Isotope effects on L-H threshold and confinement in tokamak plasmas

Abstract

The dependence of plasma transport and confinement on the main hydrogenic ion isotope mass is of fundamental importance for understanding turbulent transport and, therefore, for accurate extrapolations of confinement from present tokamak experiments, which typically use a single hydrogen isotope, to burning plasmas such as ITER, which will operate in deuterium-tritium mixtures. Knowledge of the dependence of plasma properties and edge transport barrier formation on main ion species is critical in view of the initial, low-activation phase of ITER operations in hydrogen or helium and of its implications on the subsequent operation in deuterium-tritium. The favourable scaling of global energy confinement time with isotope mass, which has been observed in many tokamak experiments, remains largely unexplained theoretically. Moreover, the mass scaling observed in experiments varies depending on the plasma edge conditions. In preparation for upcoming deuterium-tritium experiments in the JET tokamak with the ITER-like Be/W Wall (JET-ILW), a thorough experimental investigation of isotope effects in hydrogen, deuterium and tritium plasmas is being carried out, in order to provide stringent tests of plasma energy, particle and momentum transport models. Recent hydrogen and deuterium isotope experiments in JET-ILW on L-H power threshold, L-mode and H-mode confinement are reviewed and discussed in the context of past and more recent isotope experiments in tokamak plasmas, highlighting common elements as well as contrasting observations that have been reported. The experimental findings are discussed in the context of fundamental aspects of plasma transport models.

Isotope identity experiments in JET-ILW with H and D L-mode plasmas

“…The scaled energy confinement times of the H and D isotope identity pair yield Ω i τ E,th ~ A 0.05±0.1 , indicating no isotope mass dependence for the dimensionless thermal energy confinement time. We may therefore ask if this result is consistent with the finding τ E,th ~ A 0.15±0.02 derived from the JET-ILW L-mode power scans (with NBI) at constant n e , B T and I P in H and D [7]. We first note that the atomic mass exponents in the two scalings may be different.…”

“…In dimensional L-mode experiments in JET-C, with NBI heating (6 MW) and limiter discharges (3.1 MA/2.9 T), f saw (H) was approximately twice than f saw (D) [14]. In the JET-ILW L-mode NBI power scans at constant density in H and D, at 2.5MA/3.0T, reported in [7], the sawteeth were more frequent in H than in D at low P NBI , while f saw became similar in the two isotopes at the highest powers in the scan (7-9 MW). These different results warrant further investigation, which is however outside the scope of this paper.…”

Isotope effects in self-organization of internal transport barrier and concomitant edge confinement degradation in steady-state LHD plasmas

“…In the case of the ETB, there are several well-defined criteria, i.e., a sharp drop of the edge recycling emission, the pedestal profile formation at the edge, and others 2 . Thanks to them, the diagram of the transition condition is routinely reproduced 5–7 . In contrast, no commonly used definition of the ITB exists so far, although some practical approaches, i.e., the major radius divided by the temperature gradient length exceeding a critical value or a sharply suppressed diffusion coefficient at some specific regimes, are used to identify the ITB structure 3,8–13 .…”

“…In particular, the hydrogen isotope effect is more obvious in properties of the ETB. For example, the threshold power above which the ETB formation occurs is lowered by a factor with the heavier hydrogen isotope fuel 5–7 . In contrast, isotope effect in ITB has not been systematically discussed to date.…”

Dependence of the turbulent particle flux on hydrogen isotopes induced by collisionality

“…The understanding of the differences in the properties of transport and confinement of tokamak plasmas when changing the main ion species is a critical component towards the capability of predicting of fusion reactor plasma, where a mixture of deuterium D and tritium T will be used. Many studies have been dedicated to the comparison of the global confinement properties when changing the hydrogen (H) isotope as main ion species [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16], with increasing emphasis recently also dedicated to the understanding of changes in the transport properties more than just in the global confinement, also from a theoretical perspective [17][18][19][20]. The observation that the confinement increases with increasing mass of the H isotope is in contraddiction with simple expectations based on an overall gyro-Bohm scaling of local transport, but allows more optimistic expectations when considering the D-T fuel mixture as compared to present experiments which are predominantly performed in D. The recent and the forthcoming campaigns at JET with different H isotopes, including T, have strongly renewed the interest in achieving a deeper understanding of the physics of transport and confinement related to a change of the H isotope as main ion.…”