  High-brightness beams generated by particle sources based on advanced
accelerator concepts have the potential to become an essential part of future
accelerator technology. High-gradient accelerators can generate and rapidly
accelerate particle beams to relativistic energies while minimizing
irreversible detrimental effects to the beam brightness that occur at low beam
energies. Due to the high accelerating gradients, these novel accelerators are
also significantly more compact than conventional technology. The beam
parameters of these particle sources are largely determined by the injection
and subsequent acceleration processes. While there has been significant
progress crucial parameters that are required for a future collider or more
near-term applications, including X-ray free-electron lasers (XFELs), such as a
sufficiently small energy spread and small emittance for bunches with a high
charge and at high pulse repetition rate. Major research and development
efforts are required to realize these approaches for a front-end injector for a
future collider in order to address these limitations. In particular, this
includes methods to control and manipulate the phase-space and spin
degrees-of-freedom of ultrashort LWFA electron bunches with high accuracy,
methods that increase the laser-to-electron beam efficiency and increased
repetition rate. This also includes the development of high-resolution
diagnostics, such as full 6D phase-space measurements, beam polarimetry and
high-fidelity simulation tools. A further increase in beam luminosity can be
achieve through emittance damping. For future colliders, the damping rings
might be replaced by a substantially more compact plasma-based approach. Here,
plasma wigglers are used to achieve similar damping performance but over a two
orders of magnitude reduced length.
