This review focuses on one of the fundamental phenomena that occur upon application of sufficiently strong electric fields to gases, namely the formation and propagation of ionization ...waves–streamers. The dynamics of streamers is controlled by strongly nonlinear coupling, in localized streamer tip regions, between enhanced (due to charge separation) electric field and ionization and transport of charged species in the enhanced field. Streamers appear in nature (as initial stages of sparks and lightning, as huge structures—sprites above thunderclouds), and are also found in numerous technological applications of electrical discharges. Here we discuss the fundamental physics of the guided streamer-like structures—plasma bullets which are produced in cold atmospheric-pressure plasma jets. Plasma bullets are guided ionization waves moving in a thin column of a jet of plasma forming gases (e.g., He or Ar) expanding into ambient air. In contrast to streamers in a free (unbounded) space that propagate in a stochastic manner and often branch, guided ionization waves are repetitive and highly-reproducible and propagate along the same path—the jet axis. This property of guided streamers, in comparison with streamers in a free space, enables many advanced time-resolved experimental studies of ionization waves with nanosecond precision. In particular, experimental studies on manipulation of streamers by external electric fields and streamer interactions are critically examined. This review also introduces the basic theories and recent advances on the experimental and computational studies of guided streamers, in particular related to the propagation dynamics of ionization waves and the various parameters of relevance to plasma streamers. This knowledge is very useful to optimize the efficacy of applications of plasma streamer discharges in various fields ranging from health care and medicine to materials science and nanotechnology.
Here, we show that the ponderomotive force associated with laser speckles can scatter electrons in a laser-produced plasma in a manner similar to Coulomb scattering. Analytic expressions for the ...effective collision rates are given. The electron-speckle collisions become important at high laser intensity or during filamentation, affecting both long- and short-pulse laser intensity regimes. As an example, we find that the effective collision rate in the laser-overlap region of hohlraums on the National Ignition Facility is expected to exceed the Coulomb collision rate by 1 order of magnitude, leading to a fundamental change to the electron transport properties. At the high intensities characteristic of short-pulse laser-plasma interactions (I ≳ 1017 W cm–2), the scattering is strong enough to cause the direct absorption of laser energy, generating hot electrons with energy scaling as E ≈ 1.44 (I/1018 W cm–2)1/2 MeV , close to experimentally observed results.
This article presents the results of experimental studies on radiation flux generation in the submillimeter wavelength range due to a strong beam-plasma interaction. A relativistic electron beam ...(REB) with parameters 0.5 MeV/12 kA/<inline-formula> <tex-math notation="LaTeX">6 \mu \text{s} </tex-math></inline-formula> pumps plasma waves in a plasma column with a length of ~2 m at a plasma density of <inline-formula> <tex-math notation="LaTeX">\sim 10^{15} {\mathrm{ cm}}^{-3} </tex-math></inline-formula> in a magnetic field of ~4 T. In the presence of density gradients in the plasma column, direct measurements of the energy content of the radiation flux 18 cm in diameter leaving the plasma column into the atmosphere showed that its value reaches 5-7 J. The pulse duration of the flux at half of its amplitude was about <inline-formula> <tex-math notation="LaTeX">0.5 \mu \text{s} </tex-math></inline-formula>, and therefore, the pulse power was at the level of ~10 MW. In this series of experiments, the spectral composition of the radiation flux in the frequency range of 0.1-0.5 THz and the energy distribution function of the beam electrons passed through the plasma have been measured.
Plasma cleaning of extreme ultra-violet (EUV) optics for the semiconductor industry requires atomic-level precision. Low-energy ions and neutrals can be highly beneficial for this purpose. However, ...ion energies in many industrial capacitively or inductively coupled plasmas may be too high for atomic precision processing so that ions can cause sputtering and re-deposition of materials and produce vacuum system contamination. We discuss two sources that create low-energy ions: a capacitively coupled (CCP) plasma source operating at several tens of MHz and an electron beam gas ionization source. The goal here is to minimize the contamination by limiting the ion impact energy to a few tens of electron-volts. Ion energy and flux measurements on a grounded surface are characterized with a compact retarding field ion spectrometer. Plasma-induced contamination is quantified using X-ray photo-electron spectroscopy (XPS). Low ion energy plasma sources introducing little surface contamination may be interesting for cleaning and accelerated testing in EUV lithography (EUVL)-related research and for cleaning of front-end optical mirrors in fusion reactor diagnostics.
PDF
