PEP-II LLRF Status PEP-II MAC Review October 25, 2006 - PowerPoint PPT Presentation

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PEP-II LLRF Status PEP-II MAC Review October 25, 2006

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PEP-II LLRF Status PEP-II MAC Review October 25, 2006 D. Van Winkle, John Fox, Themistoklis Mastorides, Claudio Rivetta, Dmitry Teytelman, Jiajing Xu – PowerPoint PPT presentation

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Title: PEP-II LLRF Status PEP-II MAC Review October 25, 2006


1
PEP-II LLRF Status PEP-II MAC Review October 25,
2006
D. Van Winkle, John Fox, Themistoklis Mastorides,
Claudio Rivetta, Dmitry Teytelman, Jiajing Xu
2
Outline
  • 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)

4
RF 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.

5
RF Status (Aborts)
RF fault analysis over 483 days (04/23/2005 -
08/18/2006), 1942 faults Average 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
6
RF Status (Aborts)
230kV line Down
RF Flange Region 4
IR2 Work Q4 chamber
IR2 Work Q2 chamber and bellows
BPMs Fall Off
7
RF 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.

8
RF 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.

9
RF 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.

10
RF 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.

11
RF Status (Tuning)
  • Example of poorly and well configured RF feedback
    loops.

Poorly Configured
Well Configured
12
RF 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.

13
RF 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.

14
Klystron Linearzier
15
Klystron 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.

16
Klystron 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.

17
Klystron Linearizer
  • Final MD Results

18
Klystron 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).

19
Klystron 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.

20
Klystron 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
21
Klystron 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.

22
PEPII Klystron Preamps
23
Pre-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.

24
Pre-Amplifiers
  • Typical Klystron output with beam
  • Sidebands at /- n136 kHz.
  • 50 to 60 dB down from carrier.

25
Pre-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.
26
Pre-Amplifiers
  • All Amps in LER, HER and SPEAR have been measured

27
Pre-Amplifiers
  • Intermodulation Performance (MPE Amp)
  • This characteristic is not well controlled (nor
    specified)

Pre-Amplifier
Intermodulation Distortion
28
Pre-Amplifiers
  • LR4-2, HR12-3 and HR12-6 are especially bad in
    small signal response

29
Pre-Amplifiers
  • Pulse Response is undesirable

30
Pre-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.
31
Pre-Amplifiers
  • What to do?
  • Run as is
  • Modify/Replace existing amplifiers

32
Pre-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.

33
Pre-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

34
Pre-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.

35
Conclusions
  • 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.

36
End
37
Extra Info (Aborts)
  • HR12-1 Aborts

38
Extra Info (Aborts)
  • Cavity 1 Arcs (RE reported)

39
Pre-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.

40
Pre-Amplifiers
  • Intermodulation Performance of existing amps

41
Pre-Amplifiers
Evaluations
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