Francesco Fanale

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.

An innovative cooling strategy for the mirrors of the transmission line of the electron cyclotron heating (ECH) of a fusion research reactor, the divertor tokamak test (DTT) facility, is presented and investigated here based on triply periodic minimal surfaces (TPMSs). After the check on the manufacturability of gyroid and Split-P lattices of unit cell size ranging from 3 to 8 mm, extensive numerical investigations by conjugate heat transfer analyses have been performed. The heat removal capability has been assessed together with the pressure drop for all heat sinks. The temperature field in the mirror has been evaluated and used for the assessment of the deformation of the mirror reflective surface. The results show that all tested TPMSs are highly efficient in the heat removal, with low hotspot temperatures, minimal pressure drop, and low deformation of the reflective surface.

The divertor tokamak test (DTT) facility is designed to study the power exhaust methods in scenarios similar to those expected in future fusion plants like DEMO. The project foresees 45 MW of external additional heating power to the plasma, provided by electron cyclotron heating (ECH), ion cyclotron heating, and neutral beam injector. This article focuses on the present status of the design of the transmission lines (TLs) and antennas for the ECH system. In the first operational phase, the system shall include 16 gyrotrons (1 MW/170 GHz/100 s), grouped into two clusters of eight sources each. In a later stage, the total installed power shall be doubled for a total of four clusters. Quasi-optical TLs connect the EC sources, located in a dedicated building, to the tokamak, via mirrors transmitting the microwave beams inside vacuum pipes to reduce losses and risks of arcing in air. Each TL consists of three main sections: two single-beam (SB) sections with small mirrors dedicated to a single beam at the beginning and the end of the line and a midsection of multibeam (MB) mirrors for the transmission of all the eight beams of the cluster. The beams at the output of the TL match the antennas installed in the equatorial and upper ports of the same vacuum vessel sector, through corrugated circular waveguides (WGs). The WGs are needed to reduce the vacuum conductance between the DTT vessel and TLs environments, being the connection with the vacuum vessel managed by all-metal gate valves without diamond windows. The two antennas consist of independent launching modules for each beam composed by two mirrors, one fixed and one steerable. All the components of the system are actively water-cooled to limit deformations of the mirrors surface to ensure the required quality of the beams and to reduce the transmission losses.

The EU DEMO Tokamak is foreseen to be equipped with an electron cyclotron (EC) system for plasma heating, magnetohydrodynamic (MHD) control, and thermal instability suppression. Up to six launchers will be installed into equatorial ports with the aim to inject a maximum of 130 MW millimeter wave power at frequencies of 136/170/204 GHz toward dedicated positions into the plasma. This article presents the current layout of the typical EC system launcher components within the available space reservation areas in the equatorial level of the DEMO baseline model. Beside the general arrangement of the EC launcher components, preliminary design features of the launcher port plug modules, an active cooling system approach, integration concepts for mirrors and waveguides (WGs) of the optical system, the shielding component formation, and safety important elements of the first containment barrier of the launcher are presented. Furthermore, relevant aspects of assembly and potential remote maintenance (RM) procedures are discussed.