1. R. Assmann - LHCCWG Two Beam Operation R.W. Aßmann LHCCWG 8.5.2007 Acknowledgements to W. Herr, V. Previtali, A. Butterworth, P. Baudrenghien, J. Uythoven, J. Wenninger, … Not a final presentation  depending on your comments and input a next iteration might be needed…

2. R. Assmann - LHCCWG Commissioning Stage This concerns phase A.6: “450 GeV – Two Beam Operation” Essentially this means: –No crossing angle required (at maximum 156 bunches). –Do not worry about collisions (this is a different step). 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.

3. R. Assmann - LHCCWG Entry Conditions 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. –Stored beams are characterized (emittance, lifetime, …) and reasonably close to nominal (for example, emittance will affect effective separation of beams). Automatic machine protection is operational (impossible to baby-sit two independent beams at all times).

4. R. Assmann - LHCCWG Major Issues for Two Beams 1.Set up separation bumps to have roughly the correct separation between the beams in the IP. 2.Clean up corrector settings to have consistent values for the two beams. First common orbit correction. 3.Commission “common” beam diagnostics (common BPM’s, BLM’s) with two beams. 4.Determine BPM to quad offsets in triplets with k-modulation for each beam separately. 5.Equalize radial offsets in case of different ∫BdL for the two beams (see presentations A. Butterworth and G. Arduini). 6.Adjust injection timing such that we inject bunches into roughly the right buckets (beams should collide in the IP´s if put into collision). 7.Equalize beam characteristics (intensity, emittance, optics, lifetime) if necessary. 8.Verify and adjust separation bumps to maximize aperture in triplet. 9.Set up two beam collimators in IR2 and IR8 (TCLIA and TCTVB).

5. R. Assmann - LHCCWG 1 – Setup of Separation Bumps Already done in A.4.9? The two beams would collide in the IP´s (or close to them) without separation bumps. 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 deterministically once the beam is centered in the triplet and once BPM offsets are known. Plenty of aperture should be available at injection (no crossing angle). Check aperture after roughly putting crossing bumps. Separation constant in normalized coordinates (fields not ramped) during the energy ramp.

6. R. Assmann - LHCCWG X-planeY-plane

7. R. Assmann - LHCCWG X-planeY-plane

8. R. Assmann - LHCCWG Horizontal Dispersion with Crossing Bumps

9. R. Assmann - LHCCWG 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. Put two beams with safe intensities. Do a first orbit correction with common correctors with both beams present in the machine. Increase of intensity is possible after this step (if A.5 has been done).

10. R. Assmann - LHCCWG 3 – 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.

11. R. Assmann - LHCCWG 4 – Determine BPM to QUAD Offsets in Triplets K-modulation in common triplets for the two beams : –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: x 0,beam1, x 0,beam2, y 0,beam1 and y 0,beam2. 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 for each single beam independently (faster with two beams at the same time).

12. R. Assmann - LHCCWG 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.

13. R. Assmann - LHCCWG 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.

14. R. Assmann - LHCCWG 6 – 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.

15. R. Assmann - LHCCWG 7 – Verify and Adjust Crossing 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.

16. R. Assmann - LHCCWG 8 – 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.

17. R. Assmann - LHCCWG 9 – Other Issues Machine Protection has some features related to two beams (beyond BLM coupling between the beams): –Will a normal MP interlock always dump both beams? If yes, check with two beams. –Is it possible to dump one beam only, keeping the other? A plus for understanding issues but a safety risk? Sequence of injection and possibility of interleaved injection: –If injection is done first for one beam and then for other beam: When to retract injection protection elements for each beam? Sequence of injection inhibits and permits? Intensity difference between beams? –If interleaved injection: Problems for large injection oscillations on the other beam? Sequence of injection inhibits and permits?

18. R. Assmann - LHCCWG Major Issues for Two Beams 1.Set up separation bumps to have roughly the correct separation between the beams in the IP. 2.Clean up corrector settings to have consistent values for the two beams. First common orbit correction. 3.Commission “common” beam diagnostics (common BPM’s, BLM’s) with two beams. 4.Determine BPM to quad offsets in triplets with k-modulation for each beam separately. 5.Equalize radial offsets in case of different ∫BdL for the two beams (see presentations A. Butterworth and G. Arduini). 6.Adjust injection timing such that we inject bunches into roughly the right buckets (beams should collide in the IP´s if put into collision). 7.Equalize beam characteristics (intensity, emittance, optics, lifetime) if necessary. 8.Verify and adjust separation bumps to maximize aperture in triplet. 9.Set up two beam collimators in IR2 and IR8 (TCLIA and TCTVB).

19. R. Assmann - LHCCWG Conclusion This is not the most complicated stage of commissioning but will certainly be very exciting. This commissioning step can be performed quickly but should be done properly to have a good base for further work. 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. 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, … Some proposals: early k-modulation in triplets, timing signal at injection (from fast BCT), selective beam dump.