Papers by Sergei Krasheninnikov
arXiv: Plasma Physics, 2019
The electron dynamics in counter-propagating laser waves is investigated by employing a novel app... more The electron dynamics in counter-propagating laser waves is investigated by employing a novel approach, where the new Hamiltonian is time-independent when the perturbative laser wave is absent. The physical picture of stochastic electron dynamics is clearly revealed and the threshold values of the amplitude of the perturbative laser field for triggering stochastic electron acceleration are derived for different laser polarization directions and initial electron momentum. It demonstrates that the dephasing rate (new Hamiltonian) between the electron and the dominant laser can be randomly reduced if the amplitude of the perturbative laser is above the threshold such that the electron could be accelerated by the dominant laser well beyond the ponderomotive energy scaling. The impact of a superluminal phase velocity is examined, which slightly changes the stochastic region in Hamiltonian space if the superluminal phase velocity is under a threshold value but significantly decreases the ...
Physics of Plasmas, 2019
The mechanism of stochastic electron acceleration in colliding laser waves is investigated by emp... more The mechanism of stochastic electron acceleration in colliding laser waves is investigated by employing proper canonical variables and effective time, such that the new Hamiltonian becomes time independent when the perturbative laser wave is absent. The performed analytical analysis clearly reveals the physical picture of stochastic electron dynamics. It shows that when the amplitude of the perturbative laser field exceeds some critical value, stochastic electron acceleration occurs within some electron energy range. The conditions, at which the maximum electron energy gained under stochastic acceleration greatly exceeds the ponderomotive energy scaling based on the amplitude of the dominant laser, are derived.
Plasma Physics and Controlled Fusion, 2019
By deriving the 3/2 dimensional Hamiltonian equations for electrons in the intense laser radiatio... more By deriving the 3/2 dimensional Hamiltonian equations for electrons in the intense laser radiation and quasi-static transverse electric and longitudinal magnetic fields, the electron heating mechanisms are examined both for low harmonic resonance of electron frequency in the static fields with the laser frequency and for high harmonic resonances where the overlapping of broadened resonances causes the stochastic heating. For both cases, the maximum electron kinetic energies, well beyond the ponderomotive scaling, depend only on a small parameter combining the laser amplitude, electrostatic field strength and the conserved dephasing rate.
Cross-field transport asymmetries in unbalanced double-null divertor configurations. A.YU. PIGARO... more Cross-field transport asymmetries in unbalanced double-null divertor configurations. A.YU. PIGAROV, UCSD, T.D. ROGNLIEN, LLNL, B. LABOMBARD, MIT, S.I. KRASHENINNIKOV, UCSD -A key feature of anomalous cross-field transport in the tokamak SOL is strong poloidal asymmetry where the plasma flux is much larger on low field side (LFS) than on high field side (HFS). As shown, this asymmetry can cause an enhanced main chamber recycling at LFS and the large, M∼1, parallel plasma flows. Recent experiments performed in Alcator C-Mod with an unbalanced double null (DN) configuration show (as measured by reciprocating probes at the LFS and HFS midplanes) that plasma density profiles have a much shorter cross-field decay length in the region outside the secondary separatrix on the HFS compared to the same region on the LFS. In well-balanced DN discharges, the density e-folding length measured at the HFS midplane is substantially smaller than that in single null (SN) configuration. These data independently indicate a strong ballooning-like asymmetry. We use UEDGE code to simulate plasma transport in unbalanced DN shots obtained in C-mod. From matching experimental profile data, we infer an asymmetry factors for the anomalous cross-field plasma transport (diffusive and convective) which are indicative of strong asymmetry. The effect of secondary separatrix on radial plasma profiles, parallel plasma flows, and impurity migration will be discussed. Simulated transport asymmetries in unbalanced DN, balanced DN, and SN shots will be compared. Work supported by DoE grant DEFG0204ER54739.
Transport of dust particles in fusion devices A.YU. PIGAROV, S.I. KRASHENINNIKOV -Large productio... more Transport of dust particles in fusion devices A.YU. PIGAROV, S.I. KRASHENINNIKOV -Large production rates and spreading of dust throughout the device volume are very important safety issues for next-step fusion projects (both: magnetic and inertial). A physical model for dust transport includes the dynamics dust-plasma, dust-turbulence, and dust-surface interactions. The dynamics is strongly coupled to heating, charging, erosion, evaporation, and thermo-chemical properties of dust particles. Recent developments in the model and their impact on the dust motion are highlighted. The model is incorporated into the DUSTT code [A.Pigarov, PoP 12 (2005) 122508]. Results of carbon-dust dynamics and transport in plasma are shown for NSTX and DIII-D tokamaks. The simulations demonstrate that dust particles can be very mobile, accelerate to large velocities (>100 m/s), and penetrate deeply toward plasma core. Predictions for dust penetration into ITER plasma are also presented. Under standard tokamak plasma conditions, dust particles experience the net erosion or sublimation. However, as discussed, in some cases (for example, parasitic plasmas that occur underneath a dome), the dust particles can even grow from net deposition when a low-Te low-density plasma contains significant concentrations of impurities. Modeling is presented for transport of dust particles outside the working chamber, i.e. in pumping or diagnostics ports. Results of multi-parametric analysis of dust penetrability are discussed. Work supported by USDoE grant DE-FG02-04ER54739.
Nuclear Fusion, 2002
In inertial fusion energy (IFE) power plant designs, the fuel is a spherical layer of frozen DT c... more In inertial fusion energy (IFE) power plant designs, the fuel is a spherical layer of frozen DT contained in a target that is injected at high velocity into the reaction chamber. For direct drive, typically laser beams converge at the centre of the chamber (CC) to compress and heat the target to fusion conditions. To obtain the maximum energy yield from the fusion reaction, the frozen DT layer must be at about 18.5 K and the target must maintain a high degree of spherical symmetry and surface smoothness when it reaches the CC. During its transit in the chamber the cryogenic target is heated by radiation from the hot chamber wall. The target is also heated by convection as it passes through the rarefied fill-gas used to control chamber wall damage by x-rays and debris from the target explosion. This article addresses the temperature limits at the target surface beyond which target uniformity may be damaged. It concentrates on direct drive targets because fuel warm up during injection is not currently thought to be an issue for present indirect drive designs and chamber concepts. Detailed results of parametric radiative and convective heating calculations are presented for direct-drive targets during injection into a dry-wall reaction chamber. The baseline approach to target survival utilizes highly reflective targets along with a substantially lower chamber wall temperature and fill-gas pressure than previously assumed. Recently developed high-Z material coatings with high heat reflectivity are discussed and characterized. The article also presents alternate target protection methods that could be developed if targets with inherent survival features cannot be obtained within a reasonable time span.
Modeling of Dust Dynamics in Tokamaks 1 R.D. SMIRNOV, A. YU. RIGAROV, S.I. KRASHENINNIKOV, M. ROS... more Modeling of Dust Dynamics in Tokamaks 1 R.D. SMIRNOV, A. YU. RIGAROV, S.I. KRASHENINNIKOV, M. ROSENBERG, D.A. MENDIS, UCSD -The experimentally proved presence of dust in fusion devices is a recognized issue that may have large impact on tokamak operation and can present safety threat for fusion energy. In this work, transport and distribution of dust particles in tokamaks are studied using computer simulations with the dust transport DUSTT code. Recent developments of the DUSTT code are reported. The improved model accounts for thermionic and secondary electron emission; heat fluxes on the dust are refined using the Orbital Motion Limited theory; corrections for small body thermal radiation and dust-impurity interaction are also introduced. The effect of these processes on dust dynamics is evaluated. Reproducing of experimentally obtained dust tracks in NSTX tokamak allows tuning and verification of the code. Statistical averaging over ensemble of dust trajectories is used to obtain volume distributions of dust characteristics. It is predicted that transport of dust accompanied with ablation can cause significantly enhanced penetration of impurities toward the core in comparison with impurity ion transport. As shown, collisions of the dust particles with walls are critical for dust transport due to loss of kinetic energy and mass. It is shown that dust can experience net mass deposition in relatively cold contaminated plasma regions.
Dust in Fusion Plasmas: Modeling Approach 1 R.D. SMIRNOV, A.YU. PIGAROV, S.I. KRASHENINNIKOV, M. ... more Dust in Fusion Plasmas: Modeling Approach 1 R.D. SMIRNOV, A.YU. PIGAROV, S.I. KRASHENINNIKOV, M. ROSENBERG, D.A. MENDIS, UCSD -The confinement of enormous energy in the plasma of future fusion devices presents numerous challenges associated with unavoidable interactions of the plasma with bounding walls. The interactions both continuous and intermittent lead to destruction of the walls with the release of a significant amount of wall material in the form of atoms, clusters and dust into the plasma. It appears that the dust is not directly affected by the magnetic field and may have a shorter path toward the plasma core, delivering impurities to it. Part of the impurities re-deposits on the plasma facing surfaces as thin films, stimulating further dust production. While in the plasma, the dust particles driven by plasma flows acquire large speeds, this leads to dust spreading over the volume and the surfaces of a fusion device. Taking into account the high reactivity of the dust and its ability to retain tritium, dust also presents important safety issue in the next-step fusion devices. In this work we report on recent progress in numerical simulation efforts with the DUSTT code toward understanding the dust behavior in fusion plasmas. Dust dynamics, transport, statistics, and impact on the plasma are considered.
The intermittent turbulent transport in the scrape-off-layers of Alcator C-Mod, DIII-D, and NSTX ... more The intermittent turbulent transport in the scrape-off-layers of Alcator C-Mod, DIII-D, and NSTX is studied experimentally. On DIII-D the fluctuations of both density and temperature have strongly non-Gaussian statistics, and events with amplitudes above 10 times the mean level are responsible for large fractions of the net particle and heat transport, indicating the importance of turbulence on the transport. In C-Mod and NSTX the turbulence is imaged with a very high density of spatial measurements. The 2-D structure and dynamics of emission from a localized gas puff are observed, and intermittent features (also sometimes called "blobs") are typically seen. On DIII-D the turbulence is imaged using BES and similar intermittent features are seen. The dynamics of these intermittent features are discussed. The experimental observations are compared with numerical simulations of edge turbulence. The electromagnetic turbulence in a 3-D geometry is computed using non-linear plasma fluid equations. The wavenumber spectra in the poloidal dimension of the simulations are in reasonable agreement with those of the C-Mod experimental images once the response of the optical system is accounted for. The resistive ballooning mode is the dominant linear instability in the simulations.
The goal of the Lithium Tokamak eXperiment (LTX) is to produce tokamak discharges with near-zero ... more The goal of the Lithium Tokamak eXperiment (LTX) is to produce tokamak discharges with near-zero recycling, and investigate the consequences of operating under this extreme condition for plasma transport and stability. A major component of LTX is a conducting copper shell with a dynamically-bonded stainless steel liner as the plasma facing component (PFC). Fabrication of a prototype shell section has demonstrated that the tolerances required for conformity to the plasma shape can be maintained. A lithium coating will provide the low recycling PFC on LTX. The goal of depositing 100 nm lithium layers within the five minute interval between discharges has been achieved. Details of the progress in these areas and the status of other LTX tasks will be reported.
Physical Review Letters, 1998
Molecular activated recombination (MAR) has been clearly observed for the first time in a diverto... more Molecular activated recombination (MAR) has been clearly observed for the first time in a divertor plasma simulator. A small amount of hydrogen gas puffing into a helium plasma strongly reduced the ion particle flux along the magnetic field, although the conventional radiative and three-body recombination processes were quenched. Careful comparison of the observed helium Balmer spectra with collisional radiative atomic and molecular data indicates that the population distribution over the atomic levels with relatively low principal quantum numbers can be well explained by taking the MAR effects into account. [S0031-9007(98)
Nuclear Fusion, 2005
The major objective of the National Spherical Torus Experiment (NSTX) is to understand basic toro... more The major objective of the National Spherical Torus Experiment (NSTX) is to understand basic toroidal confinement physics at low aspect ratio and high β T in order to advance the ST concept. In order to do this, NSTX utilizes up to 7.5 MW of Neutral Beam Injection, up to 6 MW of High Harmonic Fast Waves, and it operates with plasma currents up to 1.5 MA and elongations of up to 2.6 at a toroidal field up to 0.45 T. New facility, diagnostic and modeling capabilities developed over the past two years have enabled the NSTX research team to make significant progress towards establishing this physics basis for future ST devices. Improvements in plasma control have led to more routine operation at high elongation and high β T (up to ∼40%) lasting for many energy confinement times. β T can be limited by either internal or external modes. The installation of an active error field correction coil pair has expanded the operating regime at low density and has allowed for initial resonant error field amplification experiments. The determination of the confinement and transport properties of NSTX plasmas has benefited greatly from the implementation of higher spatial resolution kinetic diagnostics. The parametric variation of confinement is similar to that at conventional aspect ratio but with values enhanced relative to those determined from conventional aspect ratio scalings and with a B T dependence. The transport is highly dependent on details of both the flow and magnetic shear. Core turbulence was measured for the first time in an ST through correlation reflectometry. Non-inductive startup has been explored using PF-only and transient co-axial helicity injection techniques, resulting in up to 140 kA of toroidal current generated by the latter technique. Calculated bootstrap and beam-driven currents have sustained up to 60% of the flattop plasma current in NBI discharges. Studies of HHFW absorption have indicated parametric decay of the wave and associated edge thermal ion heating. Energetic particle modes, most notably TAE and fishbone-like modes result in fast particle losses, and these instabilities may affect fast ion confinement on devices such as ITER. Finally, a variety of techniques has been developed for fueling and power and particle control.
Journal of Nuclear Materials, 1999
The structural change of the detached plasmas associated with the molecular activated recombinati... more The structural change of the detached plasmas associated with the molecular activated recombination (MAR) has been studied in a high density helium plasma with helium or hydrogen pu in the linear divertor plasma simulator, NAGDIS-II. The ion particle ¯ux with the hydrogen gas pu starts to decrease in the upstream region close to the plasma source, compared to that with the helium pu. The reduction of the ion particle ¯ux along the magnetic ®eld is found to depend on the plasma density. With the hydrogen gas pu, the attenuation of the ion particle ¯ux is getting smaller as increasing plasma density, which is opposite tendency in pure helium detached plasma where the conventional electron±ion recombination (EIR) is dominating.
Journal of Nuclear Materials, 2013
Dust will have severe impact on ITER performance since the accumulation of tritium in dust repres... more Dust will have severe impact on ITER performance since the accumulation of tritium in dust represents a safety issue, a possible reaction of dust with air and steam imposes an explosion hazard and the penetration of dust in core plasmas may degrade plasma performance by increasing radiative losses. Investigations were performed in TEXTOR where known amounts of pre-characterized carbon, diamond and tungsten dust were mobilized into plasmas using special dust holders. Mobilization of dust changed a balance between plasma-surface interactions processes, significantly increasing net deposition. Immediately after launch dust was dominating both core and edge plasma parameters. Remarkably, in about 100 milliseconds after the launch, the effect of dust on edge and core plasma parameters was vanished: no increase of carbon and tungsten concentrations in the core plasmas was detected suggesting a prompt transport of dust to the nearby plasma-facing components without further residence in the plasma.
Springer Series in Plasma Science and Technology, 2020
Plasma Science and Technology covers all fundamental and applied aspects of what is referred to a... more Plasma Science and Technology covers all fundamental and applied aspects of what is referred to as the "fourth state of matter." Bringing together contributions from physics, the space sciences, engineering and the applied sciences, the topics covered range from the fundamental properties of plasma to its broad spectrum of applications in industry, energy technologies and healthcare. Contributions to the book series on all aspects of plasma research and technology development are welcome. Particular emphasis in applications will be on hightemperature plasma phenomena, which are relevant to energy generation, and on low-temperature plasmas, which are used as a tool for industrial applications. This cross-disciplinary approach offers graduate-level readers as well as researchers and professionals in academia and industry vital new ideas and techniques for plasma applications.
Springer Series in Plasma Science and Technology, 2020
Plasma Science and Technology covers all fundamental and applied aspects of what is referred to a... more Plasma Science and Technology covers all fundamental and applied aspects of what is referred to as the "fourth state of matter." Bringing together contributions from physics, the space sciences, engineering and the applied sciences, the topics covered range from the fundamental properties of plasma to its broad spectrum of applications in industry, energy technologies and healthcare. Contributions to the book series on all aspects of plasma research and technology development are welcome. Particular emphasis in applications will be on hightemperature plasma phenomena, which are relevant to energy generation, and on low-temperature plasmas, which are used as a tool for industrial applications. This cross-disciplinary approach offers graduate-level readers as well as researchers and professionals in academia and industry vital new ideas and techniques for plasma applications.
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Papers by Sergei Krasheninnikov