Healing plasma current ramp-up by nitrogen seeding in the full tungsten environment of WEST

Abstract

Achieving a successful plasma current ramp-up in a full Tungsten tokamak can be challenging due to the large core radiation (and resulting low core temperature) that can be faced with this heavy metallic impurity if its relative concentration is too high. Nitrogen injection during the plasma current ramp-up of WEST discharges greatly improves core temperature and Magneto-Hydro-Dynamic stability. Experimental measurements and integrated simulations with the RAPTOR code complemented with the Qualikiz Neural Network for computing turbulent transport allow a detailed understanding of the mechanisms at play. Increased edge radiation during this transient phase is shown to improve confinement properties, driving higher core temperature and better MHD stability. This also leads to increased operation margins with respect to Tungsten contamination.

Operating a full tungsten actively cooled tokamak: overview of WEST first phase of operation

“…In WEST, during the early plasma current ramp-up, tungsten central radiation very often cools down the plasma in the centre, resulting in a hollow electron temperature (T e ) profile and the triggering of MHD 2/1 modes. In order to prevent T e profile hollowness from occurring, nitrogen (N 2 ) injection during the current ramp-up has been employed (figure 8) [33]. N 2 injection induces an increase of the plasma resistivity in the peripheral region that leads to faster current diffusion (see plasma inductance in figure 8), hence higher ohmic heating in the cen- tre which compensates for the plasma cooling due to W core radiation.…”

Developing high performance RF heating scenarios on the WEST tokamak

“…Although most of the discharges with initial temperature greater than 1.5 keV were performed without nitrogen gas injection during the current ramp-up, this low-Z impurity was found efficient in raising the temperature of the OH phase. It results in an increase of the plasma resistivity in the peripheral region that leads to faster current diffusion and a strong peaking of the temperature with a central temperature increasing from ∼1 keV to ∼2 keV [26].…”

Predictive JET current ramp-up modelling using QuaLiKiz-neural-network

“…Within the previous JET H discharges, the core plasma density was controlled by injecting neutral gas into the SOL. While other methods are available for controlling the core impurity accumulation [2,34,35], this was ultimately chosen due to experimental constraints but remains suitable for this study due to its simplicity in modelling. By empirically extrapolating the gas flow rate necessary to produce the paired experiments in H [17], it was estimated that an increase of 20%-30% was needed in the gas injection system in order to achieve the same q profile in T. This Comparison of the electron temperature (leftmost), Te, ion temperature (left middle), T i , electron density (right middle), ne, and safety factor (rightmost), q, profiles at t = 7 s for the electron density BC scan of the T extrapolation simulation (blue) based on JET#97776.…”

“…Accurate modelling of this phase is required for detailed pulse design and preparation of ITER scenarios, starting from the prefusion-power-operation (PFPO) campaigns [1]. The lessons learned from the WEST device [2] highlight the necessity and the challenges of successfully developing the current rampup phase, from both an operational and a plasma physics perspective. An examination of the validity of integrated plasma models in this phase helps to identify any gaps in the existing code base and enables these tools to assist in the development and interpretation of these campaigns [3].…”