Characterization of energy gain and efficiency for guided and unguided laser-plasma accelerator stages*

The viability of next generation plasma-based linear colliders relies on the possibility of accelerating high-charge and low-emittance bunches to high energies over short distances with high efficiency, while keeping a small relative energy spread. Laser-plasma accelerators (LPAs) can operate in different regimes, namely, linear or mildly nonlinear stages, where laser guiding is achieved by means of an external wave guide such as a plasma channel (guided LPA), or highly nonlinear stages where the laser is self-guided through the plasma (unguided LPA). For the same laser driver energy, guided and unguided LPAs are characterized by different average accelerating gradients, dephasing/depletion lengths, optimal bunch length, shape, and charge, and acceleration efficiency. In this contribution, we present a systematic investigation of the properties of guided and unguided LPA stages with fixed laser energy. Constraints imposed by using a fixed bunch emittance will also be discussed.