Cu,Mg Photoinjectors & LAL projects

Laboratoire de l’Accélérateur Linéaire ~300 people – activity : particule Physics&detectors ~ 30 in DEPACC Local plateform  PHIL : R&D photoinjectors + users  ThomX : Compton X ray source : 2017 ! Futur projects :  PRAE (300 MeV), ESCULAP (laser/plasma)

PHIL PHoto-Injector at LAL :R&D + beam for users Rf gun tests Cathodes : Cu, Mg, Transfer arm Cs 2 Te Test stand for other projects (ThomX) Student formation

PHIL today ICT1 ICT2 YAG1YAG2 YAG3 Ti window cerenkov YAG4 Bending magnet Focusing magnet Photo cathode Transfer arm (CERN/CTF collaboration) 2,5 cell RF gun Beam caracteristics today 10 pC < Q < 200 pC (Cu) with Mg Q ~ 1.4 nC 1.5 MeV < E < 5 MeVdE/E = 0.2% for 100pC 3 MeV F = 5 Hz Emittance (rms) 5 to 10 mm.mrad pulse duration 10ps

PHIN RF Gun Cathode change possible Canon 2.5 cells F = MHz Rf coupling on the side Change gun in feb 2012 Ez max reach = 90 MV/m  Same as CERN/CTF

Cu cathode tests 22/07/14 : few arcing < 10. conditioning last ~ 4h diamond polished cathode Ra~50 nm HT max = 12,6 kV  Ez max = 90 MV/m Cu

Charge-phase Laser : sx = 0,42 mm sy = 0,33 mm Elas max = 55 µJ (37 µJ on cathode) 24/07/14 Charge on ICT1 (0.25 nC/V) iB3 = 100 A Cu

Charge =f(Elaser,Ez) 24/07/14 Phase for max charge Extracting all the charge : no saturation Charge sur ICT1 (0.25 & 0,1 nC/V) iB3 = 100 A Cu

QE=f(Ez) Phase for max charge Cu

Dark current HT = 12,7 kV PA = 300 W iB3 = 100 A ict1 = 5,73 V (.10 nC/V)  570 pC ict1 = 2,26 C (.25 nC/V)  565 pC iB3 = 190 A iB5 = 30 A YAG2 iB3 = 180 A iB3 = 160 A iB3 = 150 A Cu

Mg photocathode tests Cathode Candidate for ThomX (Q ~ 1 nC) Tests with apha-x and PHIN Rf gun Mg : QE Mg ~ to : high charges > 1 nC Easy machining, polishing Life time ? Stability of emission ? Mg

Alpha-x Rf gun Dark charge (nC) Emitted charge (Cu) Emitted charge (Mg) QE ~ 4,5 x QE ~ 4,5 x Photocathode = backplate Cathode Mg n°1 : conditioning difficult : no polish was done : high dark charge Ez~60 MV/m Ez max reach = 90 MV/m P ~ 9 MW F = MHz Ref: NIMA 797 (2015) Ez~80 MV/m Mg1Cu (MV/m) (µJ) Charge (pC)

Mg n°2 Préparation : inclusion in Cu substrat Diameter visible = 5 mm Surface lathing Cu+Mg & diamond polish (OPA Opticad) tested on PHIL october 2013 Mg2 with PHIN RF Gun

Conditionning : ~ 3h30 Mg n°2 Few arcing ( < 10) Ezmax ~ 62 MV/m P ~ 5 x mbar after 1 week Mg2 RGA monitored : no Mg seen on spectrum (masses 0 to 50)

Cathode « cleaning » - Focused laser sweeping the cathode surface with RF ON Initial Position sx ~ sy (mm) Exposure time Position reading start vacuum Fixed miror gaussian distribution : sx ~ sy = 0,25 mm Elas ~ 30 µJ Surface energy density ~ 100 µJ/mm² (damage threshold ~ 300 µJ/mm²) RF = ON +/- 4sigma Movable miror Laser 266 nm cathode vacuum Mg2

Effect of « cleaning » Before after Cleaning = RF ON + focused laser + laser deplacement with last miror note : test with RF OFF : no improvement Elas = 30 µJ Ez ~ 60 MV/m Exposure ~ 2s Mg2

Mg n°2 emission/cleaning cleaning n°1 cleaning n°2 cleaning n°3 cleaning seems to recover max charge Charge goes down with time 1 month Beam production conditions are not constant Mg2

Ligne Transport actuelle Mg cathode n°2 * A diamond polished Mg photocathode has been mounted between October 2013 and July 2014 in the RF-Gun * Laser cleaning of the cathode surface increases by a factor 5 the QE (up to 0.13%) and by a factor 2 the maximal beam charge (up to 2.2nC). Cleaning effect disappears in roughly one week, but can easily be recovered by a new cleaning. Mg2

Surface images Cu Mg 100 µm Mg n°2 O-K Mg-K Photocathode-Mg-5kV(2)_pt Photocathode-Mg-5kV(2)_pt Photocathode-Mg-5kV(2)_pt Photocathode-Mg-5kV(2)_pt Photocathode-Mg-5kV(2)_pt Mg, MgO, Mg(OH), C? Mg2

Surface image Zone with laser impact Zone out of laser impact 5  m Mg n°1 ~ 300  m laser impact has modified the surface : fusion ? More C out of laser impact More Mg on laser impact -> Mg, MgO, C Mg1

Mg test conclusions Mg cathode easily prepared and monted High charge > 1 nC possible Charge decreasing with time : Mg pollution ? Vacuum quality Emission not stable with time --> run every day ! Life time : depends on admissible charge Cleaning recover QE Need cleaning 1/week => CsTe cathode better : collaboration with Cern/CTF3 for transfert arm + cathode fabrication

Cathode transfert for PHIL Photocathodes transfer Collaboration with CERN/CTF Transfer arm Cathodes holder

Cathode transfert for PHIL

Cathode transfert RF gun cathode

Photocathodes transfer arm -Transfert arm+cathode holder mounted -Transfert test done -Installation on PHIL -First Cs 2 Te test ? -Cathode holder identical for CERN RF gun position 4 Cathodes reservoir Transfer arm Cathodes holder Collaboration with CERN/CTF holder

Ligne Transport actuelle Transfert arm * A photocathodes transfer arm has been mounted in July 2014, in order to quickly change the cathode without breaking the RF-gun vacuum. The aim is to compare in the same conditions several cathode types (Cu, Mg, Cs 2 Te …) * A problem appeared during the extraction of the first cathode (it remained blocked), causing damages to the back of the RF-Gun (remachining of this part was necessary).  problem solved now

NEG activation 200°C – 24 h Pf = 1.5 x mbar Cathode Non poliePoli mécaniquePoli electrolytiquePercée UnpolishedDiamond Powder polishedElectrolytic polishDrilled 1234 Cs2Te - #196 QE = 14.2 % Plug Cu CERN 6A26 Cs2Te - #197 QE = 14.0 % 4 Cathode Holder Photocathodes transfer arm No tests of CsTe has been done yet on PHIL !

Ligne Transport actuelle Conclusions and prospects * Studies of alpha-x RF gun and CERN/CTF3 gun * Mg cathode tests : allows high QE (0.13%), high charge (> 2nC) for long time (more than 6 months) if laser cleaning done (recover QE) * First test of photocathodes (Cu) transfer arm didn’t work and damaged the back of the RF-Gun  solved now, other tests to be done * CsTe cathode are waiting to be tested * Futur projects (ThomX, PRAE) will help the developpement of PHIL

Tomorrow’s PHIL changes –Shorter laser pulses (100 fs) need new laser : Laserix installation + ESCULAP project –Add booster to reach 9 MeV RF studies done –4.5 cell RF gun for 9 MeV Reach 9MeV without booster (rf studies done) –Reduce charge to few electrons users demand (calibration detector study) LEETECH project –Upgrade to 10 Hz then 50 Hz decrease acquisition time and for PRAE project –Cathode preparation chamber –LAL own production for PHIL and ThomX ? –…

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2017? : PHIL improvements/4,6 cells RF gun (P.Chen) A compact way to reach 9MeV Parameters 4.6 cells RF gun Frequency /MHz Rsh/ Mohm/m 67 Q Pc/W Esurf/Ezmax MV/m 9MeV