Session ZI1: Transport and Invited Post Deadline

Correlation of Increased Electron Heat Transport With Modifications in ETG to ITG Scale Turbulence During Electron Cyclotron Heating on DIII-D

Increased high-k, electron temperature gradient (ETG)-scale ($k_\perp\sim\,$35-40~cm$^{-1}$, $k_\perp\rho_i=$ 4-10, $\rho_i=$ ion gyroradius) plasma density turbulence has been found to correlate with increased electron thermal flux during electron cyclotron heating experiments on the DIII-D tokamak. In contrast, low k ($k_\perp\sim\,$1~cm$^{-1}$) ITG-scale turbulence levels remain virtually unchanged indicating that the high-k fluctuations ($k_\perp\sim\,$35-40~cm$^{-1}$) are potentially responsible for driving the observed increase in electron heat transport and further that they are not simply remnants of low-k turbulence. Depending on the model utilized and/or plasma studied recent theoretical work indicates that high-k turbulence at the ETG scale can drive either significant or negligible electron heat transport. This highlights the critical need for turbulence measurements in this spectral region as well as detailed comparisons with transport properties and theoretical predictions. Quantitative measurement of low-k and high-k $\tilde{n}$ shows a much larger level of low-k, however integration over the appropriate $k_\perp\rho_e$ range could significantly increase the high k level. Observed changes in low-k measurements are consistent with linear gyrokinetic growth rate calculations only if the effects of electric field shear are accounted for. Intermediate and high-k measurements show somewhat better consistency. This indicates a differing effect of electric field shear on low and high k which is of theoretical interest. Nonlinear simulations utilizing diagnostic filters to simulate experimental diagnostics are underway.   

Particle and Impurity Transport in JET and ASDEX Upgrade: Experimental Observations and Theoretical Understanding

Cross-field transport in tokamak plasmas determines the peaking of both the density and the impurity profiles, with important consequences on both plasma stability and confinement. Experiments indicate that, in most conditions, a turbulent (non-neoclassical) particle pinch is needed to explain the observed peaking of electron and impurity density profiles. Theory has shown that several turbulent mechanisms contribute to the formation of a pinch velocity. For the first time, an empirical scaling for density peaking in H-mode plasmas is presented, by combining data from two different devices, AUG and JET. Correlations among plasma parameters are reduced, and collisionality is identified as the statistically most relevant plasma parameter in multiregression analyses. On the basis of this scaling, the density profile in a burning plasma is predicted to be peaked. The empirically determined dependence on collisionality is found to be consistent with the predictions of the gyrofluid model GLF23, namely with the collisional reduction of the turbulent particle pinch. However, the most advanced gyrokinetic models are found in quantitative disagreement with the experiments, since they predict the reduction of the pinch to take place over a range of collisionality which is well below the lowest collisionality experimentally achieved. Observations in JET and AUG indicate that auxiliary electron heating is the most efficient method to suppress heavy impurity accumulation and impurity pinches reversing the direction from inwards to outwards in response to localized auxiliary electron heating are observed. Such a reversal is not explained by neoclassical effects. A newly identified mechanism of turbulent pinch, which does not vanish for highly charged impurities and which reverses its direction as a consequence of the reversal of the direction of propagation of the turbulence is proposed as the best candidate to explain the experimental observations.   

Transport with Reversed Shear in the National Spherical Torus Experiment

In NSTX plasmas with strongly reversed magnetic shear $(dq/dr<0)$ in the plasma core we can observe a marked improvement in electron confinement compared to otherwise similar plasmas with positive or only weakly reversed shear. The shear profile is determined by the early evolution of the plasma current, the plasma cross-section, and the neutral-beam heating power. Although the q-profile is sensitive to the development of MHD instabilities during the current ramp, once established, the strongly reversed shear can be maintained for periods greater than 0.2 s. In the region of shear reversal, the inferred electron thermal diffusivity can be significantly reduced. Detailed experimental investigation of this phenomenon has been made possible by the successful development of a motional Stark effect (MSE) polarimetry diagnostic suitable for the low magnetic field in NSTX, typically 0.3 -- 0.5 T. This innovative system currently measures the magnetic field line pitch at 12 locations over a region extending from the plasma center to near the outboard edge of the plasma. Several of the unique features of this system will be highlighted. Measurements of the electron and ion temperature, density, and plasma toroidal rotation profiles are also available with high spatial and temporal resolution. Integrated analysis of these data with equilibrium, transport, and stability codes has been performed to assess the results in terms of theoretical models for the turbulence postulated to be responsible for anomalous transport.   

Reduction of Particle and Heat Transport in HSX with Quasisymmetry

The Helically Symmetric Experiment (HSX) has a helical direction of symmetry in the magnetic field strength. As a result of this symmetry, the theoretical neoclassical transport is reduced to the level of an axisymmetric device. This neoclassical transport is in addition to the anomalous contribution to the total transport. Experimentally, the electron collisionality is in the long mean free path regime so that neoclassical electron transport can be studied as a function of the magnetic field spectrum. Here we report experimental measurements of differences in electron temperature and density profiles between the quasihelically symmetric configuration (QHS) and configurations with the symmetry broken. The central electron temperature in QHS is significantly higher than that in the configuration without symmetry ($\sim $450 vs. $\sim $250 eV) under the same conditions. Ray tracing calculations show that the absorbed power is localized to within r/a$\sim $0.2 for both configurations, and the total absorbed power is scaled to match diamagnetic measurements. The resulting electron thermal conductivity in the core increases from $\sim $3 m$^{2}$/s for QHS up to $\sim $8 m$^{2}$/s as the symmetry is broken. With central heating and peaked temperature profiles, the density profile is centrally peaked in QHS, while for nonsymmetric plasmas the profile is flat or hollow as has been typically observed in conventional stellarators with ECH. Decreasing the temperature gradient in nonsymmetric plasmas results in a peaking of the density profile. The experimental particle flux has been inferred using a set of absolutely calibrated H$_{\alpha }$ detectors coupled with 3D neutral gas modeling. In the core of the plasma the neoclassical particle flux in the nonsymmetric configuration is comparable to the experimental flux. This neoclassical flux is dominated by particle flux driven by the temperature gradient, verifying that the flattening of the density profile in the nonsymmetric configuration is caused by neoclassical thermodiffusion. In QHS, the neoclassical particle flux (including thermodiffusion) is reduced, leading to a peaked density profile in the presence of a peaked temperature profile.   

Generation of periodically modulated plasma optical fibers and applications

We report the generation of up to $\sim $ 3cm long periodically modulated plasma waveguides that allow $\mu $m-scale control of the instantaneous intensity and phase velocity of a guided ultra-intense femtosecond laser pulse. This is accomplished by focusing an auxiliary spatially modulated laser pulse onto cluster jets. We can control the depth and period of the waveguide axial modulations, as well as the plasma ionization stage, electron density and guided spot size. The method is highly tunable and extremely stable, with controllable modulation periods as short as 35 $\mu $m and as long as 3 mm. Single-mode propagation at $>$10$^{17}$ W/cm$^{2}$ in these guides has been demonstrated using argon, nitrogen, and hydrogen clusters. A high degree of waveguide uniformity and shot-to-shot consistency is attained. We review several applications of our new device. Efficient direct electron acceleration by a radially polarized laser pulse can be achieved by quasi-phase matching in an axially modulated plasma waveguide [1]. High-harmonic generation could be quasi-phase matched [2] at extremely high orders. Finally, recent calculations [3] show that these channels could convert multi-millijoule femtosecond laser pulses into terahertz radiation with high efficiency. [1] A. York et al., Advanced Accelerator Concepts Conference 2006; [2] H. M. Milchberg et. al., Phys. Plasmas \textbf{3}, 2149 (1996); [3] T. Antonsen et al. Bull. Am. Phys. Soc. \textbf{50} (2005).   

Resistive Wall Mode Stabilization by Slow Plasma Rotation in DIII-D Tokamak Discharges with Balanced Neutral Beam Injection

Recent DIII-D experiments show that, with low magnetic field errors, the plasma rotation threshold for stable operation above the no-wall beta limit is more than a factor of 2 lower than previously reported. A key feature is that slow plasma rotation is achieved by reducing the neutral beam torque, using DIII-D'$\rm s$ new capability for mixed co and counter injection. In the presence of a nearby resistive wall, the ideal kink instability that often limits beta in tokamaks becomes a slowly growing resistive wall mode (RWM), which can be stabilized by sufficiently rapid rotation of the plasma. Previous experiments [1-4] indicated that RWM stability required a minimum plasma rotation velocity, evaluated at the $q=2$ surface, of at least 0.6\% of the local Alfven speed, consistent with theoretical modeling [2,4,5]. Those experiments combined strong, uni-directional neutral beam torque with magnetic braking by intrinsic or applied magnetic perturbations to slow the plasma, and resonant effects of the perturbation may have led to a larger rotation threshold. In contrast, recent DIII-D discharges with low torque and minimal nonaxisymmetric field errors have stably exceeded the no-wall beta limit by more than 30\% with ion rotation velocity (from C-VI emission) less than 0.3\% of the Alfven speed at the $q=2$ surface. These results challenge the leading RWM stabilization theories and suggest that wall stabilization of high beta plasmas in ITER and other future devices may be possible with modest rotation, if resonant perturbations are minimized. \vskip4pt \par\noindent [1] AM Garofalo, Phys. Plasmas 9, 1997 (2002).\par\noindent [2] RJ La Haye, Nucl. Fusion 44, 1197 (2004).\par\noindent [3] AC Sontag, Phys. Plasmas 12, 056112 (2005).\par\noindent [4] H Reimerdes, Phys. Plasmas 13, 056107 (2006).\par\noindent [5] Y Liu, Nucl. Fusion 45, 1132 (2005).