R2E – identifying problems, mitigating risks

During LS1, the R2E project team will be working on a task as painstaking as it is crucial: to achieve a sixfold reduction in the number of electronic malfunctions caused by radiation. On their success depends the ability of the accelerator to function correctly at nominal energy. No mean challenge, considering it comes on top of the tenfold reduction already achieved since 2009.

The graph plots the rate of LHC beam dumps due to single-event effects against beam luminosity. An indication of the challenge that faces the R2E project teams during LS1!

The origins of the project known as R2E (Radiation to Electronics) go back to 2007, when the CNGS (CERN Neutrinos to Gran Sasso) experiment was being commissioned. "Right from the outset, some CNGS control systems were causing problems. They would regularly break down in operations with beam," recalls Markus Brugger, head of the R2E project. "Even though the beam intensity was very low, we began to suspect that radiation was behind the problem." Their suspicion was soon borne out: the electronics turned out to be vulnerable to disruption caused by the impact of individual energetic particles, which can cause logic errors and other single-event effects*. For CNGS the problem was solved by installing thick radiation shields in situ and moving all of the control systems into a protected area; but it highlighted a potential issue for the LHC, scheduled to enter operation one year later.

"Most of the equipment installed in the LHC tunnel was designed with sufficient radiation hardness built in, so theoretically they should not have been affected," notes Markus Brugger. "But the same was not true for off-the-shelf equipment installed in the shielded areas located near the tunnel; it remained vulnerable, despite the shielding." But how was the vulnerable equipment to be identified? A detailed inventory was drawn up in 2008 and 2009. At the same time, testing began of the instrumentation that was at risk. This was done in a test area set up next to the CNGS experiment, which had an environment similar to that of the LHC, and at the Paul Scherrer Institute (PSI). In 2010, for practical reasons, a dedicated test area called H4irrad was finally set up on the H4 beam line in the North Area. It in turn will soon be replaced by a bigger facility, currently under construction on the site of the old DIRAC experiment. It should come into operation in mid-2014.

The purpose of the equipment testing is to identify critical components so that they can be replaced, improving the radiation hardness of the hardware and ensuring that all of the electronics installed in the accelerator chain and exposed to radiation will operate correctly. The R2E project team has been working on this for more than three years, in close collaboration with all of the groups responsible for LHC equipment and with the invaluable assistance of the service groups.

So what can be done when an item is at risk but there is no way of improving its characteristics? "In that case, you have three possibilities," explains Markus Brugger. "You can build new, more heavily shielded caverns, but you don’t always have the time or the resources to do this. You can provide additional shielding in the existing protected areas. Or you can move the equipment further away." In the time since LHC start-up, several relocation and shielding campaigns have already been conducted. In parallel, the Radiation Working Group performs post-mortems on all failures with beam loss, in order to determine which ones are due to radiation. "The major relocation and shielding work that was done in the framework of the R2E project was only possible thanks to the energetic assistance of the groups in charge of the LHC machinery and the EN/EL, EN/CV, EN/HE and EN/MEF groups, among others," says Anne-Laure Perrot, in charge of integration of the R2E mitigation measures in the underground installation of the LHC. "I want to thank them all for the excellent work done."

*The phenomenon of single-event effects is a stochastic one, affecting electronics in a random way.

by Anaïs Schaeffer