Contents lists available at ScienceDirect Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes Fourier-domain post-processing technique for Digital Focus Array imaging with Matrix phased array for ultrasonic testing of ITER components D.O. Dolmatov a, ⁎ , D.G. Demyanyuk a , A.H. Ozdiev a , R.V. Pinchuk b a National Research Tomsk Polytechnic University, 634050 Tomsk, Russia b ACS-Solutions GmbH, Science Park 2, 66123 Saarbrücken, Germany ARTICLE INFO Keywords: Ultrasonic nondestructive testing Ultrasonic imaging Matrix phased arrays Quality control ABSTRACT The high demands on the quality of the components of the International Thermonuclear Experimental Reactor (ITER) create the need to apply methods of nondestructive testing which are able to provide accurate and reliable results. One such technique is Digital Focus Array ultrasonic imaging with Matrix phased arrays. The application of this approach is linked with the need to process huge sets of data in order to obtain images of controlled objects. In this article, we propose the technique of computationally efficient Fourier-domain post- processing. This algorithm is based on Fast Fourier transformations and calculations in frequency domains and can be applied in immersion and contact testing. The performance of this technique was examined experi- mentally. 1. Introduction The fabrication of International Thermonuclear Experimental Reactor (ITER) components demands strict quality control. An im- portant part of this quality control is the application of nondestructive testing methods. Immersion pulse-echo ultrasonic testing is considered as one of the methods which can be used for the inspection of ITER components [1,2]. Advanced approaches can be used to increase the versatility, pre- cision and accuracy of immersion ultrasonic testing. The application of Local Immersion technique allows obtaining the consistent coupling in case of big objects inspection. The Local Immersion technique is based on the formation of the water layer between the ultrasonic transducer and the controlled object by the application of a local water bath; this provides the required acoustic contact between the surface of controlled object and the transducer. This technique could be useful for the testing of objects which cannot be inspected through the conventional method of immersion ultrasonic testing due to their size or in the scenario of in- site inspections [3]. Furthermore, it is possible to increase the precision and reliability of ultrasonic inspections by applying systems which process the measured echodata. Such systems allow obtaining high resolution images of the flaws in a controlled object. These images allow determining size of the flaws, their shape and location in specimen. This information can be used for reliability assessment of the controlled objects. Ultrasonic post-processing techniques aim to solve the inverse scattering problem. The quality of results is determined by the accuracy of applied post-processing technique and the size of measured data. Phased array probes can be used to obtain comprehensive data about the internal structure of a controlled object. These type of ultrasonic probes consist of numerous elements (which are mounted in a singular casing). Multi-channel electronic units operate transmission and re- ception of ultrasonic signals for each element. Application of Digital Focus Array or Full Matrix Capture techniques by using phased arrays presupposes the utilization of all combinations of transmitter and re- ceiver elements for the sampling of ultrasonic data [4]. The main ad- vantage of applying the Digital Focus Array technique is linked with the ability to obtain high-resolution images of the flaws in a controlled object. All varieties of existing phased array probes could be divided into linear and matrix arrays. Matrix phased arrays are ultrasonic probes which use the two-dimensional locating of elements; this allows us to obtain a three-dimensional image in a single position. These probes in immersion testing could provide us with precise results and reduce inspection time. However, huge sets of data need to be processed in order to obtain such an image. In this case, the computationally effi- cient post-processing techniques could be applied for obtaining the high-resolution images of the flaws in controlled objects with accep- table speed. https://doi.org/10.1016/j.fusengdes.2017.11.020 Received 21 July 2017; Received in revised form 9 November 2017; Accepted 22 November 2017 ⁎ Corresponding author. E-mail address: dolmatovdo@tpu.ru (D.O. Dolmatov). Fusion Engineering and Design 126 (2018) 124–129 0920-3796/ © 2017 Elsevier B.V. All rights reserved. T