Utilization of D2 molecular band emission for electron density measurement

D₂ molecular band emission observed at a wavelength range of $\lambda \sim 557-643\mathrm{nm}$ is utilized to measure electron density, $n_e$, in D plasmas of the PISCES-A and PISCES-RF linear plasma devices. The D₂ band is divided at $\lambda =593\mathrm{nm}$ to make an intensity ratio, D${}_{\mathrm{2L}}$ ($\sim 557$–$593\mathrm{nm}$)/D${}_{\mathrm{2R}}$ ($\sim 593$–$643\mathrm{nm}$), where D${}_{\mathrm{2L}}$ consists predominantly of the $g{}^3{\Sigma }_g^{+}3d\sigma$, $h{}^3{\Sigma }_g^{+}3s\sigma$, $i{}^3{\Pi }_{g}3d\pi$, $j{}^3{\Delta }_{g}3d\delta$ $\to$ $c{}^3{\Pi }_{u}2p\pi$ transitions, while D${}_{\mathrm{2R}}$ mainly includes the $d{}^3{\Pi }_{u}3p\pi$ $\to$ $a{}^3{\Sigma }_g^{+}2s\sigma$ Fulcher band emission. It is experimentally found that D${}_{\mathrm{2L}}$/D${}_{\mathrm{2R}}$ depends strongly on $n_e$ with little $T_e$ dependence in ranges of $n_e\sim (0.031-6.1)\times 10^{18}$ m⁻³ and $T_e\sim 2.3-13.9\mathrm{eV}$. This observed trend is consistent with collisional-radiative model calculations using Yacora on the Web.