756 research outputs found
Microturbulence studies in RFX-mod
Present-days Reversed Field Pinches (RFPs) are characterized by quasi-laminar
magnetic configurations in their core, whose boundaries feature sharp internal
transport barriers, in analogy with tokamaks and stellarators. The abatement of
magnetic chaos leads to the reduction of associated particle and heat transport
along wandering field lines. At the same time, the growth of steep temperature
gradients may trigger drift microinstabilities. In this work we summarize the
work recently done in the RFP RFX-mod in order to assess the existence and the
impact upon transport of such electrostatic and electromagnetic
microinstabilities as Ion Temperature Gradient (ITG), Trapped Electron Modes
(TEM) and microtearing modes.Comment: Work presented at the 2010 Varenna workshop "Theory of Fusion
Plasmas". To appear in Journal of Physics Conference Serie
Isotope dependence of energy, momentum and particle confinement in tokamaks
The isotope dependence of plasma transport will have a significant impact on the performance of future D-T experiments in JET and ITER and eventually on the fusion gain and economics of future reactors. In preparation for future D-T operation on JET, dedicated experiments and comprehensive transport analyses were performed in H, D and H-D mixed plasmas. The analysis of the data has demonstrated an unexpectedly strong and favourable dependence of the global confinement of energy, momentum and particles in ELMy H-mode plasmas on the atomic mass of the main ion species, the energy confinement time scaling as τE∼A0.5 (Maggi et al., Plasma Phys. Control. Fusion, vol. 60, 2018, 014045; JET Team, Nucl. Fusion, vol. 39, 1999, pp. 1227–1244), i.e. opposite to the expectations based only on local gyro-Bohm (GB) scaling, τE∼A−0.5 , and stronger than in the commonly used H-mode scaling for the energy confinement (Saibene et al., Nucl. Fusion, vol. 39, 1999, 1133; ITER Physics Basis, Nucl. Fusion, vol. 39, 1999, 2175). The scaling of momentum transport and particle confinement with isotope mass is very similar to that of energy transport. Nonlinear local GENE gyrokinetic analysis shows that the observed anti-GB heat flux is accounted for if collisions, E × B shear and plasma dilution with low-Z impurities (9Be) are included in the analysis (E and B are, respectively the electric and magnetic fields). For L-mode plasmas a weaker positive isotope scaling τE∼A0.14 has been found in JET (Maggi et al., Plasma Phys. Control. Fusion, vol. 60, 2018, 014045), similar to ITER97-L scaling (Kaye et al., Nucl. Fusion, vol. 37, 1997, 1303). Flux-driven quasi-linear gyrofluid calculations using JETTO-TGLF in L-mode show that local GB scaling is not followed when stiff transport (as is generally the case for ion temperature gradient modes) is combined with an imposed boundary condition taken from the experiment, in this case predicting no isotope dependence. A dimensionless identity plasma pair in hydrogen and deuterium L-mode plasmas has demonstrated scale invariance, confirming that core transport physics is governed, as expected, by the 4 dimensionless parameters ρ*, ν*, β, q (normalised ion Larmor radius, collisionality, plasma pressure and safety factor) consistently with global quasi-linear gyrokinetic TGLF calculations (Maggi et al., Nucl. Fusion, vol. 59, 2019, 076028). We compare findings in JET with those in different devices and discuss the possible reasons for the different isotope scalings reported from different devices. The diversity of observations suggests that the differences may result not only from differences affecting the core, e.g. heating schemes, but are to a large part due to differences in device-specific edge and wall conditions, pointing to the importance of better understanding and controlling pedestal and edge processes.EUROfusion Consortium grant agreement No 63305
On the mutual effect of ion temperature gradient instabilities and impurity peaking in the reversed field pinch
The presence of impurities is considered in gyrokinetic calculations of ion
temperature gradient (ITG) instabilities and turbulence in the reversed field
pinch device RFX-mod. This device usually exhibits hollow Carbon/Oxygen
profiles, peaked in the outer core region. We describe the role of the
impurities in ITG mode destabilization, and analyze whether ITG turbulence is
compatible with their experimental gradients.Comment: 19 pages, 9 figures, accepted for publication in Plasma Phys.
Control. Fusio
Neon seeding effects on two high-performance baseline plasmas on the Joint European Torus
We present the JETTO-QuaLiKiz-SANCO fully predictive modelling of two JET-ILW
high-performance baseline plasmas, a Ne seeded shot and an equivalent unseeded one. The
motivation of the work lies in the experimental observation of a slightly higher confinement and
performance of the Ne seeded shot with respect to the unseeded one, despite sharing the same
main plasma parameters and heating powers. Moreover, the neon seeded shot shows a lower
pedestal electron density and a higher core ion temperature with respect to the unseeded one.
Integrated modelling is performed in order to understand if the cause of the improved
confinement has to be ascribed to the improved pedestal parameters with neon seeding or if an
impurity-induced turbulence stabilization is at play. The QuaLiKiz transport model is used for
predicting the electron density, electron and ion temperatures and rotation in the core up to the
pedestal top, while the pedestal is empirically modelled to reproduce the experimental kinetic
profiles. The thermal diffusivities of the two shots, computed by QuaLiKiz, are compared, as
well as the turbulence spectra, suggesting that the reduced transport found in the neon seeded
shot is due in part to the stabilization of ion temperature gradient and electron temperature
gradient modes. Further modelling is performed in order to disentangle the neon seeding effects,
which are a direct effect on the turbulence stabilization and an indirect effect on the pedestal
parameters. The results suggest that the improved performance with neon is due to a
combination of turbulence stabilization and improved pedestal parameters
Impurity transport studies in multiple helicity and enhanced confinement QSH regimes in RFX-mod
The most recent impurity transport studies in RFX-mod are presented. Results from enhanced confinement quasi-single helicity (QSH) and multiple helicity (MH) are compared. The transport parameters are determined by comparing a 1-dimensional collisional radiative simulation with experimental spectroscopic data. Transport parameters obtained for Ni particles injected by the laser blow-off method are described and discussed in relation to results from Ne injection and previous studies
Equilibrium reconstruction for Single Helical Axis reversed field pinch plasmas
Single Helical Axis (SHAx) configurations are emerging as the natural state
for high current reversed field pinch (RFP) plasmas. These states feature the
presence of transport barriers in the core plasma. Here we present a method for
computing the equilibrium magnetic surfaces for these states in the force-free
approximation, which has been implemented in the SHEq code. The method is based
on the superposition of a zeroth order axisymmetric equilibrium and of a first
order helical perturbation computed according to Newcomb's equation
supplemented with edge magnetic field measurements. The mapping of the measured
electron temperature profiles, soft X-ray emission and interferometric density
measurements on the computed magnetic surfaces demonstrates the quality of the
equilibrium reconstruction. The procedure for computing flux surface averages
is illustrated, and applied to the evaluation of the thermal conductivity
profile. The consistency of the evaluated equilibria with Ohm's law is also
discussed.Comment: Submitted to Plasma Physics and Controlled Fusio
Turbulent transport stabilization by ICRH minority fast ions in low rotating JET ILW L-mode plasmas
The first experimental demonstration that fast ion induced stabilization of thermal turbulent transport takes place also at low values of plasma toroidal rotation has been obtained in JET ILW (ITER-like wall) L-mode plasmas with high ( 3 He)-D ICRH (ion cyclotron resonance heating) power. A reduction of the gyro-Bohm normalized ion heat flux and higher values of the normalized ion temperature gradient have been observed at high ICRH power and low NBI (neutral beam injection) power and plasma rotation. Gyrokinetic simulations indicate that ITG (ion temperature gradient) turbulence stabilization induced by the presence of high-energetic 3 He ions is the key mechanism in order to explain the experimental observations. Two main mechanisms have been identified to be responsible for the turbulence stabilization: a linear electrostatic wave-fast particle resonance mechanism and a nonlinear electromagnetic mechanism. The dependence of the stabilization on the 3 He distribution function has also been studied.</p
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