Electron critical gradient scale length measurements of ICRF heated L-mode plasmas at Alcator C-Mod tokamak.

A profile for the critical gradient scale length (<italic>L_c</italic>) has been measured in L-mode discharges at the Alcator C-Mod tokamak, where electrons were heated by an ion cyclotron range of frequency through minority heating with the intention of simultaneously varying the heat flux and changing the local gradient. The electron temperature gradient scale length (<italic>L_Te</italic>⁻¹ = |∇<italic>T_e</italic>|/<italic>T_e</italic>) profile was measured via the B_T-jog technique [Houshmandyar <italic>et al.</italic>, Rev. Sci. Instrum. <bold>87</bold>, 11E101 (2016)] and it was compared with electron heat flux from power balance (TRANSP) analysis. The <italic>T_e</italic> profiles were found to be very stiff and already above the critical values, however, the stiffness was found to be reduced near the <italic>q</italic> = 3/2 surface. The measured <italic>L_c</italic> profile is in agreement with electron temperature gradient (ETG) models which predict the dependence of <italic>L_c</italic>⁻¹ on local <italic>Z_eff</italic>, <italic>T_e</italic>/<italic>T_i</italic>, and the ratio of the magnetic shear to the safety factor. The results from linear Gene gyrokinetic simulations suggest ETG to be the dominant mode of turbulence in the electron scale (<italic>k</italic>_⊥<italic>ρ_s</italic> > 1), and ion temperature gradient/trapped electron mode modes in the ion scale (<italic>k</italic>_⊥<italic>ρ_s</italic> < 1). The measured <italic>L_c</italic> profile is in agreement with the profile of ETG critical gradients deduced from Gene simulations. [ABSTRACT FROM AUTHOR]