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Recent results of charge exchange recombination diagnostics on TCV : sawteeth resolved measurements and L-H transition studies.

par Caroline CHAMPENOIS - publié le

séminaire du laboratoire
lundi 6 novembre, 11:00, service 361 campus Saint-Jérôme

C. Marini

EPFL, Swiss Plasma Center (SPC), CH-1015 Lausanne, Switzerland.

abstract :

The charge exchange recombination spectroscopy (CXRS) diagnostic TCV (Tokamak à Configuration Variable) routinely measures carbon rotation, density and temperature. Recent upgrades of the CXRS diagnostic, with the development of the high throughput EDGE CXRS system and refurbishing of the legacy systems cameras, allow measurements of unprecedented accuracy in TCV, where a low power diagnostic neutral beam injector permits localised measurements of intrinsic plasma rotation.
The upgraded CXRS diagnostic is used to study the impact of sawteeth (ST) on plasma rotation in L-mode limited configurations and on the changes in edge impurity parameters at the onset of the H-mode in diverted discharges. The evolution of plasma rotation and kinetic profiles during an averaged ST are studied with a 2 ms time resolution for ST periods ranging from 8 to 36 ms, obtained by locking the ST period with the electron cyclotron heating modulation. The plasma rotation profile suddenly changes at the ST crash, from a peaked profile in the counter-current direction before the crash to a hollow co-current profile after the crash, showing evidence of a co-current torque of increasing magnitude with the ST period. This is in agreement with the 1/Ip scaling observed in TCV L-mode plasmas [1], where the maximum rotation is limited by ST dynamics. The crash is also characterised by expulsion of carbon from the core, resulting in hollow post-crash carbon density profiles.
The EDGE system allows accurate measurements of the changes in impurity poloidal flow across the transition from L-mode to ELM-free H-mode. A narrow and deep poloidal rotation well develops at the last closed flux surface at the transition, together with a density pedestal and an increase of the impurity temperature. From the kinetic profiles the radial electric field Er is computed, revealing the development of the well characteristic of H-mode as observed in other devices, whose width agrees with inter-machine scaling [2]. The evolution of Er components suggests involvement of the poloidal rotation in the triggering of the transition, although the time resolution is insufficient to prove the causality.
The EDGE spectrometer is also exploited for the characterisation of the spectral (H and D) Balmer and molecular emission in the resonant antenna ion device (RAID), where a new helicon source was developed for negative ions production. Line emissivities are interpreted using the collisional-radiative code YACORA [3], resulting in estimates of the dissociation degree and the distribution of the different atomic and molecular species, including the density of negative ions [4], and confirming the high efficiency of the new source.

[1] B. P Duval et al., Phys. Plasmas 15, 056113 (2008)
[2] R. M. McDermott et al., Phys. Plasmas, 16, 056103 (2009)
[3] D. Wunderlich, S. Dietrich, U. Fantz, J Quant Spectros Radia Transfer 110, 62 (2009) [4] C. Marini et al., Nucl. Fusion 57, 036024 (2017)

contact : Alexandre Escarguel


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