Operational requirements of the ion cyclotron wall conditioning in DTT

The Divertor Tokamak Test facility (DTT), a new superconducting tokamak (B0=6T, Ip=5.5MA, R0=2.19 m, a = 0.7 m, pulse length=100 s) (Ambrosino et al., 2021) under realization at the ENEA Frascati Research centre, needs an efficient first wall conditioning technique suitable for the use in the presence of a permanent magnetic field. The Ion Cyclotron Wall Conditioning (ICWC), based on Radio-Frequency (RF) discharges (Lyssoivan et al., 2012), with waves in the ion cyclotron range of frequencies, is one of the most efficient technique available (between discharges) for routine inter-pulse and overnight conditioning. ICWC will be performed using the first Ion Cyclotron Resonance Heating (ICRH) RF module foreseen during the first phase of DTT and constituted by two Antennas, four or two External Matching Units (EMUs) and four RF solid-state transmitters, operating in the frequency range (60-90) MHz. In the paper by means of the HFSS and CST codes, an eigenvalue analysis of the electromagnetic waves (E/H modes) excited inside the vacuum DTT volume has been carried out. Then full-wave simulations have been run for the entire DTT torus injecting a RF signal with a single and a pair of three-strap antennas, currently in the design phase for DTT, evaluating the S-parameters and the parallel electric field (E//) aligned to the toroidal magnetic field, responsible of the initial gas breakdown. On this basis the most appropriate choice for the RF feeding, in monopole phasing, as well as for the frequency of ICWC discharges in order to maximize E// has been identified and presented in the paper, demonstrating for the first time the usefulness of using an H mode at the frequency of 62.5 MHz instead of the lower frequency of 60 MHz minimizing the RF power required to activate the ionization process. Finally operational parameters for DTT, extrapolated from the analysis of ICWC experiments documented in literature, will be presented.