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Register a new userMultiple-isotope pellet cycles captured by turbulent transport modelling in the JET tokamak
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For the first time the pellet cycle of a multiple-isotope plasma is successfully reproduced with reduced turbulent transport modelling, within an integrated simulation framework. Future nuclear fusion reactors are likely to be fuelled by cryogenic pellet injection, due to higher penetration and faster response times. Accurate pellet cycle modelling is crucial to assess fuelling efficiency and burn control. In recent JET tokamak experiments, deuterium pellets with reactor-relevant deposition characteristics were injected into a pure hydrogen plasma. Measurements of the isotope ratio profile inferred a Deuterium penetration time comparable to the energy confinement time. The modelling successfully reproduces the plasma thermodynamic profiles and the fast deuterium penetration timescale. The predictions of the reduced turbulence model QuaLiKiz in the presence of a negative density gradient following pellet deposition are compared with GENE linear and nonlinear higher fidelity modelling. The results are encouraging with regard to reactor fuelling capability and burn control.
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Core turbulent particle transport with multiple isotopes can display observable differences in behaviour between the electron and ion particle channels. Experimental observations at JET with mixed H-D plasmas and varying NBI and gas-puff sources [M. Maslov et al., Nucl. Fusion 7 076022 (2018)] inferred source dominated electron peaking, but transport dominated isotope peaking. In this work, we apply the QuaLiKiz quasilinear gyrokinetic transport model within JINTRAC flux-driven integrated modelling, for core transport validation in this multiple-isotope regime. The experiments are successfully reproduced, predicting self consistently $ j $, $ n_{e} $, $ n_{Be} $, $ T_{e} $, $ T_{i} $, $omega_{tor}$ and the isotope composition. As seen in the experiments, both H and D profiles are predicted to be peaked regardless of the core isotope source. An extensive sensitivity study confirmed that this result does not depend on the specific choices made for the boundary conditions and physics settings. While kinetic profiles and electron density peaking did vary depending on the simulation parameters, the isotope ratio remained nearly invariant, and tied to the electron density profile. These findings have positive ramifications for multiple-isotope fuelling, burn control, and helium ash removal.
Nonlinear gyrokinetic simulations have been conducted to investigate turbulent transport in tokamak plasmas with rotational shear. At sufficiently large flow shears, linear instabilities are suppressed, but transiently growing modes drive subcritical turbulence whose amplitude increases with flow shear. This leads to a local minimum in the heat flux, indicating an optimal E x B shear value for plasma confinement. Local maxima in the momentum fluxes are also observed, allowing for the possibility of bifurcations in the E x B shear. The sensitive dependence of heat flux on temperature gradient is relaxed for large flow shear values, with the critical temperature gradient increasing at lower flow shear values. The turbulent Prandtl number is found to be largely independent of temperature and flow gradients, with a value close to unity.
The results of flux-driven, two-fluid simulations in single-null configurations are used to investigate the processes determining the turbulent transport in the tokamak edge. Three turbulent transport regimes are identified: (i) a developed transport regime with turbulence driven by an interchange instability, which shares a number of features with the standard L-mode of tokamak operation, (ii) a suppressed transport regime, characterized by a higher value of the energy confinement time, low-amplitude relative fluctuations driven by a Kelvin-Helmholtz instability, a strong E x B sheared flow, and the formation of a transport barrier, which recalls the H-mode, and (iii) a degraded confinement regime, characterized by a catastrophically large interchange-driven turbulent transport, which reminds the crossing of the Greenwald density limit.We derive an analytical expression of the pressure gradient length in the three regimes. The transition from the developed to the suppressed transport regime is obtained by increasing the heat source or decreasing the collisionality and vice versa for the transition from the developed transport regime to the degraded confinement regime. An analytical expression of the power threshold to access the suppressed transport regime, linked to the power threshold for H-mode access, as well as the maximum density achievable before entering the degraded confinement regime, related to the Greenwald density, are also derived. The experimental dependencies of the power threshold for H-mode access on density, tokamak major radius, and isotope mass are retrieved. The analytical estimate of the density limit contains the correct dependence on the plasma current and on the tokamak minor radius.
Understanding of the transport in a Tokamak plasma is an important issue. Various mechanisms have been reported in the literature to relate the core phenomenon to edge phenomenon. Sawtooth and Mirnov oscillations caused by MHD instabilities are generally observed in Tokamak discharges. Observation of these effects in the visible radiation from outer edge may offer a possible means to understand the transport.Oscillations in the visible radiation from outer region of the plasma have been observed during recent Aditya discharges. Percentage modulation of these oscillations vary with the Lines of Sight (LOS) of the chords and surfaces on which they terminate. This has been found in both the low frequency (~1 kHz) oscillations that seem to correlate with sawteething in SXR signals and the higher frequency (~10 kHz) oscillations that correlate well with Mirnov signals indicative of m/n=2/1 mode rotation. This suggests that the extent to which the MHD instabilities in the central region of the plasma column are reflected in the edge radiation depends on the interaction of the plasma with the surface at the extremity of the LOS. The release of particle/ energy accompanying the MHD instabilities leads to a large influx of particles from such surfaces. Cross-bispectral analysis suggests that a mode (having frequency of ~20 kHz) is also generated due to the interaction of m/n=1/1 (~10 kHz, seen in SXR) and m/n=2/1 (~10 kHz, seen in Mirnov, Visible & Microwave Interferometer signals). By possible selection rules, this mode seems to be a m/n=3/2 mode. This mode is seen in Mirnov, Visible & Interferometer signals. Behaviour of these oscillations on various LOS and their relation to SXR&Mirnov signals can lead to an understanding of the transport phenomenon. These observations and our interpretations will be presented.
Long-lived, highly localized structures called palm tree modes (PTM) are observed in the edge plasma of the JET tokamak. Although PTMs are well documented, little is known about the mechanisms which produce these structures. In the case of the PTM, an ELM-postcursor, its genesis is usually explained by ergodisation of the magnetic field due to edge localized modes and the appearance of a seed magnetic island which evolves into a PTM later. In this study we try to invoke a creation mechanism based on the concepts and observations in edge plasma turbulence. An interesting aspect of plasma turbulence is the occurrence of coherent, long-lived structures in the scrape-off-layer (SOL). These localized and magnetic-field-aligned regions with higher or lower plasma densities are called blobs and holes. Measurements show that these filaments carry parallel currents. We thus here interpret ELM-filaments as massive blobs and the interspace between these filaments as holes. We demonstrate that a forward-modelled closed current filament on a q=3 surface produces similar magnetic fluctuations as measured by the JET in-vessel magnetic pickup coils if a PTM is present. From that we deduce that if a hole is captured on a q=3 surface after an ELM-crash, a PTM equivalent signature is generated. If the ELM-filament itself is captured on a q=4 surface, a signature equivalent to an outer mode appears.
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