Post on 20-Feb-2016
description
Base-Band Tune (BBQ) Measurement System 1M.Gasior, CERN-AB-BI
Base-Band Tune (BBQ)
Measurement System
Marek Gasior
Beam Instrumentation Group, CERN
Base-Band Tune (BBQ) Measurement System 2M.Gasior, CERN-AB-BI
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)
Base-Band Tune (BBQ) Measurement System 3M.Gasior, CERN-AB-BI
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
Base-Band Tune (BBQ) Measurement System 4M.Gasior, CERN-AB-BI
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
Base-Band Tune (BBQ) Measurement System 5M.Gasior, CERN-AB-BI
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)
Base-Band Tune (BBQ) Measurement System 6M.Gasior, CERN-AB-BI
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)
Base-Band Tune (BBQ) Measurement System 7M.Gasior, CERN-AB-BI
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
Base-Band Tune (BBQ) Measurement System 8M.Gasior, CERN-AB-BI
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
Base-Band Tune (BBQ) Measurement System 9M.Gasior, CERN-AB-BI
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
Base-Band Tune (BBQ) Measurement System 10M.Gasior, CERN-AB-BI
Architecture of the Base Band Q (BBQ) Measurement System
Analog front-end box (2 channels)Detector box (for one PU electrode)
Base-Band Tune (BBQ) Measurement System 11M.Gasior, CERN-AB-BI
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
Base-Band Tune (BBQ) Measurement System 12M.Gasior, CERN-AB-BI
SPS BBQ
Base-Band Tune (BBQ) Measurement System 13M.Gasior, CERN-AB-BI
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
Base-Band Tune (BBQ) Measurement System 14M.Gasior, CERN-AB-BI
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)
Base-Band Tune (BBQ) Measurement System 15M.Gasior, CERN-AB-BI
PS
EASTB, regular kick every 10 ms TOF, regular kick every 10 ms
Base-Band Tune (BBQ) Measurement System 16M.Gasior, CERN-AB-BI
PSB
LHC25A, R3, no kick Same, kick 20 V (a % of the standard kick)
Base-Band Tune (BBQ) Measurement System 17M.Gasior, CERN-AB-BI
LEIR
NOMINAL, regular kick 500 V, every 10 ms Same, no kick
Base-Band Tune (BBQ) Measurement System 18M.Gasior, CERN-AB-BI
(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
Base-Band Tune (BBQ) Measurement System 19M.Gasior, CERN-AB-BI
Spectra of the RHIC BBQ signals
Hor. Ver.
Base-Band Tune (BBQ) Measurement System 20M.Gasior, CERN-AB-BI
RHIC BBQ measurements – Collisions
Store beginning
H plane
V plane
5 hours later (end of the store)
H plane
V plane
Base-Band Tune (BBQ) Measurement System 21M.Gasior, CERN-AB-BI
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)
Base-Band Tune (BBQ) Measurement System 22M.Gasior, CERN-AB-BI
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
Base-Band Tune (BBQ) Measurement System 23M.Gasior, CERN-AB-BI
Extra slides: PS BBQ – Detector DC voltages
AD
SFTPRO
TOF
Base-Band Tune (BBQ) Measurement System 24M.Gasior, CERN-AB-BI
Extra slides: RHIC BBQ – Tune scan
Mea
sure
men
t by
P. C
amer
on (B
NL)
Base-Band Tune (BBQ) Measurement System 25M.Gasior, CERN-AB-BI
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
Base-Band Tune (BBQ) Measurement System 26M.Gasior, CERN-AB-BI
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
Base-Band Tune (BBQ) Measurement System 27M.Gasior, CERN-AB-BI
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
Base-Band Tune (BBQ) Measurement System 28M.Gasior, CERN-AB-BI
H plane
V plane
200 ms after injection, no kicks, average on 100 cycles
Extra slides: LEIR – Beam not (too much) bunched