Transcript ILC Operation – SLAC ILC Controls meeting, 1/19/2006

ILC Operation –
SLAC ILC Controls meeting, 1/19/2006
Turning on the beam
The chronology of a trip
Separating power and luminosity
testing feedbacks
Maintaining equilibrium through transients
Positrons
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Marc Ross - SLAC
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At first:
• Extract the 1% pilot from the DR
• 10 us later begin the full train sequence
• Each bunch must traverse properly or the abort
system will be triggered.
• sensed using the
– beam position monitors,
– beam loss monitors and
– beam intensity monitors
• true single bunch response time devices.
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Damping Ring
• Before extraction: must have…
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no coherent motion
decent lifetime
appropriate gaps
designated pilot bunch ready to be first
tested kicker pulse in the gap
RF within tols
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Abort systems – rtml, linac/undu, bds
• The minimal abort system consists of a spoiler /
collimator / absorber block (copper) and a kicker.
– Rise time should be fast enough to produce a guaranteed
displacement of more than the pipe radius in an inter-bunch
interval. In any given fault, at most 450 bunches would then
strike the copper block.
• Assuming the latency for detecting the fault is
500 ns, the upstream signal effective
propagation speed is 0.7 c, and the abort kicker
latency time is 1 us, the maximum kicker
spacing should be 1000m.
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MPS abort dumps
• In the baseline configuration five abort systems
are needed on the electron side (four on the e+
side): 2 upstream of the linac, one upstream of
the undulator and 2 in the beam delivery.
– An alternative is an additional abort per kilometer of linac.
– may depend on the linac straightness.
• The required kicker deflection is 10 mm, for the
radius, and a relatively small additional amount
for margin. With a kicker volume of 20 * 20 mm,
about 25 MW of peak power would be required
for a 50 m long kicker system
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Linac failure modes and time scales
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•
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Quads,
RF phase and amplitude – during the pulse
Cryo
slow
valves slow
dipoles
• fast time scale energy drop
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Energy / Energy spread stabilization
• Nominal plan: end of linac monitoring system
• Backup plan: use residual beta oscillation
wavelength
– May need additional BPM’s (HOM?)
– Chirp bunch train a small amount
– High resolution BPM’s needed
• To avoid  mid-linac spectrometers.
– These are justified when the linac will be operated with narrow
energy bandpass (not this linac)
– expected bandpass ~ 50%, depending on straightness?
– expect undulator to be narrow - band
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Collimation
• 10KW/m max ; with very optimistic halo
assumptions
• About 10x SLC max
– Mechanical tests, tolerances
• energy collimation likely to demand most care:
– narrower than BDS optical bandwidth (0.2%?)
• energy variations on the ‘slits’
– intra-train feedback
– fast local abort
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Marc Ross - SLAC
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Expected energy variability:
• LLRF LO
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Seen at TTF (250kHz)
Mixing intermodulation 1300 / 52 MHz ?
Interbunch spacing == 400/1300 (=16/52) us
Should be ok.
• Check for intermodulation with digitizer clock – high harmonic
relationship
• ‘slow’ quenches outside of feedback correction
range
– the loss in gradient cannot be compensated by single klystron
vector sum feedback
– often seen at TTF
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MPS – average power loss
• For stability, it is important to keep as much of the
machine operating at a nominal power level.
– including the source, damping ring injector and the damping ring itself.
• Segmentation is the key  beam shut off points.
– Each of these segmentation points is capable of handling the full
beam power, i.e. both a kicker and dump are required.
– also fast abort locations
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Begin
End
1
e- injector
Source (gun)
e- Damping ring injection
(before)
2
e- damping ring
Ring injection
e- Ring extraction (after)
3
e- RTML
Ring extraction
e- Linac injection (before)
4
e- linac
Linac injection
Undulator (before)
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Undulator
Undulator
BD; e+ target
6
e- BDS
BD start
e- Main dump
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e+ target
e+ target
e+ damping ring injection
8
e+ damping ring
Ring injection
e+ ring extraction
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e+ RTML
ring extraction
e+ linac injection
10
e+ linac
linac injection
e+ BDS
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e+ BDS
e+ BDS
e+ main dump
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Low Power operation
• intra-train b/b feedback limitations
– Pilot bunch + one nominal I bunch?
– What is the minimum beam power for ‘nominal operation’?
• beam-sensor performance degradation
– LLRF/BPM systematics
– Collimation: esp. energy. Does the pilot bunch go through the
slits?
• Reduced repetition rate
– 0.1 Hz pulse rate
– 10 KHz bunch spacing
• Reduced RF power operation
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Example low power
operation:
pilot +1 @ 1Hz
• 800W / 11.3 MW  factor 15000 reduction
– Compelling to test lumi/background/tuning procedures
– How many bunches at what intensity / spacing are needed for
systems that MUST have intra-train feedback?
– Pilot + 1 at 10 us?
– Laserwire scan will take ~1 minute; x y + coupling phase space
15m unless scans can be done in parallel, at both ends of the
machine, for example.
• Can electricity use be reduced?
– Marx allows controllable pulse length
– Baseline?
– Klystron thermal stabilization  another transient for LLRF to
handle
Equilibrium
• Where are the ‘fields that depend on preceding beam pulses’?
– There are (at least) 3 primary subsystems whose configuration depends on
average beam power:
– 1) damping ring alignment,
– 2) positron capture system phases,
– 3) collimation
– Klystrons – (depending on power saving strategies)
• In each of these cases, beam heating is a significant part of the total
heat flow and will necessarily have some impact.
– At SLC, the beam power on target was ~30KW, about 20% of this was absorbed
in the positron capture RF section.
– Much can be done to reduce these effects using more careful initial engineering,
• beam power is much more than 30KW; neutral beam may mitigate this
• Must consider the impact of residual temperature changes carefully and
assume they will be a problem.
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Damping ring stored current
• How to keep the DR full under all variations
downstream & upstream?
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Lifetime?
Off-axis injection (aka accumulation)?
Abort & fill cycles; low repetition rate
most ring users recommend ‘top up’ for maintaining equilibrium
• Full power dumps are needed in the damping
ring (complex) and at the entrance to the linac.
– to keep the DRs as warm as possible.
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Tune up and steady-state dumps
• 1) purpose for additional high power dumps results from
the desire to keep upstream systems in equilibrium
during short interruptions.
– Other functions include the desire to have beam instrumentation and
related feedback / stabilization systems in operation during the
interruptions
• (soft requirements in comparison).
– The critical parameters are the degree to which the upstream machine
configuration (includes field strength, phase, alignment etc) depend on
the average beam power in those locations.
• If it is guaranteed that there is no difference between full power
operation and very low power operation, then additional high power
dumps are not needed.
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MPS Transients
• two basic kinds of interruptions,
– 1) short (MPS or beam tuning) driven where it would be useful if the system
recovered more or less instantly and
– 2)longer interruptions involving access etc where upstream thermal time scales
are unimportant.
• High power beam auxiliary beam dumps are only needed for 1) (not
2).
• The most logical place to dump the full rep rate/n_bunch beam is
before the entrance to the linac, not after it.
– recommend removing the baseline requirement for full power dumps at the
entrance to the beam delivery.
– These dumps are important but need not take full power, only the full bunch train.
A much lower power, lower cost dump could be implemented, for example one
capable of 0.1Hz full train operation.
– expect that 0.5MW dumps will be much cheaper and easier to deal with than full
power beam dumps.
– full power dump will cost ~ 50M$ (DESY).
• Lower power dumps may cost 1/10 of this, based on the SLC design.
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Full power Dumps
• The undulator positron system should also remain operating at full
power. This requires a full power charged beam dump at that
location. In principle, if there were a problem on the positron side,
the electron beam could be transported to the main BDS dumps.
• 6) During access to the BDS area, where the interruption is long
compared to these thermal time scales, the power in the entire
machine, except the stored beam in the DR, should be scaled back
to reasonable levels.
• 7) This is the 'minimum dump' configuration. There are 6 1/2 MW
class dumps, one 15 MW (at the e+ source) and 2 nominal full
power 20MW dumps. Not including dumps needed in the injector,
undamped, system.
• Positron capture
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Operation with the ‘keep-alive’
• ring population
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both rings full
full e- ring / one e+
full e+ / one e- (?)
both rings one (or small)
• accumulation (aka off-axis injection) from the
keep-alive
– full ring fill takes ~ 30,000 10% bunches (100 min @ single
bunch)
– lifetime ~ 10 minutes
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Pilot control
• Will the pilot bunch go through the energy
collimation?
• Coupling vs intensity – two different ways to
make a pilot bunch.
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Kicker operation
• Feedback
– stabilizing the voltage
– stabilizing the residual kick
• Feedforward
– across the extraction hairpin
• Single point failures
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Single point failures
• critical, high power, high speed devices:
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damping ring kicker,
DRRF,
linac front end RF,
bunch compressor RF and
dump magnets systems
• redundancy needed.
• extraction kicker, a sequence of independent power supplies and
stripline magnets that have minimal common mode failure
mechanisms.
• front end and bunch compressor RF, more than one klystron /
modulator system powering a given cavity through a tee.
–
LLRF feedback must stabilize the RF in the event that one of sources fails ‘midpulse’.
– alternate : using a sequence of modestly powered devices controlled completely
in parallel,
• There are several serious common mode failures in the timing and
phase distribution system that need specially engineered controls.
– frozen unless the system is in the benign – beam tune up mode.
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Control limits
• Depending on the state of the machine, 
– programmed (perhaps at a very low level) ramp rate limits that
keep critical components from changing too quickly.
– may have an impact on the speed of beam based feedback.
• Some devices, such as collimators should be
effectively frozen in position at the highest beam
power level.
• There may be several different modes, basically
defined by beam power, that indicate different
ramp rate limits.
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The Baseline Machine (500GeV)
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