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Pigarov, A. Yu.

Publications and source records attributed to Pigarov, A. Yu..

11 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Observation of D 2 molecule line emission after massive D 2 injection into runaway electron plateaus in DIII-D

Molecular deuterium line emission is observed in both the visible and ultraviolet (UV) wavelength ranges after massive (> 100 Torr-L) injection of D 2 gas into post-disruption runaway electron (RE) dominated plasmas in the DIII-D tokamak. D 2 UV line emission is found to be the dominant source of radiated power, surpassing D Lyα. Interpretive modeling with a collisional-radiative model (CRM) indicates that D 2 radiation surpasses D radiation because Lyα is strongly trapped, while D 2 UV lines are mostly untrapped. The CRM also indicates that the D 2 line emission is completely dominated by RE impact (rather than thermal electron impact), so the D 2 line emission can serve as a good diagnostic for the spatial localization of REs. Analysis of D 2 visible lines indicates that the D 2 molecules in the plasma are thermally equilibrated with the background plasma, with vibrational, rotational, and kinetic temperatures all near 0.3 eV. D 2 spectroscopy therefore serves as a convenient diagnostic of background plasma temperature. As a result, measurement of D 2 radiated power also appears to serve as a useful diagnostic for constraining neutral transport modeling.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2020

Study of argon expulsion from the post-disruption runaway electron plateau following low-Z massive gas injection in DIII-D

Herein, a 1D radial diffusion model is developed to study the observed rapid expulsion of argon from the runaway electron plateau in the DIII-D tokamak following secondary massive low-Z (D 2 or He) gas injection. The expulsion of argon is found to be caused by further cooling of the background plasma due to the added neutrals, accompanied by recombination of argon ions and greatly increased outward radial transport rate of argon (now dominantly in neutral form) out of the runaway electron beam. After argon expulsion, power loss out of the runaway electron plateau is found to be dominated by neutral transport to the wall (rather than by radiation); this result resolves the power balance discrepancy highlighted in previous work on argon expulsion out of the runaway electron plateau.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

Radiative detached divertor with acceptable separatrix Z eff

The feasibility study is performed for the radiative detached divertor (RDD) concept, which characterizes a variety of detached plasmas with impurity amounts providing the highest levels of divertor radiation without X-point MARFE (XPM), based on a set of restrictive criteria for the leading plasma parameters (LPPs) including, e.g., tolerable peak power loads below 1 MW/m2, low separatrix densities, allowable impurity concentrations, and acceptable Z eff values for a DIII-D like tokamak. For this, extensive simulations with the 2-D edge plasma transport code were done scanning the deuterium and impurity inventories practically for all impurity elements from beryllium to neon and the analysis of LPP variations in these scans is presented. It is shown that, for a given D inventory, the total radiation fraction with an increase in the impurity inventory reaches a flat top level, f rad = 0.85 ± 0.01, whereas the higher f rad corresponds to XPM. This critical fraction is the same for all elements and values of the D inventory. Successful RDD solutions with a flat top radiation meeting all ad hoc LPP criteria are found for some elements. Boron and nitrogen are shown to be the most promising elements for seeding, since they are capable of providing alone the successful RDD at the lowest concentrations. Several important effects on impurity radiation are considered including: cross-field impurity transport in regions with strong temperature gradients, multi-species thermal force, charge-exchange of impurity ions with D atoms originating from recombination, impurity entrainment by parallel flows, flows caused by inner/outer divertor asymmetries, and Mach ∼ 1 flows reached inside radiation-ionization fronts. The impurity radiation profiles of various elements are analyzed suggesting three patterns differing in the radiation front position with respect to the D ionization source. The modeled relocation of D from the pedestal into divertor regions; an enhanced pedestal enrichment for impurities with high first ionization potential; and RDD capability of confining inside the divertors the large D inventories as neutral atoms, are considered as effects of increasing impurity inventory. The responses of a flat-top RDD to the input parameters variation are studied. For example, dependencies of LPPs on the input power increase, which express the sharp and even bifurcation transitions between two distinct branches corresponding to the detached and attached divertors, are discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Impurity-induced divertor plasma oscillations

Two different oscillatory plasma regimes induced by seeding the plasma with high- and low-Z impurities are found for ITER-like divertor plasmas, using computer modeling with the DUSTT/UEDGE and SOLPS4.3 plasma-impurity transport codes. The oscillations are characterized by significant variations of the impurity-radiated power and of the peak heat load on the divertor targets. Qualitative analysis of the divertor plasma oscillations reveals different mechanisms driving the oscillations in the cases of high- and low-Z impurity seeding. The oscillations caused by the high-Z impurities are excited near the X-point by an impurity-related instability of the radiation-condensation type, accompanied by parallel impurity ion transport affected by the thermal and plasma friction forces. The driving mechanism of the oscillations induced by the low-Z impurities is related to the cross-field transport of the impurity atoms, causing alteration between the high and low plasma temperature regimes in the plasma recycling region near the divertor targets. The implications of the impurity-induced plasma oscillations for divertor operation in the next generation tokamaks are also discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Tungsten dust impact on ITER-like plasma edge

The impact of tungsten dust originating from divertor plates on the performance of edge plasma in ITER-like discharge is evaluated using computer modeling with the coupled dust-plasma transport code DUSTT-UEDGE. Different dust injection parameters, including dust size and mass injection rates, are surveyed. It is found that tungsten dust injection with rates as low as a few mg/s can lead to dangerously high tungsten impurity concentrations in the plasma core. Dust injections with rates of a few tens of mg/s are shown to have a significant effect on edge plasma parameters and dynamics in ITER scale tokamaks. The large impact of certain phenomena, such as dust shielding by an ablation cloud and the thermal force on tungsten ions, on dust/impurity transport in edge plasma and consequently on core tungsten contamination level is demonstrated. It is also found that high-Z impurities provided by dust can induce macroscopic self-sustained plasma oscillations in plasma edge leading to large temporal variations of edge plasma parameters and heat load to divertor target plates.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Multi-fluid transport code modeling of time-dependent recycling in ELMy H-mode

Simulations of a high-confinement-mode (H-mode) tokamak discharge with infrequent giant type-I ELMs are performed by the multi-fluid, multi-species, two-dimensional transport code UEDGE-MB, which incorporates the Macro-Blob approach for intermittent non-diffusive transport due to filamentary coherent structures observed during the Edge Localized Modes (ELMs) and simple time-dependent multi-parametric models for cross-field plasma transport coefficients and working gas inventory in material surfaces. Temporal evolutions of pedestal plasma profiles, divertor recycling, and wall inventory in a sequence of ELMs are studied and compared to the experimental time-dependent data. Short- and long-time-scale variations of the pedestal and divertor plasmas where the ELM is described as a sequence of macro-blobs are discussed. It is shown that the ELM recovery includes the phase of relatively dense and cold post-ELM divertor plasma evolving on a several ms scale, which is set by the transport properties of H-mode barrier. The global gas balance in the discharge is also analyzed. The calculated rates of working gas deposition during each ELM and wall outgassing between ELMs are compared to the ELM particle losses from the pedestal and neutral-beam-injection fueling rate, correspondingly. A sensitivity study of the pedestal and divertor plasmas to model assumptions for gas deposition and release on material surfaces is presented. The performed simulations show that the dynamics of pedestal particle inventory is dominated by the transient intense gas deposition into the wall during each ELM followed by continuous gas release between ELMs at roughly a constant rate.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Investigation of mechanisms for He-I emission radial profile broadening in a weakly ionized cylindrical helium plasma with recombining edge

Spatially resolved spectroscopic measurements of He-I line emission are used to study the causes of emission profile broadening radially across the cylinder of a weakly ionized helium plasma. The plasma consists of an ionizing core (r < 2 cm) surrounded by a recombining edge (r > 2 cm) plasma. The brightness profiles of low-n EUV He-I resonance lines are shown to be strongly radially broadened due to opacity. The brightness profiles of high-n visible lines are also found to be strongly radially broadened, but dominantly due to edge recombination. Visible low-n lines are less strongly radially broadened, apparently by a combination of both recombination and EUV opacity. The low-n visible He-I line ratio method with central opacity correction is found to calculate central electron density and temperature well, with poor agreement at the edge, as expected for recombining plasma. In the recombining edge, high-n Boltzmann analysis is found to accurately measure the cold (T{sub e} < 0.2 eV) edge temperature. Near the core, however, high-n Boltzmann analysis can be complicated by electron-impact excitation, giving incorrect (T{sub e}≈ 0.1 eV) apparent temperatures. Probe measurements were not able to capture the cold edge temperature accurately, probably due to large potential fluctuations, even when using fast triple probe measurements. Fast spectroscopic measurements show that this discrepancy is not explained by recombining plasma alternating with ionizing plasma in the edge region.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Dynamic Models for Plasma-Wall Interactions

In theory and modeling of plasma-wall interactions, very different spatial and temporal scales need to be re- solved. Hierarchy of material models (e.g. macroscopic transport, Kinetic Monte Carlo, Molecular Dynamics, and quantum models) used in fusion applications is reviewed and a way of their integration is highlighted. Physics background, theory and applications of macroscopic transport models, which are the main focus of the paper, are thoroughly discussed. The results on self-consistent macroscopic modeling of major physics phe- nomena, e.g. hydrogen recycling, retention, chemistry, chemical sputtering, and radiation enhanced sublimation for fusion related materials are presented. An example of coupled plasma and material transport simulations is given. The results on Molecular Dynamics modeling of deuterium irradiation of carbon leading to formation of supersaturated surfaces and chemical sputtering are presented.

Pigarov, A. Yu.↗