# Presentation: 102930

## 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*

- 1/19/2006

## 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.

- 1/19/2006

## 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*

- 1/19/2006

## 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.

- 1/19/2006

## 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

- 1/19/2006

## Linac failure modes and time scales

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

- 1/19/2006

## 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

- 1/19/2006

## 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

- 1/19/2006

## 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

- 1/19/2006

## 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

- 1/19/2006

|  | 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  |

- 1/19/2006

## 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

- 1/19/2006

## 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

- 1/19/2006

## 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.

- 1/19/2006

## 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.

- 1/19/2006

## 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.

- 1/19/2006

## 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.

- 1/19/2006

## 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

- 1/19/2006

## 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 @ single bunch)
- lifetime ~ 10 minutes

- 1/19/2006

## 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

- 1/19/2006

## 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.

- 1/19/2006

## 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.

- 1/19/2006

## The Baseline Machine (500GeV)

- 1/19/2006