Accurate measurement of magnetic field strengths is critical in many plasma environments, ranging from astrophysical systems to fusion energy research. In this work, a non-perturbative laser-based optical diagnostic known as quantum beat spectroscopy is demonstrated to be good alternative for measuring the magnetic field strength in low-pressure laboratory plasmas. The technique is investigated using both ns and fs pulsed lasers in an argon plasma. Preliminary results for a helium plasma are also given. Zeeman-split J = 1 electron states with transitions from metastable states were identified and tested for neutral argon (2P1/2o)4p 2[1/2] and neutral helium 1s3p 1P1o. Magnetic fields are measured with sub-Gauss precision at near single laser pulse acquisition rates.

Laser induced fluorescence is used to measure argon ion heating during magnetic reconnection in the PHase Space MApping experiment (PHASMA). Sufficient signal-to-noise ratio (SNR) of the processed signal with pulsed laser injection is a delicate balance between saturation of the absorption line and injecting enough laser power to overcome the spontaneous emission of the plasma at the fluorescence wavelength. Averaging over many laser pulses and integrating over the fluorescence lifetime improves the SNR of the processed signal (processed SNR) when the SNR of the laser pulse time series is small (pulse SNR), but for laser powers small enough to avoid saturation, averaging over hundreds of pulses is needed to obtain an appreciable processed SNR over the entire Doppler-broadened absorption line. Here, we describe a matched filter processing method that significantly improves the SNR of the final measurement with fewer shots averaged. Investigation of simulated measurements validated by experimental results suggests that the matched filter method provides up to a 20% improvement in the processed SNR, resulting in less uncertainty in distribution function fits.

Experiments have demonstrated that ion phenomena, such as the lower hybrid resonance, play an important role in helicon source operation. Damping of the slow branch of the bounded whistler wave at the edge of a helicon source (i.e. the Trivelpiece-Gould mode) has been correlated with the creation of energetic electrons, heating of ions at the plasma edge, and anisotropic ion heating. Here we present ion velocity distribution function measurements, electron density and temperature measurements, and magnetic fluctuation measurements on both sides of an m = | 1 | helical antenna in a helicon source as a function of the driving frequency, magnetic field strength, and magnetic field orientation relative to the antenna helicity. Significant electron and ion heating (up to two times larger) occurs on the side of the antenna consistent with the launch of the m = + 1 mode. The electron and ion heating occurs within one electron skin depth of the plasma edge, where slow wave damping is expected. The source parameters for enhanced particle heating are also consistent with lower hybrid resonance effects, which can only occur for Trivelpiece-Gould wave excitation.

The small signal-to-noise ratio (SNR) of conventional laser induced fluorescence (LIF) measurements using a continuous wave laser, either diode or dye, is typically overcome by amplitude modulating the laser at a specific frequency and then using lock-in amplification to extract the signal from measurement noise. Here, we present LIF measurements of the neutral helium velocity distribution function in an rf plasma using frequency modulated (FM) laser injection. A pulse train of 100% amplitude modulation is generated synthetically with a random sequence of pulse lengths. The FM signal then drives an acoustic optic modulator placed in the path of the injection beam in an LIF measurement. The signal from a fast photomultiplier tube is digitized and cross-correlated with the known modulation signal. The resultant FM-based LIF signal outperforms a conventional lock-in-based LIF measurement on the same plasma in terms of SNR and precision.

A regime of reconnection, called electron-only reconnection since there is insufficient time or space for ions to couple with reconnection structures, provides an opportunity to study electron heating mechanisms more directly than in fully ion-coupled reconnection. Electron-only reconnection is thought to be important for energy dissipation at kinetic scales in Earth’s turbulent magnetosheath, bow shock and the quiet magnetotail. Recent simulations of turbulent near-Sun plasmas suggest that electron-only reconnection with a large guide field plays a key role in the dissipation of turbulent energy at the kinetic scale and results in parallel, i.e. anisotropic, electron heating.

Here, we present laser-induced fluorescence measurements of an ion beam extraction angle and speed through a plasma-vacuum boundary as a function of plasma source parameters and bias potential applied to a wafer simulacrum outside the plasma. Ion temperature, velocity, and relative density are calculated from the measured ion velocity distribution function and are compared to a particle-in-cell model of the system. The measurements demonstrate that beam steering is feasible by varying plasma source density and extraction bias voltage. The focal point of the extracted beam, resulting from the plasma meniscus at the plasma-vacuum interface, depends on source density and extraction bias in a manner consistent with computational predictions.

Experiments in helicon sources have shown that more efficient wave-plasma coupling (and therefore higher temperatures and densities) occurs when the antenna excites the m = +1 (i.e., right-hand polarized) helicon wave. Here we present ion and electron temperature measurements, and electron density measurements, on both sides of a helical antenna in a helicon source as a function of antenna frequency and magnetic field strength. These measurements were obtained for two different background magnetic field directions. For both field directions, significant ion heating at frequencies near the lower hybrid frequency was observed. A remarkable and unexpected density difference (~ 2 orders of magnitude) was observed in the downstream region when the magnetic field direction was reversed. Radial wave field profiles and phase measurements were measured to identify the polarization of the helicon waves for both background magnetic field directions, on both sides of the antenna. It was confirmed that the region of hotter ion temperature and higher plasma density were on the m = +1 side of the antenna.

We report on the simulation of temperature gradients in tamped NaFMgO target-foil plasma, heated and backlit by z-pinch dynamic hohlraum radiation. Our approach compares the spectroscopic output of a collisional-radiative model (prismspect) with soft X-ray absorption spectra collected on Sandia National Laboratories’ (SNL) Z Pulsed Power Facility. The pattern of minimum χ2 is seen to agree with an efficient, three-parameter model. Results show that a negligible gradient in electron temperature Te is consistent with experimental data, justifying the assumptions of previous work. The predicted sensitivity of line spectra to the gradient-aligned profile of Te is documented for each spectral feature, so that the line-area ratio between a pair of spectral features may be assessed as a proxy for the existence and quantification of such gradients.

Remotely situated diagnostics are desirable for fusion devices since electromagnetic interference (EMI) and radiation are becoming more of an issue as the field moves into an era of burning plasmas. Specifically, diagnostics capable of measuring absolute neutral densities are critical to controlling fueling rates and maintaining transport barriers in the plasma edge. Two-photon absorption laser induced fluorescence (TALIF) non-perturbatively measures spatially resolved neutral velocity distribution functions (NVDF) to determine absolute, ground state neutral densities of hydrogenic species if the measurements are calibrated with a noble gas, commonly krypton or xenon. However, TALIF injects deep ultraviolet light (~205 nm) that is easily absorbed in air and difficult to couple into vacuum chambers. These limitations restrict the location of the TALIF systems to regions of potentially high EMI and eliminate use of fibers and common optical materials. A three-photon laser induced fluorescence (3pLIF) technique has the potential to provide similar measurements, while injecting a more near-visible wavelength, 300–308 nm, alleviating the requirements for special optics, allowing the use of high-power fibers, and allows the laser system to be situated further from the intense EMI environment. In this work, a Quantel Qscan pulsed dye laser produces ~300 nm light over ~7 ns at a repetition of 10 Hz to excite ground state krypton through three-photon excitation. Fluorescence is fiber coupled to detecting electronics. Here krypton spectral lineshapes measured using 3pLIF are presented. The measured lineshapes are unexpectedly broad and blue-shifted. Possible mechanisms responsible for these spectral features are discussed.

Laser-induced fluorescence diagnostics are used routinely for measurements of ion velocity distribution functions (IVDFs) for a wide variety of gases used in processing plasmas. This work explores the viability of a related technique known as two photon absorption laser-induced fluorescence (TALIF) for ions produced in a 1-Torr chlorine reference cell placed in an Evenson microwave cavity driven at 2.45 GHz. Common diagnostic techniques for processing plasmas lack spatial localization (e.g., passive optical emission spectroscopy) or are perturbative to the system (e.g., Langmuir probes). TALIF is a spatially localized, non-perturbative diagnostic suitable for diagnosing a range of plasmas but is rarely implemented on processing plasma systems due to their low ion density production and limited optical access. When performed with a confocal approach, TALIF requires only a single point of optical access making it ideal for probing processing plasma reactors. Initial passive spectroscopy measurements of the chlorine discharge used in this work show sufficient ion production inside the microwave cavity region. This work provides a brief introduction to the TALIF technique and explores its applicability on chlorine ions used for processing-relevant plasmas.

Iodine has been an element of recent interest for commercial use as fuel in electrostatic propulsion systems. A lingering problem when investigating ionized iodine using non-perturbative, laser-based techniques is determining the spectral width, i.e., the species temperature, of iodine. To this end, the hyperfine structure must be well understood to develop a spatially resolved diagnostic technique capable of ion flow and temperature measurements. Previous work investigated the lineshape of the transition between the D-5(4)o and P-5(3) states of singly-ionized atomic iodine (I II) with laser induced fluorescence (LIF), but the hyperfine structure of the transition was unresolved in those measurements [Steinberger and Scime, Journal of Propulsion and Power, 34, 2018]. In this work, an intermodulated LIF technique is used to measure an enhanced lineshape of the same I II transition. A linear least squares fitting algorithm is used to fit the transition lineshape, where hyperfine transition locations and theorized relative amplitudes are constrained by theory. A lineshape model that incorporates hyperfine transition amplitude enhancement introduced from an intermodulated laser technique is implemented into the fitting function, as well as a nonlinear laser saturation effect. We report converged hyperfine coupling coefficients for these I II states. (C) 2021 Elsevier Ltd. All rights reserved.

Using incoherent Thomson scattering, electron heating and acceleration at the electron velocity distribution function (EVDF) level are investigated during electron-only reconnection in the PHAse Space MApping (PHASMA) facility. Reconnection arises during the merger of two kink-free flux ropes. Both push and pull type reconnection occur in a single discharge. Electron heating is localized around the separatrix, and the electron temperature increases continuously along the separatrix with distance from the X-line. The local measured gain in enthalpy flux is up to 70% of the incoming Poynting flux. Notably, non-Maxwellian EVDFs comprised of a warm bulk population and a cold beam are directly measured during the electron-only reconnection. The electron beam velocity is comparable to, and scales with, electron Alfvén speed, revealing the signature of electron acceleration caused by electron-only reconnection. The observation of oppositely directed electron beams on either side of the X-point provides “smoking-gun” evidence of the occurrence of electron-only reconnection in PHASMA. 2D particle-in-cell simulations agree well with the laboratory measurements. The measured conversion of Poynting flux into electron enthalpy is consistent with recent observations of electron-only reconnection in the magnetosheath [Phan et al., Nature 557, 202 (2018)] at similar dimensionless parameters as in the experiments. The laboratory measurements go beyond the magnetosheath observations by directly resolving the electron temperature gain.

For diagnosing temperatures of high-energy-density plasmas, relying on the ratio of a pair of isoelectronic spectral lines provides the alternative of "matched charge-state transits in different elements"to the more conventional, "unmatched charge-state transits in the same-element"spectral-line-ratio technique. In contrast to a novel previous establishment of isoelectronic emission-line ratio determination of plasma temperature, this report determines plasma temperature from the ratio of isoelectronic absorption spectral line pairs. The feasibility of this technique is assessed from experimentally acquired transmission spectra, spanning the 7- 15 angstrom range, through a 0.4 mu m MgO-NaF foil, tamped with CH, x-ray-radiation heated by a z-pinch dynamic hohlraum (ZPDH), and backlit by the brief X-ray burst upon imploding-wire stagnation on the Z pulsed -power facility at Sandia National Laboratories. As expected from the slight difference between interstage and isoelectronic absorption processes, a quantitative comparison between the value of isoelectronic-absorption-derived temperature and the value of inter-stage-absorption-derived temperature is shown to yield a well correlated, slight difference in inferred values of plasma temperature associated with local thermodynamic equilibrium.

A new experiment, called the PHAse Space MApping (PHASMA) experiment, features laser induced fluorescence diagnostics for ion measurements, Thomson scattering diagnostics for electron velocity distribution function measurements, and a microwave scattering system for turbulence measurements. PHASMA is designed to enable the direct measurement of ion and electron vdfs in space-relevant plasma phenomena including reconnection, shocks, and turbulence. To create the conditions necessary for different experimental regimes, PHASMA employs a 2 kW, steady-state helicon source capable of generating variable-density background hydrogen, helium, argon, krypton, and xenon plasmas with controllable plasma pressure (relative to the magnetic pressure), collisionality, and azimuthal flow shear. Reconnecting flux ropes arise through the merging of discharges from two pulsed plasma guns.

In laboratory plasmas, arrays of probes have typically been used to measure the evolution of the magnetic field topology. Here, we present initial image-based measurements of the magnetic topology in a low-temperature plasma using a purely optical diagnostic. Laser induced fluorescence measurements of neutral velocity distribution functions are made using a fast camera, imaging the Zeeman splitting of σ-peaks in neutral argon. The separation of σ-peaks provides spatially resolved magnetic field magnitude measurements with a detection threshold on the order of 10 G.

As diagnostic groups are increasingly called upon to participate in experimental campaigns at remote facilities, there is a need to develop portable versions of plasma diagnostic systems. One such diagnostic is laser induced fluorescence (LIF). Here, we describe a portable LIF apparatus that eliminates the need for an optical table, beam splitters, and an optical chopper. All of the light exiting the laser system is coupled through optical fibers to the experiment and housekeeping diagnostics. The collected light is coupled through an optical fiber as well. A key feature is modulation of the tapered amplifier current instead of physical modulation of the laser output. Using this portable LIF system, measurements of ion temperature, ion flow, and relative metastable ion density are reported for two different remote experiments.

Magnetic flux ropes have been successfully created with plasma guns in the newly commissioned PHAse Space MApping (PHASMA) experiment. The flux ropes exhibit the expected m = 1 kink instability. The observed threshold current for the onset of this kink instability is half of the Kruskal–Shafranov current limit, consistent with predictions for the non-line tied boundary condition of PHASMA. The helicity, paramagnetism, and growth rate of the observed magnetic fluctuations are also consistent with kink instability predictions. The observed fluctuation frequency appears to be a superposition of a real frequency due to a Doppler shift of the kink mode arising from plasma flow (∼2 kHz) and a contribution from a wave mode (∼5 kHz). The dispersion of the wave mode is consistent with an Alfvén wave. Distinct from most previous laboratory studies of flux ropes, the working gas in PHASMA is argon. Thus, the ion cyclotron frequency in PHASMA is quite low and the frequency of the Alfvénic mode plateaus at ∼0.5 of the ion gyro frequency with increasing background magnetic field strength.

Neutral particle control is critical for fusion fueling and confinement. Neutral diagnostics for fusion-relevant plasmas are commonly restricted to line-integrated or ex situ methods. A non-perturbative, two-photon absorption laser induced fluorescence (TALIF) diagnostic is implemented on the Prototype Material Plasma Exposure eXperiment (Proto-MPEX) to probe neutral atomic deuterium in a fusion-relevant plasma at 1 cm intervals along the radius of the vacuum vessel. The diagnostic is situated ∼20 m from the vacuum vessel, and a signal is collected along the laser injection axis, requiring only one line-of-sight. TALIF measurements are absolutely calibrated using xenon and krypton. Absolute atomic densities derived from xenon calibration are compared to absolute atomic densities derived from krypton calibration. Here, preliminary measurements of absolute atomic deuterium density, temperature, and local bulk flow dependence on radial location and input power in Proto-MPEX are presented. Neutral atomic deuterium velocity distribution functions are measured throughout a one-second plasma pulse with a time resolution of 250 ms.

Ion temperature anisotropy in an expanding magnetized plasma is investigated using laser induced fluorescence. Parallel and perpendicular ion velocity distribution functions (IVDFs) were measured simultaneously with high spatial resolution in the expanding plasma. Large ion temperature anisotropies (T⊥i/T∥i∼10) are observed in a conical region at the periphery of the expanding plasma plume. A simple 2D Boris stepper model that incorporates the measured electric field structure is able to reproduce the gross features of the measured perpendicular IVDFs. A Nyquist stability analysis of the measured IVDFs suggests that multiple instabilities with k⊥ρi∼1 and k||ρi∼0.2 are likely to be excited in these plasmas.