Evaluation of finite orbit width effect on alpha and NBI ions heating in CFETR scenarios

Impact of finite orbit width (FOW) effect on fast ion heating and fusion performance in CFETR scenarios is investigated using a test particle code PTC and integrated modelling (IM) within the OMFIT framework. Simulation results from PTC code indicate that when the FOW effect is considered, a higher safety factor q profile, lower magnetic field, or larger initial fraction of trapped particles each leads to a broader slowing-down deposition profile of fast ions, primarily due to the increased orbit width. The broadening effect is more pronounced in regions with steep gradients, such as the deep core and ITB regions for alpha ions, and near the magnetic axis and inflection point for NBI ions. To enhance the predictive accuracy of IM for hybrid and steady-state CFETR scenarios, the PTC code is incorporated into the kinetic-EFIT workflow in OMFIT platform, replacing the simplified delay model for alpha heating, which ignored the FOW effect. In the CFETR hybrid scenario, integrated simulations show that the significant reduction in electron temperature in the deep core region weakens inward particle transport driven by TEM, which leads to a decrease in density and a subsequent 6.4% reduction in the fusion gain factor Q. In the CFETR steady-state scenario, the weakening of temperature ITB strength and the further reduction of temperature in the deep core region lead to lower fusion power. Moreover, to sustain the fully noninductive condition, auxiliary heating is increased to compensate for the decline in bootstrap current. These factors ultimately lead to a 17% reduction in the Q value. The results reveal the importance of considering the FOW effect of fast ions heating in the physics design of CFETR.