For the past two decades, the interaction of ultra-intense lasers with nano-foils has been motivated by acceleration of proton/ions for radiobiological applications. Despite some progress, the realization of a stable high-charge narrow-energy-spread protons of hundreds MeV remains a challenge. One promising scheme is the “light sail” acceleration, where laser pressure directly pushes the whole target, providing a strong accelerating force. One major drawback of such scheme is that it will suffer significant transverse instabilities that can break up the target, but the underlying mechanism has still not been clarified. Here we present a theoretical model that clarifies the origin of this long-standing problem, and support it with 2/3D PIC simulations for a wide range of parameters. Based on this understanding, we propose a new concept of relativistically intense laser tweezer for stable ion acceleration. In this scheme, two counter-propagating lasers of different colors collide on a nano-foil. By dragging plasma electrons out of the foil, such a tweezer simply avoids the electron-ion coupled instability and at the same time produces a strong micro-capacitor with a nearly uniform strong electrostatic field. This field preferentially accelerates protons with a narrow energy spread from a hydrocarbon target, indicating a new route for future compact laser driven ion accelerator.
One direction towards compact Free Electron Laser is to replace the conventional linac by a laser plasma driven beam, provided proper electron beam manipulation to handle the large values of the energy spread and of the divergence. Applying seeding techniques enable also to reduce the required undulator length. The rapidly developing LWFA are already able to generate synchrotron radiation. With an electron divergence of typically 1 mrad and an energy spread of the order of 1 % (or few), an adequate beam manipulation through the transport to the undulator is needed for FEL amplification. Electron beam transfer follows different steps with strong focusing variable strength permanent magnet quadrupoles, an energy demixing chicane with conventional dipoles, a second set of quadrupoles for further dedicated focusing in the undulator. A test experiment for the demonstration of FEL amplification with a LWFA is under preparation and progress on the equipment preparation and expected performance are described.