Collisionality dependence and ion species effects on heat transport in He and H plasma, and the role of ion scale turbulence in LHD

Abstract

The surveys of the ion and electron heat transports of Neutral Beam (NB) heating plasma were carried out by power balance analysis in He and H rich plasma of LHD. Collisionality was scanned by changing density and heating power. The characteristics of the transport vary depending on collisionality.In low collisionality with low density and high heating power, ion internal transport barrier (ITB) was formed. The ion heat conductivities (ci) is lower than electron heat conductivities (ce) in the core region at r < 0.7. On the other hand, in high collisionality with high density and low heating power, ci is higher than ce in the entire region of the plasma. These different confinement regimes are associated with different fluctuation characteristics. In ion ITB, fluctuation has a peak at r =0.7, and in normal confinement, fluctuation has a peak at r = 1.0. The two confinement mode changes gradually depending on the collisionality. The scan of concentration ratio between He and H were also performed. The ion confinement improvements were investigated by gyro-Bohm normalization taking account of the effective mass and charge. The concentration ratio affected the normalized ci only in the edge region ( r ~ 1.0 ). This indicates ion species effects vary depending on collisionality. Turbulence was modulated by the fast ion loss instability. The modulation of turbulence is higher in H rich than in He rich plasma.

Role of Helium–Hydrogen ratio on energetic interchange mode behaviour and its effect on ion temperature and micro-turbulence in LHD

“…Since the diagnostic measures only the component of the velocity perpendicular to the vertical probing beam, and since turbulence propagates in the poloidal direction (including E × B and diamagnetic rotation components), the component of measured phase velocity is high near the edge of the plasma, reducing towards zero at the tangency radius, and inverting on the other side. The phase velocity is dominated by E × B rotation, as has been shown in [9]. The key interpretation here is that turbulence phase velocity is dominated by E × B rotation and so this can be a monitor at high time resolution of changes in electric field.…”

“…The average turbulence level is the same in the H and He-rich discharges [9], and, because of the larger occurrence of bursts in H-rich discharges, where the fluctuations are suppressed, the fluctuation-level in-between bursts is in-fact higher. Comparing the electron temperature in Fig.…”

“…(IV B) and (d) a rough estimate of the fast ion loss current j r from the EIC events, obtained from PCI data, computed using Eq. (9). For increasing He concentration, the ion temperature increases and the EIC burst rate goes down.…”

“…A detailed analysis of the T i profile, given in [9], shows L −1 T i is rather constant (and the same in He and H discharges) in the core regions (ρ < 0.9), and increases in an "edge-pedestal" region. However, the height of the edge pedestal is larger in He discharges, leading by invariance of the core L −1 T i to a higher core T i .…”

“…(12), contributing to lower CX losses. This will impact on the power delivered to ions from the perpendicularly-injected beams and to our knowledge has not been included in the power balance analysis presented in [9,11].…”