Time-dependent experimental identification of inter-ELM microtearing modes in the tokamak edge on DIII-D

Abstract

In a series of discharges on the DIII-D tokamak, fast vertical plasma jogs are used to induce current perturbations in the steep gradient region of the H-mode edge. These current perturbations directly influence the edge q profile, decoupling the resonant location and instability drive of pedestal-localized microtearing modes (MTMs). By exploiting this effect, we develop and apply a new experimental technique to track the dynamical frequency evolution of MTMs in the pedestal region, providing a compelling validation of the MTM model. The frequency of potential MTMs is calculated as the Doppler-shifted electron diamagnetic frequency at rational q = m/n surfaces, showing remarkable agreement with chirped frequency behavior of n = 3, 4 and 5 modes detected with fast magnetics. Data is collected throughout multiple ELM cycles in order to build robust statistics describing the time-dependent frequency evolution of MTMs, which can be explained by examining the recovery of pedestal gradients after an ELM event. MTMs have a dominant transport contribution in the electron thermal channel, so the presented results indicate that reduced models of pedestal transport must be electromagnetic in nature and constructed with accurate calculations of MTM stability; inclusion of this physics is essential for accurate predictions of the electron temperature pedestal profile. Supporting measurements of mode saturation, propagation direction and transport fingerprints are made to support the dynamic frequency determination, unambiguously and experimentally identifying MTMs in the pedestal region of DIII-D.

DIII-D research advancing the physics basis for optimizing the tokamak approach to fusion energy

“…On the longer timescale of the electron temperature gradient (ETG) recovery, the TEM turbulence (figure 7(b)) exhibits a threshold in ETG (figure 7(c)) and then saturates later in the ELM recovery. MTM scale electro-magnetic modes (figure 7(d)) driven by grad-T e can also contribute to the anomalous Q e through to the end of the ELM cycle as suggested by non-linear simulations [32][33][34], although their experimental identification is not yet conclusive in this set of experiments. Finally, simulations predict that ETG modes also contribute to Q e between ELMs [36], although the spatial scales are so short that no direct measurements of ETG scale fluctuations in the pedestal are available.…”

“…The density gradient and E r well reform rapidly and ITG is suppressed by E × B shear, consistent with the decrease of pedestal ion heat flux (Q i ) from anomalous to near neoclassical. Main ion CER measurements indicate pedestal Q i becomes increasing anomalous at low collisionality, but at high collisionality Q i in the pedestal region remains closer to neoclassical [31,32]. On the longer timescale of the electron temperature gradient (ETG) recovery, the TEM turbulence (figure 7(b)) exhibits a threshold in ETG (figure 7(c)) and then saturates later in the ELM recovery.…”

“…Advances in pedestal physics through new measurements of density and internal magnetic fluctuations, and advances in non-linear simulations, suggest that variations of the electron and ion heat fluxes are consistent with the evolution of multi-scale turbulence in the pedestal. These studies [30][31][32][33][34][35][36] identify possible roles for ion temperature gradient (ITG), micro-tearing and trapped electron modes (MTMs and TEMs) in DIII-D pedestal transport. In DIII-D experiments with pedestal ion collisionality ν i * = 0.9, observations immediately after the ELM crash show that ITG scale density fluctuations (figure 7(a)), predominantly at the bottom of the pedestal, drive ion and electron thermal transport [30].…”

H-mode inhibition in negative triangularity tokamak reactor plasmas

“…Even so, further experimental validation of these results is required, and should be conducted on capable machines as soon as possible. During actual operation of a reactor, the edge pressure profiles may not be constrained a simple mtanh form with fixed pedestal width and may instead vary in complicated ways to maximize turbulent transport [45]. To develop some intuition for how arbitrary profile changes may influence access to the 2 nd stability region, a pedestal width scaling is presented as function of δ in figure 16.…”

Three-Dimensional Inhomogeneity of Electron-Temperature-Gradient Turbulence in the Edge of Tokamak Plasmas

“…These turbulent fluxes constrain the pedestal's magnetohydrodynamic stability [30][31][32][33][34], neoclassical transport [35], and scrape-off-layer processes [36]. Extensive experimental, numerical, and analytic results suggest that iontemperature-gradient (ITG) [3,37,38], ETG [4,39], microtearing [40], kinetic-ballooning [41,42], and trappedelectron modes [43] are responsible for anomalous heat losses in the pedestal [26,[44][45][46][47][48][49][50][51][52][53][54][55][56]. Pedestal instability and turbulence peaking away from the outboard midplane has been observed for ETG [57][58][59], ITG [60], microtearing [48,53,61], and trapped-electron modes [61].…”