Spectroscopic measurements and numerical simulations of the electrode plasma of an electrode microwave discharge in hydrogen

The structure of an electrode microwave discharge in hydrogen at pressures of 1–8 torr and incident powers of 20–100 W is studied using optical spectroscopy. A two-dimensional computer code is developed for self-consistently simulating a self-sustained steady-sate electrode microwave discharge ignited at the end of the inner conductor of a coaxial line. The model is based on simultaneously solving time-dependent Maxwell’s equations, the balance equations for charged particles, and a homogeneous Boltzmann equation. The numerical results referring to the electrode region of the discharge are in fair agreement with the experimental data. This confirms the early suggestion (inferred from experimental data) of the combined “self-sustained-non-self-sustained” character of the electrode discharge. It is shown that the self-sustained discharge domain is located in the electrode region of the discharge.     

Gas temperature in the plasma of a low-pressure electrode microwave discharge in hydrogen

The gas temperature in an electrode microwave discharge in hydrogen at pressures of 1–8 torr and input powers of 20–90 W is determined from the relative intensities of the rotational lines of the electronically excited molecules of the Fulcher α system of molecular hydrogen. It is found that the gas temperature in the discharge is no higher than 800 K over the entire range of the experimental conditions under study. For this reason, plasma resonance cannot be regarded as a factor determining the physical processes in the discharge over the entire pressure range. Since the discharge unit is a nonuniform gas-dynamic system (the gas is fed through a small hole into a chamber of limited size), there is a possibility of generating vortex flows that intensively mix the gas. This results in a uniform distribution of the gas temperature throughout the entire volume of the spherical plasma structure produced in the experiment.     

The calculation of optical properties of cesium plasma under conditions of a pulse-periodic discharge

The calculations of the radiation spectrum, the color rendering index R _a , the color temperature T _c , and the color coordinates X _c and Y _c of cesium plasma radiation under the conditions characteristic for a pulse-periodic discharge in cesium are presented. The plasma pressure range of 30–1500 Torr and the range of temperatures on the axis of 3200–6000 K are considered. The color rendering index is shown to be R _a > 90 in the entire analyzed plasma parameter ranges. The color temperature of plasma radiation varies within a broad range of 2300–5500 K. The color coordinates of the cesium plasma radiation are close to the X _c and Y _c coordinates of a black body.     

Laser spectroscopy measurements of the effective temperature of argon ions in the PNX-U plasma neutralizer

Laser spectroscopy measurements of the effective temperature of Ar¹⁺ ions in the PNX-U multipole trap, in which argon plasma is ionized and heated by microwaves under electron-cyclotron-resonance conditions, are performed using a narrow-band tunable dye laser. The absorption profile of the 611.5-nm line is examined. In a microwave power range of 5–50 kW, the observed behavior of the effective temperature of argon ions T_{i, eff} indicates an anomalous mechanism for ion heating. It is shown that certain information about the electron temperature can be deduced from measurements by the laser-induced fluorescence (LIF) technique. The measurements performed also serve to test the laser technique and apparatus that is presently being developed for diagnosing additives to the ITER divertor plasma by the LIF method.     

Hydrocarbon plasma chemistry in a continuous microwave discharge

A study is made of the relation between the kinetic processes involving carbon-containing species and the intensity ratios of different emission lines in synthesizing diamond films in a microwave discharge plasma. The intensity ratios of the emission lines are measured as functions of the pressure, composition, and flow rate of the gas mixture. The kinetic processes involving carbon-containing components are simulated under conditions close to the experimental ones. It is shown that the intensity ratios of different pairs of lines can be used to control diamond film deposition.     

Generation of plasma oscillations in a low-pressure microwave discharge

The resonant excitation of plasma (Langmuir) oscillations during the microwave breakdown of a low-pressure gas is studied both analytically and numerically using the simplest uniform model. It is shown that, because of a significant delay in electron heating and cooling, this effect ensures that the plasma density increases at a high (resonant) rate, even after exceeding a critical value, and can reach a very high (overcritical) level.     

Spectroscopic measurements of the parameters of the helium plasma jets generated in the plasma focus discharge at the PF-3 facility

The spectroscopic technique used to measure the parameters of the plasma jets generated in the plasma focus discharge and those of the plasma of the immobile gas through which these jets propagate is described. The time evolution of the intensities and shapes of spectral lines in experiments carried out with helium at the PF-3 facility was studied by means of electron-optical streak cameras. The plasma electron temperature, T ≈ 4–5 eV, was determined from the intensity ratio of two spectral lines, one of which (λ₁ = 5876 Å) belongs to neutral helium, while the other (λ₂ = 4686 Å), to hydrogen-like helium ions. The plasma density at different time instants was determined from the Stark broadening of these lines in the electric fields of different nature. The plasma density is found to vary from 4 × 10¹⁴ to 2 × 10¹⁷ cm^?3.     

Study of the optical properties of a pulse-periodic high-pressure cesium discharge

Results of theoretical and experimental studies of the optical spectrum of a pulse-periodic high-pressure cesium discharge are presented. The results of calculations are in good agreement with experimental data. The possibility of creating an efficient light source based on recombination emission from the discharge plasma is demonstrated. The formation mechanisms of the continuous spectrum of discharge radiation are considered.     

Brain temperature measurements in rats: a comparison of microwave and ambient temperature exposures

In an effort to understand microwave heating better, regional brain and core temperatures of rats exposed to microwave radiation (2450 MHz) or elevated air temperatures were measured in two studies. In general, we have found no substantial evidence for temperature differentials, or "hot spots," in the brain of these animals. In the first study, after a 30-min exposure, no temperature differences between brain regions either after microwave or ambient air exposure were found. However, a highly significant correlation between brain and core temperatures was found and this correlation was the same for both microwave and ambient air heating. In the second study, time-temperature profiles were measured in rats exposed to either 30 mW/cm2 or 36.2 degrees C. In this study, the 30-min exposure period was divided into seven intervals and the change in temperature during each period was analyzed. Only the cortex showed significantly different heating rates between the air heating and microwave heating; however, this difference disappeared after the initial 5 min of exposure.     

Dissociation of nitrogen in a pulse-periodic dielectric barrier discharge at atmospheric pressure

Nitrogen molecule dissociation in a pulse-periodic atmospheric-pressure dielectric barrier discharge is numerically analyzed. It is shown that the quenching rate of predissociation states at atmospheric pressure is relatively low and the production of nitrogen atoms in this case can be adequately described using the cross section for electron-impact dissociation of N₂ molecules taken from the paper by P.C. Cosby [J. Chem. Phys. 98, 9544 (1993)].     

Coagulation of dust grains in the plasma of an RF discharge in argon

Results are presented from experimental studies of coagulation of dust grains of different sizes injected into a low-temperature plasma of an RF discharge in argon. A theoretical model describing the formation of dust clusters in a low-temperature plasma is developed and applied to interpret the results of experiments on the coagulation of dust grains having large negative charges. The grain size at which coagulation under the given plasma conditions is possible is estimated using the developed theory. The theoretical results are compared with the experimental data.     

Determination of the electron density and electric field in the plasma of a low-pressure microwave electrode discharge in hydrogen from the measured spectral line intensities

The parameters of the plasma of a microwave electrode discharge in hydrogen at pressures of 1–8 torr and incident powers of 20–80 W are measured by the so-called “relative intensity” method. The method allows one to determine the electron density and electric field in plasma by measuring the relative intensities of the H_α, H_β, and 763.5-nm Ar line emission and calculating the electron-impact rate constants from the homogeneous Boltzmann equation. The measurements show that there are regions in the discharge where the electron density is higher (a bright electrode sheath) and lower (a spherical region) than the critical density for the frequency 2.45 GHz (n_cr～7×10¹⁰ cm^?3). Inside the spherical region, the electric field varies slightly over the radius and the electron density increases as the discharge boundary is approached. The observed discharge structure can be attributed to the presence of a self-sustained discharge zone (electrode sheath); a non-self-sustained discharge zone (spherical region); and a decaying plasma region, which is separated from the active discharge zone by an electric double layer.     

Electrode microwave discharge in nitrogen: Structure and gas temperature

Electrode microwave discharges in nitrogen at pressures of 1–16 Torr and input microwave powers of 30–180 W have been studied by space-resolved emission spectroscopy. It is shown that the discharge is highly nonuniform. The relative intensities of the first and second positive nitrogen bands, as well as of the first negative band of nitrogen ions, are found to vary significantly throughout a discharge because, in different discharge regions, emitting particles are excited by different mechanisms. The gas temperature was determined by the method of the unresolved rotational structure of different sequences of the emission spectra of the second positive system of nitrogen.     

Propagation of a surface microwave along the afterglow plasma column of a high-current pulsed discharge

It is demonstrated experimentally that the lifetime of the afterglow plasma of a high-current pulsed discharge in a dielectric tube filled with a mixture of argon with saturated mercury vapor is longer than 1 ms. Such a long lifetime, during which the electron density decreases from 10¹⁴ to 10¹² cm⁻³, is explained by the chemi-ionization of mercury vapor by long-lived metastable argon atoms. During this time, the afterglow plasma can serve as a microwave waveguide for a weakly damped low-noise E ₀-type axisymmetric surface mode, which allows one to use it for transmission of signals in the centimeter wavelength range.     

Simulation of toluene decomposition in a pulse-periodic discharge operating in a mixture of molecular nitrogen and oxygen

The kinetic model of toluene decomposition in nonequilibrium low-temperature plasma generated by a pulse-periodic discharge operating in a mixture of nitrogen and oxygen is developed. The results of numerical simulation of plasma-chemical conversion of toluene are presented; the main processes responsible for C₆H₅CH₃ decomposition are identified; the contribution of each process to total removal of toluene is determined; and the intermediate and final products of C₆H₅CH₃ decomposition are identified. It was shown that toluene in pure nitrogen is mostly decomposed in its reactions with metastable N₂(A₃?? _u ⁺ ) and N₂(a??¹?? _u ^? ) molecules. In the presence of oxygen, in the N₂ : O₂ gas mixture, the largest contribution to C₆H₅CH₃ removal is made by the hydroxyl radical OH which is generated in this mixture exclusively due to plasma-chemical reactions between toluene and oxygen decomposition products. Numerical simulation showed the existence of an optimum oxygen concentration in the mixture, at which toluene removal is maximum at a fixed energy deposition.     

The use of a high-current electron beam in plasma relativistic microwave oscillators

Relativistic microwave electronics faces the problem of using high currents of relativistic electron beams; i.e., it is possible to use beams the current of which is lower than that of actually existing high-current accelerators. We show the possibility of increasing the power of radiation generated in a plasma relativistic microwave oscillator (PRMO) due to an increase in the absolute value of current. For the beam currents close to the value of limiting vacuum current, the efficiency of microwave generation decreases; therefore, we study PRMO schemes with a high value of limiting vacuum current, i.e., schemes with a small gap between a hollow relativistic electron beam and the waveguide wall. The results of the experiment and numerical simulation are discussed.     

Effect of the dc field on the near-surface plasma of a highly nonuniform microwave discharge

The effect of the dc electric field on the near-surface plasma of an electrode microwave discharge at pressures of 1?C5 Torr was studied by the emission spectroscopy method. It is shown that the dc field weakly affects the vibrational distribution of nitrogen molecules in the C₃??_u state, but changes the structure of the near-surface plasma (shifting the intensity maxima of the emission bands) and the strength of the microwave field near the electrode surface. It is also found that the ratio between the intensities of bands of different sequences of the second positive system of nitrogen radiated from the same state depends on the position along the discharge axis.     

Measurements of the electron density and degree of plasma ionization in a plasma target based on a linear electric discharge in hydrogen

Laser interferometry methods were used to measure the density of free electrons and degree of plasma ionization in a hydrogen target intended for experiments on determining energy losses of heavy ion beams in an ionized matter. It is shown that the linear electron density can be varied in the range from 3.3 × 10¹⁷ to 1.3 × 10¹⁸ cm^?2 by varying the initial plasma parameters (the hydrogen pressure in the target and the discharge current). The error in measuring the linear electron density in the entire range of the varied plasma parameters was less than 1%. The maximum degree of plasma ionization achieved at the initial gas pressure of 1 mbar was 0.62 ± 0.05.     

A multicouple probe for temperature gradient measurements in biological materials

An easy-to-make, sensitive, thin, flexible, multisensor probe for in vivo tissue temperature profile measurement is described. It is essentially a multijunction thermocouple (i.e., a multicouple) of type-T composition. Enamel-insulated copper wires (38 gauge) were soldered 5 mm apart to one common uninsulated constantan wire (36 gauge) and introduced into a polyethylene tube sealed at one end. The total outside diameter of the multicouple probe is less than 1 mm, and the maximum number of junctions using the specified wire sizes is approximately 16. This design permits the instantaneous measurement of a tissue temperature profile at 5-mm intervals over a distance of approximately 8 cm. An extensive calibration for the thermal conductivity effect (k effect) along the multicouple wires by means of a limb model is presented. The results show that the temperature readings of the individual junctions are significantly affected by the k effect when a thermal gradient exists along the multicouple, as is usually the case during tissue temperature measurements. However, calibration of the multicouple for the k effect yields a measurement accuracy of +/- 0.1 degree C under a wide range of gradients. This probe can be implanted in tissues to measure thermal gradients under different physiological conditions.     

Estimation of the electron density and radiative energy losses in a calcium plasma source based on an electron cyclotron resonance discharge

The parameters of a calcium plasma source based on an electron cyclotron resonance (ECR) discharge were calculated. The analysis was performed as applied to an ion cyclotron resonance system designed for separation of calcium isotopes. The plasma electrons in the source were heated by gyrotron microwave radiation in the zone of the inhomogeneous magnetic field. It was assumed that, in such a combined trap, the energy of the extraordinary microwave propagating from the high-field side was initially transferred to a small group of resonance electrons. As a result, two electron components with different transverse temperatures—the hot resonance component and the cold nonresonance component—were created in the plasma. The longitudinal temperatures of both components were assumed to be equal. The entire discharge space was divided into a narrow ECR zone, where resonance electrons acquired transverse energy, and the region of the discharge itself, where the gas was ionized. The transverse energy of resonance electrons was calculated by solving the equations for electron motion in an inhomogeneous magnetic field. Using the law of energy conservation and the balance condition for the number of hot electrons entering the discharge zone and cooled due to ionization and elastic collisions, the density of hot electrons was estimated and the dependence of the longitudinal temperature T _e∥ of the main (cold) electron component on the energy fraction β lost for radiation was obtained.