Plasma Physics Reports

The influence of refined parameters of the upgraded system for 6-MW-power neutral beam injection into the T-15MD tokamak plasma was analyzed numerically. The efficiency of toroidal current drive in plasma has been determined, and the profiles of absorbed power of the injected neutral beam, the safety factor, and the bootstrap current have been calculated. The possibility of replacing the ohmic current in plasma with the current driven by the upgraded neutral beam injection system has been investigated. The results of calculations performed for neutral beams with monoenergetic distribution and more complicated three-component energy composition are compared. For the first time, for simulating future T-15MD shots with neutral beam injection, the NUBEAM code was used in combination with the ASTRA code. The calculations were performed with allowance for the real geometry of injectors and the parameters of ion sources. The calculation results obtained using the NUBEAM code are compared with the calculation results obtained using the new version of the ASTRA-NBI neutral beam injection module, as well as the BTR code, which optimizes the parameters of neutral beam injection. The calculations show that the fraction of current driven by the upgraded neutral beam injection system was approximately 30%.

The article presents the results of experiments and radiation magnetohydrodynamic simulations of the electrical explosion of flat copper conductors in megagauss magnetic fields. The experiments were carried out on the terawatt-range MIG generator at the current of amplitude up to 2.5 MA and the rise time of 100 ns. The thickness (along the y-axis) of the conductors used was much less than theirs width (along the x-axis) and the current flowed in the z-axis. It was experimentally shown and confirmed by RMGD simulations, that the conductor expands in thickness (along the Mi>y-axis), while plasma expansion along the x-axis is suppressed. This is due to the increased magnetic field at the conductor edges, which also causes earlier explosion of the side faces. A matter from the side faces, that is low-density plasma, flows toward the center of symmetry of the wide faces of the plate. As a result, a plasma channel forms on the wide face of the conductor (at the xz plane) along z-axis about 75 ns after the onset of current flow. X-ray patterns of the conductors were obtained with radiation from the X -pinch “hot spot”. The recorded expansion of the conductor along the y-axis is in good agreement with the results of numerical simulations.

The threshold of absolute parametric decay instability developing in inhomogeneous plasma in the presence of two decay points was studied as a function of the width of pump waves beam.

The article presents calculations of the electron temperature based on the spectral lines of iron and platinum atoms in the plasma-chemical process of obtaining platinum catalysts: platinum nanoparticles on aluminum oxide microparticles as a carrier. The process was initiated by a microwave discharge in a powder mixture consisting of platinum microparticles and γ-Al₂O₃, as a result of treatment with powerful (up to 400 kW) microwave radiation pulses, the source of which was a gyrotron operating at a frequency of 75 GHz. It is shown that the process leads to the formation of platinum nanoparticles on the surface of aluminum oxide microparticles; the spectral characteristics of the process were measured, and the temperatures were estimated. The data obtained indicate the possibility of transferring platinum particles to the surface of aluminum oxide particles through the gas phase.

The charging of micron-sized dust particles in a multicomponent plasma of humid air and H₂O:O₂:N₂ gas mixtures at various gas ionization rates by an external ionization source is considered. The dust particle charge is determined using a kinetic model of ion-molecular processes encompassing over 600 processes for electrons, negative and positive ions, including hydrated ions containing up to 12 water molecules. It is established that, despite the negligible electron number density far from the dust particle in an undisturbed plasma, the charge of dust particles in humid air is determined by the electron flux when the gas ionization rate exceeds a critical value. This leads to relatively high dust particle charges, comparable to those in electropositive gases. The influence of the gas ionization rate and its composition on the dust particle charge is studied. A comparative analysis of the obtained data with the results of the analytical theory of particle charging at elevated pressures is performed. The issue of the influence of particle charge polarization under the action of a point charge of an ion (electron) on the dust particle charge is considered.

The paper presents the results of measuring the spectral and dose characteristics of pulse-periodic X-ray radiation generated by a plasma accelerator. The device is based on the principle of electron autophasing in the oscillating electric field of a microwave cavity, implemented in the scenario of automatic maintenance of electron cyclotron resonance in a magnetic field that slowly increases in time in the interaction region (gyromagnetic autoresonance—GA). It was found that electrons of generated plasma bunches during acceleration reach energies of up to 0.5 MeV. Due to multiple acceleration and deceleration of electrons in the microwave field, hard X-ray radiation is generated, which is of interest for radiation technologies. It is shown that the bremsstrahlung of bunches has a pronounced anisotropy and spatial regions with its maximum output are experimentally determined. The measured average absorbed dose rate for biological tissue was 2 mGy/s. Conditions for increasing the accelerator’s bremsstrahlung power with an increase in the number of accelerated electrons and their energy are determined. The accompanying characteristic radiation of the plasma-forming gas and the accelerator’s structural elements facing the plasma is investigated.

The paper considers the main physical processes that determine flows of charged particles onto a negative metal electrode immersed in a fully ionized isotropic plasma. Equations are found describing the charged particle motion in the plasma both near the metal electrode under a constant negative potential Ψ₀ and far from it. The charged particle flows from the plasma to the electrode in the charge separation region near the electrode surface are calculated for large ratios of the electrode electric potential Ψ₀ to the plasma electron temperature T_e: eΨ₀/T_e ≫ 1. The ion and electron current densities from the plasma to the electrode are calculated. It is shown that, in the specific case of interaction of a negative titanium electrode with a natural oxide film about 10 nm thick with a pulsed plasma with a density n_i = 10¹³ cm^–3, as a result of charge transfer to the film surface by the ion flow from the plasma, electric voltages of about 6 V and a corresponding strong electric field of about 6 MV/cm arise inside the film over characteristic times of 5–8 µs. Such a strong electric field leads to electrical breakdown of the thin film and excitation of microplasma discharges on titanium. A reduction in the plasma density significantly reduces the probability of excitation of microplasma discharges on the surface of the metal electrode.

Emission of low-frequency barrier discharge plasma in neon at a pressure of 1.4–1.7 Torr formed by transitions between excited states of the Ne⁺ ion is studied in the wavelength range of 290–450 nm using kinetic spectroscopy methods. The difference in the relative intensities of ion lines at different stages of the plasma evolution is discussed: from direct excitation with ionization from the ground state of the atom by electron impact in the active stage (discharge), followed by a transition to the recombination afterglow as the electron temperature relaxes. The latter is due to the collisional-radiative recombination of doubly charged Ne²⁺ ions with electrons, creating at a density of the latter of [e]≈10¹¹ cm⁻³ the population flux of some Ne⁺* levels comparable to excitation by electrons at the stage of the plasma creation. A significant number of ion lines corresponding to transitions from states with the principal quantum number n = 3 contain an intermediate stage, the explanation of which is based on the experimentally confirmed hypothesis that the long-lived neon atoms in metastable states participate in their excitation. In contrast, the spectral lines of transitions from excited states of the Ne⁺* ion of 2s²2p⁴(³P₂)4f configurations do not have this stage.