89 / 2023-04-13 12:45:22

Experiment of laser plasma instability under new ignition path

laser plasma instabilities,multi-beam LPIs,,novel ignition path

In laser-driven inertial confinement fusion (ICF), a new ignition approach combines the advantages of direct-drive (DD) and indirect-drive (ID), which improves the utilization rate of laser energy and reduces the risk of hydrodynamic instability. In this path, the lasers are divided into DD beams and ID beams. The ID beams have relatively low intensities, they will interact with the hohlraum to generate X-rays, which will firstly compress the capsule to a certain density. Then the DD beams with higher intensities will directly irradiate the capsule for further compression, finally realizing ignition. However, before the DD beams reach the pre-compressed capsule, they will pass through the large-scale inhomogeneous plasma generated by the ID beams. Meanwhile, the intensities of the DD beams are much higher than the threshold of laser plasma instability (LPI), so the LPI risk is very high. In order to evaluate the LPI risk under this ignition path (mainly for DD beams), we carried out experiments on SG hundreds-kJ laser facility. In the experiments, twelve ID beams were used to generate X-rays and four overlapped beams were used to act as a DD beam. The intensity of an ID beam is ~4.6×10¹⁴W/cm² and the overlapped intensity of the DD beams is ~2.1×10¹⁵W/cm². A cylindrical hohlraum was used and a CH spherical crown was pasted on the bottom of the hohlraum to act as the capsule. The experimental results showed that multi-beam LPIs in the plasma produced by ablating the polyimide film were excited by DD beams. However, if the DD beams had toes, the multi-beam LPIs could be weakened. The results also showed that the toes could not only reduce the multi-beam LPIs of the ID beam, but also change the plasma conditions (for example, the density could be decreased) that may influence LPIs of the DD beam. The spectrum of the stimulated Raman scattering (SRS) showed that SRS were excited in plasmas with lower density when toes of the DD beams were added. Other processes such as two-plasmon-decay could also be suppressed if the plasma density was decreased. These changes can reduce the risk of hot electrons, which was also shown in the experiment. This research is of great significance to the design of this new ignition scheme in the future.