Status and future plans for C3 R&D
Authors: Nanni Emilio A. , Breidenbach Martin , Li Zenghai , Vernieri Caterina , Wang Faya , White Glen , Bai Mei , Belomestnykh Sergey , Bhat Pushpalatha , Barklow Tim , Berg William J. , Borzenets Valery , Byrd John , Dhar Ankur , Dhuley Ram C. , Doss Chris , Duris Joseph , Edelen Auralee , Emma Claudio , Frisch Josef , Gabriel Annika , Gessner Spencer , Hast Carsten , Jing Chunguang , Klebaner Arkadiy , Kim Dongsung , Krasnykh Anatoly K. , Lewellen John , Liepe Matthias , Litos Michael , Lu Xueying , Maxson Jared , Montanari David , Musumeci Pietro , Nagaitsev Sergei , Nassiri Alireza , Ng Cho-Kuen , Othman Mohamed A.K. , Oriunno Marco , Palmer Dennis , Patterson J. Ritchie , Peskin Michael E. , Peterson Thomas J. , Power John , Qiang Ji , Rosenzweig James , Shiltsev Vladimir , Shumail Muhammad , Simakov Evgenya , Snively Emma , Spataro Bruno , Tantawi Sami , van der Graaf Harry , Weatherford Brandon , Wu Juhao , Wootton Kent P. 
Published: September 2023

Abstract
C3 is an opportunity to realize an e
+
e
- collider for the
study of the Higgs boson at √s = 250 GeV, with a well
defined upgrade path to 550 GeV while staying on the same short
facility footprint [2,3]. C3 is
based on a fundamentally new approach to normal conducting linear
accelerators that achieves both high gradient and high efficiency at
relatively low cost. Given the advanced state of linear collider
designs, the key system that requires technical maturation for C3
is the main linac. This paper presents the staged approach towards a
facility to demonstrate C3 technology with both Direct (source and
main linac) and Parallel (beam delivery, damping ring, ancillary
component) R&D.
The primary goal of the C3 Demonstration R&D Plan is to reduce
technical and cost risk by building and operating the key components
of C3 at an adequate scale. This R&D plan starts with the
engineering design, and demonstration of one cryomodule and will
culminate in the construction of a 3 cryomodule linac with
pre-production prototypes. This R&D program would also demonstrate
the linac rf fundamentals including achievable gradient and gradient
stability over a full electron bunch train and breakdown rates. It
will also investigate beam dynamics including energy spread,
wakefields, and emittance growth. This work will be critical to
confirm the suitability of the C3 beam parameters for the physics
reach and detector performance in preparation for a Conceptual
Design Report (CDR), as well as for follow-on technology development
and industrialization.
The C3 Demonstration R&D Plan will open up significant new
scientific and technical opportunities based on development of
high-gradient and high-efficiency accelerator technology. It will
push this technology to operate both at the GeV scale and mature the
technology to be reliable and provide high-brightness electron
beams.
The timeline for progressing with C3 technology development will
be governed by practical limitations on both the technical progress
and resource availability. It consists of four stages: Stage 0)
Ongoing fundamental R&D on structure prototypes, damping and
vibrations. Stage 1) Advancing the engineering maturity of the
design and developing start-to-end simulations including
space-charge and wakefield effects. This stage will include testing
of strucutres operating at cryogenic temperatures. Beam tests would
be performed with high beam current to test full beam loading.
Stage 2) Production and testing of the first cryomodule at cryogenic
temperatures. This would provide sufficient experimental data to
compile a CDR and it is anticipated for Stage 2 to last 3 years and
to culminate with the transport of photo-electrons through the first
cryomodule. Stage 3) Updates to the engineering design of the
cryomodules, production of the second and third cryomodule and their
installation. Lower charge and lower emittance beams will be used to
investigate emittance growth. The successful full demonstration of
the 3 cryomodules to deliver up to a 3 GeV beam and achieve the
C3five gradient will allow a comprehensive and robust evaluation
of the technical design of C3 as well as mitigate technical,
schedule, and cost risks required to proceed with a Technical Design
Report (TDR).