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This work presents an overview of the integrated strategy developed, as part of the DEMO Key Design Integration Issue 1 (KDII1), to protect the EU-DEMO first wall (FW) from planned and unplanned plasma transients by employing discrete limiters. The present Breeding Blanket (BB) FW design, which aims at minimizing the loss of neutrons while travelling to the breeding zone, is able to withstand steady state heat fluxes up to ≈1-1.5 MW/m2 [1], which is not sufficient to guarantee its integrity for most plasma-FW direct contact. This is different from ITER, which has a FW designed for peak heat loads up to 4.6 MW/m2 [2], and it does not have the DEMO BB breeding related requirement. A series of documents was compiled in the DEMO Pre-Conceptual Design Phase, in support of the KDII1. The work presented here was presented at the 2020 DEMO Gate 1 (G1) review, and collects also the comments of the panel and the relative additional studies triggered by them. The design process, presented in this paper was adopted to systematically evaluate the impact of design changes, or new physics inputs, on the FW protection strategy and integration issues. It includes compiling the list of transients, and performing the relative plasma simulations, the design of discrete limiters and the evaluation of their capability to reduce the heat flux density on the FW, and finally a preliminary analysis of the heat loads effects on the Plasma Facing Components (PFC). All these aspects, together with preliminary limiter design, where considered since the beginning, in an integrated way.

The effect of pulsing on the first wall of a future fusion power plant is manifested mainly as thermal fatigue of the structure. Due to pulsing the temperature cycles result in varying stress in the first wall which is lifetime limiting as opposed to a steady-state mode where the number of heat-up–cool-down cycles is negligible. A series of analyses have been carried out on a “generic” DEMO outboard first wall using different parameters. The estimated lifetime from thermal fatigue point of view has been calculated. Both water and helium cooling have been considered. The effect of wall thickness between the channels and the front face as well as the coolant inlet temperature has been investigated and compared for both coolants.

Porous-Si-micromachining technique was used for the formation of single-crystalline force-sensor elements, capable of resolving the three vector components of the loading force. Similar structures presented so far are created from deposited polycrystalline Si resistors embedded in multilayered SiO2/Si3N4 membranes, using surface micromachining technique for a cavity formation. In this paper, the authors implanted four piezoresistors in an n-type-perforated membrane, having their reference pairs on the substrate in order to form four half bridges for the transduction of the mechanical stress. They successfully combined the HF-based porous-Si process with conventional doping and Al metallization, thereby offering the possibility of integration with readout and amplifying electronics. The 300times300 mum2 membrane size allows for the formation of large tactile arrays using single-crystalline-sensing elements of superior mechanical properties. They used the finite-element method for modeling the stress distribution in the sensor, and verified the results with real measurements. Finally, they covered the sensors with different elastic silicon-rubber layers, and measured the sensor's altered properties. They used continuum mechanics to describe the behavior of the rubber layer

Silicon based three-dimensional (3D) force sensor was designed and fabricated for detection of normal and shear forces. The sensor contains a rectangular rod emerging out of the centre of the membrane. The technology involves ion-implanted piezoresistors formation on the backside and a novel anisotropic etching on the front side of an n-type wafer. The two-component, two-step anisotropic etching process forms the rectangular side-wall rod and the membrane simultaneously, using a double layer silicon oxide and nitride mask. A mask-less alkaline etching in the second step results in the central rod protruding over the chip surface while leaving a frame of reduced thickness around the membrane providing mechanical stability. The cavity underneath the membrane was formed by bonding, facilitating also the signal read-out from the ion-implanted strain gauge resistors at the bottom (backside) of the membrane. The results of the mechanical test of the realised 3D Si force sensors are in good agreement with the theoretical predictions.

The paper deals with the analytical estimation of bandwidth and device sensitivity of cantilever- and bridge-type monolithic piezoresistive and capacitive accelerometers using different beam models and of their design sensitivity due to the change of geometrical parameters. Using a simplified two-beams model, closed-form formulae have been derived to give the relationship between the rates of length, width and thickness of the beams and the lowest eigenfrequency characterizing the bandwidth as well as the physical sensitivity of the accelerometers. By means of symbolic derivation, a structural design sensitivity analysis has been carried out to obtain information for the optimal selection of the geometrical parameters.

The deformation of Si/SiN x based square membrane structures are measured by Makyoh topography. The results are compared to finite-element modelling of the structure, and the thermal expansion coefficient of SiN x is extracted.