Effect of the electron heating transition on the proton acceleration in a strongly magnetized plasma

A novel scheme assisted by an external axial magnetic field is proposed to accelerate and collimate protons when a right-hand circularly polarized laser irradiates on an overdense plasma. We find that the transition of heating electron mode plays an important role in proton acceleration. First, the electrons are accelerated by stochastic heating in the case of no external magnetic field. Second, when the ratio of electron cyclotron frequency in the external magnetic field to the laser frequency is smaller than the relativistic factor ω ce / ω 0 ≤ γ, the cyclotron resonance absorption can occur and a laser front sharpening mechanism greatly improves the energy conversion from the laser to electrons. Meanwhile, the external magnetic field also restrains electrons' transverse motion. Finally, for ω ce / ω 0 > γ, there is a time delay in the electron heating, which can be divided into two stages. In the case of B = 2, a high quality proton beam can be harvested whose cut-off energy is enhanced by a factor of 4 and beam width is reduced to one fifth of that in the case of B = 0. These results may be helpful to understand the electron heating and proton accelerated process in a strongly magnetized plasma.