FP420 Alignment WithBeam Position Monitors

Jo Pater (Manchester)

14-16 July 2008

July 2008 J.Pater - BPM-based Alignment 2

FP420 Alignment Plan

• LHC button-style BPMs on fixed beampipe + Wire Positioning System (WPS)– Alignment wire is absolute reference

• Similar but modified (larger-aperture) BPM on each Hamburg pipe– Referenced to detector by knowledge of mechanics

• Offline track-based alignment using exclusive dileptons (M.Albrow et.al)

July 2008 J.Pater - BPM-based Alignment 3

Hamburg pipeHamburg pipe

BP

M

BP

M beam

LHC beampipe

Alignment wire

WPS sensors

bracket

Fixed BPMs + WPS

Overall accuracy of ~10 challenging: tolerances of individual components add up quickly:– WPS sensors: known to be accurate to < 1– Mechanics: <10 tolerances possible but not easy !– Complicated by the moving bit– BPMs: need micron-scale accuracy and resolution

July 2008 J.Pater - BPM-based Alignment 4

BPM issues for FP420

– Preliminary study (JP) suggests that BPMs are capable of it (see following slides)– Will need carefully designed readout electronics (see following slides)– UCL engineer (A.Lyapin) on board, experienced with BPMs for linear colliders

• Electrode design could be tailored to give better performance if necessary– Two larger electrodes (instead of four smaller) would give better performance but only

in one dimension

Workshop April 2007: what BPMs are the best choice for us?

– 1st choice: LHC BPMs (electrostatic button-type)

• Already used in large numbers in LHC– minimises integration issues

• Can be optimised to special diameters – e.g. to mount on Hamburg pipe

• Micron-level precision/resolution believed possible, although not demonstrated, by LHC team:

July 2008 J.Pater - BPM-based Alignment 5

Manchester, Cockcroft Alignment Test Bench• Damped, floating optical table 1.2m wide x

3m long (see next slide)• 3 WPS sensors + wire + readout• 2 LHC BPMs (i.e. the fixed BPMs)

– Horizontal setup: • beam wire stretched by hanging weight over

pulley• ends of wire on micro-positioners

– Read out with network analyser (courtesy of Cockcroft Laboratory):

• 40 MHz CW on beam wire• Electrode signals (unamplified) compared

internally with reference• calculated offline

– NB: as yet no large-aperture BPMs (i.e. the ones that will move with the Hamburg pipes)

• 2 Schaevitz LVDTs and signal conditioners– Read out via DMM + Labview

July 2008 J.Pater - BPM-based Alignment 6

LHC BPMs in FP420?Preliminary Study (JP, July 2008)

• Resolution– A quick study:

• 6 repeated measurements of under identical conditions, at two different wire positions, yields standard deviations of 0.00025 and 0.00035.

• Corresponds to spatial displacement of the wire of about 5 microns.

• Taken as a measure of what the BPM itself is capable of, this can be considered a ‘worst possible’ resolution as specialised readout electronics can only help.

• Linearity– See next slides

July 2008 J.Pater - BPM-based Alignment 7

LHC BPM Linearity± 6mm either side of centre

July 2008 J.Pater - BPM-based Alignment 8

LHC BPM Linearityin 50 steps ~2.5mm from centre

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LHC BPM Linearityin 10 steps around centre

July 2008 J.Pater - BPM-based Alignment 10

LHC BPM Linearityfurther from the centre

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LHC BPM Performance

• Based on preliminary study– Resolution (~ a few microns) looks acceptable– Linearity

• Very good over short distances near centre of BPM• Less good further away from centre

– As expected!– Should be repeatable and therefore correctable

• Further work needed to determine– Repeatability correctability accuracy– Other corrections, e.g. temperature dependence

July 2008 J.Pater - BPM-based Alignment 12

Possible Hardware Solutions for BPM Processing Electronics

A. Lyapin (UCL)

• Narrow bandwidth electronics commercially available (i-tech) high resolution as noise is rejected (down to a few um) gain/offset drifts compensation implemented (stable over hours and days!) averaging over a few hundred consequent bunches

• Wide bandwidth electronics single bunch measurement poor single bunch resolution (LHC electronics: ~100 m) Averaging turn-by-turn could improve resolution by sqrt(N) e.g. standard LHC front-end electronics + custom next-level board need to take care of drifts

LHC frontend electronics + specialised next-level board

July 2008 J.Pater - BPM-based Alignment 13

BPM tests: next steps

Have in hand front-end LHC readout boards1) Commission them

• need to bricolage connection to power supplies (don’t have the custom backplane)

2) Test them• Use e.g. LabView to simulate averaging over individual

bunches

3) If that works well, AL to design next-level board.

July 2008 J.Pater - BPM-based Alignment 14

Potential BPM Calibration Scheme• On bench:

– Attach fiducials to outside of BPM– Survey --> position of fiducials wrt WPS sensor and

beam-wire, fold in BPM response– Must be temperature-dependent (e.g. BPM expansion)

• In-situ:– Mount some BPMs on positioners

• calibrate them by offsetting a known amount• Cross-calibrate the others by fitting the orbit

– Inject pulse to compensate for gain/offset drifts (it should last for at least one normal fill) - method studied by T-474 at SLAC ESA (A.Lyapin)

FP420: two ofour BPMs

already move

July 2008 J.Pater - BPM-based Alignment 15

• WPS sensors use a capacitive measurement technique along 2 perpendicular axes.

• On each axis the wire lies between 2 electrodes

• Proven resolution ~0.1-0.3 microns

• LEP energy spectrometer study

• Reproduced on Manchester bench

Wire Positioning Sensors

July 2008 J.Pater - BPM-based Alignment 16

The Moving BitNeed to relate detector position precisely to alignment wire, while allowing detector (on Hamburg pipe) to move freely

–LVDT is an obvious potential solution, but off-the-shelf examples not accurate enough:

• best are ~0.25% of full scale i.e. ~100 on 4cm

–Schaevitz® designed special (rad-hard, very accurate) LVDTs for LHC collimator alignment (see next slide)

• 0.1-0.04% of full scale i.e. 16-40 on 4cm• Compact package (20cm)• Rad-hard to 50 MGy, very good temperature stability• Company confident they can provide shorter version with significantly

better accuracy (at least at one end of stroke.)• Have 2 examples of LHC device at Manchester…

July 2008 J.Pater - BPM-based Alignment 17

July 2008 J.Pater - BPM-based Alignment 18

LVDT study at Manchester

• Resolution

• Accuracy

• Temperature dependence and compensation

July 2008 J.Pater - BPM-based Alignment 19

LVDT Resolution• As expected, resolution is a function of

displacement – (plots show resolution in volts; 10V=25mm)

At 25mm, ~115nm

At 10mm, ~60nm

At centre, ~30nm

July 2008 J.Pater - BPM-based Alignment 20

LVDT Accuracy:• Calibrate by scanning across length of

LVDT, plotting voltage against nominal x position; fit a line to this data.

July 2008 J.Pater - BPM-based Alignment 21

Can get better accuracy near centre by fitting to central points

More work needed: e.g. calibrate at constant temperature

July 2008 J.Pater - BPM-based Alignment 22

LVDT Temperature Dependence• Data taken at several displacements…

– several days per displacement– Tracking room temperature

• …shows clear temperature dependence– Probably more than one effect, e.g.

• Difference in CTEs of support components

• Effects of temperature on electrical characteristics of LVDT (e.g. wire resistance)

July 2008 J.Pater - BPM-based Alignment 23

LVDT Temperature dependence (2)

• Should be correctable. First try:

• Needs more work– Different correction factors for e.g. dT/dt– Better temperature control --> better calibration

• Have programmable ‘oven’ at Manchester

July 2008 J.Pater - BPM-based Alignment 24

Other BPM-based Alignment Jobs

• Integration– Must put together working group to integrate alignment

hardware. Action JP to coordinate, someone from each relevant area.

• DAQ requirements (inputs/outputs) need to be defined

Reserve Slides

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July 2008 J.Pater - BPM-based Alignment 27

Resolution/Precision/AccuracyA. Lyapin (UCL)

• Resolution – the smallest change of the measured value an instrument can see– depends on the sensitivity, noise/adc bit

resolution• Precision – if multiple measurements of the

same value are taken, how far they fall from each other– mainly depends on resolution and scale

calibration• Accuracy – how far the averaged measured

value is from the true value– depends on the offset calibration, drifts and

non-linearities• Precision and accuracy are usually defined

over some period as they degrade

High accuracy, low precision

From Wikipedia:

High precision,low accuracy

July 2008 J.Pater - BPM-based Alignment 28

Narrow- vs. Wide-Band Electronics

At 420m, will individual bunches be…– …same size, same orbit as overall beam?

• Then use narrow-band solution, nothing to be gained from bunch-by-bunch analysis

– …smaller than beam, each bunch having a stable individual orbit?

• Could win by using wide-band electronics, averaging over ~hundreds of turns for each bunch

• At IP, individual bunch orbits vary by ~1 (for an r.m.s. beam size of 16)

July 2008 J.Pater - BPM-based Alignment 29

Gain/offset drifts compensation A. Lyapin (UCL)

• T-474 experiment at SLAC ESA active monitoring system sending CW burst

into processing electronics when there is no beam induced signal

clear gain drifts have already been observed compensation hasn’t yet been demonstrated method under study