Abstract Viable magnetic fusion devices necessitate combining good confinement with effective power flux handling. A major concern for ITER, and devices beyond, is the divertor heat load width, which ...sets peak boundary heat loads on the plasma-facing materials. Current estimates of the heat flux width are narrow for future reactors. Here, we demonstrate how pedestal turbulence can expand into, or entrain, the stable scrape-off-layer and so broaden the heat flux width beyond these neoclassical predictions. Employing combined theoretical, computational, and experimental approaches, we focus on quiescent high confinement discharges on the DIII-D tokamak, but the results are of broader significance. Our findings uncover common trends in the edge turbulence intensity flux, the pressure perturbation skewness, and the turbulence mixing length, which together determine the heat flux width. This research demonstrates the physics of scrape-off-layer broadening by turbulence and highlights the promise of a turbulent pedestal for successful core-edge integration in ITER and future fusion devices.
The trapping and release of hydrogenic particles is a crucial issue in the application of tungsten (W) as plasma-facing materials (PFM) in current fusion devices. To study the interaction of ...low-energy atomic hydrogen with W surface, molecular dynamics (MD) simulations were performed adopting a bond-order interatomic potential. Energetic hydrogen atoms with a series of energies ranged from 0.5
eV to 50
eV were projected on the W (0
0
1) surface at normal incidence. The particle and energy reflection coefficients as well as mean range distribution as a function of incident energy have been discussed. Also calculated were the interstitial hydrogen formation energies and the migration energy. For an interstitial hydrogen atom in W, the most favorable configuration is in the tetrahedral position and the migration energy is obtained as about 0.30
eV, in agreement with experimental data.
Abstract
Measurements of the turbulent density wavenumber spectrum, δn(k
⊥
), using the Doppler Back-Scattering (DBS) diagnostic are reported from DIII-D H-mode plasmas with electron cyclotron ...heating (ECH) as the only auxiliary heating method. These electron-heated plasmas have low collisionality, ν
*
e
< 1, T
e
/T
i
> 1, and zero injected torque – a regime expected to be relevant for future fusion devices. We probe density fluctuations in the core (ρ ≈ 0.7) over a broad wavenumber range, 0.5 ≤ k
⊥
≤ 16 cm
−1
(0.1 ≤ k
⊥
ρ
s
≤ 5) to characterize plasma instabilities and compare with theoretical predictions. We present a novel synthetic DBS diagnostic to relate the back-scattered power spectrum, P
s
(k
⊥
) – which is directly measured by DBS – to the underlying electron density fluctuation spectrum, δn(k
⊥
). The synthetic DBS P
s
(k
⊥
) spectrum is calculated by combining the SCOTTY beam-tracing code with a model δn(k
⊥
) predicted either analytically or numerically. In this work we use the quasi-linear code TGLF to approximate the δn(k
⊥
) spectrum. We find that TGLF, using the experimental profiles, is capable of closely reproducing the DBS measurements. Both the DBS measurements and the TGLF-DBS synthetic diagnostic show a wavenumber spectrum with variable decay. The measurements show weak decay (k
−0.6
) for k < 3.5 cm
−1
, with k
−2.6
at intermediate-k (3.5 ≤ k ≤ 8.5 cm
−1
), and rapid decay (k
−9.4
) for k > 8.5 cm
−1
. Scans of physics parameters using TGLF suggest that the normalized ∇T
e
scale-length, R/LT
e
, is an important factor for distinguishing microturbulence regimes in these plasmas. A combination of DBS observations and TGLF simulations indicate that fluctuations remain peaked at ITG-scales (low k) while R/LT
e
-driven TEM/ETG-type modes (intermediate/high k) are marginally sub-dominant.
Abstract
We report on the observation of spatially asymmetric turbulent structures with a long radial correlation length in the core of high-collisionality \emph{H}-mode plasmas on DIII-D tokamak. ...These turbulent structures develop from shorter wavelength turbulence and have a radially elongated structure. The envelope of turbulence spans a broad radial range in the mid-radius region, leading to streamer-like transport events. The underlying turbulence is featured by intermittency, long-term memory effect, and the characteristic spectrum of self-organized criticality. The amplitude and the radial scale increase substantially when the shearing rate of the mean flow is reduced below the turbulent scattering rate. The enhanced LRRC transport events are accompanied by apparent normalized energy confinement time degradation. The emergence of such LRRC transport events may serve as a candidate explanation for the degrading nature of H-mode core plasma confinement at high-collisionality on DIII-D tokamak.
Measurements of the turbulent density wavenumber spectrum, δne(k⊥), using the Doppler Back-Scattering (DBS) diagnostic are reported from DIII-D H-mode plasmas with electron cyclotron heating (ECH) as ...the only auxiliary heating method. These electron-heated plasmas have low collisionality, ν*e < 1, Te/Ti > 1, and zero injected torque – a regime expected to be relevant for future fusion devices. We probe density fluctuations in the core (ρ ≈ 0.7) over a broad wavenumber range, 0.5 $\leqslant$ k⊥ $\leqslant$ 16 cm–1 (0.1 $\leqslant$ k⊥ρs $\leqslant$ 5) to characterize plasma instabilities and compare with theoretical predictions. We present a novel synthetic DBS diagnostic to relate the back-scattered power spectrum, Ps(k⊥) – which is directly measured by DBS – to the underlying electron density fluctuation spectrum, δne(k⊥). The synthetic DBS Ps(k⊥) spectrum is calculated by combining the SCOTTY beam-tracing code with a model δne(k⊥) predicted either analytically or numerically. In this work we use the quasi-linear code TGLF to approximate the δne(k⊥) spectrum. We find that TGLF, using the experimental profiles, is capable of closely reproducing the DBS measurements. Both the DBS measurements and the TGLF-DBS synthetic diagnostic show a wavenumber spectrum with variable decay. The measurements show weak decay (k–0.6) for k < 3.5 cm–1, with k–2.6 at intermediate-k (3.5 $\leqslant$ k $\leqslant$ 8.5 cm–1), and rapid decay (k–9.4) for k > 8.5 cm–1. Scans of physics parameters using TGLF suggest that the normalized ∇Te scale-length, R/LTe, is an important factor for distinguishing microturbulence regimes in these plasmas. A combination of DBS observations and TGLF simulations indicate that fluctuations remain peaked at ITG-scales (low k) while R/LTe-driven TEM/ETG-type modes (intermediate/high k) are marginally sub-dominant.
Abstract
The integration of a high-performance core and a dissipative divertor or the so-called “core-edge integration” has been widely identified as a critical gap for the design of future fusion ...reactors. In this letter, we report, for the first time, the direct experimental evidence of electron turbulence at DIII-D H-mode pedestal that correlates with the broadening of the pedestal and thus facilitates core-edge integration. In agreement with gyrokinetic simulations, this electron turbulence is enhanced by high ηe (ηe =Ln/LTe, where Ln is the density scale length and LTe is the electron temperature scale length), which is due to a strong shift between density and temperature pedestal profiles associated with a closed divertor. The modeled turbulence drives significant heat transport with lower pressure gradient and that may broaden the pedestal wider than the empirical and theory-predicted pedestal width scaling. Such a wide pedestal, coupled with a closed divertor, enables us to achieve a good core-edge scenario which integrates high-temperature low-collisionality pedestal (pedestal top temperature Te,ped>0.8keV and pedestal top collisionality *ped <1) with detached divertor conditions. This paves a new path in solving core-edge integration issue for future fusion reactors.
Abstract Sawteeth are one of the concerning instabilities in ITER and future burning plasma experiments. Sawtooth dynamics and its interaction with broadband plasma turbulence has been a challenge ...for predictive simulations of core transport in future fusion devices. This study provides new observations of core turbulence behavior during sawtooth oscillations in DIII-D hydrogen L-mode neutral beam injection heated plasmas in an inner wall limited configuration. A strong correlation of electron temperature and density turbulence levels with the sawtooth oscillation phase has been observed at locations inside the T e inversion radius and/or safety factor q = 1 magnetic surface. The T e turbulence amplitude in the core during the sawtooth ramp exhibits a critical T e gradient behavior inside but not near the T e inversion radius/ q = 1 magnetic surface. The most unstable mode calculated from the trapped gyro-landau fluid turbulence simulations reveal a change from low- k ion-type to low-k electron-type modes from pre- to post- sawtooth crash time periods.
Abstract We report on comprehensive experimental studies of turbulence spreading in edge plasmas. These studies demonstrate the relation of turbulence spreading and entrainment to intermittent ...convective density fluctuation events or bursts (i.e. blobs and holes). The non-diffusive character of turbulence spreading is thus elucidated. The turbulence spreading velocity (or mean jet velocity) manifests a linear correlation with the skewness of density fluctuations, and increases with the auto-correlation time of density fluctuations. Turbulence spreading by positive density fluctuations is outward, while spreading by negative density fluctuations is inward. The degree of symmetry breaking between outward propagating blobs and inward propagating holes increases with the amplitude of density fluctuations. Thus, blob-hole asymmetry emerges as crucial to turbulence spreading. These results highlight the important role of intermittent convective events in conveying the spreading of turbulence, and constitute a fundamental challenge to existing diffusive models of spreading.
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