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.
The REX-ISOLDE project
The Radioactive Beam Experiment REX-ISOLDE [1–3] is a pilot experiment at ISOLDE (CERN) testing the new concept of post acceleration of radioactive ion beams by using charge breeding of the ions in a high charge state ion source and the efficient acceleration of the highly charged ions in a short LINAC using modern ion accelerator structures. In order to prepare the ions for the experiments singly charged radioactive ions from the on-line mass separator ISOLDE will be cooled and bunched in a Penning trap, charge bred in an electron beam ion source (EBIS) and finally accelerated in the LINAC. The LINAC consists of a radiofrequency quadrupole (RFQ) accelerator, which accelerates the ions up to 0.3 MeV/u, an interdigital H-type (IH) structure with a final energy between 1.1 and 1.2 MeV/u and three seven gap resonators, which allow the variation of the final energy. With an energy of the radioactive beams between 0.8 MeV/u and 2.2 MeV/u a wide range of experiments in the field of nuclear spectroscopy, astrophysics and solid state physics will be addressed by REX-ISOLDE.