We propose to use a combination of Hermite-Gaussian laser modes to generate a nonlinear ring bubble with a large longitudinal accelerating field and a transverse focusing field suitable for positron beam focusing and acceleration. The nonlinear bubble can provide higher accelerating gradients compared with a linear plasma wake. PIC simulations have been used to demonstrate control of the focusing force by changing the relative intensity ratio of the two laser modes, enabling matched positron beam propagation for emittance preservation. In addition, in order to improve phase space characteristics of laser wakefield produced electron beams, in particular, to reduce their transverse emittance, we propose to generate femtosecond electron beams with ultralow emittance using a two-color laser-ionization injection method. By controlling the amplitude and the duration of the injection pulse, the emittance can be controlled to the level of 10-2 mm mrad.
Ultra-low emittance (tens of nm) beams can be generated in a plasma accelerator using ionization injection of electrons into a wakefield. An all-optical method of beam generation uses two laser pulses of different colors. A long-wavelength drive laser pulse (with a large ponderomotive force and small peak electric field) is used to excite a large wakefield without fully ionizing a gas, and a short-wavelength injection laser pulse (with a small ponderomotive force and large peak electric field), co-propagating and delayed with respect to the pump laser, to ionize a fraction of the remaining bound electrons at a trapped wake phase, generating an electron beam that is accelerated in the wake. The trapping condition, the ionized electron distribution, and the trapped bunch dynamics are discussed. Expressions for the beam transverse emittance, parallel and orthogonal to the ionization laser polarization, are derived. An example is presented using a 10-µm CO2 laser to drive the wake and a frequency-doubled Ti:Al2O3 laser for ionization injection.
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