The ion-channel laser (ICL) has been proposed as an alternative to the free-electron laser (FEL), replacing the deflection of electrons by the periodic magnetic field of an undulator with the periodic betatron motion in an ion channel. Ion channels can be generated by passing dense energetic electron bunches or intense laser pulses through plasma. The ICL has potential to replace FELs based on magnetic undulators, leading to very compact coherent X-ray sources. In particular, coupling the ICL with a laser plasma wakefield accelerator would reduce the size of a coherent light source by several orders of magnitude. An important difference between FEL and ICL is the wavelength of transverse oscillations: In the former it is fixed by the undulator period, whereas in the latter it depends on the betatron amplitude, which therefore has to be treated as variable. Even so, the resulting equations for the ICL are formally similar to those for the FEL with space charge taken into account, so that the well-developed formalism for the FEL can be applied. The amplitude dependence leads to additional requirements compared to the FEL, <i>e.g.</i> a small spread of betatron amplitudes. We shall address these requirements and the resulting practical considerations for realizing an ICL, and give parameters for operation at UV fundamental wavelength, with harmonics extending into X-rays.
The ion-channel laser (ICL) has been proposed as an alternative to the free-electron laser (FEL), replacing the
deflection of electrons in an undulator by betatron oscillations in an ion channel. The aim of this study is to
describe the ICL in terms of the well-developed formalism for the FEL in the steady-state, while taking into
account the dependence of the resonance between oscillations and emitted field on the oscillation amplitude. Numerical
solutions for experimentally relevant parameters show similarities and differences between both devices.
The ICL has potential to replace FELs based on magnetic undulators, leading to very compact coherent X-ray
sources. Furthermore, coupling the ICL with a laser plasma wakefield accelerator would reduce the size of a
coherent light sources by several orders of magnitude.
Electron acceleration using plasma waves driven by ultra-short relativistic intensity laser pulses has
undoubtedly excellent potential for driving a compact light source. However, for a wakefield accelerator to
become a useful and reliable compact accelerator the beam properties need to meet a minimum standard. To
demonstrate the feasibility of a wakefield based radiation source we have reliably produced electron beams
with energies of 82±5 MeV, with 1±0.2% energy spread and 3 mrad r.m.s. divergence using a 0.9 J, 35 fs 800
nm laser. Reproducible beam pointing is essential for transporting the beam along the electron beam line. We
find experimentally that electrons are accelerated close to the laser axis at low plasma densities. However, at
plasma densities in excess of 10<sup>19</sup> cm<sup>-3</sup>, electron beams have an elliptical beam profile with the major axis of
the ellipse rotated with respect to the direction of polarization of the laser.