Requirements and modelling of fast particle injection in RFX-mod tokamak plasmas

The planned upgrade of the RFX-mod device is a good opportunity to widen the operational space of the machine, in both RFP and tokamak configurations. Installation of a power neutral beam injector (NBI) is also envisaged and a NBI system compatible with RFX-mod is already available on site. It was previously installed in TPE-RX (Tsukuba, Japan), it has a nominal power of 1.25 MW, a nominal current of 50A and it can operate at a voltage of 25kV for 30ms (or 15 kV for 100ms). A porthole in the modified vacuum toroidal support structure is planned to be adapted to host this injector, which would operate in the equatorial plane and in a perpendicular direction with respect to the plasma column. This geometry is the only one possible due to mechanical constraints, mainly linked to toroidal field coils configuration.In this work, the METIS simulator is used to study the feasibility of TPE-RX injector integration in RFX-mod tokamak plasmas. METIS code allows the simulation of a full tokamak discharge in a time of the order of a few minutes by using relations coming from scaling laws coupled with simplified source models. The neutral beam injection in METIS is described by a decay equation applied in a simplified geometry and an analytical solution of the Fokker-Planck equation. In order to estimate a set of requirements for an NBI system to be applied to RFX-mod tokamak plasmas the beam shine-through has been carefully considered for a wide range of scenarios. Finally some indications of the physical effects to be expected from NBI-plasma coupling will be highlighted, with special attention to the beam energy absorption (both in terms of time dependent quantities and of spatial profiles) and to the possibility of inducing a transition from L to H confinement modes.