1. Actuation and Alignment Challenges of LHC Collimators M. Garlaschè on behalf of the EN\MME collimation team 1 st PACMAN Workshop CERN, Geneva, Switzerland – 2÷4 February, 2015

2.  LHC collimators  Actuation system  Beam-relative alignment  Summary M. Garlaschè (CERN EN/MME) Outline

3. Primary Collimator Secondary halo p p e  Core Unavoidable losses Shower Beam propagation Primary halo (p) Secondary Collimator e  Shower p Tertiary halo The collimation system must satisfy 2 main functions: Multi-stage Beam Cleaning, i.e. removing stray particles which would induce quenches in SC magnets. Machine Protection, i.e. shielding the other machine components from the catastrophic consequences of beam orbit errors. The collimation system must satisfy 2 main functions: Multi-stage Beam Cleaning, i.e. removing stray particles which would induce quenches in SC magnets. Machine Protection, i.e. shielding the other machine components from the catastrophic consequences of beam orbit errors. Absorber C/C WW Super- conducting magnets SC magnets and particle physics exp. What is a LHC collimator M. Garlaschè (CERN EN/MME) Absorber Courtesy: R. Assmann – CERN

4. Collimators in the LHC 02.02.20154M. Garlasché (CERN EN/MME)  ~100 collimators in the LHC  Mostly positioned at IP3 and IP7  Also present before the experiments, to reduce signal background in the detectors

5. Collimator: General Layout 02.02.20155M. Garlasché (CERN EN/MME) Collimator assembly Overall length: 1480mm Tank width: 260mm Overall length: 1480mm Tank width: 260mm Quick Plug-in Adjustable stand Orientation Fine positioning Beam Loss Monitor (BLM)... High levels of radiation...

6. Collimator: Main Subsystems 02.02.20156M. Garlasché (CERN EN/MME) Actuation system Jaw Assembly (2x) Vacuum Tank Independent actuation of the 2 jaws!...The cleaning element...

7.  LHC collimators  Actuation system  Beam-based alignment  Summary M. Garlaschè (CERN EN/MME) Outline

8. Actuation system 02.02.20158M. Garlasché (CERN EN/MME) Frontal cross Section Motor Gap opening (LVDT) Gap position (LVDT) Resolver Resolver Vacuum tank + switches for IN, OUT, ANTI-COLLISION Left Jaw Movable table Vertical movement (5 th axis) +/- 10mm (1 motor, 2 switches, 1 LVDT) Courtesy R. Losito (EN/STI) Fixed table Beam Right Jaw

9. Actuation System: Functional Requirements Requirements Jaw stroke+30/-5 mm Jaws auto-retractionYes Motors per jaw2 Stepper Motor min. Pull-in Torque2.5 Nm Stepper motor max Detent Torque60 mNm Tolerance on expected actuator position± 25 μm Repeatability on actuator position± 10 μm Minimum required lifetime20000 cycles over 20 years Component radiation hardness (cumulated)10 MGy 02.02.20159M. Garlasché (CERN EN/MME)

10. Actuation System... a closer look... 02.02.201510M. Garlasché (CERN EN/MME) Fixed Table Stepper Motor Mobile Table Linear Bearing Spring Roller Screw Nut Roller Screw Shaft

11.  High resolution Stepper-motor:  400 (sub)steps/turn  Recirculation Roller Screw:  Stainless steel (AISI420) components with graphite coating of shaft plus radiation hard grease  High precision (<6  m on total screw travel)  Lead 2 mm/rev. From Circular to Linear Motion 02.02.201511M. Garlasché (CERN EN/MME)  5 μm jaw displacement per each motor step

12. Next Step: Ball Screws  End of 2013, an increase of demanded torque in some of the motors was detected  This was due to the evaporation of roller screw liquid lubricant, which caused an increased in friction  Regular maintenance impossible due to high radiation levels! Dry screws Dust and debris in the end cap and the housing 02.02.201512M. Garlasché (CERN EN/MME)  Alternative solution under testing: hybrid stainless steel/ceramic ball screws  Low friction & wear between the rolling elements, no need of lubrication  Radiation-hard! Courtesy O. Aberle (EN/STI)

13. Minimizing Play... Preloaded spring, in order to:  Recover screw mechanical gaps  Move the jaws away from the beam axis in case of short-circuit  The torque of each movable table is qualified with a dedicated test bench M. Garlaschè (CERN EN/MME)

14. Minimizing Play... Iperstatic motor-screw fitting:  Adds complexity to assembly procedure  Eventual decoupling would allow for play Preloaded crossed roller configuration All-metal, corrosion resistant, for use in non-lubricated conditions Linear Bearing M. Garlaschè (CERN EN/MME)

15. Position Sensors LVDT LVDT and Resolvers:  Radiation hardness  Lifetime: infinite since contactless (no mechanical stresses)  ‘Zeroing’ performed at CERN metrology during last assembly steps 02.02.201515M. Garlasché (CERN EN/MME)

16. BLM-based alignment  How to align collimator jaws once installed in the tunnel?  First method (indirect): use the signal coming from Beam Loss Monitors (BLMs) 02.02.201516M. Garlasché (CERN EN/MME) A jaw is aligned when the characteristic loss spike is seen in the Beam Loss Monitoring (BLM) detector signal. Courtesy G. Valentino (BE/ABP)  Two main limitations of the system:  Time consuming (tens of hours in the worst cases for manual alignment!)  Beam instabilities & other effects may impair cleanness of signals

17. BPM-based alignment 02.02.201517M. Garlasché (CERN EN/MME)  In order to speed-up the alignment procedure, new collimators with embedded Beam Position Monitors (BPMs) have been installed in the LHC and will be operational starting from the 2015 run  BPM alignment duration: few tens of seconds!  The device performance has been tested in the SPS with a mock-up BPM-equipped collimator jaw  BPMs provide a direct measurement of the beam orbit at the collimator locations

18. Summary 02.02.201518M. Garlasché (CERN EN/MME)  Collimators are key elements of the LHC, providing beam cleaning and protection to the machine most delicate components, such as the magnets  The collimator jaws actuation system has to guarantee precision, reliability and robustness in a harsh environment (radiation-induced degradation, corrosion, low or no accessibility/maintenance)  How do we obtain it? Linear displacement given by μm precise stepper motor + roller screws  How do we guarantee it? Preloaded elements and minimized play. Minimization of components degradation (corrosion, wear...)  How do we control it? In operation, it is of paramount importance to precisely determine the jaw position with respect to the particle beam. Two methods are adopted:  BLM-based alignment (indirect, time consuming)  BPM-base alignment (direct, rapid, adopted on last-generation collimators)

19. M. Garlaschè (CERN EN/MME)