Inner-wall Impurity Density Measurements on Alcator C-Mod

Recent findings on Alcator C-Mod imply a poloidal impurity density asymmetry in the pedestal region, inferred from boron ($B^{5+}$) velocity measurements at the inner- and outer-wall combined with neoclassical transport theory. In an effort to confirm these findings, the direct measurement of the boron density was made at the inner-wall pedestal using simultaneous spectroscopic views of boron charge exchange recombination spectroscopy (CXRS) ($B^{4+},\, n=7-6,\,\lambda=494.467nm$) and Balmer-$\alpha$ ($D^{0+},\, n=3-2,\,\lambda=656.6nm$). Since the inner-wall CXRS measurement utilizes a deuterium gas puff as the source of neutrals to induce charge exchange, detailed modeling is required in order to calculate the neutral density profile from the Balmer-$\alpha$ measurement. Additionally, due to the low interaction energies the $B^{5+}+D^{0+}(n=2)\rightarrow B^{*(4+)}+D^{1+}$ is the dominant boron CX process, so a collisional-radiative model is required to calculate the $n=2$ deuterium density from the $n=3$ density derived from the Balmer-$\alpha$ measurement. The radial shape of the resulting boron density profile agrees with the profile predicted by the impurity density asymmetry. Supported by USDoE award DE-FC02-99ER54512.