View source for TJ-II:L-H transition studies: characterization of plasma turbulence using Gas Puff Imaging, Probes, Doppler reflectometry and HIBP diagnostics

== Experimental campaign ==
Spring 2022

== Proposal title ==
'''L-H transition studies: characterization of plasma turbulence using Gas Puff Imaging, Probes, Doppler reflectometry and HIBP diagnostics'''

== Name and affiliation of proponent ==
Teresa Estrada

CIEMAT 

https://orcid.org/0000-0001-6205-2656

== Details of contact person at LNF ==
Teresa Estrada

https://orcid.org/0000-0001-6205-2656

== Description of the activity ==
At TJ-II, spontaneous L-H transitions are achieved in neutral beam injection (NBI) heated plasma. Abrupt as well as gradual transitions are achieved depending, among other plasma parameters, on the heating power and magnetic configuration topology. Doppler reflectometry (DR) measurements show an increase in the negative radial electric field (Er) together with a reduction in plasma turbulence at the transition [1]. These measurements together with measurements obtained using a dual Langmuir probe system [2] indicate that the trigger of the L-H transition is more correlated with the development of fluctuating radial electric fields than steady-state Er effects. This conclusion is further stressed when operating close to the L-H transition threshold conditions, where pronounced oscillations in both, Er and density turbulence measured by DR show a characteristic predator-prey relation [3]. These experimental observations are consistent with L-H transition models based on turbulence-induced sheared/zonal flows.  In addition, HIBP measurements show a reduction in the plasma turbulence and associated flux not only in the plasma edge region but also in the plasma core [4].
Recently a Gas Puff Imaging system (GPI) has been installed and tested at TJ-II.  The new gas injection system is used with a camera system that allows 3 simultaneous filtered frames to apply the He I ratio technique, getting 2D measurements of the edge plasma electron density ne and temperature Te with spatial resolution of 3 mm and temporal resolution down to 10 microseconds [5]. This new diagnostic will permit the 2-D characterization of edge and SOL plasma turbulence and is expected to provide additional valuable information on the behaviour of the turbulence during the L-H transition.
The whole set of diagnostics operating simultaneously will allow the characterization of the plasma turbulence evolution from the SOL (GPI and probes) to the plasma gradient (DR) and plasma core (HIBP) regions at the L-H transition.

[1] T. Estrada et al., Plasma Phys. Control. Fusion 51, 124015 (2009)

[2] C. Hidalgo et al., EPL 87, 55002 (2009) 

[3] T. Estrada et al., Phys. Rev. Lett. 107, 245004 (2011) 

[4] A. Melnikov et al., Nucl. Fusion 53, 092002 (2013) 

[5] E. de la Cal and TJ-II Team, Nucl. Fusion 56, 106031 (2016)

== International or National funding project or entity ==
Include funding here (grants, national plans)

== Description of required resources ==
Required resources:
* Number of plasma discharges or days of operation: 
Experiments in March 2022 focussed on GPI & DR: To prepare the experiment we need first to reproduce the conditions for L-H transitions. Once the proper conditions are found, He puffing will be introduced for GPI measurements.

Experiments in May 2022 focussed on HIBP & DR & Probes

Experiments in June 2022 focussed on simultaneous measurements using the whole set of TJ-II diagnostics

* Essential diagnostic systems: GPI, Doppler reflectometer, Langmuir probes, HIBP, microwave interferometer, Thomson scattering, Hα detectors, diamagnetic, Rogosky and Mirnov coils, SXR, bolometry

* Type of plasmas (heating configuration): NBI plasmas with plasma target created by ECH in the standard (or 101_42_64) magnetic configuration.

* Specific requirements on wall conditioning if any: Fresh Li is required for a good density control during the NBI phase.

* External users: need a local computer account for data access: no

* Any external equipment to be integrated? Provide description and integration needs:

== Preferred dates and degree of flexibility ==
Preferred dates: experiments splitted in three blocks in March (DR and GPI), May and June (availability of HIBP)

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== References ==
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