The propagation characteristics of intense femtosecond laser pulses underwater are numerically investigated and modulated by the input energy, lens focal length and beam waist width.The results indicate that, when the system parameters are appropriately selected, the generation of filament can be effectively controlled by the focal length of lens in range of 1 meter to 10 meters underwater, and the filament length reaches the meter scale. With increase of the focal distance (such as f=10 m), the generated plasma filament will oscillate strongly, which is disadvantage to the underwater detection of spectrum. At this time, by increasing the waist width of the beam, the filament can be transmitted more stably at a distant target position underwater. The attenuation effect of the impurities in seawater on pulse energy can be balanced by increasing input power, so as to realize the long-distance transmission of filaments.

For the unique magnetic structure of a compact fusion reactor, confinement dynamics of a single high-energy charged particle was studied with Monte Carlo method. Taking into account the local flatness characteristics of the magnetic field configuration, a charged particle moves basically in a constant magnetic field for a sufficiently small area or a short enough timestep. Therefore, a point-by-point analytical solution of particle motion equation that guarantees accurately conservation of energy is proposed, which has long-term tracking capabilities. Simulation results show that for high-energy deuterium particles with 1 keV energy distributed randomly in initial position and velocity direction, there is about a 7% probability that can be constrained to an order of 10 ms. Since the method for solving the motion equation is independent of magnetic configuration, it can be generalized naturally to arbitrary magnetic confinement setup.