Armor Thickness Assessment for the Divertor Tokamak Test Facility (DTT) Divertor Targets

The Divertor Tokamak Test (DTT) facility is a fusion device under construction in Italy. The mission of DTT is to test alternative divertor concepts under integrated physics and technological conditions that can reliably be extrapolated to DEMO. Due to the plasma core characteristics with relevant edge and scrape-off layer (SOL) parameters and a wall entirely in tungsten (W), DTT will provide an extensive set of information useful to select the most appropriate strategy for the power exhaust in DEMO. Several divertors, which may differ in design or/and technologies or/and poloidal profile, will be tested during the life of the machine. The first divertor to be installed will have to accommodate a multitude of strike points, located at various positions according to the different magnetic configurations, which will be tested in the first operational phases of the machine with the aim to identify the most promising. The first divertor will not test innovative technological solutions but will mainly take advantage of the technologies already qualified for the ITER divertor production. Thus, the entire divertor plasma-facing surface is designed to be used as targets: it will be made of W monoblocks joined on CuCrZr pipes (plasma-facing units, PFUs) similar to the ITER targets. With the purpose to increase the flexibility in operational scenarios by maximizing the allowable thermal load for the PFUs, the possibility of using monoblocks with a plasma side reduced thickness was investigated. By reducing the thickness of the armor, it is possible to limit plastic deformation of the monoblock and to preserve the characteristics of the plasma-facing surface during the component lifetime. A thickness between 3 and 4 mm is compatible both the erosion estimates in the DTT divertor area and the manufacturing constraints and therefore proposed for the DTT PFUs. Several mock-ups based on monoblock design were in the past tested under thermal fatigue, confirming the reliability of the monoblock design and the manufacturing processes, but with larger armor thicknesses (6-8 mm). The experimental verification of the monoblock performance with the proposed reduced thickness has been verified in the GLADIS facility at IPP Garching with a thermal load of 20 MW/m(2) applied for 1000 cycles of 10 s. The results showed the absence of plastic deformation and negligible increase in surface roughness.