82 results found
Simons L, Cowley C, Fuller P, et al., 2021, Modelling dust transport in DIII-D with DTOKS-Upgrade, Plasma Physics and Controlled Fusion, Vol: 63, Pages: 1-14, ISSN: 0741-3335
Comprehensive upgrades to the dust transport code Dust in TOKamaks (DTOKS) that extend the plasma-dust interaction model are presented and compared with recent measurements of dust transport in DIII-D. Simulations incorporating variation in physical properties of graphite dust with temperature and size in a stationary plasma background suggest a substantial decrease in lifetimes due principally to thermal expansion. The trajectories of 53 dust grains identified from analysis of visible camera data taken across two similar shots were used to measure the dust particle velocity distributions. Dust tracks terminated mostly at the outer divertor strike point having a mean observation time of 2.1 ± 0.4 ms. Stochastic modelling of 200 graphite dust particles in the DIII-D tokamak performed with DTOKS-U using plasma simulations generated by OEDGE found similar behaviour, with particles ablating rapidly after acquiring a positive charge in the region close to the outer strike point, creating an acute source of neutral carbon atoms. The simulated mean lifetime, 11 ± 2 ms, showed approximate agreement with experimental observation when corrected by accounting for dust visibility and ignoring the longest trajectories 1.5 ± 0.2 ms. Synthetic diagnostic data generated from coupling the results of DTOKS-U with the visualisation software Calcam offers a powerful new tool for validation of simulations and predictive calculations of dust dynamics.
James L, Coppins M, 2020, Suppression of the ion drag force on dust in magnetized plasmas, Physics of Plasmas, Vol: 27, Pages: 1-9, ISSN: 1070-664X
Modeling the transport of “dust” particles in a magnetically confined plasma device is an area of active research and requires a detailed understanding of the forces experienced by dust immersed in a plasma. One of the most significant of these is the “ion drag force.” Dust transport codes employ a model of this force that was not specifically designed for fusion plasmas and so does not consider the relevance of strong magnetic fields. However, it is shown here that the effect of magnetic fields on the ion drag force is significant for such plasmas. In this work, the Monte Carlo code DiMPl is employed to perform the first detailed characterization of the dependence of the ion drag force on magnetic fields. A semi-empirical model of this dependence is fitted onto the simulation data, so that these magnetic effects may be straightforwardly captured by dust transport codes. The limiting behavior of the ion drag force in the case of very strong fields is derived analytically and shown to be consistent with the simulation results. The validity of the results is further motivated through a novel theoretical treatment of the ion drag force at intermediate magnetic field strengths.
Simons L, Coppins M, 2020, Modeling of spherical dust charging in collisionless magnetized plasmas with DiMPl, Physics of Plasmas, Vol: 27, Pages: 1-10, ISSN: 1070-664X
Determining the equilibrium charge of conducting spheres in plasmas is important for interpreting Langmuir probe measurements, plasma surface interactions, and dust particle behavior. The Monte Carlo code Dust in Magnetised Plasmas (DiMPl) has been developed for the purpose of determining the forces and charging behavior of conducting spheroids under a variety of conditions and benchmarked against previous numerical results. The floating potentials of spheres in isothermal, collisionless, hydrogen plasmas as a function of magnetic field strength and size relative to the Debye length are studied using DiMPl and compared with new results from the N-body tree code (pot) and recent particle in cell measurements. The results of all three simulations are similar, identifying a small range at modest ion magnetization parameters over which the electron current is reduced relative to the ion current. The potential as a function of magnetic field strength is found to be relatively insensitive to dust size for dust smaller than the Debye length. The potential of large dust is found to depend less strongly on flow speed for modest magnetic field strengths and to decrease with increasing flow speed in the presence of strong magnetic fields for smaller dust. A semi-empirical model for the potential of small dust in a collisionless plasma as a function of magnetic field strength is developed, which reproduces the expected currents and potentials in the high and low magnetic field limit.
Abbas G, Allen JE, Coppins M, et al., 2020, A study of the propagation of a solitary wave along the magnetic field in a cold collision-free plasma, PHYSICS OF PLASMAS, Vol: 27, ISSN: 1070-664X
Holgate JT, Coppins M, 2020, Enhancement of droplet ejection from molten and liquid plasma-facing surfaces by the electric field of the sheath*, JOURNAL OF PHYSICS D-APPLIED PHYSICS, Vol: 53, ISSN: 0022-3727
Holgate J, Coppins M, Allen J, 2019, Simulated dynamics of a plasma-sheath-liquid interface, New Journal of Physics, Vol: 21, ISSN: 1367-2630
The discovery of a highly-charged sheath region at the boundary between a plasma and a surface is one of the earliest and most important discoveries in plasma science. However sheath physics has almost always been omitted from studies of the dynamics of plasma-facing liquid surfaces which are rapidly assuming a pivotal role in numerous industrial and fusion applications. This paper presents full simulations of the plasma-sheath-liquid interface and finds good agreement with theoretical stability limits and experimental observations of cone formation and pulsed droplet ejection. Consideration of sheath physics is strongly encouraged in all future studies of plasma–liquid interactions.
Somboonkittichai N, Kijamnajsuk P, Fukuyama A, et al., 2019, Numerical tracking of impurities by dust ablation in HT-6M plasma, International Nuclear Science and Technology Conference (INST), Publisher: IOP PUBLISHING LTD, ISSN: 1742-6588
Gibson J, Coppins M, 2018, Theory of electron density in a collisionless plasma in the vicinity of a magnetic dipole, PHYSICS OF PLASMAS, Vol: 25, ISSN: 1070-664X
Holgate J, Coppins M, 2018, Shapes, stability and hysteresis of rotating and charged axisymmetric drops in vacuum, Physics of Fluids, Vol: 30, ISSN: 1070-6631
The behavior of rotating and/or charged drops is a classic problem in fluid mechanics with a multitude of industrial applications. Theoretical studies of such liquid drops have also provided important insights into fundamental physical processes across nuclear and astrophysical lengthscales. However, the full nonlinear dynamics of these drops are only just beginning to be uncovered by experiments. These nonlinear effects are manifest in the high sensitivity of the breakup mechanisms to small perturbations of the initial drop shape and in observations of hysteresis in the transition between different drop shape families. This paper investigates the equilibrium shapes and stability of charged and rotating drops in a vacuum with an energy minimization method applied to spheroidal shapes and with numerical simulations using a finite-difference, level-set method. A good working formula for the stability limit of these drops is given by Lmax = 1.15 − 0.59x − 0.56x2, where L is the dimensionless angular momentum and x is the charge fissility parameter. These methods also provide a firm explanation for the hysteresis of rotating and charged drops.
Holgate J, Coppins M, 2018, Numerical implementation of a cold-ion, Boltzmann-electron model for nonplanar plasma-surface interactions, Physics of Plasmas, Vol: 25, ISSN: 1070-664X
Plasma-surface interactions are ubiquitous in the field of plasma science and technology. Much of the physics of these interactions can be captured with a simple model comprising a cold ion fluid and electrons which satisfy the Boltzmann relation. However, this model permits analytical solutions in a very limited number of cases. This paper presents a versatile and robust numerical implementation of the model for arbitrary surface geometries in cartesian and axisymmetric cylindrical coordinates. Specific examples of surfaces with sinusoidal corrugations, trenches, and hemi-ellipsoidal protrusions verify this numerical implementation. The application of the code to problems involving plasma-liquid interactions, plasma etching, and electron emission from the surface is discussed.
Holgate JT, Coppins M, 2018, Electron emission from electrically isolated spheres, JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B, Vol: 36, ISSN: 1071-1023
Holgate J, Coppins M, Allen JE, 2018, Electrohydrodynamic stability of a plasma-liquid interface, Applied Physics Letters, Vol: 112, ISSN: 1077-3118
Many plasma applications involve the plasma coming into contact with a liquid surface. Previous analyses of the stability of such liquid surfaces have neglected the presence of the sheath region between the bulk plasma and the liquid. Large electric fields, typically in excess of several MV m−1, and strong ion flows are present in this region. This paper considers a linear perturbation analysis of a liquid-sheath interface in order to find the marginal condition for instability. This condition shows that molten metal surfaces in tokamak edge plasmas are stable against the electric field, if a normal sheath is formed, due to the impact of ions on the surface. The stabilization of the liquid surface by ion bombardment is encouraging for the ongoing development of plasma-liquid technologies.
Somboonkittichai N, Coppins M, 2017, Conceptual Study of Possibility for Droplets to Achieve Superheated in Edge Tokamak Plasmas, Siam Physics Congress (SPC), Publisher: IOP PUBLISHING LTD, ISSN: 1742-6588
Holgate J, Coppins M, 2017, Murphy-Good emission theory for earthed spherical nanoemitters, 30th International Vacuum Nanoelectronics Conference (IVNC), Publisher: IEEE, Pages: 238-239, ISSN: 2164-2370
Holgate JT, Coppins M, 2017, Field-Induced and Thermal Electron Currents from Earthed Spherical Emitters, PHYSICAL REVIEW APPLIED, Vol: 7, ISSN: 2331-7019
Holgate JT, Coppins M, 2016, Charging of nonspherical macroparticles in a plasma, PHYSICAL REVIEW E, Vol: 93, ISSN: 2470-0045
Rizopoulou N, Robinson APL, Coppins M, et al., 2014, Electron emission in a source-collector sheath system: a kinetic study, Physics of Plasmas, Vol: 21, ISSN: 1070-664X
The classical source-collector sheath system describes a plasma that forms between a Maxwellian source and an absorbing wall. The plasma is assumed to be collisionless and without ionization. Two distinct areas are being formed: the collector sheath, an ion-rich region in contact with the absorbing boundary, and the source sheath, which is an electron-rich area near the Maxwellian source. In this work, we study a modified version of the classical source-collector sheath system, where the wall is no longer absorbing but emits electrons. As a result, we have two different types of collector sheath, one where a potential well is formed and one without a potential well. We examine the effect of electron emission for a range of conditions for the plasma and the emitted electrons. In the first part of this work, we study the problem analytically, and in the second, using our kinetic Vlasov code, Yggdrasil. The simulation results are in very good agreement with the predictions of our theoretical model.I. INTRODUCTION
Thomas DM, Coppins M, 2013, Equilibrium probability distribution of a conductive sphere's floating charge in a collisionless, drifting Maxwellian plasma, PHYSICAL REVIEW E, Vol: 88, ISSN: 1539-3755
Rizopoulou N, Robinson APL, Coppins M, et al., 2013, A kinetic study of the source-collector sheath system in a drifting plasma, PLASMA SOURCES SCIENCE & TECHNOLOGY, Vol: 22, ISSN: 0963-0252
Willis CTN, Coppins M, Bacharis M, et al., 2012, Floating potential of large dust grains in a collisionless flowing plasma, PHYSICAL REVIEW E, Vol: 85, ISSN: 1539-3755
Willis CTN, Allen JE, Coppins M, et al., 2011, Wakes formed by dust grains in supersonically flowing plasmas, PHYSICAL REVIEW E, Vol: 84, ISSN: 1539-3755
Willis CTN, Coppins M, Bacharis M, et al., 2010, The effect of dust grain size on the floating potential of dust in a collisionless plasma, PLASMA SOURCES SCIENCE & TECHNOLOGY, Vol: 19, ISSN: 0963-0252
Willis CTN, Coppins M, Bacharis M, et al., 2010, The floating potential of large dust grains in a collisionless, flowing plasma, Pages: 1086-1089
Many dusty plasma environments have some degree of flow associated with them. Shifted orbital motion limited (SOML) is the most widely used theory for predicting the floating potential of dust in a flowing plasma. The applicability of SOML is investigated via PIC simulation. Flow velocities up to five times the cold ion acoustic speed are simulated for dust grains of varying size.
Bacharis M, Coppins M, Allen JE, 2010, Critical issues for modeling dust transport in tokamaks, PHYSICAL REVIEW E, Vol: 82, ISSN: 1539-3755
Bacharis M, Coppins M, Allen JE, 2010, Dust grain charging in RF discharges, PLASMA SOURCES SCIENCE & TECHNOLOGY, Vol: 19, ISSN: 0963-0252
Bacharis M, Coppins M, Allen JE, 2010, Dust in tokamaks: An overview of the physical model of the dust in tokamaks code, PHYSICS OF PLASMAS, Vol: 17, ISSN: 1070-664X
Coppins M, 2010, Electrostatic Breakup in a Misty Plasma, PHYSICAL REVIEW LETTERS, Vol: 104, ISSN: 0031-9007
Zimmermann TMG, Coppins M, Allen JE, 2010, Coaxial discharge with axial magnetic field: Demonstration that the Boltzmann relation for electrons generally does not hold in magnetized plasmas, PHYSICS OF PLASMAS, Vol: 17, ISSN: 1070-664X
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