Vibration Stabilization of NIF-scale Direct Drive Cryogenic Targets

Ethan Burnham-Fay, PhD Defense

Hopeman 224

Experience at the LLE with cryogenic direct drive inertial confinement fusion targets has shown that there are stability errors once targets are positioned at target chamber center. Excitation sources for target motion include broadband base excitations, coupled into the target stalk from the target inserter, and shock loads, for example from shroud removal. Plans for polar direct drive experiments at the NIF are expected to have similar positioning errors which must be mitigated to ensure the quality of the experimental results. The dynamic performance of NIF and LLE cryogenic targets was model to assess their performance. Various passive vibration stabilization techniques were considered, but an active stabilization system was pursued. Suitable metrology systems for target position measurements to act as a feedback sensor  were investigated, including direct target shell interferometric measurements, fiber optic strain probes coupled with a fiber contained heterodyne interferometer, and fiber Bragg gratings. Due to metrology system issues, a passive method of adding damping to structures down to -180 ^oC was investigated using shunted piezoelectric elements to remove energy from the system. Future work on how to achieve higher positioning stability of cryogenic direct drive targets is discussed. Additional research into electronic-poling assisted manufactured (EPAM) piezoelectric polyvinylidene fluoride (PVDF) is presented showing displacements and strains orders of magnitude larger than commercially prepared samples.