Beam Stay-Clear (BSC) Apertures in LCLS-II June 24, 2015 P. Emma Take up work Jim Welch started (LCLSII-TN-14-15, Jan. 23, 2015) Goal is to define stay-clear everywhere to avoid high-power beam (non-issue in LCLS-I) and to set energy apertures Injector and SC-linac not included here (APEX & CM-design) Try to keep BSC compatible with existing hardware (magnets, BPMs, etc), especially at dispersive locations Develop code for fast re-calculation of all BSC apertures with each MAD version (updated: 19JUN15) BSC PRD is out for review now (6/24/15, LCLSII-2.1-PR-0442)

BSC originally defined by Jim very reasonably as… Start with  16-   apertures Add energy apertures based on 300 pC (see Table 1) Include additional aperture requirements (steering, mismatch errors, RF cavity trip, beam chirp, etc) Compare BSC’s to apertures of existing components When necessary, re-evaluate BSC, or find/build new components to match apertures where necessary Defining the BSC

Betatron BSC (i.e., a monochromatic beam) …along the lines developed by Jim Use a conservative emittance of  n = 1  m. BSC apertures are  16-times the rms beam size,  . Allow a worst-case beta-mismatch, f   (  = f    ). Add steering clearance:  x =  2 mm (  1 mm in FEL). Betatron BSC full diameter, D , for an accelerator component is: Example: f   2,    50 m,  n  1  m,  mc   100 MeV,  x  4 mm  BSC full-width is 27 mm (minimum here). 

X Y ID X Y Two geometries to consider… Cylindrical or Rectangular BSC If aperture is cylindrical, such as quadrupole magnets, then the ID is: If rectangular, such as dipole magnets, then BSC is rectangle based on X and Y values.

Full Energy Widths at 300 pC At laser heater (100 MeV) At BC1 exit (250 MeV) At BC2 exit (1.6 GeV) At dog-leg-1 (4.0 GeV) At HXR or SXR und. entrance (4.0 GeV) 0.6% 12% 3.6% 1.6% 1.0%

Energy Apertures Start with the full “core energy width” (prev. slide) for each area Allow a possible energy chirp over e  bunch (beyond nominal) Add a possible energy-vernier scan capability (only after linac) Add a max. FEL energy loss in main dump line (after undulators) Add energy jitter (  4-times the expected rms) for each location Add a possible single RF cavity trip (but not in L0 or L1 linacs) Add a worst-case dispersion error of |  |/|  0 | = 1.25 Defined using the following recipe:

Energy Aperture Budget

Overall Beam Stay-Clear The two BSC components (“betatron” and “energy” apertures) are added linearly (more conservative). A minimum BSC is set for each area (Table 2), including the two S-band RF deflectors in the off-axis diagnostic line. Also must include adequate apertures for X-band RF deflectors in main dump line (see next slide)…

RF Deflector Beam Stay-Clear in Dump Line X-band (11.4 GHz) RF deflector installed after each FEL for time-resolved e  and photon diagnostics Deflector streaks and kicks the beam horizontally Requires large hor. apertures, especially near main dump FEL RF deflector Main Dump screen Stay-clear

Beam Stay-Clear Results – Linac SC-linac and bunch compressors BC2 BC1 LH L3-end

Beam Stay-Clear Results – Diag. Line Off-axis diagnostic line (DIAG0) TCAVX TCAVY Kicker

Beam Stay-Clear Results – Bypass-SXR Linac extension through to SXR dump Spreader Dog-leg-1Dog-leg-2 TCX01

Beam Stay-Clear Results – Bypass-HXR Linac extension through to HXR dump spreader Dog-leg-1 TCX01

Beam Stay-Clear Results – BSY Dump Kicker to BSY dump