exotic bunch spacing for scrubbing
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Exotic Bunch spacing for scrubbing. RF considerations P. Baudrenghien , J. Esteban Mueller, D. Valuch BE-RF. Sure we can deal with any bunch spacing multiple of 2.5 ns. The LHC RF is a single-frequency system at 400 MHz and can therefore deal with bunches spaced by multiples of 2.5 ns - PowerPoint PPT PresentationTRANSCRIPT
LBOC
EXOTIC BUNCH SPACING FOR SCRUBBING
RF considerationsP. Baudrenghien, J. Esteban Mueller, D. Valuch
BE-RF
Nov 26, 2013
Sure we can deal with any bunch spacing multiple of 2.5 ns
• The LHC RF is a single-frequency system at 400 MHz and can therefore deal with bunches spaced by multiples of 2.5 ns
• However the subsequent questions are• Can this be done without major upgrade to the LLRF system that
“assumed” a 25 ns bunch spacing? Subject of this talk.• Can the SPS and/or the SPS-LHC transfer be adapted so that
“exotic” beams are captured with reasonable losses? Subject of Juan’s talk, next LBOC
Nov 26, 2013 LBOC
The LHC RF system (IP4)Nov 26, 2013 LBOC
Faraday Cages
Kly
Ant
to SUM
Cavity Controller
Cav
Kly
Ant
to SUM
Cavity Controller
Cav
Kly
Ant
to SUM
Cavity Controller
Cav
Phase PU
SUM
Fiber (400 M
Hz and F
rev ref)
Tunnel IP4
Beam 1
UX45 cavern
Kly
Ant
Cav
Kly
Ant
Cav
Kly
Ant
Cav
Phase PUBeam 2
to SUM
Cavity Controller
to SUM
Cavity Controller
to SUM
Cavity Controller
SUM
RF Synchronization Beam Control beam 2Beam Control beam 1
Fiber (400 M
Hz and F
rev ref)
Fib
ers
to S
PS
Surface building SR4
cabl
e
cabl
e
cable
cabl
e
Distance ~ 500 m
One Beam Control system per ring.· I t averages over all bunches· I t updates once per turn· I t generates a fixed amplitude RF reference signal that tracks the
momentum ramp· I t uses signals f rom a phase PU to minimize the eff ect of RF noise
For each cavity, a f ast local loop keeps the voltage at the desired set-point f or each bunch
Nov 26, 2013 LBOC
Faraday Cages
Kly
Ant
to SUM
Cavity Controller
Cav
Kly
Ant
to SUM
Cavity Controller
Cav
Kly
Ant
to SUM
Cavity Controller
Cav
Phase PU
SUM
Fiber (400 M
Hz and F
rev ref)
Tunnel IP4
Beam 1
UX45 cavern
Kly
Ant
Cav
Kly
Ant
Cav
Kly
Ant
Cav
Phase PUBeam 2
to SUM
Cavity Controller
to SUM
Cavity Controller
to SUM
Cavity Controller
SUM
RF Synchronization Beam Control beam 2Beam Control beam 1
Fiber (400 M
Hz and F
rev ref)
Fib
ers
to S
PS
Surface building SR4
cabl
e
cabl
e
cable
cabl
e
Distance ~ 500 m
One Beam Control system per ring.· I t averages over all bunches· I t updates once per turn· I t generates a fixed amplitude RF reference signal that tracks the
momentum ramp· I t uses signals f rom a phase PU to minimize the eff ect of RF noise
For each cavity, a f ast local loop keeps the voltage at the desired set-point f or each bunch
The fast local loop in UX45 is clocked at 40 MSPS but it does not use beam signal. It will work with any bunch pattern.
Nov 26, 2013 LBOC
Faraday Cages
Kly
Ant
to SUM
Cavity Controller
Cav
Kly
Ant
to SUM
Cavity Controller
Cav
Kly
Ant
to SUM
Cavity Controller
Cav
Phase PU
SUM
Fiber (400 M
Hz and F
rev ref)
Tunnel IP4
Beam 1
UX45 cavern
Kly
Ant
Cav
Kly
Ant
Cav
Kly
Ant
Cav
Phase PUBeam 2
to SUM
Cavity Controller
to SUM
Cavity Controller
to SUM
Cavity Controller
SUM
RF Synchronization Beam Control beam 2Beam Control beam 1
Fiber (400 M
Hz and F
rev ref)
Fib
ers
to S
PS
Surface building SR4
cabl
e
cabl
e
cable
cabl
e
Distance ~ 500 mThe beam control in SR4 uses PU signal and will be affected by the bunch pattern.
Beam Control• The LHC Beam Phase Loop:
• With 25 ns spaced bunches, we measure the phase of each bunch individually
• Then we average over the desired pattern using a mask
• The result updates the RF frequency at every turn
Nov 26, 2013 LBOC
R1
Low-levelLoops
Processor
Radial PUFront-end
DDSor
VCXO
Phase Discri
F RF Prog 1
Radial loop
Phase loop
Synchro loop
10 MHz ref.
DDS1 DDS2
Sync
F1,P
1
F2,P
2
F1 F2
1/h divider
Frev Prog
Master F rev
To Ring 1 Cavity Controllers (fibers)
Dual Frequency Program and
Rephasor FPGABeam
Parameters Processor
Phase Noise Generator.
Function Gen. Function Gen.
F RF Prog 2
DUAL FREQUENCY
PRGM
Master F RF
Beam 1
Ib
Rad. PU
RF Summing Network
Vt
Phase PU
X
AD
C
Var. Gain IF Amplifier
Digital IQ Demod
CORDIC +
AGC
Gain cntrl
Bunch/RF phase
X
Var. Gain IF Amplifier
Digital IQ Demod
CORDIC +
AGC
Gain cntrl
Vt/RF phase
Phase Difference
and Averaging
PHASE DISCRI MODULE
Stable Phase
Beam/Vt phase
Beam parameters: s, bucket size, ...
Vt avg
Pin
k no
ise
RF/Fprog phase
F out
Rad
Pos
.
Encoder
Fiber Optic TX
BEAM CONTROL
LOOPS MODULE
BEAM PARAMETERS
MODULE
Beam Control. Simplified block diagram.
Technology: DSP
FPGA or CPLD (40 or 80 MHz)
Analog RF
Signals: Digital Analog
s
AD
C
Cavities
Radial steering with radial loop
See also alternative with analog I/Q demod
Coarse F1
B field
PU
XRF
BPF@RF
LPF S/Hf
Ck
SPS beam phase loop• Beam phase = phase of the RF component of beam
current• Single bunch response:
Nov 26, 2013 LBOC
The 200 MHz BPF has a short impulse response (250 ns FWHM). It effectively averages over 10 bunches (25 ns spacing)
PU
XRF
BPF@RF
LPF S/Hf
Ck
SPS (cont’d)• Batch response:
Nov 26, 2013 LBOC
The phase measurement is an average over the last ~ 10 bunches before the sampling
In the SPS, the averaging over bunches is done in the RF domain, before extraction of the phase information
PU
RF
BPF@RF
LPF ADCI
Ck
IQ Demod
LPF ADCQ
Ck
LHC bunch phase loop• Bunch phase = phase of each bunch, if 25 ns spacing (or multiple)• Beam phase = average over the individual phases, using a dynamic
bunch mask
Nov 26, 2013 LBOC
The 400 MHz BPF has a short impulse response (20 ns FW), so that bunches spaced at 25 ns do not couple
The ADCs are clocked ~ 20 ns after the passage of the bunch. They give clean (I,Q) coordinates of the corresponding wavelet, from which the phase is extracted
The LPF reduces the noise added by the demodulation. Their step response is 10-20 ns
The IQ Demodulator gives the coordinates of the wavelet, in a system with the RF on the x-axis
In the LHC, the averaging is done on the phase measurements
PU
RF
BPF@RF
LPF ADCI
Ck
IQ Demod
LPF ADCQ
Ck
5 + 20 ns spacing
Nov 26, 2013 LBOC
Below, the BPF output caused by bunch1 alone (assumed 0 degree phase @ 400 MHz) and bunch2 alone (assumed 90 degrees phase shift @ 400 MHz)
The measurement correctly averages over the two bunches if the risetime of the LPF is below 15 ns.
Atop, the BPF output sum (blue) and the equivalent with a single bunch at 45 degrees phase shift (red).
Sample here
Two bunches, 5 ns spacing
The averaging is first done on the RF signal for bunch pairs, then on the individual phase measurements
PU
RF
BPF@RF
LPF ADCI
Ck
IQ Demod
LPF ADCQ
Ck
2.5 + 22.5 ns spacing
Nov 26, 2013 LBOC
Bunch1 (assumed 0 degree phase @ 400 MHz) is followed by bunch2 (assumed 90 degrees phase shift @ 400 MHz) after 2.5 ns
The measurement correctly averages over the two bunches if the risetime of the LPF is below 17.5 ns.
Atop, the BPF output sum (blue) and the equivalent with a single bunch at 45 degrees phase shift (red).
Two bunches, 5 ns spacing
Sample here!
The use of a 400 MHz BPF gives a usable wavelet for any bunch spacing compatible with the RF buckets
Conclusions (1)• With 2.5+22.5 or 5+20 bunch spacing the existing phase
measurements (every 25 ns) will average over the bunch pairs
• To perform best, the LPF BW must be increased. This can be prepared in the lab in 2014
• A small “leaking” in the next 25 ns measurement must be expected as the BPF impulse response is not strictly zero after 20 ns
• The phase loop can be ON at injection with 5+20 ns scheme. It must be OFF with 2.5+22.5 (bunch splitting at LHC injection) for the incoming batch, but ON for the circulating batches. This “dynamic masking” was tested during MD in 2013 (Batch by batch blow-up at injection)
Nov 26, 2013 LBOC
Conclusions (2)
• Capture losses are an issue with both schemes. The capture voltage can be increased (12 MV)
• With 5+20 ns spacing, the bunch length must stay below 1.8 ns (4 sigma). If that is possible capture losses will not be larger than with standard 25 ns spacing
• The 2.5+22.5 ns scenario calls for non-adiabatic splitting in the LHC (injection on the 0 degree stable phase) and capture losses cannot be avoided. RF gymnastics are proposed to limit these losses. See Juan’s talk at the next meeting
Nov 26, 2013 LBOC
THANK YOU FOR YOUR ATTENTION
Nov 26, 2013 LBOC