Base-Band Tune (BBQ)
Measurement System
Marek Gasior
Beam Instrumentation Group, CERN
M.Gasior, CERN-AB-BI
Base-Band Tune (BBQ) Measurement System
1
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)
M.Gasior, CERN-AB-BI
Base-Band Tune (BBQ) Measurement System
2
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
M.Gasior, CERN-AB-BI
Base-Band Tune (BBQ) Measurement System
3
Classical approach – “One betatron harmonic filtering”
3 dB




sc (t )  cos(2π f b t ) sb1 (t ) 
 (t  nT )   so1 (t ) 
 (t  nT )


n  
n  




Sc ( f ) 



1
n
n 1
n



Sb1 ( f  f b )
  f  f b    Sb1 ( f  f b )
  f  f b    So1 ( f )
  f 
2T
T
T
T
T






n 
n  
n 


bunchspectrum
3 dB cut-off


0.133
bunchlength

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
M.Gasior, CERN-AB-BI
Base-Band Tune (BBQ) Measurement System
4
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)
M.Gasior, CERN-AB-BI
Base-Band Tune (BBQ) Measurement System
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Direct Diode Detection – The principle
sb1 (t )  sb (t ) (1   )1   cos(2π f b t ) 
sb 2 (t )  sb (t ) (1   )1   cos(2π f b t ) 
sd(t)
beam relative offset  = 0.1
betatron oscillation relative amplitude  = 0.05
simulated tune value q = 0.1
filter time constant  = 10T (T – revolution period)
storage capacitor Cf = Cpu (PU electrode capacitance)
Electrode 1
signal
Electrode 2
signal
Signals
of both
peak
detectors
Detector
signal
difference
M.Gasior, CERN-AB-BI
Base-Band Tune (BBQ) Measurement System
6
Direct Diode Detection – The principle
Signals of both peak detectors
Detector signal difference
 = 0,  = 0.01
q = 0.1, Cf = Cpu
 =T
 = 100 T
4 bunches
 = 100 T
M.Gasior, CERN-AB-BI
Base-Band Tune (BBQ) Measurement System
7
Direct Diode Detection – Diodes not perfect
Signals of both peak detectors
Detector signal difference
 = 0,  = 0.01
q = 0.1, Cf = Cpu
4 bunches
 = 100 T
beam
not centered
one bunch
10 % larger
M.Gasior, CERN-AB-BI
Base-Band Tune (BBQ) Measurement System
8
Direct Diode Detection – Base band spectrum
q  0.5
q  0.5
T

1  exp   j 2π f T  


S f ( f ) 
1  j 2π f T
M.Gasior, CERN-AB-BI
f 
Sf  r 
T 
~
3T 2
2
     4  2 2 2 coth 
S f  fr 
T π 
 2 
Base-Band Tune (BBQ) Measurement System
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Architecture of the Base Band Q (BBQ) Measurement System
Detector box (for one PU electrode)
M.Gasior, CERN-AB-BI
Analog front-end box (2 channels)
Base-Band Tune (BBQ) Measurement System
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BBQ systems at CERN
LHC, SPS
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
PS, PSB, LEIR
Normalized magnitude [dB]
0
SPS
PSB
-20
-40
PS
-60
-80
-100
3
5
1
2 3
5
10
2 3
5
100 2 3
5 1000 2 3
5 10000
Frequency [kHz]



BBQ system operational at RHIC
Tested at Tevatron
Will be operational at the CNAO hadrontherapy machine
M.Gasior, CERN-AB-BI
Base-Band Tune (BBQ) Measurement System
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SPS BBQ
M.Gasior, CERN-AB-BI
Base-Band Tune (BBQ) Measurement System
12
SPS BBQ – Transverse damper noise
1 bunch LHC pilot,
 5109 p+,
26-450 GeV
Damper system ON
No explicit beam excitation
Damper system OFF
M.Gasior, CERN-AB-BI
Base-Band Tune (BBQ) Measurement System
13
SPS BBQ – Low-pass filtering
(no filters)
 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
M.Gasior, CERN-AB-BI
Base-Band Tune (BBQ) Measurement System
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PS
EASTB, regular kick every 10 ms
M.Gasior, CERN-AB-BI
TOF, regular kick every 10 ms
Base-Band Tune (BBQ) Measurement System
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PSB
LHC25A, R3, no kick
M.Gasior, CERN-AB-BI
Same, kick 20 V (a % of the standard kick)
Base-Band Tune (BBQ) Measurement System
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LEIR
NOMINAL, regular kick 500 V, every 10 ms
M.Gasior, CERN-AB-BI
Same, no kick
Base-Band Tune (BBQ) Measurement System
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(Sound card) Record of the RHIC BBQ signals
ramp
squeeze
RF switching
injections
Q' too small
Horizontal plane (L)
Vertical plane (R)
about 10 minutes
M.Gasior, CERN-AB-BI
Base-Band Tune (BBQ) Measurement System
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Spectra of the RHIC BBQ signals
Hor.
M.Gasior, CERN-AB-BI
Ver.
Base-Band Tune (BBQ) Measurement System
19
RHIC BBQ measurements – Collisions
Store beginning
5 hours later (end of the store)
H plane
V plane
M.Gasior, CERN-AB-BI
Base-Band Tune (BBQ) Measurement System
H plane
V plane
20
Conclusions
BBQ advantages
BBQ disadvantages
Sensitivity (noise floor in the nm range for intense beams)

Interference: operation in the low frequency range


It is sensitive to the "bunch majority“
(gating needed to measure separate bunches)
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)
M.Gasior, CERN-AB-BI
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)
Base-Band Tune (BBQ) Measurement System
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Extra slides: Direct Diode Detection – SNR limits
 = 100 T

GD 
2
VnC T R f C f C pu 1  exp  j 2π q  T ( R f C f )

1  j 2π q
π C pu  C f



4k 
T 2 R f 2  2 e I RD 
 I nA 2 


R
f


VnA 2 
2
2
T  ( 2π q R f C f )
M.Gasior, CERN-AB-BI
Base-Band Tune (BBQ) Measurement System
22
Extra slides: PS BBQ – Detector DC voltages
AD
SFTPRO
TOF
M.Gasior, CERN-AB-BI
Base-Band Tune (BBQ) Measurement System
23
Measurement by P. Cameron (BNL)
Extra slides: RHIC BBQ – Tune scan
M.Gasior, CERN-AB-BI
Base-Band Tune (BBQ) Measurement System
24
Extra slides: RHIC BBQ – Mains ripple in the beam spectrum
180 Hz
360 Hz
720 Hz
BBQ near transition
Million turn BPM near transition
The BBQ sensitivity was
estimated to be better than
10 nm
f [Hz]
Measurement by P. Cameron (BNL)
M.Gasior, CERN-AB-BI
RHIC BBQ compared to a million turn BPM
Base-Band Tune (BBQ) Measurement System
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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 a
50 Hz multiple increases beam oscillations!
50Hz
No explicit
excitation
M.Gasior, CERN-AB-BI
Base-Band Tune (BBQ) Measurement System
26
Extra slides: PS BBQ – Mains ripple in the beam spectrum
10 lines spaced
by 100 Hz
2 injections, 6 bunches,  81012 p+/ bunch,
1.4-26 GeV, splitting into 72 bunches
M.Gasior, CERN-AB-BI
Base-Band Tune (BBQ) Measurement System
27
Extra slides: LEIR – Beam not (too much) bunched
H plane
V plane
200 ms after injection, no kicks, average on 100 cycles
M.Gasior, CERN-AB-BI
Base-Band Tune (BBQ) Measurement System
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