Thermo-gravitational convection in a continuous optical discharge

The relative importance of such processes as the free convective motion of the gas, the absorption of the laser radiation and radiative heat transfer is discussed. The burning of a continuous optical discharge under experimental conditions [6] is theoretically investigated. The two-dimensional problem of the convective motion of the gas in an optical discharge burning in a vertical CO₂ laser beam inside a cylindrical chamber is solved. The principal characteristics of thermogravitational convection of the radiating air under conditions of local thermodynamic equilibrium at atmospheric pressure are studied on the temperature interval from 300 to 20 000°K.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 4, pp. 124–129, July–August, 1989.     

Recombination of electrons in a plasma expanding into a vacuum

As noted in a paper by one of the authors [1], when a hot ionized gas expands into a vacuum, at a certain moment ionization equilibrium must necessarily break down. Shortly after this point, which may be found by the method indicated in [1], ionizing events become very rare and only recombination occurs in the gas. In [1] photorecombination and triple collisions with the capture of an electro to the ground level of the atom were considered. Here the recombination did not proceed to the end: on expanding to infinity and cooling to zero the gas remained partially ionized.Papers have recently appeared [2–7] in which the significant role of triple collisions with the capture of electrons to upper atomic levels is noted. The recombination process has a cascade character at low temperatures and densities which are not excessively small. At first, the electron is captured by one of the upper atomic levels in a triple collision with an ion and another electron. Subsequently, as a result of electron collisions of the second kind, and later also as a result of radiative transitions, the bound electron descends through the energy levels to the atomic ground state. The recombination coefficient for such a process depends much more strongly on the electron temperature T than for a triple collision with capture directly by the ground level (as T^–9/2 as opposed to T^–1), and at low temperatures cascade recombination proceeds much more quickly than capture to the ground level. Since this casts doubt upon the conclusions of [1] regarding the residual ionization when a plasma expands into a vacuum, we were led to re-examine the question, which, as will be clear from what follows, is not considerably more complicated.     

Application of the theory of excursions of a random field to the analysis of radiation from a rough surface in the quasistatic approximation

A phenomenological model is proposed and implemented for calculating the electrodynamic and radiative characteristics of a small-scale statistically rough surface. Elements of the theory of excursions of a random field are used, providing a simple means for determining the effective parameters of the transition layer from the given characteristics of the random surface. A numerical example is discussed, illustrating the main functional dependences of radiation from a Gaussian surface with a broad spectrum of roughness elements (corrugations). Restrictions are not imposed on the slopes of the corrugations.Institute of Cosmic Research, Academy of Sciences of the USSR. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 34, No. 2, pp. 147–156, February, 1991.     

Monte Carlo method for finding the ionization and secondary emission coefficients and <Emphasis Type="Italic">I–V</Emphasis> characteristic of a Townsend discharge in hydrogen

Secondary cathode emission coefficients for hydrogen are determined from experimental data on breakdown by calculating the number of ionization events in gaps using the Monte Carlo method. Other parameters being calculated are the ionization rate, Townsend ionization coefficients (with demonstration of their nonlocality), velocity of electron drift toward the anode, and probabilities of electron return to the cathode. The calculated and measured values are in reasonable agreement. Gaps with pd = 0.37–17.0 Torr cm are considered. It is shown that the observed negative differential resistance of the Townsend discharge can be related to a decrease in the probability of electron return to the cathode with increasing E/N only if the value of pd is near the minimum in the Paschen curve.     

Quantification of cerebral perfusion using the "bookend technique": an evaluation in CNS tumors

We present a method of quantifying cerebral blood volume using dynamic susceptibility contrast. Our approach combines T₂-weighted echo planar imaging (EPI) pulse sequences and reference scans that determine the parenchymal T₁ changes resulting from an injection of a gadolinium chelate. This combined T₂- and T₁-weighted approach (the “bookend” technique) has been shown to be effective in the quantification of gradient-echo (GRE) (T₂*-weighted) perfusion images but has not been applied to spin-echo EPI (SE-EPI) (T₂-weighted) images. The physics related to blood volume measurement based on T₂- and T₂*-weighted EPI sequences is known to be different, and there is a question as to whether the bookend approach is effective with SE-EPI. We have compared the quantitative SE-EPI with GRE-EPI in a series of patients with central nervous system (CNS) tumors. We found that quantitative cerebral blood volume (qCBV) values for SE-EPI and GRE-EPI are in agreement with each other and with historical reference values. A subjective evaluation of image quality showed that image quality in the SE-EPI scans was high and exhibited high interreader agreement. We conclude that measuring qCBV using the bookend technique with SE-EPI images is possible and may be a viable alternative to GRE-EPI in the evaluation of CNS tumors.