A high-power laser pulse guided through a relativistically underdense plasma channel generates a strong quasistatic magnetic field, confining the transverse motion of electrons inside the channel. This confinement allows the laser field to efficiently accelerate electrons transversely which are then deflected in forward direction and collimated by the channel's magnetic field, establishing the novel forward-sliding swing acceleration (FSSA) mechanism. Its advantage is a threshold behaviour, yielding high electron energies for sufficiently strong laser fields or initial electron momenta, regardless of a fine-tuned resonance. The achievable electron energies are demonstrated to be two orders of magnitude higher than the vacuum limit of direct laser acceleration. We study the electrons' dynamics by a simplified model analytically and confirm this model's predictions by numerical simulations. Specifically, we derive and confirm simple relations between the channel's magnetic field strength, the particles' initial transverse momenta and the laser's field strength indicating in what parameter regimes high electron energies can be achieved.
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