@article{zafar_martin_shannon_2019, title={Doppler-free, Stark broadened profiles at low plasma densities in helium}, volume={230}, ISSN={0022-4073}, url={http://dx.doi.org/10.1016/j.jqsrt.2019.03.020}, DOI={10.1016/j.jqsrt.2019.03.020}, abstractNote={This work utilizes Doppler-free saturation spectroscopy to measure Doppler-free, Stark broadened spectral profiles for the π-polarization of the HeI 21P → 61D transition in a low density (ne=7×1010−2×1012 cm−3) helium plasma. The measurements were performed in an electron-cyclotron resonance discharge at 23 mTorr with electron density being diagnosed using a combination of Langmuir probe and microwave interferometry techniques. The Doppler-free profiles were observed to be nearly symmetric at ne < 1011 cm−3 but markedly asymmetric above this transition point. Electron density is extracted from the spectral data via fitting to a spectral model based on quasi-static Stark broadening. The fit results are compared to ne measurements obtained using a combination of Langmuir probe and microwave interferometry techniques. The fit and measured ne are shown to agree within 20% on average. Finally, the quasi-static model is shown to be valid in the low-density regime for 21P → 61D helium transition.}, journal={Journal of Quantitative Spectroscopy and Radiative Transfer}, publisher={Elsevier BV}, author={Zafar, Abdullah and Martin, Elijah and Shannon, Steve}, year={2019}, month={Jun}, pages={48–55} }
 @article{zafar_martin_shannon_2018, title={High resolution magnetic field measurements in hydrogen and helium plasmas using active laser spectroscopy}, volume={89}, ISSN={0034-6748 1089-7623}, url={http://dx.doi.org/10.1063/1.5039334}, DOI={10.1063/1.5039334}, abstractNote={Passive spectroscopic measurements of Zeeman splitting have been used reliably to measure magnetic fields in plasmas for decades. However, a requirement is that the field magnitude must be sufficiently strong to be resolved over Doppler and instrument broadening (typically >10 000 G). A diagnostic for measuring magnetic fields spectroscopically well below this limit (>20 G) with high sensitivity has been developed at the Oak Ridge National Laboratory. The diagnostic relies on measuring a high resolution spectral profile using Doppler-free saturation spectroscopy (DFSS) and then fitting the spectrum to a quantum mechanical model. DFSS is an active, laser based technique that greatly reduces the influence of Doppler broadening and eliminates instrument broadening. To date, the diagnostic has been successfully employed to measure the magnetic field in magnetized (550-900 G), low-temperature (5-10 eV), low-density (1010–1012 cm−3), hydrogen and helium plasmas in the 5-200 mTorr pressure range using a low power (25 mW) diode laser. Implementing an approximate crossover resonance model, the measurements are shown to be accurate within 5 G for helium and 83 G for hydrogen. The accuracy in hydrogen can be improved to 39 G if the crossover resonances are neglected. A more robust crossover model can decrease this error to <1 G.}, number={10}, journal={Review of Scientific Instruments}, publisher={AIP Publishing}, author={Zafar, Abdullah and Martin, Elijah and Shannon, Steve}, year={2018}, month={Oct}, pages={10D126} }
 @article{klepper_martin_isler_colas_goniche_hillairet_panayotis_pegourié_jacquot_lotte_et al._2014, title={Probing the plasma near high power wave launchers in fusion devices for static and dynamic electric fields (invited)}, volume={85}, ISSN={0034-6748 1089-7623}, url={http://dx.doi.org/10.1063/1.4890247}, DOI={10.1063/1.4890247}, abstractNote={An exploratory study was carried out in the long-pulse tokamak Tore Supra, to determine if electric fields in the plasma around high-power, RF wave launchers could be measured with non-intrusive, passive, optical emission spectroscopy. The focus was in particular on the use of the external electric field Stark effect. The feasibility was found to be strongly dependent on the spatial extent of the electric fields and overlap between regions of strong (>∼1 kV/cm) electric fields and regions of plasma particle recycling and plasma-induced, spectral line emission. Most amenable to the measurement was the RF electric field in edge plasma, in front of a lower hybrid heating and current drive launcher. Electric field strengths and direction, derived from fitting the acquired spectra to a model including time-dependent Stark effect and the tokamak-range magnetic field Zeeman-effect, were found to be in good agreement with full-wave modeling of the observed launcher.}, number={11}, journal={Review of Scientific Instruments}, publisher={AIP Publishing}, author={Klepper, C. C. and Martin, E. H. and Isler, R. C. and Colas, L. and Goniche, M. and Hillairet, J. and Panayotis, S. and Pegourié, B. and Jacquot, J. and Lotte, Ph. and et al.}, year={2014}, month={Nov}, pages={11E301} }
 @article{klepper_hillis_isler_hillairet_martin_colas_ekedahl_goniche_lotte_colledani_et al._2013, title={Direct, spectroscopic measurement of electric fields in a plasma-RF antenna interaction region in Tore Supra}, volume={438}, ISSN={["1873-4820"]}, DOI={10.1016/j.jnucmat.2013.01.273}, abstractNote={Balmer-series spectral line profiles of deuterium emission near a lower-hybrid (3.7 GHz) wave, high power (1–4 MW) launcher were measured with high-spectral resolution in the Tore Supra tokamak and fitted to an atomic physics model which includes both Zeeman and dynamic Stark effects. The magnetic field is static and the electric field is assumed to be monochromatic at 3.7 GHz. The determined strength and direction of the high-frequency electric field is found to be in good agreement with the results of a simulation that computes the propagation of these lower hybrid waves into the plasma in the region around the launch antenna and specifically in the region of estimated peak emission contributing to the measurement. This agreement indicates feasibility for the use of dynamic Stark effect spectroscopy to study interaction at the plasma antenna interactions in a fusion plasma environment.}, journal={JOURNAL OF NUCLEAR MATERIALS}, author={Klepper, C. C. and Hillis, D. L. and Isler, R. C. and Hillairet, J. and Martin, E. H. and Colas, L. and Ekedahl, A. and Goniche, M. and Lotte, Ph and Colledani, G. and et al.}, year={2013}, month={Jul}, pages={S1232–S1236} }
 @article{klepper_isler_hillairet_martin_colas_ekedahl_goniche_harris_hillis_panayotis_et al._2013, title={Dynamic Stark Spectroscopic Measurements of Microwave Electric Fields Inside the Plasma Near a High-Power Antenna}, volume={110}, ISSN={["0031-9007"]}, DOI={10.1103/physrevlett.110.215005}, abstractNote={Fully dynamic Stark effect visible spectroscopy was used for the first time to directly measure the local rf electric field in the boundary plasma near a high-power antenna in high-performance, magnetically confined, fusion energy experiment. The measurement was performed in the superconducting tokamak Tore Supra, in the near field of a 1–3 MW, lower-hybrid, 3.7 GHz wave-launch antenna, and combined with modeling of neutral atom transport to estimate the local rf electric field amplitude (as low as 1–2 kV/cm) and direction in this region. The measurement was then shown to be consistent with the predicted values from a 2D full-wave propagation model. Notably the measurement confirmed that the electric field direction deviates substantially from the direction in which it is launched by the waveguides as it penetrates only a few cm radially inward into the plasma from the waveguides, consistent with the model.}, number={21}, journal={PHYSICAL REVIEW LETTERS}, author={Klepper, C. C. and Isler, R. C. and Hillairet, J. and Martin, E. H. and Colas, L. and Ekedahl, A. and Goniche, M. and Harris, J. H. and Hillis, D. L. and Panayotis, S. and et al.}, year={2013}, month={May} }
 @article{hahn_martin_bourham_2005, title={Characterization of arc generated plasma interactions with a liquid metal medium}, volume={47}, DOI={10.13182/fst05-a850}, abstractNote={Abstract Plasma interaction with first wall and interior reactor chamber components is an influencing factor in the design of inertial fusion facilities. The concept of a liquid metal wall, in which a circulating lithium curtain would be used, has been considered in many studies. The interaction of plasmas with moving liquid metals is a complex subject due to the influence of hydrodynamics, evaporation and droplet formation, nucleation and agglomeration of condensed particulates. To gain an understanding of some of the specific details of this interaction an experimental setup of an arc-generated plasma interacting with a liquid lead pool has been designed, constructed and operated. This simulation of the plasma-liquid interaction focuses on the particle condensation of the liquid metal after plasma interaction. The experiment generates transient high-density plasma over 50 μs pulse duration. Plasma characteristics are determined by various diagnostics. A set of collection substrates are arranged to collect nucleated particulates condensing from the evolving plume. Particulate size and distribution are analyzed numerically using digital images.}, number={4}, journal={Fusion Science and Technology}, author={Hahn, G. C. and Martin, E. H. and Bourham, Mohamed}, year={2005}, pages={1197–1201} }