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Stabilization Projects at SLAC

Discusses beam stabilization methods like interferometer-based feedback and challenges in achieving nanometer-level stability for ground motion control in linear colliders. Explores environmental effects on beam stabilization systems.

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Stabilization Projects at SLAC

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  1. Eric Doyle, Leif Eriksson, Josef Frisch, Linda Hendrickson, Thomas Himel, Thomas Markiewicz Richard Partridge NLC Project, SLAC Stabilization Projects at SLAC

  2. Beam Stabilization • Goal: Stabilize beams to ~1nm at a Linear Collider IP • Slow Beam Based Stabilization (luminosity) • Fast Beam Based Stabilization (IP deflection) • Magnet position Stabilization: • Interferometer, Inertial Sensor based. • Very fast Beam Based Stabilization: Feather / Font • Nanometer BPMs

  3. Ground Motion

  4. Beam Based Stabilization • Beam based measurements are the only long term measurement of beam positions • Mechanical objects are not stable to nanometers! • For Timescales > 10 minutes, Luminosity Optimization feedback • 120 Hz Feedback (for NLC) based on deflection scans. • Note that 120Hz feedback has unity gain at ~10Hz.

  5. Calculated Gain for 120Hz Beam feedback

  6. Magnet Position Stabilization • Interferometer based feedback • Measures magnet position relative to ground • Work ongoing at UBC (Tom Mattison). • Accelerometer based feedback • Measures magnet position relative to "fixed stars" • Work ongoing at SLAC (this talk). • Ground referenced (Interferometer) and inertial feedback both work in simulation. Effectiveness depends on ground motion spectrum.

  7. Commercial Interferometer Technology • Heterodyne system provides immunity to ambient light, and high resolution phase measurement.

  8. Interferometer Measurement Limits • Zygo company ZMI-4004 Measurement resolution 1/2048 Fringe • 0.31 Nanometer single pass • 4 axis / VME module • Data rate 10MHz. • Zygo #7712 Laser Head • 0.5ppb Stability 1 Hour • OK for 1nm to > 1Meter

  9. Environmental Effects - Air • Air tpemerature and Pressure: • 1ppm/°C • 1ppm/2.8mm Hg pressure, • 1ppm/90% Humidity • Compensation • 0.1ppm to 1ppm from calculation • < 0.1ppm from refractometer compensation • Difficult to get 1nm over 1M in Air.

  10. Other Environmental Effects • Even Vacuum not ideal - windows • Fused Silica has small temperature coeficient, but index variation with temperature is large ~10ppm/°C • For 1 cm path in fused silica, need .01°C • May be difficult to provide vacuum paths for interferometers. • Assuming 10cm between reflector and center of magnet / BPM, need .001°C short term stability.

  11. Interferometer Overall • Performance typically limited by environmental issues. • Commercial heterodyne systems available from Zygo, Agilent, probably other companies • Provide stabilization to the GROUND • Cannot do better than a perfectly rigid mechanical support. • Need to decide how to evaluate performance

  12. Inertial Stabilization Work at SLAC • Stabilize a simple block using low sensitivity commercial seismometers (done) • Stabilize an “extended object” with mechanical properties similar to a final focus magnet using low sensitivity commercial seismometers. • Stabilize an “extended object” with high sensitivity seismometers • Construct a high sensitivity non-magnetic seismometer suitable for use in a detector.

  13. Hard Support Small motion without feedback Couples high frequencies: will excite internal modes Requires high actuator forces: 10 N Soft Support Large motion at support resonance without feedback Attenuates high frequencies, minimal excitation of higher modes Low actuator forces: .01 N Used for this project Magnet Suspension

  14. Actuator, Sensor • With soft supports, actuator strengths can be low ~.01 N (100Kg, 100nm, 5Hz Resonance) • Use “electrostatic Actuators” • Capacitive gap, ~100cm2, 1mm, gap, 1KV • Low stiffness, Fast response time • Force proportional to V2, not dependant on position (if motion << 1mm). • Sensor: Low cost, low sensitivity geophones for now

  15. Data Acquisition System • DSP (Old TI TMS320C40), for closed loop feedback • May upgrade to modern DSP if needed (C6000 series) • So far not a performance limit • 24 channel A-D, D-A. • 16 bit, 250KHz hardware, Typically operated at a few KHz • Variable gain input amplifiers • Variable frequency input filters for anti-alias. • Hardware: MIX bus / VME / Ethernet / Sun • Software: DSP C, VxWorks, (EPICS), Solaris, Matlab

  16. Feedback Algorithm • Characterize system • Drive all actuators, measure all sensors, all frequencies • Find normal modes • Find sensor resonances • Find couplings • ~96 parameter fit (works!) • Six independent feedback loops • State-space type feedback.

  17. Single Block Stabilization System Note: frequencies below 2 Hz filtered out

  18. Spectrum, Feedback On / Off

  19. Integrated spectrum with simulated beam / beam feedback

  20. “Extended Object” • Designed for same resonant frequencies and masses as a real magnet support. • Magnet support tube replaced by support beam under magnet for convenience • Use “Soft” supports ~ 3-7 Hz. • Use 8 sensors, 6 for solid body modes, 2 for first higher modes • Use 8 electrostatic actuators

  21. Extended Object Drawings

  22. Extended Object Actuator Sensor Support Spring

  23. Extended Object

  24. Extended Object

  25. Characterization of Extended Object

  26. Extended Object Status • Sensors, actuators, DAQ operating • 6 solid body, and 2 internal modes identified • Feedback software requires minor modifications from single block system • 6 to 8 sensors and actuators • “Code Rot” since single block tests • Attempt to close loop soon

  27. Possible Technical Issues • Extended object is far from symmetric – expect wide range of couplings to sensors, actuators and modes. • Very weak control over “roll” mode • Internal modes are high frequency (75, 120Hz), probably not excited. • Sensor tilt sensitivity: Tilt indistinguishable from transverse acceleration • Orthogonalization now frequency dependant. • May need to solve fully coupled problem (more computation)

  28. Why Build Our Own Sensor? • Want ~3x10-9M/s2/sqrt(Hz) noise at F > 0.1Hz. • Compact sensors for machinery vibration measurements (used for single block test) have noise ~300X larger • Geo Science seismometers have good noise < 10-9M/s2/sqrt(Hz), but are magnetically sensitive and physically large • Could not find commercial sensors which met our requirements

  29. General Seismometer Design • Thermal mechanical noise sets ultimate limit • Readout noise can be low • Thermal noise limited acceleration given by

  30. Vertical Sensors Difficult • Need to measure 3x10-9M/s2/sqrt(Hz) on top of Earth's gravity 9.8M/s2. • Spring "sag" under gravity is large for low frequency suspension • Small changes in suspension spring length or spring constant will appear as acceleration signals • Thermal changes typically limit low frequency performance - typically operate in vacuum • Material creep can be a serious issue

  31. Suspension Design • Want low fundamental resonance frequency in a compact geometry. • Simple mass on spring frequency goes as f=(1/2p)sqrt(g/L): f = 1.5Hz (our design) L = 11cm • Pre-bent spring gives high second order mode f.

  32. Feedback Seismometers • High suspension mechanical Q improves sensitivity - but results in large amplitude motion at resonance • Below resonance sensitivity decreases as w2 - leads to dynamic range problems • Use feedback to keep suspended mass motionless relative to sensor housing. (Standard technique) • Can use feedback force as acceleration signal • Optionally use force and residual error as signal

  33. Sensor Parameters • Suspended mass 40 grams • Resonant frequency 1.46Hz • Next mode ~96Hz, ANSYS simulation (not seen) • Mechanical Q ~50 • Theoretical Thermal Noise 2.5x10-10 M/s2/sqrt(Hz) • 10X better than needed • Theoretical electrical noise X2 smaller than mechanical thermal noise

  34. Electrodes on PCB Spring Cantilever

  35. Mechanical Design Issues • BeCu spring (high tensile strength, non magnetic) • Pre-bent, operated at high stress to increase higher mode frequencies • Extensive creep measurements done at SLAC • Thermal effects very large!! • ~10-8Co corresponds to (0.1Hz) noise limit • Use multiple "thermal filters", Gold plating to reduce temperature variations. Operate in < 1 um vacuum. • Expected to be ultimate low frequency noise limit

  36. Spring Cantilever RF Out RF IN Electrodes, Test Mass

  37. Sensor Status • Construction of prototype sensor complete • RF system operational, but with kludged control of out of phase signal. • Sensor mounted on 30 Ton Shielding block on elastomer supports. • Two Streckheisen STS-2 Seismometers mounted on block to provide reference signals. • Data very very preliminary!!!

  38. Sensor Testing • Do not have a location sufficiently quiet to measure sensor noise • Compare sensor with STS-2 seismometer • STS-2 noise much better than we need in this frequency range • Look for correlation with STS-2 • Compare with correlation between two STS-2s. • Data analysis very preliminary

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