Overview of the isotope effects in the ASDEX Upgrade tokamak

Abstract

In recent years, measurements on the ASDEX Upgrade tokamak and modelling performed for plasmas with hydrogen (H) and deuterium (D) as the main gas have improved our understanding of the ion mass dependencies in fusion plasmas. The observed isotope effects can be explained with established physics processes which highlight the importance of treating heat transport with coupled electron and ion heat channels. In the core of electron heated L-mode plasmas, the mass dependence of the electron–ion equipartition results in a reduction of q i / q e with increasing ion mass. Combined with higher profile stiffness in the ions compared to the electrons, this results in improved core confinement for higher ion masses. At the edge of L-mode plasmas where a higher collisionality is observed, parallel electron dynamics is fundamental for turbulence. The parallel electron dynamics term in the gyrokinetic equations directly depends on m i / m e , resulting in a different kinetic response with different ion mass. Higher turbulent fluxes are expected with lower ion mass. This is consistent with the difference in L n e observed in the experiment. The mass dependence of turbulent transport in the L-mode edge has direct consequences for the L–H transition. More heating power is required to enter the H-mode at lower mass ( P L-H H ∼ 2 P L-H D ). This is expected if the critical E × B shearing rate γ E × B is important for the transition from L to H mode. In the H-mode pedestal, γ E × B remains important to regulate the turbulent transport. The electrons do not contribute to γ E × B and the enhanced equipartition for lower ion masses causes a shift from the ion channel to the electron channel in the absolute heat fluxes. Consequently, the inter edge localised mode (ELM) transport is found to be higher with lower isotope mass. This enhanced transport in H can prevent the pedestal from reaching the peeling–ballooning stability boundary with engineering parameters where D plasmas are peeling–ballooning unstable. Increasing the triangularity reduces the inter ELM transport in H stronger than in comparable D plasmas. For matched pedestal top and matched heat sources, the core heat transport is found to be similar for H and D when the fast-ion content is low. When ion temperature gradient turbulence stabilisation by fast ions becomes relevant, the mass dependent fast-ion slowing down results in higher fast-ion content in D and therefore in a reduction of ion heat transport in the core. Then, even for matched pedestals τ E D > τ E H .

Validation of quasi-linear turbulent transport models against plasmas with dominant electron heating for the prediction of ITER PFPO-1 plasmas

“…As shown in figure 15, in the predictions of the ion temperature, the replacement of hydrogen mass with deuterium mass leads to a small increase of the TGLF ion heat conductivity in the confinement region, by about 10% on average in the present AUG dataset. This weak positive dependence can be still considered to be acceptable in reproducing the AUG experimental observations in these conditions of dominant electron heating, where the experimental power balance ion heat conductivities are observed not to show a strong dependence on mass, consistent with previous studies [47,51]. This behaviour of the TGLF model is also satisfactory in H-mode conditions with comparable ion and electron heating, where the TGLF modelling of H plasmas with H mass only leads to a small overprediction of the ion temperature [46].…”

“…The database contains AUG plasmas in hydrogen as well as in deuterium which allows one to examine the predicted mass dependence in ASTRA-TGLF simulations. Experimentally, isotope effects in ASDEX Upgrade were extensively studied in [47] where several isotope mass dependent mechanisms were identified which can influence the energy confinement. Considering the presence of such a mass dependence in turbulent transport, it is considered to be problematic for ITER PFPO-1 simulations that TGLF is exclusively calibrated on deuterium plasmas although all discharges in PFPO-1 will be performed either in hydrogen or in helium [25].…”

Confinement properties of L-mode plasmas in ASDEX Upgrade and full-radius predictions of the TGLF transport model

“…The ion heating by thermal equipartition is stronger in H, due to the inverse mass dependence of the thermal exchange term, which was already identified to be the dominant effect which explains differences in core confinement in H and D plasmas in AUG [67]. The TGLF-SAT2 simulation results also point out that the TGLF-SAT2 mass dependence of ion heat transport has properties which more closely follow a gyroBohm dependence, whereas the experiment has a behaviour which is closer to Bohm like [68].…”

Overview of initial negative triangularity plasma studies on the ASDEX Upgrade tokamak

“…Clear turbulence activity is observed in the inter-ELM periods, which is sometimes interrupted by comparably quiescent behavior. This kind of inter-ELM activity has been observed in AUG H-modes [74,75].…”