New Bunch-Current Monitors,Fast IP Luminosity Dither,

and IP Vibration

Alan Fisher

PEP-II MAC Review

2007-11-16

Participants

Bunch-Current Monitors Matt Boyes Ron Chestnut Boni Cordova-Grimaldi Alan Fisher Mike Laznovsky Jeff Olsen Mark Petree Dmitry Teytelman

Fast IP Luminosity Dither Stan Ecklund Clive Field Alan Fisher Steve Gierman Phyllis Grossberg Karey Krauter Ed Miller Mark Petree Nancy Spencer Kiran Sonnad Mike Sullivan Ken Underwood Uli Wienands

Architecture of the BCMs

Pick-Ups One set of four BPM-type buttons, each giving short (~300 ps) bipolar pulses.

Stripline Filter/Combiner (improved) In the tunnel. Combines 4 button pulses into a single period of 1428 MHz.

Analog stage: Downconverter (improved) Mix bunch signal with a 1428-MHz sinusoidal reference. Four phases: Shift this reference in four 90° steps for cosine and sine data.

Corrects for pedestal and for shift of synchronous phase across bunch train. Digital stage: Bunch-by-Bunch Processor (BxBP, replacing VXI crate)

Digitizes and averages many turns for each bucket and each of the four phases. BCM Computer (replacing VXI crate processor)

Takes data from BxBP. Removes pedestal by subtracting opposite phases. EPICS interface to BIC and to users, for measurements and settings.

Bunch Injection Controller (BIC) Computes quadrature sum of cosine and sine data to get true bunch amplitudes. Normalizes bunch-current counts to the total ring currents from the DCCTs. Plans injection sequence for fills and trickle injection based on BCM data.

Purpose of Building New BCMs

Replace the digital stage. Boards in VXI crates built over 10 years ago at LBNL

Used obsolete chips and other parts that are now unavailable. Has given trouble in the past. Difficult to repair.

New design is simpler, with improved performance. Improve the analog stage.

Less crosstalk between buckets. Previously compensated (incompletely) with a deconvolution algorithm.

Better tuning of the phase adjustments. 476-MHz phase shifters with a 120° range, supplemented by cable delays. 1428-MHz 4-step phase shifter had not-quite-90° steps.

Just in time (almost): Old HER VXI boards began to die in early July. Repairs were ineffective.

Completion of new BCMs had been planned for September downtime. Resorted to open-loop fill and coast for HER during part of July. New BCMs were rushed into service by late July, ran through August.

Full installation and minor fixes being completed during this downtime.

Old Downconverter

New Downconverter

Test Pulse In, Downconverted Pulse Out

Test pulse: 1 period of 1428 MHz (3fRF).

Beam pulse: Fast bipolar button spike, followed 350 ps later by an inverted spike. Choice of processing frequency:

If lower: crosstalk between output pulses. If higher: more sensitive to bunch phase.

Ringing is due to pulse generator.

350 ps

Slower: Can’t digitize a button spike, but can digitize the downconverted peak. Start to finish < 1 bucket (2.1 ns).

Avoids crosstalk with next bucket.

Pulser’s ringing causes the small ripple.

Time (ns) Time (ns)

Volt

s

Old vs. New Downconverter Output

Time (ns) Time (ns)

Volt

s

Old downconverter’s response to test pulse Wider than the desired 2.1 ns. Significant ringing afterward. Crosstalk into subsequent buckets.

New downconverter’s response to test pulse Improvement due mostly to filters:

Faster near-Gaussian low-pass Notch filter removing leakage of LO through mixer while avoiding ringing.

Measure at Four Reference Phases

Digitize near peak (for cosines) and zero crossing (for sines). Subtract opposite phases to remove pedestal while doubling signal. Sum these cosine and sine terms in quadrature to get amplitude.

0°

270°180°

90°

Cosine Sine

LER BCM Phases across Bunch Train

90°

270°

180°

0°

Empty buckets

Sine difference(showing shift of synchronous phase)Cosine

difference

New Downconverter Chassis

Power supplies

Digitally-controlled phase

shifters

Interface to BxBP

Step attenuator

Filters

Frequency tripler

(1428 MHz)

Test-pulse generator

Mixer

Output amplifier

Bunch-Signal Input

Downconverted output to

BxBP

Changes to the Digital Stage

Old BCM New BCM

Hardware (per ring) 6 modules in a VXI crate 1 board in a chassis

Digitizers Two 8-bit ADCs at 238 MHz One 8-bit ADC at 476 MHz

FPGAs 12 FPGAs on two boards One FPGA (Virtex-II Pro)

Downsampling 1/8 of the buckets per turn All buckets on every turn

Measurements of each bucket at each phase

250 1000

Repetitions of 4 phases 15 16

Total measurement time 0.9 s 0.5 s

BCM computer (per ring) VXI crate processor Linux PC in rack, via USB

Connection to BIC Shared memory VXI module EPICS

Other applications Single purposeMany: Includes an analog output

for use in bunch feedbacks

VXI Modules of Old Digital Stage

Dual 238-MHz digitizers in alternation

One of two modules, each with 6 FPGAs

Digitizer Decimator

New BxBP

FPGAADC

Input from downconvert

er

To BxBP interface

USB to compute

r

Status of BCMs

New hardware has been installed in place of old, inside a temperature-controlled rack.

Running now with a test-pulse generator, and ready for timing with beam signals.

Some firmware and software improvements are still underway.

Luminosity Dither Feedback

Constantly move one beam with small local bumps at the IP, seeking to maximize luminosity. Feedback loop seeks best overlap. Compensates for drift and for IP motion due to tuning

elsewhere (steering, sextupole bumps,…). Since the beam is flat at the IP, we dither in three

coordinates: x, y, and y'. We dither the HER, since its IP optics are simpler. Feedback rate:

Must maintain collisions during fills. Shouldn’t delay operator tuning by slow corrections.

The Old Slow-Dither Feedback

Coordinates (x, y, and y') 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 best spot within ±d. Start next coordinate.

9 seconds to complete one tweak of all coordinates: 12 steps per cycle 750 ms/step = Inductance of correctors

+ Field penetration into copper chambers+ Bitbus delays in links to power supplies+ Luminosity measurement

The New Fast-Dither Feedback

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.

1 s/cycle = 300 ms for measurement+ 700 ms for adjusting correctors

Luminosity with Sinusoidal Dithering

Dithered beam position

Luminosity

Overlapped width (µm)

Dithered luminosity

Small second-harmonic term, independent of offset Lock-in detects component at ωx

2

0 2( ) exp

2 x

xL x L

0 cos xx x x t

2 2 2x x x

2220 0

0 2 2 2( ) 1 cos cos exp

2 2x xx x x

x x xxL x L t t

Newton’s Method for the Next Step

Lock-in output (V):

Sign of Vx gives direction of offset x0.

Slope of lock-in output (V/µm):

As corrections are taken, keep an exponential average of Since we know and , the average slope lets us compute

The next correction uses:

This is essentially Newton’s method for finding the zero of the function dL/dx, but for stability using a gain g < 1 and the average slope.

20 0

0 22exp

22x L

xx

x x xV C L

0 20 2x

L

x

dV xC L

dx

x0L x

0

x

x

Vx g

dV dx

xV x x

New Coils for Sinusoidal Dithering

Four sets of xy air-core Helmholtz coils for low inductance. Can shake the beams in small 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

SRS 830 Digital Lock-In Amplifier

Digital processing of input luminosity signal: Two mixers: Multiply signal by cos(t+) and sin(t+)

Phase offset 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.

Provides reference for the lock-in and for the dither drive.

Choosing Dither Frequencies

Low-frequency limitations: A 1-s cycle needs a shorter measurement time.

Measurement must settle after the corrector move finishes. The low-pass filter of the lock-in is set to a 100-ms time constant.

With this 100-ms filter, dither frequencies must be well above 10 Hz. Go above 60-Hz power line (and avoid 120 Hz, etc.).

High-frequency limitations: 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 has modulation 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.

Dither Control

Due to coupling, all 8 magnets are driven at each frequency to make a closed bump for each coordinate. Each Helmholtz coil is driven by a sum of 3 sines. 24 coefficients (A/µm or A/µrad) computed from the HER

model, and then scaled by specified dither amplitude. Algorithm decreases amplitude with increasing luminosity.

24 DAC voltages control programmable-gain amps. Magnets are driven by bipolar amplifiers acting as

voltage-controlled current sources. Kepco BOP 20-20M, ±20 A, ±20 V

Gain Control (1 of 8 Magnets)

Commissioning: Detect Slow Dithers

First tests of fast dither were passive.

Can it detect steps made by the slow-dither feedback in each coordinate?

Dithers used here: Slow y dither,

step size = ±0.42 µm Fast y dither,

sine amplitude = ± 0.30 µm

This gave a large signal. The full-scale output of the

lock-in amplifier is ±10 V.

Re[y

] (V

)

Time (s)

Luminosity Spectrum while Dithering

50 150

60

120

Lum

inosi

ty (

5 d

B/d

iv)

Frequency (Hz)

77 93

127

Fast Dither in Feedback: Two Speeds

Slow Dither Luminosity drops with each

dither, especially in y. Fast Dither in “Tweak” Mode:

For fast response while tuning or filling

1-s adjustments of x, y, and y′ 9 times faster than slow dither

0.1-s time constant for lock-in Larger dithers

Fast Dither in “Peak” Mode: For best peak and integrated

luminosity 3-s adjustments of x, y, and y′

3 times faster than slow dither 1-s time constant for lock-in Dithers reduced by factor of 3

for a smaller dither penalty

SlowDither Fast Dither

Switching,

no dithers

TweakMode

PeakMode

Time (s)

Lum

inosi

ty (

10

33 c

m-2 s

-

1)

Dither Penalty

Recall the expression for the dithered luminosity:

Even for centered beams, dithering reduces the time-average luminosity . Define the dither penalty (for 3 coordinates):

Dithering by Σ/10 in all 3 coordinates reduces luminosity by 0.75%. Compare to slow dither, with its consecutive steps on and off peak.

For the same amplitudes, the calculated penalty is 3 times lower. Peak mode beats this using smaller dithers and longer integration.

L

2220 0

0 2 2 2( ) 1 cos cos exp

2 2x xx x x

x x xxL x L t t

2 22

0

11

4 x y y

L x y y

L

Status of Fast-Dither Feedback

Fast dither was commissioned, and was in regular use for the final two months of the run.

Easy to switch between modes, and between fast and slow dither for comparison.

Operators fight for peak luminosity, and so they generally prefer Peak mode. It’s fast enough and avoids the dither penalty of Tweak mode.

Evidence for Vibration at the IP

In first tests, fast dither could not control the beam. We saw the vertical position and correctors oscillating…

Even with a low loop gain. Even when feedback corrections were halted.

We also saw: 9.7 Hz on the luminosity Vertical motion at ~10 Hz on the BPMs

9.7 Hz on Luminosity Spectrum

Peak at 9.7 Hz and its 2nd and 3rd harmonics

Larger than our dither peaks

Sidebands at ±9.7 Hz around 60 Hz

0 100Frequency

(Hz)

Lum

inosi

ty (

5 d

B/d

iv)

9.7

29.1

60

19.4

30

69.7

50.3

10 Hz Vertical Motion on Both Beams

Old LER BPM Processor(FFT of PR02-2092-Y)

New HER BPM(PR02-8012-Y)

Franz-Josef Decker Walter Wittmer

Frequency is typical of vibrations and affects both beams:Magnets inside support-tube of BABAR?

Geophone at Nose of Forward Raft

10-Hz motion at one end of the support tube.How much do the beams move?

1-µm Relative Motion at the IP

Old luminosity signal was filtered for slow dither. Didn’t see 10 Hz.

Fast dither uses a new high-bandwidth luminosity signal.

When slow dither moves HER vertically off the center of LER:

Luminosity drops. 10-Hz modulation appears.

With 1-µm dithers, the top of each 10-Hz period restores the luminosity to the baseline (when seen with DC coupling).

Y dither lowers luminosity, but vibration restores it.

Elsewhere, the hash is at 20 Hz. 2nd harmonic as vibration drops the

luminosity on both sides of peak.

1 s/div

Filtered lumi(DC coupled)

Wideband lumi

(AC coupled)

2-µm Slow Dithers in Y

10 Hz20 Hz

“Noise-Canceling Headphones”

Initially, slow dither only. Start vertical fast-dither,

driving HER with a 0.6-µm amplitude at 9.75 Hz.

Electrons move in a beat relative to the vibration.

Immediate luminosity gain of 4% at peak of beat.

Change to 1-µm dither at 9.70 Hz.

Beat frequency changed. Operators began tuning

the drive frequency to hold this peak.

9.75 Hz0.6 µm

Slow dither only

9.70 Hz1 µm

Brace Added under Nose of Raft

Stuart Metcalfe

Change in Vertical Vibration

Before Bracing: A=0.209 µm

After Bracing:A=0.053 µm

Vibration reduced by a factor of 4, recovering the missing luminosity.

Status of Vibration Monitoring

New 3-axis vibration sensors are now installed at both ends of the support tube. Designed to withstand BABAR’s 1.5-T field.

Multichannel data acquisition hardware with FFT software expected to arrive by end of this month.

If other significant vibrations are found, they might be corrected mechanically or with a closed-loop feedback using the IP air-core correctors.