Dynamics of the LH-transition in TUMAN-3M tokamak in the scenarios with cryogenic pellet injection

Abstract

In the experiments on tangential fuel pellet injection in the TUMAN-3M tokamak an initiation of LH-transition or, in several scenarios—temporal (1–2 ms) confinement improvement with the following backwards transition was observed. To understand the possibility of the transitions, a model calculating the evolution of density and ion temperature profiles under the effect of source profile perturbation and plasma cooling created by pellet evaporation was developed. In the model, a diffusion coefficient depending on radial electric field shear value was used. Turbulence parameters are defined using a gyrokinetic simulation of the experiments with ELMFIRE code. Modeling results are in good agreement with experiments. Using the data obtained from the modeling, non-linear particle flux dependency on density gradient was analyzed; the existence of two stabile solutions for two confinement modes dependent on particle flux value was proved, in agreement with experiments.

LH-transition initiation and dynamics in a conventional tokamak

“…In FT-2 low-density regimes with GAM LH-transition was not observed [14]. Another scenario is pellet-injection in TUMAN-3M [11].…”

“…Fuel pellet tangential injection was performed in TUMAN-3M tokamak [11,17], two characteristic scenarios with confinement improvement were observed: if pellet was partially broken up in the pellet-guide and gas cloud was injected in plasma alongside with pellet, LH-transition occurred. In case of deeper evaporation of solid pellet only transient confinement improvement was observed.…”

“…These two cases were modeled [11]. In the model turbulence suppression affects only particle diffusion, ion heat conductivity is considered neoclassical.…”

“…Another factor necessary for LH-transition is threshold heating power [4,5] and threshold density (and thus particle source) [1,6]. Experiments on Ioffe Institute tokamaks TUMAN-3M and FT-2 provide wide range of scenarios with radial electric field and particle source perturbation [6,[7][8][9][10][11]. The interplay between the particle and heat source, Er shear and turbulence level could be analyzed using the theory of LH-transition initiation [12].…”

Tokamak research at the Ioffe Institute

“…Transport quantities and turbulence parameters were provided by the ELMFIRE gyrokinetic code. The modeling results have shown satisfactory agreement with the experiments: (1) in TUMAN-3M the GAM parameters threshold for L-H transition is determined (figure 4), however for FT-2 the transition was not possible for experimental GAM parameter sets [18]; (2) in the case of the pellet injection, L-H transition occurred if a part of the pellet was disintegrated in the pelletguide, resulting in a gas/'snow' puff so that the pellet ablation is peripheral; on the contrary, if the pellet is evaporated deeper in the plasma, there is no L-H transition [19].…”