Low Pressure Helium Discharge Research Articles

PiC/MC-simulation of a low-pressure discharge in helium initiated by a relativistic electron beam in a coaxial chamber with a magnetic cusp

A gas discharge is simulated and numerically studied. The discharge is excited by a tubular relativistic electron beam in gaseous helium in a coaxial gas-discharge chamber. A magnetic field in the form of a cusp was imposed on the chamber. The code based on the PiC/MC-method was used. Electron beam dynamics, time evolution of phase portraits of particles, physical parameters, and spatial-time characteristics of plasma were calculated. It is shown that a dense plasma appears in the cavity under the action of an electron beam in the form of a thin-walled tube of a large length; its concentration can be nHe+≈2×1010 cm−3. At the same time, it turned out that plasma is considerably non-uniform in length; its denser part corresponds to the parts where beams stop. Time dependence of the charge compensation of electrons on the helium atom concentration in the chamber was calculated. The dependence turned out to be decreasing since the rate of the discharge development is increasing with the growth of the helium pressure in the cavity.

Absolute radiometric calibration of a VUV spectrometer in the wavelength range 46–300 nm

Vacuum ultraviolet (VUV) radiation corresponds to photon energies exceeding 6.2 eV and provides access to atomic and molecular resonant transitions in various plasmas. Determining population densities of electronic levels directly connected to the ground state requires an absolute intensity calibration of the VUV spectrometer. For this purpose, synchrotron radiation from electron storage rings is the typical primary standard source associated with transport effort and operating costs. Alternatively, a complex procedure using several secondary standards and discharges directly at the setup in use can be applied. In this work, an absolute intensity calibration avoiding synchrotron radiation to cover the wavelength range 46–300 nm accessible with two detectors—a photomultiplier tube and a channel electron multiplier—is presented. A combination of different standard sources—two deuterium arc lamps, branching-ratios of a nitrogen plasma and a high current hollow cathode—is applied for the relative calibration. The absolute calibration is based on simultaneous measurements of atomic lines in a low pressure helium discharge with the VUV spectrometer and an absolutely calibrated optical emission spectrometer. Special focus is laid on the uncertainties of the relative and absolute calibration depending on the wavelength range. The applicability of the intensity calibration is demonstrated at a hydrogen plasma in the wavelength range 80–300 nm. Using the emissivity of the Lyman series, population densities of the first six excited atomic states are calculated considering opacity effects. The excellent agreement with results from the Balmer series by optical emission spectroscopy manifests the high validity of the performed absolute intensity calibration.

Kinetic simulation of a low-pressure helium discharge with comparison to experimental measurements

Modern computational validation efforts rely on comparison of known experimental quantities such as current, voltage, particle densities, and other plasma properties with the same values determined through simulation. A discrete photon approach for radiation transport was recently incorporated into a particle-in-cell/direct simulation Monte Carlo code. As a result, spatially and temporally resolved synthetic spectra may be generated even for non-equilibrium plasmas. The generation of this synthetic spectra lends itself to potentially new validation opportunities. In this work, initial comparisons of synthetic spectra are made with experimentally gathered optical emission spectroscopy. A custom test apparatus was constructed that contains a 0.5 cm gap distance parallel plane discharge in ultra high purity helium gas (99.9999%) at a pressure of 75 Torr. Plasma generation is initiated with the application of a fast rise-time, 100 ns full-width half maximum, 2.0 kV voltage pulse. Transient electrical diagnostics are captured along with time-resolved emission spectra. A one-dimensional simulation is run under the same conditions and compared against the experiment to determine if sufficient physics are included to model the discharge. To sync the current measurements from experiment and simulation, significant effort was undertaken to understand the kinetic scheme required to reproduce the observed features. Additionally, the role of the helium molecule excimer emission and atomic helium resonance emission on photocurrent from the cathode are studied to understand which effect dominates photo-feedback processes. Results indicate that during discharge development, atomic helium resonance emission dominates the photo-flux at the cathode even though it is strongly self-absorbed. A comparison between the experiment and simulation demonstrates that the simulation reproduces observed features in the experimental discharge current waveform. Furthermore, the synthesized spectra from the kinetic method produces more favorable agreement with the experimental data than a simple local thermodynamic equilibrium calculation and is a first step towards using spectra generated from a kinetic method in validation procedures. The results of this study produced a detailed compilation of important helium plasma chemistry reactions for simulating transient helium plasma discharges.

Plasma cleaning of multilayer mirrors in EUV lithography from amorphous carbon contaminations

The feasibility of plasma cleaning of the multilayer mirrors used in 13.5-nm EUV lithography from carbon contaminations is studied. Experiments conducted in electrodeless plasma of the surface-wave low-pressure discharge in helium and hydrogen demonstrated the high rate, efficiency, and selectivity of this cleaning without any damage of the mirror’s upper protection layer, even at the atomic level. The optimal working parameters of plasma cleaning are determined and its possible mechanism is discussed.

On the mechanism of electric breakdown under low-pressure discharge in helium

It us shown that the left branch of the Paschen curve is determined by the gas phase breakdown, which is caused by the electron avalanche at pD ≥ (pD)e and by the gas ionization by fast positive ions at pD < (pD)e. The (pD)e threshold value is evaluated, and it is established that the E/p ratio is the main parameter determining the gas breakdown.

Precision Measurement of the Three23PJHelium Fine Structure Intervals

The three 2(3)P fine structure intervals of 4H are measured at an improved accuracy that is sufficient to test two-electron QED theory and to determine the fine structure constant alpha to 14 parts in 10(9). The more accurate determination of alpha, to a precision higher than attained with the quantum Hall and Josephson effects, awaits the reconciliation of two inconsistent theoretical calculations now being compared term by term. A low pressure helium discharge presents experimental uncertainties quite different than for earlier measurements and allows direct measurements of light pressure shifts.

Description of a surface-wave-produced microwave discharge in helium at low pressure in the presence of a gas flow

The characterisation of surface-wave-produced discharges is extended to the case where there is a neutral density gradient along the discharge. Experimental results for a low-pressure helium discharge at 2.45 GHz with a gas flow are presented. In order to model the electron density along the discharge, a law giving the variation of the mean power theta needed to maintain an electron with the neutral density is experimentally established. A good agreement is obtained between the measured and calculated electron densities. Furthermore the variations of theta are explained by a simple energy balance model.

Structure of the Hel 43D, 3F-2 3P (λ = 447.15 nm, λ = 447.0 nm) Spectral Line Emitted from a Low-Pressure Linear Discharge

= The line profile of the HeI line at λ447.15 nm has been measured during the ionisation phase and during the afterglow of a condensed low-pressure helium discharge by means of a 10-channel spectrum analyzer having a channel width of 0.1 Å. The temporal resolution was I.6 μs (e-folding time). The filling pressure ranged from one to twenty Torr. The same relative peak structure was observed during the plasma build-up phase and the afterglow, only the intensities of the peaks were different in the two cases. Our measured profiles are quite similar to those obtained under different experimental conditions by other authors who explained the structure as being due to Baranger-Mozer plasma satellites. In contrast to this we did not find any correlation between electron density and peak position, i.e. the interpretation as Baranger- Mozer plasma satellites has to be excluded. The wavelengths of a number of peaks agree well with already identified He2 molecular lines, and there are sufficient experimental indications to conclude that the prominent part of the satellite structure originates from He0 molecular transitions most of which have not yet been identified.

Characteristics of O(I) and N(I) resonance line broadening in low pressure helium discharge lamps

O(I) (130 nm) and N(I) (120 nm) resonance line broadening in a variety of rf- and microwave- excited low-pressure helium discharge lamps of the type used in laboratory and atmospheric experiments has been characterized by a line-absorption-titration technique. A line-absorption model with variable emission line width is used to fit experimentally observed absorptions of ground state O and N produced in a discharge-flow apparatus where the concentrations of the absorbing atoms are indepedently determined by chemical titration. The observed broadening is in general non-thermal and the effect is more pronounced for the O(I) lamps, for which the degree of line broadening increases with decreasing lamp pressure and discharge power. The O(I) results are interpreted in terms of an excitation mechanism which includes electron-impact excitation of O and dissociative excitation of O 2 by helium metastables.

Ionization waves (moving striations) in a low pressure helium discharge — results of measurements compared with a direct solution of the electron Boltzmann equation

The Boltzmann equation expanded in the angular dependence in the velocity space is solved numerically in one-dimensional approximation to obtain the perturbed distribution function. This perturbed distribution is integrated to construct the rate coefficients entering the atomic ion and the metastable continuity equations. From these balance equations the dispersion equations of the fast as well as slow ionization wave varieties are recovered. The space resonances of the electron gas described earlier give rise to the fast s and slow s' wave varieties observed in the positive column of the low current helium DC discharge. The existence regions of these waves are numerically evaluated and compared with experiment. Furthermore, the results on the wave phase velocities and dependence of the characteristic potential on the electric field are confronted with the experimental data. The theory, which involves no free or adjustable parameters, renders a resonable agreement in some points. Possible reasons for the observed discrepancies are also discussed.

Saturation of the Ion-Acoustic Instability in a Weakly Ionized Plasma

The spatial saturation and the frequency dependence of the energy spectrum of ionacoustic waves in a low-pressure discharge in helium in the pressure range 0.03-0.2 Torr has been investigated. In the high-pressure region the spectrum is peaked around the most unstable frequency predicted by linear theory [i.e., ${\ensuremath{\omega}}_{\mathrm{Peak}}\ensuremath{\sim}(\frac{1}{\sqrt{3}}){\ensuremath{\omega}}_{i}$], and the wave energy saturates at the modest value of $\frac{W}{n{T}_{e}}\ensuremath{\simeq}{10}^{\ensuremath{-}5}$. At low pressures the wave energy increases and varies with frequency according to $W(\ensuremath{\omega})\ensuremath{\propto}{\ensuremath{\omega}}^{\ensuremath{-}\ensuremath{\alpha}}$, with $\ensuremath{\alpha}$ assuming values between 2.2 and 2.8.

Parametric excitation of ion-acoustic waves by a high-frequency electric field

The theory of parametric coexcitation of ion-acoustic and electron plasma waves by a high-frequency electric field is reviewed. Numerical calculations of the parametric excitation are reported which include the effect of steady-state drift velocities and plasma-neutral collisions to simulate an actual physical situation. The solutions are compared with experimental results obtained by the interaction of a 1.5-GHz electromagnetic signal with a low-pressure helium discharge.

Comparison of electron densities measured with Langmuir probes and with two different microwave devices

Electron densities in the positive column of a low-pressure helium discharge are measured 1) with cylindrical and spherical Langmuir probes; 2) with a precision microwave interferometer and 3) with a microwave cavity. The resulting electron densities agree very closely for the two microwave diagnostics; the results obtained with the probes are about 30% lower. The origin of this difference is discussed. Electron mobilities are calculated from the measured electron densities and electric field. The agreement with theoretical and experimental results of other authors is good for results obtained at 0.5 torr; at lower pressures a gradually increasing difference is found.

Screened Probe Measurements in the Helium Negative Glow

Detailed examination of the electron energy and density distribution in the negative glow region of a low pressure helium discharge has been made with the aid of a screened probe. This structure eliminates much of the inaccuracy inherent in the use of simple Langmuir probes. Earlier results by Emeleus indicating the existence of three discrete electron groups, have been confirmed and extended and a detailed picture obtained of the conditions near the cathode dark space edge. Spectrophotometric measurements of the intensities of the helium spectral lines confirm the screened probe measurements.

Positive Ions in the Afterglow of a Low Pressure Helium Discharge

The production and loss of ${\mathrm{He}}^{+}$ and $\mathrm{He}_{2}^{+}$ ions in the afterglow of a low pressure helium discharge are studied using a mass spectrometer to analyze the positive ions and microwave techniques to determine the electron density. The change of positive ion and electron density with time is explained by considering three dominant processes in the afterglow: the production of ${\mathrm{He}}^{+}$ ions and electrons by collisions between pairs of metastable atoms, the ambipolar diffusion of the ions and electrons, and the conversion of ${\mathrm{He}}^{+}$ ions to $\mathrm{He}_{2}^{+}$ ions by three-body collisions with neutral atoms. The time constants for the decay of the electron density at low pressures yield the ambipolar diffusion coefficient for ${\mathrm{He}}^{+}$ ions of 560 ${\mathrm{cm}}^{2}$/sec and a frequency for the conversion of ${\mathrm{He}}^{+}$ ions into $\mathrm{He}_{2}^{+}$ ions of 65 ${\mathrm{sec}}^{\ensuremath{-}1}$, both at 1 mm pressure and 300\ifmmode^\circ\else\textdegree\fi{}K. The mobility coefficient for ${\mathrm{He}}^{+}$ ions in helium obtained from the measured ambipolar diffusion coefficient is 14 ${\mathrm{cm}}^{2}$/volt-sec at 300\ifmmode^\circ\else\textdegree\fi{}K and 760-mm pressure and agrees satisfactorily with the value of 12 ${\mathrm{cm}}^{2}$/volt-sec calculated using the quantum-mechanical interaction of the ${\mathrm{He}}^{+}$ ion and the neutral helium atom.