Transcript Talk
M.Gasior, CERN-AB-BI
Base-Band Tune (BBQ) Measurement System
Marek Gasior Beam Instrumentation Group, CERN 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
s c
(
t
) cos( 2 π
f b t
)
s b
1 (
t
)
n
(
t
nT
)
s o
1 (
t
)
n
(
t
nT
)
S c
(
f
) 1 2
T S b
1 (
f
f b
)
n
f
f b
n T
S b
1 (
f
f b
)
n
f
f b
n T
1
T S o
1 (
f
)
n
n T
bunch spectrum 3 dB cut off 0 .
133 bunch length 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”)
f rev
content converted to the DC and removed by series capacitors
f b
modulation moved to a low frequency range (as after the diodes
f b
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
f rev
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 5
Electrode 1 signal
Direct Diode Detection – The principle
s d
(
t
)
s s b
1 (
t
)
b
2 (
t
)
s b s b
(
t
) ( 1 (
t
) ( 1 ) ) 1 1 cos( 2 π cos( 2 π
f b f b t t
) ) beam relative offset = 0.1
betatron oscillation relative amplitude = 0.05
simulated tune value
q
= 0.1
filter time constant = 10
T
(
T
storage capacitor
C f
– revolution period) =
C pu
(PU electrode capacitance) Electrode 2 signal Signals of both peak detectors M.Gasior, CERN-AB-BI Base-Band Tune (BBQ) Measurement System Detector signal difference 6
Direct Diode Detection – The principle
Signals of both peak detectors Detector signal difference = 0, = 0.01
q
= 0.1,
C f
=
C pu
= T
M.Gasior, CERN-AB-BI Base-Band Tune (BBQ) Measurement System
= 100 T 4 bunches
= 100 T
7
Direct Diode Detection – Diodes not perfect
Signals of both peak detectors Detector signal difference = 0, = 0.01
q
= 0.1,
C f
=
C pu
4 bunches
= 100 T
M.Gasior, CERN-AB-BI Base-Band Tune (BBQ) Measurement System
beam not centered one bunch 10 % larger
8
Direct Diode Detection – Base band spectrum
q
0.5
q
0.5
S f
(
f
) 1 exp 1 j j 2π 2π
f f T T
T
M.Gasior, CERN-AB-BI ~
S f S f f r f
2
r
4
T
2 3
T
2 π 2 2 coth
T
2 Base-Band Tune (BBQ) Measurement System 9
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 10
BBQ systems at CERN Machine LHC SPS PS LEIR PSB Front-End “constant f
rev
type” “constant f
rev
type” “constant f
rev
type” “varying f
rev
type” “varying f
rev
type” Acquisition 24 bits (up to 100 kHz) 24 bits 16 bits (up to 40 MHz) 16 bits 16 bits
0 SPS PSB -20 -40 -60 -80 -100 PS 3 5 1 2 3 5 10 2 3 5 100 2 3 Frequency [kHz] 5 1000 2 3 5 10000
LHC, SPS
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
PS, PSB, LEIR
11
SPS BBQ
M.Gasior, CERN-AB-BI Base-Band Tune (BBQ) Measurement System 12
SPS BBQ – Transverse damper noise
Damper system OFF 1 bunch LHC pilot, 5 10 9 p + , 26-450 GeV Damper system ON 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 14
PS
M.Gasior, CERN-AB-BI
EASTB, regular kick every 10 ms TOF, regular kick every 10 ms
Base-Band Tune (BBQ) Measurement System 15
PSB
M.Gasior, CERN-AB-BI
LHC25A, R3, no kick Same, kick 20 V (a % of the standard kick)
Base-Band Tune (BBQ) Measurement System 16
LEIR NOMINAL, regular kick 500 V, every 10 ms
M.Gasior, CERN-AB-BI Base-Band Tune (BBQ) Measurement System
Same, no kick
17
(Sound card) Record of the RHIC BBQ signals Horizontal plane (L)
injections M.Gasior, CERN-AB-BI ramp squeeze
Vertical plane (R)
about 10 minutes Base-Band Tune (BBQ) Measurement System 18
Spectra of the RHIC BBQ signals Hor.
Ver.
M.Gasior, CERN-AB-BI Base-Band Tune (BBQ) Measurement System 19
RHIC BBQ measurements – Collisions Store beginning 5 hours later (end of the store)
H plane H plane M.Gasior, CERN-AB-BI V plane Base-Band Tune (BBQ) Measurement System V plane 20
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) M.Gasior, CERN-AB-BI Base-Band Tune (BBQ) Measurement System 21
Extra slides: Direct Diode Detection – SNR limits
= 100
T G D
V nC T
π
R C f C pu f C pu
C f
1 exp j 2π
q
T
(
R f
1 j 2π
q V nA
2
T
2
R f
2 2
e I RD
4
k
R f
I nA
2
T
2 ( 2 π
q R f C f
) 2
C f
) 2 M.Gasior, CERN-AB-BI Base-Band Tune (BBQ) Measurement System 22
M.Gasior, CERN-AB-BI
Extra slides: PS BBQ – Detector DC voltages
AD SFTPRO TOF Base-Band Tune (BBQ) Measurement System 23
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 Measurement by P. Cameron (BNL)
M.Gasior, CERN-AB-BI
f [Hz] RHIC BBQ compared to a million turn BPM
Base-Band Tune (BBQ) Measurement System 25
Extra sides: SPS BBQ – mains ripple in the beam spectrum
72 LHC bunches, 10 11 coasting (RF on) p + / bunch, 270 GeV, 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
M.Gasior, CERN-AB-BI 10 lines spaced by 100 Hz 2 injections, 6 bunches, 8 10 12 p + / bunch, 1.4-26 GeV, splitting into 72 bunches Base-Band Tune (BBQ) Measurement System 27
Extra slides: LEIR – Beam not (too much) bunched
H plane V plane
M.Gasior, CERN-AB-BI
200 ms after injection, no kicks, average on 100 cycles
Base-Band Tune (BBQ) Measurement System 28