Ionization induced electron injection in laser wakefield acceleration

In this chapter we describe and discuss a special technique in laser wakefield acceleration (LWFA): electron injection by use of field ionization of atoms to high orders. Compared with other electron injection schemes in LWFA, this scheme shows the merits of relatively simple operation and controllable final beam quality. In the single-color laser ionization injection scheme, we show quasi-monoenergetic electron acceleration is possible through the control of laser self-focusing, which includes self-focusing accommodated ionization injection or ionization injection suppression. In these processes the effective injection length can be controlled within a hundred micrometers range and the absolute energy spread of the beam can be controlled within tens of MeV. In the two-color laser ionization injection scheme, we show the effective injection length can be further reduced to tens of micrometers length, and the absolute energy spread of the electrons can be reduced to a few MeV, i.e. the relative energy spread can be less than 0.5 %. A further interesting result is the generation of multi-color electron bunches by use of two-color lasers . These electrons can be used for multi-color X-ray generation through laser beam Thomson scattering. It is also found that the transverse emittance of the electron beam can be significantly reduced by a different two-color ionization injection scheme, in which the wake excitation and electron injection are made by using lasers with different wavelength, respectively. The final transverse emittance of the beam can be about 10 nm rad. Some recent progresses worldwide in experimental demonstrations of ionization injection are reviewed briefly. Some prospects of ionization injection for practical applications in laser wakefield accelerators are also discussed.