Neutral Gas Depletion Research Articles

Response to “Comment on ‘The effects of radio-frequency bias on electron density in an inductively coupled plasma reactor’” [J. Appl. Phys. 132, 156101 (2022)

The rf-bias-induced decreases in plasma electron density observed by us [J. Appl. Phys. 102, 113302 (2007)] and others [Fox-Lyon et al., J. Vac. Sci. Technol. A 32, 030601 (2014)] are better explained by changes in gas composition, rather than neutral gas depletion.

Convergent neutral gas injection using supersonic gas puffing (SSGP) method for propellant feeding system in RF electric propulsion.

A convergent gas feeding method is proposed to alleviate neutral gas depletion near the central plasma region in typical electrodeless radio-frequency (RF)/helicon plasma thrusters. To achieve further performance improvement, the SuperSonic Gas Puffing (SSGP) system is one of the methods that is expected to overcome the above-mentioned depletion and the density limit. This study discovered that the spatiotemporal profiles of the neutral pressure and the estimated gas diffusion angle vary depending on the SSGP gas feeding condition, i.e., the nozzle size, filling pressure, and the valve opening time. Convergent gas feeding is successfully conducted using the SSGP method in a vacuum. As a preliminary study, high-density plasma is also obtained in the vicinity of the gas injection region using the developed SSGP system. The effects of the gas feeding position and an external divergent magnetic field on the plasma density are investigated. A suitable gas feeding position/region exists for plasma generation using the RF/helicon plasma thruster.

Operation of Large RF Driven Negative Ion Sources for Fusion at Pressures below 0.3 Pa

The large (size: 1 m × 2 m) radio frequency (RF) driven negative ion sources for the neutral beam heating (NBI) systems of the future fusion experiment ITER will be operated at a low filling pressure of 0.3 Pa, in hydrogen or in deuterium. The plasma will be generated by inductively coupling an RF power of up to 800 kW into the source volume. Under consideration for future neutral beam heating systems, like the one for the demonstration reactor DEMO, is an even lower filling pressure of 0.2 Pa. Together with the effect of neutral gas depletion, such low operational pressures can result in a neutral gas density below the limit required for sustaining the plasma. Systematic investigations on the low-pressure operational limit of the half-ITER-size negative ion source of the ELISE (Extraction from a Large Ion Source Experiment) test facility were performed, demonstrating that operation is possible below 0.2 Pa. A strong correlation of the lower pressure limit on the magnetic filter field topology is found. Depending on the field topology, operation close to the low-pressure limit is accompanied by strong plasma oscillations in the kHz range.

Study on Plasma Uniformity Using 2-D Measurement Method in Argon Inductively Coupled Plasmas

In this paper, 2-D plasma density profile is measured in an inductively coupled plasma (ICP) with changing rf power and gas pressure. As the ICP power increases at a fixed argon gas pressure of 100 mtorr, increase in the plasma density is observed at the radial edge. It could be understood by the neutral gas depletion in the discharge center and the step-ionization effect.

Effect of neutral gas heating in argon radio frequency inductively coupled plasma

Heating of neutral gas in inductively coupled plasma (ICP) is known to result in neutral gas depletion. In this work, this effect is considered in the simulation of the magnetic field distribution of a 13.56 MHz planar coil ICP. Measured electron temperatures and densities at argon pressures of 0.03, 0.07 and 0.2 mbar were used in the simulation whilst neutral gas temperatures were heuristically fitted. The simulated results showed reasonable agreement with the measured magnetic field profile.

Dynamics of neutral gas depletion investigated by time- and space-resolved measurements of xenon atom ground state density

The dynamics of neutral gas depletion in high-density plasmas is investigated by time- and space-resolved measurements of the xenon ground state density. Two-photon absorbed laser induced fluorescence experiments were carried out in a helicon reactor operating at 10 mTorr in xenon gas. When the plasma is magnetized, a plasma column is formed from the bottom of the chamber up to the pumping region. In this situation it is found that two phenomena, with different time scales, are responsible for the neutral gas depletion. The magnetized plasma column is ignited in a short (millisecond) time scale leading to a neutral gas depletion at the discharge centre and to an increase of neutral gas density at the reactor walls. This is explained both by neutral gas heating and by the rise of the plasma pressure at the discharge centre. Then, on a much longer (second) time scale, the overall neutral gas density in the reactor decreases due to higher pumping efficiency when the magnetized plasma column is ignited. The pumping enhancement is not observed when the plasma is not magnetized, probably because in this case the dense plasma column vanishes and the plasma is more localized near the antenna.

Neutral depletion in an H− source operated at high RF power and low input gas flow

On an RF driven H− ion source the temperature of the H2 background gas was measured via optical emission spectroscopy as a function of flow and applied RF power at two locations of the source. The source operates at an applied RF power of 30–100 kW and at input gas flows of less than 1 Pa m3 s−1. At the same time the pressure was measured by Baratrons connected to the source body via tubes of length much greater than their diameter. Using these two data sets the background gas density and degree of depletion were determined. The neutral gas depletion was found to be relatively high (50–70%) as compared with the gas density before plasma ignition. The results obtained for the temperatures show the background gas is colder (maximum 1200 K) in the area where the RF power is coupled into the plasma than in the expansion region near the extraction grid (maximum 2100 K). The degree of depletion is highest where the power is coupled in, so the background gas density is lowest there.

Model of an inductively coupled negative ion source: II. Application to an ITER type source

The injection of energetic neutral deuterium atoms will be one of the major heating methods of the ITER plasma. The 1 MeV, 16.5 MW neutral atom beam will be obtained by acceleration and collisional neutralization of negative ions extracted from an inductively coupled low-temperature plasma source. This negative ion source is composed of driver volumes where the RF (radio-frequency) power is inductively coupled to the plasma electrons, an expansion chamber including a magnetic filter, and the extraction grids. In this paper we present the first results of a 2D fluid model of a single-driver prototype of the source, for an H2 plasma under realistic ITER-relevant conditions. We discuss the general plasma properties: plasma density, electron and neutral particle temperatures, ion composition (H+, , ), the dissociation degree of H2 and the effect of the magnetic filter, in a large range of input powers (10–80 kW) and source pressures (0.2–0.8 Pa). Negative ions are not described self-consistently in this first approach. The results show a decrease in the gas density when the plasma is turned on, due to gas heating and to the neutral gas depletion induced by ionization. The low gas density leads to a high electron temperature in the driver, and to the saturation of the plasma density growth with power for pressures below 0.3–0.4 Pa. The H2 temperature is in the 0.1 eV range while the H temperature is much higher (up to 1 eV) because hydrogen atoms are generated at high energies by the dissociation of H2 or ion recombination at the wall surface. The simulation results are globally consistent with recent experiments on the negative ion source developed at IPP Garching. Because of the large Hall parameter in the magnetic filter, electron transport across the filter is complex and the ability of a 2D fluid model to grasp this complexity is discussed.

Competitive effects of an axial magnetic field and of neutral gas depletion in a positive column

Neutral gas dynamics has been incorporated in plasma transport equations in recent studies of nonmagnetized plasma discharge equilibrium. It was found that when the plasma density increases, the neutral gas density becomes depleted in the discharge center, leading to plasma deconfinement. Consequently, larger electron temperature, flatter plasma density profiles, and larger edge-to-center plasma density ratios were observed. In this paper, we investigate the effect of adding a static axial magnetic field to the discharge. We find that at fixed plasma density at the center, the magnetic field reduces the calculated neutral depletion and all the associated effects. Nevertheless, the action of the magnetic field is less pronounced if one keeps the power deposited into the discharge fixed instead. This is because at fixed power, the plasma density increases with the magnetic field.

Plasma diagnostics by optical emission spectroscopy on argon and comparison with Thomson scattering

A novel optical emission spectroscopy (OES) technique for the determination of electron temperatures and densities in low-pressure argon discharges is compared with Thomson scattering (TS). The emission spectroscopy technique is based on the measurement of certain line ratios in argon and a collisional–radiative model (CRM) including metastable transport. The investigations are carried out in a planar inductively coupled neutral loop discharge (NLD) over a wide range of pressures, p = 0.05 Pa–5 Pa. This discharge is a weakly magnetized novel radio-frequency (rf) plasma source, proposed for plasma etching. The NLD is operated in pure argon at a frequency of f = 13.56 MHz and powers varied between P = 1 kW and 2 kW. Both diagnostics, OES and TS, are applied in parallel. The electron energy distribution functions obtained by TS are clearly Maxwellian at low pressures but exhibit a certain enhancement of the energetic tail at higher pressures. Electron densities and temperatures obtained by both diagnostic techniques are compared. Further, absolute numbers of the metastable densities derived from the measurement by the CRM are compared with earlier measurements under similar conditions. Excellent agreement is found throughout if depletion of the neutral gas density by increasing gas temperature and electron pressure is included in the CRM. Electron pressure is the dominant depletion mechanism at gas pressures p ⩽ 0.1 Pa and rf powers P > 1 kW. There, the electron pressure exceeds more than 3 times the neutral pressure and the ionization degree approaches 7% while at pressures p > 1 Pa the degree of ionization is relatively low (<10−3) and neutral gas depletion is dominated by gas heating.

Plasma dynamics in an inductively coupled magnetic neutral loop discharge

Magnetic neutral loop discharges (NLDs) can be operated at significantly lower pressures than conventional radio-frequency (rf) inductively coupled plasmas (ICPs). These low pressure conditions are favourable for technological applications, in particular anisotropic etching. An ICP–NLD has been designed providing excellent diagnostics access for detailed investigations of fundamental mechanisms. Spatially resolved Langmuir probe measurements have been performed in the plasma production region (NL region) as well as in the remote application region downstream from the NL region. Depending on the NL gradient two different operation modes have been observed exhibiting different opportunities for control of plasma uniformity. The efficient operation at comparatively low pressures results in ionization degrees exceeding 1%. In this regime neutral dynamics has to be considered and can influence neutral gas and process uniformity. Neutral gas depletion through elevated gas temperatures and high ionization rates have been quantified. At pressures above 0.1 Pa, gas heating is the dominant depletion mechanism. At lower pressures neutral gas is predominantly depleted through high ionization rates and rapid transport of ions by ambipolar diffusion along the magnetic field lines. Non-uniform profiles of the ionization rate can, therefore, result in localized neutral gas depletion and non-uniform processing. We have also investigated the electron dynamics within the radio-frequency cycle using phase resolved optical emission spectroscopy and Thomson scattering. In these measurements electron drift phenomena along the NL torus have been identified.

Neutral gas depletion mechanisms in dense low-temperature argon plasmas

Neutral gas depletion mechanisms are investigated in a dense low-temperature argon plasma—an inductively coupled magnetic neutral loop (NL) discharge. Gas temperatures are deduced from the Doppler profile of the 772.38 nm line absorbed by argon metastable atoms. Electron density and temperature measurements reveal that at pressures below 0.1 Pa, relatively high degrees of ionization (exceeding 1%) result in electron pressures, pe = kTene, exceeding the neutral gas pressure. In this regime, neutral dynamics has to be taken into account and depletion through comparatively high ionization rates becomes important. This additional depletion mechanism can be spatially separated due to non-uniform electron temperature and density profiles (non-uniform ionization rate), while the gas temperature is rather uniform within the discharge region. Spatial profiles of the depletion of metastable argon atoms in the NL region are observed by laser induced fluorescence spectroscopy. In this region, the depletion of ground state argon atoms is expected to be even more pronounced since in the investigated high electron density regime the ratio of metastable and ground state argon atom densities is governed by the electron temperature, which peaks in the NL region. This neutral gas depletion is attributed to a high ionization rate in the NL zone and fast ion loss through ambipolar diffusion along the magnetic field lines. This is totally different from what is observed at pressures above 10 Pa where the degree of ionization is relatively low (<10−3) and neutral gas depletion is dominated by gas heating.

Plasma transport under neutral gas depletion conditions

It was previously shown that plasma transport is enhanced by neutral gas depletion when the plasma pressure is not negligible compared with the neutral gas pressure. Consequently, the plasma flux leaving the discharge is not a linear function of the central plasma density. The latest theoretical treatments of this problem have assumed isothermal fluids in pressure balance, discarding neutral gas heating. We present a model that incorporates the effects of neutral gas heating on the plasma transport enhancement. This model shows that (i) neutral gas depletion is more pronounced than previously calculated due to gas heating and (ii) as a consequence the plasma transport is further enhanced and the non-linearity of the plasma flux leaving the discharge is more pronounced.

Two-dimensional simulation of an electron cyclotron resonance plasma source with power deposition and neutral gas depletion

Using an improved two-dimensional hybrid model with microwave absorption and neutral gas transport, the properties of an electron cyclotron resonance (ECR) plasma have been investigated. Results indicate that there are two typical profiles of microwave power deposition and neutral gas density over the entire calculated parameter range: one is concentrated near the centre, and decreases with radial positions; another is a hollow distribution. The average value of neutral gas density in the source area is about 50-60% of that in the downstream region. The dependence of plasma parameters such as plasma density, electron temperature and ionization rate, and the ion angle distribution function at the substrate surface on the position and pressure were discussed. Results of our simulation can be compared qualitatively with many experimental measurements.

A self-consistent global model of neutral gas depletion in pulsed helicon plasmas

The time-dependent global model is employed to examine the temporal behavior of the electron density and temperature in helicon plasmas. The power absorption calculated from the solutions of the Maxwell equations is used in solving the power balance equation and a balance model for the neutral gas is considered to find its density self-consistently. The numerical results successfully explain neutral gas depletion and the occurrence of two distinct modes of pulsed helicon discharge, which have been observed experimentally.

Two-dimensional simulation of compact ECR plasma sources

In the present paper, argon plasma behaviour in a compact electron cyclotron resonance (ECR) plasma reactor is systematically studied by means of 2D simulation. Using a hybrid electron fluid - article ion plasma and particle neutral model, we have obtained the plasma potential, density, electron temperature, ionization rate, ion kinetic energy and velocity distributions and have found that the neutral gas pressure plays an important role in affecting all these distributions. In addition, non-uniform neutral gas depletion effects are examined by using a 2D Monte Carlo method. The depletion can obviously improve the plasma parameter uniformity. The average ion kinetic energy impacting on the substrate increases while the neutral gas pressure decreases. It is found that the plasma density is decreased by a factor of two and the profile becomes more uniform if non-uniform gas density is considered. Meanwhile, the ionization rate is sensitive to the non-uniform neutral density. The ion random kinetic energies of both the parallel and the perpendicular components are obtained and the ion rotating directed movement is observed. The perpendicular random energy near the top of the reactor can be much larger than that near the bottom.

Hydrodynamic Theory of Low‐Pressure Multi‐Component Discharges with Neutral Gas Depletion

AbstractA hydrodynamic description of a low‐pressure discharge with singly and doubly‐charged positive ions in the high ionization regime is given. Modified hydrodynamic equations are used to include the neutral gas depletion by ionization processes under free‐flight conditions. Analytical formulas for the concentrations of the two species of ions at the discharge axis are derived.Numerical results for the radial drift velocities, the radial distributions of the densities and the neutral gas temperature are presented. The nature of the dip in the density profile of the singly‐charged ions and the decrease in their radial drift velocity near the axis with increasing degree of ionization are discussed.

The Theory of the Low Pressure Discharge in the Strong Ionization Regime

AbstractA system of integro‐differential equations describing the velocity distribution functions of the neutral atoms and of the ions and the radial profiles of the electric potential and the particle densities in the positive column at low pressures and high degrees of ionization is derived. This system is solved for a simplified, but self‐consistent, plane model. Both the initial velocity of the ions and the neutral gas depletion caused by the ionization processes are taken into account. For a cylindrical model a solution is obtained for the region near the axis. With rising neutral gas temperature the radial ion flux density is slightly increasing. The pressure tensor, the heat flux tensor, and several components of the corresponding tensor of the fourth order for the ion gas are calculated. It is shown that on the axis the radial and the azimuthal ion temperatures are smaller than the neutral gas temperature because the ions generated out of neutral atoms are retarded during the flight to the axis.

Stability limits of high-power ion-laser discharges

Highly ionized plasmas (degree of ionization 0.1,...,0.5) with electron temperatures kTe≳4 eV are required for high-power generation from ion lasers. In stationary low-pressure arcs, however, these requirements are limited by discharge instabilities. The stability limits are investigated in the case of Ar and Hg low-pressure discharges. It is shown that the main limitation processes are due to neutral gas depletion by ionization and axial plasma inhomogeneities. The stability ranges of ion-laser discharges are represented using the geometry-independent parameters jR and pR ( j: discharge current density; p: filling pressure; R: discharge radius). These instabilities, together with saturation effects in the laser mechanism, limit the output power of ion lasers by restricting the operational range of the discharges.

Current limitation in mercury vapour discharges II. Experiment

In part I of the present work, theoretical predictions were made of the upper and lower current limits of a low-pressure gas discharge, and the variation of the limits with gas pressure and tube diameter. Experimental confirmation of these predictions is presented. The simple relation between arc current and ion wall current employed in part I is confirmed with the use of wall probes. The total flux of heavy particles to and from the discharge walls is measured and found to be independent of arc current. The predictions of neutral gas depletion are confirmed using the technique of resonance radiation absorption. The variation of excited state densities at current limit is measured and confirms that at current limit the electron energy distribution always attains the same limiting form.