Low-frequency Density Fluctuations Research Articles

Observation of coherent nonlinear interactions in the ion velocity distribution function

Using laser induced fluorescence (LIF) and higher order spectral analysis, we present the first measurements of phase space resolved coherent nonlinear interactions among the components of low frequency density fluctuations, (ω⩽ωci), in a linearly magnetized device. The bicoherence calculations employing the two point correlation technique suggest that there are two different coherent nonlinear wave-wave interactions in the measured spectrum. The first one, having a short correlation length and existing for slow moving ions, for which ∣υi‖∣⩽∣υith∣, is an interaction between fluctuations below the electron drift frequency, ω*. The second one is the strongest for fast moving ions, for which ∣υi‖∣⩾∣υith∣, and is a mode coupling between the azimuthal drift wave modes, m=1 and m=2. Combining these bispectral results with earlier linear analysis based on the power spectra of the fluctuations, we suggest that the nonlinear coupling observed between the spectral components below ω* for the case of slow moving ions is associated with the anomalous kinetic component. For slow moving ions, as we increase the neutral collision frequencies, the nonlinear interaction observed for spectral components below ω* decreases and the harmonic mode coupling for ω* takes over.

Energy confinement scaling in the low aspect ratio National Spherical Torus Experiment (NSTX)

Systematic and statistical studies have been conducted in order to develop an understanding of the parametric dependences of both the global and thermal energy confinement times at low aspect ratio in high power National Spherical Torus Experiment discharges. The global and thermal confinement times of both L- and H-mode discharges can exceed values given by H-mode scalings developed for conventional aspect ratio. Results of systematic scans in the H-mode indicate that the confinement times exhibit a nearly linear dependence on plasma current and a power degradation weaker than that observed at conventional aspect ratio. In addition, the dependence on the toroidal magnetic field is stronger than that seen in conventional aspect ratio tokamaks. This latter trend is also evident in statistical analyses of the available dataset. These statistical studies also indicate a weaker parametric dependence on plasma current than found in the systematic scans, due to correlations among the predictor variables. Regressions based on engineering variables, when transformed to dimensionless physics variables, indicate that the dependence of BτE on βt can range from being negative to null. Regressions based directly on the dimensionless physics variables are inexact because of large correlations among these variables. Scatter in the confinement data, at otherwise fixed operating parameters, is found to be due to variations in ELM activity, low frequency density fluctuations and plasma shaping.

Low Frequency Fluctuations in ICRF-Heated Plasmas on GAMMA 10

The high ion-temperature (> 10 keV) plasma has been performed in ion-cyclotron range of frequency (ICRF) heating experiments on the GAMMA 10 tandem mirror. In such a high performance plasma, low frequency density fluctuations are observed in relation to the applied ICRF powers. A drift-type fluctuation, that is excited when high power ICRFs for the plasma production are applied, is observed in the whole of GAMMA 10 with the finite wave length. The amplitude of the drift-type fluctuation depends strongly on the gas puffing rate, the ICRF antenna configuration and so on. The radial transport of high energy ions due to these fluctuations is studied.

Application of the Gyrotron FU II Submillimeter Wave Radiation Source to Plasma Scattering Measurements

Gyrotron FU II has been successfully applied as a submillimeter wave radiation source to plasma scattering measurements on the Compact Helical System (CHS) in National Institute for Fusion Science (NIFS) in Japan. The gyrotron operates in a long pulse mode (the pulse width is about 600 ms) at a frequency of about 350 GHz (the corresponding wavelength is 0.85 mm). The output power is about 110 W. The output power is transmitted along a circular waveguide system and converted to a Gaussian-like beam by a quasi-optical antenna. After that, the beam is directed onto the CHS plasma and the scattered signal is detected by a homodyne detection system. The frequency and the wave number of the scattered signal are analyzed. The results suggest that a broad band low frequency (several tens to several hundreds kHz) density fluctuation is excited in the CHS plasma only during neutral beam injection (NBI) or ion cyclotron resonance (ICR) heating.

Influence of external density fluctuations on parametric 3-wave interaction

Recent satellite observations seem to invalidate the classical scenario of Langmuir turbulence. Among the possible candidates which may play an important role to be taken into account are external density fluctuations. Parametric three-wave interaction is studied in the presence of quasi-monochromatic large-scale and low-frequency density fluctuations.

RADAR upper hybrid resonance scattering diagnostics of small-scale fluctuations and waves in tokamak plasmas

The upper hybrid resonance (UHR) scattering technique possessing such merits as one-dimensional probing geometry, enhancement of cross section, and fine localization of scattering region is modified in the new diagnostics under development to achieve wave number resolution. The fluctuation wave number is estimated in the new technique from the scattering signal time delay measurements. The feasibility of the scheme is checked in the proof of principal experiment in a tokamak. The time delay of the UHR scattering signal exceeding 10 ns is observed. The small scale low frequency density fluctuations are investigated in the UHR RADAR backscattering experiment. The UHR cross-polarization scattering signal related to small scale magnetic fluctuations is observed. The lower hybrid (LH) wave propagation and both linear and nonlinear wave conversion are investigated. The small wavelength (λ⩽0.02 cm) high number ion Bernstein harmonics, resulting from the linear wave conversion of the LH wave are observed in a tokamak plasma for the first time.

Production of an Intense, Well-Collimated Submillimeter Wave Beam and Its Application to Plasma Scattering Measurements

Intense submillimeter wavelength radiation (f=354 GHz, P=110 W) from Gyrotron FU II is converted into a well-collimated, linearly-polarized beam by a quasi-optical transmission line consisting of a quasi-optical antenna and several mirrors. The observed focusing of the beam agrees with calculations based on the Huygens equation. We have used this intense beam to study low frequency density fluctuations by plasma scattering in Compact Helical System (CHS) plasmas at the National Institute for Fusion Science.

Plasma scattering measurement using a submillimeter wave gyrotron (gyrotron FU-II) as a power source

Electromagnetic waves of medium power ( P = 110 W) in the submillimeter wavelength range ( f= 354 GHz) are generated in long pulses (τ = 0.6 s) by the GYROTRON FU-II and they have been applied to plasma scattering measurements in compact helical system plasma. Use of a gyrotron as a power source improves the S/N ratio of the measurement. The results of low frequency density fluctuations correspond to those of reflectometry. A new transmission line is designed for conversion of gyrotron outputs into a high quality beam. Analysis using Huygen's equation supports the view that this system can produce a high quality beam. The final goal is to improve the performance of the scattering measurement by applying an optimal probe beam.

Stochastic wave growth, power balance, and beam evolution in type III solar radio sources

Energy-balance arguments are combined with the stochastic-growth theory of type III radio sources to determine the properties of the source in average dynamical equilibrium with the beam, and the beam's long-term evolution. Purely linear stochastic-growth theory has previously emphasized that the beam evolves to a state close to marginal stability. Small mean residual deviations from marginal stability are present at dynamical equilibrium and these lead to residual energy flows that feed the waves observed in situ and by remote receivers; consequently the beam energy is depleted. Here, dynamical equilibrium beam and wave levels are estimated for the first time and it is found that the main sink of beam-driven Langmuir waves is either via electrostatic decay into product Langmuir and ion-sound waves or via scattering by short-wavelength density fluctuations, depending on the conditions. Improved estimates of energy branching ratios imply that, at 1 AU from the Sun, typically 20% of the beam energy is converted to Langmuir waves that are scattered off low-frequency density fluctuations and then dissipated, with almost all the remaining waves undergoing electrostatic decay, although as little as one-third of the Langmuir waves may decay in atypical circumstances. Of order 10−3 of the beam energy is converted into sound waves, which are mostly dissipated, and of order 10−5 is converted into potentially observable electromagnetic waves. The mean lifetime of the Langmuir waves at 1 AU is 1–40 s, while that of the beam is of order 1000 s. The beam density decreases relative to that of the background as the beam propagates. For most parameters, analysis of energy losses from the beam to the waves shows that the beam velocity decreases at roughly the same rate as the thermal velocity of the background plasma. It is argued from these considerations, and from in situ observations at 1 AU, that these trends imply that only the densest and fastest type III beams will be able to penetrate much past 1 AU from the Sun. This implies a low-frequency cutoff to type III emission at roughly 10 kHz, in good agreement with recent Ulysses remote observations, showing their consistency with in situ measurements.

Characterization of heat transport dynamics in laser-produced plasmas using collective Thomson scattering: Simulation and proposed experiment

We propose an experiment in which the collective Thomson scattering lineshape obtained from ion acoustic waves is used to infer the spatial structure of local heat transport parameters and collisionality in a laser-produced plasma. The peak-height asymmetry in the ion acoustic wave spectrum will be used in conjunction with a recently developed model describing the effects of collisional and Landau damping contributions on the low-frequency electron density fluctuation spectrum to extract the relative electron drift velocity. This drift arises from temperature gradients in the plasma. The local heat flux, which is proportional to the drift, can then be estimated, and the electron thermal conductivity will be inferred from the relationship between the calculated heat flux and the experimentally determined temperature gradient. Damping of the entropy wave component at zero mode frequency is shown to be an estimate of the ion thermal conductivity, and its visibility is a direct measure of the ion-ion mean free path. We also propose to measure thermal transport parameters under dynamic conditions in which the plasma is heated impulsively by a laser beam on a fast (∼50 ps) time scale. This technique will enable us to study heat transport in the presence of the large temperature gradients that are generated by this local heating mechanism. Deviations of the inferred local thermal conductivity from its Spitzer-Harm value can be used to study the transition to a nonlocal heat transport regime. We have constructed a simple numerical model of this proposed experiment and present the results of a simulation.

Parametric decay of an electromagnetic wave near electron cyclotron harmonics

A system of equations describing the nonlinear coupling of high frequency electron Bernstein (EB) and upper hybrid (UH) waves near harmonics of the electron cyclotron frequency with low frequency lower hybrid (LH) waves in a homogeneous, weakly magnetized, and weakly collisional plasma is derived. The EB and UH modes are described by a single second order equation, taking into account the interaction with low frequency density fluctuations. The ponderomotive force of the high frequency oscillations increases near the cyclotron harmonics due to the resonance with the electron motion. The obtained equations are used to study the parametric decay of an infinite wavelength electromagnetic pump wave into EB or UH waves and LH waves. The threshold electric fields are sufficiently low to be exceeded in high frequency ionospheric modification experiments. However, the instability cannot be excited for pump frequencies near the cyclotron harmonics. For the decay into EB waves, the resulting forbidden frequency range depends on the harmonic number in a power law manner, consistent with observations of stimulated electromagnetic emissions in ionospheric modification experiments. Further, for sufficiently high pump electric fields the instability is also suppressed, when the frequency mismatch around the eigenfrequencies at which the interaction can occur is of the order of the frequency separation between the EB and UH modes near the cyclotron harmonics.

Magnetic Configuration Scans in H-1 Heliac

The results of the experimental study of the magnetic configurations in the H-1 heliac are presented. The shape of the flux surfaces and the rotational transform in H-1 can be controlled by varying external coil currents. Electron beam magnetic mapping has been performed to show the existence of closed nested flux surfaces and to observe the effect of small errors in coil alignment on the vacuum magnetic structure in H-1. Langmuir probes have been used to study the electron density profiles in a current-free collisional RF-sustained plasma (n{sub e}<=4 x 10{sup 12} cm{sup -3}, T{sub e}<= 15 eV). In standard magnetic configuration and for the present moderate RF power levels, the highest central density is achieved at rather low magnetic field (0.07 T). This regime is characterized by peaked density profiles that appear to have a maximum coincident with the position of the vacuum magnetic axis. When a lowest-order m=1, n=1 resonance is introduced inside the outermost magnetic surface a strong asymmetry in both the vacuum magnetic structure and the plasma density profiles is observed. We observed low frequency (2-3 kHz) density fluctuations having low radial mode numbers and internal parallel plasma current localized in the regions of highest densitymore » gradient. These fluctuations are effectively suppressed by an increase of the magnetic field. 4 refs., 6 figs.« less

Long-pulse operation of a submillimeter wave gyrotron and its application to plasma scattering measurement

Electromagnetic waves of medium power (P=110 W) in the submillimeter range (f=354 GHz) are generated in long pulses (τ=0.6 s) by a submillimeter wave gyrotron. The fluctuation levels of output power are suppressed within 8% by optimizing the operating parameters. This output has been applied to plasma scattering measurements. Preliminary results of low-frequency density fluctuations in Compact Helical System plasma are presented.

Study of lower-hybrid wave propagation in the presence of low-frequency fluctuations

Phase-sensitive probe diagnostics as well as two-point-correlation analysis of radio-frequency probe signals are applied to investigate small-amplitude lower-hybrid (LH) waves launched into a linear nonuniform plasma column by a slow-wave antenna. The LH wave propagation in the inhomogeneous plasma is described by a model taking into account the wavenumber spectrum of the antenna. Special attention is focused on the interaction of the LH waves with the low-frequency density fluctuations. The LH frequency and wavenumber spectra observed can be explained by scattering of the LH waves from the (incoherent) low-frequency density fluctuations. Moreover, the enhanced damping, which is found to be much higher than expected from collisional and electron Landau dampings, is correlated with the level of low-frequency fluctuations.

Connection between ambient density fluctuations and clumpy Langmuir waves in type III radio sources

A recent stochastic-growth theory of clumpy Langmuir waves in type III sources is shown to imply that the clumps will have the same size distribution as the ambient low-frequency density fluctuations in the solar wind. Spectral analysis of Langmuir-wave time series from the ISEE 3 plasma wave instrument confirms this prediction to within the uncertainties in the spectra. The smallest Langmuir clump size is inferred to be in the range 0.4-30 km in general, and 2-30 km for beam-resonant waves, and it is concluded that the diffusion of waves in the source is anomalous.

Structural dynamics of proteins, scaling behaviour and liquid glass transition

Inelastic neutron scattering was used to study liquid-like motions and the nature of a dynamical transition in myoglobin, a small globular protein. The signature of the transition is a strong enhancement of low-frequency density fluctuations and a corresponding non-linear increase in atomic mean squared displacement above 180 K. The inelastic scattering function displays two power-law regions, whereby the crossover energy decreases with temperature. It is shown that the rescaled spectrum approximates a temperature and wave vector independent master function. Such features have been predicted for simple undercooled liquids in the vicinity of the glass transition by recent mode coupling theories.

Density fluctuations and confinement in the Nagoya Bumpy Torus (NBT-1M)

The stabilization of low-frequency density fluctuations is evidenced in the Nagoya Bumpy Torus (NBT-1M) [Plasma Physics and Controlled Nuclear Fusion Research (IAEA, Vienna, 1985), Vol. 2, p. 551] in the presence of microwave-heated hot-electron rings. Flute-type fluctuations, which are considered to be stabilized by the charge-uncovering effect of the rings, are found to cause large plasma losses, and to affect radial density profiles in the way that the lower fluctuation level yields the steeper density gradient. The particle confinement is, therefore, improved by the hot-electron rings to some extent, but is mainly determined by the plasma convection, which is expected from the discrepancy between density and potential profiles. It is also found that fluctuations in a toroidal plasma inside the ring grow when a weak negative ambipolar potential and a steep density gradient are formed, and are reduced to a low level when a deep potential well is achieved.

On the response of the horizontal mean vertical density distribution in a fjord to low-frequency density fluctuations in the coastal water

On the response of the horizontal mean vertical density distribution in a fjord to low-frequency density fluctuations in the coastal water

Dynamic instability of liquidlike motions in a globular protein observed by inelastic neutron scattering.

Inelastic neutron scattering was used to investigate liquidlike motions and the nature of a dynamic transition in myoglobin, a small globular protein. The signature of the transition is a strong enhancement of low-frequency density fluctuations and a corresponding decrease in elastic scattering above 180 K. It is shown that the line shape of the inelastic-scattering function approximates the scaling behavior predicted for a simple liquid by mode-coupling theories in the vicinity of the liquid-glass transition.

Study of microturbulence on the KT-5 tokamak by CO2 laser scattering

We report our observation of low frequency (LF) density fluctuation of microturbulence in KT-5 tokamak. Broad frequency and wavenumber power spectra are observed. The density fluctuation frequencies are found to be around a few hundred kHz, and at higher frequencies the fluctuation level rolls off as ñ2(f) ∝ f-β with β = 1 - 3. The wavenumber power spectrum S(κ) can be fitted into S(κ) ∝ κ-α with α around 3.0.