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SRF Gun Development for BERLinPro

This workshop focuses on the development of superconducting radio-frequency (SRF) guns for BERLinPro, an integrated energy recovery linac (ERL) demonstrator. Topics include beam dynamics, cathode performance, and high power operation.

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SRF Gun Development for BERLinPro

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  1. T. Kamps | kamps@helmholtz-berlin.de Ultra Bright Sources Workshop | Cockroft Institute 29.06. – 01.07.2011 SRF Gun Development for BERLinPro

  2. Helmholtz-Zentrum Berlin build and operates accelerator facilities for fundamental and applied research • Synchrotron radiation sources Bessy I (now Sesame), Bessy II and MLS for the generation of soft X-rays: 2500 users doing 700 experiments / year • Vicksi, an ion and proton accelerator complex currently used for eye cancer treatment: several hundred patients treated • HoBiCaT, a facility for the development of Superconducting radio-frequency (SRF) components for continous-wave operation of linear accelerators We will build BERLinPro, a feasibility study for an Energy-Recovery Linac (ERL) Thorsten Kamps | kamps@helmholtz-berlin.de

  3. BERLinPro – an accelerator science and technology experiment – fully integrated ERL demonstrator 1.5 MeV spent beam 5-10 MeV 5-10 MeV SRF photo gun merger section beam dump SRF booster main linac SRF modules recirculation 100 MeV beam manipulation funded by the Bundesminesterium für Bildung und Forschung and the State of Berlin Joint Accelerator Developments | T. Kamps | HZB

  4. After evaluation of several options during CW Source Workshop in 2008 we concluded that SRF is the way to go for the electron source NCRF Courtesy S. S. Kurennoy / LANL DC SRF is the right choice to reach an average current of 100 mA with 1 mm mrad emittance, maximum control and flexibility Courtesy D. Dowell / Boeing / SLAC SRF Courtesy B. Dunham / Cornell Courtesy J. Lewellen / NPS Courtesy A. Burril / BNL / AES Ultra Bright Electron Sources Workshop | Cockroft Institute | 29.06 - 01.07.2011

  5. The longest running SRF gun: The HZDR/ELBE SRF injector. First beam in 2007, injection into ELBE in 2010 Tuner ½ cell RF / HOM ports • 3.5 cell geometry to achieve 9.5 MeV beam energy • 10 kW average power required for 1 mA average current • Sacrifice on transverse brightness, ok for the IR FEL application. • Require 1 mA of average current  normal-conducting Cs2Te cathode on LN2 cathode stock. • No degradation in cavity performance due to cathode stock and material. From J. Teichert, see NIM A 577 (2006) Cathode stock Choke filter 3 full cells Setup by collaboration between HZDR, MBI, HZB, DESY and funded by BMBF, CARE, EuCARD Ultra Bright Electron Sources Workshop | Cockroft Institute | 29.06 - 01.07.2011

  6. The cathode: Motivation to choose CsK2Sb as baseline photocathode material • QE of 4…12 % at 532 nm compatible with our requirements, • need at least 10% of QE in the VIS to relax requirements on laser system • fast, robust (see Cornell results), rel. low thermal emittance • Experience in other labs shows good performance • delivered 32 mA in Boeing NCRF gun • delivered >22 mA in DC gun at U Cornell, proved to be more robust than GaAs against RF trips in booster module • Need to establish comprehensive R&D programm • undestand correlation between preparation and beam parameters • determine reliable means and procedures to operate this type of NC cathode inside SRF cavity Ultra Bright Electron Sources Workshop | Cockroft Institute | 29.06 - 01.07.2011

  7. What are the challenges and current limits for SRF guns with respect to our target beam parameters? • HZDR/ELBE gun shows feasiblity of the SRF gun concept  SRF cavity with normal-conducting cathode works! • For our application, we need different performance: • Beam dynamics: How can we have more control on the transverse and longitudinal beam parameters? Mainly determined by field on cathode and setup of any focusing elements. • Average current: How can we generate 100 mA? Need cathode with high QE at VIS, which can operate in SRF environment. • Average power: How can we couple Pavg = 100 mA x Eb power into the SRF cavity? Ultra Bright Electron Sources Workshop | Cockroft Institute | 29.06 - 01.07.2011

  8. Put these three questions into three stages Gun0 is a beam demonstrator experiment with SC Pb cathode (2011), study beam dynamics, cavity performance For Gun1 add NC cathode with high QE at VIS, study cathode lifetime, slice/projected emittance performance (2013) For Gun2 add RF input power coupler for 200 kW (2014), study high power operation Ultra Bright Electron Sources Workshop | Cockroft Institute | 29.06 - 01.07.2011

  9. Gun0 takes advantage of the work done by Jacek Sekutowicz & Co for the hybrid Nb/Pb gun cavity • Utilitzes a thin Pb film on the backwall of cavity as photo-electron emitter • Pb is also a superconductor of type I with Hcrit = 8 mT at 1.3 GHz and 2K, and has QE at least one order of magnitude higher than bare Nb backwall • Discussed first during Source workshop in Sep 2008, sketched during ERL09 at Cornell and then agreed on in June 2009. • Jacek and HZB elevate cavity design from sketch to engineering design • Peter Kneisel builds and tests a cavity ready for beam tests at HoBiCaT • Robert Nietubyc coats backwall of cavity with Pb film. • HZB prepares HoBiCaT for beam tests J. Smedley et al., PRST-AB 11, 013502 (2008) Ultra Bright Electron Sources Workshop | Cockroft Institute | 29.06 - 01.07.2011

  10. Beam dynamics simulation of gun performance: UV laser on Pb cathode, thermal emittance sqrt(2) higher than VIS on CsK2Sb (MV/m) ASTRA simulation of energy and emittance versus injection phase. Electron bunch has 10 pC charge in 2.5 ps FWHM pulse length. Similar peak current as for Gun1/2. Learn about slice beam dynamics Ultra Bright Electron Sources Workshop | Cockroft Institute | 29.06 - 01.07.2011

  11. Turning the physics design of the SRF cavity into engineering design compatible with operation in HoBiCaT Need to add stiffening to back-plane as Helium pressure fluctuations cause frequency shift of fundamental mode Target for em design was to optimze electric field on cathode (All values are given in Hz/mbar) With Ib < 2 µA and Eb < 3.8 MeV can run with 700 W forward power from solid state amp. A. Neumann et al, Proc. of LINAC 2010 Ultra Bright Electron Sources Workshop | Cockroft Institute | 29.06 - 01.07.2011

  12. The cavity collected some 20.000 air miles… keeping its Q vs Epk performance… before EM Design at DESY in Hamburg Cathode depo at Soltan INS at Swierk after Test of cold mass and beam operation at HZB in Berlin Production, RF-tests, Cleaning at JLAB in Newport News Ultra Bright Electron Sources Workshop | Cockroft Institute | 29.06 - 01.07.2011

  13. Results from vertical tests at JLAB, from assembly of cavity to delivery for Berlin 02/10 Initial test after assembly, tuning, BCP etching and rinsing of the cavity. The field flatness was only 66%. 02/10 Further tuning improved field flatness to 94%, the following BCP treatment improved the RF performance. 03/10 After installation of the helium vessel, limitation by moderate field emission. 07/10 With first cathode coating 07/10 Test after accidential loss of lead cathode and removal of remnants by grinding and BCP. 10/10 With second cathode coating Ultra Bright Electron Sources Workshop | Cockroft Institute | 29.06 - 01.07.2011 P. Kneisel et al, Proc. of PAC 2010

  14. Installation of cold mass into HoBiCaTSRF cavity, solenoid and steerer package Solenoid sits close to cavity. Both cavity and solenoid are shielded. No performance degradation with respect to Q of cavity found, OK. Solenoid gets warm if driven > 5 A due to heat transfer from current leads. Improved cooling of those leads during last week. Check performance of this measure. HZB Accelerator Physics Seminar

  15. Drive laser and diagnostics beamline for Gun0 Robust and reliable beam diagnostics to study cavity and cathode performance Short pulses of 1 ps rms length  less than 1 degL  slice equals projected beam dynamics Cathode jobs: measure QE, QE map, emission surface, thermal emittance ( master thesis) before and after laser cleaning Cavity jobs: study microwave properties, Q vs E, LLRF, microphonics, dark current 0.26 µm, 30 kHz, 2…3 ps, 0.15 µJ pulses from Yb:YAG oscillator + regen. amp. + 4th harm conv. (m) Ultra Bright Electron Sources Workshop | Cockroft Institute | 29.06 - 01.07.2011

  16. The drive laser: cw with short pulses in UV to do beam diagnostics experiments with the Pb/Nb system • Requirements from Gun0 operation • UV light for metallic cathode / existing cavity design • short pulses (2 ps) • easy and straight forward design of laser • avg. power high enough to do beam diagnostic (QE !) • Specifications: • Wavelength: 260 nm (UV) • Pulse length: 2-3 ps • Pulse Energy: 0,15 µJ • Reprate: 120 Hz – 30 kHz • Synchronized to 1.3 GHz • Good for average currents of 0.1 µA or bunch charges up to 20 pC SESAM modelocked oscillator Regenerative amplifier Ultra Bright Electron Sources Workshop | Cockroft Institute | 29.06 - 01.07.2011

  17. Q vs Epk and dark current measurements at HoBiCaT • Qo slightly lower compared to last measurement at JLAB • So far we set safety limit to 15 MV/m • Dark current kicks in above 10 MV/m, increases exponentially  field emission from cathode spot? • SEM/EDX reveal Pb droplets and flawed Pb film on Nb surface • Energy spectrum of dark current suggests emission around 90 deg of RF R. Barday, T. Kamps, A. Neumann, S. Schubert et al Proc. of DIPAC 2011 Ultra Bright Electron Sources Workshop | Cockroft Institute | 29.06 - 01.07.2011

  18. First beam of photoelectrons from Pb cathode generated and accelerated at 21st Aptil 2011, < 2 years after project approval Switch on RF, adjust phase, steer and focus with solenoid Measure QE of 1*10^(-5) at 260 nm Next lasercleaning of cathode Start with steering of laser beam spot on backwall of cavity First beam 21st April 2011 T. Kamps et al, Proc. of IPAC 2011 (planned) Ultra Bright Electron Sources Workshop | Cockroft Institute | 29.06 - 01.07.2011

  19. In parallel with commissioning of Gun0: Start activities for Gun1/2 • Gun0 was more akin to a task force project. • Two years from project approval by lab director to first beam. • Gun1/2 will be embedded inside BERLinPro project structure. • Main activities in the next two years (for first beam with Gun 1 in 2013) • Setup of preparation and analysis lab for • Beam dynamics simulation to fix parameters • Development of new gun cavity • Design and engineering of cathode insert, cold mass and cryomodule • Get new drive laser • Change diagnostics beamline setup • Put all this together in GunLab  gun test area inside HoBiCaT bunker • For Gun2 start thinking about RF feeds for high average power Ultra Bright Electron Sources Workshop | Cockroft Institute | 29.06 - 01.07.2011

  20. Use surface and material science techniques to study growth of CsK2Sb photocathodes The use of surface science already showed high potential to improve the injector performance Bessy II: XPS: determination of growth behaviour and stoichiometry UPS: determination of valence band maximum ARPES: angular distribution of emitted electrons and depending on the film appearence maybe band structure Bessy II infrastructure: PEEM: determination of areas of high and low work function IPES: determination of conduction band minimum White light interferrometry Ion source mounted on UHV chamber: LEIS: determination of the formation of 1 monolayer, the deposition rate and composition of topmost layer Other: SEM/EDX: surface topography and composition S. Lederer et al, Proc. of PAC99 Ultra Bright Electron Sources Workshop | Cockroft Institute | 29.06 - 01.07.2011

  21. Setup preparation system and DC setup to chase photoelectron properties from growth to use Mass spectrometer Transfer and transportation U=60 kV Photocathode X-ray tube QE Mapper Thermal emittance Evaporation sources Ion gun Response time Ion and electron Energy analyser Ultra Bright Electron Sources Workshop | Cockroft Institute | 29.06 - 01.07.2011

  22. beam diagnostics for gunlab build and comission Gun2 build and commission Gun0 2009 2010 2011 2012 2013 build and comission Gun1 high QE cathode new cavity, cold mass and cryomodule Ultra Bright Electron Sources Workshop | Cockroft Institute | 29.06 - 01.07.2011

  23. work done by T. Kamps, W. Anders, R. Barday, D. Böhlick, A. Frahm, M. Dirsat, F. Hoffmann A. Jankowiak, S. Klauke, J. Knobloch, O. Kugeler, J. Rudolph, A. Matveenko, A. Neumann, T. Quast, M. Schenk, M. Schuster, S. Schubert , P. Kneisel (JLAB), R. Nietubyc (Soltan), J. Sekutowicz (DESY), J. Smedley (BNL), J. Teichert (FZD), V. Volkov (BINP), I. Will (MBI) supported by the Accelerator Physics and SRF institutes and Experimental Support groups at HZB. Ultra Bright Electron Sources Workshop | Cockroft Institute | 29.06 - 01.07.2011

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