Transcript RF System LLRF, Noise and Commissioning at cold CERN AB-RF

RF System
LLRF, Noise and Commissioning at cold
Reported by P. Baudrenghien
CERN AB-RF
Dec 6, 2007
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Outline

Introduction




Cavities
Klystrons
LLRF Layout
RF noise and the Low Level RF

Cavity Controller
 RF feedback


Cavity field phase and amplitude noise. Lifetime
Effective Cavity impedance. Instability threshold
Klystron Polar Loop
Beam Control
 Phase Loop, Radial loop and Synchro Loop



Commissioning at cold
Dec 6, 2007
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1. Introduction
Dec 6, 2007
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1.1 Cavities


Two independent rings
8 RF cavities per ring at 400.790 MHz [1]:


Super Conducting Standing Wave Cavities, single-cell, R/Q = 45
ohms, 5.5 MV/m nominal
Movable Main Coupler (12000 < QL < 180000)



Cavities in LSS4
1 MV /cavity at injection with QL = 20000
2 MV/cavity during physics with QL = 60000
Mechanical Tuner range > 200 kHz
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1.2 Driver+Klystron CW
Pout
Phase
shift
Pin
Klystron power sweep CW @ 400.8 MHz

1 klystron per cavity




Dec 6, 2007
330 kW max (58 kV, 8.4 A)
130 ns group delay (~ 10 MHz BW)
CW gain 39 dB @ 200 kW, 36 dB @ 300
kW
In operation < 200 kW CW
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Driver + Klystron Frequency Sweep
200 kW
gain
300 kW
Network
Analyzer
Synthesizer
-40
dB
Combiner
200 kW
phase
added
small freq
sweep
CW @ 400.8 MHz
300 kW
circ
Klystron freq response





Freq response measurement set-up
Small signal frequency response with CW power 200 kW and 300 kW
Gain drops by ~5 dB
Phase shift ~ 25 degrees @ 400 MHz
Strong (20 dB) resonance around 404.8 MHz (bunching cavity)
Acceptable for inclusion in a feedback loop…
Dec 6, 2007
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Fibers to SPS
1.3 LLRF Layout
Beam Control beam 1
RF Synchronization
Beam Control beam 2
cable
UX45 cavern
SUM
Cavity
Controller
to SUM
Cavity
Controller
Kly
to SUM
Cavity
Controller
Kly
Cavity
Controller
Cavity
Controller
to SUM
Ant
Cav
Beam 1
Cav
Cavity
Controller
to SUM
Kly
Kly
Ant
Cav
Ant
Ant
Cav
Cav
Tunnel
Phase PU
Dec 6, 2007
to SUM
Kly
Kly
Ant
cable
SUM
Faraday Cages
to SUM
Rad PU
cable
cable
cable
Distance ~ 500 m
Fiber (400 MHz and Frev ref)
Fiber (400 MHz and Frev ref)
cable
Surface building SR4
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Ant
Cav
Beam 2
Phase PU
Rad PU
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2. RF noise and the Low Level RF
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2.1 Cavity Controller



One system per cavity (2 VME
crates)
Located in a Faraday Cage in the
UX45 cavern
Control phase and amplitude of
cavity voltage. Disturbances come
from





HT ripples from Power Converters:
1 % HT ripple -> 8.4 degrees @
400.8 MHz
Transient Beam loading
Cavity Controllers in FC
Keep demanded klystron power
reasonable (300 kW max).
Largely digital implementation
Operates at the bunch rate (40
Msps)
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Cavity Controller Loops [2]







RF Feedback Loop: Reduces the cavity impedance at the fundamental
(by 20 linear for Q=20000, by 180 at Q = 180000). Precision of RF
voltage, transient beam loading and longitudinal stability
Klystron Polar Loop: Compensates for the klystron gain/phase changes.
(HT drifts and ripples, 50 Hz components and multiple).
1-T Feedback: Adds factor 10 reduction on the revolution frequency
side-bands. (Transient beam loading + longitudinal stability)
Tuner Loop: Minimizes klystron current. (Half detuning keeps the
modulus of klystron current constant during beam segments and gaps)
Set Point: Customizes the voltage for each bunch, phase and
amplitude. (Each bunch slightly displaced with respect to a constant
spacing [3])
Conditioning: Automatic conditioning system integrated.
Longitudinal damper: Damps the injection phase and momentum errors.
Acts on 400 MHz cavities.
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RF permitted
SWITCH/LIMITER
CONDITIONING DDS
LIMITER
SWITCH
Circ
Pin
TUNER LOOP
Master F RF
Phase
Shift
Ic fwd
Digital
IQ
Demod
Fwd
Digital
IQ
Demod
X
Ic rev
Dir.
Coupler
Tuner
Processor
Rev
Digital
IQ
Demod
X
Tuner Control
Digital
IQ
Demod
Digital RF feedback (FPGA)
60 dB
DAC
1 kHz
SUM
SET
POINT
SUM
40 dB
20 dB
SUM
Set Point
Generation
dV
I0
Q0
SUM
1 kHz
1-Turn Feedback
Cavity Servo Controller.
Simplified Block Diagram
DAC
Phase
Equalizer
Vcav
dp
From long.
Damper
RF FEEDBACK
Voltage
fct
Analog IQ
Demodulator
I Q
Analog RF feedback
Single-Cell
Superconducting
Cavity
X
Klystron
Polar Loop
(1 kHz BW)
Ic fwd
Digital
IQ
Demod
Gain Cntrl
X
ADC
SUM
Ig fwd
X
RF
Phase Shifter
noise
Digital
IQ
Demod
DAC
DAC
RF MODULATOR
X
Var Gain RF
Ampifier
Analog IQ
Modulator
Baseband
Network
Analyzer
300 kW Klystron
Pout
ANALOG
DEMOD
ADC
1-Turn Feedforward
Analog:
Digital I/Q pair:
Analog I/Q pair:
DAC
Technology:
DSP
CPLD or FPGA (40 or 80 MHz)
Analog RF
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ADC
Analog IQ
Demodulator
Signals: Digital:
Wideband
PU
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2.1.1 Performances of the RF Feedback
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RF Feedback Open Loop




Hybrid feedback: Digital part provides
precision at low speed while wideband
analog part provides bandwidth.
Optimal Open-Loop gain = 20 (Q=20000),
120 linear (Q=180000)
Notches at +-4.3 MHz offset.
Compensates for the klystron bunching
cavity resonance.
20 dB (10 linear) gain increase in 4 kHz
band around centre frequency for
precision
Full Fdbk OL response 10 MHz band
Dig Fdbk OL response 10 kHz band
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Noise and ripple reduction with RF feedback
Phase
Vcav/Ref
Open Loop 10 mV/div -> 5 dg pk-pk, 5
Closed Loop 2 mV/div -> 0.1 dg pk-pk, 5
ms/div (File PhaseMeasOpen 14 March 2007) ms/div (PhaseMeasAtt_0A 14 March 2007)





Phase noise Vcav vs ref.
Measured in the SM18 test stand, March 2007
Calibration: 10 mV/dg @ 400 MHz
Q=60000, 1 MVacc, 35 kW
Tuner Loop On, Klystron Polar Loop Off
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



Power Spectral Density of
cavity phase noise (Vcav vs
Ref) with and without RF
feedback
ZLW1 mixer and Spectrum
Analyzer
300 mV/dg @ 400 MHz
50 dB reduction of 600 Hz line
Open Loop vs Close Loop. 50 dB
reduction @ 600 Hz
(File PhaseNoise3 15 March 2007)
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Expected lifetime from simulations
(J. Tuckmantel [4])

RF feedback alone (further improvement with Klystron
Polar Loop)

Coast at 7 TeV/c with 16 MV, 2.5 eVs (fs0=23 Hz).



Crossing the 50 Hz line during ramp: during ~ 1 min, 50 Hz falls
inside fs band. Dangerous



Blow-up rate below the 24 h synchrotron radiation damping
time.
1 ps rms white noise just compensates synchrotron radiation
damping. We measure 2.4 10-2 ps rms from DC to frev =11250 Hz .
0.2 % rms emittance increase
If amplitude of 50 Hz line is increased by 10 linear, we get 27 %
emittance increase with bunch centre reduced in population
Circulating beam at 450 GeV/c with 8 MV, 0.7 eVs (fs0=63 Hz).


Dec 6, 2007
50 Hz line multiples do not hit the populated synchrotron frequency
band -> no significant effect observed in simulations
1 ps rms white noise now gives 0.1 % loss after 1 hour.
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Step response
Vcav Q
Vcav I
Ig
400 kV in 10 ms, 2 ms per div
(File StepQ6 15 March 2007)
100 kV in 3 ms, 2 ms per div
(File VcavStep100kV_Q 14 March 2007)




1 MV in I, step in Q
Q=60000, 1 MV asks for ~ 40 kW
Observe Vcav I and Q plus klystron drive @ 400 MHz
Linear regime: 70 kV in 1 ms for Q=20000
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Small Signal Freq Resp (Fdbk close)
Gain
Q=20000
Q=180000
Phase
Centre 400.8 MHz, 2 MHz span



Red: 1 MVacc, Q=20000, G=20
Blue: 2 MVacc, Q=180000, G=120
Close Loop Q = 700, 600 kHz -3 dB Bw, independent of cavity Q
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Longitudinal Instability threshold

With RF feedback alone we get 45 kohm/cavity. This gives a stable
current limit (E. Shaposhnikova [7]) of




1/6 th nominal (0.56 A DC or 1.1 1011 per bunch) for 0.7 eVs up to 7 TeV
1/3 rd nominal for 1 eVs up to 7 TeV
2 times nominal for optimal blow-up 0.7-2.5 eVs during ramp. In optimal
blow-up the emittance should increase proportional to the square root of the
energy: From 0.7 eVs at 450 GeV to 2.5 eVs at 7 TeV. Controlled emittance
blow-up must be done with phase noise injected in the 400 MHz cavities. To
be optimized …
1-T feedback adds a further 20 dB reduction in ~ 1 MHz, 2-sided BW
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2.1.2 Performances of the Klystron Polar
Loop
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Adjusting the Klystron loop gains
Gain Cntrl
Phase Cntrl
Gain control: Response to a step in
klystron gain. Bottom trace =
phase control. 20 ms /div
Phase control: Response to a 90
degrees phase shift. 20 ms /div


Gain and phase loops react in ~ 10 ms. (Reminder: RF fdbk time constant ~
1 ms, Tuner loop time constant ~1 s).
Klystron Polar loop is automatically switched OFF if Switch&Protect module
limits the drive.
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Static behavior (phase)



Measured on SM18 test stand
Phase shift @ 400 MHz function of klystron HV
Klystron Polar Loop open. Theory: 8.4 degree @
400.8 MHz per percent HV drift @ 50 kV
52.9 kV -> 34 degree @ 400.8 MHz
51.9 kV -> 17.4 degree @ 400.8 MHz
50.9 kV -> 0 degree @ 400.8 MHz
49.8 kV -> -36.3 degree @ 400.8 MHz
47.8 kV -> -74.4 degree @ 400.8 MHz

Klystron Polar Loop close
52.9 kV -> -0.1 degree @ 400.8 MHz
51.9 kV -> 0.0 degree @ 400.8 MHz
49.9 kV -> 0.0 degree @ 400.8 MHz
47.9 kV -> 0.0 degree @ 400.8 MHz
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Static behavior (gain)

Keep modulator input constant, observe klystron output power @
400 MHz function of klystron HV and Icath.
Dec 6, 2007
Loop open
Loop closed
Icath=6.4 A
HV
51.5 kV
46.4 kV
41.3 kV
Pg
123 kW
117 kW
102 kW
Pg
109 kW
109 kW
109 kW
HV=50 kV
I cath
4.4 A
5.1 A
5.8 A
6.3 A
Pg
44 kW
67 kW
94 kW
126 kW
Pg
109 kW
109 kW
109 kW
109 kW
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Compensation for HV ripples (phase)
Phase
Noise
Phase
compensation
Loop open. Phase noise Ig-Ref:
Mainly 100 Hz and 600 Hz due to
HV ripples. Calib 10 mV/dg @
400 MHz. ~3.5 dg pkpk (10
mV/div, 5 ms /div)


Loop closed. Red trace = phase
noise Ig-Ref. Calib 10 mV/dg @
400 MHz. ~0.2 dg pkpk (2 mV/div,
5 ms /div). Blue trace = phase
compensation.
Measured with HP8508 Vector voltmeter
Calibration: 10 mV/dg @ 400 MHz
Dec 6, 2007
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Spectrum of klystron output phase noise
PSD in dBV2/Hz, 10 dB/div, DC to
1 kHz. Phase noise Ig-Ref: Bright
trace = klystron loop On,
background trace = loop off.
Measured reduction 30 dB @ 600
Hz



PSD in dBV2/Hz, 10 dB/div, DC to
200 Hz. Phase noise Ig-Ref: Bright
trace= klystron loop On, background
trace = loop off. Measured reduction
50 dB @ 100 Hz
Measured with HP8508 Vector voltmeter plus HP3562A spectrum analyzer
Calibration: 10 mV/dg @ 400 MHz
Observe 30 dB reduction @ 600 Hz and 50 dB reduction @ 100 Hz
Dec 6, 2007
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Compensation for HV ripples (amplitude)
Modulus
Ig
Loop open. Amplitude noise Ig:
42 Vpkpk ripple on a 3 kV
klystron output (50 ohm). (2
mV/div, 10 ms /div)

Loop closed. Amplitude noise Ig.
(2 mV/div, 10 ms /div)
Measured with HP8508 Vector voltmeter
Dec 6, 2007
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Spectrum of klystron output amplitude noise
PSD in dBV2/Hz, 7 dB/div, DC to 1 kHz. Amplitude noise Ig-Ref: Bright trace=
klystron loop On, background trace = loop off. Measured reduction 35 dB @ 600 Hz


Measured with HP8508 Vector voltmeter plus HP3562A spectrum analyzer
Observe 35 dB reduction at 600 Hz
Dec 6, 2007
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2.2 The Beam Control
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Beam Control

Goal:



Keep beam centered
Minimize emittance blow-up due to phase noise (rigid beam mode)
Implementation:




One system per ring
Located on the surface (SR4)
Generates the Master RF @ 400.8 MHz of fixed amplitude but with a phase
that is adjusted continuously
Update rate at the 11 kHz revolution frequency
Dec 6, 2007
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Beam Control Architecture



A very strong phase loop compares the beam phase (Wide-Band
PU) with the Cavity Sum (RF Sum of the Antenna signals of the 8
cavities). It acts back on the input of a Voltage Controlled Crystal
Oscillator (VCXO) generating the reference RF (one per beam).
We do not use a radial loop during normal operation as this was
found too noisy in the SPS p-pbar. (But it is implemented, ready to
be used for commissioning and MDs).
At injection and during the acceleration ramp a synchronization
loop locks the VCXO output onto a Low-noise Direct Digital
synthesizer (DDS) whose frequency tracks the B field. Loop gains
are adjusted so that this latter loop is much weaker than the phase
loop.
Dec 6, 2007
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Beam parameters:
s, bucket size, ...
Radial steering with radial loop
BEAM B field
PARAMETERS
MODULE
Pilots the
rephasing
Beam/Vt phase
Radial loop
BEAM POS
MODULE
CORDIC
Gain cntrl
Var. Attn
Ib
 mag
1/h
divider
CORDIC
Master Frf
Vt
Master F RF
Master Frf
Var. Attn
Gain cntrl
Var. Attn
Fiber Optic
TX
VCXO for clean
spectrum
Signals:
Digital
Analog
Technology:
b
Beam 1
Rad. PU
F rev 2
Beam Control. Simplified
block diagram.
180 deg
hybrid
Phase PU
F rev 1
To Ring 1 Cavity Controllers
(fibers)


RF Summing Network
TDC
Master F rev
SYNCHRO
MODULE
Analog I/Q
demod
a
DSP
FPGA or CPLD (40 or 80 MHz)
Analog RF
Cavities
Dec 6, 2007
7 TeV synthesizer
Frev Prog
ADC
ADC
40 MHz
Analog I/Q
demod
Gain cntrl
Delay adjust
Ref during
physics
Check before
rephasing
Master Frf
Var. Attn
Phase
shifter
Signed
Ratio and
Averaging
F RF Prog 2
Sync
Phase
Discri
VCXO
CORDIC
Analog I/Q
demod
Master Frf
Gain cntrl
DDS1 DDS2
40 MHz
ADC
Analog I/Q
demod
ADC
ADC
ADC
ADC
 mag
AD9959
4-CH DDS
10 MHz ref.
F out
ADC
CORDIC
LF
switch
RF/Fprog phase
Rad Pos.
Bunch/RF
phase
Phase loop
Dual Frequency
Program and
Rephasor FPGA
F RF Prog 1
Phase
Discri
Synchro loop
Low-level
Loops
Processor
s
Stable Phase
Phase
Difference
and
Averaging
Vt/RF
phase
Phase Noise
Generator.
F2
F2,P2
Pink noise
Vt avg
DUAL
FREQUENCY
PRGM
Dphi Synchro
Dphi Rephasing
BEAM
CONTROL
LOOPS
MODULE
Beam
Parameters
Processor
F1
Function Gen.
F1,P1
BEAM PHASE
MODULE
Function Gen.
R1
Bunch by
bunch phase
meas. Average
over relevant
bunches
To Long damper
Coarse F1
“optimal” 400
MHz BPF
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Coarse F1
R1
Phase Loop
1 Gbps serial links
Analog I/Q
demod
ADC
ADC
CORDIC
ADC
Radial loop
CORDIC

40 MHz
Analog I/Q
demod
Master Frf

Master Frf

Gain cntrl
Var. Attn
Gain cntrl
Var. Attn

Delay adjust
Vt

Ib
RF Summing Network

Phase PU
Cavities
Dec 6, 2007
Pink noise
Phase loop
Beam/Vt phase
ADC
Bunch/RF
phase
“optimal” 400
MHz BPF
Phase Noise
Generator.
Low-level
Loops
Processor
s
Phase
Difference
and
Averaging
Vt/RF
phase
Rad Pos.
BEAM PHASE
MODULE
BEAM
CONTROL
LOOPS
MODULE
Bunch by bunch
phase meas.
Average over
relevant bunches
Bunch/RF phase
To
Long damper
Bunch heigth to
Long damper
Radial steering with radial loop
RF/Fprog phase
F out
Phase loop input = phase difference between cavity sum (8
cav) and PU
Measures the phase of each bunch (3564 possible buckets)
Applies threshold on bunch intensity to discard marginal
bunches
Averages overall the relevant bunches. A mask identifies
the relevant bunches
ON successive injections the newly injected bunches
become part of the average only after stabilization by the
longitudinal damper
The phase loop is switched ON at injection and remains ON
all the time
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Synchro Loop
Coarse F1
Function Gen.
R1
Radial steering with radial loop
Rad Pos.
Dphi Synchro
Dphi Rephasing
LF
switch

Phase
shifter
F2
AD9959
4-CH DDS
F2,P2
F1,P1

DUAL
FREQUENCY
PRGM
F1
A Dual DDS driven by function
BEAM
Dual Frequency
(magnetic field plus radial
CONTROL
Program and
LOOPS
Rephasor FPGA
steering) generates 400 MHz
MODULE
Reference for each ring.
Phase Noise
Generator.
For each ring a VCXO
DDS1 DDS2
F RF Prog 1
generates Master 400 MHz RF
Phase
Synchro loop
Sync
Low-level
Discri
Loops
sent to Cavity Controllers (fiber Phase loop Processor
10 MHz ref.
RF/Fprog phase
optic links).
Phase
F out
Discri
VCXO
A weak Synchro Loop locks the Radial loop
Master RF onto the DDS
1/h
Frev Prog
divider
Reference.
VCXO cleans the
TDC
Master F rev
During physics the DDS output
Master F RF
DDS spectrum
is replaced by a low noise
SYNCHRO
MODULE
Synthesizer.
Fiber Optic
TX
Synchro Loop is always ON
except for MD when
To Ring 1 Cavity Controllers
(fibers)
acceleration is performed with
radial loop.
Pink noise

Pilots the
rephasing
Function Gen.
F RF Prog 2
Ref during
physics
7 TeV synthesizer
Check before
rephasing


Dec 6, 2007
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F rev 1
F rev 2
33
3. Commissioning at cold
Dec 6, 2007
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UX45
Dec 6, 2007
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Conditioning in progress!
Dec 6, 2007
Courtesy of P. Maesen
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Cavity Commissioning









Warm commissioning done
Pressure test 2.1 bar done
Klystron collectors cooling system modified
Start cooling 2 modules (sector 45) on Nov 20th
2 modules cold on Nov 22nd
Low power measurements. Frequency ranges correct
except for 2 cavities. Fixed.
Vacuum OK
Cryogenics reasonably stable so far
Start conditioning one cavity Dec 4. Good progress.
Quickly reached 2 MV/m in pulsed mode
Dec 6, 2007
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Cavity Commissioning plans







Automatic conditioning (several cavities in parallell) up to
8 MV/m (nominal=5.5 MV/m)
Set up the Tuner Loop
Set up the Klystron Polar Loop
Set up the RF feedback
Set up the 1-T feedback
Goal: set the above 4 loops on one cavity before warmup (mid January).
Sector 34 cold only in March 2008
Dec 6, 2007
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References
[1] LHC Design Report, Vol 1, the LHC Main Ring, Chapter 6, CERN-2004-003V1
[2] The LHC Low Level RF, P. Baudrenghien and al., EPAC 2006, Edinburgh
[3] Adaptive RF Transient Reduction for High Intensity Beams with Gaps,
J.Tuckmantel, EPAC 2006, Edinburgh
[4] Simulation of LHC Bunches under Influence of 50-Hz multiple Lines on the
Cavity Field, Joachim Tuckmantel, LHC Project Note-404, June 6, 2007
[5] Digital Design of the LHC Low Level RF: The Tuning system for the
Superconducting Cavities, J. Molendijk, EPAC2006, Edinburgh
[6] The Tuning Algorithm of the LHC 400 MHz Superconducting Cavities, P.
Baudrenghien, CERN, AB-Note-2007-011
[7] Longitudinal beam parameters during acceleration in the LHC, E.
Shaposhnikova, CERN, LHC Project-Note-242
Dec 6, 2007
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Thank you…
Dec 6, 2007
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Additional material if questions arise
Dec 6, 2007
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Radial Loop
Coarse F1
R1
Radial steering with radial loop
BEAM
CONTROL
LOOPS
MODULE
Pink noise
Rad Pos.
40 MHz
Analog I/Q
demod
Analog I/Q
demod
Master Frf
Gain cntrl
ADC
Bunch by bunch pos to
Transverse Damper
ADC

ADC

Phase Noise
Generator.
Transverse position of
each bunch measured at
Synchro loop
Low-level
each turn
Loops
Phase loop
Processor
RF/Fprog phase
Threshold and mask
F out
VCXO
applied to give an average
Radial loop
Averaged radial position BEAM POS
Signed
 mag
Ratio and
can replace synchro loop MODULE  mag Averaging
during commissioning or
CORDIC
CORDIC
1 Gbps serial links
MDs
ADC

Master Frf
Var. Attn
Gain cntrl
Var. Attn


180 deg
hybrid
a
b
Beam 1
Rad. PU
Dec 6, 2007
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Radiation issues for Low-Level RF electronics
A. Butterworth

15 FPGAs in each Cavity Controller, 1 or 2M gate Virtex-2

Single-event upset (SEU) in configuration latch might cause failure of
feedback loop

At nominal beam intensity, failure of feedback in any cavity will result in
beam instability and dump

FLUKA simulations performed to estimate the hadron fluence at the RF
equipment racks*
LEEC06, 29th September 2006
– simulate hadrons due to beam-gas interactions in vacuum chamber
– nominal beam intensity & vacuum conditions

Try to estimate the overall failure rate for the whole system
– only configuration upsets in FPGAs considered
* (K. Tsoulou, V. Vlachoudis, A. Ferrari)
43
Radiation: Simulations
A. Butterworth
Simulation
geometry
RF
Racks
Simulations indicated that:
 Risks from total integrated dose and
displacement damage insignificant
LEEC06, 29th September 2006
 Possible risk of SEUs from
hadrons > 20MeV
– fluence ~5x106/cm-2/year
– expected failure rate between 20
and 200 per year
RF Racks
(simulations by K. Tsoulou & V. Vlachoudis)
1 MeV n equivalent (cm-2 year-1)
44
LEEC06, 29th September 2006
Radiation: Modification of RF layout
New location inside
Faraday cage
24 m
Factor 5 reduction in predicted SEUs
Possibility to install extra shielding if necessary
A. Butterworth
5m
Old rack
location
45
Radiation: Estimation of SEU failure rate: details
A. Butterworth

Based on
– SRAM SEU cross-section of 1x10-13 cm-2/bit (estimated)
– total number of effective configuration bits: ~ 4x107 (estimated for sum
of exposed Low-Level RF equipment)
– application-dependent mitigation factor of between 10 and 40 since in
any given application only a small proportion of configuration latches are
used (from Xilinx: “90% to 97% of the configuration latches in any given
application are ‘don't care’”)
– a safety factor of 10 for the simulation
LEEC06, 29th September 2006

We arrived at an expected failure rate due to SEUs of between 20 and 200
per year, which was deemed unacceptable for reliable machine operation
46
Single Event Upset cross-sections
A. Butterworth
Xilinx SRAM-based XC4036XLA FPGA
(Radiation hardened FPGA)
Fuller et al. 200?
Hitachi HM628512P 512K x 8 SRAM
Buchanan & Gingrich 200?
LEEC06, 29th September 2006
Protons
Neutrons
• Published data rather scarce for low
LET radiation
• Not much data for neutrons
Berger et al. 1997
47
Limiter on transient: Switch&Protect module
Developed by T. Rohlev
Vcav Q
Vcav I
Mod in Q
Ig
Limiter ON
(File StepQ9 15 March 2007)




Limiter disabled
(File StepQ10 15 March 2007)
1 MV in I, step in 400 kV in Q
Q=60000, 1 MV asks for ~ 40 kW
Observe Vcav I and Q, Mod in Q and klystron output
Without limiter it takes more time to fill cavity
Dec 6, 2007
MAC22
48
The LHC beam
72 bunches
0.94 ms
•
•
•
•
•
0.94 ms
3 ms
High beam current: 0.6 A DC (nominal)
Very unevenly distributed around the ring: many gaps …
2808 bunches, 25 ns spacing, 400 MHz bucket
bunch length (4 s): 1.7 ns at injection, 1 ns during physics.
Longitudinal emittance: 1.0 eVs (injection), 2.5 eVs (physics)
– growth time due to IBS: 61 hours (physics)
– damping time due to synchrotron radiation: 13 hours (physics)
• Frequency swing (450 Gev -> 7 TeV):
– < 1 kHz for protons
– 5.5 kHz for Pb
Dec 6, 2007
MAC22
Bottom line: high beam current,
low noise electronics…
49
RF feedback Theory
• RF Feedback theory [6],[7]
– Minimal cavity impedance (with
feedback) scales linearly with T
Rmin 
2 R
0T
Q
– Achieved for a gain value proportional
to Q
G opt
Q
≈
ωo T
assumed
single-cell
– Achievable fdbk BW inversely
proportional to T
1.3
Δω =
T
Dec 6, 2007
MAC22
50
Tuner
•
•
Abort gap ~ 3 ms
Half detuning principle:
– If cavity tuned for beam current
Ib, Ig minimal when beam
present, but huge power surge
during abort gap
– If cavity tuned for zero beam
current, Ig minimal during abort
gap, but huge power surge during
beam segment
• Half detuning = detuned for Ib/2:
– Modulus of Ig constant
– Imaginary part (w.r.to Vacc)
changes sign with/without beam
Dec 6, 2007
MAC22
51
RF fdbk, and
1fdbk crate
Conditioning
and Tuner
crate
RF signals
fan-out
Dec 6, 2007
MAC22
52