Magnetically tapered plasma channels for laser wakefield accelerators

To partially overcome the dephasing problem for the accelerated electron beam in the laser wakefield accelerators, a certain radial and axial plasma density distribution is desired. To obtain such a two-dimensional density profile, a magnetical tapering scheme is proposed and numerically tested. By using this scheme, a precisely tapered channel can be formed. For given laser parameters, the accelerated electrons would stay in phase with the accelerating field over many dephasing lengths. Our preliminary simulation results show the feasibility of such a magnetically tapered plasma channel, and help to identify potential experimental difficulties. For practical implementation, a combination of externally controlled axial magnetic field distributions and wall temperature quenching is desirable.