Modelling of RFX-mod2 tokamak equilibria with DEMO-like shape conditions and negative triangularity

Abstract

In this paper, DEMO-like shaped plasma equilibria are defined for RFX-mod2 tokamak operations by using the Inverse Equilibrium Tool (IET code). IET allows for the computation of the coil currents needed to obtain a predetermined plasma shape with well defined plasma global parameters (i.e. total plasma current and total poloidal magnetic flux at the boundary) by solving a constrained minimization problem. The new shape conditions would allow achieving higher plasma current and plasma density values at the same toroidal magnetic field and safety factor limits of previous RFX-mod tokamak operations. The feasibility of these new equilibria is explored in terms of coil current requirements and vertical stability analysis. This study shows that RFX-mod2 is a flexible device, able to perform DEMO-like shaped tokamak operations with low requirements on both magnitude and distribution of active coil currents.

A brief history of negative triangularity tokamak plasmas

“…For example a double-null with strong δ < 0 was shown to be compatible with the TCV PF-coil limit and to feature a controllable γ n=0 ∼ 500 s −1 with a small LFS gap [59]. In [60], δ < 0 plasmas in RFX-mod2 are considered and compared with PT DEMO-like shapes. They find a similar vertical growth rate, within controllable range of their control system and mainly sensitive to elongation.…”

Design of a new reflectometric system for real time plasma position control on the RFX-mod2 device

“…Beside these technical issues, RFX-mod2 can offer a very high flexibility in terms of attainable plasma shapes. A good example is shown in figure 5(b), where plasma parameters (triangularity δ = 0.49 and elongation κ = 1.52) are very close to one expected on a single null DEMO scenario (other examples are shown in [24]). In DEMO, the implementation of a reflectometry based plasma position control is under development: preliminary studies [15] suggest a minimum of 16 poloidally distributed measurements in a single poloidal sector to correctly reconstruct the plasma shape; 2D/3D simulation tools [25], however, suggest a rapid downgrade of the performance for the antennas located far from the midplane, and, at present, the only mitigation strategy is represented by the increasing of the number of the receiving antennas per measurement position.…”

“…Thanks to the number and arrangement of poloidal field coils and flexible power supply system, RFX-mod2 can also operate with different plasma shapes and, in particular, different triangularities and X-point configurations [24]. As an example, in figure 5(b), we show a possible elongated plasma scenario, a single null configuration with a high triangularity δ = 0.49 and elongation κ = 1.52.…”

Positive and negative triangularity in RFX-mod2: a comparative analysis

“…The device is planned to be very flexible in terms of magnetic field shaping and control, and will be able to perform shaped tokamak operations with modest requirements of both magnitude and distribution of active coil currents. The definition of DEMO-like equilibria in RFX-mod2 was previously described in [13], also including NT plasmas, with the main focus on achieving elongation and triangularity parameters compatible with DEMO. On the other hand, the peculiar features of the (ohmic) RFX-mod2 tokamak will be the potential to run high triangularity experiments with low, almost vanishing elongation and to explore alternative confinement regimes at low q.…”