Value Of Plasma Potential Research Articles

Influence of the size of the Langmuir probe insulator on the measurements of inhomogeneous plasma parameters in a discharge with a hollow cathode

The influence of the insulator of a cylindrical Langmuir probe on the results of the plasma potential, electric field, electron energy distributions and their concentration measurements in a discharge in helium at a pressure of 3 mbar maintained by a hollow cathode was studied. The measurements were carried out using moving probes with spatial resolution along the cathode-anode axis. Two probes with the same geometry of the conductive part and different diameters of the insulators were used. It was determined that the effect of the insulator is different for different plasma parameters. An increase in its diameter leads to a decrease in the measured value of plasma potential and to an increase in the electron concentration and the average electron energy. The degree of this influence is different in various regions of inhomogeneous plasma and correlates with the local values of the average electron energy. Under experimental conditions, the differences are maximum near the cathode, weaken as the electron energy decreases with distance from it, and are negligible at average energies less than 1.5 eV.

Langmuir probe without RF compensation

Langmuir probes with RF compensation are used for measurements of electron concentration, electron temperature and the DC value of plasma potential in RF discharges. In order to obtain all the RF components of plasma potential, simple probes without RF compensation are used. However, it has been believed that these uncompensated probes can not be used for determination of the DC value of plasma potential and of electron concentration and temperature, since their VA characteristics is distorted by the RF current. Consequently, the evaluation of data measured with uncompensated probes was not possible without additional measurement with a RF compensated Langmuir probe. This contribution analyzes the possibility to use uncompensated probes not only for measurement of RF components of plasma potential, but also for measurement of the DC component of plasma potential, electron concentration and electron temperature.

Afterglow dynamics of plasma potential in bipolar HiPIMS discharges

In bipolar magnetron sputtering, the plasma afterglow is initiated by switching the target bias from a negative to positive voltage. In the following, the plasma potential evolution in this configuration is characterized, being responsible for the ion acceleration at the substrate sheath potential fall, in particular in high power impulse magnetron sputtering (HiPIMS). A mass-energy analyzer and a Langmuir probe respectively measure the ion energies and the plasma/floating potential at different positions within HiPIMS discharges. A plasma potential drop and rise in the first 45 μs of the afterglow is observed, settling in the plasma bulk towards values below the applied positive bias. The measured ion energies agree with the plasma potential values before and after the drop-rise. To gain more comprehensive insights into the mechanisms responsible for such a potential evolution, particle-in-cell Monte Carlo 3D simulations of bipolar direct current magnetron sputtering discharges are explored in equivalent geometries. Despite their average power being orders of magnitude lower compared to the HiPIMS configuration, a similar afterglow behavior is observed. This indicates that the measured dynamics are not specific to HiPIMS, but rather a feature of bipolar magnetron sputtering. The responsible mechanisms are studied further: the effects of various system parameters are decoupled, with the magnetic field configuration emerging as crucial for the plasma potential drop-rise dynamics and the associated re-ionization close to the target.

Temperature response of laser heated emissive probe materials under vacuum and free atmospheric conditions

Precise temporal and spatial knowledge of plasma potential has been a challenging task for decades. Gradient in values of plasma potential govern local electric fields providing insight into many other bulk plasma properties like particle drifts, confinement, transport barriers etc and plays a crucial role in determining stability of magnetically confined high temperature plasmas. In high temperature devices like tokamaks, plasma tends to develop edge bifurcations and results in edge transport barriers, which are a key tool for enhancing the plasma confinement properties in magnetic fusion devices, which in turn requires knowledge of plasma potential. Conventional emissive probes (CEPs) in high temperature magnetically confined plasmas are not advisable owing to their inherent properties and tokamak parameters like high magnetic field, ultra-high vacuum pressure etc as well as tokamak geometry. A new type of emissive probe is becoming popular in recent times in such devices called the laser heated emissive probe (LHEP). Mostly, LaB6 and graphite are used as a LHEP tip owing to their inherent properties of thermal conductivity, low work function, high emissivity, higher lifetime etc. Similar with LaB6 in its mechanical and electrical properties, CeB6 is emerging as a promising candidate for LHEP. CeB6 is a better electron emitter than graphite and LaB6 at comparatively low power due to its lower work function. In this work, the heating dynamics of LaB6 and CeB6 heated by a CW CO2 laser with maximum power of 55 W have been reported. Theoretical and simulation models using Matlab and ANSYS have been developed to understand and explain the temperature gaining process of the probes. Simulation results are further validated by comparing them with experimentally measured data using an infrared camera.

Effect of plasma grid bias on extracted currents in the RF driven surface-plasma negative ion source.

Extraction of negative ions from the large inductively driven surface-plasma negative ion source was studied. The dependencies of the extracted currents vs plasma grid (PG) bias potential were measured for two modifications of radio-frequency driver with and without Faraday screen, for different hydrogen feeds and for different levels of cesium conditioning. The maximal PG current was independent of driver modification and it was lower in the case of inhibited cesium. The maximal extracted negative ion current depends on the potential difference between the near-PG plasma and the PG bias potentials, while the absolute value of plasma potential in the driver and in the PG area is less important for the negative ion production. The last conclusion confirms the main mechanism of negative ion production through the surface conversion of fast atoms.

Comparative measurements of plasma potential with ball-pen and Langmuir probe in low-temperature magnetized plasma

The ball-pen probe (BPP) is used for direct plasma potential measurements in magnetized plasma. The probe can adjust the ratio of the electron and ion saturation currents Isat−/Isat+ to be close to one and therefore its I-V characteristic becomes nearly symmetric. If this is achieved, the floating potential of the BPP is close to the plasma potential. Because of its rather simple construction, it offers an attractive probe for measurements in magnetized plasma. Comparative measurements of plasma potential by BPPs of different dimensions as well as one Langmuir probe (LP) in an argon discharge plasma of a cylindrical magnetron were performed at various experimental conditions. An additional comparison by an emissive probe was also performed. All these types of probes provide similar values of plasma potential in a wide range of plasma parameters. Our results for three different BPP dimensions indicate that the BPP can be operated in a cylindrical magnetron DC argon discharge if the value of the ratio of the magnetic field and neutral gas pressure, B/p, is greater than approximately 10 mT/Pa.

Temporal behavior of the tungsten plasma produced by 1064 nm pulsed Nd-YAG laser

A time-resolving Langmuir probe was used to investigate the expansion of tungsten plasma produced by the pulsed Nd-YAG laser (wavelength=1064nm, pulse duration=10ns). Current–voltage (IV) plots at various times after the laser shot hit the target were obtained from probe electron and ion currents. Langmuir probe theory was used to determine the temporal variation of the plasma potential, electron temperature and electron density. These plasma parameters sharply rise to a maximum value and then slowly decrease during the plume expansion. For the laser irradiance of 10.88×108W/cm2, maximum values of plasma potential, electron temperature and electron density were about 24V, 15.8eV and 2.24×1016cm−3 respectively. The measurements performed with time-of-flight electrostatic energy analyzer confirmed the presence of highly charged tungsten ions in the plasma and charge state of ions found to increase with the laser irradiance.

Langmuir probe diagnostics in the IMPF device and comparison with simulations and tracer particle experiments

The plasma parameters in the IMPF (International Microgravity Plasma Facility) device for studying complex plasmas are determined with a 2D-scanning Langmuir probe system, which is suitable for operation on the International Space Station. The probe characteristics in low density, low pressure radio frequency (rf) discharges are evaluated in the framework of the radial-motion theory with corrections for collisions. A critical comparison between experimental values for plasma potential and ion density with SIGLO simulations (Kinema Software) is made, which yields good overall agreement. In particular, the shaping of the plasma profile by different rf power in the plasma centre and in an outer ring is well described. Small amounts of dust particles are used as a novel method to mark the boundary, where ion-drag force and electric field force are balanced. Again close agreement with simulation is found and demonstrates the applicability of the tracer technique as a quantitative diagnostic means.

Langmuir Probe Measurements in the WEGA Stellarator

AbstractLangmuir probe measurements have been performed in weakly magnetized (0.06T < B0 ≤ 0.1T), lowpressure (6.× 10–4Pa ≤ p ≤ 1.0 × 10–2Pa) helium, hydrogen and argon plasmas of the WEGA stellarator. A simple cylindrical graphite probe as well as a double probe were used. The observed characteristics do not show saturation of the ion current. The reason for this anomaly can be the presence of a fast electron component in the plasma caused by the heating method ( ECRH at 2.45 GHz ). Using the model from [5] for derivation of the current to the probe for a two‐temperature Maxwellian electron distribution the characteristics were successfully fitted. Radial profiles of the electron temperature, density and plasma potential as a function of heating power, neutral gas pressure, magnetic field and rotational transform have been obtained. The plasma potential values were compared to results from emissive probe measurements. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Mass spectrometric and Langmuir probe measurements in inductivelycoupled plasmas in Ar, CHF3/Ar and CHF3/Ar/O2 mixtures

Absolute fluxes and energy distributions of ions in inductivelycoupled plasmas of Ar, CHF3/Ar, and CHF3/Ar/O2 have been measured.These plasmas were generated in a gaseous electronics conference cell modifiedfor inductive coupling at pressures of 10-50 mTorr and 100-300 W of13.56 MHz radio frequency (RF) power in various feed gas mixtures. In pure Arplasmas, the Ar+ flux increases linearly with pressure as well asRF power. In mixtures, the Ar+ flux decreases with increase in pressureand CHF3 concentration in the mixture. The loss mechanism for Ar+ isattributed to resonance charge exchange(Ar+ + CHF3→products). Total ion flux in CHF3 mixturesdecreases with increase in pressure and also CHF3 concentration. Relativeion fluxes observed in the present studies are analysed with the help ofavailable cross sections for electron impact ionization and charge-exchangeion-molecule reactions. Measurements of plasma potential, electron and ionnumber densities, electron energy distribution function, and mean electronenergy have also been made in the centre of the plasma with an RF-compensatedLangmuir probe. Plasma potential values are compared with the mean ionenergies determined from the measured ion energy distributions and areconsistent. Electron temperature, plasma potential, and mean ion energy varyinversely with pressure, but increase with CHF3 content in the mixture.

Nonself-sustained arc discharge in anode material vapors

The basic characteristics of the nonself-sustained arc discharge in a vapor of the anode material are studied. The influence of thermoemission parameters of the cathode on volt-ampere characteristics of the discharge is described. It is established that in a free mode of the discharge cathode operation, when the discharge current I/sub D/ is lower than the current of the thermoelectron emission from the discharge cathode I/sub C/, the deposition rate of the films q is directly proportional to the discharge current I/sub D/. In the compelled mode of the cathode operation, when I/sub D/>I/sub C/, q/spl sim/W/sub D//sup 2/, where W/sub D/=I/sub D/U/sub D/ with U/sub D/ being the discharge voltage. It is shown that the magnetic field increases the plasma density and changes the density profile from n(x)-1/x/sup 2/ to n(x)-1/x with x being a distance along the flow. The motion of created plasma flow is shown to have a noncollisional character with constant electron temperature of 5-7 eV along the flow. The values of plasma potential and electric field in the flow are determined; the values of cathodic and anodic potential drops in the discharge are evaluated. The angular distributions of ion and neutral fluxes in the created plasma flow are described. It is shown that the plasma flows parameters depend substantially on the working material. With use of crossed electric and magnetic fields, the flow ionization coefficient was enhanced up to 85% for the discharge in Ti vapors, and 35% for the discharge in Cu vapors.

Experimental Study on Heat Flux from an Argon RF Plasma Using Laser Interferometry Method

The plasma heat flow to a birefringent substrate (LiNbO3) in an argon rf discharge has been investigated with the help of a laser interferometry method. This method is based on monitoring the variation of natural birefringence with temperature by the interferometry. The evaluation of the temperature curve shows that heat flux from the plasma towards the substrate depends on which electrode the substrate is put on. While the value is about 8% of the applied power in the case of the substrate on the powered electrode, the heat flux decreases to less than 1% in the case of on the grounded electrode. Ion fluxes have been obtained from the measured values of plasma potential, electron density and electron temperature. In the case of low pressure the ion flux approximately agrees with the value of the heat flux. However it decreases with an increase in pressure, and is almost 35% of the heat flux at 200 mTorr. Gas temperature in the plasma has been measured using a thermocouple in order to investigate the influence of gas pressure on the heat flux by neutral particles. The result shows that the gas temperature increases by raising the gas pressure.

Ion energy distributions in SiCl 4 and Ar/O 2 dry etching discharges

Mass-resolved energy distributions (IEDs) are reported for a variety of ions striking the grounded electrodes of two dissimilar dry etching reactors operating at 13.56 MHz. All IEDs are recorded using the same electrostatic sector field energy analyser/quadrupole mass analyser combination. The chemistries studied are the etching of III–V multilayer structures in SiCl 4 plasmas and photoresist removal in Ar/O 2 plasmas. Values for plasma potential are determined and contrasted with theoretical predictions. The predominantly collisionless IEDs show peak splittings in good agreement with an analytical model, from which the dependence on ion mass and sheath width are etermined. A marked decrease in sheath width in O 2 and SiCl 4 plasmas compared with argon is inferred and attributed to the effect of negative ions.

Plasma diagnostic studies for reactive ion etching systems

Detailed understanding of rf reactive ion etching plasmas requires reliable measurements of the plasma properties. The paper describes such measurements for 13.56 MHz discharges in a parallel plate etching system, using argon as the test gas. Using a special type of Langmuir probe to determine the plasma potential together with measurements of the electron density n e and the electron temperature T e enables the plasma to be specified. The remaining parameter of importance in etching systems is the energy of the ions striking the electrodes. In this work ion energy measurements at the grounded electrode are reported and plasma potential values deduced from them may be compared with those obtained from the probe and electrode voltage measurements. The data are used to test a computer model of the motion of ions through the sheath regions.

The effect of non-Markovian terms on the Landau equation

The Landau equation can be derived as the sum of an expansion series in time provided certain non-Markovian short-time terms are neglected. The time series could be very useful for the solution of a variety of problems. It is necessary to estimate the time span over which the neglected term s are significant. The non-Markovian terms for the case of a homogeneous plasma are evaluated using a technique developed by Prigogine and Balescu. For typical values of plasma potential cut-offs the conditions under which the short-time terms may be ignored are estimated. It is found that for electrons of number density less than 1010 and protons of number density less than 1014 it may be possible to ignore the short-time effects. It is conjectured that for many situations the non-Markovian effects will be important for a time interval which has a lower bound t 0 and an upper bound several orders of magnitude greater than t .

Self-generated electrical fields in flames

When a spherical electrostatic probe is immersed in ionized flame gases, the probe floatingelectrical potential, measured relatively to a fixed electrical earth potential, varies with position. Results are presented for a stoichiometric premixed methane-air flame at a pressure of 38 torr in an axial traverse. The range of variation of floating potential was 1.3 volts and cannot be explained in terms of the calorelectric effect. It is proposed that the generated voltage is a consequence of the diffusion of positive ions and electrons from the region of ion formation. A theoretical expression is presented for the spatial variation of plasma potential. This variation is derived from experimental values of positive ion concentration. Floating potentials are in turn obtained from these values of plasma potential using electrostatic probe theory. Theoretical and experimental values of floating potential are in fair agreement. Differences in floating potential can yield an open-circuit voltage that is able to supply current through an external resistance. A generator working on these principles is termed a diffusion generator. It is shown how current-voltage characteristics for both diffusion and calorelectric generators may be obtained from theoretical dimensionless probe characteristics and a knowledge of the differences of plasma potential and temperature between electrodes. Experimental diffusion generator characteristics are in fair agreement with those of theory.

Cooperative Effects in a Tenuous Energetic Plasma Contained by a Magnetic Mirror Field

The formation and characteristics of a steady-state hydrogen plasma contained in a magnetic mirror field are described. The mean ion energy is 20 keV. The plasma is formed by ionizing and trapping a portion of a beam of energetic hydrogen atoms passing through the confining field. The methods of measurement used to determine the plasma properties are described. Measurements of the radial and azimuthal trapped-ion distributions, the average ion and electron densities, and the plasma potential are compared with the predictions of simple theory, neglecting cooperative plasma effects. The observed deviations from these simple predictions show that the plasma properties are dominated by cooperative phenomena. The plasma density is found to be limited to a low value (∼4 × 107 ions/cm3) by a flute or drift instability. This instability is characterized by a low frequency rotation of the plasma at a frequency typically close to the ▿B precession frequency of a 20-keV proton in the nonuniform mirror field. The plasma density at the unstable limit is observed to be related to the value of plasma potential. This relationship can be understood quantitatively by an extension of an earlier theoretical treatment of the drift instability to the case of unequal ion and electron densities. The flute oscillation frequency is also derived from this theory. Oscillations at the ion gyrofrequency are observed, marking the presence of an electrostatic resonance type of instability. Coupling of these modes with the flute oscillations is described, but detailed identification of the electrostatic resonance modes cannot yet be made.