Acceleration Schemes of of Modern Electron Guns Jochen

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

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 Jochen Seite 2 Teichert j. teichert@hzdr. de www. hzdr. de HZDR Mitglied der Helmholtz-Gemeinschaft

Introduction – High Brightness Beams Definition of brightness: The electron density in 6 D phase space ~ Integrating over energy spread: brightness A figure for the quality of a bunch, but not for the number of bunches/time frep 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). Jochen Seite 3 Teichert j. teichert@hzdr. de www. hzdr. de HZDR Mitglied der Helmholtz-Gemeinschaft

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. ULTRA high brightness electron sources Statement of Workshop on Future Light Sources SLAC, 2010 are still a challenge. Nevertheless we should think about it. Jochen Seite 4 Teichert j. teichert@hzdr. de www. hzdr. de HZDR Mitglied der Helmholtz-Gemeinschaft

Introduction – High Brightness Beams Emittance contribution from the gun: thermal, rf field, space charge Thermal emitt. scales as rlaser small r <-> SC force small Ephoton <-> low QE Higher frf : higher gradient εn, x scales as tb 2/λrf increasing rf-nonlinearities Jochen Seite 5 Teichert j. teichert@hzdr. de www. hzdr. de HZDR High final momentum - space charge forces scales as - ~500 ke. V reduces SC forces sufficiently Gradient at cathode - space charge limit: max. bunch charge - Ea > Esc preserves beam quality - shortening low energy path Mitglied der Helmholtz-Gemeinschaft

The three “modern” gun types DC photo guns Normal conducting RF photo guns low frequency (≤ 800 MHz) high duty factor & cw high frequency (≥ 1. 3 GHz) low duty factor Superconducting RF photo guns Jochen Seite 6 Teichert j. teichert@hzdr. de www. hzdr. de HZDR Mitglied der Helmholtz-Gemeinschaft

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) frf = 1. 3 GHz NC PC: Cs 2 Te BNL, Jlab, DESY (since 2002) frf=1. 3 GHz SC PC: Nb, Pb NC PC: Ga. As Jochen Seite 7 Teichert j. teichert@hzdr. de www. hzdr. de HZDR IHIP Peking University (since 2001) frf = 1. 3 GHz NC PC: Cs 2 Te DC-SRF Photo gun HZB, DESY, Jlab (since 2008) frf = 1. 3 GHz SC PC: Pb BERLin. Pro stage one gun Mitglied der Helmholtz-Gemeinschaft

Superconducting RF Photo Guns High current / low frequency cavity BNL, AES (since 2004) Quarter wave cavity NPS, NIOWAVE Uni Wisconsin, Niowave frf = 703. 75 MHz frf = 500 MHz SC PC: Nb frf = 200 MHz NC PC: Cs 2 Te NC PC: alkali + diamond amplifier BNL, Niowave Jochen Seite 8 Teichert j. teichert@hzdr. de www. hzdr. de HZDR frf = 112 MHz Mitglied der Helmholtz-Gemeinschaft

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

Gun accelerating gradient half cell 3 TESLA cells 2 HOM couplers cathode Design values • Bs, max = 110 m. T max. magn. surface field • Ecathode = 20 MV/m (backtracked cathode) • Epeak (1 st cell) = 30 MV/m • Epeak (TESLA)= 50 MV/m sc choke filter (to prevent RF leakage) FZD coupler & pickup ant. Mode ELBE High Charge final electron energy ≤ 9. 5 Me. V operation mode CW bunch charge 77 p. C 1 n. C repetition rate 13 MHz 500 k. Hz laser pulse (FWHM) 4 ps 15 ps transverse rms emittance 1 mm mrad 2. 5 mm mrad average current 1 m. A 0. 5 m. A 10 Jochen Seite 10 Teichert j. teichert@hzdr. de www. hzdr. de HZDR Mitglied der Helmholtz-Gemeinschaft

Gun accelerating gradient Performance of HZDR´s first 3. 5 cell cavity (3. 5 cell/2006) vertical tests @ DESY, 1. 8 K The insufficient cleaning (HPR) was the major problem – esp. choke filter & half-cell measurements in gun 2007 -2011 risk of contamination due to the NC photo cathode? After > 1000 h operation no deterioration was seen. 11 Jochen Seite 11 Teichert j. teichert@hzdr. de www. hzdr. de HZDR Mitglied der Helmholtz-Gemeinschaft

Gun accelerating gradient Gradient in elliptical 1. 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 Jochen Seite 12 Teichert j. teichert@hzdr. de www. hzdr. de HZDR Mitglied der Helmholtz-Gemeinschaft

Gun accelerating gradient peak electric field [MV/m] Cavity vertical Test CW operation 80 65 33 15 0. 7 17 3 TESLA 9 -cell cavity Rossendorf ½ cell cavity Rossendorf 3. 5 cell /2006 23 Rossendorf 3. 5 cell /2010 FG 35 3. 5 cell cavity design value vertical cryostat P. Kneisel Jochen Seite 13 Teichert 6. 5 50 PITZ/DESY 1. 6 cell NC gun (1. 3 GHz) 3. 5 cell /2010 FG fine grain RRR 300 Nb Energy gain [Me. V] 9. 5 60 (pulsed) Since gun performance mainly depends on gradient, Ea≈ 60 MV/m will give emittance < 1 µm @ 1 n. C + shaped laser + emittance compensation needed (see PITZ gun) 3. 5 cell/2010 LG large grain Nb j. teichert@hzdr. de www. hzdr. de HZDR Mitglied der Helmholtz-Gemeinschaft

Gun accelerating gradient Naval Postgraduate School (NPS) 500 MHz quarter wave resonator First beam in 2010: beam energy: 480 ke. V peak field: 8. 5 MV/m 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 14 Jochen Seite 14 Teichert j. teichert@hzdr. de www. hzdr. de HZDR Mitglied der Helmholtz-Gemeinschaft

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

SRF gun injection in ELBE for advanced beam diagnostic C 2 energy chirp energy spectrum vertical quadrupole scan • Phase scan technique for longitudinal phase • Slice emittance measurement Jochen Seite 16 Teichert j. teichert@hzdr. de www. hzdr. de HZDR bunch length correlation energy width Mitglied der Helmholtz-Gemeinschaft

fs bunches from the gun Bunch compression by means of “wrong” laser phase 17 Jochen Seite 17 Teichert j. teichert@hzdr. de www. hzdr. de HZDR Mitglied der Helmholtz-Gemeinschaft

fs bunches from the gun kinetic energy & energy spread SRF gun phase scan – optimum laser phase ? screen DV 02 (YAG) 1. 9 m from gun exit, 2. 7 m from cathode -175°, σx = 300 µm -150°, σx = 330 µm -130°, σx = 610 µm -115°, σx = 760 µm -95°, σx = 330 µm -65°, σx = 430 µm +3°, σx = 1650 µm 18 Jochen Seite 18 Teichert j. teichert@hzdr. de www. hzdr. de HZDR Mitglied der Helmholtz-Gemeinschaft

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 19 Jochen Seite 19 Teichert j. teichert@hzdr. de www. hzdr. de HZDR Mitglied der Helmholtz-Gemeinschaft

Photoelectron injectors for high-brightness beams and cw operation Gun type potential low f NC RF highest brightness in high f guns ¼ wave SC RF DC voltage best combination of brightness + aver. current high aver. current rf tests first beam routine operation LBNL NPS status rf power Tests gradient@cath. final energy *) 19. 5 MV/m 1) 0. 75 Me. V show stoppers rf heat dissipation produces beam HZDR HZB Examples present efforts elliptical SC RF Jlab FEL Cornell, KEK higher gradients reducing FE 25 MV/m 2) 1. 2 Me. V designs for higher voltage 20 -30 MV/m 3) 9. 5 Me. V NC cathode in SC cavity ? >100 m. A ERL light sources highest brightness 6. 75 MV/m 4) 0. 5 Me. V high voltage ~1 GHz rep rate best combination grad. + energy *) design 1) 2) 20 3) 4) values F. Sannibale, et al. , Proc. of FEL´ 20, Malmö, Sweden, 2010, p. 475. J. R. Harris, et al. , Phys Rev. ST AB 14, 053501 (2011). A. Arnold, et al. , NIM A 577, 440 (2007). N. Nishimori, et al. , Proc. of LINAC´ 10, Tsukuba, Japan, 2010, p. 995. Jochen Seite 20 Teichert j. teichert@hzdr. de www. hzdr. de HZDR Mitglied der Helmholtz-Gemeinschaft

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