The effect of the magnetic field applied along the laser propagation direction on the radiation efficiency of high-order harmonics generated from laser-irradiated overdense plasma is investigated theoretically and numerically. We find that the external magnetic field can increase the transmittance of the overdense target, thereby dramatically enhancing the energy coupling between the laser and target. While for the high-order harmonics of the laser reflected from the oscillating target, the radiation efficiency reaches the maximum when the cyclotron frequency of the electrons in the magnetized target approaches the laser frequency. This conclusion applies only to overdense plasmas targets. For targets with low reflectivity, the application of the magnetic field will reduce the harmonic radiation efficiency due to the decrease of both the oscillating coherence and opacity of the target. This work provides a reasonable approach to improving the radiation efficiency of high-order harmonics and a method to estimate the magnitude of the self-generated magnetic field during intense laser-plasma interactions.

Relativistic vortex laser has drawn increasing attention in the laser-plasma community owing to its potential applications in various domains, e.g., generation of energetic charged particles with orbital angular momentum (OAM), high OAM X/γ-ray emission, high harmonics generation, and strong axial magnetic-field production. However, the generation of such relativistic vortex laser is still a challenge to the current laser technology. Using micro-structure targets named axial line-focused spiral zone plate (ALFSZP), we propose a novel scheme for ultra-intense vortex laser generation. In the scheme, a relativistic Gaussian laser pulse irradiates an ALFSZP, and diffracts as it passes through the ALFSZP. Due to the focusing and radial Hilbert transform capabilities of the ALFSZP, the seed laser is converted efficiently to a vortex one which is then well focused in a tunable focal volume. Three-dimensional particle-in-cell simulations indicate that using a seed laser pulse with intensity of 1.3 × 1020 W/cm2, the vortex laser intensity achieved is as high as 1.3 × 1021 W/cm2 with the averaged angular momentum per photon up to 0.73 ℏ , promising diverse applications in various fields aforementioned.