CERN Accelerating science

Optimized first energy stage for CLIC at 380 GeV
by Daniel Schulte & Philipp Roloff (CERN)

  The CTF3 test facility at CERN, which has demonstrated CLIC’s novel two-beam acceleration technology (Image credit: Maximilien Brice)

In the post-LHC era, one of CERN’s potential options for the next flagship accelerator is an electron–positron collider at the high-energy frontier; the Compact Linear Collider (CLIC).

In August 2016 the CLIC collaboration, which consists of 75 institutes, published an updated baseline scenario. This scenario starts with a first energy stage at 380 GeV center-of-mass, followed by a second stage with an energy around 1.5 TeV, and a final step to 3 TeV.

Prior to the discovery of the Higgs boson particle, the CLIC conceptual design report (CDR) focused on the design of the 3 TeV stage and has documented the viability of the technology required for this energy. Lower energy stages have been considered with much less detail.

With the information obtained from the Higgs discovery, the optimum energy choice for the first stage was also studied. The physics programme has been evaluated, including detailed studies of realistic detector configurations. The choice of 380GeV would allow detailed measurements of the Higgs boson and the top quark.

To optimize the CLIC accelerator, a systematic design approach has been developed and used to explore a large range of configurations for the RF structures of the main linac. For each structure design, the luminosity performance, power consumption and total cost of the CLIC complex are calculated.

For the first stage, different accelerating structures operating at a somewhat lower accelerating gradient of 72 MV/m will be used to reach the luminosity goal. The design of this will have a cost and power consumption similar to earlier projects at CERN such as LHC with its injectors, whilst it ensures that the cost of the higher-energy stages is not inflated. The design should also be flexible enough to take advantage of projected improvements in RF technology during the construction and operation of the first stage.

In order to  upgrade to higher energies, the structures optimized for 380 GeV will be moved to the beginning of the new linear accelerator and the remaining space filled with structures optimized for 3 TeV operation. The RF pulse length of 244 ns is kept the same at all stages to avoid major modifications to the drive-beam generation scheme.

Data taking at the three energy stages is foreseen to last for a period of seven, five and six years, respectively. The stages are interrupted by two upgrade periods of two years, meaning that the overall three-stage CLIC programme would last for 22 years from the start of operation. The duration of each stage is derived from integrated luminosity targets of 500 fb–1 at 380 GeV, 1.5 ab–1 at 1.5 TeV and 3 ab–1 at 3 TeV.

Overview of the CLIC layout at 3 TeV, showing combiner rings (CR), delay loop, damping ring (DR), pre-damping ring (PDR), bunch compressor (BC) and beam delivery system (BDS). The red and green squares represent beam dumps. (Image Credit: CLIC collaboration). 

Further improvements are being pursued via an intense R&D programme. For instance, the CLIC study recently proposed a novel design for klystrons that could increase efficiency significantly. In addition, permanent magnets are also being developed that are tunable enough to replace the normal conducting magnets are also being developed as they could reduce power consumption even further.

The goal is to develop a detailed design of both the accelerator and detector in time for the update of the European Strategy for Particle Physics towards the end of the decade.

*A version of this article appeared in the November 2016 issue of CERN Courier.