  Laser wakefield accelerators have emerged as a promising candidate for
compact synchrotron radiation and even x-ray free electron lasers. Today, to
make the electrons emit electromagnetic radiation, the trajectories of laser
wakefield accelerated electrons are deflected by transverse wakefield,
counter-propagating laser field or external permanent magnet insertion device.
Here, we propose a novel type of undulator which has a few hundred microns of
period and tens of Tesla of magnetic field. The undulator consists of a bifilar
capacitor-coil target which sustains strong discharge current that generates
helical magnetic field around the coil axis when irradiated by a high energy
laser. Coupling this undulator with state-of-the-art laser wakefield
accelerators can, simultaneously, produce ultra-bright quasi-monochromatic
x-rays with tunable energy ranging 5-250 keV and optimize the free electron
laser parameter and gain length compared with permanent magnet based undulator.
This concept may pave a path toward ultra-compact synchrotron radiation and
even x-ray free electron lasers.
