Electron Source Design Dr Tim Noakes ASTeC, STFC Daresbury Laboratory

Overview Current position Required beam parameters Beamline considerations Outlook

History February ASTeC involvement commenced April Contributed to PPRP proposal June Became WP leader for the photoinjector October Failed to get funding under the PPRP grant November 2012 to January 2013– Became involved in ERC Synergy grant submission March 2013 – Contribute to AWAKE CDR

AWAKE Experiment Electron beam requirements Short pulse length, low repetition rate, modest bunch charge – Use normally conducting RF photoinjector with metal photocathode

Electron Beam Test Facility (EBTF) at Daresbury EBTF designed to provide ~ 5 MeV electron beams to two test areas for use in commercially important research (security, medical applications, etc) Currently undergoing RF commissioning

EBTF photoinjector based on 2.5 cell ALPHA-X S-band gun Donated by Strathclyde University!

AWAKE Injector Two and a half cell NC RF gun with booster to achieve MeV Ion Pump ? RF Gun Beam Direction Ion Pump Support Pedestal Synthetic Granite Girder Booster Linac 1m long YAG Quadrupole Magnet Slit Horizontal and Vertical Corrector Magnet YAG

AWAKE beam parameters Required properties can be inferred from plasma simulations Beam should have small enough size and angular divergence to fit into high capture efficiency region From Konstantin Lotov’s Lisbon talk

AWAKE beam parameters Beam broadening could also be important (window?) Spot size Divergence Spot size and divergence less than half the broadening in the window! Fast valve? Scott Mandry, Lisbon talk Window won’t stick or fail to synchronise!

Beam Blow Up in the Plasma Cell Reduced affect at higher energy? Horizontal beam size in the electron beamline (5 MeV electrons): From Konstantin Lotov’s CERN talk October 2012

Potential beam parameters Parameters continuously being refined by simulation work! ParameterValue Beam Energy 10 – 20 MeV Energy Spread (rms) < 1 % Bunch Length 0.3 – 5.0 ps Synchronisation (laser/RF) 0.1 ps Synchronisation to experiment 0.1 ps Free repetition Rate 10 Hz Synchronised Repetition Rate 0.03 Hz Spot Size (diameter FWHM) < 0.5 mm Angular Divergence < 3 mrad Normalised Emittance 0.5 mm mrad Bunch Charge1 – 250 pC

ASTRA simulations 1000 particles, 100 pC charge, 3 ps pulses, isotropic intrinsic emittance (0.9 mm mrad per mm of spot size) Parameters to optimise Gun phase, gun RF power Solenoid field Linac phase, linac RF power Linac Gun Solenoid

Results Simulations indicate suitable beam parameters can be obtained

Optimised parameters 10 MeV20 MeV Gun field70 MV/m Linac field7 MV/m20 MV/m Energy spread0.6%1.3% Bunch length3.0 ps Spot size0.5 mm0.2 mm Emittance0.4 mm mrad0.5 mm mrad Parameters for 3ps pulses Space charge limits allowable current to 100 pc

Space Charge Limitation Space Charge limit is determined by the minimum volume of the electron bunch Occurs at lowest velocity directly after the electrons have been ejected from the cathode Cathode Laser spot ½ a  t 2   E (keep gun field high as possible consistent with low dark current)   2 (use largest spot size consistent with high capture efficiency)   t 2 (maximise pulse length)

Pulse Length Pulse length should be small to sit in high field region of phase – limits the bunch charge! If a longer pulse can be tolerated more charge can be obtained What happens to ‘spare’ electrons? Do they affect the wakefield? 300 fs, 1 pC bunch charge 1ps, 10 pC bunch charge (3 pC in 300 fs section)

Increasing the Bunch Charge Bunch compression – requires suitable beam transport line! - Complicated and expensive!RF chopper

Photoinjector beamline Dog-leg beamline deisgned to have minimal effect on beam properties Simulations don’t include space charge Vitaly Yakimenko

Alternative design Triple (or quadruple) bend achromat may allow bunch compression –Energy chirp from booster required (longer section?) –Shorter bunch length for higher bunch charge –Simulations required! Additional focussing elements may also be required

Bunch Compression Chicane compressor reduces pulse length from 4 ps to 600 fs for IR FEL Triple bend achromat arcs at each end might achieve the same compression factor (~7) Could give rise to a factor of 50 increase in pulse charge? –Not straightforward (CSR, MBI, etc) –Need to model! ALICE Energy Recovery Linac Prototype

TargetJunction chamber / TCC4 Proton beam line TT41 Horn TSG41 TCV4 Storage gallery TSG40 TSG4 tap can be removed TSG41 tap can be moved inside TCV4 – TSG41 Layout in CNGS Area Dog-leg requires 8 to 9 m straight section! Plasma Cell 2 Dipoles 14 Quadrupoles RF gun and beamline Photoinjector laser RF modulator and klystron

TargetJunction chamber / TCC4 Proton beam line TT41 Horn TSG41 TCV4 Storage gallery TSG40 TSG4 tap can be removed TSG41 tap can be moved inside TCV4 – TSG41 Layout in CNGS Area Triple bend achromat needs 3.5m wide tunnel –Bunch compression? Plasma Cell 3 Dipoles Quadrupoles RF gun and beamline Photoinjector laser RF modulator and klystron

TargetJunction chamber / TCC4 Proton beam line TT41 Horn TSG41 TCV4 Storage gallery TSG40 TSG4 tap can be removed TSG41 tap can be moved inside TCV4 – TSG41 Layout in CNGS Area Quadruple bend achromat needs smaller shorter tunnel Existing tunnel? Compression possible Plasma Cell 4 Dipoles 20 Quadrupoles RF gun and beamline Photoinjector laser RF modulator and klystron

IR FEL at Friz-Haber-Institut Designed to deliver 15 – 50 MeV electrons with 200 pC charge in 1–5 ps and good longitudinal and transverse beam properties

Conclusions NC RF gun with metal photocathode capable of providing suitable beam For short pulses bunch charge is limited by space charge effects Bunch compression might allow up to 50 times more beam (suitable beamline required) 50 pC with 30% capture efficiency gives ~ 10 8 electrons –Is it enough?

Outlook Contribute to the AWAKE CDR – Draft already prepared! EBTF results will feed into photoinjector design Lack of current funding limits work between now and September If ERC Synergy grant successful resume simulation work Produce a final design and build!

Acknowledgements Julian McKenzie, Boris Militsyn, Yuri Savilev, Bruno Muratori, Steve Jamison, Alan Wheelhouse, Jim Clarke, Clive Hill, Steve Griffiths, Neil Bliss, Deepa Angal-Kalinin