Electron-positron linear colliders are currently being studied as potential future Higgs-factories. The two most mature studies are for the International Linear Collider (ILC) in Japan, and the Compact Linear Collider (CLIC) at CERN, Switzerland. Linear colliders rely on low emittance high intensity beams created in damping rings and ultimately being focussed to the nano-meter level at the collision point.
The current volatility in energy prices underlines the importance of reducing the power needed for operating future facilities. Both linear collider projects, collaborating in many areas, have extensively studied novel design and technology solutions to address power efficiency and reduce the environmental impact of the facilities. The sustainability considerations, in addition to the more traditional cost concern and need for developing core technologies, are today primary R&D drivers for the projects. These studies have recently been summarized in a contribution  to the International Atomic Energy Agency (IAEA) “Conference on Accelerators for Research and Development: from good practices towards socioeconomics impact”.
This article briefly summarized the studies performed and on-going within the CLIC collaboration. The CLIC RF technology is based on normal conducting 12 GHz accelerating structures. The initial 11.5 km stage provides collisions at 380 GeV at a luminosity of 2.3 x 1034 cm-2s-1. CLIC can be upgraded in energy and luminosity as part of a longer-term electron-positron collider programme.
Concerning energy consumption, the CLIC power consumption has been estimated to 110 MW at 380 GeV . Turning these power numbers into yearly energy consumption gives estimates around 600 GWh. As a reference CERN uses around 1.2 TWh of electricity yearly. The initial stage CLIC numbers are considerably lower than earlier estimates, which were largely based on scaling from the 3 TeV machine studied for the Conceptual Design Report (CDR) in 2012. The reduction is around a factor two, out of which a fraction is a trivial scaling going from 500 GeV in the CDR, to 380 GeV adapted for Higgs and top physics.
To achieve the reduced numbers several dedicated studies have been conducted to control and optimise the power consumption, in parallel with studies considering the environmental impact of the facilities in a wider sense. Many of these studies are widely applicable and generally relevant for future accelerator facilities. Among the studies carried out are:
- The designs of CLIC, including key performance parameters as accelerating gradients, pulse lengths, bunch-charges and luminosities, have been optimised for cost but also increasingly focussing on reducing power consumption. The parameter sets giving the lowest cost and power for a given luminosity have been identified and retained as baseline.
- Technical developments and studies targeting reduced power consumptions at system level, primary examples are RF system design optimisation, developments of high efficiency klystrons , and studies of permanent magnets for damping rings and linacs .
- The possibility of making use of the fact that the linear colliders are single pass, i.e. the beams and hence power are needed “shot by shot”, possibly allowing to operate in daily or weekly time-windows when power is available in abundance from suppliers and costs are reduced . Seasonal operation is already being used for energy cost reasons.
- Estimating the renewable power that can be made available for running the colliders by investing for example 10% of the overall construction costs in solar and wind energy capabilities , again profiting from the fact that single pass colliders can quickly adapt to changes in energy output from such sources.
- Technical solutions for recovering energy losses in all parts of the accelerator, to be reused for acceleration and/or for use in the local area (homes, industry) near the facility.
In many cases the studies mentioned are still on-going and further work is needed. For CLIC these studies will be included in the planned Project Readiness Report for the next European Strategy Update. Among the studies planned is an analysis of the start to end environmental impact including carbon footprints for CLIC. While one can expect that energy production in a decade or two are largely carbon free reducing the operational impact, the evaluation of raw materials, and their processing, being used for the civil engineering and accelerator will need to be carefully analysed. Decommission will also be considered. The power and energy use of CLIC at 1.5 and 3 TeV will be revised, including the saving mentioned above. Current estimates date back to the CDR in 2012 and are by now outdated and too high.
As mentioned initially many of these studies are equally applicable to ILC and many will be done together with ILC. As ILC is a green field installation there are interesting possibilities to address sustainability from the very start for the facility.
 List B. et al, Sustainability studies for Linear Colliders: https://conferences.iaea.org/event/264/contributions/21011/.
 The CLIC project, Snowmass White Paper, https://arxiv.org/abs/2203.09186.
 Cai J. and Syratchev I., Modelling and Technical Design Study of Two-Stage Multibeam Klystron for CLIC, doi: 10.1109/TED.2020.3000191.
 Shepherd B., Permanent Magnets for Accelerators, https://jacow.org/ipac2020/papers/moviro05.pdf.
 Fraunhofer CLIC power/energy study: https://edms.cern.ch/document/2065162/1.