Speaker
Description
Plasma–material interactions represent a key challenge for future nuclear fusion reactors and are widely investigated in linear plasma devices. The GyM [1] linear device, hosted at ISTP-CNR in Milan, operated at plasma densities of $10^{15}-10^{17}\text{m}^{-3}$, electron temperatures below 15eV, and ion fluxes up to $10^{21}\text{m}^{-2}\text{s}^{-1}$, reproducing tokamak main chamber conditions. To extend the parameter space toward divertor-relevant regimes, characterised by plasma densities of $\sim10^{19}\text{m}^{-3}$ and ion fluxes up to $10^{23}\text{m}^{-2}\text{s}^{-1}$, GyM is being upgraded to BiGyM, within the framework of the NEFERTARI project funded by the NextGenerationEU programme.
In BiGyM, helicon–wave–sustained plasmas will be generated by two 10kW birdcage resonant antennas [2] operating at 13.56MHz, alike those used in the RAID [3] linear machine, capable to ensure high plasma densities. The device will also feature a revised magnetic configuration, a redesigned vacuum vessel, a novel sample holder, and new in–situ surface diagnostics.
Among BiGyM design activities to support the machine optimisation, this contribution focuses on detailed plasma-neutral modelling performed with SOLPS–ITER [4], a tokamak edge simulation code recently adapted to linear geometries.
Firstly, BiGyM modelling was informed by parametric studies of a RAID plasma, examining the impact of absorbed power, power density distribution, particle absorption at pumping surfaces, particle diffusion, and energy transport coefficients. RAID simulations were validated against experimental data.
Building on these results, BiGyM simulations predict plasma parameters and explore their dependence on injected power, gas pressure, magnetic field configuration and boundary conditions. Different working gases, including helium and argon, were considered to evaluate the overall plasma performance.
For representative helium discharges (B≈20mT, p≈0.8Pa, $P_\text{injected}≈3\text{kW}$), simulations predict electron densities $n_e$ in the range $(1.5-2.0)\times10^{19}\text{m}^{-3}$ and electron temperatures $T_e$ of 4–5eV along the axis, with particle fluxes $\lesssim 3.5\times10^{22}\text{m}^{-2}\text{s}^{-1}$, consistent with divertor-relevant conditions. Under similar operating conditions, argon plasmas are predicted to reach densities of $\sim2\times10^{18}\text{m}^{-3}$ and electron temperatures of ~9eV along the axis, with particle fluxes $\lesssim 6\times10^{21}\text{m}^{-2}\text{s}^{-1}$. The results show good left–right symmetry of plasma parameters and indicate that variations in magnetic configuration modify them by less than 15%, assuming a fixed absorbed power density. By doubling the absorbed power, $n_e$ increases by about 70%, while $T_e$ increases by approximately 10%.
The predicted plasma conditions meet the performance targets set for the BiGyM upgrade, confirming that the adopted design choices are well suited to access divertor–relevant regimes and providing quantitative guidance for the finalisation of the machine layout and operational strategy.
References
[1] A.Uccello et al (2023) Front. Phys. 11, 1108175
[2] P.Guittienne et al (2021) Plasma Sources Sci. Technol. 30 075023
[3] I.Furno et al (2017) EPJ Web Conf. 157, 03014
[4] S.Wiesen et al (2015) J. Nucl. Materials 463:480–484