base-band tune (bbq) measurement system marek gasior beam instrumentation group, cern

Base-Band Tune (BBQ) Measurement System 1M.Gasior, CERN-AB-BI

Base-Band Tune (BBQ)

Measurement System

Marek Gasior

Beam Instrumentation Group, CERN


Tune measurement – The principle

Beam oscillations are observed on a position pick-up Oscillations of individual particles are incoherent – an excitation needed for “synchronization” Small beam oscillation signals in the presence of large revolution frequency content due to the fact that each

bunch appears in the pick-up only once per revolution Oscillations are usually observed in the frequency domain (separation from the strong background)


Tune measurement – Classical approach

Linear processing of position pick-up signals Dynamic range problems: revolution frequency content is large with respect to the betatron content

• large kicks required• accurate gain control needed (signal cannot be clamped)

If only small kicks are affordable (to limit beam emittance blow-up), complicated solutions needed. e.g.• resonant pick-up (does not work with single bunches)• beam centering (mechanics or electronics), the limit is the hybrid


Classical approach – “One betatron harmonic filtering”

The LHC bunch length (4) is about 1 ns and the corresponding bunch spectrum cut-off is about 500 MHz With just one bunch in the machine the revolution spectral lines are spaced by 11 kHz, so there are some 50 000 of these and some 100 000 betatron

lines When using the classical “one betatron harmonic filtering” method, one observes only 0.00001 (-100 dB) of the spectral content This results in very small signals, requiring low noise amplifiers and mixers, which have small dynamic ranges; they can be easily saturated by a huge

revolution content

no

nbbc nTttsnTttstfts )()()()()π2cos()( 11

no

nbbb

nbbbc T

nffSTT

nffffSTnffffS

TfS )(1)()(

21)( 111

length bunch133.0

off-cut dB 3spectrum bunch

3 dB


Tune measurement – Direct Diode Detection (3D)

Peak detection of position pick-up electrode signals (“collecting just the cream”) frev content converted to the DC and removed by series capacitors fb modulation moved to a low frequency range (as after the diodes fb is on much longer pulses) A GHz range before the diodes, after the diodes processing in the a kHz range Large sensitivity Works with any position pick-up Impossible to saturate (large frev suppression already at the detectors + large dynamic range) Low frequency operation after the diodes

• High resolution ADCs available• Signal conditioning / processing is easy (powerful components for low frequencies)


Direct Diode Detection – The principle

)π2cos(1)1()()(

)π2cos(1)1()()(

2

1

tftststftsts

bbb

bbb

beam relative offset = 0.1betatron oscillation relative amplitude = 0.05simulated tune value q = 0.1filter time constant = 10T (T – revolution period)storage capacitor Cf = Cpu (PU electrode capacitance)

Electrode 1

signal

Electrode 2

signal

Signalsof both peak

detectors

Detector signal

difference

sd(t)


Direct Diode Detection – The principle

= 0, = 0.01q = 0.1, Cf = Cpu

= T

= 100 T

4 bunches = 100 T

Detector signal differenceSignals of both peak detectors


Direct Diode Detection – Diodes not perfect

Detector signal differenceSignals of both peak detectors = 0, = 0.01q = 0.1, Cf = Cpu

4 bunches = 100 T

beamnot centered

one bunch10 % larger


q 0.5

Direct Diode Detection – Base band spectrum

Tf

TTffS f 2πj1

2πjexp1)(

2

cothπ

342~222

2 TT

TfS

fS

rf

rf

q 0.5


Architecture of the Base Band Q (BBQ) Measurement System

Analog front-end box (2 channels)Detector box (for one PU electrode)


BBQ systems at CERN

Machine Front-End Acquisition LHC “constant frev type” 24 bits (up to 100 kHz)

SPS “constant frev type” 24 bits

PS “constant frev type” 16 bits (up to 40 MHz)

LEIR “varying frev type” 16 bits

PSB “varying frev type” 16 bits

3 5 2 3 5 2 3 5 2 3 5 2 3 51 1 0 1 0 0 1 0 0 0 1 0 0 0 0F re q u en cy [k H z]

-1 0 0

-8 0

-6 0

-4 0

-2 0

0

Nor

mal

ized

mag

nitu

de [

dB]

S P S

P S

P S B

LHC, SPS PS, PSB, LEIR

BBQ system operational at RHIC Tested at Tevatron Will be operational at the CNAO hadrontherapy machine


SPS BBQ


SPS BBQ – Transverse damper noise1 bunch LHC pilot,

5109 p+,26-450 GeV

Damper system OFF

No

expl

icit

beam

exc

itatio

n

Damper system ON


SPS BBQ – Low-pass filtering

Measurement with the fixed target beam (a few thousand small bunches), no excitation BOSC – a homodyne tune measurement system A low-pass filter before the diodes cleans up the bunch longitudinal shape

• Important beam noise filtering at a small expense of a few dB signal loss, resulting in an important SNR improvement

• Similar effects seen on the PS and PSB

(no filters)


PS

EASTB, regular kick every 10 ms TOF, regular kick every 10 ms


PSB

LHC25A, R3, no kick Same, kick 20 V (a % of the standard kick)


LEIR

NOMINAL, regular kick 500 V, every 10 ms Same, no kick


(Sound card) Record of the RHIC BBQ signals

Horizontal plane (L)

Vertical plane (R)

about 10 minutes

injections ramp squeeze

RF

switc

hing

Q' t

oo s

mal

l


Spectra of the RHIC BBQ signals

Hor. Ver.


RHIC BBQ measurements – Collisions

Store beginning

H plane

V plane

5 hours later (end of the store)

H plane

V plane


Conclusions

BBQ advantages

Sensitivity (noise floor in the nm range for intense beams) Simplicity and low cost

• no timing, no resonant PU, no movable PU, no hybrid, no mixers, it can work with any PU

Very robust for saturation Base band processing and acquisition

• excellent 24 bit audio ADCs available• Signal conditioning / processing is easy

(powerful components for low frequencies)• Independence of the machine filling pattern

Flattening out of the beam dynamic range (small sensitivity to number of bunches)

BBQ disadvantages

Interference: operation in the low frequency range It is sensitive to the "bunch majority“

(gating needed to measure separate bunches)

Future development

Gating a bunch or a group of bunches(successful proof of principle done with beam)

Continuous head-tail chromaticity measurement(tests with beam and some theoretical studies done)


Extra slides: Direct Diode Detection – SNR limits

22

222

2

2

)π2(

42

2πj1)(2πjexp1

π

ff

nAf

RDf

nA

ff

fpu

puffnC

D

CRqT

IRkIeRT

V

qCRTq

CCCCRTV

G

= 100 T


Extra slides: PS BBQ – Detector DC voltages

AD

SFTPRO

TOF


Extra slides: RHIC BBQ – Tune scan

Mea

sure

men

t by

P. C

amer

on (B

NL)


Extra slides: RHIC BBQ – Mains ripple in the beam spectrum

Million turn BPM near transition

BBQ near transition

180 Hz 720 Hz360 Hz

f [Hz]Measurement by P. Cameron (BNL) RHIC BBQ compared to a million turn BPM

The BBQ sensitivity was estimated to be better than

10 nm


Extra sides: SPS BBQ – mains ripple in the beam spectrum

72 LHC bunches, 1011 p+/ bunch, 270 GeV,coasting (RF on)

Even around 5kHz, placing the tune on a50 Hz multiple increases beam oscillations!

50Hz

No explicitexcitation


Extra slides: PS BBQ – Mains ripple in the beam spectrum

10 lines spacedby 100 Hz

2 injections, 6 bunches, 81012 p+/

bunch,1.4-26 GeV, splitting into 72 bunches


H plane

V plane

200 ms after injection, no kicks, average on 100 cycles

Extra slides: LEIR – Beam not (too much) bunched

base-band tune (bbq) measurement system marek gasior beam instrumentation group, cern

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