ILC Operation

1
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

2
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.

3
Damping Ring

• Before extraction must have
• no coherent motion
• decent lifetime
• appropriate gaps
• designated pilot bunch ready to be first
• tested kicker pulse in the gap
• RF within tols

4
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.

5
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

6
Linac failure modes and time scales

• Quads,
• RF phase and amplitude during the pulse
• Cryo slow
• valves slow
• dipoles
• fast time scale energy drop

7
Energy / Energy spread stabilization

• Nominal plan end of linac monitoring system
• Backup plan use residual beta oscillation
  wavelength
• May need additional BPMs (HOM?)
• Chirp bunch train a small amount
• High resolution BPMs 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

8
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

9
Expected energy variability

• LLRF LO
• 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

10
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

11
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)
5 Undulator Undulator BD e target
6 e- BDS BD start e- Main dump
7 e target e target e damping ring injection
8 e damping ring Ring injection e ring extraction
9 e RTML ring extraction e linac injection
10 e linac linac injection e BDS
11 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 operationpilot 1 _at_ 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

14
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.

15
Damping ring stored current

• How to keep the DR full under all variations
  downstream upstream?
• 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.

16
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.

17
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.

18
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

19
Operation with the keep-alive

• ring population
• 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 _at_ single bunch)
• lifetime 10 minutes

20
Pilot control

• Will the pilot bunch go through the energy
  collimation?
• Coupling vs intensity two different ways to
  make a pilot bunch.

21
Kicker operation

• Feedback
• stabilizing the voltage
• stabilizing the residual kick
• Feedforward
• across the extraction hairpin
• Single point failures

22
Single point failures

• critical, high power, high speed devices
• 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 mid-pulse.
• 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.

23
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)