LCLS-II Injector layout design and study Feng Zhou 8/19/2015

CDR: No space is available for BPM2 & valve2 (short of 50 cm) 2014 layout: added 50 cm between SOL2 and CAV1 for missing components with shortened CM endcap May2015 baseline layout: added extra 21 cm with standard CM endcap May2015 baseline layout 3.505m SOL2SOL1Buncher CAV1 Feng Zhou, 8/19/15

Major concerns for May2015 layout Emittance increases up to 0.65um (>desired 0.6um) for 300 pC due to the increased drift between SOL2 and CAV1 Small half physical apertures – 2.5  x at SOL1/BPM1: beam starts to be lost at SOL1 when the laser beam is transversely steered 1mm off- center on cathode – ( )  x at laser injection and ICT Can we reduce emittance and also increase physical apertures?  new layout study Feng Zhou, 8/19/15

New proposed layout (July2015) Buncher SOL2 SOL1 BPM1BPM2 CAV1 Feng Zhou, 8/19/15 Laser injection ICT YAG Valve1 valve2

Changes for the new layout Modified a SLAC BPM: – Shorten length from 20cm to 15cm  23um BPM resolution – Increase physical aperture from 30mm to 57mm Designed a new solenoid (LBNL) – Field quality is similar to Cornell solenoid  improve emittance – Reduce length from 29cm to 14cm  shortened distance between SOL2 and CAV1  improve emittance – Shifted SOL1 to cathode direction with 5cm  improve emittance – Increase physical aperture from 30mm to 47mm; Small z-shifts of buncher/SOL2 for mechanical installations Feng Zhou, 8/19/15

New solenoid designed by LBNL New solenoid field map and quality factor are similar to Cornell solenoid, ~100/m2 Considering space charge, new solenoid is slightly better than old in term of emittance (see Chad talk) Feng Zhou, 8/19/15

Comparisons Proj.  x (um) (100%)  z (mm) HO dE (keV) h/  SOL1 h/  SOL2 Baseline-300pC(May2015) (30A) New-300pC(July2015) (30A) New-100pC(July2015)0.271(12A) All simulations for layouts use following laser/cathode conditions for fair comparisons. Trapezoid temporal laser: 2ps rise/fall time and flattop spatial uniform laser; emittance can be further reduced if using truncated Gaussian 1um/mm of thermal emittance; all emittance for 100% particles; Feng Zhou, 8/19/15

Sensitivity of the major component positions for 300pC emittance SOL1BuncherSOL2Cav1  x (um) m0.895 m1.651 m3.355 m cm cm cm cm-5cm cm -5cm cm increment Feng Zhou, 8/19/15

Physical apertures All half physical apertures >4.5  x instead of previous 2.5  x except: – 3.2  x at laser injection – 3.8  x at ICT Replace laser injection 2cm pipe: – with 3cm ID  4.3  x & 88.5  injection – with 4cm ID  5.7  x & 88.3  injection (LBL engineer is working on it) Can choose larger aperture for commercial ICT (LBL engineer is investigating) Feng Zhou, 8/19/15

Longitudinal phase spaces 300 pC 100 pC Feng Zhou, 8/19/15

Time-sliced emittances with 100% particles 300 pC100 pC Have been close to thermal emittance 0.2um and 0.32um for 100pC and 300pC respectively Feng Zhou, 8/19/15

Matching concern Beam from CM needs to be matched to emittance station/laser heater through two doublets Long drift from the last cavity to the matching quads requires small divergence to reduce chromatic emittance: – Achieving small divergence may compromise emittance (~5%); – If  =20m,  =-2.5  0.6% emittance growth at matching quad (Mark Woodley) So far twiss at entrance of the CM quad –  =12m,  =-1.86 (300 pC) –  =15m,  =-2.1 (100 pC) Feng Zhou, 8/19/15  <0.4% emittance growth requirement

Possible addition to the new layout ~20cm z-space enough for a low energy spectrometer: Minimum energy spread (gun phase) Gun amplitude calibrations Feng Zhou, 8/19/15

CM Cavity gradients Final energy: 95-96MeV  may eat extra laser heater power to get desired energy modulation due to off-resonant beam energy: – Cavity1: ~8MV/m – Cavity2 & 3: powered off – Cavity 4-8: ~16.2MV/m (i.e., 32MV/m peak field) – No cavity spares  a potential risk for operations We need to know availability data for CM operation Alternate layout can power all cavities without compromising emittance (ie, have 1-2 cavities spares) Feng Zhou, 8/19/15

Alternate layout All 8 cavities can be powered on without impacting emittance 1-2 cavities as spares Emittance station and energy spectrometer to measure 6d phase spaces Emittance improved 5-10% Capture cavity Standard CM Feng Zhou, 8/19/15 emittance station Energy spectrometer

Novel RF couplers correction 8-cavity CM couplers located at low energy <1 MeV increases emittance significantly for large beam size of 3-4 mm rms: A skew quadrupole can cancel the coupling terms (Dowell): Simulations Feng Zhou, 8/19/15

Summary New layout has a few advantages over baseline layout – Reduced slice emittance <0.25um (<0.4um) for 100pC (300pC)  close to thermal emittance – Increased half physical apertures >4.5  x – 15-20cm space is available to install an essential energy spectrometer for 750 keV – So far no disadvantages New layout is well mechanically modeled – ready for baseline replacement Alternate layout is even better than the new layout but needs $$$ RF couplers can be completely corrected with a weak skew quadrupole Feng Zhou, 8/19/15