Using a crystal to extract beam?

These taut wires, 60 μm in diameter, are part of the electrostatic septa in the SPS. More than 10 000 wires are needed to extract protons at 400 GeV. The study group is attempting to reduce losses on these wires using bent silicon crystals. (Image: CERN)

The aim of the SHiP (Search for Hidden Particles) experiment, a proposed project currently being studied by CERN's Physics Beyond Colliders (PBC) group, is to search directly for hidden particles that could explain certain beyond-Standard-Model phenomena.

If approved, SHiP will use 400-GeV beams from the Super Proton Synchrotron (SPS), which will be directed at a fixed target at the end of a 1-kilometre beam transfer line.

Configuration of the future SHiP experiment. The SPS beams will be extracted slowly at long straight section 2 (LSS2) and sent towards transfer line TT20. (Image: CERN)

However, to have any chance of identifying the hidden particles, the researchers have to control the background noise generated when the beam hits the target. Slow extraction of the beam from the SPS achieves this by significantly reducing the number of protons that hit the target each second.

The standard slow-extraction method is known as “resonant extraction”. It allows the production of long spills lasting for seconds in the extraction line. This is done using extraction sextupoles that cause the particles in the beam to move in an unstable but controlled way. An electrostatic septum then catches the most unstable protons and deflects them towards the extraction line.

Unfortunately, this method has one major and unavoidable drawback: a small but significant amount of beam is lost during the extraction. The SHiP experiment requires no fewer than 4 x 1019 protons on target per year, which can only be achieved by reducing these beam losses by a factor of four. “Most beam losses occur on the electrostatic septum,” explains Brennan Goddard, TE-ABT (Accelerator Beam Transfer) group leader. “Reducing the transverse beam density upstream of the septum is a promising technique for minimising these losses, but, up until now, we haven't been able to meet the requirements for SHiP with the tools available.”

The TE/ABT group, in close collaboration with the UA9 collaboration and the BE/OP and EN-STI groups, has been investigating the possibility of using a tool already used by UA9 for the LHC collimation project: a bent silicon crystal. When placed upstream of the septum, this crystal channels the beam very effectively, resulting in significantly reduced beam losses.“In 2016, we carried out the first tests with a 2-mm-long silicon crystal. In November, we successfully completed a slow-extracted spill, lasting many minutes, of a very-low-intensity 270-GeV beam into the TT20 transfer line,” Goddard explains. “The next steps will be to quantify the beam losses at higher intensities and develop crystals better suited to the extraction process.”

To this end, further studies and tests will be carried out in the SPS in 2017 and 2018. If the tests are successful, it will be excellent news not only for SHiP but also for other CERN facilities that could make use of the technology.

For more information about the SHiP experiment, see this article published in issue 28-29/2015 of the CERN Bulletin.