The applications of laser wake field accelerators (LWFA) rely heavily on the quality of produced high energy electron beams and X-ray sources. We present our recent progress on this issue. Firstly we propose a bichromatic laser ionization injection scheme for obtaining high quality electron beams. With the laser pulse combinations of 800 nm and 267 nm, or 2400 nm and 800 nm in wavelengths, electron beams with energy spread of 1% or lower can be produced. Secondly we propose polarization tunable X-ray sources based on LWFA. By shooting a laser pulse into a preformed plasma channel with a skew angle referring to the channel axis, the plasma channel can act as a helical undulator for elliptically polarized X-rays.
By employing a pair of partially overlapped supersonic gas jets, we separated injection and acceleration stages of laser wakefield acceleration to produce stable, monoenergetic, and tunable electron beams. The first jet (injector) utilized a He/N<sub>2</sub> mixture and resulted in electrons injected into the wake via ionization-assisted injection. These electrons were then accelerated in the second jet (accelerator) using pure He. By changing length and plasma density of the accelerator jet, we were able to tune electron energy in the 50 – 300 MeV range with energy spread of 10-30% and 20 pC charge. Simulations show that the injection region is limited within the overlap of the jets.