Isotope Effect and Multiscale Physics in Fusion Plasmas

Abstract

The mechanism governing the impact of the mass isotope on plasma confinement is still one of the main scientific conundrums facing the magnetic fusion community after more than thirty years of intense research. We have investigated the properties of local turbulence and long-range correlations in hydrogen and deuterium plasmas in the TEXTOR tokamak. Experimental findings have shown a systematic increasing in the amplitude of long-range correlations during the transition from hydrogen to deuterium dominated plasmas. These results provide the first direct experimental evidence of the importance of multiscale physics for unraveling the physics of the isotope effect in fusion plasmas. Introduction.-There is clear experimental evidence that at comparable plasma discharge parameters deuterium (D) discharges have improved confinement properties as compared with hydrogen (H) ones [1,2]. The isotope effect has been observed in many different tokamaks under different plasma conditions with a degree of confinement improvement in energy, particle, and momentum depending on plasma regimes. Interestingly, the isotope effect seems to be weaker, and eventually reversed, in stellarators than in tokamaks [1,2]. Understanding the physics of the isotope effect in plasma transport and confinement remains a fundamental open question confronting the fusion community since more than 30 years of intense research with direct impact in the confinement properties of fusion D-T reactors.Considering that the characteristic step size of collisional transport and turbulent structures both increase with the plasma gyroradius s [3], increasing the mass of the isotope would imply a deleterious effect on transport. Then assuming an estimation of the plasma diffusivity (D 0 ) as the ratio of the square of a characteristic radial scale length (L r ) over a characteristic time scale ( c ), D 0 / L 2 r = c , the isotope effect is a counterintuitive phenomenon if the typical radial length scales as L r % s . In addition, while Bohm and gyro-Bohm behaviors are widely used to describe the empirical confinement time, they have, however, the wrong isotopic mass dependence.Contemporary studies of transport phenomena in a wide range of research areas, including atmospheric flows [4], astrophysics [5], and fusion plasmas [6], have identified a common and fundamental feature of the physics of farfrom equilibrium systems-the ''multiscale'' physics, i.e., how large-scale structures can be developed by small-scale

Isotope effect on filament dynamics in fusion edge plasmas

“…In various tokamak experiments the confinement properties have been shown to scale favourably with increasing mass of the main (fusion relevant) ion plasma species, specifically hydrogen isotopes and helium [1,2,3,4]. The radial cross-field transport of coherent filamentary structures (commonly denoted "blobs") in the scrape-off layer (SOL) of tokamaks accounts for a significant part of particle and heat losses [5,6] to the plasma facing components.…”

Microturbulence study of the isotope effect

“…In recent TEXTOR experiments, 8 it has been measured that the correlation length in the poloidal direction of the electric potential increases with the effective mass in H/D plasmas. Thus, it is likely that the zonal flows became stronger and the turbulence was reduced.…”

Impurity effects on residual zonal flow in deuterium (D)-tritium (T) plasmas

“…Furthermore, we find that non-trace impurities can weaken the trend of stronger residual ZF level in D-T plasmas with higher , i eff A , but trace W does not affect the trend. From experiment, the amplitude of long-range correlation (LRC) is increased during the transition from H to D plasmas in TEXTOR [39]. Therefore, it is also worth investigating the effects of impurities on the amplitude of LRC in different hydrogenic isotope plasmas by injecting medium-mass impurities.…”