Session GI02: Invited: Magnetic Fusion: Pedestal and Transport

Live

Rossby/Drift-zonal flow turbulence frequently occurs in a background of a static stochastic (tangled) magnetic field. Tangled fields that coexist with an ordered mean field play a key role in turbulence in the solar tachocline\footnote{ Chen, C. C., \& Diamond, P. H. (2020). \emph{Potential Vorticity Mixing in a Tangled Magnetic Field}. The Astrophysical Journal, \textbf{892(1)}, 24 }, and in magnetic confinement devices. and in magnetic confinement devices. In the case of weak-field perturbations, quasi-linear theory predicts that the Reynolds and magnetic stresses will balance, as turbulence Alfv\'enizes for a larger mean magnetic field. However, even a modest mean magnetic field leads to large magnetic perturbations for large magnetic Reynolds number. Thus, the physically relevant case is that of a strong but disordered field. We present a novel double-average theory of potential vorticity (PV) mixing in the context of $\beta$-plane MHD, with a special focus on the physics of momentum transport in the stably stratified, quasi-2D solar tachocline. We present numerical calculations indicate that the Reynolds stress is modified well before Alfv\'enization — i.e. before fluid and magnetic energies balance. Calculations indicate that mean-square fields strongly reduce Reynolds stress phase coherence and also induce a magnetic drag on zonal flows. The physics of transport reduction by tangled fields is elucidated and linked to effects in other channels. We propose a physical picture of the system as a resisto-elastic medium threaded by a tangled magnetic network. Reynold stress decoherence is identified as the main effect. Applications of the theory to momentum transport in the tachocline and other systems are discussed in detail. Related experiments\footnote{Kriete, D. M., McKee, G. R., Schmitz, L., Smith, D. R., Yan, Z., Morton, L. A., \& Fonck, R. J. (2020). \emph{Effect of magnetic perturbations on turbulence-flow dynamics at the LH transition on DIII-D}. Physics of Plasmas, 27(6), 062507. } indicate that RMP fields can reduce fluctuation-driven Reynolds forces and so inhibit the initiation of the L-H transition. We present a theory of PV flux (Reynold force) decoherence and its implications for zonal flow evolution. Using the techniques discussed above, we calculate the decoherence of the PV flux due to stochastic magnetic field scattering. Decoherence requires the stochasticity-induced decorrelation rate to exceed the bandwidth of the ambient electrostatic micro-instabilities, as well as the bandwidth of Alfv\'en waves (i.e. Kubo number $Ku \geq1$ ). These inequalities define a critical value of $\langle (\delta B)^2 / B^2 \rangle$ for an effect on the transition. The implications for decoherence of the particle and heat flux are discussed, as well.   

Live

The inferred ion heat flux (Qi) in the pedestal region on DIII-D is approximately neoclassical at high ion collisionality ($\nu^*$) but becomes increasingly anomalous as $\nu^*$ is lowered towards values expected on ITER, challenging the assumptions made in predictive models and extrapolations. In dedicated experiments on DIII-D, beam emission spectroscopy measurements in the steep gradient region of the H-mode pedestal reveal increased broadband, long wavelength ion scale fluctuations for the low $\nu^*$ discharges. Ion scale fluctuations are known to increase anomalous multi-channel transport in the plasma core, and have significant implications for confinement and the accessibility of ELM free transport limited pedestals if they dominate in the edge transport barrier region of future reactors. Unique pedestal main-ion temperature measurements on DIII-D are essential when calculating Qi in TRANSP to reveal this anomalous transport. These new observations are consistent with gyrokinetic calculations using CGYRO that show increased growth rates of long wavelength ion temperature gradient (ITG) and trapped electron modes (TEM). Taken together, increased fluctuations, power balance calculations and gyrokinetic simulations show that the anomalous Qi at low $\nu^*$ may be due to weakly suppressed ion scale turbulence. This analysis provides a bridge between impurity temperature based approximately neoclassical Qi results on ASDEX-Upgrade [E. Viezzer NF 2017] at higher $\nu^*$, and anomalous transport contributions from ITG/TEM [D.R. Hatch NF 2017] at lower $\nu^*$ cases on JET-ILW. These new results are based on world first inferred ion and electron heat fluxes in the pedestal region of deuterium plasmas using direct measurements of the deuterium temperature for power balance across ion collisionalities covering an order of magnitude between ITER relevant $\nu^*$ of 0.1 up to 1.2.   

Live

Experimental analysis and modeling of recent super H-mode experiments on DIII-D show that driven high toroidal rotation, not high pedestal pressure, plays an essential role in achieving very high energy confinement $H_{98y2}>$1.5. Understanding the mechanisms leading to improved confinement in the super H-mode experiments is essential to the ability to extrapolate to a future reactor. For fixed plasma shape and rotation, the energy confinement time for discharges analyzed here with different plasma current, density, injected power, is proportional to the $\tau_{E,98y2}$ scaling, i.e. the energy confinement quality $H_{98y2}$ is constant, despite different pedestal pressure (up to $\times$2), and plasma stored energies. For fixed plasma shape but different toroidal rotation, which varies according to injected neutral beam torque per particle, $H_{98y2}$ varies linearly with rotation, independent of pedestal pressure. A transient phase of very high confinement quality, $H_{98y2}\sim$2 (well above standard H-mode, $H_{98y2}\sim$1), is only achieved at very high level of core rotation, e.g. 400 km/s at $\rho$=0.4, and is independent of the pedestal pressure. At moderate rotation on DIII-D (similar to levels expected in ITER) very high super H-mode pedestal pressure yields a lower confinement quality improvement ($H_{98y2}\le$1.2) if no core MHD modes are present. Linear gyrofluid and nonlinear gyrokinetic transport modeling confirms that rotation-driven E$\times$B shear is responsible for confinement quality significantly above standard H-mode, and that E$\times$B shear turbulence stabilization is far stronger than EM stabilization, so-called hot-ion stabilization ($T_i$/$T_e$), or fast ions effects. Gyrokinetic simulations also show a potential approach to improve confinement at low rotation: higher impurity (carbon) gradient in the plasma core can efficiently suppress ITG turbulence, and improve confinement in the super H-mode scenario.   

Live

This study marks the first achievement of confirmed biasing H-modes on HBT-EP, as well as the first characterization of the edge turbulence as ion temperature gradient mode (ITG) dominated. Measurements of the radial wavenumber spectrum of floating potentials at the edge show that the turbulence intensity decreases with increasing shift in the spectrum average $\langle k_{r} \rangle$ when increasing amounts of bias probe voltage (and increasing amounts of flow shear) is applied. These measurements extend previous findings on EAST and TCABR, which support the spectral shift model proposed by Staebler et al.\footnote{Staebler G. M. et al. 2013 \textit{Phys. Rev. Lett.} \textbf{110} 055003} for turbulence suppression via sheared flow, through detailed local measurements of L-H transitions on HBT-EP. A shift in the wavenumber spectrum occurs at applied electrode voltages and currents below the threshold needed for an L-H transition and furthermore, a dithering transition is obtained when biasing near the threshold. Additionally, the suppression of blob-filament turbulence in the scrape-off layer (SOL) precedes the L-H transition, and the SOL turbulence remains low throughout the entire dithering phase, despite the modulation of turbulence levels in the nearby edge. In this way, the SOL turbulence ``decouples" from the edge turbulence. The shift in the measured radial wavenumber is corroborated by the direct measurement of eddy tilt angle using a novel time delay analysis technique\footnote{Pinz\'{o}n J. R. et al. 2019 \textit{Plasma Phys. Control. Fusion} \textbf{61} 105009} first developed for Doppler reflectometry but adapted here for floating potential measurements.   

Live

Edge turbulence in L-mode plasmas plays a key role in the understanding of the L-H transition as well as of some ELM-free high-confinement regimes such as the I-mode. This work combines observations in ASDEX Upgrade and JET-ILW and related gyrokinetic turbulence simulations. We explore both the role of the isotope mass as well as that of the temperature and density profiles in driving or stabilizing the edge turbulence. Recent experiments in ASDEX Upgrade and JET-ILW have given some new indications on these two aspects. For both devices pairs of D and H L-modes with matched profiles and with heating power scan have been performed. It is observed that the normalized temperature logarithmic gradients R/L$_{\mathrm{Te}}$ and R/L$_{\mathrm{Ti}}$ are free to evolve towards the L-H transition while, despite the changes in n$_{\mathrm{e}}$ and in its gradient, R/L$_{\mathrm{n}}$ shows a more limited variation. A local gyrokinetic approach is demonstrated to be applicable to the study of L-mode edge turbulence, and provides critical new insights in the understanding of these observations. Results with the GENE code reproduce quantitatively the experimental fluxes and show that the edge high collisionality favors instabilities strongly affected by the parallel electron dynamics. The corresponding term in the gyrokinetic equation, proportional to (m$_{\mathrm{e}}$/m$_{\mathrm{i}})^{\mathrm{0.5}}$, leads to increased transport at lower mass, an effect which is magnified when electromagnetic effects are included. Moreover, a competition has been found between R/L$_{\mathrm{Te}}$, R/L$_{\mathrm{Ti}}$ and R/L$_{\mathrm{n}}$ in driving the turbulence. In contrast, the concomitant increases of the equilibrium ExB shear, consistent with measurements, and of the self-generated zonal flow shear stabilize the turbulence. These results suggest a path towards edge turbulence stabilization in which the evolution of T$_{\mathrm{i}}$, connected with an increased ion heat flux, leads to increased shearing without driving the turbulence.   

Live

We demonstrate for the first time that fast electric field transients triggering the L-H transition are quantitatively consistent with the combined radial polarization (displacement) currents due to Reynolds stress, thermal ion orbit loss, and ion viscosity. These $E_{\mathrm{r}}$ transients (typically 0.05-1 ms) can produce large \textbf{\textit{E}}x\textbf{\textit{B}} shear and can trigger L-H transitions when the L-mode ``equilibrium'' shear flow due to the ion pressure gradient is insufficient to suppress edge turbulence. Typical examples are plasmas with unfavorable grad-$B$ drift direction and/or strong toroidal co-current rotation. Edge turbulence is suppressed once the transient \textbf{\textit{E}}x\textbf{\textit{B}} shearing rate exceeds the plasma frame turbulence decorrelation rate [1]. \quad Initial experiments indicate that the L-H transition power threshold $P_{\mathrm{LH}}$ can be reduced at low ion collisionality via Neoclassical Toroidal Viscosity (NTV) from applied n$=$3 non-resonant magnetic fields (NRMF). CER data confirm that the applied NTV counter-current torque locally reduces L-mode edge toroidal co-rotation, increasing the shear in the $\mbox{v}_{\phi } B_{\theta } $ term in the radial ion force balance. The well-known increased $P_{\mathrm{LH}}$ with unfavorable grad-$B$ drift direction is attributed to reduced shear flow in the outer shear layer due to higher (intrinsic) edge co-rotation. This increase is often mitigated in ITER-similar-shape plasmas in DIII-D via localized rotation reversals in the inner shear layer, triggered by sawteeth or transport avalanches. \quad These new insights can open up paths for reducing $P_{\mathrm{LH}}$ during the initial ITER hydrogen campaign with limited auxiliary power, by generating edge NTV [via the planned (partial) 3-D coil set], by exploiting edge magnetic topology modifications due to MHD modes, or by localizing power deposition to critical edge layers. [1] L. Schmitz et al., Phys. Rev. Lett. \textbf{108}, 155002 (2012).