Uniform irradiation of a direct drive target by optimizing the beam parameters

Abstract

Laser driven fusion requires a high-degree uniformity in laser energy deposition in order to achieve the high-density compression required for sustaining a thermonuclear burn. Nowadays, uniform irradiation of capsule is still a key issue in direct drive inertial confinement fusion. The direct drive approach is to drive the target with laser light, by irradiating it with a large number of overlapping laser beams. In the direct drive scheme, the laser deposition pattern on the target can be decomposed into a series of Legendre spherical harmonic modes. The high mode (shorter wavelength) nonuniformity can lead to Rayleigh-Taylor instability, which may result in the failure of target compression. This nonuniformity can be suppressed by thermal conduction and beam conditioning technologies, such as continuous phase plate, smoothing by spectral dispersion and polarization smoothing. The low mode (longer wavelength) nonuniformity is related to the number, orientation and power balance of laser beams, which is hard to suppress by thermal conduction and beam conditioning technologies. Generally, the nonuniformity of laser irradiation on a directly driven target should be less than 1% (root mean square, RMS), to meet the requirement for symmetric compression. Several methods have been proposed to optimize the irradiation configuration in direct drive laser fusion, such as truncated icosahedron with beams at the 20 faces and 12 vertices of an icosaherdron, dodecahedron-based irradiation configurations, self-organizing electrodynamic method, etc. However, limited by the different parameters of incident beams, the irradiation uniformity is often not satisfactory. Therefore, it is necessary to find new way to improve the irradiation uniformity and make it more robust. According to the analytical result, the irradiation nonuniformity can be decomposed into the single beam factor and the geometric factor. Simulation results show that the single beam factor is mainly determined by the parameters of the incident beams, including beam pattern, beam width and beam wavelength. By analyzing and simulating the single beam factor with different incident beam parameters, and comparing the single beam factor with the geometric factor, a matching relationship between them is found by using the optimized parameters. Based on the simulation results, a method to optimize the incident beam parameters is proposed, which is applied to the 32-beam and 48-beam irradiation configurations. The results show that there is a set of optimal incident beam parameters which can attain the highest irradiation uniformity for a given configuration. The feasibility to achieve more uniform irradiation by optimizing the incident beam parameters is proved. When the single beam factor is optimized in a directly driven inertial confinement fusion system, the restrictions on the beam pointing error and power imbalance between incident beams can be relaxed. The results provide an effective method of designing and optimizing the uniform irradiation system of direct drive laser facility.