Mitglied der Helmholtz-Gemeinschaft Jochen Teichert HZDR Acceleration Schemes of of Modern Electron Guns Jochen Teichert ULTRA BRIGHT Electron Sources Workshop 29 June – 1 July 2011, The Cockcroft Institute Daresbury

Seite 2 Mitglied der Helmholtz-Gemeinschaft Jochen Teichert HZDR Overview Introduction – high brightness beams Overview of modern electron guns superconducting RF photo guns The superconducting RF photo gun at ELBE gun acceleration gradient emittance compensation in SC guns messurements ultra short pulses (idea from V. Volkov et al.) Summary

Seite 3 Mitglied der Helmholtz-Gemeinschaft Jochen Teichert HZDR Introduction – High Brightness Beams Definition of brightness: The electron density in 6D phase space Integrating over energy spread: brightness A figure for the quality of a bunch, but not for the number of bunches/time f rep Using the average: For a high mean value (light sources): high rep. rate and low bunch charge, Formula contains no bunch length: DC is the best (electron microscope). ~

Seite 4 Mitglied der Helmholtz-Gemeinschaft Jochen Teichert HZDR Introduction – High Brightness Beams SRF High Brightness Electron Guns The BES Photon Workshop held on October 2009 concluded that for ultimate performance in future radiation sources MHz repetition rate is needed. The workshop also noted that the realization of such sources “is also hindered by the lack of technical developments as far as gun performance is concerned.” These recommendations lead naturally to CW operating electron guns, since no pulsed system with a sufficient stored energy can operate at MHz rates. Superconducting RF guns are one of three contenders in this arena. Statement of Workshop on Future Light Sources SLAC, 2010 ULTRA high brightness electron sources are still a challenge. Nevertheless we should think about it.

Seite 5 Mitglied der Helmholtz-Gemeinschaft Jochen Teichert HZDR High final momentum - space charge forces scales as - ~500 keV reduces SC forces sufficiently Gradient at cathode - space charge limit: max. bunch charge - E a > E sc preserves beam quality - shortening low energy path Higher f rf : higher gradient ε n,x scales as t b 2 /λ rf increasing rf-nonlinearities Introduction – High Brightness Beams Emittance contribution from the gun: thermal, rf field, space charge Thermal emitt. scales as r laser small r SC force small E photon low QE

Seite 6 Mitglied der Helmholtz-Gemeinschaft Jochen Teichert HZDR The three “modern” gun types DC photo guns Normal conducting RF photo guns high frequency (≥1.3 GHz) low duty factor low frequency (≤ 800 MHz) high duty factor & cw Superconducting RF photo guns

Seite 7 Mitglied der Helmholtz-Gemeinschaft Jochen Teichert HZDR HZB, DESY, Jlab (since 2008) Superconducting RF Photo Guns At present, a lot of different approaches depending on application: average current (ERL type guns), bunch charge and brightness leak of konowledge: over/under estimation of problems SRF guns with TESLA-style elliptical cavities HZDR/Rossendorf (since 1998) IHIP Peking University (since 2001) BNL, Jlab, DESY (since 2002) f rf = 1.3 GHz NC PC: Cs 2 Te f rf = 1.3 GHz NC PC: Cs 2 Te f rf =1.3 GHz SC PC: Nb, Pb NC PC: GaAs f rf = 1.3 GHz SC PC: Pb DC-SRF Photo gun BERLinPro stage one gun

Seite 8 Mitglied der Helmholtz-Gemeinschaft Jochen Teichert HZDR BNL, AES (since 2004) Superconducting RF Photo Guns High current / low frequency cavity Quarter wave cavity f rf = MHz NC PC: alkali + diamond amplifier NPS, NIOWAVE f rf = 200 MHz NC PC: Cs 2 Te BNL, Niowave f rf = 112 MHz f rf = 500 MHz SC PC: Nb Uni Wisconsin, Niowave

Seite 9 Mitglied der Helmholtz-Gemeinschaft Jochen Teichert HZDR 9 LASER NC Cs 2 Te photo cathode helium port SC Nb 3½ -cell cavity e-e- liquid He vessel cathode cooling (77 K) & support system photo cathode alignment cavity tuners rf power coupler SRF Gun Cryomodule The superconducting RF photo gun at ELBE

Seite 10 Mitglied der Helmholtz-Gemeinschaft Jochen Teichert HZDR 10 cathode sc choke filter (to prevent RF leakage) half cell FZD coupler & pickup ant. 2 HOM couplers 3 TESLA cells ModeELBEHigh Charge final electron energy≤ 9.5 MeV operation modeCW bunch charge77 pC1 nC repetition rate13 MHz500 kHz laser pulse (FWHM)4 ps15 ps transverse rms emittance 1 mm mrad2.5 mm mrad average current1 mA0.5 mA Gun accelerating gradient Design values B s,max = 110 mT max. magn. surface field E cathode = 20 MV/m (backtracked cathode) E peak (1 st cell) = 30 MV/m E peak (TESLA)= 50 MV/m

Seite 11 Mitglied der Helmholtz-Gemeinschaft Jochen Teichert HZDR 11 Gun accelerating gradient vertical DESY, 1.8 Kmeasurements in gun risk of contamination due to the NC photo cathode? After > 1000 h operation no deterioration was seen. Performance of HZDR´s first 3.5 cell cavity (3.5cell/2006) The insufficient cleaning (HPR) was the major problem – esp. choke filter & half-cell

Seite 12 Mitglied der Helmholtz-Gemeinschaft Jochen Teichert HZDR Gradient in elliptical1.3 GHz gun cavities: The challenge is the field in TESLA (Flash, XFEL, ILC) cavities DESY vertical testcryostat D. Reschke, et al. SRF´09, Berlin DESY horizontal CW test D. Kostin, et al. SRF´09, Berlin Gun accelerating gradient

Seite 13 Mitglied der Helmholtz-Gemeinschaft Jochen Teichert HZDR Gun accelerating gradient 3.5 cell /2010FG fine grain RRR 300 Nb Cavity peak electric field [MV/m]Energy gain [MeV] vertical TestCW operation TESLA 9-cell cavity Rossendorf ½ cell cavity150.7 Rossendorf 3.5 cell / Rossendorf 3.5 cell /2010FG cell cavity design value509.5 PITZ/DESY 1.6 cell NC gun (1.3 GHz)60 (pulsed) 3.5 cell/2010LG large grain Nb vertical cryostat P. Kneisel Since gun performance mainly depends on gradient, E a ≈ 60 MV/m will give emittance < 1 1 nC + shaped laser + emittance compensation needed (see PITZ gun)

Seite 14 Mitglied der Helmholtz-Gemeinschaft Jochen Teichert HZDR 14 Advantages of quarter wave resonators: low frequency and small size DC like field in the gap > high transit time factor, longer pulses low rf losses allow 4.2 K operation low rf losses at the cathode Drawback comp. to elliptical cavities: no multi cell design Gun accelerating gradient Naval Postgraduate School (NPS) 500 MHz quarter wave resonator First beam in 2010: beam energy: 480 keV peak field: 8.5 MV/m

Seite 15 Mitglied der Helmholtz-Gemeinschaft Jochen Teichert HZDR Retracted or shaped cathode Down stream solenoid focusing Emittance compensation methods Solenoid field like axial field Additionally excited TE mode better : SC solenoid in cryostat NPS & HZB

Seite 16 Mitglied der Helmholtz-Gemeinschaft Jochen Teichert HZDR C2 energy chirp energy spectrum vertical quadrupole scan Slice emittance measurement SRF gun injection in ELBE for advanced beam diagnostic Phase scan technique for longitudinal phase phase correlation energy width bunch length

Seite 17 Mitglied der Helmholtz-Gemeinschaft Jochen Teichert HZDR 17 Bunch compression by means of “wrong” laser phase fs bunches from the gun

Seite 18 Mitglied der Helmholtz-Gemeinschaft Jochen Teichert HZDR 18 SRF gun phase scan – optimum laser phase screen DV02 (YAG) 1.9 m from gun exit, 2.7 m from cathode -175°, σ x = 300 µm -150°, σ x = 330 µm -130°, σ x = 610 µm -115°, σ x = 760 µm -95°, σ x = 330 µm -65°, σ x = 430 µm +3°, σ x = 1650 µm kinetic energy & energy spread ? fs bunches from the gun

Seite 19 Mitglied der Helmholtz-Gemeinschaft Jochen Teichert HZDR 19 fs bunches from the gun ASTRA simulation, 1.5-cell gun, 15 MV/m peak field looping in half-cell experimental verification still needed: energy width, emittance, bunch length

Seite 20 Mitglied der Helmholtz-Gemeinschaft Jochen Teichert HZDR 20 Gun type low f NC RF ¼ wave SC RF elliptical SC RF DC voltage potential highest brightness in high f guns best combination of brightness + aver. current high aver. current status rf testsfirst beam produces beam routine operation Examples LBNLNPS HZDR HZB Jlab FEL Cornell, KEK present efforts rf power Tests higher gradients reducing FE designs for higher voltage final energy *) 19.5 MV/m 1) 0.75 MeV 25 MV/m 2) 1.2 MeV MV/m 3) 9.5 MeV 6.75 MV/m 4) 0.5 MeV show stoppersrf heat dissipation NC cathode in SC cavity ? high voltage >100 mA ERL light sources ~1 GHz rep rate highest brightness best combination grad. + energy Photoelectron injectors for high-brightness beams and cw operation *) design values 1) F. Sannibale, et al., Proc. of FEL´20, Malmö, Sweden, 2010, p ) J.R. Harris, et al., Phys Rev. ST AB 14, (2011). 3) A. Arnold, et al., NIM A 577, 440 (2007). 4) N. Nishimori, et al., Proc. of LINAC´10, Tsukuba, Japan, 2010, p.995.

Seite 21 Mitglied der Helmholtz-Gemeinschaft Jochen Teichert HZDR 21 The ELBE Crew visiting the German Watch Museum Glashütte/Sa. December 2010 Thank you for your attention Thanks to my colleagues at ELBE and all collaborators. Apologies for the “stolen” pictures in the talk.