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Les Sources d’ É lectrons de Forte Intensit é et de Faible Emittance

Les Sources d’ É lectrons de Forte Intensit é et de Faible Emittance. T. Garvey, Laboratoire de l’Acc é l é rateur Lin é aire – Orsay. Journ é es Acc é l é rateurs de la SFP – Roscoff, 10 octobre, 2005. Motivation for new high brightness injectors – new synchrotron radiation sources.

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Les Sources d’ É lectrons de Forte Intensit é et de Faible Emittance

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  1. Les Sources d’Électrons de Forte Intensité et de Faible Emittance T. Garvey, Laboratoire de l’Accélérateur Linéaire – Orsay. Journées Accélérateurs de la SFP – Roscoff, 10 octobre, 2005.

  2. Motivation for new high brightness injectors – new synchrotron radiation sources • SASE Free Electron Lasers at VUV and X-ray wavelengths • High peak brightness • Energy Recovery Linacs • High average brightness synchrotron radiation source. Not an exhaustive review of all existing or planned projects! Will evoke major issues in the design of injectors for FEL and ERL Projects.

  3. Single-pass Free Electron Lasers Gain Length: Beam size: r small high electron density for maximuminteraction with radiation field Emittance  ≤  Energy: Peak current inside bunch: Î > 1 kA feasible only at ultrarelativistic energies, or may dilute emittance  bunch compressor Energy width: Narrow resonance E/E ≤ r

  4. SASE FEL Projects • TTF VUV-FEL (6 – 30 nm) • European X-FEL (0.1 – 6 nm) • SPARC (500 nm) • LCLS (~ 0.15 nm) • SCSS (~ 3.6 nm) • BESSY (50 – 1 nm) • DUV-FEL…….

  5. Gun types Also – Field emission guns for intense e-beams.

  6. Advantages of laser triggered RF Gun RF gun – electrons are “born” in high field (~ 100 MV/m) region (c.f. DC guns ~ few MV/m) ⇒quickly accelerated to high energy ⇒ reduces detrimental effect of space-charge forces on the beam emittance. Laser-driven ⇒ allows generation of beams with same temporal structure as laser ⇒ rapid ( ~ pico-seconds, c.f. gridded DC guns ~ 1ns). CTF-3 gun under designed at LAL/Orsay

  7. RF Photo-injectors use many technologies • Photo-cathodes • Quantum efficiency, thermal emittance, dark current, aging…… • Cathode lasers • Long pulse train, intensity stability, synchronisation, temporal and spatial homogeneity….. • Gun cavity • Geometric form, symmetric coupling, surface fields…. • Analytic theory and numerical simulations • Longitudinal bunching, transverse emittance compensation, wake-fields…. • Diagnostics • Ultra-short pulses, low emittances, E.O. techniques, RF deflectors……

  8. Gun geometry design Waveguide coupler in exit cell – Coupling from both sides – reduces dipole “kick” Form of irises – reduce peak surface field for given accelerating field

  9. Bunch compression Need high peak currents (~ 1 - 2 kA) for SASE FEL. RF gun limited to < 100 A (1nC, 4 ps for TTF) to mitigate space charge effects ⇨ need to perform longitudinal compression. • Magnetic compression not necessarily part of the injector • Ballistic compression → “classical” injector approach • “Velocity” (or RF) bunching → incorporates bunching into the injector.

  10. Powerful radiation generates energy spread in bends • Causes bend-plane emittance growth (DESY experience) bend-plane emittance growth coherent radiation for > z z  E/E = 0 L0 s R x e– E/E < 0 x = R16(s)E/E overtaking length: L0  (24zR2)1/3  Magnetic Chicane Compresser

  11. Ballistic bunching • Usually at low energy, typical in injectors with DC guns. • Buncher cavity imparts energy chirp to give compression in downstream drift space.

  12. Velocity Bunching For non-rigid bunch, relative movements take place within bunch to reduce phase spread. Observed on DUV-FEL linac (BNL) Technique considered for CTF probe beam linac

  13. Emittance Compensation: Controlled Damping of Plasma Oscillation Space-charge effects and non-linear RF fields play a role in the evolution of the RMS emittance. Must determine at what point to accelerate and ‘freeze’ the emittance oscillations. L. Serafini and J.B. Rosenzweig, Phy Rev. E, Vol. 55, 1997. Space charge effects can dominate to high energies for these intense beams and low emittances. “Injector” can be > 100 MeV 100 A => 150 MeV

  14. Matching onto the Local Emittance Max., Final emittance = 0.4 mm Example of an optimized matching M. Ferrario et al., “HOMDYN Study For The LCLS RF Photo-Injector”, Proc. of the 2nd ICFA Adv. Acc. Workshop on “The Physics of High Brightness Beams”, UCLA, Nov., 1999, also in SLAC-PUB-8400

  15. emittance envelope Movable Emittance-Meter for the SPARC project

  16. Free-Electron Laser at the TESLA Test Facility value Beam energy (during FEL operation) 220-270 MeV Bunch charge ~ 3 nC Transverse projected emittance (rms) ~ 8 mm-mrad Bunch length (rms) ~ 1 ps Peak current 1.2 - 1.5 kA Long. emittanceσE σl 60 mm-keV The TESLA Test Facility at DESY: TTF-FEL I bunch compressor booster magnet undulator rf-gun FEL beam Photon Diagnostics Area superconducting cavities electron beam parameter cathode laser electron dump

  17. S-band Photo-injectors – high peak currents • Waveguide coupling to cavity • Tuners for frequency regulation ELYSE Gun – LAL/Orsay Waveguide and tuners break circular Symmetry of gun ⇒ bad for emittance. CTF-2 Gun - CERN

  18. Free-Electron Laser at the TESLA Test Facility 1800 pulses The TTF-1 photo-emission electron source • UV laser impinging a Cesium Telluride cathode provide electrons via photo-emission • The cathode is located in a 1-1/2 cell rf-gun (f=1.3 GHz) with peak E-field of 40 MV/m • The laser is a 4w ND:YLF laser (262 nm)

  19. TTF injector II typical parameters for TTF 1-FEL: repetition rate: 1 Hz bunch frequency: 1 - 2.25 MHz bunch charge: 1- 3 nC bunch length (rms): ~3 mm (1 nC, after booster ) norm. emit., x,y: ~ 4 µm ( @ 1nC) dp/p: 0.13 % rms ( @ 16 MeV ) injection energy: 16 MeV

  20. Free-Electron Laser at the TESLA Test Facility The new electron source • Coaxial input coupler prevents transverse rf- kicks (and the associated emittance growth) • Laser will be upgraded to generate plateau-like distributions • In its final upgrade the source will generate 7200 bunches stack on a 800 msec rf-pulse • RF-gun commissioned at the DESY-Zeuthen PITZ facility • Requirements : e < 2 mm-mrad at the undulator, Q=1 nC

  21. Collaborating Institutes: PITZ Collaboration BESSY Berlin, CCLRC Daresbury, DESY (HH + Z), INFN Frascati, INFN Milano, INR Troitsk, INRNE Sofia, LAL Orsay, MBI Berlin, TU Darmstadt, U Hamburg, YERPHI Yerevan (1.3 GHz)

  22. VUV-FEL Gun: Longitudinal Phase Space Q = 1 nC max. mean momentum:4.72 MeV/c min. rms momentum spread:33 keV/c good agreement with simulations ! Minimum bunch length: FWHM = (21.04 ± 0.45stat ± 4.14syst) ps = (6.31 ± 0.14stat ± 1.24syst) mm bunch length:

  23. Transverse Emittance Measurements Single Slit Scan Technique beam spot at screen 2 beamletsatscreen 3 single slit positions beamlet size is measuredfor 3 slit positions:

  24. Q = 1 nC, F = Fm – 5° Imain = 305 A Main Solenoid Bucking Solenoid 1.3 GHz 1 1/2 cell RF Gun VUV-FEL Gun: Transverse Emittance requirement for VUV-FEL (30 nm) =3 requirement for VUV-FEL (6 nm) = 2 requirement for XFEL=0.9 Start-up requirement of TTF2 is clearly fulfilled !

  25. ATF at 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

  26. PITZ: Cathode Laser Pulse Profile On 23.06.2003 longitudinal shape changed to flat top Until 23.06.2003 - Gaussian longitudinal laser shape: FWHM  18-23 ps rise and fall time about 5-8 ps FWHM = 7 ± 1ps Minimum measured emittance ≥ 3 mm mrad 1.6 mm mrad

  27. Outlook PITZ2 study emittance conservation principle further improvement of beam quality • Towards to the XFEL Photo Injector: • 60 MV/m at the cathode • Cathode laser improvement booster cavity (TESLA, CDS - Cut Disc Structure) work on laser, cavities, photo cathodes, developments on simulation tools Booster Cavity Simulations for 60MV/m at the cathode

  28. Free-Electron Laser at the TESLA Test Facility The new injector configuration rf-gun ACC. MOD. ACC. MOD. BC Diagnostics section dump 3rd harm rf-structure Matching section • Injection directly into a TESLA accelerating module makes the emittance compensation scheme more efficient (e~1.3 mm-mad) • Use of a 3rd harm. RF-section to correct the longitudinal phase space distortions

  29. BESSY FEL 2.2 GeV linac using TESLA cavities 1 kHz repetition rate, 40 MV/m cathode field → 75 kW average dissipation !! c.f. needs of Arc-en-Ciel. Gun being developed in framework of EUROFEL

  30. A variation of the RF-gun concept: the pulsed photodiode 2 MV HV 1 ns pulse on a 2 mm diode gap: 100 pC @ 100 fs bunch Bn=1.2.1015 M. Van der Wiel et al., T.U. Eindhoven,

  31. Advanced Laser-Plasma High Energy Accelerators towards X-rays University of Strathclyde, Glasgow. J. Rodier – LAL/Orsay

  32. •ERL – combines LOW transverse emittance beam properties with HIGHaverage (high duty cycle / CW) current. •Beam properties at high energy are determined by injector quality and not by dynamic equilibrium properties of a Storage Ring Energy Recovery Linac Injectors High brightness light source

  33. 4GLS – Daresbury Laboratory

  34. High Current ERL Injector Requirements • Output energy ~ 7 – 10 MeV • CW average current ~ 100 – 500 mA • Transverse emittance < 6 mm-mrad • Longitudinal emittance ~ 150 keV – ps (rms) • Bunch length ~ 2 – 7 ps • Energy spread < 0.5 % @ 7 MeV • High Power RF feed-through possibilities.

  35. JLab High Brightness Gun Extremely reliable Delivered 5.5 kiloCoulombs from one cathode Same cathode for 2 years Modest improvements for 10 kW Upgrade

  36. Jefferson Lab Gun Approach • Highest average brightness is produced by lowest charges at highest frequency (J.B. Rosenzweig PAC95 pp. 957-960) • However FEL needs high peak current, contrary to this scaling • Compromise: highest frequency for which charge (when later bunched to high peak current) has sufficient small signal gain and linac is stable to BBU, etc. • Highest brightness is achieved by high gun gradient and fast acceleration • IR Demo operated 3.8 to 4.2 MV/m limited by cesium on gun ball • Upgrade should achieve >6 MV/m with cesium only on GaAs • Get to >10 MeV as quickly as possible using srf high gradient cavities • Long cathode lifetimes (measure in coulombs/cm2 not time!) • 5,500 Coulombs from one cathode demonstrated (3 mm spot radius) • Limited by ion back bombardment from background gas • Vacuum pumping in DC gun 100 times rate in rf gun

  37. JLab 10 kW Upgrade IR FEL Injector demonstrated performance The injector is driven by a 350 kV DC GaAs Photocathode Gun • Pulsed operation at 8 mA/pulse (110 pC/bunch) in 16 ms-long pulses at 2 Hz repetition rate • CW operation at 9.1 mA (75 MHz) with 122 pC/bunch • Routinely delivers 5 mA CW and pulse current at 135 pC/bunch for FEL operations • 400 A peak current at wiggler Beam

  38. Hybrid DC gun /SRF cavity photo-injector in design engineering • SBIR development by AES Corp. partnered with JLab • 750 MHz operation 133 pC / RF bucket → 100 mA @ 10 MeV 100 kW of beam power!!! JLab concept of a high voltage DC gun married to a low frequency rf cavity (shown here with SNS cavities)

  39. Twin Coupler Attached to the CORNELL ERL Injector Cavity Modified version of TTF-III Coupler. Mechanical changes to Increase average power – 75 kW

  40. Normal (Cs2Te) and superconducting (Nb) photo-cathodes under study. SC cavity prohibits use og magnetic field for Emittance Compensation.

  41. Conclusions • Recent years (~ 10) have seen impressive progress in the development of photo-injectors for short wave-length SASE FEL’s. • Many subjects need to be developed further – cathode life-time, dark current levels, shaping of laser pulse (temporally and spatially), gun cavity technology for high duty cycle, longitudinal beam compression techniques which leave transverse emittance un-disturbed. • Increasingly sophisticated BD simulations are guiding injector design for low emittance. • DC guns, originally investigated for nuclear physics, are being up-graded for ERL requirements. • marriage of DC gun and SC RF technolgy • The development of superconducting RF technology has had a major influence on the design of new synchrotron light sources.

  42. Acknowledgements I thank the following people for providing material : Ph. Piot (FNAL) K. Flöttmann (DESY Hamburg) F. Stephan (DESY Zeuthen) M. Krasilnikov (DESY Zeuthen) F. Marhauser (BESSY) D. Jaroszynski (Univ. Strathcyde) M. Ferrario (INFN Frascati) G. Neil (JLab)

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