Charged Plasma Components Research Articles

Features of processes in a microwave discharge in water vapor

A zero-dimensional steady-state simulation of microwave discharge in water vapor at atmos-pheric and reduced pressures and a constant gas temperature has been carried out. A model of a continuous stirring reactor is used. A joint solution of the balance equations for neutral and charged plasma components, the Boltzmann equation for plasma electrons, and the equation for the stationary distribution of the microwave field in a volume filled with plasma is carried out. The dependences of various parameters of thedischarge (the magnitude of the microwave field, the concentrations of all components) on the input specific power WVare obtained. It is shown that at reduced pressure the magnitude of the microwave field in the plasma is signifi-cantly lower, and the electron concentration is higher than at atmospheric pressure at the same applied specific power. At atmospheric pressure the water plasma is electronegative, and quasi-neutrality is maintained by the negative OH-ion in the range of the considered WV values. Transition from electronegative to electropositive plasma occurs at pressure of 30 Torr and ap-plied specific power of 60–70 kW/cm3

Effect of charging solid particles on their growth process and parameters of microwave discharge in liquid n-heptane

A zero-dimensional non-stationary model of a microwave discharge in liquid n-heptane at atmospheric pressure with continuous argon injection into the plasma region is presented. The model includes equations for the kinetics of neutral and charged plasma components, equations describing the formation and growth of solid particles from the decomposition products of n-heptane, and an equation for the microwave field strength in plasma. The description of the coagulation process takes into account the electrostatic repulsion of solid particles due to their charge. The kinetics of charged particles is described taking into account their death on the surface of negatively charged solid particles formed in plasma. To determine the coefficients of the reaction rates in mechanisms of direct electron impact, the electron energy distribution function obtained by solving the Boltzmann equation is used. Calculations have shown that the plasma quasineutrality is mainly supported by the charge of solid particles, and the electron concentration is 1–2 orders of magnitude less than the total ion concentration. An increase in the averaged microwave field was noted in comparison with the case that does not take into account charging. Charging of the solid particles does not affect the composition of the main gas-phase products, but leads to the suppression of the coagulation process for large solid particles, which leads to a change in the size distribution function of the solid particles formed in the plasma.

An assessment of the first wall and limiter erosion during limiter discharge stages in DEMO

The model elaborated on previously (Tokar 2018 Nucl. Fusion 58 016016) for the transport of neutral and charged plasma components in the DEMO scrape-off layer (SOL) with a divertor is reworked to describe limiter discharge phases. It provides radial profiles of the plasma density and temperatures of electrons and ions, averaged over flux surfaces, which are calculated self-consistently at the edge of the confined region and in the SOL, without prescribing any decay lengths at the last closed flux surface. It is assumed that on the limiter discharge phase in question the plasma will be in the L-mode of the energy confinement and the radial transport in the SOL is modeled with coefficients of the Bohm type. Computations give the plasma particle flows both to the limiters and to the first wall (FW). This allows us to assess the erosion rates by plasma ions of these reactor components. In the case of the FW physical sputtering with hot atoms generated in charge-exchange collisions of ions with neutral species, recycling from the wall and limiters is also taken into account.

Preliminary characteristics of magnetic field and plasma performance in the Magnetized Dusty Plasma Experiment (MDPX)

The Magnetized Dusty Plasma Experiment (MDPX) device is a newly constructed research instrument for the study of dusty (complex) plasmas. The MDPX device is envisioned as an experimental platform in which the dynamical behavior of all three charged plasma components, the electrons, ions, and charged microparticles (i.e., the ‘dust’) will be significantly influenced by the magnetic force. This brief paper will provide a short overview of the design, magnetic performance, and initial plasma measurements in the MDPX device.

Supernonlinear Waves in Plasma

A new class of nonlinear waves in plasma—supernonlinear waves (SNWs) characterized by the nontrivial topology of their phase portraits—has been revealed. The topological classification of such waves is given, and suitable notation for them is proposed. It is demonstrated using several examples that SNWs can exist in the form of plasma waves of different physical nature, e.g., electrostatic (ion-acoustic) and MHD (Alfven) waves. It is shown that a necessary condition for the existence of SNWs is the presence of at least three different charged plasma components (electrons, positrons, ions, dust grains, etc.). As the number of plasma components increases, the topology of the SNW phase portrait becomes more complicated. Typical indications of SNWs are given, which make is possible to easily reveal such waves experimentally.

Study of the efficiency of magnetic island macroparticle filters for different vacuum arc configurations

In the present work, we studied the efficiency of a magnetic island filter for two vacuum arc designs, pulsed and continuous. The magnetic island filter consisted in a straight duct with an external solenoid and a magnet enclosed in a housing (magnetic island) located inside the duct on its axis, and both magnetic fields are in opposite direction. The housing of the magnetic island obstructs the line of sight between the cathode and the substrate. In this arrangement, the charged plasma components move along the curved magnetic field lines, around the magnetic island, but the macroparticles move in straight paths and deposit on the wall of the magnetic island housing. The performance of the filter was characterized for different external and internal field strengths. The plasma transport efficiency was analyzed by measuring the ion saturation current with Langmuir probes and the deposited mass rate. The ion transmission efficiency around the system axis achieved values around 25%. Observation of the coating surface morphology with optical microscopy determined that the macroparticles were effectively removed.

Structure of the charged sheath at the plasma-charged body boundary

Based on a hydrodynamic plasma model that incorporates the inertia and temperatures of the plasma components, we have analytically and numerically solved the problem on the structure of the charged sheath at the plasma-charged body boundary. We formulate a criterion for the existence of a stationary charged sheath at the plasma-charged body boundary for both charge signs and with allowance made for the inertia and temperatures of the charged plasma components. We determine the minimum and maximum electric potentials and densities of the charged plasma components in the sheath. We show that the charge profile in the sheath can form a double-layer structure in which the ion-depleted and electron-depleted sublayers are adjacent to the plasma and the charged body, respectively, with the electric potential in the sheath remaining monotonic.

The effect of electron detachment on the diffusive decay of a low-pressure electronegative plasma

There are two possible scenarios in the diffusion decay of a low-pressure electronegative plasma in a system featuring the electron detachment effect. In the case of a weak detachment, the system exhibits an initial sharp drop in the electron concentration followed by the ion-ion plasma formation. A small admixture of electrons appearing as a result of the detachment keeps all negative ions within the volume. In this stage, the densities of all charged plasma components decay according to the same exponential law with a characteristic detachment time. In the limit of a strong detachment, a principally different plasma decay regime takes place. In this limiting case, a usual (free of negative ions) rather than ion-ion plasma is formed in the second stage. This can be accompanied by paradoxical phenomena such as the growth of electron density with time, build-up of the near-wall potential jump, and a considerable decrease (up to complete vanishing) of the diffusion electron cooling usually dominating in the electron gas energy balance.