Validity of using heat pulse diffusivity as a proxy for incremental diffusivity in validation studies

One metric which can be compared between plasma physics models and experiments in validation work is the incremental diffusivity, χ_e^inc(Creely et al., 2016). However, the quantity which can be experimentally measured is actually the heat pulse diffusivity, χ_e^HP­. In past work, the effective diffusivity, χ_e^eff, is assumed to be independent of the ion temperature gradient, ∇T_i, when proving equivalence between the incremental diffusivity, χ_e^inc, and the heat pulse diffusivity, χ_e^HP(Tubbing et al., 1987). However, we expect that in strongly driven ITG transport, the electron heat transport would be sensitive to ion temperature gradients. Here, we use the ASTRA power balance code (Pereverzev et al., 1991) with TGLF SAT2 (Staebler et al., 2021) to test whether the heat pulse diffusivity, χ_e^HP, changes depending on whether or not we hold ∇T_i fixed. We explore which plasma conditions cause a discrepancy between the calculation of the heat pulse diffusivity, χ_e^HP, and the incremental diffusivity, χ_e^inc, suggesting that validation utilizing their comparison should be avoided in these regimes.