[1B1] A novel thickness measurement method for extreme environments using laser ultrasound in the thermoelastic regime
M Riding, R Pyle, P Lukacs, D Pieris, G Davis, C MacLeod and T Stratoudaki
University of Strathclyde, UK
Measuring the thickness of material between two parallel, acoustically reflective surfaces is a standard technique in ultrasonic non-destructive evaluation (NDE). Such a measurement would be valuable for monitoring the thickness of the plasma-facing components (PFCs) of future tokamak power plants in situ, which may change due to the erosion and deposition of material. The standard ultrasonic method for measuring backwall thickness in parallel-sided components is enabled by the strongly normally directed directivity pattern of longitudinal acoustic waves emitted by traditional single-element piezoelectric transducers. Deploying such transducers will not, however, be possible within the radioactive ultra-high vacuum and ultra-high cleanliness environment of a tokamak vacuum chamber. In contrast, generating and detecting ultrasound using lasers enables fully non-contact couplant-free ultrasonic inspection, allowing ultrasonic NDE techniques to be deployed in environments that preclude the use of piezoelectric transducers, such as the tokamak. The standard ultrasonic method for thickness measurement has already been successfully replicated using laser-based ultrasound transduction, but the required directivity pattern is only obtained when the generation laser is operated in the ablation regime, wherein a small amount of material is evaporated from the surface of the component each time the laser is fired. Operation in the fully non-destructive ‘thermoelastic’ regime would be preferable in applications such as the in-situ inspection of PFCs, where component damage and material mobilisation must be minimised. Thermoelastic excitation, however, generates very little ultrasonic amplitude normal to the illuminated surface in most materials, with the highest amplitude waves instead emitted in lobes angled away from the surface normal. This directivity pattern is not compatible with the standard ultrasonic thickness measurement method. In this work, techniques used in reflection seismology have been applied to the problem of measuring backwall thickness using thermoelastic laser ultrasound. It is shown that an ‘off-end’ scan strategy, where A-scans are collected at a fixed detection point for a periodic, linear array of source points with increasing surface off-set (x) from the detection point, enables the normal moveout (NMO) hyperbolae associated with specular backwall reflections to be mapped over a range limited by the directivity pattern of the thermoelastic source and the ratio of the reflection depth to the maximum surface offset achieved in the scan. Backwall thickness and ultrasonic speed are determined simultaneously via straight line fitting to the NMO echo peaks in an x²-versus-time² plot. A common depth point (CDP) scan strategy, which enables localised backwall thickness measurements, is also described. This scan strategy is used to demonstrate thickness profiling for samples featuring step-like thickness changes. The effect of varying the offset range captured in each CDP scan on the accuracy of the output parameters is explored, alongside the limitations resulting from surface wave crosstalk. The values of backwall thickness and ultrasound speed obtained from this method are evaluated against standard high-precision measurements and the associated errors are discussed.
