Isotope dependence of energy, momentum and particle confinement in tokamaks

Abstract

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 ${\tau _E}\sim {A^{0.5}}$ (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, ${\tau _E}\sim {A^{ - 0.5}}$ , 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 ${\tau _E}\sim {A^{0.14}}$ 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.

Isotope dependence of the type I ELMy H-mode pedestal in JET-ILW hydrogen and deuterium plasmas

“…In type I ELMy H-mode plasmas a positive scaling of τ E,th with the isotope mass was observed: τ E,th ∝ A 0.4 [1,18,19]. The gradient length (R/L T , where R is the major radius, L T = T /∇T and T is the temperature) of core temperature profiles was similar in H and D, despite large variations in the heat flux [1,18,19]. This suggests that the isotope effect may originate in the pedestal in conditions where ions and electrons are collisionally coupled and electron and/or ion transport are stiff [1].…”

Overview of the isotope effects in the ASDEX Upgrade tokamak

“…In most observations, the isotope effects manifest in a reversal of the ideal gyroBohm scaling, i.e. the confinement improves with mass [3]. Consequently, when studying different isotope masses, the gyroBohm scaling can be a poor frame of reference and might complicate the interpretation of observations.…”

“…Then, even for matched pedestals τ D E > τ H E . 3 See author list of Meyer H et al 2019 Nucl. Fusion 59 112014 4 See author list of Labit B et al 2019 Nucl.…”

A new quasilinear saturation rule for tokamak turbulence with application to the isotope scaling of transport

“…for k y > 0, where c 0 and c 1 are case-dependent parameters. When allowing these parameters to be fitted to the NL CGYRO data, as shown for various cases in figure 8, it can be seen that the two curves adhere to one another extremely closely 5 . Per case, this constitutes only two fitted parameters for 39 data points, rendering there no question of over-fitting.…”