TJ-II: Density limit and ZFs in TJ-II
Experimental campaign
2020 Spring
Proposal title
TJ-II: Density limit and ZFs in TJ-II
Name and affiliation of proponent
Ulises Losada, Daniel Fernández-Ruiz, Carlos Hidalgo
Details of contact person at LNF
Carlos Hidalgo
Description of the activity
Density limit is manifested in tokamaks, stellarators and RFPs. In stellarator the density limit is related to radiation collapse mechanisms [1] whereas in tokamaks edge transport can play an important role [2].
LRC in floating potential fluctuations has been investigated in the proximity of the density-limit in the NBI plasma scenarios in TJ-II. The density limit decreases with the magnetic field (B = 0.7 – 1 T) in qualitative agreement with the empirical stellarator scaling law for density limit [5]. At low densities the LRCs, measured at rho ≈ 0.9 for frequencies below 20 kHz, are quite large (≈ 0.8) and vary only slightly with increasing density. When approaching the density-limit the amplitude of LRC reduces rapidly with increasing plasma density, in agreement with previous results [3]. The reduction in the LRC as approaching the density limit is accompanied by a reduction in edge mean radial electric field and the level of plasma turbulence. Results point to the role of collisionality, mean E × B flows and level of turbulence on the amplitude of zonal flows in the proximity of the density limit. TJ-II findings suggest that at a threshold radiation value for the density limit the degradation of confinement would be partially due to the damping of Zonal Flows as recently pointed out [4]. If this is the case, density limit in stellarators would depend on the transport optimization criteria, a prediction that could be validated experimentally.
International or National funding project or entity
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Description of required resources
Required resources:
- Number of plasma discharges or days of operation: 1
- Essential diagnostic systems: Dual probes for LRC studies
- Type of plasmas (heating configuration):NBI + ECRH (edge heating)
- Specific requirements on wall conditioning if any:
- External users: need a local computer account for data access: yes/no
- Any external equipment to be integrated? Provide description and integration needs:
Preferred dates and degree of flexibility
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