Laser Etched Fiducial Marks for Detonation Front Curvature Measurements

Front-curvature measurements provide information on the detonation wave shape, which is an essential experimental data component used in high explosive model calibration. We typically perform these measurements on rate-sticks and other test geometries using a mirror destruct technique. Typically, the mirror is a layer of vapor deposited aluminum, 0.45 micrometers thick, applied to a clear plastic window placed in intimate contact with the breakout surface of the charge. During the experiment, light is reflected from this mirror surface to a streak camera until the detonation wave arrives at the aluminum coating, destroying the mirror. An important feature of our front-curvature windows is the placement of fiducial marks beyond the edges of the high explosive charge. These fiducial marks are small defects in the mirror where light is not reflected, and are placed at a known distance apart from each other, allowing for accurate spatial scaling of the streak camera record. The marks are visible in the still images taken prior to firing the shot, and also produce lines on the streak camera dynamic record since they interrupt the reflected light. Previously, we've created these fiducial marks by center point milling, which requires a very sharp tool cutter and using the knee of the milling machine to bring the window into contact with the cutter. To keep the mark small and precise, the tool should only cut about 75 microns deep. Producing fiducial marks of consistent size and shape requires a skilled machinist, and is a time consuming process. Here, we describe a process to generate front curvature fiducial window marks using a laser etching technique. We show that the consistency of the fiducial mark size and shape as well as positioning accuracy are improved relative to our prior technique. In addition, front curvature results for both methods are shown and compared.