Title: PEP-II LLRF Status PEP-II MAC Review October 25, 2006
1PEP-II LLRF Status PEP-II MAC Review October 25,
2006
D. Van Winkle, John Fox, Themistoklis Mastorides,
Claudio Rivetta, Dmitry Teytelman, Jiajing Xu
2Outline
- RF Status
- Abort Status
- Key contributors to aborts
- RF Tuning
- Klystron Linearizer Wrap-up
- Klystron Pre-Amp Investigations
- Plans and Conclusions
3- RF Status (Aborts and Tuning)
4RF Status (Aborts)
- Understanding the root causes and patterns of RF
station aborts is critical for operating
efficiency - RF Aborts are tracked on a daily basis in a an
attempt to find root cause. - Sorting out the true RF aborts from the reported
RF aborts is an ongoing issue. - The goal of doing this RF abort tracking is to
reduce the number of RF aborts. - The pool of knowledge for this RF abort tracking
is very limited (1 person). - New staff is being identified/hired in the
accelerator operations department to share this
RF diagnosis skill as well as increase operations
efficiency in identifying and resolving RF
related issues.
5RF Status (Aborts)
RF fault analysis over 483 days (04/23/2005 -
08/18/2006), 1942 faultsAverage of 4.0 aborts
per day (1.9 (RF events))
Station Faults
her 566
ler 479
HR21 110
LR44 84
LR43 82
HR25 81
HR26 80
HR22 80
HR83 78
LR42 77
HR81 75
HR41 52
HR85 41
HR23 32
LR45 25
Last event Count Stations Description
03/03/2006 425 ler(4),her(421) IP Vacuum event with/without radiation
01/27/2006 234 ler(215),her(19) didt with transverse
09/05/2005 89 multiple Transverse instability (TFB)
08/14/2006 81 multiple Unknown
08/14/2006 66 multiple Cavity 1 arc (RE reported)
07/16/2006 51 multiple Klystron arc/HVPS Arc
08/17/2006 48 ler(29),her(19) Longitudinal instability
07/30/2006 43 multiple Power dip (Region 8 or 4)
08/05/2006 41 multiple Slow or stuck tuners
08/16/2006 40 multiple Site Power Dip
Top 10 events account for 1118 faults (58) Top 10 events account for 1118 faults (58) Top 10 events account for 1118 faults (58) Top 10 events account for 1118 faults (58)
Color code being debugged not actively pursued non-RF
Subsystem Faults
HER RF 629
LER RF 268
HER other 566
LER other 479
7 RF related aborts per day for Run 3 2.9 RF
related aborts per day for Run 4 1.9 RF related
aborts per day for Run 5
6RF Status (Aborts)
230kV line Down
RF Flange Region 4
IR2 Work Q4 chamber
IR2 Work Q2 chamber and bellows
BPMs Fall Off
7RF Status (Aborts)
- Key Abort Contributors
- Cavity Arcs
- We are limited in what we can do to address
cavity arcs. Our strategy to date has been to
re-partition the gap voltage so that arcing
cavities are running at lower gap voltages - Stuck Tuners
- This has been an ongoing issue for many years.
The tuner motor drivers have been self shutting
down on hot days sometimes generating an abort.
Claudio Rivetta dug in and has figured out
exactly what the problem is (Voltage drop to
driver logic) and has designed a fix to resolve
this issue permanently. - HVPS Dips
- We have been seeing HVPS dips in two regions of
PEP. At this point it is unclear what is causing
these dips. We have also seen an increase in
site wide dips. The SLAC HVPS people are
currently working to understand and resolve this
issue. - Transverse Feedback Issues (see Ron Akres talk)
- We saw an increase in aborts do to the transverse
feedback system this run. Ron Akre should have
some insight into what caused these. - Longitudinal Issues
- At high currents we saw loss of control in the
low modes. This was likely due to the HER being
run very close to the RF power limit at the end
of the run. The higher order modes are in
control but there has been thermal issues with
the LER kicker cable plant and absorptive filter
connections. These issues should be resolved for
the next run.
8RF Status (Aborts)
- Longitudinal Issues
- Low-modes (cavity fundamental) are fastest
growing instabilities in HER and LER - HOM driven instabilities are 1/3 to 1/5 as fast -
well controlled by broadband feedback - Continued measurement of growth rates as currents
increase, estimation of adequate HOM damping
margins in the future - In cavity modes -higher currents in the LER
require LLRF system improvements per Claudio's
talk. (LGDW max damping rate may be exceeded
otherwise) - Thermal management
- Cavity-style kickers in LER work well - still
have had connector and absorptive filter
connector thermal problems. - HER- installation of "old" LER drift tube kickers
with better cooling in progress. - Monitoring of load powers, amplifier powers, etc.
is very important in operation of the systems.
9RF Status (Tuning)
- RF Tuning
- The LER and HER RF stations are routinely tuned
as currents are increased. - A measure of how stable the RF system is running
is how often the RF stations need to be tuned.
10RF Status (Tuning)
- What is meant by RF tuning?
- The LLRF system uses a time-domain excitation to
inject noise and measure the closed-loop
frequency response with the system in operation
(and beam in the machine). - A Model-based technique is used to fit the closed
loop data. The model is open-looped and adjusted
for best gain and phase margin ( best RF station
stability). - The model based corrections are applied to the
station and measured again to ensure convergence. - The Klystron operating point is not constant with
current (resulting in small-signal gain and phase
response variations). We configure the loops to
operate best at the maximum running current with
the idea that that is where the impedance needs
to be controlled the most. - As the loops cannot be opened with beam in the
machine, this technique allows adjustment as
currents increase, and brings insight into the
dynamics of the station operating point.
11RF Status (Tuning)
- Example of poorly and well configured RF feedback
loops.
Poorly Configured
Well Configured
12RF Status (Tuning)
- Why is Tuning important?
- If stations are mistuned, low mode longitudinal
growth rates can increase beyond our capability
to control them with the low group delay woofer. - Stations may oscillate causing beam jitter or
loss. - Gap voltage regulation could be degraded.
- Other loops need tuning as well
- Tuner loops, Drive set point loops, Gap voltage
loops, Tuner position control, etc. These loops
have variations station to station because of
Klystron and other implementation-specific
details.
13RF Status (Tuning)
- During Run 5 and especially in the last 3-4
months of running, the RF system required very
little tuning. - Also during run 5, the LLRF hardware was much
more reliable (Much fewer module replacements
than prior runs) - It is essential that the diligence of maintaining
a known good set of spare modules and the
replacement procedures be maintained throughout
the remaining running of PEP-II. This will be
even more important as currents are pushed to new
limits! - It is also essential the tuning and monitoring of
the LLRF system continue. If the LLRF system is
neglected, we will likely see an increase of beam
aborts attributable to the LLRF system. The
accelerator operations department is the right
place for these activities to be based.
14Klystron Linearzier
15Klystron Linearizer
- In the last MAC review I gave an in depth talk on
a klystron linearizer we had been developing with
the goal or reducing the low mode longitudinal
growth rates. - The last slide of that talk was Next Steps.
- MD2 with beam (higher klystron Saturation).
- Modeling used to specify LER operating point to
show saturated effects (similar to HER). - Careful measurements of growth rates with and
without linearizer. Klystrons will be in heavily
saturated state. - This MD will be the decision point for before
final production linearizer development.
16Klystron Linearizer
- The goal of the linearizer MD was to run the LER
with all stations linearized, and to carefully
compare the station dynamics and instability
growth rates with and without the linearizer. - To do this we took grow/damp data at various beam
currents with and without the linearizer at the
same klystron operating points.
17Klystron Linearizer
18Klystron Linearizer
- Key Findings from Final MD
- The action of the linearizer as an small signal
gain ( amplitude) linearizer was confirmed,
though we found that one station could not be
linearized. The MD was done with 1 parked
station. - The linearizer does help with the loop frequency
response, and loop stability margins ( RF station
stability). - The linearizer does NOT seem to effect the
low-mode growth rates ( when consistent operating
points are chosen).
19Klystron Linearizer
- This discrepancy led to studies using the
nonlinear station simulation, and in hindsight we
can now explain why the technique helps with
station stability via the gain and phase margins,
but does not improve growth rates. This
understanding came from the analysis of the
linearizer MD data. - With this insight, we have decided not to invest
resources in developing production linearizers.
20Klystron Linearizer
- We are still investigating the HVPS ripple
reduction possibilities in SPEAR. This is
possible because spear does not push its LLRF
system nearly as hard as PEP-II.
Linearized
Bypass
21Klystron Linearizer
- During the final MD, we had troubles configuring
the linearizer with LR4-2. It was unclear at the
time as to what was causing the problem. - In addition and around the same time Claudio
Rivetta speculated that something in the LR42
transfer function was helping to increase the low
mode growth rates. - Some of our noise file measurements had confirmed
this result. - It finally became apparent that something was
wrong with the pre-amp in LR42. As we
investigated further we found some interesting
effects with most all of the Klystron pre-amps in
both the LER and HER.
22PEPII Klystron Preamps
23Pre-Amplifiers
- The pre-amplifier used in the PEP-II RF system is
under specified. The Klystron Pre-Amp was
specified and selected based on CW operation. - We operate the preamp in a mode which demands a
large amount of dynamic range. Large CW signal
with small modulation sidebands.
24Pre-Amplifiers
- Typical Klystron output with beam
- Sidebands at /- n136 kHz.
- 50 to 60 dB down from carrier.
25Pre-Amplifiers
Significant Amplitude and Phase Distortion exists
in a /1 MHz band around the carrier.
Small Signal
Large Signal
Large Signal
Small Signal
When carrier is removed (Blue Trace), gain looks
flat as it should.
26Pre-Amplifiers
- All Amps in LER, HER and SPEAR have been measured
27Pre-Amplifiers
- Intermodulation Performance (MPE Amp)
- This characteristic is not well controlled (nor
specified)
Pre-Amplifier
Intermodulation Distortion
28Pre-Amplifiers
- LR4-2, HR12-3 and HR12-6 are especially bad in
small signal response
29Pre-Amplifiers
- Pulse Response is undesirable
30Pre-Amplifiers
- LR4-2 Distortion affects ability to implement
comb rotation (see Claudio Rivettas talk)
LR42 is nearly unstable with 20 degrees of comb
rotation. Simulations show this is due to
non-linear pre-amplifier response.
31Pre-Amplifiers
- What to do?
- Run as is
- Modify/Replace existing amplifiers
32Pre-Amplifiers
- Run as is?
- Not a good scenario.
- Modeling shows that the RF stations will be
difficult to configure at beam currents above
3100 mA.
33Pre-Amplifiers
- Modify/Replace existing amplifiers
- To evaluate required changes we have tested
- Class A amplifier
- Modified existing (Class AB) amplifier
- Hybrid (replace RF part of existing amplifier)
- Other class AB amplifier
- In the works
- Second hybrid option
- Second class AB amplifier
34Pre-Amplifiers
- Conclusions/Plans
- We are actively searching for a
replacement/upgrade path for the existing
klystron pre-amplifiers. - A detailed specification will need to be worked
out which includes an intermodulation distortion
spec. - We plan to complete this work in the next three
weeks and then generate a plan/proposal for
pre-amplifier upgrades.
35Conclusions
- The RF system is performing fairly well.
Reducing the number of aborts below current rate
of 2/day will require consistent operations
support and careful attention to system
configuration. - Transfer of tuning and fault analysis to
operations is key to maintaining the LLRF
performance - The modeling effort in conjunction with what we
have learned from the Klystron linearizer effort
has improved our ability to predict growth rates
and understand the overall RF system operation - The pre-amps have been identified as a source of
low mode growth rate increase. - We are currently working on a modification plan
for these preamps. - We hope to have amplifiers retrofitted before the
down is over, but it may end up being a swap and
go program. - More testing is required before submitting plan.
- The RFP module has its own set of distortions
which are just beginning to characterize. This
may or may not end up being a significant
contributor to low mode longitudinal growth
rates.
36End
37Extra Info (Aborts)
38Extra Info (Aborts)
- Cavity 1 Arcs (RE reported)
39Pre-Amplifiers
- Amplifiers specified for only CW operation.
- We devised a way to test the small signal in
presence of large signal response and found
un-expected results.
40Pre-Amplifiers
- Intermodulation Performance of existing amps
41Pre-Amplifiers
Evaluations