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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 > 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 > 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– < 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 .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.

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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 SupportLarge 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 << 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

Support Spring

Sensor

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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 > 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

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General Seismometer Design• Thermal mechanical

noise sets ultimate limit

• Readout noise can be low

• Thermal noise limited acceleration given by

0

04

mQ

TkA b

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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=(1/2)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 2 - 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 Housing

Adjust motor

Cantilever

Suspension flexure spring (pre-bent to be flat under gravity load)

Slow adjustflexure

Electrostaticfeedback pusher~50V, 500um

Cable delay1 nanosecond

Signal split

Signal combine

Mass

I/Q

Signal nullat center

ADC - get positionand phase mismatchinformation

VCO~20dBm

DACphase match feedback

DACposition feedback

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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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Spring

Cantilever

Electrodes on PCB

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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 < 1 um vacuum.– Expected to be ultimate low frequency noise limit

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Spring

Cantilever

Electrodes, Test Mass

RF IN

RF Out

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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: <0.25nm 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 < .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 <1Hz, probably to

<0.1Hz.● Need to understand how to use beam to evaluate

system performance

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Inertial Stabilization Issues

● Inertial sensor: Low beam rate (< 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 <1nm for timescales of seconds

● Not pushing state of the art!● Ground motion at single point >>1nm at 0.1Hz.

– At SLAC see ~300nm at > 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