Level population densities and line intensities in helium discharges at intermediate pressures

Dothan, F.; Kagan, Yu.

doi:10.1088/0022-3727/14/2/011

Cited by 10 publications

(16 citation statements)

References 28 publications

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“…equations for the excited atoms is simplified accounting for the major processes: (i) direct excitation from the ground state of the metastable 2 1 S and 2 3 S states and of the resonant 2 1 P and 2 3 P states; (ii) contribution to the population densities of the 2 1 P and 2 3 P levels by a stepwise excitation from the 2 1 S and 2 3 S states; (iii) a stepwise excitation of the higher n = 3, 4 levels from the metastable levels and; (iv) step ionization from the metastable and resonant states. The assumption for a predominant contribution of stepwise excitation and ionization is in accordance with the collisional-radiative models [4,11,[30][31][32][33][34] of discharges in helium developed numerically.…”

Section: Modelmentioning

confidence: 80%

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Optical spectroscopy diagnostics of a helium surface wave sustained discharge. II: modelling and evaluation of experimental data

Dountchev

Koleva

Shivarova

1996

Plasma Sources Sci. Technol.

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A theoretical model for the kinetics of the processes for production and destruction of atom excited states in microwave discharges sustained in helium gas at reduced pressures by surface wave propagation is developed. The excited state population densities and the line intensities at the axis of the discharge are obtained from the stationary Boltzmann equation and the set of the rate balance equations for the n ≤ 4 excited states solved in a semi-analytical-semi-numerical manner. The numerical evaluation of the results accounts for the experimental conditions investigated in part I of the study. The comparison of the theoretical results with the experimental data (part I) for the population densities of the 2 1 P, 2 3 P, 4 1 S and 4 3 S excited states allows one to deduce the values of the mean electron energy and the electron density at the axis of the discharge. By these means, the two parts of the study complete a procedure for determination of these two plasma parameters in discharges sustained by propagating surface waves. This procedure for plasma diagnostics is extended to obtaining the radial and axial profiles of the electron density. The measured radial and axial distribution of the 504.7 nm line emitted intensity and radial distribution (through line absorption) of the 2 1 P and 2 3 P population densities (part I) are used. The axial density distribution obtained from completing the optical spectroscopy measurements with the theoretical model is in good agreement with that evaluated from radial decay measurements of the surface wave electric field intensity.

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Section: Modelmentioning

confidence: 80%

“…An approximate solution of the Boltzmann equation is obtained analytically by introducing an analytical approach of the total cross section [29][30][31][32] for excitation of the 2 1 S, 2 3 S, 2 1 P and 2 3 P levels. The set of the rate balance [12] are given; the numbering of the states is according to that used in the text.…”

Section: Modelmentioning

confidence: 99%

Optical spectroscopy diagnostics of a helium surface wave sustained discharge. II: modelling and evaluation of experimental data

Dountchev

Koleva

Shivarova

1996

Plasma Sources Sci. Technol.

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“…The rate coefficients of the corresponding processes are obtained as in Ref. 47 by using the data for the cross sections and for the diffusion coefficient of the neutrals from [48][49][50][51][52]. The comparison of the results in figures 9(a) and 9(b) with the corresponding results (figures 3(b) and 4(b)) obtained in hydrogen discharges shows that all the effects discussed regarding the magnetic filter operation in hydrogen discharges appear also in argon discharges.…”

mentioning

confidence: 86%

“…This is confirmed by the results shown in figure 9, for the space distribution of the electron temperature and the density in an argon discharge, at B 0 = 50 G. The charged-particle production is via direct and step ionization and electron-energy losses in collisions are through both inelastic and elastic collisions. The rate coefficients of the corresponding processes are obtained as in [47] by using the data for the cross sections and for the diffusion coefficient of the neutrals from [48][49][50][51][52] hydrogen discharges shows that all the effects discussed regarding the magnetic filter operation in hydrogen discharges also appear in argon discharges. The values of the electron temperature in the argon discharge are lower than those in the hydrogen discharge.…”

Section: D-model Descriptionmentioning

confidence: 99%

Magnetic filter operation in hydrogen plasmas

Kolev

Lishev

Shivarova

et al. 2007

Plasma Phys. Control. Fusion

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Abstract. Fluid-plasma model description of the operation of a magnetic filter for electron cooling in gas-discharge plasmas is presented in the study. Directed to the use of weak magnetic fields in the sources of negative hydrogen ion beams for additional heating of fusion plasmas, hydrogen discharges have been considered. The numerical results obtained within a 2D model are stressed. The 1D model presented aims at showing main trends whereas the results obtained within the 3D model, also developed, come to confirm the 2D-model description. The models outline importance of the transport phenomena: electron-energy and charged-particle fluxes. Reduction of the thermal flux across the magnetic field together with thermal diffusion and diffusion, acting in a combination, are in the basis of the electron cooling and of the spatial distribution of the electron density. Effects due to the ) ( B E × -drift and to the diamagnetic drift form the fine spatial structure of the plasma-parameter variations. IntroductionThe development of sources of negative ion beams for fusion plasma heating by neutral beam injection [1-4] is one of the stimuli motivating the active research on low-pressure hydrogen discharges. In general, the sources of negative hydrogen ions with volume-production based processes as well as the hybrid sources where surface production is employed are tandem-type sources with a construction ensuring space separation of regions of high and low electron temperature [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. Thus, electron cooling in the discharge is needed and this is provided by a magnetic filter.The magnetic filter is a localized transverse magnetic field. Its effect for cooling the electrons has been proved both experimentally [3,6,7,13,[20][21][22][23][24][25][26], by probe diagnostics and measurements of the current density of the extracted negative ions, and theoretically [6][7][8][9][10][11][12]15,27] by modelling and discussions on the mechanisms governing the operation of the filter. The fluid-plasma models [6][7][8][9][10]15] of the magnetic filter involve importance of the transport processes. However, the different models stress different aspects as mechanisms of the filter operation: (i) reduction of the electron mobility and of the diffusion in magnetized plasmas acting in a combination with the temperature dependence of the Coulomb collision frequency, the latter considered as a factor ensuring lower diffusion of the hot electrons; (ii) thermal conductivity effects, again acting together with Coulomb collisions; (iii) importance of the Lorentz force showing evidence due to cancelled effects of the -) ( B E × and diamagnetic drifts; (iv) importance of the diamagnetic drift; (v) diffusion acting together with elastic electron-neutral collisions. Both 1D and 2D models have been developed, however, as it has been usually stressed, 2D models are needed for proper description of the problem. Due to the complexity of the description, simplifying assumptions, like neglecting of col...

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“…The balance equation of the block includes its excitation (cross section 0j [279]) and deactivation through diffusion (diffusion time j = (R/2.4) 2 D j with D j [cm 2 /s] = 1.29 × 10 18 N −1 0 [cm −3 ] according to [280,281]), ionization (with cross section ji [282]) and transitions to the ground state (with rate coefficient v j0 calculated according to its relation [283] to the rate coefficient v 0j for excitation).…”

Section: Appendix a Argon Dischargesmentioning

confidence: 99%