Theoretical interpretation of the operational density limit and comparison to experimental data

The operational density limit represents the maximum plasma density that can be achieved in magnetic fusion devices before the plasma develops a strong magnetohydrodynamic activity that leads to a disruption. Experimental observations have pointed out a strong link between the cooling of the plasma at the tokamak edge and the crossing of the density limit, suggesting the important role played by edge physics on setting the maximum achievable density. In fact, the crossing of the density limit is usually preceded by a strong and localized radiation in the tokamak edge, denoted as MARFE.

Three-dimensional turbulent simulations carried out by using the GBS code have pointed out the presence of a regime of degraded confinement at high values of collisionality and low values of heat source [1]. This regime, characterized by a large turbulent transport driven by resistive ballooning modes, has been associated to the crossing of the density limit. By leveraging these simulation results, a theory-based scaling law of the crossing of the density limit has been derived. While the theoretical scaling law shares with the Greenwald density limit the main dependences on the plasma current and tokamak minor radius, it also shows a significant dependence on the power crossing the separatrix. 

The scaling law derived in this work has been validated against experimental data taken within a multi-machine effort involving ASDEX Upgrade, JET and TCV tokamaks. This database is composed also of discharges that use advanced active control techniques that avoid the disruption after the onset of the MARFE as the density limit is approached. Therefore, the edge density was measured near the onset of the MARFE with a very high time resolution, allowing for an accurate comparison to the theoretical predictions. The predicted value of the density limit shows a remarkable good agreement with the density measured at the onset of the MARFE.

[1] Giacomin & Ricci, J Plasma Phys (2020)