gun Fulfilled with previous conditions? Energy spread effect?
Space charge • Space charge induced error in the solenoid • scan method, could be evaluted by virtual experiment performed with numerical simulation codes (PARMELA, ASTRA, GPT…) • The lowest limit of the charge used for meaasurement is usually the one that gives a S/N ratio acceptable for beam profile measurements: • Dark current • Single shot or multiple shot Xiang et al.,PRSTAB, 12, 022801 (2009)
Cu and Nb @ PSI GaN @ Cornell GaAs @ Cornell Cs2Te @ PITZ Mg @ UCLA Cu @ BNL RF or DC gun? Sub-pC bunch charge
px x In case of RF Gun (i.e.Pegasus Blow-out @UCLA) Laser pulse duration 50 fs Beam Energy 3.55 MeV with σdE ~ 20 keV Energy spread Solenoid and quadrupoles are chromatics: The focal length depends on the beam energy In case of DC gun (i.e. LEG @PSI) Beam Energy 160 KeV with σdE~ 0.08 eV Energy spread is 104 lower!!!
Metal cathodes (AFM @ Tsinghua University) GaAS surface (AFM @ Cornell) Roughness of the surface Thermal emittance arising from momentum conservation in photoemission is usually evaluated considering an ideally flat surface barrier between photocathode and vacuum
Roughness of the surface Krasilnikov (FEL 2006) developed a simple mathematical model taking account cathode surface roughness aiming to predict the TE increase due to local surface orientation Electric field lines near the surface bumps may deviate electrons from ideal trajectories focusing or defocusing the beam and increasing the effective TE by Schottky lowering of the vacuum level
Alignment in magnets and slice emittance If the beam is not well aligned inside magnetic elements different slices will suffer a transverse kick that is energy dependent Lower Energies Higher Energies Projected emittance could be increased by systematic shift of slices centroids Filippetto @ FEL2009
Data taking and analysis • Beam profiles analisys: • Background removal; • -Profile fitting; • Phase space reconstruction: • -% of the charge considered; • -Trace space interpolation;
QE measurements • Emission curves to determine QE at appropriate wavelengths; • Emission curves at different electric field to determine enhancement coefficient; • QE maps to characterize the uniformity of emission: • Imaging electron beam; • Scanning small laser spot over the surface;
Emission curves Taking care to measure far from space charge saturation (Cs2Te measured @ INFN of Milano) What about laser polarization and AOI? QE at common solid state laser wavelengths should be measured: could be of great interest for other groups Cs2Te has about 1% QE @ TiSa 2nd harmonic (measured @ INFN of Milano) Workfunction measure obtained form spectral response could be used to validate thermal emittance models
QE maps to determine uniformity Cathode imaging using the solenoid as lens (GaAs @ JLAB presented at PESP2008) Scanning the laser beam over the photocathode Resolution limited by laser spot size (Cs2Te @ INFN presented at PAC07) (Cu @ INFN presented at DIPAC2009)
Cu @ PEGASUS-UCLA Resolution of about 50 fs Laser pulse was few tens of fs Mg @ PEGASUS-UCLA Response time and RF deflector GaAS @ JLAB-U.Mainz Response time tens of ps Laser pulse was 5 ps
Lifetime • Usually defined as the time needed for QE to degrade at 1/e of initial value • What limits the life of a photocathode? • QE (GaAs and multialkali) • Uniformity (Metals) • Dark current (Cs2Te photocathodes) • Could they be rejuvenated? • GaAs (recesiation out of the gun) • Dispenser W cathodes (cesiation on site) • Metals (laser cleaning on site)
Cathode ID common database • “Birthdate” • Preparation date • Start of operation • Recipe • Preparation recipe • Storage condition • Relevant Properties • QE vs wavelength • Thermal emittance • Uniformity of emission • Death date • Reasons of death and history of operation • Autopsy (surface morphology and composition characterization)
…Waiting to be filled with numbers Dowell et al., NIM-A, 622, (2010) 685-697
ARPES Cu (111) with 21.2 eV photon J.Osterwalder et al, PRB, 53 (1996) 10209
The three screens method • The transfer matrix for a free drift space is • Knowledge of L1, L2 and L3 with the measure of the beam profiles at these three locations allows the evaluation of the beam emittance Converging beam Focused beam Diverging beam L1 L2 L3
Quadrupole and Solenoid Scan • Considering the expression of b deduced from transfer matrix • Remebering that s2=be and substituting • Finally we got that a parabolic relationship exists between s2and (1-L/f)