Z. Yan

Studies of blob formation, propagation and transport mechanisms in basic experimental plasmas (TORPEX and CSDX)

The findings of previous blob studies in the interchange-dominated regime of TORPEX helium (Muller et al 2007 Phys. Plasmas 14 110704) and hydrogen plasmas (Furno et al 2008 Phys. Rev. Lett. 100 055004) are summarized and compared. The onset of blobs is studied as a function of the vertical magnetic field Bz, proving the existence of blobs also in the drift-interchange-dominated regime characterized by Bz < 1 mT. These blobs, despite being inherently three-dimensional and sheath-disconnected, exhibit statistical properties similar to the blobs in the interchange regime. Using conditionally averaged density and potential measurements, the entire time line of an interchange instability leading to the formation of wave-like structure patterns and blobs could be experimentally observed. These results show that a background E × B shear flow is not essential for the generation of blobs and that the phase shift between density and potential during the generation of blobs is π/2 in all studied cases, demonstrating the interchange nature of blobs in TORPEX. Fast-camera and Langmuir probe measurements of blobs in the linear device CSDX (Tynan et al 2004 Phys. Plasmas 11 5195) support the hypothesis that an interchange instability is also responsible for the generation of blobs in the linear geometry, where the necessary effective gravity is provided by centrifugal forces.

Study of nonlinear spectral energy transfer in frequency domain

A method for measuring nonlinear energy transfer in the frequency domain using a two-field model of drift turbulence is proposed, and the theoretical motivation and experimental results are presented. The approach is based on the cross-bispectral analysis of quadratic nonlinearities in the turbulent internal and kinetic energy balance equations directly derived from the fluid plasma continuity and momentum equations. Application of the technique to data from a laboratory plasma experiment reveals the nonlinear energy transfer in weak collisional plasma drift turbulence; the results show a transfer of density fluctuation energy toward higher frequency (which correspond to smaller azimuthal spatial scales) and a transfer of kinetic energy to lower frequencies (corresponding to larger azimuthal scales).

Statistical analysis of the turbulent Reynolds stress and its link to the shear flow generation in a cylindrical laboratory plasma device

The statistical properties of the turbulent Reynolds stress arising from collisional drift turbulence in a magnetized plasma column are studied and a physical picture of turbulent driven shear flow generation is discussed. The Reynolds stress peaks near the maximal density gradient region, and is governed by the turbulence amplitude and cross-phase between the turbulent radial and azimuthal velocity fields. The amplitude probability distribution function (PDF) of the turbulent Reynolds stress is non-Gaussian and positively skewed at the density gradient maximum. The turbulent ion-saturation (Isat) current PDF shows that the region where the bursty Isat events are born coincides with the positively skewed non-Gaussian Reynolds stress PDF, which suggests that the bursts of particle transport appear to be associated with bursts of momentum transport as well. At the shear layer the density fluctuation radial correlation length has a strong minimum (∼4–6mm∼0.5Cs∕Ωci, where Cs is the ion acoustic speed and Ωci ...

Fourier-domain study of drift turbulence driven sheared flow in a laboratory plasma

Frequency-resolved nonlinear internal and kinetic energy transfer rates have been measured in the Controlled Shear Decorrelation Experiment (CSDX) linear plasma device using a recently developed technique [Xu et al., Phys. Plasmas 16, 042312 (2009)]. The results clearly show a net kinetic energy transfer into the zonal flow frequency region, consistent with previous time-domain observations of turbulence-driven shear flows [Tynan et al., Plasma Phys. Controlled Fusion 48, S51 (2006)]. The experimentally measured dispersion relation has been used to map the frequency-resolved energy transfer rates into the wave number domain, which shows that the shear flow drive comes from midrange (kθρS>0.3) drift fluctuations, and the strongest flow drive comes from kθρS≈1 fluctuations. Linear growth rates have been inferred from a linearized Hasegawa–Wakatani model [Hasegawa et al., Phys. Fluids 22, 2122 (1979)], which indicates that the m=0 mode is linearly stable and the m=1–10 modes (corresponding to kθρS>0.3) are l...

Experimental characterization of multiscale and multifield turbulence as a critical gradient threshold is surpassed in the DIII-D tokamaka)

A critical gradient for long wavelength (kθρs≲0.4) electron temperature fluctuations has been observed in an experiment in the DIII-D tokamak [J. L. Luxon, Nucl. Fusion 42, 614 (2002)], where below a threshold value of LTe−1=|∇Te|/Te electron temperature fluctuations are constant and above they steadily increase. Above the critical gradient, the electron heat flux inferred by power balance also increases rapidly. Critical gradients are a predicted attribute of turbulence arising from linear instabilities and are thought to be related to transport stiffness. The presented results are the first direct, systematic demonstration of critical gradient behavior in turbulence measurements in a tokamak. The experiment was performed by changing the deposition location of electron cyclotron heating shot-to-shot to locally scan LTe−1 at r/a = 0.6 in L-mode plasmas; rotation was also varied by changing the momentum input from neutral beam injection. Temperature fluctuations were measured with a correlation electron cy...

Shear flow and drift wave turbulence dynamics in a cylindrical plasma device

The experimental observations of the dynamics of the coupled drift wave turbulence (DWT)/sheared zonal flow (ZF) system in a cylindrical plasma device using a combination of Langmuir probe and fast-framing imaging measurements are reported. The results show the presence of an azimuthal ZF that exhibits low frequency (∼250 Hz) fluctuations. The envelope of the higher frequency (above 5 kHz) floating potential fluctuations associated with the DWT, the density gradient, and the turbulent radial particle flux are all modulated out of phase with the strength of the ZF. The divergence of the turbulent Reynolds stress is also modulated at the same slow time scale in a phase-coherent manner consistent with a turbulent-driven shear flow sustained against the collisional and viscous damping. The radial turbulence correlation length and cross-field particle transport are reduced during periods of strong flow shear. The results are qualitatively consistent with theoretical expectations for coupled DWT-ZF dynamics.

Scaling properties of turbulence driven shear flow

The characteristics and scaling properties of the turbulence driven shear flow are investigated in a cylindrical laboratory plasma device. For a given plasma pressure, the density fluctuation amplitude and radial particle flux increase with the applied magnetic field. Strong flow shear is found to coexist at high magnetic fields (>700G) with ∼10kHz drift wave turbulence, but not at low magnetic fields (<700G). The absolute value of the divergence of the turbulent Reynolds stress at the shear layer is shown to increase with the magnetic field as well. For a fixed magnetic field, the shear flow is found to decrease as the discharge gas pressure is increased. The density fluctuation amplitude and divergence of the turbulent Reynolds stress also decrease with the plasma pressure. For both situations the cross phase between the radial and azimuthal components of the velocity is found to be a key factor to determine variations in the turbulent Reynolds stress at different magnetic fields and discharge pressures...

Simulations of drift resistive ballooning L-mode turbulence in the edge plasma of the DIII-D tokamaka)

Results from simulations of electromagnetic drift-resistive ballooning turbulence for tokamak edge turbulence in realistic single-null geometry are reported. The calculations are undertaken with the BOUT three-dimensional fluid code that solves Braginskii-based fluid equations [X. Q. Xu and R. H. Cohen, Contrib. Plasma Phys. 36, 158 (1998)]. The simulation setup models L-mode edge plasma parameters in the actual magnetic geometry of the DIII-D tokamak [J. L. Luxon et al., Fusion Sci. Technol. 48, 807 (2002)]. The computations track the development of drift-resistive ballooning turbulence in the edge region to saturation. Fluctuation amplitudes, fluctuation spectra, and particle and thermal fluxes are compared to experimental data near the outer midplane from Langmuir probe and beam-emission-spectroscopy for a few well-characterized L-mode discharges in DIII-D. The simulations are comprised of a suite of runs in which the physics model is varied to include more fluid fields and physics terms. The simulatio...

Measurement of cross-field power loss due to rovibrationally excited H2 in a detached hydrogen divertor plasma simulator

The cross-field power loss due to radiation, plasma, and neutrals are measured for hydrogen discharges in a linear divertor simulator experiment. Radiation appears to be the dominant power loss channel; however, power loss due to heating of H2 neutrals is found to be quite significant, being only 2× weaker than radiation in the higher neutral pressure experiments. The H2 vibrational temperature Tvib is found to be the most important channel for carrying neutral energy out of the plasma—more important than either kinetic temperature Tkin or rotational temperature Trot. Power carried radially to the wall by plasma cross-field transport is found to be negligible when compared to neutral and radiation losses. These results demonstrate the importance of including of H2 neutrals in understanding power balance in detached tokamak divertors.

The Physics of Zonal Flow‐Drift Wave Turbulence Interactions: A Synthesis of Time‐domain, Fourier Domain, and Direct Visualization Studies

The nonlinear interaction between drift turbulence and zonal flows plays a key role in determining the turbulence amplitude, spatiotemporal scale and subsequent transport and is therefore of significant interest in magnetic fusion research. We summarize multipoint time‐domain probe studies, frequency‐domain studies, and direct visualization studies of this important process. The fluctuation‐induced Reynolds stress is shown to be consistent with the observed shear flow; spatially resolved direct measurement of nonlinear exchange of energy between drift fluctuations and zonal flows and direct imaging show how the drift instability generates isotropic vortex structures which then propagate outwards towards the zonal flow shear layer, where they are stretched and thinned, and then incorporated into the shear layer, thereby maintaining it against damping mechanisms. The drift fluctuation‐zonal flow nonlinear interactions, shearing rate, and cross‐field transport are consistent with a critical gradient transpor...