Pulsed high-current discharge in water: adiabatic model of expanding plasma channel and acoustic wave

This paper presents the results of a theoretical and experimental study of the use of a pulsed discharge in water to obtain a strong acoustic wave in a liquid medium. A discharge with a current amplitude of 10 kA, a duration of 400 ns, and an amplitude pulsed power of 280 MW in water at atmospheric pressure created an expanding acoustic wave with an amplitude of more than 100 MPa. To describe the formation of the discharge channel, an isothermal plasma model has been developed, which made it possible to calculate both the expansion dynamics of a high-current channel and the strong acoustic wave generated by it. Our calculations show that the number density of plasma in the channel reaches 10^20 cm^(–3), while the degree of water vapour ionization is about 10%, and the channel wall extends with a velocity of 500 m/s. The calculations for the acoustic wave are in good agreement with measurements

Hot X-ray onsets of solar flares

ABSTRACT The study of the localized plasma conditions before the impulsive phase of a solar flare can help us understand the physical processes that occur leading up to the main flare energy release. Here, we present evidence of a hot X-ray ‘onset’ interval of enhanced isothermal plasma temperatures in the range of 10–15 MK over a period of time prior to the flare’s impulsive phase. This ‘hot onset’ interval occurs during the initial soft X-ray increase and definitely before any detectable hard X-ray emission. The isothermal temperatures, estimated by the Geostationary Operational Environmental Satellite X-ray sensor, and confirmed with data from the Reuven Ramaty High Energy Solar Spectroscopic Imager, show no signs of gradual increase, and the ‘hot onset’ phenomenon occurs regardless of flare classification or configuration. In a small sample of four representative flare events, we tentatively identify this early hot onset soft X-ray emission to occur within footpoint and low-lying loop regions, rather than in coronal structures, based on images from the Atmospheric Imaging Assembly. We confirm this via limb occultation of a flaring region. These hot X-ray onsets appear before there is evidence of collisional heating by non-thermal electrons, and hence challenge the standard modelling techniques.

Influence of metal vapours on radiation characteristics of air arc plasmas

This paper deals with the evaluation of radiation properties of air arc plasma with various admixtures of Cu, Ag, and Fe, respectively. Under assumption of isothermal plasma cylinder, the net emission coefficients were calculated for various arc radii as a function of the plasma temperature up to 30000K. For plasma with prescribed temperature profile, the equation of radiation transfer was solved in the P1approximation, and the radiation flux and its divergence were calculated.

Spectral gradient of the thermal millimetre continuum as a diagnostic for optical thickness in the solar atmosphere

Aims. In this Letter we aim to show how the gradient of the thermal millimetre continuum spectrum, as emitted from the quiet solar atmosphere, may be used as a diagnostic for the optical thickness regime at the centre of the observing frequency band. Methods. We show the theoretical derivation of the gradient of the millimetre continuum for both logarithmic- and linear-scale spectra. We compare this expression with the empirical relationship between the slope of the millimetre continuum spectrum and the plasma optical thickness computed from both isothermal and multi-thermal two-dimensional cylindrical radiative transfer models. Results. It is found that the logarithmic-scale spectral gradient provides a clear diagnostic for the optical thickness regime for both isothermal and multi-thermal plasmas, provided that a suitable correction is made for a non-constant gaunt factor over the frequency band. For the use of observers we present values for this correction at all ALMA bands and at a wide range of electron temperatures. Conclusions. We find that the spectral gradient can be used to find (a) whether the source is fully optically thin, (b) the optical thickness of the source if it lies within the transitional regime between optically thin and thick plasma (τ ≈ 10−1−101), or (c) whether the source is fully optically thick for an isothermal plasma. A multi-thermal plasma will act the same as an isothermal plasma for case (a), however, the transitional regime will only extend from τ ≈ 10−1 to τ ≈ 100. Above τ = 1 the slope of the continuum will depend increasingly on the temperature gradient, as well as the optical thickness, reducing the reliability of the diagnostic.

Thermodynamic and dynamical properties of dense ICF plasma

In present work, thermodynamic expressions were obtained through potentials that took into consideration long-range many-particle screening effects as well as short-range quantum-mechanical effects and radial distribution functions (RDFs). Stopping power of the projectile ions in dense, non-isothermal plasma was considered. One of the important values that describe the stopping power of the ions in plasma is the Coulomb logarithm. We investigated the stopping power of ions in inertial confinement fusion (ICF) plasma and other energetic characteristics of fuel. Calculations of ions energy losses in the plasma for different values of the temperature and plasma density were carried out. A comparison of the calculated data of ion stopping power and energy deposition with experimental and theoretical results of other authors was also performed.