Multichannel visible spectroscopy diagnostic for particle transport studies in the H-1 heliac

A multichannel spectroscopy diagnostic has been developed to study cross-field particle transport in the radiation-dominated low-temperature plasmas (Te<100 eV) in the H-1 heliac. The optical setup covers the full plasma minor radius in the poloidal plane collecting light from ten parallel chords arranged tangentially to the flux surfaces. The light collected from the plasma is coupled into optical fibers and through interference filters into photomultipliers. Two such ten-fiber arrays are aligned parallel to one another to allow the simultaneous monitoring of two different spectral lines. The net radial electron particle flux is determined from the continuity equation by integrating over the ionization source term in the steady-state partially ionized plasma. The diagnostic measures the neutral line intensities and their ratios (in case of helium using the line ratio technique) and also measures excited neutral and ion spectral lines (in case of the argon plasma transport studies). A comparative analysis of the radial particle transport in the low- and high-confinement regimes is presented

Fluctuation studies using combined Mach/triple probe

A probe consisting of two poloidally separated triple probes and a Mach probe (TMT probe) has been designed and installed on the H-1 heliac to study fluctuations. Mach probes are shown to be sensitive to the fluctuations in the electron density, electron and ion temperatures, and ion drift velocity. If the ion Larmor radius is much larger than the characteristic probe dimension, then the Mach probe is insensitive to the magnetic field. When the Mach probe is oriented such that the two tips are separated radially, it becomes sensitive to the radial velocity of the ions. A model has been devised to allow the above mentioned time-resolved plasma parameters to be reconstructed from the data obtained using the TMT probe. One of the important results of these studies is that ion temperature fluctuations cannot be neglected

Strong ExB shear flows in the pedestal region in H-mode plasma

We report the first experimental observation of stationary zonal flows in the pedestal region of the H-mode plasma in the H-1 toroidal heliac. Strong peaks in E_r shear mark the top and foot of the density pedestal. Strong m=n=0 low-frequency (f < 0.6 kHz) zonal flows are observed in regions of increased E_r, suggesting substantial contribution of zonal flows to the spatial modulation of E_r radial profiles. Radial localization of zonal flows is correlated with a region of zero magnetic shear and low-order (7/5) rational surfaces.Comment: 4 pages, 5 figure

Spectrally condensed turbulence in thin layers

We present experimental results on the properties of bounded turbulence in thin fluid layers. In contrast with the theory of two-dimensional (2D) turbulence, the effects of the bottom friction and of the spectral condensation of the turbulence energy are important in our experiment. Here we investigate how these two factors affect statistical moments of turbulent fluctuations. The inverse energy cascade in a bounded turbulent quasi-2D flow leads to the formation of a large coherent vortex(condensate) fed by turbulence. This vortex, depending on its strength, can substantially affect the turbulence statistics, even at small scales. Up to the intermediate strength of the condensate, the velocity moments similar to those in isotropic 2D turbulence are recovered by subtracting the coherent component from the velocity fields. A strong condensate leaves a footprint on the underlying turbulence; it generates stronger non-Gaussianity and reduces the efficiency of the inverse energy cascade. Remarkably, the energy flux in the cascade derived from the third-order structure function using the Kolmogorov flux relation gives physically meaningful values in a broad range of experimental parameters regardless of the condensate strength. This result has important implications for the analysis of the atmospheric wind data in upper troposphere and lower stratosphere.This work was supported by the Australian Research Council Discovery Projects funding scheme Grant No. DP0881544, Israeli Science Foundation Grant No. 671/09, and Minerva Einstein Center

Collective microwave scattering diagnostic on the H-1 heliac

A multichannel microwavescatteringdiagnostic has been developed and installed on the H-1 heliac. The purpose of the new diagnostic is to study small-scale plasma fluctuations in H-1, which are believed to be responsible for the loss of particles and energy from the plasma. The diagnostic is a 132 GHz, four-channel superheterodyne system. The transmitter and receiver antennas (consisting of horns and focusing bispherical mirrors) are located inside the vacuum vessel of H-1. A radial resolution of Δr/a∼0.2 is achieved. The scattering volume is positioned in the density gradient region at r/a∼0.6. At present, the system is aligned to measure fluctuations in the poloidal wave number range from approximately 10 to 25 cm⁻¹. The use of the heterodyne detection system allows the fluctuation propagation direction to be determined. The low frequency bandwidth of the system is 1 MHz. The instrument sensitivity is about Ps/Pi∼10⁻⁶

Measurements of poloidal rotation velocity using cross-correlation spectroscopy in the H-1 heliac

A correlation spectroscopy diagnostic [M.G. Shats and J. Howard, Fusion Eng. Des. 34–35, 271 (1997)] measures fluctuation spectra and local fluctuation intensities in a radiation-dominated plasma, such as the low-temperature plasma in the H-1 heliac (Te<50 eV, ne<2×10¹⁸ m⁻³). When the fluctuation coherence lengths in the poloidal and radial directions are shorter than the plasma radius, the cross-correlation function of the two crossed-sightline fluctuating intensities contains information about the fluctuations amplitude and their phase in the intersection volume. The optical setup on the H-1 heliac uses two nearly orthogonal views to image 20 optical fibers arranged into two linear arrays in the plasma poloidal cross section. A matrix of 10×10 cross-correlation functions is then analyzed to determine the poloidal phase velocity of the fluctuations, poloidal and radial correlation lengths, and the radial profiles of the fluctuations intensity. The results on the poloidal propagation velocity measured using the cross-correlation technique (time delay of the cross-correlation functions) are compared with the poloidal velocity measured using poloidally separated probes in the plasma. Both velocities are found to be in good agreement and also agree well with the E×B drift velocity in this plasma

Self-organization in turbulence as a route to order in plasma and fluids

Transitions from turbulence to order are studied experimentally in thin fluid layers and magnetically confined toroidal plasma. It is shown that turbulence self-organizes through the mechanism of spectral condensation. The spectral redistribution of the turbulent energy leads to the reduction in the turbulence level, generation of coherent flow, reduction in the particle diffusion and increase in the system's energy. The higher order state is sustained via the nonlocal spectral coupling of the linearly unstable spectral range to the large-scale mean flow. The similarity of self-organization in two-dimensional fluids and low-to-high confinement transitions in plasma suggests the universality of the mechanism.Comment: 5 pages, 4 figure

Turbulence-condensate interaction in two dimensions

We present experimental results on turbulence generated in thin fluid layers in the presence of a large-scale coherent flow, or a spectral condensate. It is shown that the condensate modifies the third-order velocity moment in a much wider interval of scales than the second one. The modification may include the change of sign of the third moment in the inverse cascade. This observation may help resolve a controversy on the energy flux in mesoscale atmospheric turbulence (10-500 km): to recover a correct energy flux from the third velocity moment one needs first to subtract the coherent flow. We find that the condensate also increases the velocity flatness.Comment: 4 pages, 4 figure

Metamorphosis of plasma turbulence-shear flow dynamics through a transcritical bifurcation

The structural properties of an economical model for a confined plasma turbulence governor are investigated through bifurcation and stability analyses. A close relationship is demonstrated between the underlying bifurcation framework of the model and typical behavior associated with low- to high-confinement transitions such as shear flow stabilization of turbulence and oscillatory collective action. In particular, the analysis evinces two types of discontinuous transition that are qualitatively distinct. One involves classical hysteresis, governed by viscous dissipation. The other is intrinsically oscillatory and non-hysteretic, and thus provides a model for the so-called dithering transitions that are frequently observed. This metamorphosis, or transformation, of the system dynamics is an important late side-effect of symmetry-breaking, which manifests as an unusual non-symmetric transcritical bifurcation induced by a significant shear flow drive.Comment: 17 pages, revtex text, 9 figures comprised of 16 postscript files. Submitted to Phys. Rev.