First LHC heavy ion run in 2010
We talked with John Jowett, in charge of the heavy ion beams at LHC, about the preparation of the LHC heavy ion run. Here is a summary of the main points.
- The LHC heavy-ion programme will start with collisions of lead nuclei in November 2010. This will be a large step in energy, a factor of 13.5, with respect to RHIC, the heavy ion collider in Brookhaven. This is one of the biggest ever, to be compared with the factor of 3.5 for LHC proton-proton collisions with respect to the Tevatron.
- The beam energy will be that of the proton beam multiplied by the charge of the lead ion (Z=82, A=208)
- 3.5 Z TeV = 287 TeV = 1.38 A TeV = “1.38 TeV/nucleon”.
The centre of mass energy is twice this, 2.76 TeV per colliding nucleon pair, and this is most relevant for physics.
- The injector chain is different from the system used for protons into the PS - see “LHC ion injector chain”, to appear in the next issue of ALICE Matters. Lead ions from the ECR ion source are first accelerated in Linac3, sent to the LEIR ring for cooling, to prepare the high density beams for LHC. Stripping of ions happens at various transfer stages (Pb29+, Pb54+); finally bare lead nuclei (Pb82+) are sent to SPS and then to LHC.
- The LEIR commissioning started in August. Final work on the rather complicated RF system in the SPS is ongoing.
- Luminosity of 10^25 cm-2 s-1 is the initial target. This is lower than the nominal 10^27 cm-2 s-1 because the energy is lower than the nominal (which means bigger beam emittance), β* is 3.5 m (instead of 0.5 m for the 5.5 TeV per colliding nucleon pair); also in this first ion run there will be fewer bunches (about 120 instead of 592).
- The plan is to commission HI (Heavy Ion) operation of LHC during the first week, building on the fact that we have a machine that already works with protons. Everything to do with magnetic fields should be similar – and this is the main reason we think we can go quite fast. The RF frequency will have to be modified slightly since massive nuclei are slightly slower than protons, especially at injection energy.
- The collimation setup will require some time; the physics of nuclei interacting with collimators is very different from protons. This is where things will begin to look different. Later with higher luminosity, we expect to be limited by ultra-peripheral interactions at the collision point – losses that can quench magnets.
- The priority is to provide integrated luminosity to ALICE, CMS and ATLAS; but we must go carefully to protect the machine, since the <0.5 MJ stored energy per beam is still significant.
- We can not allow the bunch intensity to drop too much (> 25% of the nominal) since we are close to the lower limits of the beam position monitors to measure the orbit; this constrains the commissioning somewhat.
- The net crossing angles in ALICE will be zero – our orbit bump will cancel the ALICE spectrometer bump.