Stability limits for tokamak plasma with negative triangularity
Negative triangularity tokamak plasmas are subject of an increased interest both in existing experiments and in studies of core physics as well as power handling relevant to the fusion demonstration power reactor. The ideal MHD stability calculations for the negative triangularity plasma extend the investigation of the TCV tokamak beta limits and edge stability to double null shapes, lower aspect ratio and negative shear. In case of the negative triangularity, the 2nd stability access is closed for ballooning modes and a well defined limiting pressure gradient profile exists in the positive shear region. The external kink mode stability limit can be obtained by rescaling the limiting profiles. H-mode discharges with upper negative triangularity in TCV demonstrate significant mitigation of type I ELM peak power losses. This is consistent with lower edge stability limits for the pedestal. For double null negative triangularity configurations higher pedestals can be stable due to higher shear at low pedestal current density. At the same time a destabilization of a whole range of fixed boundary modes takes place, once the Mercier criterion for localized modes is violated, implying possible changes in the ELM characteristics. In particular, the diamagnetic stabilization on the edge stability is much less effective than in a standard case when peeling-ballooning modes set pedestal height limit.
Beta limits and Edge Stability for Negative Triangularity Plasma in TCV Tokamak
In L-mode TCV tokamak discharges the electron heat transport has been found to decrease towards negative triangularity [1], leading to a factor of two improvement in confinement from δ = 0.4 to -0.4. This trend of confinement with triangularity in L-mode stands in opposition to H-mode confinement, where the confinement improves with increasing positive triangularity (ASDEX-UPGRADE, JET), due to pedestal pressure increase with positive triangularity. Also, ELM induced heat loads are a severe threat for the first wall of a reactor. It is thus crucial to find ways to reduce or to avoid large ELMs. Varying the triangularity, inclusive over the negative triangularity range, will bring new elements to H-mode physics and transport. The beta limits against both localized Mercier and ballooning modes and global pressure driven kink modes are lower for negative triangularity configurations. However, it gives a possibility to study resistive wall modes at lower power. When the X-point is placed on the LFS, where the curvature of the magnetic field lines is unfavorable, the current density needed to access the second stability domain increases significantly and may even exclude it altogether. The edge kink-ballooning mode stability limits follow the changes in the high-n limits behavior [2]. It gives lower pressure pedestal height attainable in the negative triangularity configurations. It could potentially lead to different ELM behaviors in positive and negative triangularity configurations.
Computational spectroscopy of carbon monoxide isotopomers in helium clusters.
The rotational excitation spectrum, including the vibrational shift of the rotational band, of several CO isotopomers solvated in He clusters has been calculated. Reptation quantum Monte Carlo simulations are used in conjunction with an accurate He-CO potential energy surface, which quantitatively describes the rovibrational spectrum of the binary complex. Our simulations, when compared with number-selective infrared spectra taken for different isotopomers, help discriminate among the alternative assignments proposed for cluster sizes around 15 He atoms. The origin of the vibrational band has a red shift that is nearly linear with the cluster size within the first solvation shell and is almost constant up to the largest cluster studied, well beyond completion of the second solvation shell. A blue upturn at even larger sizes would be needed to attain the nanodroplet limit, as recently estimated from the isotopic dependence of the measured R(0) transitions.