Emission mechanisms in photocathode RF guns

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1 Emission mechanisms in photocathode RF guns Jang-Hui Han (DESY) 3 June 2005 Research seminar, Experimental high energy physics, Humboldt university

2 Motivation Electron beam quality in linear accelerators is determined at the gun. The emission processes in the gun limits the highest performance of the accelerator. Present understanding on the emission mechanisms is not sufficient to explain all empirical observations.

3 Contents FEL, SASE, TTF-FEL, PITZ Cs 2 Te photocathodes Photoemission Secondary emission Field emission

4 Free-electron laser (FEL) FELs are powerful sources of coherent EM radiation with high peak power and brightness. In a FEL, the magnetic field of the undulator magnet causes the electrons to oscillate transversely and generates the photons at resonant wavelength λ ph. λ w K H w γ electron beam λ ph = λ w 1+ K 2γ 2 2 radiation

5 Self-amplified spontaneous emission (SASE) At high phase space density of an electron bunch the FEL instability develops in a single pass through the undulator.

6 TESLA Test Facility (TTF) - FEL RF gun M1 M2 M3 M4 M5 M6 M7 6 undulator modules 1 st bunch 2 nd bunch collimator laser compressor compressor 4 MeV 130 MeV 350 MeV MeV 250 m bypass

7 Photoinjector Test Facility (PITZ)

8 Gun cavity coaxial coupler (RF input) photocathode electron beam cavity geometry absolute RF field in the cavity

9 Cs 2 Te photocathode φ 16 mm Cs 2 Te φ 5 mm Mo plug ~ 30 nm side view of the cathode plug top view of the cathode Why Cs 2 Te? - High quantum efficiency ~ 10% for fresh one ~ 0.5% for used one in normal operation (still enough) - Long lifetime ~ several months

10 Photoemission process photons vacuum solid electrons Process of photoemission: (1) absorbed photons deliver their energy to electrons in the material (2) the motion of the energized electrons through the material, losing some of their energy (3) the escape of the electrons over the surface barrier into vacuum

11 Space charge force during the emission F RF When F SC is greater than F RF, the electrons travel backward and cannot escape from the material. emissive material electrons F SC At the PITZ operation condition, the RF field is typically on the order of 10 MV/m. With the bunch charge of 1 nc, the space charge field induced the emitted electrons is comparable to the RF field.

12 Laser driven electron emission in RF guns highest energy at ~37 Operating RF phase at 40 MV/m: 37 - Smallest transverse emittance - Highest energy kinetic energy (MeV) MV/m rf electric field at the cathode (MV/m) RF longitudinal electric field at the cathode during electron beam emission = 40 (MV/m) * sin (37 ) = 24 (MV/m) rf phase (degree) RF electric field at the cathode and kinetic energy of the beam after gun Vs. RF phase

13 Longitudinal space charge field 20 longitudinal laser profile taken with a streak camera FWHM Transverse laser size: x rms = y rms =0.5 mm space charge field to the cathode (MV/m) nC simulation with ASTRA relative emission emission time (ps) phase longitudinal space charge force to the cathode

14 Synchronization between electron beam and E z in full cell beam velocity / c beam starts at distance from cathode (m) beam velocity and RF phase advance Vs. distance from cathode rf phase Beam velocity in the half cell is much smaller than the speed of light. to synchronize electron beam and the longitudinal electric field in the full cell, the electron beam has to start earlier than 90

15 Bunch extraction from the gun 4.0 beam charge (nc) μj rf gradient (MV/m) 0.12 μj μj beam extraction from gun cavity; two lines are the Schottky effect fits. Space charge force is still higher than the RF electric field. Some electrons emitted by the laser hit back the cathode. Secondary electron can be generated! At low gradient region the Schottky effect fits do not work because the longitudinal space charge field effect is dominate.

16 Schottky effect Under the RF field, the surface barrier of the emissive material is deformed to be lower. The generated electrons can be liberated to the vacuum more easily. vacuum level x electric field image field effective potential φ - efx - e 2 16πε 0 x metal V(x) = φ - efx - e 2 16πε 0 x

17 charge (nc) Measurement of the charge and momentum vs. the emission phase (1) Qmeas (2) Qsimulated Pmean, simulated [MeV/c] rf phase (deg) (3) <Pz> (MeV/c) (1) Space charge dominated (2) Schottky effect dominated (3) Aperture effect

18 Beam dynamics at low charge Beam charge: max. 5 pc, Gradient: 21 MV/m b) a) e) beam charge momentum c) d)

19 Secondary electrons in the RF gun photoelectrons secondary electrons photoelectrons secondary electrons measurement rf phase simulation

20 Secondary electron emission true secondary electrons primary electrons back-scattered secondary electrons re-diffused secondary electrons secondary emission mechanism δ ( E p δ = secondary emission yield (δ ): # of secondary electrons # of impact (primary) electrons δ dependence on the primary energy: ) = δ max E E p p, max s 1+ ( E E ) s p s p, max Emission of true secondary electrons: (1) Production by kinetic impact of the primary electrons (2) Transport toward the surface (3) Escape through the solid-vacuum interface

21 Multipacting at the cathode RF pulse multipacting at the beginning and the end of the RF measured multipacting - photo-emitted electron - 1 st generation - 2 nd generation - 3 rd generation - 4 th generation secondary electrons simulated multipacting on cathode

22 Field emission Even in the absence of the drive laser, electrons are generated with the high RF field. This is un-wanted signal and called dark current. from the Cu cavity from the Mo plug from the Cs 2 Te Dark current from the Mo cathode plug and the Cu cavity Dark current from the Cs 2 Te cathode, the Mo cathode plug and the Cu cavity

23 Dark current and beam spectra count (arbitrary unit) dark current electron beam momentum (MeV/c) Small part of the dark current overlaps with the electron beam. Nevertheless, the small part can survive till the end of the accelerator and make a damage at several vacuum components. RF field and field emission Spectra of the dark current and the electron beams Emission phases of the dark current and the electron beams RF phase (degree)

24 Summary Photo-emission process is determined with the space charge force as well as the Schottky effect. Secondary electrons are generated by electron beams or field emitted electrons. In the former case, the secondaries can arise with the beam. In the latter case, multipacting can take place. The geometry and the material of the cathode influence the dark current generation. The understanding of the emission processes is crucial for the next generation electron gun required for SASE-FEL or linear colliders.

11th International Computational Accelerator Physics Conference (ICAP) August 19 24, 2012, Rostock-Warnemünde (Germany)

Numerical Modeling of RF Electron Sources for FEL-Accelerators Erion Gjonaj Computational Electromagetics Laboratory (TEMF), Technische Universität Darmstadt, Germany 11th International Computational Accelerator