The challenges faced by the Large Hadron Collider’s (LHC) ion injector chain are similar to those involved in proton operations: the generation of high-density bunches and acceleration of these beams through successive accelerators without a degradation in the beam quality.
The lead ion source
The chain starts off with pellets of Pb208; very pure, at 99.57 per cent, these are purchased from CEA Saclay at a cost of 500 Euros per gram - which is ten times more expensive than the cost by weight of gold. These are inserted in a micro-oven, where they are heated to around 500°C, and transformed into lead vapour.
From here, the vapour is injected into the Plasma Chamber of an Electron Cyclotron Resonance. The lead vapour is mixed with oxygen and ionised by an electron current, producing different charged states (O2+, Pb27+, Pb28+ and Pb29+.)
A magnetic spectrometer selects Pb29+ ions and sends them via the radiofrequency quadrapole onto Linac3; here they are accelerated to 4.2 MeV per nucleon. After acceleration, the beam is focused onto a carbon foil, a few microns in thickness, which further strips the ions. This is followed by another spectrometer, which selects Pb54+ for transfer to LEIR.
Several pulses from Linac3 are accumulated in the Low Energy Ion Ring (LEIR), and transformed into dense bunches, which are suitable for heavy ion operation in the LHC.
This bunching is accomplished by means of a technique called electron cooling. Here, an exchange of energy between electrons and the ions results in a reduction of the energy spread and size of the ion beam, making it denser.
For nominal LHC ion operation, four or five Linac3 pulses - each one lasting about 200 milliseconds, or 70 revolutions within LEIR - are accumulated to generate four ion bunches in LHC. These are then accelerated to 72 MeV per nucleon.
Next, in the proton synchrotron (PS), the beam is not only accelerated to 5.9 GeV per nucleon, but is also arranged into the basic bunch structure. After acceleration, four bunches are ejected from the PS and transferred through another stripper. This removes the remaining electrons to obtain Pb82+ ions, which are then injected into the super-proton synchrotron (SPS), where they are accelerated to 177 GeV per nucleon. These lead beams are then sent to the LHC. 12 batches from the SPS can fill the LHC with 592 bunches.
Note that the LHC ion run this year will not be carried out with the nominal scheme described above. The so-called ‘early’ scheme, adopted for the first LHC ion run, will feature a smaller number of bunches - 121 - and, as a consequence, lower luminosities. The advantage of this is that the scheme is less challenging, and safer, for the injector chain.