Description
Recent records for triple-product at Wendelstein 7-X (W7-X) have been achieved using a combination of neutral beam injection (NBI) and electron cyclotron resonance heating (ECRH). To push the boundaries of operation and achievable performance parameters further, an upgrade of the NBI from 4 to 6 and possibly later to 8 sources is under consideration. In this study, we assess the extended operational space based on both empirical extrapolation and transport modelling.
The upgrade corresponds to an approximate additional 3-6 MW total auxiliary heating and a significant increase in particle fueling. Derived from the database of discharges with the present NBI configuration, empirical scalings for key performance observables are established and extrapolated to the upgraded power range. The study projects the evolution of diamagnetic energy, line-integrated density, and core density as a function of total heating power. The analysis separates core and edge density responses to increased beam fueling, thereby enabling a preliminary estimate of the edge density rise that is anticipated at constant gas puff and pumping conditions, as well as the attendant alterations in operational margins.
The plasma scenarios of the highest performance employ NBI heating but the particle fueling and inherent density peaking must be balanced by the density pump-out effect of ECRH. This imposes a minimum fraction of ECRH to NBI to sustain a stable plasma density and the favorable temperature profiles. This study therefore provides critical guidance for integrated actuator planning (NBI and ECRH) given the planned NBI upgrades.
To complement the empirical extrapolation with physics-based predictions, NTSS transport simulations are performed using projected density and heating profiles to estimate ion temperature evolution in prospective high-performance scenarios. The simulations indicate a moderate increase in core ion temperature with the additional NBI power, consistent with the scaling-based rise in stored energy, while remaining sensitive to assumptions on density peaking and edge density constraints. Taken together, the empirical scalings and NTSS results provide quantitative expectations for diamagnetic energy, density partitions (core/edge), and ion temperature in a 6 or 8-source NBI configuration. Furthermore, they identify the additional ECRH power needed for stable access to the projected high-performance operational space.