Present status of the design of the reflectometric system for DTT

The Divertor Tokamak Test (DTT) facility [1] is a new fusion device conceived to investigate alternative power exhaust solutions for DEMO. The study and the control of the plasma-wall interactions is, thus, of particular importance and will be also possible thanks to different diagnostic systems. In this contribution, the present status of the design of the reflectometric system for DTT will be described. DTT reflectometer will involve three different sub-systems to be installed in a dedicated toroidal section: a reflectometer for density profile reconstruction (DPR), a Doppler reflectometer for fluctuation and turbulence studies (TR) and a plasma position reflectometer (PPR). The DPR will be installed in the equatorial port (Low Field Side) and will be designed to cover from the Scrape Off Layer to the core density layers. To accomplish this task a fast sweep (~?s) among the K, Ka, U, V, W and D bands either in O or X-mode will be necessary. The measurement provided by the DPR in the equatorial port will be also used in the PPR system. Two TR will be installed in the oblique upper and lower ports (with respect to the midplane) where O-mode and X-mode wave will be launched respectively. The use of W and V bands will guarantee a good coverage of the SOL, edge and pedestal layers. The installation of steerable mirrors is under consideration and should allow the access to a larger spectrum of fluctuation wavenumber k?. In general, TR systems will be used for the study of the electric radial field behavior with respect to the main discharge parameters and the different plasma equilibria that DTT is expected to explore. A PPR is also being investigated for the installation on DTT with the aim of testing its capabilities in the view of a possible reactor application as a complement/back-up system with respect to magnetic sensors. The system should involve four lines-of-sight: one in the low field side midplane (the DPR), one in the upper vertical port and two in the high field side region. According to the physical space available, in each line-of-sight a configuration involving a full band (K, Ka, U, V, W and D band) or a subset will be considered to monitor the full density radial profile or the regions around the last closed flux surface. Due to the strong impact on the first wall and vessel high field side components the design study of the PPR is of primary importance. At present status, the integration with the DTT structure in the high field side has requested the removal of a cooling tube to accommodate the waveguides conveyed through the upper vertical port. To meet the very severe space limitations, hog-horn antennas will be used. A simulation activity will accompany the definition of the system: the performance of the PPR system in the low field side ports has been already assessed through the 2D REFMULF code [2]; a similar activity in the high field system will allow a better definition of the PPR layout.