RF System Improvements for
Performance and Reliability
Dan Van Winkle
Kirk Bertsche, John Fox, Themis Mastorides,
Claudio Rivetta, Heinz Schwarz
Brief Outline
• Current performance and future plans
• Brief RF System Review
• What keeps us up at night
– Longitudinal Growth Rates
– Aborts
• General ongoing global issues
• Conclusions and Outlook
Current and Planed
Performance
Current performance and future plans
• Run 6 Parameters (max achieved)
LER
HER
Beam Current 3.026 (1.4X) 1.96 A (2X)
RF Voltage
4.05
16.5 MV
# Cavities
8
28
# Klystrons
4
11 (1.8X)
Voltage/Cav 506
590 kV
Klystron Pwr 755.1
845.9kW
Current performance and future plans
• Run 7 Parameters (max planned)
LER
HER
Beam Current 4.00 (1.9X)
2.2 A (2.2X)
RF Voltage
5
17.5 MV
# Cavities
8
28
# Klystrons
4
11 (1.8X)
Voltage/Cav 625
625 kV
Klystron Pwr 1005
980 kW
Brief Overview of PEP-II
RF System
The PEP-II LLRF
STATION REF (EPICS)
STATION
REF
Gap
Loop
(EPICS)
+
SAT LOOP
476
MHz
REF

-
IQ
MOD
HVPS
120W
Driver
RF
CAV
-
DIRECT
LOOP
•15 Stations
•4 LER
•11 HER
•8 Cavities LER
•28 Cavities HER
Klystron
TUNER
LOOP
IQ
DEMOD
BEAM
COMB
LOOP
Slow Loop (EPICS)
Fast Loop (Electronics)
476 MHz
Baseband
The PEP-II LLRF
• LLRF Station
LLRF VXI Crate
1.2 MW Klystron
Fast Interlock Chassis
120W Klystron
Pre-Amplifier
Temperature Controlled
LLRF “Blue Box”
Insomnia Producing
Problems
What Keeps us up at Night
• Longitudinal Growth Rates
– Grow Damp measurements in 2003 showed
5-10X greater growth rates than predicted by
a linear model.
Longitudinal Growth Rates
• Previous MAC talks have addressed
various ideas we’ve tried which include:
– Non-Linear Modeling Effort (Claudio Rivetta
Talk)
– Klystron Linearizer (MAC Oct ’06 Talk)
– Klystron Pre-Amplifiers (MAC Oct ‘06 Talk –
Preliminary)
Solution Flow Diagram
High Growth
Rates Observed
Linearizer Project
Non-linear Model
Pre-amp Evaluation
New Pre-Amps
Reduced Growth
Rates Observed
Comb
Rotation
Longitudinal Growth Rates
• Today, I’ll Discuss:
– Progress since last MAC on:
• Identifying key parameters and measurements for
replacement amplifiers
• Progress on purchasing and replacement
• Results of new installations
Pre-Amplifier Specification
and Measurements
Klystron Pre-Amplifiers
Small (network analyzer
swept) signal injected
along with large carrier to
simulate small signal
modulation on CW carrier
Full Power Carrier
Swept Low Level Carrier
Klystron Pre-Amplifiers
Unusual (distorted)
response seen when
carrier is present
Since we use the
amplifier with carrier and
small signal modulation,
the modulation “sees”
this response rather than
the flat (desirable)
response.
Klystron Pre-Amplifiers
• Old Data showed LR4-2, HR12-2 and HR12-6
were especially bad in small signal response
Klystron Pre-Amplifiers
•
LR4-2 Distortion affected ability to implement comb rotation
LR42 was nearly unstable with 20 degrees of
comb rotation. Simulations show this is due to
non-linear pre-amplifier response.
Klystron Pre-Amplifiers
Non-linear distortion is also a key parameter of these amplifiers. Rather than
use two large tones as is typically done in a TOI measurement, we decided to
try a new technique similar to our network analyzer technique. Namely, a
small signal in the presence of a large signal.
Klystron Pre-Amplifiers
Klystron Pre-Amplifiers
Klystron Pre-Amplifiers
In this case,
AmpC was a
class A amplifier
powered off
240V AC.
Amp B was a
class AB
amplifier
powered of
120V.
We chose amp
B based upon
“good enough”
performance
and much less
expensive price
New Pre-Amp
Performance In Station
Klystron Pre-Amplifiers
STATION REF (EPICS)
476
MHz
REF
Gap Loop
IQ
DEMOD
ADC
+

IQ
MOD
120W
Driver
Klystron
RF
CAV
-
+
-
DIRECT
LOOP
DAC
IQ
DEMOD
BEAM
COMB
LOOP
Built in stimulus driver allows for unique
in-situ measurements
Klystron Pre-Amplifiers
• New Amps installed in all stations
Klystron Pre-Amplifiers
Klystron Pre-Ampliers
• Since Last October
– Significant Time spent characterizing and
specifying amplifiers in a “new” way.
– Found Vendor who met specs
– Bought 17 and installed 15 new pre-amplifiers
– Amplifiers show much improved response and
allow for 20 degrees of comb filter rotation
Longitudinal Growth Rates
Conclusions
• We now feel system is prepared to move
to higher current realms in terms of
longitudinal growth rates
• Further work may include:
– Asymmetric Combs
– RFP asymmetry calibrations
What else keeps us up?
• Aborts
– Biggest Contributors are cavity arcs
during startup after down (Heinz S.)
– We also had many issues with the
HVPS systems (things breaking)
– Things we plan to work on are:
• PG&E Power Dips (55 Aborts during run 6)
• LR4-4 Drive Glitches (41 Aborts during run 6)
HVPS Dips
What’s Going On?
Pout vs. Pin
Constant Running
1100
Po (kW)
1050
1000
950
900
850
77
kVkV
75.5
800
77.5
kV
76 kV
750
76.5
kV
75 kV
700
650
600
0
10
20
30
40
50
60
70
Pin (W)
Swing
Power
Required
can not
of reach 900
5W to
kW
keep
andoutput
goes over the top
constant at 900 kW
For 76.5
77 kVkV
77.5
@@
900
900
kW
kW
Output,
Output,
~24
~22
~27
WW
Input
Input
HVPS Dips
• What to do?
– Lower drive power on Klystron
• Upside:
– Allows for greater head room
– More linear running
• Downside
– Can’t reach as high power
– Higher Collector Power
HVPS Dips
• Since these dips appear to be coming
from PG&E, there is very little we can do
locally to mitigate them.
– Constant monitoring of drive set-points will be
required
– For peak currents, we may need to live with
the occasional power dip abort
LR4-4 Drive Dips
Ongoing problem since 2005
•AIM HVPS monitor shows noise burst
•Followed by Klystron Forward “wiggle”
•Followed by very short dropout in drive
signal
•Followed by beam abort…
LR4-4 Drive Drop
• Drive mysteriously drops (or rises)
• Cavity responds after delay
LR4-4 Drive Dips
• Things Tried so far:
– Replaced several modules (not well controlled study)
– Moved drive set point
– Low trip rate and missing measurement points makes
this difficult to diagnose
• Plans
– New klystron being installed. Will let run for some
time to see if this makes any difference (not likely)
– Begin plan for swapping various modules. Likely
culprits are gap module and RFP module. Will start
with one then wait 2-3 weeks, then swap another.
– Extra Monitoring on HVPS signals to attempt to
understand mysterious HVPS noise burst
– More plans to be developed in LLRF ongoing
meetings. This will become high priority.
General Issues
General Ongoing Issues
• Cavity Tuning Polynomials
– Polynomial fits for cavity de-tuning vary with
temperature
– Occasionally cavity temperatures have been varied
without concern for these polynomials (RF experts not
notified)
– Net result is constant tuning of RF stations as current
is pushed.
– Machine seems to run better after stations are given
“tune cavities” and “make polynomials” tune ups.
– This process take time without beam so
administration is reluctant to do except when
necessary
General Ongoing Issues
• Cavity Tuning Polynomials
– We are working on scheme to make this
process run-able by operators.
– Process must be “bullet proof”
– Will remind operators to run at opportunistic
times when we are without beam for 30
minutes
Summary and
Conclusions
Summary and Conclusions
•
Much progress has been made over the last 3-5 years in improving the reliability and performance of the PEP-II
RF systems. Some highlights are:
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Re-designed RFP modules
Re-designed IQ&A modules
Fixed “Stuck Tuner” Problem
Better temperature control on blue boxes
Designed new R2 COMB Module (reduced two VXI modules to one)
New pre-amplifiers
Comb rotation for better beam stability
Filters on RE signals
Non-linear modeling for low order model longitudinal damping improvement
Low Group Delay Woofer for increase low order mode damping
Gage Board diagnostics for transverse and longitudinal troubleshooting
Matlab GUIs for fault files
Matlab GUIs for “tune cavities” and “make polynomials”
RF training for operators
Model based RF station tuning
AIM module diagnostics of HVPS signals
SLAC Klystrons in most stations
Klystron Linearizer Development
Re-designed VXI Clock module
Re-designed VXI AIM Module
Found & Fixed old VXI COMB stuck overflow problem
Upgraded VXI CPU from 40MHz 68040 to 350MHz PPC (and re-designed VXI interface on all old VXI modules)
Re-wrote RFP DSP Ripple loop code to reduce phase ripple
Added RFP analog ripple loop
Found & fixed AC power wiring issue with VXI Crate power supplies, which was causing intermittent problems
Upgraded 476Mhz RF Phase Reference System to reduce phase drift and improve stability as well as added more diagnostics
Developed full-fledged RF station mock-up in lab for development & troubleshooting
Insulated Blue VXI Racks and added heaters with closed-loop control for better temperature regulation
Improved RFP Calibration routine
Improved IQA calibration method
Summary and Conclusions
• The PEP-II RF system is running relatively
well (2.5 aborts per day) considering the
complexity of the overall system.
• This rate is still too high, but there will be
difficulties in reducing this rate due to the
lack of “big ticket” aborts to go after.
• We will (of course) continue working on
these issues to the last day of running.
Summary and Conclusions
• Finally
– Running at the highest currents will require
constant vigilance
• If we lose a station, we WILL NOT be able to
continue running at full current
Acknowledgements
Technical Review and Discussions:
Dmitry Teytelman, Mike Browne, John Dusatko, Jim
Sebek, Ron Akre, Vojtech Pacak, Alan Hill, Kirk Bertsche
Original Concept and Design
Paul Corredoura, Rich Tighe and Flemming Pedersen
Support and Permission
Uli Wienands, John Seeman, Mike Sullivan
All this work was done under contract #DE-AC02-76SF00515 from the U.S.
Department of Energy
The PEP-II RF TEAM
High Power RF
Alan Hill, Heinz Schwarz, Vojtech Pack, Al Owens, Ron Akre
Accelerator Research Department
John Fox, Claudio Rivetta,
Controls Department
Mike Browne, John Dusatko, David Brown, Bill Ross
High Voltage
Marc Larrus, Dick Cassel, Paul Bellomo, Serge Ratkovsky
Control Software
Mike Laznovsky
Accelerator Dept
Mike Sullivan, Uli Wienands, William Colocho, Franz-Josef Decker, Alan Fisher, Stan Ecklund, Mat
Boyes, Kirk Bertsche
Management
John Seeman, Ray Larsen, Sami Tantawi
References
• Dan Van Winkle – MAC ’06 Talk
– http://www.slac.stanford.edu/~dandvan/mac_1006_dvwR3.ppt
• Dan Van Winkle – MAC ’04 Talk
– http://www.slac.stanford.edu/~dandvan/MAC_12_04.ppt
• Dan Van Winkle – Internal Linearizer Review
– http://www.slac.stanford.edu/~dandvan/project_review_3_08_06.ppt
• Claudio Rivetta et al – PRST Longitudinal Simulation Paper
– http://prst-ab.aps.org/pdf/PRSTAB/v10/i2/e022801
• Dan Van Winkle - EPAC ’06 Klystron Linearizer
–
http://www.slac.stanford.edu/pubs/slacpubs/11750/slac-pub-11945.pdf
• John Fox – RF Amplifier Selection PAC ’07
– http://www.slac.stanford.edu/pubs/slacpubs/12500/slac-pub-12636.pdf
• Dan Van Winkle – LLRF Workshop 2007 Invited Talk
– http://www/~dandvan/llrf07.ppt