LHC Report: Cloudy with sunny spells

The LHC is continuing its 25 ns intensity ramp-up and has now reached 1465 bunches per beam. Performance is reasonable and the experiments have seen some long fills with steadily increasing luminosity delivery rates. Some now familiar issues continue to make life interesting.


The image shows the heat load evolution as measured in specially equipped dipoles. (Image: Giovanni Iadarola). Top frame: energy and intensity. Middle frame: measured heat load in W/m. Bottom frame: heat load normalised to total beam intensity.

One of the key challenges of 2015 was always expected to be electron clouds. The two scrubbing runs that were performed in the summer successfully qualified the LHC for up to around 1500 bunches. However, the final phase of the scrubbing, which saw the move from regular 25 ns beam to the doublet beam, proved difficult, and the scrubbing team concluded that the machine was not yet well-enough scrubbed for the doublets to be used effectively.

The 25 ns intensity ramp-up has thus had to contend with significant but manageable electron clouds. The main consequence of this has been heat load to the beam screens in the cold sectors of the machine. The beam screens’ primary function is to intercept beam-induced heat loads at a temperature well above that of the magnets. They are actively cooled by a forced flow of supercritical helium with a regulated outlet temperature (of the beam screens) between 17 K and 20 K. The additional and quite considerable heat load from the electron clouds is proving a challenge for the regulation of the cryogenics system, in particular during the sharp transitions at injection and during the energy ramp. This has led the operators to opt for a non-aggressive approach, allowing the cryogenics team time to fully adjust following each increase in the number of bunches. On the positive side, by operating with electron clouds, the machine continues to scrub slowly.

UFOs are still present in considerable numbers, but thanks to the judicious beam-loss monitor thresholds, most are below the dump level and only a few fills have been lost to UFOs since resuming the intensity ramp-up after the last technical stop.

With 1465 bunches per beam, the peak luminosity is around 3.5 x 1033 cm-2s-1. This is around half the luminosity of the peaks seen with the 50-ns beam in Run 1. However, the beam seems happy at 6.5 TeV and is enjoying the benefits of a relatively modest bunch population, synchrotron radiation damping and a relaxed squeeze. Because of these factors, the instantaneous luminosity is falling slowly, allowing for some gratifyingly long fills. For example, Tuesday to Wednesday saw a 17-hour fill that delivered around 160 inverse picobarns to both ATLAS and CMS. 

by Lionel Herblin & Mike Lamont for the LHC team