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Basic Facilities
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Safe Setting Management - limits loaded to front-ends
- orbit, emittance, beta dependance...
- temperatures
Some discussion about how to synchronise the movement of the collimators.Either force synchronicity at high level by asynchronously applying very small steps to each collimator in turn, or possibly command to low level controller (go from here to here in this time). FUNCTIONS.
TRIM - but not at constant speed. Want to avoid boing oscillations.
Synchronicity requirements between the 2 beams were also questioned.
Frequencies of read out:
Frequencies of motor: 100 Hz minimum, aim for 1kHz, motors (1, 1.5 kHz steps a second) 1 to 5 mm/s. 10 micron step size minimum, 2.5 microns wished for
Frequencies of Beam Loss readings plus what's the delay? 25 kHz maximum
PLC limited to around 100 Hz,
Trouble with motors
? synchronicity between jaws, between collimators
? range of movements - max out
? auto retraction
? speed maximum
Is this over specified at the moment?
Pilot is essentially "safe without protection". (5 10^9 per bunch is not able to provoke quench). Will need an intensity inhibit via SPS BCT. If mode = pilot and total intensity greater than x don't inject into LHC. Clearly needed to avoid equipment damage.
The collimators will be "all out". What's out? Greater than 10 sigma or on the switches? This clearly might vary as experience grows.
Acquire and correct closed orbit. Asynchronously position collimators coarse settings [8-10 sigma?] with respect to closed orbit. Presumably IR by IR. Presumably as fast as possible with jaw synchronisation. All out is 60 mm. Beam size ~ 1.2 mm at collimators. 10 sigma ~ 12 mm, therefore of the order of 50 mm movement required if starting from switches.
What is the real beam size at collimators? How do we take care of the effects of beta beating?
Note en passant: during commissioning will need bumps and BLMs to home on aperture limits...
At least some collimators will be able to action a beam dump if losses greater than a variable threshold are sustained. For example that incurred if the emittance are too large. Thresholds to be determined but figure of 1% beam loss mentioned. Thresholds will clearly have to be adjustable.
After injection process has finished, the momentum collimators will move in to finer settings and then stay where they are during the ramp.
Secondary collimator movement has to shadow primary collimator movement.
Orbit feedback will be required in cleaning sections (3 & 7) hold to hold collimator positions fixed with respect to closed orbit (average position of bunches).
Move during the ramp:
One plan to keep primary at 5.7 sigma (quite a challenge), with secondaries at a constant distance from the primary. The TCDQ would follow the secondary collimators being place at a constant absolute distance.
Note: if, during the ramp, the primaries have tracked the beam size reduction and are at 5.7 sigma, or thereabouts, then they are in position for the squeeze. The secondary collimators will need to brought in during the squeeze
The collimators need to be positioned to 0.1 sigma or 10 microns (1 sigma ~ 0.4 mm at beta ~ 200 m.) Tolerance relaxed a bit for phase 1. Defining requirement -relative retraction of primary and secondary collimators.
Adiabatic, clearly have to follow intermediate steps. 10 mm. in 10 minutes,
| beta* | beta in triplets (v. approx!) | n (sigma) |
| 18 | 300 | 32.5 |
| 11 | 250 | |
| 7 | 350 | |
| 5 | 500 | 25 |
| 4 | 600 | 23 |
| 1.5 | 1600 | 14 |
| 0.55 | 4500 | 8.4 |
Limits on rate of change of power converters (see OB Cham 05)
So:
1. Squeeze will be slow
2. Aperture reduction at triplets only really becomes apparent in the final
section
3. Move collimators in anticipation
The set-up and optimization of the collimation system will be done in several
beam-based steps, relying
on the measurements from Beam Loss Monitors (BLM’s) which will be installed near
every collimator [24].
Following set-up procedures at other colliders the following logic could apply:
1. Separate beam-based calibration of each collimator: After producing a
well-defined cut-off in the beam distribution (e.g. with a scraper), the two
ends of each collimator jaw are moved until the beam edge is touched (witnessed
by a downstream beam loss signal). This step defines an absolute reference
position and angle for each jaw, which is valid for given and hopefully
reproducible orbit and optics functions.
2. System set-up: After restoring the reference beam conditions all collimators
are set to their target gaps and positions, directly deduced from the absolute
reference positions obtained in step 1. The cleaning inefficiency is observed in
a few critical BLM’s in the downstream areas.
3. Empirical system tuning: The cleaning inefficiency is minimized by empirical
tuning on the few relevant BLM’s where quenches can occur. The most efficient
collimators are optimized first. The optimization is orthogonal if the beam
direction is followed. Possible cross-talks between beams can be avoided by
single beam optimization.
4. Automatic tuning algorithms: Once some experience has been gained with the
collimation system a more advanced automatic tuning algorithm may be envisaged,
taking into account collimator response matrices. The detailed process of set-up
and optimization of the collimation system requires further studies and work.
Some effort has already been invested in understanding the BLM response to beam
loss in the cleaning insertions. Considering advanced scenarios (all collimators
used simultaneously for optimization) it was found that the data recorded near
collimators is difficult to use and to interpret. At high energy, the cascade
developed in a jaw and in the surrounding material will induce signals in all
monitors which are installed nearby and downstream. In order to understand how
to use the signals, a preparatory simulation was done with MARS, which
develops cascades into the entire momentum cleaning section, including 7
collimators and BLM monitors, vacuum chambers, magnets with their field, tunnel,
ground, etc [62]. A primary impact map was generated. The partial fluences as
issued from every collimator were recorded at each monitor, which allows the
computation of the normalized rate si at every monitor as a function each
collimator. For nominal working condition at injection energy,
for ~s = M~r, M is M =
.0178 .0 .0 .0 .0 .0 .0
.4662 1.19 .0 .0 .0 .0 .0
.0268 .0291 1.081 .0004 .0 .0 .0
.0432 .0389 1.085 1.044 .0 .0 .0
.0079 .0036 .138 .3245 .9891 .0 .0
.0036 .0017 .03858 .1187 .513 .9848 .0
.0012 .0007 .0099 .0349 .1642 .5093 .9445
Further work will include a variation of the jaw depth in one by one, in order
to
M may be constructed by sending a pilot bunch on each jaw sequentially. With the
terms in M, it is not yet sure that unambiguous calculations of the loss rate on
deduced with this approach. This will only become an issue once it is tried to
tune
once, e.g. trying to speed up optimization procedures after a few years of
operation.will result in easily understandable response matrices.