Frequency domain laser ultrasound for inertial confinement fusion target wall thickness measurements

In inertial confinement fusion experiments hollow, spherical mm-sized capsules are used as a container for nuclear fuel. To achieve maximum implosion efficiency, a perfect capsule geometry is required. This paper presents a wall thickness measurement method based on zero-group velocity guided elastic wave resonances. They are measured with a non-destructive, contactless frequency domain laser ultrasound microscopy system. Wall thickness measurements along the equator of a high-density carbon capsule with a diameter of around <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mn>2</mml:mn> <mml:mspace></mml:mspace> <mml:mstyle><mml:mi>m</mml:mi> <mml:mi>m</mml:mi></mml:mstyle> </mml:mrow> </mml:math> and a wall thickness of around <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mn>80</mml:mn> <mml:mspace></mml:mspace> <mml:mstyle><mml:mi>µ</mml:mi> <mml:mi>m</mml:mi></mml:mstyle> </mml:mrow> </mml:math> excellently agree with infrared interferometry reference measurements. In addition, the multi-resonant nature of a spherical shell is studied by complementing experimental observations with plate dispersion calculations and finite element wave propagation simulations. The presented method is scalable and can be applied to a broad range of target materials, including metals, or metal-doped targets.