Laser-induced fluorescence study of a xenon Hall thruster

2 ⁰→6p[3/2]₂(³P₂-¹D₂) transition at 823.2 nm and the xenon-ion 5d[3]_7/2→6p[2]_5/2 ⁰(⁴D_7/2-⁴P_5/2) transition is used to measure plasma parameters in the plume of a laboratory-model xenon Hall thruster. The Hall discharge operates nominally at 62 V, 4.2 A, and 3.2 mg s^-1 xenon flow, with an overall thruster power of 320 W. A tunable semiconductor diode laser and an Ar⁺-pumped dye laser are used to probe the respective excited-state transitions. Axial velocity measurements are made at a number of axial and radial locations up to 4.5 cm downstream of the thruster-exit plane and under a variety of thruster operating conditions. Neutral velocities from 100 m s^-1 to 400 m s^-1 and ion velocities as high as 12 km s^-1 are calculated from measured Doppler shifts. The charge-exchange phenomenon evidently does not significantly affect the xenon neutrals. The spectral-line shapes of the ion indicate a spread in ion energies through a non-Maxwellian distribution of axial velocities. Neutral kinetic temperatures of 500 (±200) K are observed under standard operating conditions. Zeeman and Stark effects on the spectral-line shapes, from the thruster’s magnetic and electric fields, are not substantial. The measured line center of the ion transition is 16521.23 (±0.02) cm^-1. Received: 20 January 1997/Revised version: 12 May 1997     

Laser-induced-fluorescence characteristics of a helium arcjet flow

Laser-induced fluorescence is used to characterize the axial velocity and temperature field at the exit plane of a low-power helium arcjet. Two cases were examined, one in which the mass-flow rate was changed at a constant current, and the other where the current was changed at a constant mass-flow rate. At constant mass-flow rate, the velocity scales with the increase in power. At constant current, a higher specific energy results in an unexpected lower mean exit velocity. The temperature profiles show that the nozzle-wall temperature is greater than the mean exit temperature. Along the axis of the arcjet, these measurements indicate the presence of a shock less than one nozzle diameter downstream of the exit. Received: 10 July 2002 / Published online: 22 November 2002 RID="*" ID="*"Corresponding author. Fax: +1-650-723-1748, E-mail: qwalker@stanford.edu     

Pulsed X-ray diode with a long-lifetime plasma cathode

6 shots, the X-ray dose, uniformity, and ionisation rate values guarantee an effective preionisation of excimer laser discharges. Owing to both the long lifetime and the substantial absence of maintenance, this X-ray diode seems suitable to preionise commercial gas lasers, such as excimer and TEA CO₂ lasers. Received: 28 August 1998 / Revised version: 16 November 1998 / Published online: 24 February 1999     

Quantitative comparison of different preionization schemes of KrF laser discharges

* fluorescence radiation from the ion–ion recombination process in the gas volume during the preionization phase, therefore allowing spatial resolution by partial imaging of the volume. Volume-integrated and temporally resolved measurements are carried out to determine the average of absolute preionization densities by comparison with a theoretical model of the temporal behaviour of the recombination process. Spatially resolved measurements reveal the distribution of the preionization density. The preionization densities determined from spark and sliding/corona discharges schemes are considerably higher (n_e ⁰?10¹² cm^-3) than those obtained from pulsed X-ray preionization (n_e ⁰?10⁷ cm^-3). Received: 3 October 1997/Revised version: 6 January 1998     

Equipment for measurements of radial profiles of excited species in DC glow discharge

The original experimental device for studying the radial dependences of emitted radiation, generated by the cylindrically symmetric positive column of the DC glow discharge, is proposed. Principle of detection of the radial intensity profiles of the spectral lines by means of the special scanner with two movable conic mirrors is described. The function of the scanner was verified experimentally and by the method of raytracing. As an example, the radial profiles of lines emitted by the positive column of the neon glow discharge are presented. No significant changes were found. This work is a part of the research plan MSM 0021620834, that is financed by the Ministry of Education of the Czech Republic.     

Application of laser-guided discharge to processing

Received: 5 May 1998/Accepted: 14 August 1998     

Diagnosis of Methane Plasma Generated in an Atmospheric Pressure DBD Micro-Jet by Optical Emission Spectroscopy

Diagnosis of methane plasma, generated in an atmospheric pressure dielectric barrier discharge （DBD） microplasma jet with a quartz tube as dielectric material by a 25 kHz sinusoidal ac power source, is conducted by optical emission spectroscopy （OES）. The reactive radicals in methane plasma such as CH, C2, and Ha are detected insitu by OES. The possible dissociation mechanism of methane in diluted Ar plasma is deduced from spectra. In addition, the density of CH radical, which is considered as one of the precursors in diamond-like （DLC） film formation, affected by the parameters of input voltage and the feed gas flow rate, is emphasized. With the Boltzmann plots, four Ar atomic spectral lines （located at 675.28nm, 687.13nm, 738.40nm and 794.82nm, respectively） are chosen to calculate the electron temperature, and the dependence of electron temperature on discharge parameters is also investigated.     

Measurement of the gas temperature in a glow discharge by fluorescence spectroscopy

From the measurement of the distribution of sputtered iron atoms over the fine structure levels of the iron groundstate (a ⁵ D), the gas temperature of an argon glow discharge (p=1 Torr) has been derived under collision dominated conditions at distances larger than 30 mean free paths from the iron cathode.     

Collective behavior of laser-produced plasma from a multicomponent YBa2Cu3O7 target in air

2 Cu₃O₇, using a Q-switched Nd:YAG laser is investigated by time-resolved emission-spectroscopic techniques at various laser irradiances. It is observed that beyond a laser irradiance of 2.6×10¹¹ W cm^-2, the ejected plume collectively drifts away from the target with a sharp increase in velocity to 1.25×10⁶ cm s^-1, which is twice its velocity observed at lower laser irradiances. This sudden drift apparently occurs as a result of the formation of a charged double layer at the external plume boundary. This diffusion is collective, that is, the electrons and ions inside the plume diffuse together simultaneously and hence it is similar to the ambipolar diffusion of charged particles in a discharge plasma. Received: 30 January 1998/Revised version: 12 June 1998     

Investigations on ionic processes and dynamics in the sheath region of helium and hydrogen discharges by laser spectroscopic electric field measurements

Temporally and spatially resolved measurements of the electric field distribution in the sheath region of RF and dc discharges provide a detailed insight into the sheath and ion dynamics. The electric field is directly related to the sheath ion and electron densities, the sheath voltage, and the displacement current density. Under certain assumptions also the electron and ion conduction current densities at the electrode, the ion current density into the sheath from the plasma bulk, the ion energy distribution function, and the power dissipated in the discharge can be inferred. Furthermore, the electric field distribution can give an indication of the collision-induced conversion between different ion species in the sheath. Laser spectroscopic techniques allow the noninvasive in situ measurement of the electric field with high spatial and temporal resolution. These techniques are based on the spectroscopic measurement of the Stark splitting of Rydberg states of helium and hydrogen atoms. Two alternative techniques are applied to RF discharges at 13.56 MHz in helium and hydrogen and a pulsed dc discharge in hydrogen. The measured electric field profiles are analyzed, and the results discussed with respect to the ion densities, currents, energies, temporal dynamics and species composition. Received: 26 July 2000 / Accepted: 12 December 2000 / Published online: 3 April 2001     

Hollow cathode discharge (HCD) dark space diagnostics with laser photoionization and galvanic detection

We report on a new technique for measuring the cathode dark space width and the variation of ground state atomic density within it by measuring the ionization current generated by laser photoionization of neutral ground state metallic atoms. The technique is supported by a theoretical model of charge displacement in the dark space based on the assumption of a Lorentzian gas and thus on Langevin equations. After verifying the applicability of the theoretical approach, measurements of dark space width with respect to pressure, current and nature of the buffer gas are presented for an uranium HCD. Results of variation in density of ground state neutral uranium, in the dark space, versus current and pressure in Xe are also given. These last results are of interest when using photoionization currents in the HCD dark space for laser spectroscopy, such as photoionization studies of elements like uranium.     

Investigation of radial profiles of vibrational and rotational temperatures in nitrogen DC glow discharge

Spatially resolved measurements of vibrational and rotational temperature determined from the N₂(C) nitrogen bands intensities have been performed by means of optical scanner of original construction. It has been found that radial variations of studied bands are independent of pressure and discharge current under our experimental conditions, i.e. in the pressure range (100–300) Pa and for discharge current up to 40 mA. Moreover, it has been found that vibrational as well as rotational temperatures stay almost constant in the radial direction. No radial changes of both temperatures can be explained by good thermal conductivity of the positive column of DC glow discharge. This research was supported by grants: Charles University No. GAUK 194/01, Ministry of Education of Czech Republic MSM 11320002, and Grant Agency of Czech Republic GAČR 202/03/0827. The theme of presented article was included in the EU project No. G1RT-CT-2002-05083 “Plasmatech”.     

Investigation of XeCl vibrational and quenching kinetics: Measurements of gain and laser spectra in a discharge-pumped oscillator module

The small signal gain, amplified spontaneous emission and laser spectra of a transverse discharge-excited XeCl laser have been measured. Several gas mixtures, total gas pressures and electron densities have been investigated. From these measurements it is concluded, that bound-free transitions and transitions to high-lying vibrational levels of the ground state contribute significantly to the gain and laser emission. For the upper laser level a vibrational population ratio [XeCl(B, =1)]/[XeCl(B, =0)] corresponding to a temperature of about 370 K has been determined. The intensity ratio of about 1 observed for the two laser lines may be explained by the vibrational and quenching kinetics of the lower laser level together with the upper state kinetics. A table summarizing the result of the small gain measurement has been included for model comparison.     

Optical Emission Spectroscopy Investigation of a Surface Dielectric Barrier Discharge Plasma Aerodynamic Actuator

The optical emission spectroscopy of a surface dielectric barrier discharge plasma aerodynamic actuator is investigated with different electrode configurations, applied voltages and driving frequencies. The rotational temperature of N2 （C^3 Ⅱu） molecule is calculated according to its rotational emission band near 380.5 nm. The average electron energy of the discharge is evaluated by emission intensity ratio of first negative system to second positive system of N2. The rotational temperature is sensitive to the inner space of an electrode pair. The average electron energy shows insensitivity to the applied voltage, the driving frequency and the electrode configuration.     

Rotational and Vibrational Temperatures of Atmospheric Double rc Argon--Nitrogen Plasma

The spectroscopic technique is employed to study the emission ofatmospheric argon--nitrogen plasma jet generated by an original dcdouble anode plasma torch. The molecular bands of the N₂⁺first negative system are observed at the torch exit and chosen toevaluate the rotational and vibrational temperatures in comparison withthe simulated spectra. The excitation temperature (T_exc ≈9600K) is determined from the Boltzmann plot method. The results show that the rotational, vibrational, electron and kinetic temperatures are in good agreement with one another, which indicates that the core region of atmospheric double arc argon--nitrogen plasma jet at the torch exit is close to the local thermodynamic equilibrium state under our experimental conditions.     

The influence of Exciting Frequency on N2 and N2+ Vibrational Temperature of Nitrogen Capacitively Coupled Plasma

By using optical emission spectroscopy （OES）, N2 and N2^＋ vibrational temperatures in capacitively coupled plasma discharges with different exciting frequencies are investigated. The vibrational temperatures are acquired by comparing the measured and calculated spectra of selected transitions with a least-square procedure. It is found that N2 and N2^＋ vibrational temperatures almost increase linearly with increasing exciting frequency up to 23 MHz, then increase slowly or even decrease. The pressure corresponding to the maximum point of N2 vibrational temperature decreases with the increasing exciting frequency. These experimental phenomena are attributed to the increasing electron density, whereas the electron temperature decreases with exciting frequency rising.     

Impulse-scaling in a laser-driven in-tube accelerator

The laser-driven in-tube accelerator (LITA) is a unique device for laser propulsion. It is characterized by the acceleration of a projectile in a tube. The thrust performance can be improved by exploiting a confinement effect. In the experiment, a 3.0-g projectile is vertically launched, and the momentum coupling coefficient is measured for various monoatomic gases. The measured coupling coefficient is almost proportional to the reciprocal of the speed of sound. The same impulse generation characteristics are obtained in simplified situations that are analyzed based on conservation relations. Received: 26 August 2002 / Accepted: 8 February 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +81-22/217-5284, E-Mail: sasoh@ifs.tohoku.ac.jp RID="**" ID="**"Present address: also at Institute of Advanced Aerospace Technology, Seoul National University, Seoul, 151-742, Korea     

Determination of Ionization Coefficient of Atmospheric Helium in Dielectric Barrier Discharge

A weakly luminous layer close to the anode is observed at time far ahead of the current pulse in dielectric barrier discharge of helium at atmospheric pressure and it is considered as the result of a very weak Townsend discharge. Based on the assumption that the space charge produced by this Townsend discharge is too small to distort the uniform electric field in the gas gap, the electrons have more or less the same energy over the entire gap and the spatial distribution of the discharge light is proportional to the distribution of electron density. This light distribution is obtained by processing side-view photograph of discharge gap using an intensified charge coupled device camera with an exposure time of 20ns. By fitting a theoretically derived formula with the measured curve of light distribution, the Townsend electron ionization coefficient α is determined to be 31 cm^-1 at E/p = 3.6 V·cm^-1·Torr^-1, which is much higher than that obtained by solving the Boltzmann equation of pure helium. It is believed that penning ionization of helium metastables with impurity of nitrogen molecules makes great contribution to the experimentally determined α value. The contribution of this penning ionization to α is roughly estimated.     

Numerical modeling of a XeCl laser excited by microwave discharge

12 , 169 (1987). Reasonably good agreements in the peak output power and laser efficiency have been achieved. Model calculations also predict that an efficiency as high as 2.7% can be obtained once the conditions of the above-mentioned experiments have been optimized. From the consideration that the skin depth effectively limits the absorption length of the microwave pumping and hence the excitable volume, it is concluded that high input power densities (>2 MW/cm³) and higher gas pressures (between 3 and 10 atm) are the preferable conditions to achieve higher efficiency. Preliminary calculations on CCl₄ containing XeCl gas mixtures show that improvement in laser efficiency by several folds may be achieved as a result of the higher intrinsic efficiency of excimer formation. Received: 23 September 1996 / Revised version: 25 March 1997     

λ=2537 Å line from a low-pressure mercury discharge lamp emission profile and line absorption by a gas containing a mercury vapor

By measuring the emission-line intensity of Hgi =2537 Å (6³ P6¹ S) from a low-pressure mercurcury lamp, we have determined the dependence of the upper-level population on the discharge current and the mercury vapor density. From the radial profile of the intensity the spatial distribution function of the population has been determined to be the zero-th order decay mode for the trapped radiation. Line absorption by mercury vapor in a cell has been measured with various mercury densities and Lorentzian widths. The results are consistent with the upper-level population distribution as determined above. On the basis of these findings we calculate the emission-line profile and its change during the absorption in the absorption cell. The amount of absorption at an arbitrary depth of the absorption cell is calculated, and the optimum cold-spot temperature of the lamp of 40–50°C is suggested for the maximum absorption under the typical condition of the photo-CVD experiment.