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Review of experimental results on photo-emission electron sources

Review of experimental results on photo-emission electron sources. Introduction RF-guns DC-guns Conclusions. Ph. Piot, DESY Hamburg. Introduction.

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Review of experimental results on photo-emission electron sources

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  1. Review of experimental results on photo-emission electron sources Introduction RF-guns DC-guns Conclusions Ph. Piot, DESY Hamburg Ph. Piot, DESY

  2. Introduction • Application of high-brightness photo-injectors: - high energy linear colliders (needs flat beam ey/ex <<1) - radiation sources (FELs, linac-based SR) - X-rays production (XTR, Thomson) - plasma-based electron sources-drivers,… • Many accelerator test facilities in operation based on photo-injectors: - dedicated to beam physics (BNL, UCLA, DESY-Z, NERL...) - drive user-facility (Jlab, DESY-HH,…) • Figure-of-merits: emittance (FELs requires e<l) , peak current, average current (photon flux), local energy spread, bunch length (e.g. for probing ultra-fast phenomena)… Ph. Piot, DESY

  3. Photo-emission from metals and semi-conductors semi-conductor metal Ph. Piot, DESY (from Spicer et al. SLAC-PUB 6306)

  4. Few words on Lasers • For metal, typical laser energy required: 5-500 mJ/pulse • For semi-conductor: 0.5 mJ/pulse • Metallic cathodes are bad candidates for high-average power machine [one might need an FEL-based photo-cathode laser to have 100 W level in the UV. E.g. see Zholents’s talk at BNL PERL workshop 01/2001] Ph. Piot, DESY

  5. Emittance and Brightness Phase-space emittance (Liouvilian invariant) (what codes give) Trace-space emittance (experimentally measurable) Normalized brightness beam current Ph. Piot, DESY

  6. 1.2 1 0.8 (mm-mrad) 0.6 N e 0.4 0.2 0 0 0.2 0.4 0.6 0.8 1 1.2 Horizontal RMS laser size (mm) Thermal Emittance Electrons are emitted with a kinetic energy Ek laser spot assumed uniform with radius r Example of measurement for Cu-cathode (Courtesy of W. Graves) Nonlinear fit gives brf=3.1+/-0.5, Fcu=4.73+/-0.04 eV, and Ek=0.40 eV Linear fit gives Ek=0.43 eV Ph. Piot, DESY

  7. Thermal Emittance (CNT’D) To date no thermal emittance measurement for Cs2Te cathodes has beenperformed [plan at INFN Milano are underway] Several groups have measured thermal emittance of GaAs: * Duhnam et al., on the Illinois/CEBAF polarized beam (PAC1993) at room temperature * Orlov et al., at Heidelberg (Appl. Phys. Lett. 78: 2171 (2001)) at 70 K The measurements indicate that a reduction of the cathode temperature results in a lower transverse kT for the emitted e-. This is particular to NEA cathodes where electrons from thermalized population can escape. The price to pay is the long emission time of 10-20ps Ph. Piot, DESY

  8. Generic photo-injectors Split injectors • 1-1/2, 2-1/2 cell cavity with high E-field • booster section downstream of the gun • E.g. BNL-gun, FNAL, AWA, DESY,… gun booster Integrated injectors • typically 10-1/2 cell cavity with moderate E-field • long solenoid lens • E.g. AFEL, PEGASSUS gun DC-gun • DC column with HV 500 kV and higher achieved • Solenoids + rf-buncher • Booster section • E.g. IR-Demo gun booster Ph. Piot, DESY

  9. Frequency Scaling of photo-injectors (Rosenzweig and Colby PAC95 Also L C.-L. Lin et al., PAC95) • If the operating parameters are scaled following the Table, one would expect:Brightness~ w2 • this assumes: E-field~ w1 • Naively scaling the present BNL gun (120 MV/m) e.g. to 17 GHz would imply: E-field~ 720 MV/m!!! Ph. Piot, DESY

  10. MIT 17 GHz gun Mission:Advanced ultra-bright accelerator developments 1/ has commissioned a 1.5 cell gun 2/ work on a 2.4 cell gun (>2 MeV) (PR. ST. AB vol. 4:083501 (2001)) Ph. Piot, DESY

  11. MIT 17 GHz gun • Measured emittance at 50 pC to be 1mm-mrad at the gun exit • Brightness=80 A/(mm-mrad)2 • It will be boosted to ~800 A/(mm-mrad)2 after emittance compensation • Emittance compensation presently non effective (velocity spread) need to increase the beam energy at gun exit (will use a 2.4 cell gun) (PR. ST. AB vol. 4:083501 (2001)) Ph. Piot, DESY

  12. 3 GHz CLIC drive beam photo-injector Operational since 1996. About 1000h of running each year since, mainly for CLIC 30 GHz power production Ph. Piot, DESY (Courtesy of H Braun)

  13. BNL/UCLA/SLAC gun • Popular design, used at BNL (ATF & SDL), SLAC (GTF), ANL (LEUTL), Tokai (NERL),… • Since its first design the gun has undergone improvements; latest foreseen are: a mode-lock system and a split symmetric RF input coupler Ph. Piot, DESY

  14. Recent results from ATF, BNL Example of fitted envelope at 70 MeV • Beam based alignment of quad to center beam in the TWS • Optimized optics (with a high-b) to overcome problems inherent to the screen resolution • Measured beam emittance using the multi-monitor technique • Obtained: e=0.8 mm-mrad for Q=0.5nC and I=200 A centered beam mis-steered beam 30 um wire focused spot (Courtesy of V. Yakimenko) Ph. Piot, DESY

  15. Recent results from ATF, BNL • Measurement of impact of transverse non-uniformity on emittance • Used a mask • Q=0.5 nC (kept constant) • Emittance for uniform beam is about 1.5 mm-mrad • Long. Length is 3 ps FWHM (extracted from ATF News Letter 03/2002) 50 60 70 80 90 100 100 % 90 % 60 % 50 % • As predicted by simulation, uniform beam gives the best emittance • Emittance doubles for the 50 % modulation case Ph. Piot, DESY

  16. undulators linac zero-phased linac 5 MeV 75 MeV 75 MeV dump dump Recent results from SDL, BNL y Slice emittance measurements • Parametric study of emittance (projected + slice) vs various parameters • Preliminary data indicate brightness improves as charge is decreased t (Courtesy of W. Graves et al.) 200 pC 10 pC Ph. Piot, DESY

  17. Recent results from SDL, BNL Observation of sub-picosecond compression by velocity bunching • Used the TWS tank downstream of the rf-gun as a buncher (operated far off-crest) [see M. Ferrario’s talk] • Measurement were performed using both frequency- and time-domain technique (Piot’s talk in Working Group I) Ph. Piot, DESY

  18. Recent results GTF, SLAC • Parametric study of emittance versus bunch charge • Achieved LCLS project parameters (1.5 mm-mrad for I~100 A) • Reconstructed the longitudinal phase space from a set of energy profile measurement. • For Q=200 pC, FWHM d=8%, FWHM t=3 ps (initial laser FWHM=4.3 ps) (Courtesy of J. Schmerge) Ph. Piot, DESY

  19. 2.856 GHz PWT gun (under commissioning at UCLA) • Integrated injector installed at the PEGASUS facility, UCLA • Exit energy ~20 MeV • E-field 40-60 MV/m • Charge 1 nC • Input power 20 MW Proposed to be used to produced polarized electron beam using GaAs (which requires 1E-11 T vacuum) because of the better vacuum conductance compared to usual cavity-based photo-injector [Clendenin et al. SLAC-PUB-8971] (Telfer et al., PAC 2001) Ph. Piot, DESY

  20. FNPL(FNAL) & TTF injector II (DESY) typical parameters for TTF 1-FEL: [see also S. Schreiber’s talk] repetition rate: 1 Hz pulse train length: 1-800 µs bunch frequency: 1-2.25 MHz bunch charge: 1-3 nC bunch length (rms): ~3 mm ( 1 nC, after booster ) norm. emit., x,y: 3-4 µm ( @ 1nC) dpp: 0.13 % rms ( @ 17 MeV ) injection energy: 17 MeV (Schreiber et al. EPAC2002) Ph. Piot, DESY

  21. Results at TTF Injector 2 (1nC setup) Emittance measurements Bunch length measurement (streak cam.) (Schreiber et al. PAC2001) (Honkaavara et al. PAC2001) Ph. Piot, DESY

  22. VUV-FEL driven TTF injector • Primary electron bunches (charge 3nC) are produced by laser-driven rf gun • During single pass of the undulator primary bunch produces powerful VUV radiation (l=95 nm) • Radiation is reflected by plane SiC mirror and is directed back to the photocathode of rf gun • Electron bunch produced by SASE radiation (charge up to 0.5 nC) is accelerated Ph. Piot, DESY (Faatz et al. FEL2002)

  23. Results at FNPL, FNAL Transverse Emittance Studies • Systematic optimization of the rf-gun parameters (solenoids, laser radius) for various charges • Estimate of brightness indicates it improves with decreasing charge Production of Flat beams • Used the inverse Derbenev transform to convert a magnetized round beam in a flat beam [see S. Lydia’s talk] • High ratio of ex/ey~50 demonstrated (Courtesy of J.-P Carneiro) Ph. Piot, DESY

  24. DESY 1.3 GHz gun • Second generation of gun for TTF user facility • Fully symmetrized cavity using a coaxial input-coupler • Test facility at DESY-Z just commissioned • Cs2Te thermal emittance measurement are foreseen Ph. Piot, DESY

  25. LANL AFEL Facility Mission: Advanced free-electron laser experiment at Los Alamos. The gun has driven a IR SASE-FEL • 1.3 GHz, 10+1/2 cells • E-field=20 MV/m • Typical charge 1 to 4 nC • Exit energy 15-20 MeV • Macropulse current up to 400 mA (from Nguyen’s talk at PERL workshop BNL, Jan 2001) Ph. Piot, DESY

  26. Results, LANL AFEL • Measure slice emittance using a combined quadrupole scan with a streak camera • Measured slice emittance of 1.6 mm-mrad at 1nC • PARMELA predicts 0.6 mm-mrad (without thermal emittance) (from S. Gierman’s Thesis -- UCSD) Ph. Piot, DESY

  27. SRF gun (DROSSEL collaboration) First phase: proof-of-principle: observe photo-emission of a cathode in a superconducting rf-cavity Later: built a “real” gun that could be used for CW operation of the ELBE free-electron laser based at Forschungszentrum Rossendorf • frequency=1.3 GHz • Number of cell~ 0.5 • Half-cell is a TESLA cavity shape with a shallow cone • Use a Cs2Te • No solenoid => focusing provided by rf (conic-shaped back plate) • First photo-electrons observed last March (Courtesy of P. Janssen et al.) Ph. Piot, DESY

  28. SRF gun (DROSSEL collaboration) (Courtesy of P. Janssen et al.) Ph. Piot, DESY

  29. SRF gun (DROSSEL collaboration) (Courtesy of P. Janssen et al.) Ph. Piot, DESY

  30. The APLE BOEING (decommissioned) • 0.433 GHz, 2 cells • E-field=25 MV/m • Typical charge 1 to 5 nC • Exit energy ~2 MeV • Laser: 53 ps (FWHM), 5 mm radius • K2CsSb cathode Bucking coil coil • duty cycle: 25% • Macropulse frequency: 30 Hz • Macropulse length: 8.3 ms • Micropulse frequency: 27 MHz (Courtesy of D. Dowell) Ph. Piot, DESY

  31. Recent results, ELSA-2 Bruyeres-le-chatel • 0.144 GHz, 2 cells • E-field=25 MV/m • Typical charge 1 to 10 nC • Exit energy ~2.6 MeV • Laser: 60 ps (FWHM), 4 mm radius • Macropulse frequency: 10 Hz • Macropulse length: 150 ms • Micropulse frequency: 14.4 MHz (Courtesy of Ph. Guimbal) Ph. Piot, DESY

  32. DC-GUN, JLab IR-Demo insulating ceramic • DC gun with GaAs photo-cathode • Buncher needed despite the 20 ps laser • In the Ir-Demo gun is coupled to a ¼ cryounit (2 CEBAF-type 5-cell SRF cavities at 10 and 9 MV/m) • advantage: ran CW at 75 MHz (1/80th of 1497 MHz) • Recently developed laser (M. Poekler PAC 2001) allows CW ope. @ 1.5GHz photo-cathode anode solenoid (D. Engwall et al. PAC1997, Ph. Piot et al. EPAC1998) laser Ph. Piot, DESY

  33. DC-gun, JLab IR-Demo • High voltage operation of DC-gun limiter by field-emission • Collaboration Jlab + College of William & Mary: study reduction of field-emission by Nitrogen ions implantation on the electrodes • Experiment performed in a test chamber demonstrate the benefits of ion implantation: up to 25 MV/m DC-field could be achieved with less than 40 pA “dark” current. (C.K. Sinclair et al. PAC2001) Ph. Piot, DESY

  34. Comparison of Peak brightness Ph. Piot, DESY

  35. Conclusions • ATF at BNL has set new record in brightness • Both BNL-type and DESY-type gun have driven short wavelength single-pass FELs to saturation (LEUTL, TTF-1). • ELSA-2 at Bruyeres-le-Chatel has demonstrated the targeted emittance number of 1 mm-mrad at 1 nC (to the expense of bunch length) • Presently achieved performances with a DC gun are comparable to rf-gun running with high duty cycle (in term of brightness). - better candidate to drive high photon-flux based on ERL? - largest average brightness - and E-field of 25 MV/m have been achieved in experiment • Many other developments I have not addressed (hybrid DC/RF guns, hybrid plasma/photo-emission guns, needle cathodes, etc…) Ph. Piot, DESY

  36. Grazie Mile! # Thanks to all the individual aforementioned for their contributions # To M. Ferrario, K. Floettmann , W. Graves , P. Hartmann, C. Sinclair for discussions Ph. Piot, DESY

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