R. Assmann - LTC Two Beam Operation R.W. Aßmann with W. Venturini and V. Kain LTC 4.7.2007 Acknowledgements to W. Herr, V. Previtali, A. Butterworth, P. Baudrenghien, J. Uythoven, J. Wenninger, … LHCCWG presentation on May 8 th
Jan 17, 2016
R. Assmann - LTC
Two Beam Operation
R.W. Aßmann
with W. Venturini and V. Kain
LTC 4.7.2007
Acknowledgements to W. Herr, V. Previtali, A. Butterworth, P. Baudrenghien, J. Uythoven,
J. Wenninger, …
LHCCWG presentation on May 8th
R. Assmann - LTC
Commissioning Stage
• This concerns phase A.6: “450 GeV – Two Beam Operation”
• Essentially this means:
– No crossing angle required (at maximum 156 bunches).
• Intensity:
– Two beams should first be commissioned with safe beam in both
rings.
– Phase A.5 (“450 GeV, increasing intensity”) must be done before
increasing intensity.
R. Assmann - LTC
Entry Conditions• With 5×109 p to 3×1010 p:
– Both beams have completed single-beam commissioning (phases A.1 to A.4):
• Orbit and optics have been adjusted.
• Instrumentation is operational for single beam.
• The two RF systems are operational.
• Aperture is understood at the 0.5-1.0 mm level. Available n1 known.
• Stored beams are characterized and reasonably close to nominal behavior (lifetime > 5h, emittance < 3.75 m).
– Injection bucket monitor application commissioned (for verification of injection) and “bucket tagging” done (A.2) for both rings (collision points known).
– Radial position adjusted and consistent for both rings (A.2, A.3).
– Separation bump knobs prepared in LSA.
– Operational tools for “simultaneous” beam measurements in beam 1 and beam 2.
• At higher intensities:
– Phase A.5 completed.
– Automatic machine protection is operational (impossible to baby-sit two independent beams at all times).
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Summary Commissioning Plan1. Preparation and verification of injection. (A.6.1)
2. Clean up corrector settings in all IR’s to have consistent values for the two beams. (A.6.2)
3. IR set-up for and with two low intensity beams:
a) Separation bump set-up. (A.6.3)
b) “Common” beam diagnostics (BPM’s, BLM’s) commissioning and checks (A.6.5)
c) Triplet alignment check. (A.6.4)
d) IR aperture characterization and safety check. (A.6.3)
e) Passive protection set-up: injection and tertiary collimators. (A.6.6)
f) Machine protection set-up (e.g. software interlock on separation bumps) and verification. (A.6.3)
g) RF phasing. (A.6.5)
h) Collisions at 450 GeV.
4. Two-beam multi-bunch operation without crossing angles:
a) Interleaved injection process. (A.6.7)
b) Equalization of beam characteristics. (A.6.8)
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2 – Clean Up Corrector Settings
• Settings of common correctors might be different for the two beams after single beam commissioning.
• Zero or minimize settings of common correctors.
• If we cannot get through without common correctors, try beam1 corrector settings with beam2 or vice versa.
• Apply orthogonal beam1/2 orbit correction with 1 beam (enforce zero change for not filled beam).
• If problems encountered to store two beams (very unlikely):
– Longitudinally separate the two beams (injection into different buckets).
– Inject and correct on two beams (1000 turn data and/or orbit).
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3a – IR Set-up: Separation • Already done in A.4.9?
• The two beams can collide in the IP´s (or close to them) without separation.
• Separation bumps must be set up before putting two beams. Can be done for individual single beams.
• Knobs for the different IP´s exist can be put into place once the beam is centered in the triplet and once BPM offsets are known:
– Base bump: Uses common correctors. Not orthogonal (separation in both beams). Is set up deterministically.
– Tuning bumps: Orthogonal for beam1/2 and for x/y. Get correct orbit and separation.
– Degrees of freedom (DOF) per IR: 5 around the ring 20 DOF for separation.
• Plenty of aperture should be available at injection (no crossing angle).
• Dispersion is changed.
• Check aperture after putting separation bumps.
• Separation constant in normalized coordinates (fields ramped with √) during the energy ramp.
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X-plane Y-plane
R. Assmann - LTC X-plane Y-plane
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Horizontal Dispersion with Separation Bumps
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3b – Commission Common Beam Diagnostics
• Common BPM´s:
– There are no beam-beam effects or other electro-magnetic couplings expected between the two beams (low intensity, 156 bunches, no crossing angle).
– Each beam during two beam operation should have the same position readings as the single beam.
– Can be checked by dumping one of the two beams (avoids uncertainties from drifts).
• BLM‘s:
– There can be cross-talk from beam losses to BLM‘s located for the other beam.
– Effect should be measured and compared to expectations (small effect is predicted). Can have impact on BLM thresholds. Compare with single beam results for cross-talk.
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3c – Determine BPM to QUAD Offsets in Triplets• K-modulation in common triplets for the two beams (if not done before):
– Relies on the fact that the beam orbit is insensitive to the quadrupole strength if the beam is in the magnetic center of the quadrupole!
– Changes in quad strength (k-modulation) will reveal any beam offsets in the quadrupole.
– Output are the BPM readings for beams in the magnetic center of the quadrupole: x0,beam1, x0,beam2, y0,beam1 and y0,beam2.
– Output is also * in the IR (tune measurement).
• Knowledge of this data will allow:
– Centering beams in the triplet aperture.
– Cross-check of BPM offsets for beam1 and beam2.
– Deterministic set-up of separation bumps (relying on relative BPM readings). If problems realign triplets!?
• Can be done manually or automatically.
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3d – IR aperture Characterization
• Minimum available IR aperture is easily determined via standard techniques:
– Static closed orbit bumps until beam loss is measured (edge can be defined by collimators at x).
– Separation bumps can be used.
– Must be done with local bumps, as IR must not be the limiting aperture at injection.
• Does not mean a full determination of IR aperture versus s.
• Can be skipped if done carefully before this phase, if common BPM’s are performing well and if separation bumps are understood.
• Details not described here. See other presentations.
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3e – Adjust Two-Beam Collimators
• Two-beam collimators are common to two beams, collimating one in the
vertical plane and leaving the other free.
• In IR 2 and IR 8:
– Each IR has 1 two-beam collimator for injection protection (TCLIA). Even if
not used at this stage, adjust their positions to the beam such that we get
nominal aperture.
– Each IR has 2 two beam collimators for triplet protection (TCTVB). Even if
not required at injection, adjust their positions to the beam such that we get
nominal aperture.
R. Assmann - LTC
Procedure 2-Beam IR: Check Basics
• Up to 3×1010 p (safe beam). Select IR (one IR at a time):
• Prepare and verify proper injection (correct buckets for collisions in selected IR, consistent radial offsets for both beams).
• Fill B1.
• Put in base separation bump and correct orbit B1 without common correctors.
• Dump B1.
• Fill B2.
• Correct orbit B2 without common correctors.
• Dump B2.
• Fill B1+B2.
• Perform common orbit correction.
• Dump B1. Check change in B2 meas. of common BPM’s and BLM’s in the IR’s.
• Dump B2.
• Fill B1+B2.
• Dump B2. Check change in B1 meas. of common BPM’s and BLM’s in the IR’s.
• Dump B1.
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Procedure 2-Beam IR: Alignment, Orbit, Separation
• Fill B1+B2.
• Check offsets triplet magnetic center versus BPM’s (k-modulation).
• Triplet realignment if a bad surprise was encountered (preferably delay any needed realignment to planned access period).
• Fine tune separation bumps in x, y, B1, B2 to achieve nominal separation orbit.
• Measure dispersion.
• Record reference settings.
• Set up and/or check passive protection from IR collimators: TCTH, TCTVA, TCTVB, TCLIA, TCLIB, TDI.
• RF phasing.
• Collapse separation bump and observe changes in orbit and loss maps.
• Correct closed orbit and record required correction (incorporate in separation bumps, if strong corrections are needed).
• Parasitic collisions.
• Dump.
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Procedure 2-Beam IR: Protection Checks IR
• Fill B1.
• Measure available IR aperture for B1.
• MP check: Try to put 7 TeV separation bump at 450 GeV. Abort before quench triggered from BLM system. If needed, adjust BLM thresholds.
• Fill B2.
• Measure available IR aperture for B2.
• MP check: Try to put 7 TeV separation bump at 450 GeV. Abort before quench triggered from BLM system. If needed, adjust BLM thresholds.
• Fill B1+B2.
• Start software interlock on separation bumps and check functionality.• Ready for increased intensities.
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4a – Interleaved Injection
• Goal is to have two beams stored at 450 GeV with equal properties, like
intensity and emittance. It is known:
– Many beam parameters are functions of time.
– Two stored beams can have inter-dependencies, especially for more than
156 bunches.
• It is therefore preferable to set up an interleaved injection process.
Advantages also for machine protection (sanity of two beams constantly
monitored during injection process).
• If this interleaved injection is prepared, it should be applied early on as a
standard filling mode.
• Verification of procedure is logically included in this commissioning
phase.
• Details: LHCCWG discussions (SPS supercycle with 2 LHC cycles or change supercycle, …)
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4b – Equalize Beam Characteristics
• Measure major beam characteristics (emittance, lifetime, intensity) and
optical parameters (tune, chromaticity, …).
• Should be equal for two beams or one beam stored. Can be checked by
dumping one of the two beams.
• Should be equal from one beam to the other.
• If unequal between the beams then equalize the two beams.
• Tolerances relaxed at lower intensities and larger beta* (much smaller
beam-beam effect) but good to diagnose potential issues early on.
• Will facilitate the diagnostics of beam behavior during ramp, squeeze
and collision.
• Important if unequal luminosity between experiments results.
R. Assmann - LTC
Summary Commissioning Plan1. Preparation and verification of injection. (A.6.1)
2. Clean up corrector settings in all IR’s to have consistent values for the two beams. (A.6.2)
3. IR set-up for and with two low intensity beams:
a) Separation bump set-up. (A.6.3)
b) “Common” beam diagnostics (BPM’s, BLM’s) commissioning and checks (A.6.5)
c) Triplet alignment check. (A.6.4)
d) IR aperture characterization and safety check. (A.6.3)
e) Passive protection set-up: injection and tertiary collimators. (A.6.6)
f) Machine protection set-up (e.g. software interlock on separation bumps) and verification. (A.6.3)
g) RF phasing. (A.6.5)
h) Collisions at 450 GeV.
4. Two-beam multi-bunch operation without crossing angles:
a) Interleaved injection process. (A.6.7)
b) Equalization of beam characteristics. (A.6.8)
R. Assmann - LTC
Exit Conditions
• Two 450 GeV beams safely stored with lifetime of 5-10 h.
• Separation bumps fully commissioned with common correctors.
• Beam calibrated separation bumps including corrections.
• Triplet alignment checked.
• IR aperture fully characterized and safe for both beams (n1 > 7).
• Passive protection (collimators) in place for both beams.
• The injection * of 11m fully proven. Else fallback to 17 m (very unlikely).
• Automatic machine protection checked with safe intensities (e.g. “7 TeV separation bumps” in IR’s).
• Interleaved injection process commissioned.
• Beams reasonably equalized:– ∆ Emittance ≤ 40%
– ∆ Intensity ≤ 20%
• Up to 156 bunches possible in the IR.
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Conclusion• This is not the most complicated stage of commissioning but will certainly be
very exciting.
• This commissioning step can be performed quickly (even too quickly) but should
be done properly to have a good and safe base for further work (I estimate 3-4
shifts per IR, 3-4 days in total) . Goal: Understand the IR’s early on and fix
problems (alignment) if machine access is possible.
• It will be the first time that we must have a detailed look at the experimental IR’s:
this will come back with more stringent requirements later.
• Full safety of the IR’s assessed and proven with dedicated MP tests. Remember:
Special risks associated with IR orbits and bumps!
• First 450 GeV collisions in every IR possible without significant overhead.
• Two-beam operation (450 GeV and 7 TeV) will become challenging with more
than 156 bunches, high intensities and lower beta*: long-range beam-beam,
crossing angles, reduced aperture, beam-beam effects, …
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Backup
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5 – Equalize Radial Offsets
• It is assumed that the same RF frequency and harmonic number is set up for the two beams (see presentation by G. Arduini).
• This means that the two beams have the same revolution frequency.
• Any different ∫BdL or path length for the two beams will then result in a momentum offset and a corresponding radial offset:
1 cm C 1.5 mm x in the arc
• Offsets in the two beams should be of equal magnitude. Equalize if necessary (see talks by A. Butterworth and G. Arduini). Implications on injection.
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6 – Adjust Injection Timing• LHC has 2 independent RF systems. LHC is the master.
• The bunches in beam1 and beam 2 need to be injected into the right RF buckets.
• Requirement: Bunches collide at the IP.
• Final phasing will be done with feedback from experiments.
• Rough phasing done here:
– Observe beam induced pickup signals in a common BPM and adjust the delays. Ideally done with two common BPM‘s symmetrically at both sides of the IP, using same cable length (ideal phasing condition: beam 1 left arrives at same time as beam 2 right).
– Alternatively, do rough phasing with single beam wall-current monitors in IR4 (known cable length).
– Details by P. Baudrenghien in June.
• Monitor of injection buckets (foreseen from fast BCT) should be operational.
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7 – Verify and Adjust Separation Bumps
• After detailed adjustments (radial offsets, injection timing, equalization,
BPM offsets) check again crossing bumps.
• If needed fine-adjust to the target bumps.
• Determine the aperture in the triplets in absolute and normalized terms.
• Separation constant in normalized coordinates (fields not ramped)
during the energy ramp.