Preprint number

Sustainability Considerations for Accelerator and Collider Facilities

Thomas Rosera on behalf of the ICFA Panel for Sustainable Accelerators and
Colliders

aBNL, Upton, New York, USA

> ABSTRACT
>
>
>
>
> As the next generation of large accelerator-based facilities are being
> considered at the Snowmass 2021 study high priority has to be given to
> environmental sustainability including energy consumption, natural resource
> use and the environmental impact of effluents. Typically, increased
> performance – higher beam energies and intensities – of proposed new
> facilities have come with increased electric power consumption. In the
> following we discuss the most important areas of development for the
> sustainability of accelerator-based research infrastructures in three
> categories - technologies, concepts and general aspects. To achieve the goal
> of increased performance with reduced energy consumption a focused R&D
> effort is required with the same or even higher priority as the traditional
> performance-related R&D. Such a recommendation was included in the recent
> European Strategy for Particle Physics Accelerator R&D Roadmap [1].

Submitted to the Proceedings of the US Community Study

on the Future of Particle Physics (Snowmass 2021)

##  1 Introduction

Scarcity of resources, along with climate change originating from the
excessive exploitation of fossil energy are ever growing concerns for
humankind. Particularly, the total electric power consumption of scientific
facility operations will become more important as the reliance on fossil fuels
is being reduced, carbon-neutral energy sources are still being developed and
a larger part of the energy consumption, in particular transportation, is
converted from fossil fuel to electric power.

In our accelerator community we need to give high priority to the realization
of sustainable concepts, particularly when the next generation of large
accelerator-based facilities is considered. Indeed, the much-increased
performance – higher beam energy and intensity – of proposed new facilities
comes together with anticipated increased electric power consumption. In the
following we classify the most important development areas for sustainability
of accelerator driven research infrastructures in three categories -
technologies, concepts and general aspects. We suggest investing R&D efforts
in these areas and to assess energy efficiency with an equal level of
relevance as the classical performance parameters of the facilities under
discussion.

##  2 Energy efficient technologies

Energy efficient technologies have a long history in the accelerator
facilities for particle physics since often the required performance could
only be reached with highly energy efficient devices such as superconducting
magnets and superconducting RF cavities. Below are some items, where R&D could
further improve energy efficiency.

Low loss superconducting resonators: Cryogenic losses in superconducting
resonators can be significant for linacs, particularly in CW operation. The
R&D on high Q superconducting resonators should be continued with high
priority. Resonators using Nb3Sn-coating have shown good performance [2] and
could be operated at 4.5 K. At this temperature the cryogenic efficiency is
much improved, while still reasonable Q values are achieved.

Efficient Radio Frequency (RF) sources: For many accelerators the main power
flow involves converting grid power to RF power. To improve the overall
efficiency RF sources must be optimized. Efforts should be invested for
efficient klystron concepts (e.g. adiabatic bunching and superconducting
coils), magnetrons (mode locking) and solid-state amplifiers [3] [4] [5].

Permanent magnets: Permanent magnets don’t need electrical power. As a side
effect no heat is introduced which has a positive effect on the stability of a
magnet lattice. Significant progress has been made with permanent magnets for
light sources, and for example tunable quadrupoles for the CLIC linacs [6].

Highly efficient cryogenic systems: Another important development are
efficient cryogenic sys- tems (e.g. He/Ne refrigeration), allowing to optimize
heat removal in cold systems from synchrotron radiation and other beam induced
energy deposition [7].

Superconducting electrical links: Cables using High Temperature
Superconductors (HTS) allow to power high-current devices from a distance with
no or little losses, thus enabling to install the power converters outside of
radiation areas [9].

Use of heat pumps: Heat recovery in aquifers is often done at low temperatures
with limited usefulness. But after boosting the heat to a higher temperature
level using heat pumps, this waste heat can be used for residential heating.

##  3 Energy efficient accelerator concepts

Increasing the energy efficiency of accelerator components can significantly
reduce energy consumption, but different accelerator concepts, especially with
built-in energy recycling, has the potential to drastically reduce the energy
consumption without compromising the performance.

Energy Recovery Linacs: The Energy Recovery Linac (ERL) concept was first
proposed in 1956 and it allows the recirculation of the beam power after the
beam is used by decelerating it in the same RF structures. Using this concept
for the electron and positron beam a high energy e+e- collider could be built
where more luminosity can be achieved with much less beam intensity than using
storage rings since the single beam collision can be much more disruptive. The
much lower beam intensity then results in much less energy lost to radiated
synchrotron power [10]. For a high energy collider, the energy savings can
amount to over a 100 MW. In view of the significant technical challenges this
scheme should be studied and optimized in more detail.

Intensity Frontier Machines: For Intensity Frontier Machines the conversion
efficiency of primary beam power for example to Muon/Neutrino beam intensity
is a critical parameter. With optimized target and capture schemes the primary
beam power, and thus the grid power consumption, can be minimized. Similar
arguments are valid for accelerator driven neutron sources [5].

Muon Collider: For very high parton collision energies the Muon Collider [11]
exhibits a favorable scaling of the achievable luminosity per grid power. With
constant relative energy spread bunches can be made shorter at higher
energies, allowing stronger transverse focusing at the interaction points.
Besides other arguments this is an important reason for strengthening R&D
efforts on the muon collider concept.

Energy Management: With an increasing fraction of sustainable energy sources
like wind and solar power in the future energy mix, the production of energy
will fluctuate significantly. One way to mitigate the impact of high energy
physic facilities on the public grid is to actively manage their energy
consumption using local storage or dynamic operation. Investigation of such
concepts should be an integral part of design studies.

Accelerator Driven Systems (ADS): Accelerator driven sub-critical reactors can
be used to reduce the storage time of radioactive waste (transmutation) of
nuclear power stations by orders of magnitude. Such concepts would address an
important sustainability problem of nuclear power. The development of high
intensity accelerators for ADS has synergies with applications for particle
physics or neutron sources. Another innovative accelerator-based transmutation
concept using muons is proposed in [8].

##  4 General sustainability aspects

A carbon footprint analysis in the design phase of a new facility can help to
optimize energy consumption for construction and operation. For cooling
purposes accelerator facilities typically have significant water consumption.
Cooling systems can be optimized to minimize the impact on the environment.
For the construction of a facility environment-friendly materials should be
identified and used preferentially. The mining of certain materials, in
particular rare earths, takes place in some countries under precarious
conditions. It is desirable to introduce and comply with certification of the
sources of such materials for industrial applications, including the
construction of accelerators. A thoughtful life-cycle management of components
will minimize waste. Many facilities use helium for cryogenic purposes. Helium
is a scarce resource today and with appropriate measures the helium loss in
facilities can be minimized.

Many of these issues are discussed at the workshop series on ’Energy for
Sustainable Science at Research Infrastructures’ [7].

## References

  * [1] European Strategy for Particle Physics - Accelerator R&D Roadmap, N. Mounet (ed.), 2021. https://cernbox.cern.ch/index.php/s/sGq4LyfnHUfboUC
  * [2] M. Liepe. Superconducting RF for the future: Is Nb3Sn ready for next-generation accelerators? In Proc. 10th Int. Particle Accelerator Conf., Melbourne, Australia, 19–24 May 2019.
  * [3] EuCARD2 Workshop on EnEfficient RF Sources, Cockcroft Institute, Daresbury, UK, 3–4 Jun. 2014. https://indico.cern.ch/event/297025/
  * [4] ARIES Workshop on Energy Efficient RF, Angstrom Laboratory, Uppsala, Sweden, 18–20 Jun. 2019. https://indico.uu.se/event/515/
  * [5] Workshop on Efficiency of Proton Driver Accelerators, PSI, Villigen, Switzerland, 29 Feb.–2 Mar. 2016. https://indico.psi.ch/event/3848/
  * [6] EuCARD2 Workshop on Compact and Low Consumption Magnet Design for Future Linear and Circular Colliders, CERN, Geneva, Switzerland, 26–28 Nov. 2014. https://indico.cern.ch/event/321880/
  * [7] 5th Workshop on Energy for Sustainable Science at Research Infrastructures, PSI, Villigen, Switzerland, 28–29 Nov. 2019. https://indico.psi.ch/event/6754/
  * [8] Y. Mori et al., Intense negative muon facility with MERIT ring for nuclear transmutation. JPS Conf. Proc., 21:011063, 2018. https://doi.org/10.7566/JPSCP.21.011063
  * [9] A. Ballarino, Development of superconducting links for the Large Hadron Collider machine. Supercond. Sci. Technol., 27:044024, 2014. https://doi.org/10.1088/0953-2048/27/4/044024
  * [10] V.N. Litvinenko, T. Roser, and M. Chamizo-Llatas, High-energy high-luminosity e+e- collider using energy-recovery linacs. Phys. Lett. B, 804:135394, 2020. https://doi.org/10.1016/j.physletb.2020.135394
  * [11] Muon accelerators for particle physics (MUON). JINST, (Spec. issue), 2016–2021. https://iopscience.iop.org/journal/1748-0221/page/extraproc46