Reassessment of impurity transport coefficients in Alcator C-Mod

Transport coefficients $D_{Z}$, $V_{Z}$ inferred from spectroscopic measurements of impurity injections are often used to validate simulations of turbulent transport. Existing methods for inferring $D_{Z}$, $V_{Z}$ fall short in one or more ways, leading to incorrect results and calling the use of such measurements for validation into question. For instance, the use of too simple of a function to describe the $D_{Z}$, $V_{Z}$ profiles leads to a dramatic underestimation of the uncertainty. This talk presents recent attempts to demonstrate some of these issues, estimate diagnostic requirements to determine impurity transport coefficients, and obtain rigorous uncertainty estimates on $D_{Z}$ and $V_{Z}$. In particular, transport coefficients are inferred using the MultiNest algorithm and the critical question of the level of complexity in the $D_{Z}$, $V_{Z}$ profiles which can be inferred is answered using the marginal likelihood. In addition to providing a useful framework for the solution of inverse problems relevant to the interpretation of experimental data, the use of these techniques will enable a reassessment of the ability of gyrokinetic simulations to match the impurity channel.