  Particle accelerators have enabled forefront research in high energy physics
and other research areas for more than half a century. Accelerators have
directly contributed to 26 Nobel Prizes in Physics since 1939 as well as
another 20 Nobel Prizes in Chemistry, Medicine and Physics with X-rays.
Although high energy physics has been the main driving force for the
development of the particle accelerators, accelerator facilities have
continually been expanding applications in many areas of research and
technology. For instance, active areas of accelerator applications include
radiotherapy to treat cancer, production of short-lived medical isotopes,
synchrotron light sources, free-electron lasers, beam lithography for
microcircuits, thin-film technology and radiation processing of food.
Currently, the largest and most powerful accelerator is the Large Hadron
Collider (LHC) at CERN, which accelerates protons to multi-TeV energies in a 27
km high-vacuum ring. To go beyond the maximum capabilities of the LHC, the next
generation of circular and linear particle colliders under consideration, based
on radiofrequency acceleration, will require multi-billion investment,
kilometric infrastructure and a massive power consumption. These factors pose
serious challenges in an increasingly resource-limited world. Therefore, it is
important to look for alternative and sustainable acceleration techniques. This
review article pays special attention to novel accelerator techniques to
overcome present acceleration limitations towards more compact and
cost-effective long term future accelerators.
