Isotope effects on transport in LHD

Abstract

Isotope effects are one of the most important issues for predicting future reactor operations. Large helical device (LHD) is the presently working largest stellarator/helical device using super conducting helical coils. In LHD, deuterium experiments started in 2017. Extensive studies regarding isotope effects on transport have been carried out. In this paper, the results of isotope effect studies in LHD are reported. The systematic studies were performed adjusting operational parameters and nondimensional parameters. In L mode like normal confinement plasma, where internal and edge transport barriers are not formed, the scaling of global energy confinement time (τ E) with operational parameters shows positive mass dependence (M 0.27; where M is effective ion mass) in electron cyclotron heating plasma and no mass dependence (M 0.0) in neutral beam injection heating plasma. The non-negative ion mass dependence is anti-gyro-Bohm scaling. The role of the turbulence in isotope effects was also found by turbulence measurements and gyrokinetic simulation. Better accessibility to electron and ion internal transport barrier (ITB) plasma is found in deuterium (D) plasma than in hydrogen (H). Gyro kinetic non-linear simulation shows reduced ion heat flux due to the larger generation of zonal flow in deuterium plasma. Peaked carbon density profile plays a prominent role in reducing ion energy transport in ITB plasma. This is evident only in plasma with deuterium ions. New findings on the mixing and non-mixing states of D and H particle transports are reported. In the mixing state, ion particle diffusivities are higher than electron particle diffusivities and D and H ion density profiles are almost identical. In the non-mixing state, ion particle diffusivity is much lower than electron diffusivity. Deuterium and hydrogen ion profiles are clearly different. Different turbulence structures were found in the mixing and non-mixing states suggesting different turbulence modes play a role.

Recent results from deuterium experiments on the large helical device and their contribution to fusion reactor development

“…The mechanism of this different tendency in ion thermal diffusivities at the high collisionality region is not clear yet. It must be noted that this discrepancy of χ i /Ω i from equation ( 8) at high collisionality will not affect the linear dependence of M in τ E Ω i scaling, because the contribution of the ion confinement channel to the global confinement property is small under the electron heating dominant condition by energetic N-NBI [22,23].…”

“…As mentioned in the previous section, the isotope effect on τ E of L-mode plasmas was investigated by comparing dimensionally similar plasmas of H and D ions [6,22,23]. In future fusion burning plasmas, D and T ions will coexist.…”

“…Here, dimensionally similar means that the dimensionless parameters, which are normalized Larmor radius (ρ * ), normalized collisionality (ν * ) and normalized pressure (β), are kept at the same value. In order to keep ρ * , ν * and β the same value, the magnetic field strength (B), density (n), and temperature (T ) must scale as B ∝ M 3/4 , n ∝ M, and T ∝ M 1/2 , respectively, where M is a mass number [6,[22][23][24][25]. For mixture plasmas, the effective mass (M eff ) is adopted as M. The main heating scenario of these experiments is electron-heating by high-energy tangential N-NBI because the plasma initiation at non-ECH-resonant magnetic field strength (1.64 T) is only possible by hydrogen tangential N-NBI on the LHD [2,26].…”

“…τ E ∼ a 2 /χ, and the mass dependence of equation ( 2) can be approximated by τ E Ω i ∝ M 1 , the normalized thermal diffusivity (χ/Ω i ) can be expressed as χ/Ω i ∝ M −1 . Thus, the ratio of normalized thermal diffusivity can be given by [22,23];…”

“…In figures 6(a) and (b), the normalized density fluctuation and density profiles are shown for a typical dimensionally similar pair of H and D plasmas at ν * = ∼0.2, respectively [22]. Here, density fluctuations are measured by a phase contrast imaging (PCI) diagnostic [31] and are normalized by the electron density of figure 6(b).…”