Mandrel degradation model of combined fast and slow processes

Targets are physical base of laser inertial confinement fusion (ICF) research, whose quality has extremely important influences on the reliability and degree of precision for subsequent ICF experimental results. At present, the degradable mandrel technique with poly-α-methylstyrene (PAMS) degradation as the core has become one of the key technologies for fabricating ICF target. Its general process can be divided into three steps: first, hollow PAMS microspheres are prepared as mandrel, then plasma vapor deposition technology is used to prepare a coating (glow discharge polymer, GDP) with higher thermal stability on the surface, and finally PAMS are degraded leaving the hollow GDP target. Although many reports have been devoted to the related process, there are still two key problems in the actual preparation of GDP, that is, how to reduce the thermal degradation temperature of PAMS and how to avoid residues in PAMS degradation. Considering that the general nature of degradation corresponds to the breaking of chemical bonds, it is urgent to grasp the physical laws of the complex degradation process of PAMS at the atomic level and construct the reliable model of mandrel degradation.

The research groups led by Prof. Zhigang Wang from Jilin University and Prof. Zhanwen Zhang from China Academy of Engineering Physics reported the study on degradation of PAMS mandrel based on the theoretical and experimental methods in High Power Laser Science and Engineering, Vol. 9, Issue. 1, 2021 (Yu Zhu, Zheng Liu, Famin Yu, et al. Mandrel degradation model of combined fast and slow processes[J]. High Power Laser Science and Engineering, 2021, 9(1): 010000e1).

The theoretical calculations show that degradation reaction generally includes two kinds of processes: depolymerization and hydrogen transfer-chain scission. Between them, the energy barrier for former (0.68-0.82 eV) is smaller than that for most of the latter (1.39-4.23 eV), which results in a 5-31 orders of magnitude difference in reaction rates at 550K between the two, bringing a clear distinction between the former being a fast process and the latter being a slow one. Further, thermogravimetric experiment shows that the activation energy of 2.53 eV for degradation is between those of fast and slow processes, corresponding to theoretical average value of multiple reaction pathways. Thus, the mandrel degradation model of combined fast and slow processes was established at the atomic level. The establishment of this model highlights the decisive role of slow processes in mandrel degradation, which provides a direction for effective control of degradation process. Future work will focus on the study of slow processes in PAMS degradation models to explore physical pathways for achieving effective regulation.

This work combines theoretical and experimental methods to study the degradation process of mandrel material called PAMS, and establishes the model of mandrel degradation at the atomic level. This study can provide reference for the further development of key technology of target fabricating in ICF research, and has important value for the exploration of foundation and application of polymer material assembly and degradation process.

During the fabricating of ICF target, the degradation of PAMS mandrel consists of two kinds of processes: depolymerization and hydrogen transfer-chain scission. Generally, the former is a fast process, the latter is a slow process.