Review of experimental results on photoemission electron sources

Review of experimental results on photoemission electron sources Ph. Piot, DESY Hamburg Introduction RF-guns DC-guns Conclusions ICFA/BD Sardinia July 2002 Ph. Piot, DESY 1

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)… ICFA/BD Sardinia July 2002 Ph. Piot, DESY 2

Photo-emission from metals and semi-conductors semi-conductor ICFA/BD Sardinia July 2002 metal Ph. Piot, DESY 3 (from Spicer et al. SLAC-PUB 6306)

Few words on Lasers • For metal, typical laser energy required: 5 -500 m. J/pulse • For semi-conductor: 0. 5 m. J/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] ICFA/BD Sardinia July 2002 Ph. Piot, DESY 4

Emittance and Brightness Phase-space emittance (Liouvilian invariant) (what codes give) Trace-space emittance (experimentally measurable) Normalized brightness beam current ICFA/BD Sardinia July 2002 Ph. Piot, DESY 5

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) (mm-mrad) e N 1. 2 1 0. 8 0. 6 0. 4 0. 2 0 0 0. 2 0. 4 0. 6 0. 8 1 Horizontal RMS laser size (mm) 1. 2 Linear fit gives Ek=0. 43 e. V ICFA/BD Sardinia July 2002 Nonlinear fit gives brf=3. 1+/-0. 5, Fcu=4. 73+/-0. 04 e. V, and Ek=0. 40 e. V Ph. Piot, DESY 6

Thermal Emittance (CNT’D) To date no thermal emittance measurement for Cs 2 Te cathodes has been performed [plan at INFN Milano are underway] Several groups have measured thermal emittance of Ga. As: * Duhnam et al. , on the Illinois/CEBAF polarized beam (PAC 1993) 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 k. T 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 -20 ps ICFA/BD Sardinia July 2002 Ph. Piot, DESY 7

Generic photo-injectors Split injectors gun • 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, … booster Integrated injectors • typically 10 -1/2 cell cavity with moderate E-field • long solenoid lens • E. g. AFEL, PEGASSUS gun DC-gun ICFA/BD Sardinia July 2002 • DC column with HV 500 k. V and higher achieved • Solenoids + rf-buncher • Booster section • E. g. IR-Demo booster Ph. Piot, DESY 8

Frequency Scaling of photo-injectors (Rosenzweig and Colby PAC 95 Also L C. -L. Lin et al. , PAC 95) • If the operating parameters are scaled following the Table, one would expect: Brightness~ w 2 • this assumes: E-field~ w 1 • Naively scaling the present BNL gun (120 MV/m) e. g. to 17 GHz would imply: E-field~ 720 MV/m!!! ICFA/BD Sardinia July 2002 Ph. Piot, DESY 9

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 Me. V) (PR. ST. AB vol. 4: 083501 (2001)) ICFA/BD Sardinia July 2002 Ph. Piot, DESY 10

MIT 17 GHz gun • Measured emittance at 50 p. C to be 1 mm-mrad at the gun exit • Brightness=80 A/(mm-mrad)2 • It will be boosted to ~800 A/(mmmrad)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)) ICFA/BD Sardinia July 2002 Ph. Piot, DESY 11

3 GHz CLIC drive beam photo-injector Operational since 1996. About 1000 h of running each year since, mainly for CLIC 30 GHz power production ICFA/BD Sardinia July 2002 Ph. Piot, DESY (Courtesy of H Braun) 12

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 ICFA/BD Sardinia July 2002 Ph. Piot, DESY 13

Recent results from ATF, BNL • 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. 5 n. C and I=200 A Example of fitted envelope at 70 Me. V centered beam mis-steered beam 30 um wire focused spot ICFA/BD Sardinia July 2002 Ph. Piot, DESY (Courtesy of V. Yakimenko) 14

Recent results from ATF, BNL (extracted from ATF News Letter 03/2002) 60 90 80 50 70 100 • As predicted by simulation, uniform beam gives the best emittance • Emittance doubles for the 50 % modulation case ICFA/BD Sardinia July 2002 Ph. Piot, DESY • Measurement of impact of transverse non-uniformity on emittance • Used a mask • Q=0. 5 n. C (kept constant) • Emittance for uniform beam is about 1. 5 mm-mrad • Long. Length is 3 ps FWHM 100 % 90 % 60 % 50 % 15

Recent results from SDL, BNL undulators linac zero-phased 75 Me. V linac 75 Me. V y dump Slice emittance measurements dump • Parametric study of emittance (projected + t slice) vs various parameters • Preliminary data indicate brightness (Courtesy of W. Graves et al. ) improves as charge is decreased 10 p. C ICFA/BD Sardinia July 2002 Ph. Piot, DESY 200 p. C 16

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 offcrest) [see M. Ferrario’s talk] • Measurement were performed using both frequency- and time-domain technique (Piot’s talk in Working Group I) ICFA/BD Sardinia July 2002 Ph. Piot, DESY 17

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 p. C, FWHM d=8%, FWHM t=3 ps (initial laser FWHM=4. 3 ps) (Courtesy of J. Schmerge) ICFA/BD Sardinia July 2002 Ph. Piot, DESY 18

2. 856 GHz PWT gun (under commissioning at UCLA) • Integrated injector installed at the PEGASUS facility, UCLA • Exit energy ~20 Me. V • E-field 40 -60 MV/m • Charge 1 n. C • Input power 20 MW Proposed to be used to produced polarized electron beam using Ga. As (which requires 1 E-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) ICFA/BD Sardinia July 2002 Ph. Piot, DESY 19

FNPL(FNAL) & TTF injector II (DESY) typical parameters for TTF 1 -FEL: repetition rate: pulse train length: bunch frequency: bunch charge: bunch length (rms): ICFA/BD Sardinia July 2002 [see also S. Schreiber’s talk] 1 Hz norm. emit. , x, y: 3 -4 µm ( @ 1 n. C) 1 -800 µs dpp: 0. 13 % rms ( @ 17 Me. V ) 1 -2. 25 MHz injection energy: 17 Me. V 1 -3 n. C ~3 mm ( 1 n. C, (Schreiber et al. EPAC 2002) after booster ) 20 Ph. Piot, DESY

Results at TTF Injector 2 (1 n. C setup) Emittance measurements sol. 1/2 emit. x emit. y 4. 19 0. 13 4. 58 0. 15 220 / 104 3. 02 0. 17 3. 47 0. 12 240 / 104 4. 08 0. 57 4. 52 0. 47 200 / 104 Bunch length measurement (streak cam. ) (Schreiber et al. PAC 2001) (Honkaavara et al. PAC 2001) ICFA/BD Sardinia July 2002 Ph. Piot, DESY 21

VUV-FEL driven TTF injector ü Primary electron bunches (charge 3 n. C) are produced by laser-driven rf gun ü During single pass of the undulator primary bunch produces powerful VUV radiation (l=95 nm) ü Radiation is reflected by plane Si. C mirror and is directed back to the photocathode of rf gun ü Electron bunch produced by SASE radiation (charge up to 0. 5 n. C) is accelerated ICFA/BD Sardinia July 2002 Ph. Piot, DESY (Faatz et al. FEL 2002) 22

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) ICFA/BD Sardinia July 2002 Ph. Piot, DESY 23

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 • Cs 2 Te thermal emittance measurement are foreseen ICFA/BD Sardinia July 2002 Ph. Piot, DESY 24

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 n. C • Exit energy 15 -20 Me. V (from Nguyen’s talk at PERL workshop BNL, Jan 2001) ICFA/BD Sardinia July 2002 • Macropulse current up to 400 m. A Ph. Piot, DESY 25

Results, LANL AFEL • Measure slice emittance using a combined quadrupole scan with a streak camera • Measured slice emittance of 1. 6 mm-mrad at 1 n. C • PARMELA predicts 0. 6 mm-mrad (without thermal emittance) (from S. Gierman’s Thesis -- UCSD) ICFA/BD Sardinia July 2002 Ph. Piot, DESY 26

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 Cs 2 Te • No solenoid => focusing provided by rf (conic-shaped back plate) • First photo-electrons observed last March (Courtesy of P. Janssen et al. ) ICFA/BD Sardinia July 2002 Ph. Piot, DESY 27

SRF gun (DROSSEL collaboration) (Courtesy of P. Janssen et al. ) ICFA/BD Sardinia July 2002 Ph. Piot, DESY 28

SRF gun (DROSSEL collaboration) (Courtesy of P. Janssen et al. ) ICFA/BD Sardinia July 2002 Ph. Piot, DESY 29

The APLE BOEING (decommissioned) • 0. 433 GHz, 2 cells Bucking coil • E-field=25 MV/m • Typical charge 1 to 5 n. C • Exit energy ~2 Me. V • Laser: 53 ps (FWHM), 5 mm radius • K 2 Cs. Sb cathode coil • duty cycle: 25% • Macropulse frequency: 30 Hz • Macropulse length: 8. 3 ms • Micropulse frequency: 27 MHz (Courtesy of D. Dowell) ICFA/BD Sardinia July 2002 Ph. Piot, DESY 30

Recent results, ELSA-2 Bruyeres-le-chatel • 0. 144 GHz, 2 cells • E-field=25 MV/m • Typical charge 1 to 10 n. C • Exit energy ~2. 6 Me. V • Laser: 60 ps (FWHM), 4 mm radius • Macropulse frequency: 10 Hz • Macropulse length: 150 ms • Micropulse frequency: 14. 4 MHz (Courtesy of Ph. Guimbal) ICFA/BD Sardinia July 2002 Ph. Piot, DESY 31

DC-GUN, JLab IR-Demo insulating ceramic • DC gun with Ga. As photo-cathode • Buncher needed despite the 20 ps laser • In the Ir-Demo gun is coupled to a ¼ photo-cathode cryounit (2 CEBAF-type 5 -cell SRF anode cavities at 10 and 9 MV/m) • advantage: ran CW at 75 MHz (1/80 th of 1497 MHz) solenoid • Recently developed laser (M. Poekler PAC 2001) allows CW ope. @ 1. 5 GHz laser ICFA/BD Sardinia July 2002 (D. Engwall et al. PAC 1997, Ph. Piot et al. EPAC 1998) Ph. Piot, DESY 32

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 p. A “dark” current. (C. K. Sinclair et al. PAC 2001) ICFA/BD Sardinia July 2002 Ph. Piot, DESY 33

Comparison of Peak brightness ICFA/BD Sardinia July 2002 Ph. Piot, DESY 34

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 n. C (to the expense of bunch length) • Presently achieved performances with a DC gun are comparable to rfgun 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…) ICFA/BD Sardinia July 2002 Ph. Piot, DESY 35

Grazie Mile! # Thanks to all the individual aforementioned for their contributions # To M. Ferrario, K. Floettmann , W. Graves , P. Hartmann, C. Sinclair for discussions ICFA/BD Sardinia July 2002 Ph. Piot, DESY 36