Calibration Test for Preload Control With Ultrasonic Checking for Magnet Studs During ITER Machine Assembly

The ITER superconducting magnet system comprises 18 Toroidal Field (TF) coils, one Central Solenoid (CS), 6 Poloidal Field (PF) coils, and 18 Correction Coils (CC). These superconducting coils are integrated using robust, flexible structures and over 4700 high-grade large studs ranging from M24 to M160. During assembly, these studs are preloaded from hundreds to thousands of kN with Hydraulic Tensioners or Multi-jack bolt Tensioners (MJT or Superbolts). Since preload losses are inevitable under current methods with hydraulic tensioners, and the maximum allowable loads are constrained by the material's yield strength as per ITER Magnet Structure Design Criteria, these constraints create a narrow operational window for successful assembly to the expected preload. Precise preload control is critical to ensuring proper integration of the superconducting magnets. Ultrasonic bolt load measurement emerges as a promising solution, especially in scenarios where one end of the stud is inaccessible after installation. Calibration tests at ITER are underway to validate the use of this method for preload measurement during machine assembly. This article presents calibration test results, including velocity, stress factor, load factor, preload loss, and field calibration measurements. It concludes with insights from the design and manufacturing of high-grade studs, highlighting their impact on achieving accurate preload control via ultrasonic inspection.