107 / 2025-03-14 18:55:19

Experimental Study on Laser Refocusing of Ellipsoidal Plasma mirrors

ICF diagnose,，Ellipsoidal Plasma mirrors

In inertial confinement fusion (ICF) implosion experiments, In the field of high energy density physics, the symmetry and uniformity of the target capsule during the implosion stage are crucial to the success or failure of ignition. In order to accomplish the detection of the target capsule at each of its evolutionary stages, it is necessary for us to develop a backlighting imaging technique ^[1] capable of reaching a temporal resolution of 10 picoseconds (ps) and a spatial resolution of 10 micrometers.

The current approach employs picosecond ultra-intense, ultra-short pulse lasers to drive micro-wire targets for point-source projection x-ray backlighting. This technique generates high-energy x-rays in the 50–200 keV range, enabling radiographic imaging of the core region of the hot spot.

For instance, the ARC system on the NIF facility^[2], due to its design configuration, utilizes off-axis parabolic mirrors with a long focal length of approximately 10 meters. Consequently, the laser focus spot size is on the order of several hundred micrometers, which is significantly larger than the diameter of the micro-wire target (<10 micrometers). This mismatch introduces two major challenges: the "large spot, small target" issue, where substantial laser energy is wasted, resulting in a marked reduction in energy utilization efficiency; and the "laser drift" issue, which increases the uncertainty and difficulty in accurately aligning the laser with the micro-wire target.

To deal with these problems, we use ellipsoidal plasma mirror ^[3] to conduct refocusing experiments on ultra-short, ultra-intense laser beams with focal spots in the hundreds of micrometers range. We have developed a high-precision, rapid alignment method for an ellipsoidal mirror-post-target coupling system. Experimental comparisons of the laser focal spot and x-ray focal spot before and after refocusing revealed that the focal spot size is significantly reduced after ellipsoidal plasma refocusing. The reduction in x-ray focal spot size exceeds an order of magnitude, while the laser drift is also significantly decreased, demonstrating the feasibility of this technical approach.