Thermal Emittance Measurement at PITZ Jang-Hui Han for PITZ Collaboration 4th October 2006 High QE Workshop at LASA

Thermal Emittance Measurement Contents Introduction Measurement procedure Single slit scan Quadruple scan Analysis of the measurement Schottky model Contributions to the thermal emittance measurement Summary jang.hui.han@desy.de Thermal Emittance Measurement

Thermal Emittance Measurement x x′ = px/pz two-dimensional elliptical phase space area occupied particle beam According to Liouville’s theorem, the beam emittance is invariant of the particle motion  Good indicator of the beam quality In linear accelerator (linac), the phase space area is not elliptical. Therefore, the rms emittance is useful to define the beam emittance In linac, the beam is accelerated through the accelerator. So, the transverse divergence is scaled with the longitudinal momentum. In this case, the normalized emittance is invariant. jang.hui.han@desy.de Thermal Emittance Measurement

Thermal Emittance Measurement Initial emittance of the beam, which is configured during the beam emission x x′ e- drive-laser pulse jang.hui.han@desy.de Thermal Emittance Measurement

Influence to the enormal enormal  (ethermal2 + esc2 + erf2 + ewake2 + …)½  sets the lower emittance limit of an electron source At the FLASH injector, enormal ~ 2 mm mrad & ethermal (< 1 mm mrad) not critical contribution At the European XFEL injector, enormal ~ 1 mm mrad & ethermal (< 1 mm mrad) most critical contribution jang.hui.han@desy.de Thermal Emittance Measurement

Measurement Procedure jang.hui.han@desy.de Thermal Emittance Measurement

Thermal Emittance Measurement ethermal Vs. Beam Size jang.hui.han@desy.de Thermal Emittance Measurement

Ek of emitted electrons 0.55 eV e- e- ph = 4.75 eV Ek of emitted electrons = (max DOS in the CB)  Ek e- e- e- jang.hui.han@desy.de Thermal Emittance Measurement

Effective electron affinity in RF guns electron affinity increase due to surface contamination maximum density state (0.75 eV from CBM) kinetic energy of emitted electron electron affinity decrease due to the Schottky effect Potential barrier decrease by the electric field 0.2 eV (CBM) Electron affinity variation results in 1. Bunch charge increase Kinetic energy increase of emitted electrons (thermal emittance)  : surface contamination factor ph: field enhancement factor Eemit: electric field at emission jang.hui.han@desy.de Thermal Emittance Measurement

Thermal Emittance Measurement Schottky effect: bunch charge From W. E. Spicer, PR 112. 114 (1958)  = 2.2, ph = 4 G = 11.7,  = 0.59 From C. I. Coleman, Appl. Optics 17, 1789 (1978) E (MV/m) QE (relative)  E = Emax sin  jang.hui.han@desy.de Thermal Emittance Measurement

Analysis of the measurement laser spot size ~ 0.55 mm bunch charge ~ 3 pC ~ 0.68 mm mrad (rrms = 0.55 mm) at PITZ1 & VUV-FEL (TTF2) operating condition Eemit ~ sin 35 x 42 (MV/m) ~ 24 MV/m discrepancy ~ 34% European XFEL Eemit ~ sin 44 x 60 (MV/m) ~ 42 MV/m ~ 0.60 mm mrad (rrms = 0.45 mm)  E = Emax sin  jang.hui.han@desy.de Thermal Emittance Measurement

Dependence on cathodes cathode #43.2, Nov.2004, QE ~ 3% #43.2 cathode #61.1, Apr.2004, QE ~ 1% #61.1 Thermal emittance changes depending on the surface chemistry and geometry. jang.hui.han@desy.de Thermal Emittance Measurement

Thermal Emittance Measurement Summary & Outlook Thermal emittance might be the most crucial contribution to the emittance for the European X-ray FEL. Detailed information is missing for the thermal emittance, i.e. surface properties, of the Cs2Te photocathode A more systematic study will be planned at PITZ in the beginning of next year. jang.hui.han@desy.de Thermal Emittance Measurement