nlc - the next linear collider project control and feedback for rf linacs marc ross rf control and...
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NLC - The Next Linear Collider Project
Control and Feedback for RF Linacs
Marc Ross
RF Control and MonitoringFeedback
Like most modern ‘plants’ there are loops within loops and feedforward
Special issues: precision ‘handling’ of microwave and ~high bandwidth
Author NameDate
Slide #2
Joint Accelerator School - 2002 Marc Ross – SLACNovember 14, 2002
CLIC J/NLC TESLA
Typical controls and feedback loops
• Accelerating vector – phase and amplitude– Low Level (long distance) Distribution– Source– High Power distribution– Structure beam loading & thermal… Feedback– Environment
• Transverse– Position– Emittance
• Longitudinal– Energy– Energy spread & z
…and protection systems for high power linacs
Author NameDate
Slide #3
Joint Accelerator School - 2002 Marc Ross – SLACNovember 14, 2002
CLIC J/NLC TESLA
Phase and amplitude
tolerances – NLC example
Config # 1 E1 (GeV)
2 E2 (GeV)
3
7 18 30 5 335 -30
8 20 30 7 320 -30
9 22 30 9 300 -30
A parameter that characterizes the strength of the wakefield relative to the focusing is the BNS energy spread needed for autophasing:
Autophasing is the condition where the chromatic growth of a beam performing a coherent betatron oscillation exactly cancels the wakefield growth and thus the beam oscillates as a rigid body.
BNS phase offsets imply tight phase stability tolerances (+ extra gradient)
Author NameDate
Slide #4
Joint Accelerator School - 2002 Marc Ross – SLACNovember 14, 2002
CLIC J/NLC TESLA
Phase and amplitude tolerances - NLC example (2)
Parameter Accuracy Stability& Resolution
Units
Energy profile
0.5 0.1 % voltage
Energy gain knowledge
5 0.1 % voltage
Phase readback
1 0.1 degree
Phase stability
N/A 0.1 degree
X-band:
11.424 GHz
= 26.3 mm
/360 = 73 um
/360*0.66 = 50 um
(0.66 for plastic cable signal speed)
Author NameDate
Slide #5
Joint Accelerator School - 2002 Marc Ross – SLACNovember 14, 2002
CLIC J/NLC TESLA
RF stabilization speeds
• Two kinds of linacs:– Pulse width is long compared to the transit times ‘within the
pulse’ feedback is necessary• Superconducting
– Stabilize microphonics
• Warm proton linacs– Pulse width is short compared to transit times ‘within the pulse’
feedback is not possible• Warm electron linacs• Interpulse feedback is required
– Stabilize thermal effects
• Beam loading
Long Pulse RF Control (Proton linacs and cold electron linacs)
S. Simrock - DESY
Author NameDate
Slide #7
Joint Accelerator School - 2002 Marc Ross – SLACNovember 14, 2002
CLIC J/NLC TESLA
Linac RF block diagram
3 basic loops:
1. Long baseline distribution
2. High power amplifier (Klystron)
3. Beam – based
1
1
2
3
SL
AC
RF
Dis
trib
uti
on s
chem
atic
- 1985
PAD = Phase and Amplitude Detector
Author NameDate
Slide #9
Joint Accelerator School - 2002 Marc Ross – SLACNovember 14, 2002
CLIC J/NLC TESLA
• RF phase stability at the 1 degree X-band (0.2 picosecond) level over the ~30 kilometer length of the machine.
• The RF timing requirement corresponds to a L/L stability of <2.510-9, which would be impractical without feedback.
• ( the timing distribution system will use the same hardware as the RF distribution system.)
• It is assumed that RF phase measurements relative to the electron beam will be used to obtain long term stability.
•The RF distribution system needs to maintain the RF phase to within 20 degrees X-band (<510-8) for long periods of time when the beam is not running.
RF long baseline distribution system specifications – NLC
‘Common mode’ effects are worst
NLC RF Distribution System
NL
C R
F D
istr
ibu
tion
tes
t - 2001
NL
C R
F D
istr
ibu
tion
tes
t - 2001
Author NameDate
Slide #13
Joint Accelerator School - 2002 Marc Ross – SLACNovember 14, 2002
CLIC J/NLC TESLA
High Power Phase and Amplitude Detection and Control
• High power couplers (40-70 dB)
• Cables
• Diode/Mixer detectors
• Attenuators
• Phase shifter
• Phase measurement
• Control system architecture
Hig
h P
ower
X-b
and
W
aveg
uid
e co
up
ler
– 60
dB
Hig
h P
ower
S-
ban
d B
eth
e h
ole
Wav
egu
ide
cou
ple
r
RF input
Cutaway detector diode – showing failed connection
RF Amplitude: Diode Detectors
‘Video’ out
Diode junction
Simple illustration of RF detector diode operation
Output matching network
Author NameDate
Slide #16
Joint Accelerator School - 2002 Marc Ross – SLACNovember 14, 2002
CLIC J/NLC TESLA
-10 -5 0 5 10 15100
101
102
103 10270b
dBm
mV
-10 -5 0 5 10 1510
0
101
102
103 10270a
dBm
mV
Matched Detector Diodes from Agilent
Power in
Vol
ts o
ut
Showing deviation from square law at moderate power – Low signal output is proportional to the square of the incoming RF AC voltage (V out P)
Author NameDate
Slide #17
Joint Accelerator School - 2002 Marc Ross – SLACNovember 14, 2002
CLIC J/NLC TESLA
Non-’Square Law’ detector:
Thermionic Diode
Used in original SLAC phasing system to extend dynamic range Closer to linear
Very radiation hard
Str
uct
ure
Ph
asin
g sy
stem
- S
LA
C (
1965
)
High Power RF attenuator
RF Phase shifter
In
Out
Capacitance change in ‘varactor’ diode moves effective reflection point
Conductance change in ‘PIN’ diode changes reflection coefficient
Author NameDate
Slide #20
Joint Accelerator School - 2002 Marc Ross – SLACNovember 14, 2002
CLIC J/NLC TESLA
Phase measurement
• Mixer output – if both RF / LO ports are basically the same frequency: ARF*ALO* cos()
– neglect usual mixer issues (intermod, compression)
– worry about others – offsets/diode matching
• Phase ambiguity and offsets:1. Nulling + dither to measure sign of derivative
• Wobbler (+/- 180)
• active synchronized wobbling to monitor offset
2. I/Q
• calibrated ‘double channel’
• 5 parameters – two gains/offsets and 1 angle
Author NameDate
Slide #21
Joint Accelerator School - 2002 Marc Ross – SLACNovember 14, 2002
CLIC J/NLC TESLA
‘PAD’ phase detector (and shifter) circuit
Nulling + Wobbler
Author NameDate
Slide #22
Joint Accelerator School - 2002 Marc Ross – SLACNovember 14, 2002
CLIC J/NLC TESLA
Controls Architecture
Phase and Amplitude Detector
SLAC - 1983
Sampled RF waveforms
One point digitized/pulse (120 Hz) with 30 MHz bandwidth
Cal. RF amplitude-
Fitted for energy gain estimate - lattice feedforward
RF amplitude- vs klystron drive atten. Klystron saturation
Beam Volts Modulator timing
SLAC - 1985
Author NameDate
Slide #24
Joint Accelerator School - 2002 Marc Ross – SLACNovember 14, 2002
CLIC J/NLC TESLA
motorized phase controller – 1 klystron
temp
temp
manual drive line length
What the long term feedback is doing…
Common mode error – either injection or distribution system
Single klystron – environmental (e.g. leakage through insulation)
Author NameDate
Slide #25
Joint Accelerator School - 2002 Marc Ross – SLACNovember 14, 2002
CLIC J/NLC TESLA
“Precision” microwave
• High power RF controls and monitoring + beam position monitors + beam phase monitors– SC Cavity tuning at TTF; lorentz force compensation + coupling
control
– Bunch length and Beam ‘tilt’ monitors
• programmed phase control– NLC ‘Delay Line Distribution System’
– Beam loading feedforward for short pulse linacs
2002… Modern RF controls
Author NameDate
Slide #26
Joint Accelerator School - 2002 Marc Ross – SLACNovember 14, 2002
CLIC J/NLC TESLA
Digital Low Level RF
• Precision I/Q determination– Phase/amplitude calculations with very low (bit) noise
– Use of complex math linearizes v/v amplitude and phase
1. Home-made – outgrowth of DSP based multi-bunch storage ring feedback systems
– TTF & SNS (DSP/FPGA based)
2. Commercial– Echotek ‘Digital Down-Conversion’ (Digital receiver)
• (Within the last 10 years) Biggest challenge integration with the control system &
diagnostics
Cold Linac LLRF – TESLA / TTF Simrock, DESY
System Block Diagrams
Analog (CEBAF ~1994)
Digital (TTF ~ 1998)
Author NameDate
Slide #28
Joint Accelerator School - 2002 Marc Ross – SLACNovember 14, 2002
CLIC J/NLC TESLA
Synchronous Digital Sampling – Direct down conversion
sampling clock effectively LO- importance of sampling clock stability
Digital RF How it really works
Author NameDate
Slide #29
Joint Accelerator School - 2002 Marc Ross – SLACNovember 14, 2002
CLIC J/NLC TESLA
TTF LLRF Drive Controls D
AC
DA
CReIm
Cavity 32
......8x
Cavity 25
klystronvector
modulator masteroscillator
1.3 GHz Cavity 8
......8x
Cavity 1
cryomodule 4
...cryomodule 1
. . . .
LO 1.3 GHz+ 250 kHz
250 kHz
AD
C
f = 1 MHzs
. . . . ...
vector-sum
( )aba -b
1 8( )ab
a -b
25( )ab
a -b
32( )ab
a -b
setpointtable
gaintable
feed
tableforward
+ digitallow pass
filterImReImRe ImRe
clock
LO
AD
C
LO
AD
C
LO
AD
C
S. SimrockDESY
Also have tuners, coupling etc.
TTF (DESY) DSP based I/Q controller
(Simrock)
loadableKCM
loadable
KCM
loadable
KCM +
+
Input latch
Iset
Qset
Mux,sign
Input trace
buffer512x16
++
+ +
+
restart 4-cycle accumulate
+
+
xor
+
+
feedforwardbuffer512x8
OutputDAC
saturate
reg
integrate enable
xor signfeedback enable
-
clear
regerr1 err2
samp
setp
regcum
regavg
regerr3
int1
reg
10
11
10
Host
Host
Host
Host
to 10 bitssaturate
16
+
+
regint2
loadable
KCM
loadable
KCM
loadable
KCM +
+
Input latch
Iset
Qset
Mux,sign
Input trace
buffer
512x16
++
+ +
+
restart 4-cycle accumulate
+
+
xor
+
+
feedforwardbuffer512x8
OutputDAC
saturate
reg
integrate enable
xor sign
feedback enable-
clear
regerr1 err2
samp
setp
regcum
reg
avg
regerr3
int1
reg
10
11
10
Host
Host
Host
Host
to 10 bits
saturate
16
+
+
reg
int2
Larry Doolittle – LBNLSNS Low Level RF Digital Feedback FPGA
(LINAC 2002 proceedings)
SNS Low level ‘within the pulse’ feedback Gate Array program schematic
Larry Doolittle – LBNLSNS Low Level RF Digital Feedback -1
loadable
KCM
loadable
KCM
loadable
KCM +
+
Input latch
Iset
Qset
Mux,sign
Input trace
buffer
512x16
++
+ +
+
restart 4-cycle accumulate
+
+
xor
+
+
feedforwardbuffer512x8
Output
DAC
saturate
reg
integrate enable
xor sign
feedback enable-
clear
regerr1 err2
samp
setp
regcum
reg
avg
regerr3
int1
reg
10
11
10
Host
Host
Host
Host
to 10 bits
saturate
16
+
+
reg
int2
• Input Sampler
• Diagnostic buffer
• Averaging
• Set point subtraction
loadable
KCM
loadable
KCM
loadable
KCM +
+
Input latch
Iset
Qset
Mux,sign
Input trace
buffer
512x16
++
+ +
+
restart 4-cycle accumulate
+
+
xor
+
+
feedforwardbuffer512x8
Output
DAC
saturate
reg
integrate enable
xor sign
feedback enable-
clear
regerr1 err2
samp
setp
regcum
reg
avg
regerr3
int1
reg
10
11
10
Host
Host
Host
Host
to 10 bits
saturate
16
+
+
reg
int2
Larry Doolittle – LBNLSNS Low Level RF Digital Feedback -2
• I / Q gain scaling and recombination ‘KCM’
• System calibration input
• DAC driver
• Integrator loop for fine error zeroing and feedforward input
Uses ~ 20% of the $20 FPGA
FPGA
DAC
ADC
SNS Digital LLRF prototype circuit - LBNL
Small,
Simple hardware,
~ simple software (EPICs)
Easily tested
Commercial Digital I/Q receiver
Integrated by Echotek
S. Smith, SLAC
Programmed phases/amplitudes used to switch outputs and compensate for beam loading
Delay Line Distribution System
Linac LLRF Drive
TW
T
Kly
stro
n
TW
T
Kly
stro
n
TW
T
Kly
stro
n
TW
T
Kly
stro
n
TW
T
Kly
stro
n
TW
T
Kly
stro
n
TW
T
Kly
stro
n
Modulated11.424 GHz
TimingSystem
High SpeedDDS
DDSClock
300 MHz
DDS UpdateMemory
TWT
Kly
stro
n
MIXER
IF189.25MHz
Lowpass120 MHz
TransitionTime Amplitude Frequency Phase df/dtState
LO1625 MHz
MIXER
IF2714MHz
Bandpass714 MHz
LO210.7 GHz
Bandpass11.4 GHz
xxx xxx xxx xxx xxx1
xxx xxx xxx xxx xxx2
xxx xxx xxx xxx xxx3
: : : : ::
DLDS
DDSStates
100 MHz
NLC RF compression system control using Direct Digital Synthesis – waveform memory
NLC Linac LLRF
Measurement Requirements
Parameter Value Details
Bandwidth > 100 MHz at -3 dB
Rise time < 5ns 10% to 90%
Phase resolution 1 degree At 11.424 GHz
Dynamic Range > 20 dB
Amplitude Resolution 10-3 of full scale
Beam phase wrt RF 1 degree At 11.424 GHz
Beam signal / RF -40 dB (!)
Reflected power detector max input < 100 mW Peak
Reflected power detector rise time < 10 ns
DLDS Waveforms with Beam Loading Compensation
0 500 1000 1500 2000 2500 3000
Am
plitu
de a
t eac
h D
LD
S O
utpu
t
Time (ns)
1
2
3
4
5
6
7
8
Each of 8 klystrons is programmed and combined to give independent outputs for each of 8 structure groups
Klystron 3.2 us
Structure 400 ns
Author NameDate
Slide #40
Joint Accelerator School - 2002 Marc Ross – SLACNovember 14, 2002
CLIC J/NLC TESLA
Bunch length• Streak cameras
– resolution limited to ~ 1mm– space charge, calibration
• Coherent radiation– stronger signal with shorter beams– asymmetry difficult (use power spectrum – phase info lost)
• Deflecting RF structures– promising
• Broadband microwave emission– cheap, relative – a given
• accurate monitor critical for short wave FEL
Microwave based beam diagnostics
Author NameDate
Slide #41
Joint Accelerator School - 2002 Marc Ross – SLACNovember 14, 2002
CLIC J/NLC TESLA
Transverse deflection
Brute forceCalibratedExpensiveExcellent resolution
SLAC LCLS – Krejcik/Emma (EPAC 02)SLAC/DESY TTF2
Old idea – 1965 ‘LOLA IV’Testing in linac sector 29
Krejcik / Emma EPAC 2002
Krejcik / Emma EPAC 2002
Krejcik / Emma EPAC 2002
Author NameDate
Slide #46
Joint Accelerator School - 2002 Marc Ross – SLACNovember 14, 2002
CLIC J/NLC TESLA
Beyond Bunch length Correlations
• E – z• y – z• x – y
• Proposed use of simple microwave single cell cavities to estimate correlations
• Most phase space distortions start with a linear correlation– a monitor simple, cheap and accurate compared to a
profile monitor can be more widely distributed and used to pinpoint errors
Microwave based beam diagnostics
Author NameDate
Slide #47
Joint Accelerator School - 2002 Marc Ross – SLACNovember 14, 2002
CLIC J/NLC TESLA
Response of Cavity BPMto Point Charge
Q
)sin()( taqtV
S. Smith – SLAC, Snowmass 2001
Response of BPM to Tilted BunchCentered in Cavity
q
2sincos
2)
2(sin
22)
2(sin
22)( ttt t
qat
qat
qatV
Treat as pair of macroparticles:
tq/2
q/2
Tilted bunch
• Point charge offset by
• Centered, extended bunch tilted at slope t
• Tilt signal is in quadrature to displacement
• The amplitude due to a tilt of is down by a factor of:with respect to that of a displacement of (~bunch length / Cavity Period )
2sincos
2)( t
t tqa
tV
)sin()( taqtVy
TVV tt
y
t
24
Example• Bunch length t = 200 m/c = 0.67 ps
• Tilt tolerance d = 200 nm
• Cavity Frequency F = 11.424 GHz
• Ratio of tilt to position sensitivity ½ft = 0.012
• A bunch tilt of 200 nm / 200 m (1 mrad) yields as much signal as a beam offset of 0.012 * 200 nm = 2.4nm
• Need BPM resolution of ~ 2 nm to measure this tilt
• Challenging!– Getting resolution
– Separating tilt from position
• Use higher cavity frequency?
Need 1 mrad tilt sensitivity for linac tuning
Cavity BPMFFTB (Shintake) ATF ext line (Vogel) X-band (Naito)
f 5.712 6.426 11.424 (GHz)position resolution 20 200 200 (nm)Vt/Vy (200um sig_z) 0.6% 0.7% 1.2% (.5 pi sig_t f)achieved 'projected dipole resolution' (200um sig_z) 3.3 29.7 16.7 umachieved 'tilt' angle resolution 17 149 84 mradachieved 'trajectory angle resolution' 3 26 30 uradcavity 'length' 15 15 8 mm
Angled trajectories• A trajectory that is not parallel to the cavity axis also
introduces a quadrature signal (in phase with ‘tilt’ signal)
• Projected ‘dipole’ sensitivity is increased by z/cavity length
– ~ 50
ATF z ~ 8mm gives expected tilt resolution ~ 0.1mrad
y res/y ~ 5%y’ res/y’ ~ 10x
Relative normalized precisionBeam position/beam traj angle
Very good resolution possible – 25 nm achieved in FFTB few nm possible by limiting spatial dynamic range
Wave cavity BPM X-band
12 mm bore
Naito/Li
ATF extraction lineC-band cavityL = 12mm, Radius = 26mm, f = 6426MHz, =46.6mm Movers – x, y, pitch (y-z)
ATF Cavity BPM – V. Vogel / H. Hayano
Author NameDate
Slide #54
Joint Accelerator School - 2002 Marc Ross – SLACNovember 14, 2002
CLIC J/NLC TESLA
ReferenceCavity
BPMcavity
C-bandAmplifiers15dB gain
X
X
X
Splitter
6410MHzsource
StriplineBPM
4:1 combinerX4
Scope, 250Ms/s4 channel, external trigger500 samples/ch
20MHz BW limit
Ch1
Ch2
Ch3 (Y)
Ch4 (X)
Tilt monitor electronicsJ. Frisch
Raw ‘mixed – down’ scope data from cavity BPM
Phase and amplitude wrt ref are extracted
(I and Q)
I Q response as the cavity is moved vertically using mover
The angle is arbitrary (phase offset between ref and BPM cavity)
A ‘monopole’ beam with an axial trajectory should give a (0,0) response at some point
Use the cavity ‘tilter’ to observe response to tilted trajectories
(Beam ‘tilter’ was not ready during this test – May 2002)
Compare 35 urad with 26 in table estimate
Author NameDate
Slide #56
Joint Accelerator School - 2002 Marc Ross – SLACNovember 14, 2002
CLIC J/NLC TESLA
Concluding remarks
• Digital LLRF field attractive and exciting– Wide variety of application from controls to instrumentation
• Next few years will see the application and success of critical new techniques, opening up new paths to higher brightness, low emittance transport, more stability…
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