Thermal Emittance Measurement at PITZ JangHui Han for
- Slides: 13
Thermal Emittance Measurement at PITZ Jang-Hui Han for PITZ Collaboration 4 th October 2006 High QE Workshop at LASA
Contents n n Introduction Measurement procedure q q n Analysis of the measurement q q n Single slit scan Quadruple scan Schottky model Contributions to thermal emittance measurement Summary jang. hui. han@desy. de Thermal Emittance Measurement 2
Emittance x′ = px/pz two-dimensional elliptical phase space area occupied particle beam x 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 3
Thermal Emittance Initial emittance of the beam, which is configured during the beam emission ee- ee- eee- jang. hui. han@desy. de drive-laser pulse x′ x Thermal Emittance Measurement 4
Influence to the enormal (ethermal 2 + esc 2 + erf 2 + ewake 2 + …)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 5
Measurement Procedure jang. hui. han@desy. de Thermal Emittance Measurement 6
ethermal Vs. Beam Size jang. hui. han@desy. de Thermal Emittance Measurement 7
Ek of emitted electrons 0. 55 e. V e- e- e-e- e- ee ph = 4. 75 e. V e- e- ee-e-e- eee-e-e- e- e-ejang. hui. han@desy. de Ek of emitted electrons = (max DOS in the CB) Ek Thermal Emittance Measurement 8
Effective electron affinity in RF guns maximum density state (0. 75 e. V from CBM) electron affinity increase due to surface contamination kinetic energy of emitted electron 0. 2 e. V (CBM) electron affinity decrease due to the Schottky effect Potential barrier decrease by the electric field Electron affinity variation results in 1. Bunch charge increase 2. 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 9
Schottky effect: bunch charge From W. E. Spicer, PR 112. 114 (1958) = 2. 2, ph = 4 G = 11. 7, = 0. 59 QE (relative) E = Emax sin E (MV/m) From C. I. Coleman, Appl. Optics 17, 1789 (1978) jang. hui. han@desy. de Thermal Emittance Measurement 10
Analysis of the measurement laser spot size ~ 0. 55 mm bunch charge ~ 3 p. C discrepancy ~ 34% (rrms ~ 0. 68 mm mrad = 0. 55 mm) at PITZ 1 & VUV-FEL (TTF 2) operating condition Eemit ~ sin 35 x 42 (MV/m) ~ 24 MV/m 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 11
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 12
Summary & Outlook n n n Thermal emittance might be the most crucial contribution to the emittance for the European X-ray FEL. Detailed information is missing for thermal emittance, i. e. surface properties, of the Cs 2 Te photocathode A more systematic study will be planned at PITZ in the beginning of next year. jang. hui. han@desy. de Thermal Emittance Measurement 13
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