Tsanko Vaskov Tsankov

On the OES line-ratio technique in argon and argon-containing plasmas

Optical emission spectroscopy is used to investigate capacitively coupled argon and argon–hydrogen–silane plasmas. The argon collisional–radiative model (CRM) used to extract the electron density and temperature from the spectra is presented. The electron energy distribution function, which is an input parameter to the model, is discussed in detail. Its strong variation with pressure is found to significantly influence the results for the (effective) temperature. For the analysis of the spectra the common line-ratio technique is applied. Special attention is paid to the choice of lines and a pair of line-ratios for optimum accuracy is suggested. For the argon gas mixture at high partial pressure of the admixed molecular gases the CRM reduces to a corona-like model, extended by a quenching term. The line-ratio method is found to fail under these conditions due to the strong depopulation of the argon 1s states. As a consequence, individual line intensities have to be used and an absolute calibration is required. An easy calibration method, which relies on the results obtained by the line-ratio method in pure argon, is proposed and applied.

The influence of the spatial nonuniformity on the measurement of Ar*(1s5) density by the self-absorption technique

The self-absorption technique is a simple method to determine the line integrated density of metastable atoms in low-pressure plasmas. In order to employ this technique, it is necessary to make an assumption on the spatial profiles of both the emission intensity and the absorbing species, which usually are unknown and can be highly nonuniform. Taking Ar*(1s5) atoms in a capacitively coupled plasma as an example, the influence of nonuniformity on the measurement of the line integrated density of the absorbing species is investigated analytically. It is proved that when the two spatial profiles are the same, the obtained line integrated density is independent of the functional form of this profile. This is also true if the density of the absorbing species is small (weak absorption), even though the emission profile is different from that of the absorbing species. However, if the density is high (strong absorption), the choice of the profiles has a significant influence on the deduced line integrated density. In the experiment, it is found that in argon–oxygen mixture discharges, in which Ar*(1s5) density is low (weak absorption), the measured densities by self-absorption are in very good agreement with the results obtained from laser absorption. However, in pure argon discharge, when the density is high (strong absorption), the measured densities by self-absorption are significantly smaller than that by laser absorption. Both phenomena have been predicted by the model results. The smaller densities obtained by self-absorption in the pure argon discharge are attributed to the assumption of the same spatial profile used in the model.

Rotational and vibrational temperatures in a hydrogen discharge with a magnetic X- point

A novel plasma source with a magnetic X-point has been developed to probe an alternative for cesium-free negative hydrogen ion production. This study presents first results for the gas and vibrational temperatures in the source at 1 Pa and various RF powers. The temperatures are obtained from analysis of the intensity distribution of the molecular Fulcher-α bands. The gas temperature increases with the RF power, while the vibrational temperature remains constant in the studied range of RF powers. Both quantities show no appreciable spatial dependence. The obtained high values of the vibrational temperatures indicate a high population of the vibrational levels, favourable for the volume negative ion production. A theoretical concept indicates the presence of an optimum value for the vibrational temperature at which the negative hydrogen ion yield by volume processes has a maximum. Coincidently, the measured value is close to this optimum. This indicates that the novel concept can provide certain advantages...

A discharge with a magnetic X‐point as a negative hydrogen ion source

The study presents first results from investigations of a novel low‐pressure plasma source, intended for a negative hydrogen ion production. The source utilizes a dc magnetic field, shaped to form a cusp with a magnetic null‐point (X‐point). Beside the common role of filtering out the high energy electrons, this magnetic field configuration ensures in the present case also an interesting mechanism of coupling the RF power to the plasma. Investigations performed using radio frequency modulation spectroscopy (RFMOS) reveal that the main power coupling to the electrons is confined in the region on one side of the X‐point. The modulation of the light intensity indicates also the presence of a strong dc drift close to the plane of the X‐point. Several hypothesises for its explanation are raised: an azimuthal diamagnetic drift due to strong axial gradients of the electron energy, the excitation of a standing helicon wave, which couples to the radial magnetic field in the plane of the X‐point, or a Trivelpiece‐G...

2D collisional-radiative model for non-uniform argon plasmas: with or without 'escape factor'

Collisional-radiative models for excited rare-gas atoms in low-temperature plasmas are a widely investigated topic. When these plasmas are optically thick, an ‘escape factor’ is introduced into the models to account for the reabsorption of photons (so-called radiation trapping process). This factor is usually obtained assuming a uniform density profile of the excited species; however, such an assumption is often not satisfied in a bounded plasma. This article reports for the first time a self-consistent collisional-radiative model without using an ad hoc ‘escape factor’ for excited Ar atoms in the 2p states (in Paschen’s notation). Rather, the rate balance equations—i.e. the radiation transfer equations—of the 2p states are numerically solved to yield the actual density profiles. The predictions of this self-consistent model and a model based on the escape factor concept are compared with spatially-resolved emission measurements in a low-pressure inductive Ar plasma. The self-consistent model agrees well with the experiment but the ‘escape factor’ model shows considerable deviations. By the comparative analysis the limitations and shortcomings of the escape factor concept as adopted in a significant number of works are revealed.

Hydrogen Discharge With a Magnetic

Discharges in hydrogen are actively investigated not only in connection with materials processing and surface treatment but also as sources of negative hydrogen ions for fusion applications. Current sources for H- production rely on surface reactions enhanced by cesium which can cause substantial problems in operation. Here, we present a novel design concept intended for negative hydrogen ion production. First, results on the discharge performance are shown.

X

A long-standing debate in the literature about the kinetic form of the Bohm criterion is resolved for plasmas with single positive ion species when transport is dominated by charge exchange collisions. The solution of the Boltzmann equation for the ions gives the exact form free of any divergence and contains an additional term that is not included in the classical result. This term includes collisional and geometric effects and leads to a noticeable correction. Further, the question is addressed whether the space charge argument at the bottom of the Bohm criterion can actually lead to a meaningful definition of the transition point between bulk and sheath. The analysis is supported by a numerical model and experiments, showing excellent agreement throughout. As a novelty in diagnostics, the theoretical results allow from the ion velocity distribution function (IVDF), measured at the wall, a reconstruction of the IVDF and the electric field at any point in the plasma. This property is used to reconstruct non-intrusively also the ion density, flow velocity, mean energy and effective temperature and the electron density and temperature as functions of the spatial coordinate and potential. Finally, the fluid equation for ion momentum balance is verified.

-Point

The electron density evolution is investigated in a pulsed 60 MHz capacitively coupled low pressure oxygen discharge. A 77 GHz microwave interferometer is used for the time-resolved measurement of the electron density. It is found that, as the duration of the power-off period varies in the range of 0.01–2 ms, the electron density evolution pattern changes significantly. This change is more pronounced at the power-on front, due to the competition between the dissociative-attachment and the ionization processes. A global model is developed to explain the behaviour of electron density evolution under different conditions. The model predicted results agree well with the measured ones. Moreover, it is proposed that, using the measured electron density and a simple global model in the afterglow, the electronegativity and the negative ion lifetime during the power-on phase can be estimated. The value of the electronegativity at various electron densities obtained by this method is found to be consistent with the results measured by the photo-detachment technique in a previous work (Stoffels 1995 Phys. Rev. E 51 2425) under similar discharge conditions.

Information hidden in the velocity distribution of ions and the exact kinetic Bohm criterion

A radio-frequency low-pressure ICP discharge superimposed by a static magnetic field with an X-point configuration exhibits peculiar starlike structures in the total light emission. An enhanced emission at particular azimuthal angles is observed if the axial position of the magnetic null-point plane lies within the flange ports. The rays appear at angles at which the ten flange ports are located, suggesting an azimuthally inhomogeneous electron energy distribution.

The electron density evolution in pulsed 60 MHz capacitively coupled oxygen discharges

Usually the self-absorption method is adopted to determine the metastable species density in a large number of low-temperature plasmas. In the previous works this method only gives an average metastable density from the line-integrated emission out of the plasma, assuming a uniform density profile. Instead, this work uses an optical probe immersed in the plasma. From the emission spectra measured at different positions, it is possible to obtain the actual metastable density profile. As an example, this technique is used to investigate an inductive Ar plasma at pressures 0.1–1 Pa. The spatial densities of Ar(1s5) and Ar(1s3) (in Paschens notation) are experimentally determined, being in agreement with those predicted by a self-consistent collisional–radiative model published previously. Based on these results, application of the same method to the other kinds of rare-gas discharges is discussed.