The CMS Tracker Laser Alignment System

Transcription

1 The CMS Tracker Laser Alignment System B.Wittmer, I. Physikalishes Institut RWTH Aachen on behalf of the CMS Tracker collaboration Vertex 9 Workshop, Putten (NL) 1

2 Outline Working Principle and Layout Working Principle Layout and Components Operation Commissioning Time Alignment Intensity Scans Tracker Alignment Alignment Parameters and reconstruction Expected alignment precision from MC studies First Measurements Summary and Outlook 2

3 Purpose of Tracker Laser Alignment System Deformations of Tracker could be possible due to temperature, humidity or B-field Monitor alignment of mechanical structure Align endcap discs with respect to each other Align Inner/Outer Barrel and Endcaps with respect to each other Monitor changes in alignment < 20 µm Precision of absolute alignment < 100 µm Ensures stable pattern recognition 3

4 Working Principle and Layout 4

5 Working Principle Use the Tracker micro sensors to detect the beams Send several laser beams through the Tracker From changes of the measured laser spots infer movements of the modules Light absorption in Si : 10 cm-1 at λ = 1070 nm and T = K Variant 1 (endcap alignment): Beam passes through several layers of sensors (Silicon is semitransparent to infrared light) Variant 2 (subdetector alignment): Beam is split and sent to several sensors (Mirrors have to be mounted on a rigid structure) 5

6 Overview 2 x 8 beams TEC - 2 x 8 beams TEC 1 x 8 beams Alignment tubes Outer Barrel (TOB) Endcaps (TEC) Beam Splitter Inner Barrel (TIB) Inner Discs (TID) Optical Fibre 32 laser beams in each Endcap, 8 laser beams for Barrel/Endcap alignment 434 Modules out of are hit by laser beams 6

7 Endcap Alignment Sensors Silicon sensors polished on both sides Hole in backplane metallization Antireflective coating on backside No antireflective coating on s due to effects on inter capacitance AR coating transmission Tranmission reflection improves Reduced multiple reflections -> Reduced coated uncoated interference -> reduced Sensor Backplane distortion of profiles 7

8 Beam Splitters Single mode fibre FC connector The beam splitters generate a pair of back-to-back beams: Collimator lens Beamsplitter Cube λ/4 Plate Mirror misaligned fibres lead to a kink between the beams All beam splitters have been calibrated in the lab for collinearity with the final fibre connected. This data is used to correct the LAS measurements. 8

9 Internal Endcap Beams 2 x 8 beams in each Endcap (ring 4 and 6) Ring 4 Ring 6 orientation Disc 6 Ring 6 Ring 4 BeamSplitters Laser pulses orientation orientation 9

10 Alignment Tubes Beamsplitters mounted on one end of Al tube (reduce thermal gradients) 6 semitransparent mirrors per tube (3 for Inner Barrel, 3 for Outer Barrel) Glass plates with single-sided AR-coating (5% reflection per mirror) Collinearity of beamsplitters and complanarity of AT beams have been calibrated in lab This data is used to correct the LAS measurements 10

11 Alignment Tube Calibration Nominal: Beamsplitter beams collinear, Alignment Tube beams in one plane Measure deviations previous to installation Only angles that generate displacements in Φ are relevant Use this values later to correct data x z 11 y

12 Laser Pulse Generation Laser Sources in service cavern Trigger System (TTC) fires Lasers Front End Driver (FED) tags laser events PC Farm recognizes laser events and sends them to calibration stream Experimental Cavern UXC 5 CMS Tracker Service Cavern USC 5 FED Laser Room PC Farm TTC 12

13 Laser Pulse Generation Trigger System (TTC) 'Test Enable' Trigger Board OR Laser Pulser Laser Pulser Laser LaserPulser Board CU LDD CU LDD CU LDD LDD CU LDD LM VME 'Test Enable' signal emitted at constant 100 Hz 'Test Enable' signal fires the Lasers (synchronized with bunch crossings) Laser Boards stop firing after a fixed number of triggers (0-3) Tracker Modules 100 m 8 Laser 1070 nm TTC Test Enable + Trigger 13 FED & DAQ

14 Modes of Operation Dedicated Alignment Runs Laser Alignment during Run with local or global Physics Runs DAQ 'Test Enable' signal arrives in orbit gap (3µs) No zero suppression Zero suppressed mode All tracker parameters optimized for LAS Tracker parameters optimized for particles 3 µs orbit gap 14

15 Laser Alignment Output Data Quality Monitoring Quality of LAS data Quick feedback on Tracker stability Generation of Tracker Geometry correction LAS standalone alignment Can be applied online on top of existing alignment Input to combined Alignment Combine Laser Alignment Data with track-based alignment, stability, weak modes 15

16 Commissioning 16

17 Time Alignment (coarse) Trigger Board Delay t2 APV Latency t3 'Test Enable'/Trigger Latency t1 Test Enable TTC fibres t1 Trigger Board t2 Laser Board 100m fibre Latency Sensor t3 APV Trigger t (3µs orbit gap) First time alignment on bunch crossing level (25 ns) t1: const CMS, t3: const Tracker, t2: LAS coarse timing 17

18 Time Alignment (fine) Fine delay is measured on 3 levels: Laser Board / Fibre Bundle Delay Laser Board Channel / Fibre Delay TOF of photons Laser Boards Beam Splitters Fibre bundle Tracker Modules 18

19 Reaching different TEC Layers Disc 1 Disc 2 Disc 3 Disc 4 Disc 5 Low Low OK OK Good Disc 1 Disc 2 Disc 3 Disc 4 Disc 5 Good OK High High High Low intensity: Disc 1,2 no signal Disc 5 good High Intensity: Disc 1 good Disc 2-5 satureted The laser intensity is adapted event by event to reach different layers for optimal signal quality Max. 5 layers => 5 different intensities 19

20 Intensity scans Intensity scans determine proper settings for each layer Example TECRing 6, Beam 6 Saturated signals Good signals Laser Threshold 20

21 y y Ring_6_Disc_ Ring_6_Disc_7 y Ring_4_Disc_ Ring_4_Disc_ Ring_4_Disc_8 y 600 y 800 y Ring_6_Disc_ Ring_6_Disc_ Ring_6_Disc_ Ring_6_Disc_ Ring_6_Disc_ Ring_6_Disc_ y y Ring_4_Disc_ Ring 4 Ring_4_Disc_1 Intensity adapted for each disc (20% transmission per layer) Events per intensity to increase S/N Baseline correction was performed 50 Ring 6 Disc 1 Disc Ring_4_Disc_ Ring_4_Disc_ Ring_4_Disc_ y y y y y y y y y y Ring_4_Disc_ beam splitter beam splitter Ring_6_Disc_9 Endcap Beam Profiles

22 Commissioned profiles Endcap profiles (TEC- Ring 4) Good signal on almost all modules 6 Endcap modules that were not read out are missing 5 Endcap modules (TEC+) show profiles with bad shapes All measured profiles are within the dynamic range 22

23 Required Triggers 5 different intensities for Endcap layers Alternate operation of AT/TEC beams due to beam overlap events per module to reduce noise (very conservative choice) 5 x 2 x = events This is called a 'snapshot' and allows to compute the alignment constants 100 Hz 'Test Enable' Trigger rate 'Snapshots' will be taken regularly to monitor Tracker alignment (every 10 min.) 23

24 Non-Gaussian Profiles Ring_4_Disc_5 Distortion of Beam Profiles due to Non-perfect beam optics (depends on distance to beam splitter) -> asymmetric non-gaussian profiles Baseline distortion due to large charge Laser beam profile -> asymmetric profiles next to the beam splitter Diffraction at micros (depends on amount of layers passed and pitch) -> side maxima Interference inside silicon bulk due to multiple reflections -> asymmetric non-gaussian profiles Laser beam profile after 4 layers of sensors Limits position measuring accuracy to µm ADC counts ADC counts Ring_4_Disc_

25 Tracker Alignment 25

26 Internal Endcap Alignment 2 x 8 Beams (index i, position θ, radius R0) 9 Discs (index k, position z) Consider translations in x and y and rotations φ (around z) Beams are straight, but can move (2 visible dof: θai, θbi) ymi,k y x Positions of laser spots as function of alignment parameters: Collective movements of discs cannot be distinguished from collective movements of beams! 26

27 TEC Alignment Parameters φk: Individual disc rotation around z-axis. xk: Individual x-displacement of each disc. yk: Individual y-displacement of each disc. θai: Individual beam displacement in φ on disc 1. θbi: Individual beam displacement in φ between disc 1 and disc 9. φ0: Rotation of all discs with respect to the beams. x0: Displacement in x of all discs with respect to the beams. y0: Displacement in y of all discs with respect to the beams. φt: Collective torsion of discs with respect to the beams. xt: Collective shearing in x-direction of discs with respect to the beams. yt: Collective shearing in y-direction of discs with respect to the beams. Only xk, yk, φk are used for geometry correction. Global parameters 0, t resolved with alignment tube beams 27

28 Analytical Solutions for Endcaps Example (1 ring, 4-fold symm.): Yik: measured displacement for beam i on disc k Zk: disc position L: length of endcap R0: radius of beams M: number of discs N: number of beams Minimize squares of residuals Beams stay straight but can move 144 measurements, 43 degrees of freedom Generalized solutions for 2 rings and masked beams or masked modules exist 28

29 Barrel / Endcap Alignment 8 Alignment tubes 2 x 6 spots per tube on Inner and Outer Barrel modules 5 layers per tube in each Endcap at ring 4 Beams straight but can move (2 DoF) Endfaces of each subdetector have 3 DoF (+/- parts treated separatly) 176 measurements and 52 degrees of freedom TOB TEC TIB z Global tracker movements are resolved by muon link system 29

30 Alignment MC studies Detection precision varied from 0 to 100 µm (gaussian smearing) For absolute measurements expect µm Dominated by assembly precision For relative measurements expect µm Dominated by non-gaussian profile shapes Simulate Rotations +-1 mrad, Translations +-1 mm Compare simulated and reconstructed parameters 30

31 MC reconstruction Endcap Ring 6 Zero by definition due to global parameter separation Assembly precision LAS precision 31

32 MC reconstruction Alignment Tubes Assembly precision LAS precision 32

33 First Results 33

34 Alignment parameters TEC+ Metrology survey Laser Alignment during integration CRAFT 8 alignment with tracks Laser Alignment CRAFT 8 LAS Residuals 65 µm RMS => 80 µm assembly precision (assuming χ2/ndf = 1) 34

35 Endcap Alignment from CRAFT 8 Residuals [µm] TEC+ TEC85 Ring Ring 6 64 => estimated assembly precision µm 35

36 Barrel Alignment from CRAFT 8 Outer Barrel reference frame 36 Residuals RMS: µm Results in laser beam frame Outer Barrel (TOB) well aligned Rotation of Inner Barrel (TIB-) Inner Barrels (TIB +-) are vertically inclined

37 Comparison CRAFT 8/9 PRELIMINARY TEC+ RESULTS CRAFT 8 B = 0T Peak mode CRAFT 9 B = 3.8T Deconvolution Alignment parameters reproduced within 18 µrad / 15 µm Consistent with measurement precision 30 µm and no movement 37

38 Summary and Outlook The CMS Tracker Laser Alignment System monitors the stability of the Tracker mechanical structure The system is fully integrated and commissioned First measurements confirm absolute measurement precision < 100 µm and monitoring precision < 20 µm TEC+ endcap best studied and understood, extend studies to TEC- and Barrel Detectors 38

39 Backup 39

40 Polarization in Alignment Tubes Reflection in beam splitter and AT mirrors depends on polarization Polarization mismatch generates 'ghosts' Cured with λ/2 plate 40

41 Beam Ovelap φ-position of laser beams 3 AT beams overlap with TEC beams They have to be operated alternately TEC beams AT beams 41

42 Reducing Noise Summing up events improves S/N Limitation in zero suppressed mode Extreme example (S/N < 1) : single event (S/N<1) 500 events averaged 42 effective S/N

43 Profiles for Endcap (Ring 4) Diffraction Patterns from Strips 43

44 Laser Profiles for Inner Barrel 44

45 Endcap Alignment Coordinates Separate collective movements from individual disc movements Overall Endcap movement x0 Overall Endcap skew xt k = 1..9 z 45