Session TO04: Fundamental Plasmas: Diagnostics

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Current-Voltage (I-V) characteristics of strongly emitting emissive probes are investigated in a multi-dipole filament discharge in argon. It is found that at sufficiently high neutral pressure and emission current, the variation of the I-V traces and their associated inflection points no longer follow the previous predictions of space charge limited (SCL) models. A new, steep slope region of the I-V trace appears near the plasma potential when the probe is strongly emitting, causing the inflection point and the floating potential to increase toward the plasma potential as the emission current increases, rather than staying constant. It is also found that the double inflection point structure when the probe is biased below the ionization energy of the working gas is highly likely to be an emission retardation effect from enhanced virtual cathode formation due to the increased local electron density. The dip between the virtual cathode and the bulk plasma potential also increases. This suggests effects predicted by Campanell et al's inverse sheath theory are applicable to a region of emissive probe I-V traces. Mechanisms with which virtual cathode effects limit hot cathode electron emission at high heating power are also investigated.   

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An algorithm for automated fitting of the effective electron temperature of a planar Langmuir probe I-V trace taken in a plasma with multiple Maxwellian electron populations is developed through MATLAB coding. The code automatically finds a fitting range suitable for analyzing the temperatures of each of the electron populations. The algorithm is used to analyze I-V traces from both the Diagnostic Test Source device in ASIPP, CAS and a similar multi-dipole chamber previously at UW-Madison which is now at USD. I-V traces reconstructed from the parameters fitted by the algorithm not only agreed with the measured I-V traces but also revealed physical properties consistent with those found in previous studies. Application of the algorithm to cylindrical probe I-V traces is also investigated. The major difficulties of such applications, i.e. distortion of the I-V traces by a low signal-to-noise ratio combined with greater sheath expansion, have been identified. It is recommended to use planar probes when signal-to-noise ratio is poor.   

Live

The use of Langmuir probes as a diagnostic tool in the study of both space and laboratory plasma cannot be overemphasized. Different approaches have been used in the interpretation of measurements collected using Langmuir probes in term of plasma parameters such as: density, temperature, and plasma potential. In this work, we apply a multivariate regression algorithm in constructing a predictive model for plasma parameters. Data used for constructing and validating the model are obtained by kinetically simulating the interaction of plasma with a fixed bias probes over a range of plasma parameters obtained using International Reference Ionosphere (IRI) model, of relevance to Low Earth Orbit satellites. Simulations are used to construct a solution library with currents collected by three spherical probes, with corresponding plasma parameters. An approximate expression, based on the Orbital Motion Limited (OML) approximation, is derived for each of the plasma parameters to be predicted. This expression is used to estimate the parameters of interest using training data set consisting of a subset of the solution library. The error in the predictions is determined and a second model, based on RBF regression is constructed to predict and correct for the error resulting from the first (OML) approximation. The skill of the combine model is validated using the data set not included in the training data set. By combining the two models, predictions of plasma parameters can be made from a probe measurement with better accuracy.   

Live

A combination of theory and multivariate regression is used to infer a satellite floating potential from currents collected with three or more cylindrical probes with different fixed bias voltages with respect to a spacecraft. Using a scaling law derived in the Orbital Motion Limited (OML) approximation, currents can be used to infer a satellite floating potential, as well as the density and temperature of background plasma. Corrections to these estimates can then be made using multivariate regression based on Radial Basis Functions (RBF) interpolation. Training of the RBF correction model, makes use of a synthetic data set, or solution library, constructed with three-dimensional kinetic simulations from which collected currents are calculated as a function of probe voltages, in a range of plasma environment parameters relevant to ionospheric plasma encountered by satellites in low Earth orbit (LEO). These currents in turn can be used to infer approximate values of the electron plasma density, temperature, and satellite potential using the OML estimates. The RBF model is then trained to approximate the discrepancies between OML estimates and known values from the simulations. The combination of these models leads to a significant improvement in the prediction skill for the parame   

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The information about the electron population emanating from a helicon source plasma is important in order to understand the formation of the current-free double layer (CFDL) between the source and the downstream region of a helicon plasma. The electrons need an energy higher than the potential drop across the CFDL to escape downstream from the source, and at these energies, the signal of a standard Langmuir probe is less accurate. We present measurements of the high-energy tail of the electrons by an inverted RFEA. To reach the probe, these electrons must have energies above V$_{\mathrm{p}}$, which can vary over the region of the measurement. By constructing a full distribution from the electron temperature T$_{\mathrm{e\thinspace }}$obtained from the electron IV curve and the V$_{\mathrm{p}}$ obtained from the ion IV-curve from a standard RFEA setup, we obtained a density measure of the hot distribution independent of V$_{\mathrm{p}}$. We compared the axial development of this high-energy density by a simple model of the electron density taking the product of the Boltzmann relation and magnetic flux conservation. The agreement between the measured and calculated energetic electron density development was in qualitative agreement to within \textless 10 {\%} error of the experimental values.   

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Alfven waves are of fundamental importance in magnetized plasmas, such as the solar wind and Earth's magnetosphere. When the wave amplitude is large, nonlinear interactions among waves, such as three wave resonance, dominate the dynamics. These nonlinear process may be used as a diagnosis of the plasma composition; this is because the composition affects the dispersion relation of Alfven waves, especially when the wave frequency is close to ion cyclotron frequencies. We present results from recent experiments on the Large Plasma Device at UCLA aimed at investigating nonlinear interactions of two counter-propagating Alfven waves in a plasma with both hydrogen and helium ions. A third Alfven wave is excited through three wave resonance, with frequencies and wave numbers matching the resonance condition. The measured properties of these waves are then used to estimate relative density of helium ions. The results are also compared to estimates from other methods. The outcome of this study will have implications in developing new technology to measure cold ion populations in space plasmas, which is very challenging using traditional methods. This work was performed at the Basic Plasma Science Facility supported by DOE and NSF.   

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Many micro- and nanoscale electrical phenomena depend critically on work function (WF), such as field emission and gas breakdown; however, characterizing WF variation due to surface roughness is challenging. Simple theoretical approaches assume the difference between effective (measured) and material WFs arise due to changes in capacitance as a scanning Kelvin probe (SKP) moves along a sinusoidal surface [1]. However, this simple theory neglects the spatial periodicity, which becomes critical as the SKP step size approaches the period. We extend this theory to specifically include periodicity. For a given ratio of surface roughness amplitude to the gap distance, increasing the period or reducing the SKP step size reduced the surface's effective WF. For an infinite period or infinitesimally small SKP step size, the effective WF approached the material WF. As the SKP step size approaches the period of the surface roughness, the effective WF approaches infinity. These results demonstrate the importance of accounting for surface waviness when measuring WF and in theories for field emission and microscale gas breakdown. [1] W.Li and D. Y. Li, J. Chem. Phys. 122, 064708 (2005). [2] A. L. Garner, A. M. Loveless, J. N. Dahal, and A. Venkattraman, IEEE Trans. Plasma Sci. 48, 808-824 (2020).   

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Ionospheric wind is an important physical parameter in monitoring and understanding space dynamics. A relatively simply design, the segmented flow meter (SF meter) is proposed for measuring plasma flow and other space plasma parameters. This study is carried out by first using Particle in Cell simulations to calculate the response of the SF meter to space environments in a wide range of plasma conditions representative of the ionosphere at low latitudes. A solution library containing ion currents collected by several sensors in the SF meter is then constructed under the specified conditions. The solution library, is then used to build a predictive model for the flow velocity, using regression techniques such as radial basis functions and neural networks. The input to the model is a set of ion currents collected by different segments of the flow meter, and the output is a set of plasma parameters such as plasma density. This model can be used to predict various of plasma parameters such as transverse speed, the density, the ion effective mass, etc.   

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The probability of a fusion reaction occurring within a plasma is determined by the product of the fusion cross section and the plasma ion-velocity distribution function. As the mean energy of the reacting ions increases, the fusion cross section increases while the ion velocity distribution rapidly decreases. The resulting fusion reaction probability therefore has a peak value referred to as the Gamow peak. The Gamow peak contains valuable information on the both the fusion cross section and the plasma ion-velocity distribution. Since the energy of the fusion products is determined by the mass and energy of the fusing ions, information on the Gamow peak can be inferred through measurements of the fusion products energy spectra. In this talk measurements of the first and second moments of the DT and DD neutron energy spectra are used to infer the Gamow peak in plasmas with ion temperatures from 2 to 20 keV. These measurements are compared to calculations using both Maxwellian and non-Maxwellian ion-velocity distributions. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.   

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The Cherenkov mechanism used in Gas Cherenkov Detectors (GCD) is exceptionally fast. The temporal resolution of GCDs, however, has been limited to \textasciitilde 100 ps by the current state-of-the-art photomultiplier tube (PMT) technology. The novel Pulse Dilation Photomultiplier Tube (PD-PMT) has a temporal resolution of \textasciitilde 10 ps, comparable to that of the gas cell. At 8 MeV threshold GCD measures DT fusion gammas and therefore directly the fusion reaction rate. High frequency features in the DT reaction history are visible that can improve the understanding of inertial confinement fusion. Background corrupting the DT signal was investigated and counter-measures implemented. The thorough investigation of background sources will inform future high sensitivity detector designs.   

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Characterizing the flux of charge-exchange neutrals to plasma facing surfaces in fusion devices is important to understanding wall erosion and redeposition. Palladium metal insulator semiconductor (Pd-MIS) Schottky diode sensors potentially offer a compact and inexpensive way to perform these crucial measurements. While previous Pd-MIS sensors have been tested in tokamaks, several issues prevented more widespread adoption. These sensors had a poor fabrication yield and were prone to degradation in the harsh fusion environment. This was thought to be due to high energy particles causing charge accumulation in the oxide layer, thereby distorting the measured signal. We report here on the fabrication and testing of sensors with an increased Pd layer thickness and minimized oxide thickness which are expected to mitigate these deficiencies. Titanium (Ti) and chrome (Cr) adhesion layers have been tested to prevent delamination of thick Pd layers. We report preliminary testing and calibration performed with a mass-separated 500 eV deuterium (D) ion beam with a total dose \textgreater 10$^{\mathrm{14}}$ D onto 1.5 mm diameter active area sensors. New sensors demonstrated improved fabrication yield and excellent sensitivity, serving as a pathway to future testing in a tokamak.   

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A series of recent experiments [1] on the Omega laser facility with spherical beryllium and gold targets employed Thomson scattering (TS) to measure the time evolution of plasma parameters at several locations in the underdense corona. Measurements were used to validate the thermal transport description in the large scale radiation hydrodynamic code that models inertial-fusion targets. Details of the TS theory and experimental implementation are discussed in this talk. These include plasma inhomogeneity, heating by the probe, optics effects and wave front tilt. Two directions of wave vectors, both radial and tangential to the target surface directions are measured and analyzed. Similarly to the recent study [2] thermal transport is investigated by careful matching of the TS spectra using particle distribution functions calculated in Vlasov-Fokker-Planck (VFP) code and derived in the classical and nonlocal models. Scattering on ion-acoustic fluctuations and asymmetry of the resonant peaks is used to measure return current corresponding to the heat flux. The blue shifted peak in the electron plasma fluctuation spectra constrains high velocity part of the electron distribution functions. We examine applicability of the Schurtz-Nicolai-Busquet transport model [3] in these plasmas. [1] W.A. Farmer, C. Bruulsema, G. Swadling \textit{et al}. Phys. Plasmas, \textit{submitted} (2020). [2] R.J. Henchen, M. Sherlock, W. Rozmus \textit{et al}. Phys. Rev. Lett. \textbf{121}, 125001 (2018). [3] G.P. Schurtz, P.D. Nicolai, and M. Busquet, Phys. Plasmas \textbf{7}, 4238 (2000).   

Langmuir probes don't measure plasma potentials correctly in presheaths near boundaries

It is shown that emissive probes (EPs) measure plasma potential profiles correctly in plasma presheaths, and that Langmuir probes (LPs) do not, in low temperature, low pressure plasma. It is conventional wisdom that LPs do not work in the sheath near material boundaries, but do work in quasineutral plasma. Experiments were performed in unmagnetized argon discharges, $ 0.1\leq P_n \leq 1mTorr$, with $ 1 \leq T_e \leq 5 eV,$ and $\tento{1}{9} \leq n_e \leq \tento{1}{10} cm^{-3}$, that compared plasma potential measurements made by partially coated and uncoated LPs, and cylindrical LPs, with measurements made by emissive probes. Presheaths were set up in the plasma using negatively biased electrodes. Results indicated that the EP potential measurements (in the limit of zero emission) were more negative than LP measurements in the presheath. In the sheath, most LP measurements did not go negative but rather became increasingly positive. Only the EP measurements worked in the sheath and presheath. These differences are thought to be caused by inherent, diffuse, ion flow in the presheath region toward the negatively biased electrode, characteristic of sheath formation.