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Introductory comments on: How will the low level rf systems required for bunch compression, cavity t

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The master signal also includes a 'fiducial' which indicates the start of each linac cycle. ... MHz rather than 476 MHz, and it uses a lower bandwidth fiducial. ... – PowerPoint PPT presentation

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Title: Introductory comments on: How will the low level rf systems required for bunch compression, cavity t


1
Introductory comments onHow will the low level
rf systems required forbunch compression, cavity
tuning, machine protection, etc. be designed so
as to perform reliably enough not to compromise
machine operation?
  • Giorgio Bellettini
  • ITRP Meeting
  • May 26th, 2004

2
Cold machine, 1
They address first the stability of acceleration
phase in order to ensure stable bunch timing and
energy, and no luminosity loss Each klystron
in the main linac has a LLRF system that controls
the phase and amplitude of the vector-sum of 32
cavities.. ..typical phase stability of 0.3 deg.
has been demonstrated at TTF I during beam
operation. .. For bunch compression the
specification. may be as tight as 0.1 deg.
Potential for further improvement in phase
stability exists with use of additional amplitude
detectors, better isolation between channels, and
temperature stabilization of the
detectors. COMMENT plausible, but additions
cost money.
3
Cold machine, 2
They address next the Mean Time Between Failures
of RF controls ..based on JLAB experience
where 338 LLRF systems have been operated for
more than 10 yearsthe MTBF of the digital
feed-back control for amplitude and phase is
expected to be of the order of 72 months. A
redundant simple feed-forward system will
control the rf at slightly reduced performance
the probability of both failing in the same month
is 1/721/72 1/5000and the probability of one
failure in a linac (in a month) is 1/5000280
(klystrons) 5. The probability of two such rf
failures is 2.5e-3. . ?    . Since one rf
station is permitted to fail, the availability of
the LLRF is gt 99.75 (based on failure of 2
stations) Mechanical frequency tuners have a
lifetime of gt20 years .even so, as there are a
large number of these mechanical tuners, it is
imperative that their MTBF be well understood and
sufficient COMMENT plausible. Good the
reference to experience gained at JLAB. Less
strong the argument on the probability of double
failure.
4
Cold machine, 3
They address very briefly the machine protection
system For machine protection,
exception detection (quench etc.) and handling
will be implemented. A fast rf gate (redundant)
is provided as input for interlocks from other
subsystems. COMMENT information is not
detailed. It looks to me that they did not design
the machine protection system in detail yet. It
sounds as if they consider this which is a
problem of major importance - to be of limited
complexity.
5
Warm machine, RF phase control, 1
They describe first their control system of the
acceleration phase The low-level RF system will
use a common master source of timing and phase
information for all regions and subsystems, and
distribute this information via fiberoptic links
to each sector of the collider. Each fiber
optic-link is point-to-point, which allows the
phase length of each link to be individually
stabilized. All key system design are based on
the performance of prototypes and on other R D
results. They quote the level of phase stability
needed by the various RF systems and describe how
to control and stabilize the phase to the
required levels . small changes in the
low-level RF parameters will cause luminosity
reduction the key issue is the systems
stability and whether it can reliably detect
changes of the parameters
6
Warm machine, RF phase control, 2
How the controls are distributed locally Main
distribution to Remote Receivers a prototype
system using a 15 kilometer fiber was constructed
and tested demonstrated lt 1 picosecond
variation over 1 month with a 10 degree C
temperature variation of the transmitting fiber.
This meets the requirements for systems other
than the crab cavities and the bunch compressors
Distribution from Sector Receivers to Local
Users The trigger system is very similar to the
existing SLAC trigger systemTwo 11.424 GHz
signals with equal amplitude form the drive
signal for the klystrons. By varying the relative
amounts of the two signals the phase of the
klystron with respect to the master source can be
adjusted
7
Warm machine, RF phase control, 3
Local Adjustment of Klystron Phases via
Beam-Based Signals Over longer periods the
klystrons phase is adjusted by directly
detecting the phase offset between the beam and
the klystrons RF power, and adjusting it to the
desired value.this measurement relies on the
fact that the NLC bunch train generates a very
large RF power signal in each structure, and the
phase of this signal is always purely
decelerating with respect to the beam (ie, its
phase is 180º). At low current there is not
enough signal from the beam to perform this
operation.the relative phase between klystron
and beam can be determined by measuring the phase
of a structures output power under nominal
conditions.and then measuring the output power
with the klystron disabled for one pulsethis
system was tested at S-band in the SLC where it
demonstrated approximately 1.5 degree accuracy.
8
Warm machine, RF phase control, 4
Bunch Compressors and Crab Cavities The bunch
compressor relative phase tolerance is tighter
than the main linac (0.1 versus 0.25 psec)but
it applies only for a few secondsthe phase
stability of the fiberoptic distribution
prototype over such short times was not measured
but is expected to be adequate. The crab cavity
tolerance is even tighter (0.025psec). Since the
two crab cavities are quite close to one another,
a single klystron provides the main power for
both cavities, with a small trim klystron
providing pulse-to-pulse corrections to one of
the two cavities COMMENT the phase control
system is well understood and described, clearly
based on great experience at SLAC. However, the
required stability seems difficult to reach in
some areas (note that crab cavities at IR are not
needed for head-on collisions).
9
Warm machine, machine protection
They outline their machine protection
system if a large number of stations fail
simultaneously or simultaneously change their
phases and/or amplitudes by a large amount a beam
loss event may occur drive the beam into the
vacuum chamber or the irises .Such events will
be prevented by.redundancy of the master source,
main distribution, and local distribution
systems At the RF station the two reference
signals are compared . if the two master source
phases are wildly different from one another the
phase of the Phase Comparison Unit will be used
for RF genneration on the present linac cycle and
ainhibit signal will switch off the beam prior
to the next linac cycle COMMENT machine
protection seems accurately conceived but
difficult to realize.
10
spares
  • Response
  • I. Introduction
  • Uncontrolled variation in the amplitude and phase
    of linear collider RF systems can compromise
    machine operation in the following ways
  • Small or uncorrelated changes in main linac RF
    parameters can change the energy and/or energy
    spread at the IP, leading to luminosity reduction
  • 2) Small changes in bunch compressor RF
    parameters can change the arrival times of the
    two beams at the IP, leading to luminosity
    reduction

11
3) Small changes in the phase difference between
the two crab cavities can lead to horizontal
deflection of one beam with respect to the other,
which will cause luminosity reduction 4) Large
changes in bunch compressor and/or main linac RF
parameters will change the beam energy by an
amount sufficient to drive the beam into the
vacuum chamber or the irises of accelerator
structures, either of which would be damaged by
the encounter. Issues (1) through (3) relate to
small changes in the low-level RF parameters
here the key issue is the systems stability and
whether it can reliably detect changes of the
parameters at the specified levels. Issue (4) is
qualitatively different in that it pertains to
preventing large changes from occurring in a
relatively short time. The majority of this
answer will address the stability and sensitivity
questions Section IV will discuss machine
protection issues.
12
The low-level RF system for the X-band linear
collider includes the following components 1) A
master source near the IP which generates a
sinusoidal reference signal for the entire
accelerator complex. The phase and frequency of
the signal are controlled with high precision,
since all RF systems site-wide will ultimately
derive their own phase and frequency information
from this waveform. The master signal also
includes a fiducial which indicates the start
of each linac cycle. Every device in the linac
which needs to perform an operation at a fixed
moment in each linac cycle will determine that
the moment has arrived by detecting the
fiducial (indicating start of a new linac cycle)
and counting the number of oscillations of the
waveform which follow the fiducial, until a fixed
number have elapsed. Thus, the signal from the
master source generates the baseline timing used
by all timed operations in the site, as well as
generating all RF signals used throughout the
site.
13
2) A main distribution system which transports
the master signal from its source to remote
points throughout the complex. Receivers for the
master signal will be positioned at 250 to 500
meter intervals throughout the complex. 3) A
second distribution system which transmits the
master signal from the receivers described in
(2), above, to every device which requires RF or
timing system information (klystrons, modulators,
beam position monitor processing units, pulsed
magnet controllers, etc.). 4) An assortment of
local control equipment programmable delays,
phase shifters, frequency multipliers, etc. The
most difficult specifications of the low-level RF
system are all related to phase stability.
Specifically
14
  • The relative stability of the two crab cavities
    must be at the level of 0.025º of Sband (0.025
    picoseconds) over a period of a few seconds. Over
    longer periods, a beam-based feedback can correct
    the relevant error by detecting a horizontal beam
    beam offset and correcting same.
  • 2) The relative stability of the two bunch
    compressors must be at the level of 0.4º of
    X-band (0.097 picoseconds) over a period of a few
    seconds. Over longer periods, a beam-based
    feedback can correct the relevant error by
    detecting a change in the relative arrival times
    of the two beams at the IP and correcting same.
  • 3) The stability of each klystron with respect to
    the master source signal must be at the level of
    1º of X-band (0.25 picoseconds) over a period of
    one minute. Over longer periods, a beam-based
    feedback can correct the relevant error, as
    described in Section II.D, below.

15
4) The stability of the phase reference signal at
each main receiver with respect to the master
source signal must be at the level of 5
picoseconds over periods of one month. II.
Subsystems and Performance The major components
of the timing and phase distribution system from
the master source to the individual klystron LLRF
is described in this section. Figure 2 provides
an overall schematic which summarizes the entire
system. A) Master Source The master source is an
oscillator which generates a sinusoidal waveform
at 714 MHz. Once per 120 Hz linac cycle the phase
of the oscillation is shifted this phase shift
is the fiducial, which is used by the local
systems to indicate the beginning of the next
linac cycle. The specifications and requirements
on the source are not particularly challenging,
and shall not be further discussed here.
16
B) Main distribution to Remote Receivers The
phase reference system uses a point to point
stabilized fiber link from the interaction region
to each sector to distribute the 714 MHz
reference and fiducial generated by the master
source. The long fiber links use a diode laser
modulated at the phase distribution frequency.
Part of the light is reflected from the far end
of the fiber back to the transmitting station.
The phase of the RF on the reflected light is
stabilized relative to the forward phase by using
temperature to control the phase length of a
fiber in series with the transmission fiber.A
prototype system using a 15 kilometer fiber was
constructed and tested. Phase measurements (with
an independent read back) demonstrated lt 0.2
picoseconds RMS noise for 1 minute (in a 10kHz
measurement bandwidth), and lt 1 picosecond
variation over 1 month with a 10 degree C
temperature variation of the transmitting fiber.
This meets the requirements for systems other
than the crab cavities and the bunch compressors.
Stabilization of the these systems is described
in Section III.
17
C) Distribution from Sector Receivers to Local
Users (Fanout) Within a sector, the reference
signal is transmitted on coaxial cables using an
RF interferometer system similar to that for the
3 km long SLAC main drive line. The maximum
expected length of a local coaxial connection is
approximately 500 meters, far shorter than the
successful SLAC main drive line system. 1)
Trigger Generation The system triggers are
derived from the RF distribution system. Timing
devices detect the fiducial described in II.A,
above, and count cycles of the 714 MHz RF until
the correct number have passed, at which time the
trigger is generated. The generation
and detection of a monocycle, low bandwidth
fiducial has been tested. The trigger system
is very similar to the existing SLAC trigger
system with the exception that the
reference signal is 714 MHz rather than 476 MHz,
and it uses a lower bandwidth fiducial.
18
2) Klystron RF Generation and Phasing At each RF
station, the 714 MHz reference is multiplied to
produce the 11.424 GHz main RF signal. Two 11.424
GHz signals with equal amplitude are generated at
each RF unit one of the two signals is in phase
with the incoming 714 MHz signal, while the other
is 90º out of phase. The two signals are
recombined to form the drive signal forthe
klystrons. By varying the relative amounts of the
two signals the phase of the klystron with
respect to the master source can be adjusted. The
existing 8-pack LLRF uses this system, but with
modular, rather than surface mount RF
components.
19
D) Local Adjustment of Klystron Phases via
Beam-Based Signals Over short periods (up to 1
minute) the klystron phase stability depends upon
the phase stability of the incoming signal from
the master source, and this stability is assured
by techniques described in (B) and (C) above.
Over longer periods the klystrons phase
is adjusted by directly detecting the phase
offset between the beam and the klystrons
RF power, and adjusting it to the desired value.
This measurement relies on the fact that the NLC
bunch train generates a very large RF power
signal in each structure, and the phase of this
signal is always purely decelerating with respect
to the beam (ie, its phase is 180º). The RF from
the klystron and the beam mix in the structure,
resulting in a change in phase of the structures
stored energy this phase change can be detected
by measuring the phase of the power at the output
coupler. By routinely performing this
measurement, slow drifts of the klystron phase
with respect to nominal can be detected, and a
correction applied by changing the mixture of the
two signals which determine the klystron phase
(see (C) above). The klystron phase feedback
described above relies on the potent RF signal
generated by the full bunch train. At low
current, for example single-bunch operation,
there is not enough signal from the beam to
perform this operation. In such a mode of
operation, the relative phase between klystron
and beam can be determined by measuring the phase
of a structures output power under nominal
conditions (thus measuring the klystron phase),
and then measuring the output power with the
klystron disabled for one pulse (thus measuring
the beam phase).
20
This is the technique that will be used for
initial phasing of the linac, and also for
routine phasing during low-current operation.
This second system was tested at S-band in the
SLC where it demonstrated approximately 1.5
degree accuracy. III. Bunch Compressors and Crab
Cavities The bunch compressor relative phase
tolerance is tighter than the main linac phase
tolerance (0.1 versus 0.25 psec), but the latter
tolerance must be held for approximately one
minute while the former only applies for a few
seconds. The phase stability of thefiberoptic
distribution prototype over such short times was
not measured but is expected to be adequate.The
crab cavity tolerance is even tighter than the
bunch compressor tolerance (0.025psec). Since the
two crab cavities are quite close to one another,
a single klystron provides the main power for
both cavities, with a small trim klystron
providing pulse-to- pulse corrections to one of
the two cavities. The design of the system is
described in detail in 5.
21
IV. Machine Protection The failure of a single RF
station will not change the beam energy
sufficiently to cause a beam loss event, but if a
large number of stations fail simultaneously or
simultaneously change their phases and/or
amplitudes by a large amount a beam loss event
may occur. Such events will be prevented as
follows 1) Redundancy of the master source, main
distribution, and local distribution systems. In
the event of complete loss of a master source, a
fiber, a main receiver, or a coaxial cable from a
main receiver to an RF unit, all RF units will
still be able to operate. 2) At the RF station
the two reference signals are compared to each
other, and to the RF phase from the previous
linac cycle. This is accomplished by Phase
Comparison Units, which are high-Q phase-locked
loops which do not respond to rapid changes in
phase. If the two master source phases are wildly
different from one another, or if they are both
wildly different from the phase expected by the
Phase Comparison Unit, then the phase of the
Phase Comparison Unit will be used for RF
generation on the present linac cycle and a
machine-protection inhibit signal generated which
will switch off the beam prior to the next linac
cycle. The Phase Comparison Units must maintain
X-band phase to a few degrees over 8 milliseconds
well within the performance of
standard commercial oscillators.
22
3) A request for a large change in phase or for a
large change in the complement of active
klystrons will generate a machine protection
inhibit signal. This protects the machine against
the possibility of inadvertent switching of off
vast numbers of RF stations between linac
cycles. 4) Variations in beam energy due to slow
changes in RF system performance will be trapped
by the machine protection orbit monitoring
described in the answer to question 10. 5) To
eliminate single points of failure in the
compressor high power rf system, short sections
of accelerator structures are individually
powered with separate modulators, klystrons and
LLRF. This ensures that the change from the loss
of a single section is too small to produce a
beam which could damage downstream systems.
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