Experience with Insertion Device Photon Beam Position Monitors at the APS 27.6 meters (The APS has forty sectors - 1104 meters total circumference)
Beam Position Monitors and Magnets in One Sector 18m X2 20m X2 11m X1 16m X1 Q FC BM x x x BM ID C C C C C C x C e - : Broad-band RF Beam Position Monitors (7) (Turn-by-Turn) x : Narrow-band RF Beam Position Monitors (4) (~ 300 Hz) : BM X-ray Beam Position Monitors (2 - Vertical Only) (~165 Hz) : ID X-ray Beam Position Monitors (2) (~165 Hz) FC : Fast Corrector Magnet (1) (~ 1000 Hz) C : Slow Corrector Magnets (7) (few Hz) Q : Quadrupole Magnets Bending Magnet and BPM Arrangement RFBPMs BM XBPMs #1 #2-0.5 0 2.8 11 18 (meters) (Source) Insertion Device and BPM Layout RFBPMs ID s XBPMs # 1 # 2-2.5 0 2.5 16.3 20 (meters) (Source)
Insertion Device Photon Beam Position Monitor Blade Geometry Stray Radiation Sources X-ray BPMs (16, 20 meters from source) Insertion Device ID photons Main Dipole Bend Magnet 5 meters 78 mrad Legend Positron Trajectory Focusing Quadrupole Magnet Defocusing Quadrupole Magnet Sextupole Magnet Combined Function Horz./ Vert. Steering Corrector Magnet
Re-direction of Stray Photons by Girder Alignment* Stray radiation from upstream dipole, quadrupoles, sextupoles and correctors ID photons 77 mrad 1 mrad * Phys. Rev. ST Accel. Beams 2, 112801 (1999) Stray radiation from downstream dipole, quadrupoles, sextupoles and correctors Upstream X-ray BPM Downstream X-ray BPM P1 Blade Signal Sum Unmodified Sector 1ID Modified Sector P2 Blade Signal Sum Insertion device gap Insertion device gap
Raw Corrector Settings -136 Amps = -1 mrad Geometries of Canted Undulators and decker Distortions -78 mrad ID Undistorted Distorted -1 mrad -1-77 mrad (= -78+1) -1 Canted, configuration 1-1 +0.5 +0.5-1 -77 0.67 mm -1-0.5-0.5 0 0 Canted, configuration 2-77 -77 0.33 mm -1 0 0-0.5-0.5 Canted, configuration 3
BPM Offset vs. Gap Lookup Tables (Vertical) Not distorted 200 m Not distorted BPM Offset vs. Gap Lookup Tables (Horizontal) Not distorted 400 m ID 20 Not distorted
Correction of Residual ID Photon BPM Gap-dependent Systematic Errors Background Subtraction Only Background + Exponent Corrections x (Absolute) x (Absolute) 100 microns ID 20 Gap (mm) ID 20 Gap (mm) x (Relative) x (Relative) approx. 40 microns ID 20 Gap (mm) ID 20 Gap (mm) Long Term Drift of BM and ID Photon BPM Readbacks BM ( m) Detectors 11 m from source ID ( m) Detectors 16.3 m from source
One-Week Angular Drift rad Local Steering Local Tunnel Air Temperature Impacts Pointing Stability Air Temp (deg. F) 8ID V. Angle ( rad) 8ID H. Angle ( rad)
Conceptual Design of Hard X-ray Beam Position Monitor (Top View) Cooled Cu X-ray Fluorescent Source Be. Filter Rectangular Beamline Photo-resistive Limiting Aperture Diamond Detectors (Approx. 3 x 2 mm) ~30 meters from source Side View
Summary An extensive accelerator re-alignment is near completion after a 6-year effort, resulting in reduced insertion device photon bpm stray radiation background signals. Correction of residual gap-dependent systematic errors is presently performed using lookup tables. Careful alignment, background subtraction, and algorithm refinement should further reduce systematic errors to the +- 10 to 20 micron level (~0.5-1 rad). (but depend critically on assumptions / constraints) Development of a gold standard hard x-ray bpm located 30 meters from the source should allow achieving +- 100 nrad-scale long-term pointing stability (perhaps the only way).