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).

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

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).

R. Assmann - LTC

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.

R. Assmann - LTC

X-plane Y-plane

R. Assmann - LTC X-plane Y-plane

R. Assmann - LTC

Horizontal Dispersion with Separation Bumps

R. Assmann - LTC

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.

R. Assmann - LTC

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.

R. Assmann - LTC

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.

R. Assmann - LTC

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.

R. Assmann - LTC

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.

R. Assmann - LTC

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.

R. Assmann - LTC

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, …)

R. Assmann - LTC

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.

R. Assmann - LTC

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, …

R. Assmann - LTC

Backup

R. Assmann - LTC

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.

R. Assmann - LTC

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.

R. Assmann - LTC

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.