Stabilization Projects at SLAC

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Stabilization Projects at SLAC

• Eric Doyle, Leif Eriksson, Josef Frisch, Linda
  Hendrickson, Thomas Himel, Thomas Markiewicz
  Richard Partridge
• NLC Project, SLAC

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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

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Ground Motion
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Beam Based Stabilization

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

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Calculated Gain for 120Hz Beam feedback
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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.

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Commercial Interferometer Technology

• Heterodyne system provides immunity to ambient
  light, and high resolution phase measurement.

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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 gt 1Meter

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Environmental Effects - Air

• Air tpemerature and Pressure
• 1ppm/C
• 1ppm/2.8mm Hg pressure,
• 1ppm/90 Humidity
• Compensation
• 0.1ppm to 1ppm from calculation
• lt 0.1ppm from refractometer compensation
• Difficult to get 1nm over 1M in Air.

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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 .01C
• May be difficult to provide vacuum paths for
  interferometers.
• Assuming 10cm between reflector and center of
  magnet / BPM, need .001C short term stability.

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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

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Inertial Stabilization Work at SLAC

1. Stabilize a simple block using low sensitivity
   commercial seismometers (done)
2. Stabilize an extended object with mechanical
   properties similar to a final focus magnet using
   low sensitivity commercial seismometers.
3. Stabilize an extended object with high
   sensitivity seismometers
4. Construct a high sensitivity non-magnetic
   seismometer suitable for use in a detector.

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Magnet Suspension

• 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

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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 ltlt 1mm).
• Sensor Low cost, low sensitivity geophones for
  now

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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

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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.

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Single Block Stabilization System
Note frequencies below 2 Hz filtered out
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Spectrum, Feedback On / Off
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Integrated spectrum with simulated beam / beam
feedback
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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

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Extended Object Drawings
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Extended Object
Actuator
Sensor
Support Spring
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Extended Object
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Extended Object
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Characterization of Extended Object
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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

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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)

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Why Build Our Own Sensor?

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

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General Seismometer Design

• Thermal mechanical noise sets ultimate limit
• Readout noise can be low
• Thermal noise limited acceleration given by

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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

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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.

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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

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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

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Electrodes on PCB
Spring
Cantilever
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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 lt 1 um
  vacuum.
• Expected to be ultimate low frequency noise limit

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Spring
Cantilever
RF Out
RF IN
Electrodes, Test Mass
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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!!!

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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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Data Interpretation

• All noise issues expected to be at low
  frequencies
• Expect sensor noise to be flat in acceleration
  frequency down to some frequency. Then expect 1/f
  noise to cut in (unknown frequency).
• Expect STS-2 noise to be flat in acceleration
  down to 0.01 Hz.
• Compact Geophone (used for single block test),
  expect noise to be 1/f in acceleration (velocity
  sensor).

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Sensor Noise Estimate From Correlations

• STS-2 to STS-2 Correlation good to 10-8M/s2 to
  .025Hz.
• Actual sensor limit probably 10x better, but
  indicates measurement limits in this setup
• Compact geo-sensor to STS-2 correlation good to
  7x10-7M/s2 at 0.25Hz
• New sensor to STS-2 correlation good to
  4x10-8M/s2 to 0.05Hz.

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Noise Estimates

• Use correlation and assumed frequency spectrum.
• STS-2, measured lt0.25nm at 1Hz, 25nm at 0.1Hz.
  Probably measurement limit.
• Compact Geosensor (used for block tests). 5nm at
  1Hz. 5000nm at 0.1Hz (This is a velocity sensor,
  below resonance, noise 1/F3).
• New Sensor 1nm at 1Hz, 100nm at 0.1Hz.
• With NLC style beam-beam feedback, demonstrated
  sensor noise is OK down to lt .01Hz.

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Sensor Noise Limits

• Sensor operating at low RF power. Results in x10
  reduction of ideal sensitivity. (probably not the
  limit now)
• Some evidence of spring creak small steps
  during creep. Investigating
• Sensor not magnetic immune contains low
  resistance current loop on cantilever. Being
  replaced with insulating cantilever.

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Sensor Upgrades

• Non-conducting cantilever Aluminum Oxide.
• Non-conducting mass Hafnium Oxide (dense).
• RF splitting on PC board (probably ceramic), to
  replace kludged connector.
• Various detailed mechanical changes to reduce
  size, improve manufacturability

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Stabilization for ATF Nano-BPM

• Inertial and / or interferometer stabilization
• Beam rate 1-6 Hz (compare with 120Hz for NLC),
  Need low frequency system.
• Need good stability at least to lt1Hz, probably to
  lt0.1Hz.
• Need to understand how to use beam to evaluate
  system performance

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Inertial Stabilization Issues

• Inertial sensor Low beam rate (lt 6 Hz, vs. 120Hz
  for NLC) requires very low frequency sensor.
• Sensor noise scales as 1/F2
• Present performance of SLAC sensor not good
  enough.
• May want to use 3 Streckheisen STS-2 sensors.
• Can probably measure 1nm down to 0.25 Hz.
• 1nm at 0.1 Hz very difficult
• Only interesting if beam rate few Hz.
• At best, performance is somewhat marginal

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Interferometer Stabilization Issues

• Interferometers should be good to lt1nm for
  timescales of seconds
• Not pushing state of the art!
• Ground motion at single point gtgt1nm at 0.1Hz.
• At SLAC see 300nm at gt 0.1Hz
• Need to measure 2 point relative ground motion.
• Use STS-2 or similar best measurement
• Quandary Need inertial sensor to measure ground
  motion to evaluate interferometer performance!

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Beam Issues

• Need to make 2 point comparison compare line
  fit to one (3 BPM) structure with next structure.
• Magnetic fields need micro-Gauss-M field
  variation for nm motion.
• Need to measure. Typically see mill-gauss at 50Hz
  in laboratory.
• Phase shifts relative to power line can be a
  problem!
• Must turn off all magnets between BPMs.
• May need to build magnetic field feedback system.
• Lever arm 3 BPMs projecting to more distant
  point.

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Ignoring problems

• Place inertial sensors on LLNL support frame
• Space for 3 Streckheisens, or 3 pairs of SLAC
  sensors.
• Place 6 interferometer beam lines (in vacuum) to
  ground).
• Replace LLNL support frame supports with springs,
  and electrostatic actuators.
• Use SLAC DAQ system to close loop based on both
  seismic sensors and interferometers
• Adjust frequency roll-off between inertial and
  ground

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Comments on ATF stabilization

• System is complex, and requires complex
  mechanical integration
• Light paths through support table are required
  for interferometers.
• Need to integrate LLNL support / feedback system
  with LLNL support / feedback system

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Short Term Plan

• Stabilize extended object with commercial low
  noise (but magnetic sensitive) sensors.
• Hope to meet NLC performance
• Construct an updated non-magnetic seismometer
  which meets NLC requirements.
• Work on stabilization of ATF NanoBPM system