Alessandro Bruschi

This paper reports the main design keys and the challenging issues of the heating and current drive system (HCD) of the divert tokamak test (DTT) facility that is under construction at ENEA site in Frascati with the aim to perform studies on the power exhaust in a flexible and easily modifiable environment. The selected HCD systems for DTT are those with the most consolidated technology and expected to be relevant for the future reactor. The status of each system is reported, both in terms of design and procurement, which are well advanced for the system required in the first phase of the DTT exploitation: electron and ion cyclotron resonant heating. The third system is neutral beam injector, based on negative ion acceleration, which will be installed in DTT in a second phase, after the first five years of operation. The full heating power will be reached with the doubling of the radiofrequency power to reach the 45 MW at plasma needed to test the divertor with a power density at reactor level.

The electron cyclotron heating (ECH) launchers for the Divertor Tokamak Test (DTT) facility are currently being designed. The aim is to inject power in different plasma locations with high flexibility to allow plasma current ramp-up and ramp-down, central heating with tailored deposition profiles, and neoclassical tearing modes (NTMs) stabilization. Two antennas are developed, one for the equatorial port with six beam lines and one for the upper port with two beam lines, with similar front-steerable mirror modules. Each beam-line module is composed of a cooled corrugated waveguide launching a beam toward a fixed shaping mirror (M1) and then to the plasma-facing plane mirror (M2), movable around two axes, both cooled to sustain the high heat load due to the ohmic losses of the incident 1 MW beam and the plasma radiation. Thermal loads also include plasma and microwave stray radiation. Main structural loads on the mirrors and their supports, including the launcher supporting baseplate, are forces and torques due to the induced electro-magnetic (EM) currents, during normal operation and disruptions, interacting with the high magnetic field in which the mirrors and supports are immersed. Thermal and structural challenges, in particular on the M1 and M2 mirrors, have opposite solutions that require a delicate trade-off: optimal cooling requires high-thermal-conductivity materials such as copper alloys, while low-electrically-conductive materials are needed to have lower induced currents during disruptions. Solutions under study for the mirror cooling for minimal deformations and stresses, and for drive mechanisms, are presented. The possible reduction of induced currents during disruptions using different materials, supporting structures, and layouts is discussed.

Many current and next generation fusion experiments use Electron Cyclotron Resonance Heating for current profile tailoring and mode stabilization. Millimetre-wave power is delivered to the plasma in the form of Gaussian beams, either radiated by corrugated waveguides or via quasi-optical transmission lines. The size of the beams dictates that of the ducts in the neutron shielding, with the conflicting requirements of minimizing volume -to prevent loss of shielding- and reducing power loss in the wall -to lower the cooling requirements.This paper is stating scalable criteria for estimating millimetre-wave loss, to aid in assessing a minimum duct size for given power loss requirements.

European DEMOnstration (EU-DEMO) fusion reactor is currently progressing through its conceptual design phase. Reliability assessments play a key role in demonstrating that the system architecture can fulfill its intended functions over a defined lifespan. In the current approach, availability assessments begin with a failure mode and effects analysis (FMEA) to systematically identify potential failure events affecting system performance. Fault tree (FT) models are developed for subsystems exhibiting multiple failure modes, while the overall system configuration is modeled using a reliability block diagram (RBD). This study focuses on the electron cyclotron (EC) heating system, which supports plasma heating during ramp-up and ramp-down phases, stabilizes neoclassical tearing modes, and mitigates radiative instabilities. The analysis targets the current design of the transmission line (TL) and launcher subsystems, excluding gyrotrons from the current scope. Availability simulations have been performed over a 20-year operational period. Parametric analyses are also conducted to quantify the sensitivity of availability to maintenance duration and restoration parameters, providing key insights into design robustness and operational planning.