Turbulence implies nonlinear wave–wave coupling, and determining cause and effect of either is important to understand mixing responsible for enhanced number, momentum, or energy (NME) transport. To explain the identification of parent and daughter modes via a look-up table, we sketch the framework of bispectral analysis without repeating the mathematical formalism of earlier bispectrum researchers. We then apply this technique to a test signal and plasma fluctuation data from the WVU-Q machine, where the inhomogeneous energy density-driven spectrum exhibited a degree of coupling to lower frequencies that was absent in the case of the related, single-eigenmode, current-driven spectrum.

The narrowband light from a scannable, single-mode dye laser influences the electrical properties of gas discharges. The variation in these properties as the laser wavelength λ is scanned yields the optogalvanic spectrum of the discharge (i.e., electrical conductivity vs. frequency). By connecting a neon lamp, capacitor, and power supply in parallel, an undriven relaxation oscillator is formed whose natural frequency f0 is affected by neon-resonant laser light and this λ-dependence of the relaxation oscillator frequency f0 yields a variant optogalvanic spectrum (i.e., f0 vs. frequency). In this paper, a driving force is effectively applied to an otherwise undriven oscillator when the incident light is chopped periodically at fd. For fd ≈ f0 and a sufficiently large driving force amplitude (laser intensity and the degree of neon resonance), the relaxation oscillator can be entrained so that f0 is locked on fd and is independent of λ. For the new chopped-light technique described here, fd is adjusted to be the subthreshold of the entrainment range, where the λ-dependence of f0 is advantageously exaggerated by periodic pulling, and the beat frequency |fd − f0| vs. λ provides an optogalvanic spectrum with appealingly amplified signal-to-noise qualities. Beat frequency neon spectra are reported for the cases fd < f0 and fd > f0 and are compared with spectra obtained using the unchopped-light (i.e., undriven) method.

The measurement of plasma potential and electric field by the traditional Langmuir probe method can contain a significant qualitative error due to a spurious effect from temperature fluctuations that appear in the temperature-potential cross phase. This error is separate from any systematic error in the floating potential amplitude due to cross-correlated temperature and potential fluctuations not averaging out. Conditions under which electron temperature fluctuations significantly and insignificantly affect the measurement of electric field fluctuations in plasma are considered for both effects. Actual measurements illustrate the obtained conclusions are presented. Correct direct measurement of plasma’s space potential fluctuations requires that the amplitude of electron temperature fluctuations (in eV) be at least ten times smaller than the space potential fluctuation amplitude (in volts).

We present a detailed characterization of the structure and evolution of differentially rotating plasmas driven on the MAGPIE pulsed-power generator (1.4 MA peak current, 240 ns rise time). The experiments were designed to simulate physics relevant to accretion discs and jets on laboratory scales. A cylindrical aluminium wire array Z pinch enclosed by return posts with an overall azimuthal off-set angle was driven to produce ablation plasma flows that propagate inwards in a slightly off-radial trajectory, injecting mass, angular momentum and confining ram pressure to a rotating plasma column on the axis. However, the plasma is free to expand axially, forming a collimated, differentially rotating axial jet that propagates at ${\approx }100\,{\rm km}\,{\rm s}<^>{-1}$. The density profile of the jet corresponds to a dense shell surrounding a low-density core, which is consistent with the centrifugal barrier effect being sustained along the jet's propagation. We show analytically that, as the rotating plasma accretes mass, conservation of mass and momentum implies plasma radial growth scaling as $r \propto t<^>{1/3}$. As the characteristic moment of inertia increases, the rotation velocity is predicted to decrease and settle on a characteristic value ${\approx }20\,{\rm km}\,{\rm s}<^>{-1}$. We find that both predictions are in agreement with Thomson scattering and optical self-emission imaging measurements.

Recent pulsed-power experiments have demonstrated the formation of astrophysically relevant, differentially rotating plasmas. Key features of the plasma flows are the discovery of a quasi-Keplerian rotation curve, the launching of highly collimated angular-momentum-transporting axial jets, and a hollow density structure sustained by the centrifugal barrier effect. In this communication, we discuss several features of the plasma structure in these experiments through order-of-magnitude models. First, we show that the observed rotation velocity would produce a centrifugal force strong enough to support the hollow density profile. Second, we show that the axial jet should diverge much faster than what was observed, were it not for a magnetized halo with 3 T which surrounds the jet and exerts pressure on the interface. Finally, we discuss the temperature structure in the axial jet and plasma halo. We show that a 3-T magnetic field would also suppress electron heat conduction, leading to the flat profile observed experimentally. We also find that the axial jet is efficiently radiatively cooled, whereas the halo is not, which would explain the thermal decoupling between the two regions.

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.

A single magnetically insulated baffled probe (MIBP) was used to study the potential and electron temperature oscillations in a magnetized plasma. It was shown, that, although the MIBP cluster allows a more detailed study of such oscillations, the single MIBP may provide useful information about the oscillation amplitudes and, in some cases, cross-coherency and cross-phase. These quantities can characterize oscillations and distinguish co-oscillating plasma parameters in cases where the placing of a MIBP cluster in a plasma is difficult or impossible. For the reported plasma case, in which there are two types of oscillations, it is shown how to determine the maximum and minimum possible values of the electron temperature fluctuations. Detailed analysis showed that, in the studied plasma, there are two types of oscillations, both incoherent with each other.

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.

Local fluctuations of electrostatic potential, poloidal electric field, magnetic potential and electron density are simultaneously measured in the T-10 tokamak by a heavy ion beam probe (HIBP) having a five-slit energy analyser, which allows an estimate of the turbulent particle flux and $\boldsymbol {E}\times \boldsymbol {B}$ rotation velocity in the off-minor-axis gradient zone of the toroidal plasma column. The high spatial and temporal resolution of the modern multichannel HIBP makes it an effective tool to study plasma oscillations. Motivated by previous work that has documented time-resolved interactions between measured plasma parameters using correlation analysis (coherence of $E_{\textrm {pol}}$ and density $n_e$ , and cross-phase), a new result from bicorrelation analysis (bicoherence of magnetic potential $A_\zeta$ and density $n_e$ , and biphase) is reported for documenting the evidence of wave–wave coupling and energy transfer associated with the interaction between geodesic acoustic modes (GAM) and broadband, quasi-coherent modes.

Many important plasma processes, such as heating and current drive in tokamaks and laser-plasma interactions, involve the interaction of an externally generated electromagnetic wave with the plasma. The plasma can couple the incident wave to a range of other waves. Important examples include Raman scattering, where the incident wave couples to an electrostatic Langmuir wave and a scattered electromagnetic wave, and Brillouin scattering, where the Langmuir wave is replaced by an ion sound wave 1 . Both are observed in laser plasma experiments; however, these instabilities can also be generated by intense, short pulse microwave signals in a cool, tenuous plasma. A magnetized plasma supports additional beat wave interactions with natural cyclotron motions of charged particles and with magnetized plasma waves such as upper/lower hybrid waves as well as kinetic modes such as ion/ electron Bernstein waves. Applications include heating plasma that is inaccessible at low harmonics of the cyclotron frequency in high density fusion plasmas. Enabling these experiments is a large helicon apparatus, 1 m diameter, 3m length. The stainless-steel vacuum vessel is surrounded by several electromagnets providing a B 0 up to 0.0875 T. The plasma will be ionized by m = 0 helicon waves with 3 < f < 30 MHz launched by a flat spiral antenna 2 . Plasmas of 10 15 < n e < 10 18 m -3 and T e < 10 eV will be produced in a low pressure (<100 mTorr) noble gas. R.F. compensated Langmuir probes and line integrated interferometry will be used to diagnose the plasma. These measurements will be carried out alongside numerical simulations and theoretical analysis to enhance the understanding of these interactions.

An electrostatic Langmuir probe for real-time measurements of parameters in magnetized plasma is tested in fully ionized, barium, Q-machine plasma. The small-diameter, long-length, tungsten wire sensor, i.e., the probe tip, oriented with its cylindrical axis perpendicular to the magnetic field (B), is partially shielded by ceramic baffles, or masks, that form sensor-access slots between the baffles. Adjusting the azimuthal orientation of the slots, by rotating the probe about its cylindrical axis, changes the fraction of proximity gyro-orbiting electrons, relative to the fraction of proximity gyro-orbiting ions, that can access the recessed sensor along the magnetic field. Thus, the ratio between the electron and ion saturation currents, Iesat and Iisat, can be adjusted without having affected the probe bias voltage Vb. When optimally shielded (Iesat/Iisat=1), accurate, real-time measurements of space potential Vs can be acquired. When maximally shielded (Iesat/Iisat≪1), accurate, real-time measurements of ion temperature Ti can be acquired. Subtracting the floating potential Vf of an optimally shielded baffled probe from Vf of a maximally shielded baffled probe yields Ti (and its fluctuation phase) in real time.

The application of the magnetically insulated baffled probe (MIBP) and MIBP cluster for studying properties of low-temperature and peripheral fusion plasmas is reviewed. MIBP operation principles, MIBP design strategy, and MIBP examples of measurement, data analysis, and interpretation are discussed. The implementation convenience and diagnostic usefulness, as well as the inconvenience and drawbacks, for studying plasma equilibrium and dynamics properties, are demonstrated. MIBP determination of oscillations of fluid observables, such as electron and ion temperatures, electrostatic plasma potential, and electron and ion density reveal plasma instabilities and waves. Ion and electron distribution functions, and the transport of charged-particle number, momentum, and energy can also be measured.