ABSTRACT
The power-off phase of pulsed low-pressure plasmas (the so-called afterglow) in noble gases is a rich field for both fundamental and application oriented research. The physics of these plasmas is complex and involves various processes: Initially, electrons cool rapidly to temperatures close to the gas temperature by evaporative cooling. At sufficiently high plasma densities the low kinetic electron energy strongly enhances three-body recombination into Rydberg states. Finally, subsequent collisional-radiative decay leads to emission of radiation and populates the metastable states of the atoms. The various steps are investigated experimentally and are compared to analytical models. This allows us to follow all steps throughout in a single experiment involving diagnostics of electron density, metastable density, and emission. Excellent agreement with the models is achieved. The mechanisms included are: (i) for electrons, balance between evaporative cooling and Coulomb collisions with ions leading to thermalization; (ii) consistent combination of re-ionization and microfield reduction of the ionization energy in the recombination rate; (iii) adiabatic balance of recombination and collisional and radiative de-excitation; and (iv) radiative population and diffusional and pooling collisional loss of metastable levels. Although the experiment is carried out in argon, the underlying physics is generally applicable for the afterglow of high-density low-pressure discharges in atomic gases.
ABSTRACT
A simple analytical fluid dynamic model is developed for evaporative electron cooling in a low-pressure decaying plasma and compared to a two-dimensional simulation and experimental data for the particular case of argon. Measured electron temperature and density developments are fully reproduced by the ab initio model and the simulation. Further, it is shown that in the late afterglow thermalization of electrons occurs by coupling to the ion fluid via Coulomb collisions at sufficiently high electron densities and not by coupling to the neutral background.
ABSTRACT
BACKGROUND/AIMS: To improve the diagnostic potential of ultrasound in patients with perianal fistulas, by performing a combined perineal and endorectal Doppler sonography investigation before and after contrast enhancement with hydrogen peroxide. METHODOLOGY: Ten patients (9 male and 1 female) with perianal fistulas were tested. After contrast-free transperineal and endoanal sonography, hydrogen peroxide was injected in the fistulas and the ultrasound examinations were repeated. RESULTS: All fistulas were identified by conventional endoanal and transperineal sonography. The contrast application gave more detailed information about the location of the fistula in relation to the anal lumen and sphincters. The fistulas were distributed as follows: intersphincteric - in 5 patients, transsphincteric - in 2 and extrasphincteric complicated (with 1 or more secondary tracks) fistula - in 3 patients. The findings have been confirmed intraoperatively in 9 patients that underwent an operation. Hypervascularization of the wall was detected in 4 patients by means of Doppler investigation which also differentiated arterial blood flow in 3 cases and venous blood flow in one. CONCLUSIONS: The application of the contrast-enhanced endoanal and transperineal sonography offers a more detailed perianal fistula imaging. In addition the Doppler technique can further characterize their blood supply.