FLUM:Fast-Dither Luminosity Feedback

Alan Fisher2007-06-20

The FLUM Team

Stan EcklundClive FieldAlan FisherPhyllis GrossbergSteve GiermanKarey KrauterEd Miller

Mark PetreeKiran SonnadNancy SpencerMike SullivanKen UnderwoodUli Wienands

IPXY: Slow-Dither Feedback

Maximizes luminosity by tweaking HER in x, y, and y' for best overlap with LER.

The three coordinates are adjusted sequentially.Four luminosity measurements for each coordinate:

Taken for offsets of 0, +d, 0, –d (where d is the dither size).Parabolic fit finds highest luminosity between –d and +d.Move to this peak, and then start the next coordinate.

9 seconds to complete one tweak of all coordinates:Inductance of correctors + field penetration into copper chambers + Bitbus delays in communicating with power supplies + luminosity measurement = 750 ms/step12 steps per cycle

Fast-Dither Concept

Continuous sinusoidal dither of the beam at the IP.Simultaneous dithers for x, y, and y′.

Each coordinate uses a different frequency—fx , fy , and fy′A lock-in amplifier for each coordinate:

Detects the magnitude and phase of luminosity’s response at its dither frequency.Determines distance and direction of the beam’s offset.

All 3 offsets are combined into one adjustment of the DC correctors.300 ms for measurement + 700 ms for correctors= 1 second for the full cycle.

Coils for Dithering

Four sets of xy air-core Helmholtz coils for low inductance.Can shake the beams in closed bumps for x, y, and y′.Too weak for DC changes in the orbit. Use IP corrector bumps instead.

Mounted on thin stainless chambers for rapid field penetration.

±30 m ±50 m

Do the Math

Luminosity

Dithered beam position

Overlapped width (µm)

Dithered luminosity

Lock-in output (V)

Slope of output (V/µm)

2

0 2( ) exp2 x

xL x L⎛ ⎞

= −⎜ ⎟Σ⎝ ⎠0 cos xx x x tω= + %

2 2 2x x xσ σ∗ ∗

+ −Σ = +

2220 0

0 2 2 2( ) 1 cos cos exp2 2x x

x x x

x x xxL x L t tω ω⎛ ⎞ ⎛ ⎞

= − − −⎜ ⎟ ⎜ ⎟Σ Σ Σ⎝ ⎠ ⎝ ⎠

% %

20 0

0 22exp

22x Lxx

x x xV C L⎛ ⎞

= −⎜ ⎟ΣΣ ⎝ ⎠

%

0 20 2x

Lx

dV xC Ldx

≈Σ

%

Choosing the Dither Frequencies

For a 1-s cycle, lock-in uses a 100-ms time constant.Measurement must settle after the corrector move finishes.Frequencies must be well above 10 Hz.Go above 60 Hz power line (and avoid 120 Hz, etc.).

High-frequency end limited by:Field penetration into vacuum chambers.Inductance of the Helmholtz coils.

Can’t exceed the ±20-V range of our power supplies.Highest frequency should be below 2nd harmonic of lowest.

Luminosity modulation is at 2nd harmonic with centered beams.

Avoid multiples of 5 Hz: injection harmonics.Choices: x at 93 Hz, y at 77 Hz, and y′ at 127 Hz.

SRS 830 Digital Lock-In Amplifier

Same model used for the tune trackers.Digital processing of input luminosity signal:

Two mixers: Multiply signal by cos(ωt+φ) and sin(ωt+φ)Phase offset φ lets us arrange a positive signal on the cosine channel (the output’s “real part”) for a positive beam displacement, with little signal on the sine channel (the “imaginary part”).

Digital low-pass filter with adjustable time constant and roll off.Isolates the signal component at the reference frequency.

Built-in sine-wave source.Used as lock-in’s reference ω and as source for dither control.

EPICS control (copied from tune trackers) using GPIB.For occasional use to set range, frequency, filtering.

BNC outputs for amplitude and phase.For the fast feedback loop, send these to a SAM and avoid GPIB.

EPICS Panel for the Lock-In

Conceptual Sketch of Dither Drive

Dither Drive

Due to coupling, all 8 magnets are driven at each frequency to make a closed bump for each coordinate.Each dither magnet is driven by a sum of 3 sines.The sine-wave generator inside the lock-in provides a constant-amplitude sine source.24 DAC voltages control programmable-gain amps to make 3 closed bumps of user-specified amplitudes.Magnets are driven by bipolar amplifiers (KepcoBOP 20-20M) acting as voltage-controlled current sources (±20 A, ±20 V).

Gain Control (1 of 8 Magnets)

Detecting IPXY Dithers

To test FLUM, we put it into Compute with IPXY running, to observe its dithers.IPXY vertical dither= 0.42 µmFLUM vertical dither = 0.3 µmThis is a large signal. The lock-in’s output is 10-V full scale.

Re[y] vs. Time

Integrating with FLUM, June 18

Samples 1 to 32:IPXY in Feedback,FLUM in Compute.Samples 33 to 39:Luminosity spikes because dithers stop while switching.Samples 40 to 144:IPXY in Compute,FLUM in Feedbackusing Tweak Mode.Samples 145 to 200:FLUM in Feedbackusing Peak Mode.

FLUM Escapes from a Ghost, June 18

IPXY was stuck at low luminosity.FLUM feedback was turned on for test of Peak Mode software.Luminosity dropped at first. We almost turned FLUM off, until it zoomed upward.IPXY was probably stuck on a “ghost”.FLUM moved away from the local maximum and found the peak.

Feedback Panel

IPXY to Feedback forces FLUM to Off.

FLUM to Feedback forces IPXY to Compute.

To find FLUM parameters

Feedback Magnet Panel

To find FLUM parameters

PR02 Loops Panel

To find FLUM parameters

To find IPXY parameters

FLUM Parameters Panel

If Luminosity < Min,FLUM uses Max Dithers.If Luminosity > Max,FLUM uses Min Dithers.Linear interpolation forMin < Luminosity < Max.Time (sec) between dither adjustments.

Macros: Peak & Tweak ModesPeak: integrationTweak: filling, fast tweakingMacros change dithers, FLUM step intervals, and lock-in time constants.White bar in the SCP button is local to your SCP.

FLUM Expert Panel

Time between FLUM steps.1 sec indicates Tweak Mode.3 sec indicates Peak Mode.

Button macro to reset the SAM used for the FLUM lock-ins.

FLUM “Vector” Display

Summary

FLUM has survived commissioning……and so have Karey, Steve, and I.Thanks to the operators who suffered through the tests.

Try it out. It’s easy to switch between it and IPXY.Do not make the dithers smaller and smaller to get a higher peak. You may lose control of the beam.

Peak and Tweak Modes use tested dither sizes.Remember, IPXY peaks when its dithers are all at zero.FLUM cannot get quite that high, but it doesn’t dip.Peak Mode should integrate better than IPXY.

Please give me your suggestions.