Fig. 1. Classical ion accelerator
(REX-ISOLDE, CERN)

While classical accelerators have maximum accelerating fields of 100 MeV/m (limited by sparking), laser driven plasma accelerators can realise 10⁶–10⁸ times larger accelerating fields. This reduces the length of an accelerator correspondingly from 100 m to less than a millimetre. At the same time the much larger field results in 10⁶ time larger particle densities. The longitudinal and transverse beam emittances are as good as for classical beams. For electron acceleration the concept of ``bubble'' acceleration was developed, where quasi-monoenergetic electron beams of several 100~MeV are obtained in the unidirectional field of an ion cloud -- the bubble -- generated behind the laser pulse. Recently also quasi-monoenergetic ion beams with a few MeV/u have been obtained behind foil targets. The unique features of these laser-accelerated beams have to be explored: How to cascade acceleration schemes; focus such high density beams with permanent magnet quadrupole or laser-generated ion clouds or decelerate those beams by driving a plasma wake with producing nuclear radiation.an anti-collinear setup of a laser-generated electron beam and a second laser-beam, by Compton-backscattering. By sending a laser-generated electron bunch through an undulator – an arrangement of magnets with periodic alternating dipole field direction – the oscillatory motion of the electrons results in intense undulator radiation.

To obtain an X-ray Free-Electron Laser (FEL) the SASE (Self-Amplification of Spontaneous Emission) principle is used. The spontaneously emitted undulator radiation acts back on the electrons results in a micro-bunching of the beam with bunch distance λ . Then all electrons emit coherently with wavelength λ resulting in very brilliant X-ray beams. The ratio of the gain length and the undulator length is proportional to the so-called Pierce parameter, the main parameter of an FEL.

Fig. 2. Stem of bubble accelerated electrons
inside the soliton-like ion cavity.

Since the Pierce parameter scales∝ n_e^1/3with the electron bunch densityn_e, undulators are typically a factor of 100 shorter for laser-generated electron bunches than for electron bunches from classical accelerators. Therefore we hope to realize much more compact and cheaper table top XFELs. Which open many new fields of application. Here we want to use the micro-focussed, laterally coherent X-ray beams for phase contrast imaging, where X-ray pictures of soft tissue can be generated applying much less radiation dose that with usual absorption pictures. Getting contrast for small density variations such techniques open new perspectives in medical diagnostics. We want to extend die photon energy range into the MeV range because quantum fluctuations are much less perturbing for laser generated FELs. Via γ-reactions the γ-FEL beam can be converted into a brilliant micro-neutron beam. With sufficiently intense laser fields also
quasi-monochromatic ion beams can be generated.
We want to explore how these beams can be used
in cancer therapy.