Observation of Balmer-Alpha Line Profile of Hydrogen Atoms in Plasmas by Rapid-Frequency-Scan Laser Spectroscopy

The magnitude and spatial dependence of neutral density in magnetic confinement fusion experiments is a key physical parameter, particularly in the plasma edge. Modeling codes require precise measurements of the neutral density to calculate charge-exchange power losses and drag forces on rotating plasmas. However, direct measurements of the neutral density are problematic. In this work, we proposed to construct a laser-based diagnostic capable of providing spatially resolved measurements of the neutral density in the edge of plasma in the DIII-D tokamak. The diagnostic concept is based on two-photon absorption laser induced fluorescence (TALIF). By injecting two beams of 205 nm light (co or counter propagating), ground state hydrogen (or deuterium or tritium) can be excited from the n = 1 level to the n = 3 level at the location where the two beams intersect. Individually, the beams experience no absorption, and therefore have no difficulty penetrating even dense plasmas. After excitation, a fraction of the hydrogen atoms decay from the n = 3 level to the n = 2 level and emit photons at 656 nm (the Hα line). Calculations based on the results of previous TALIF experiments in magnetic fusion devices indicated that a laser pulse energy of approximately 3 mJ delivered in 5 ns would provide sufficient signal-to-noise for detection of the fluorescence. In collaboration with the DIII-D engineering staff and experts in plasma edge diagnostics for DIII-D from Oak Ridge National Laboratory (ORNL), WVU researchers designed a TALIF system capable of providing spatially resolved measurements of neutral deuterium densities in the DIII-D edge plasma. The laser systems were specified, purchased, and assembled at WVU. The TALIF system was tested on a low-power hydrogen discharge at WVU and the plan was to move the instrument to DIII-D for installation in collaboration with ORNL researchers. After budget cuts at DIII-D, the DIII-D facility declined to support installation on their tokamak. Instead, after a no-cost extension, the apparatus was moved to the University of Washington-Seattle and successfully tested on the HIT-SI3 spheromak experiment. As a result of this project, TALIF measurements of the absolutely calibrated neutral density hydrogen and deuterium were obtained in a helicon source and in a spheromak, designs were developed for installation of a TALIF system on a tokamak, and a new, xenon-based calibration scheme was proposed and demonstrated. The xenon-calibration scheme eliminates significant problems that were identified with the standard krypton calibration scheme.

The authors examine the relative merits of laser induced fluorescence scattering at the Lyman-alpha and Balmer-alpha wavelengths for the measurement of both the spatial distribution and absolute magnitude of neutral hydrogen densities in fusion plasmas. The enhancements of the fluorescence signals for present and for saturation laser intensities are determined and signal to noise ratios are presented for a range of fusion plasma conditions. Details of the interpretation of the fluorescence signals, and the limitations, at the two wavelengths are discussed. In the hotter regions of the plasma, Balmer-alpha fluorescence scattering allows the measurement of neutral densities as low as 107 cm-3, whereas present intensities from Lyman-alpha sources permit fluorescence measurements only for neutral hydrogen densities approaching 1010 cm-3. Also presented is a technique for the spatial measurement of the electron number density based upon fluorescence scattering on the Balmer transitions, this process not being possible at Lyman-alpha for the range of electron densities in magnetically contained fusion plasmas. This may have applications in regions of the plasma otherwise difficult to diagnose, for example, near divertors and limiters.

Using a comprehensive ionosphere model, we demonstrate that satellite measurements of solar EUV irradiances, neutral densities, and temperatures are consistent with Australian ionosonde measurements of the electron density from 2002 to 2006. Our approach is to adjust the model neutral densities and temperature to determine the changes that are needed to reproduce the electron density. These model‐derived neutral densities and temperatures are found to agree well with measurements of neutral density and temperatures from the GUVI instrument on the TIMED satellite for both magnetically quiet and disturbed conditions. This technique opens up the prospect of using the vast ionosonde database to improve temporal variations of empirical models of the thermosphere during magnetic storms. It could also help validate key global general circulation models of the thermosphere and ionosphere. The model calculations also demonstrate the importance of vibrationally excited N2 in the ionosphere. It is particularly important in producing negative ionosphere storms and also helps explain the rapid recovery after the storms.

The steady-state nature of the ELMO Bumpy Torus (EBT) plasma implies that the neutral density at any point inside the plasma volume will determine the local particle confinement time. This paper describes a Monte Carlo calculation of three-dimensional atomic and molecular neutral density profiles in EBT. The calculation has been done using various models for neutral source points, for launching schemes, for plasma profiles, and for plasma densities and temperatures. Calculated results are compared with experimental observations - principally spectroscopic measurements - both for guidance in normalization and for overall consistency checks. Implications of the predicted neutral profiles for the fast-ion-decay measurement of neutral densities are also addressed.

As magnetically confined plasmas progress towards ignition and long pulse experiments, measurement and control of the neutral density in the plasma edge has become a critical issue for stability, formation of transport barriers, and fueling. Recent experiments by our research group have demonstrated that is possible to use two-photon absorption laser induced fluorescence (TA-LIF) to directly measure the density of neutral hydrogen in helicon sources and in the HIT-SI3 spheromak. While those experiments validated key elements of a diagnostic system that would enable similar measurements in tokamak plasmas, they did not fully address the issue of performance optimization in the presence of intense background light at the fluorescence wavelength and they also identified a number of other issues that must be resolved before successful TALIF neutral density measurements in a tokamak are likely to be achieved. Additional specific concerns raised during design reviews with the leadership of the DIII-D tokamak facility included: the minimum detection threshold for this TALIF diagnostic (currently ~ 5 x 1015 m-3) and the rate at which neutral density measurements could be obtained. Improving the system performance to reduce the minimum detection threshold and demonstrating a faster rate of density determination are required to advance this diagnostic to the level where it could be considered for implementation on a major tokamak facility in the USA. The key issues that will be addressed through additional technological development are: validation of a new, chromatic aberration-free, xenon calibration scheme; increasing the output power of the laser; and suppression of background emission at the fluorescence wavelength through optimization of the collection optics and gating of the photomultiplier detector with a goal of obtaining a minimum detection threshold of 5 x 1014 m-3. Another key technological development, so far only tested in helicon source experiments, is the validation of Doppler-free TALIF as a means of obtaining higher speed, calibrated, neutral density measurements in tokamak-like conditions at the full cadence of the pulsed TALIF laser. In this work, we propose to complete these technological advancements through installation and testing of our prototype TALIF system on the proto-MPEX experiment at Oak Ridge National Laboratory (ORNL). The proto-MPEX facility will provide plasma conditions similar to the edge plasma of a major tokamak experiment but with pulse lengths and pulse repetition rates ideally suited for extensive development of a TALIF neutral density diagnostic. Measurement of the absolute neutral density in the edge of a magnetically confined plasma is necessary for plasma density control; calculation of charge-exchange power losses; control of plasma-wall interactions; determination of the braking of plasma flow; determination of the fuel mixture in deuterium-tritium plasma; and understanding the dynamics of the divertor region in the plasma edge. In terms of potential impact on the worldwide fusion program, we note that in burning plasma experiments, such as ITER, a 100 mJ/pulse Doppler-free TALIF diagnostic should be capable of directly measuring the deuterium/tritium (D/T) isotope ratio in the outer 0.5 to 1.0 m of the plasma radius. The D/T ratio is a critically important parameter for the control and optimization of burning plasmas. Therefore, development of the diagnostic system proposed here is also relevant to long-term US participation in diagnostic development for large tokamaks.

Satellite drag coefficients are crucial for determining the neutral mass densities that affect spacecraft operations in the thermosphere. Many studies typically utilize a constant drag coefficient of 2.2 to calculate the neutral density. However, due to the variability of space environment, uncertainties in the drag coefficient can lead to significant systematic discrepancies in neutral density measurements. Satellite drag coefficient may fluctuate in the thermosphere under various geomagnetic activities and altitudes. For the first time, we calculate the spherical satellite drag coefficient using data from the “Orbital Atmospheric Density Detection Experimental Satellite,” referred to as the QX satellite. Our findings reveal that the drag coefficient can be estimated by thermospheric temperature and density, which are dependent on geomagnetic activity and altitude. At an altitude of ∼510 km, drag coefficients are adjusted to around 2.425, instead of the constant value of 2.2. Furthermore, the drag coefficient may decrease due to the significant influence of increasing geomagnetic activity, such as geomagnetic storms, on thermospheric density and temperature. These estimates of the drag coefficient can also be used to reduce discrepancies when deducing the ballistic coefficient. Consequently, using the estimated drag coefficient can accurately determine the QX‐derived neutral density, which agrees well with the density from Swarm‐B satellite.

Laser-induced fluorescence (LIF) measurements of plasma opacity are used as a novel diagnostic to determine the absolute density of a metastable state of neutral helium atoms in a helicon plasma. The absorption scale length at a wavelength of 587.725 nm (vacuum) is determined from measurements of fluorescence intensity as a function of distance along the laser path. With a collisional–radiative model of the state populations, the absolute ground state neutral helium density is estimated from the metastable state density measurement. This paper expands upon previous work through measurements of neutral density, temperature and flow at different radial positions. The measured neutral density decreases by two orders of magnitude from the edge of the plasma to the axis of the plasma source. When the helicon source is operated in a static mode (i.e. no active gas pumping) the on-axis neutral density decreases by 69% from the pumping case and the on-axis plasma density increases by 42%; yielding an ionization fraction of approximately 90%.

Traditional interferometric techniques have been shown to lack the resolution needed to characterize the electron and neutral densities in the diffuse plasma of the Pulsed Plasma Thruster (PPT) exhaust. In the present work, a Herriott cell is added into a standard quadrature heterodyne interferometer to increase sensitivity for electron density measurements, and allow measurements of neutral densities amidst mechanical vibration. Measurements of electron and neutral density during and after the current pulse are sought for the purposes of modeling spacecraft contamination from PPTs. Testing is performed on the UIUC PPT-4, a coaxial electrothermal PPT pulsing at 20 J. Analytical and experimental analysis are conducted to determine the integrity of the phase front and the effect of multiple passes on the density measurements taken. Up to 18-passes through the plasma are accomplished using the Herriott cell, while maintaining sufficient phase-front quality for interferometric analysis. The advantage of the cell is obvious at late times when the external mechanical vibrations induce an apparent phase shift in the same direction as for neutral particles. Due to the same dependence on wavelength, 2 laser frequencies cannot be used to separate neutral and mechanical vibration contributions. The Herriott cell allows a density resolution increase linear with the number of passes that does not increase the mechanical vibrational component. Uncertainties from both mechanical vibrational sources and shot-to-shot variations of the thruster itself are investigated and characterized for this system. Due to variations in mechanical vibrations on a day to day basis, the cell is able to characterize the neutral density of the thruster only over short time periods. At 200 (is for single tests (-20 shots averaged) at 14 and 18 passes neutral density at the exit plane was shown to be no more than 1 *10 cm. Average peak electron density (4 jis) was shown to be 5.0 ± 1.1 *10 cm, however the single shot error due to the Herriott cell is±10 cm.

The MaCWAVE/MIDAS program for investigating high‐latitude mesospheric dynamics has produced an extensive set of charged particle, plasma and neutral density measurements. We report on the results from one rocket salvo involving a MaCWAVE and two MIDAS payloads launched within a one‐hour period. Each payload carried probes for evaluating the plasma and charged particle environment, and additionally, neutral density measurements were obtained by the CONE instrument on the MIDAS payloads. With the combination of particle and electron/ion density measurements, regions with and without charged particles have been clearly identified, and the plasma measurements outside the particle layers have proven useful to characterize the turbulent activity of the neutral gas.

A two-color homodyne Mach-Zehnder optical fiber interferometer is developed for the measurement of electron and neutral particle densities in a high-density capsule θ-pinch device. The interferometer leverages the disparate contributions of distinct particles to the refractive index across two discrete wavelengths of 1310 and 1550nm and incorporates the contributions of both electron and neutral particle densities to the phase shift in the plasma. The temporal evolutions of line-integrated electron and neutral argon densities are successfully measured by the interferometer. Comparing the electron density waveforms under various working gas pressures as well as the results obtained using the monochromatic and two-color measurements, it is inferred that the influence of neutral particle density can be neglected when measuring the electron density using a long-wavelength laser. Moreover, the maximum electron density is linearly correlated with the capacitor bank voltage for the θ-pinch device (5-9kV). Overall, the proposed interferometer is capable of simultaneously measuring the electron and neutral particle densities.

A quantitative laser schlieren technique has been developed to measure the changes in neutral gas density inside a spark gap switch after breakdown. The measurement system incorporates an array of narrow laser beams which are used to probe the inter-electrode gas volume at discrete distances from the spark axis. Changes in the refractive index gradient within the switch, caused by variations in neutral gas density, result in beam deflection, which is measured using a neutral density wedge/photodiode combination. The resulting schlieren profiles yield both spatially and temporally resolved refractive index data, which enable calculation of the gas density. The use of four averaged trials to obtain the final neutral gas density profiles minimizes errors caused by the turbulent nature of the cooling gas. A typical set of results is presented for SF6 at a pressure of 0.5 bar.

Theories predict that neutrals play a role in the low to high (L-H) confinement mode transition in tokamak plasmas via charge exchange damping and other effects. Previous estimates of neutral damping have been based on calculations of the edge neutral density. This work introduces a new method of measuring the neutral density near the X point, where simulations predict it to be a maximum. The technique employed uses Dα light from a TV camera reconstructed onto a poloidal plane, along with Thomson scattering measurements of the electron temperature and density. Measured neutral densities span the range 109-1013cm-3. Good agreement,considering the neutral density error bars, is found between the measurements and the 2-D simulations. This work represents the first step in verifying previous 2-D simulations and in corroborating previous conclusions that the neutral damping is large enough to play a role in the L-H transition process.

Neutral density measurements at the plasma edge of the Tore Supra tokamak with carbon charge exchange lines

(Abstract) Predicting the orbits of space objects in low altitude orbits requires an accurate model for the atmospheric neutral density. The current accuracy of semi-empirical models limits the prediction accuracy and impacts a number of operational decisions. The current models are based on sparse measurements of the neutral density, collected over an extended period. One of the problems is observing the thermosphere density changes in response to the solar and geomagnetic variability on short temporal scales, such as those characterized by geomagnetic storms. The stochastic behavior of the solar forcing represents one of the major challenges in predicting satellite orbits. In situ measurements of the density can play a significant role in improving the structure of the neutral density models and in providing a timely measurement for enhancing the accuracy of the satellite predictions. Measurements from orbiting accelerometers carried by the twin GRACE satellites have the potential for providing accurate and timely measurements to improve the satellite prediction accuracy. The objective of this paper is to describe the procedure for using the GRACE accelerometer measurements for determining accurate density measurements and thermospheric wind fields. Finally, selected results from the analysis of four years of GRACE accelerometer measurements are described.

Electron temperature, electron density, and neutral atom density were measured in a radio-frequency (rf) inductively coupled plasma using Thomson and Rayleigh scattering of laser radiation. Measurements were made in an argon discharge for pressures from 1 to 20 mTorr and input rf powers from 100 to 500 W. Spatial distribution profiles were measured for discharges with different aspect ratios. Electron temperature was found to depend on pressure but only weakly on power. Electron density depended strongly on both pressure and power. The neutral density was found to be significantly depleted in the plasma center and this depletion was attributed to heating of the neutrals by charged particle collisions. These results were compared to a simple model of inductively coupled plasmas.