Presence Of Fast Electrons Research Articles

Effect of fast electrons on emission properties of non-equilibrium nitrogen plasma in soft x-ray spectral range

Soft x-ray emission at 2.3–4.4 nm waveband in nitrogen plasma can be observed mostly from 2p-1s and 3p-1s resonant transitions. Ions of nitrogen in plasma emitting in necessary band have a high energy excited state in comparison with plasma temperature in experiments. Discharge and laser produced plasmas used in radiation sources often have non-equilibrium electron distributions with fast electrons. In this study the radiative properties of non-equilibrium nitrogen plasma with fast electrons are examined and the plasma conditions for optimal soft x-ray emission output in the water-window region are explored. Kinetic parameters for non-equilibrium plasma including major inelastic ion interaction processes, radiation characteristics are obtained in approach based on the Hartree-Fock-Slater quantum-statistical model and the distorted wave approximation. The presence of fast electrons in plasma at relatively small portions may have a significant influence on the ion level populations and the line emission intensities. Emission generation efficiency doubles at a temperature of 60 eV with 1% portion of fast electrons with respect to equilibrium plasma and the corresponding spectral efficiency jumps to 5.5%.

Self-sustained focusing of high-density streaming plasma

We describe our observations of the transport through an electrostatic plasma lens of a wide-aperture, high-current, low energy, metal-ion plasma flow produced by a cathodic arc discharge. The lens input aperture was 80 mm, the length of the lens was 140 mm, and there were three electrostatic ring electrodes located in a magnetic field formed by permanent magnets. The lens outer electrodes were grounded and the central electrode was biased up to −3 kV. The plasma was a copper plasma with directed (streaming) ion energy 20–40 eV, and the equivalent ion current was up to several amperes depending on the potential applied to the central lens electrode. We find that when the central lens electrode is electrically floating, the current density of the plasma flow at the lens focus increases by up to 40%–50%, a result that is in good agreement with a theoretical treatment based on plasma-optical principles of magnetic insulation of electrons and equipotentialization along magnetic field lines. When the central lens electrode is biased negatively, an on-axis stream of energetic electrons is formed, which can also provide a mechanism for focusing of the plasma flow. Optical emission spectra under these conditions show an increase in intensity of lines corresponding to both copper atoms and singly charged copper ions, indicating the presence of fast electrons within the lens volume. These energetic electrons, as well as accumulating on-axis and providing ion focusing, can also assist in reducing the microdroplet component in the dense, low-temperature, metal plasma.

Effect of frequency on microplasmas driven by microwave excitation

The effect of excitation frequency on the breakdown voltage of a microwave (mw) microdischarge and its steady-state plasma parameters is studied by the self-consistent one-dimensional Particle-in-Cell Monte Carlo collisions model. It is found that for microdischarges in which the electron wall losses are significant, an increase in the mw frequency results in a decrease in the breakdown voltage. For conditions in which the electron wall losses become negligible, an increase in the frequency does not influence significantly the breakdown voltage. At the same time, for both regimes, the increase in the frequency results in an increase in the steady-state plasma density. The analysis of the steady-state plasma parameters have shown that the dominant electron heating mechanism is the Joule heating while the stochastic heating is negligible. Also, it is found that the electron energy distribution function consists of two electron groups, namely, slow and fast electrons. The presence of fast electrons in the plasma bulk indicates the non-local nature of microwave excited microdischarges.

Spatially-resolved optical emission from a bi-polar pulsed DC magnetron discharge

Using optical emission spectroscopy we have observed the origins of short bursts of visible light radiation from the bulk plasma in a pulsed magnetron discharge. These bursts occur during the fast transients in the driving bi-polar voltage waveform. In an extension to the work reported in Swindells et al. [I. Swindells, P.J. Kelly and J.W. Bradley, New J. Phys. 8 No 4 (2006) 47], we have made new spatially-resolved measurements of the light intensity from transitions in Ar(I) at 750.4 nm and 811.5 nm and from Ti(I) at 398.2 nm. The results indicate that energetic electrons (> 13.5 eV) are generated both at the target during off–on phase of the pulse and at a grounded substrate during the on–off phase and penetrate deep into the plasma. The presence of fast electrons during the on–off transition of the bi-polar pulse is correlated directly with positive exertions in the plasma potential relative to the grounded substrate, while the energetic electrons observed during the off–on voltage swings are believed to those accelerated by the advancing cathode sheath.

Effects of fast monoenergetic electrons on the ion dynamics near the cathode in a pulsed direct current plasma sheath

A detailed one-dimensional simulation of the ion dynamics of the plasma sheath near a substrate (cathode) in the presence of fast monoenergetic electrons has been carried out in this article. The sheath evolution is investigated by using a fluid model assuming that the ions, plasma electrons and monoenergetic, fast electrons act as three fluids (fluid approach). The effect of the density of fast electrons on the ion density, ion velocity, and ion energy near the cathode and the evolution of the sheath boundary in front of the cathode are separately explored. Also, the variation of the ion velocity and ion density at the vicinity of the cathode as a function of time is investigated in the absence and presence of the electron beam. Results indicate that the presence of fast electrons in the sheath causes significant change in the sheath thickness and therefore basically changes the ion velocity, ion density, and ion impact energy on the cathode compared to the absence of the electron beam case.

Lower hybrid wave coupling and propagation in the presence of nonthermal edge electrons

In order to improve our understanding of lower hybrid current drive experiments in high power regimes, where fast electrons (up to few keV) are generated in front of the lower hybrid (LH) grill, an analysis of LH wave coupling and propagation is carried out using a formulation of the dielectric tensor that incorporates the fast electron population (Thot≲5 keV). It is found that both the real and imaginary parts of the dielectric tensor are modified for waves with |N∥|≳10, which can interact with the fast electrons through Landau damping. Consequently high N∥ waves can be absorbed within less than 1 cm, or even cut-off at the grill–plasma interface. Antenna–plasma coupling code results show that the power available to accelerate electrons in front of the grill (launched power with |N∥|>10) decreases in the presence of fast electrons. Also, despite strong modifications of the dielectric tensor, and of the power transfer to high N∥ waves, the presence of fast electrons (Thot≲5 keV) leads to only moderate changes of antenna reflectivity and directivity.

Heat transport in steep temperature gradients. I - Small flaring solar loops

Results on nonlocal heat transport which properly takes into account the presence of fast electrons with mean free paths much longer than the temperature scale height L are reviewed. In terms of the mean free path for the slow bulk electrons, lambda(s), the nonlocal effects are important whenever lambda(s)/L greater than 0.001, with the following consequences. The heat flux in the hot part of the gradient is reduced relative to the Spitzer-Haerm value q(SH) which does not take into account the heat carried away by the fast electrons. The heat flux in the cold part of the gradient is enhanced relative to the value q(SH) which does not take into account the heat deposited by the fast electrons. These quite general results, which should have several applications in astrophysics, are applied to the problem of thermal hard X-ray burst models. It is shown that heat is not bottled up as effectively as in some past models, and temperatures achieved for realistic energy input rates are consequently not as high. As a result such sources can be effective only in the soft part (10-30 keV) of the hard X-ray range for energy input rates up to 6,400 ergs/cu cm s. The analysis is based on a fluid model and does not consider the X-ray signature of fast electrons which escape to distances far beyond the conduction fronts formed. It is shown that such electrons could at most be effective in the soft part of the hard X-ray range.

Conditions d'existence d'une population d'electrons rapides dans une decharge capacitive de haute frequence

Abstract A simple theoretical model is proposed to explain the presence of fast electrons in a h.f. electrodeless capacitive discharge at low pressures ( p −2 Torr).