Electrostatic Double Probe Research Articles

Measurements of beam current density and space potential in a highly focused ion beam of extremely low energy

Using an electrostatic double probe, profiles of the ion beam current density, electron density, and electron temperature were measured in the background plasma that appears by injecting a hydrogen ion beam into low-pressure hydrogen gas in a propagation chamber. The hydrogen ion beam is extracted using concave-shaped electrodes at extremely low energies (Eib = 60–120 eV). Focusing of the beam occurs when Eib exceeds a certain threshold. One probe electrode P3 is far from the beam source and is set in the shadow of another electrode in the beam trajectory. The ion saturation current in P3 is then estimated without considering the beam contribution. The measured electron densities are much larger than those of the ion beam density, the electron temperatures are very low (<1 eV), and the ion beam current densities exhibit fairly good agreement with those measured by Faraday cups. The profiles of the space potential are also estimated by measuring the potential of P3 with respect to ground with a voltage divider having an extremely high resistance. The space potentials obtained are quite low at <10 eV with focusing and ∼23 eV without focusing. The data with and without focusing are compared and conditions for focusing are examined. Focusing achieved through additional electron beam injection in the ground electrode is also examined. The results obtained indicate that a large electron source is required to balance the ion charges. Secondary electron emissions and/or small electron beam injection are effective sources.

Static elimination in vacuum using plasma jet

Vacuum processes (in typical < 10−3 Pa) are expanding in the manufacturing environment of organic LED panels and food packaging films. However, the method of static elimination of charged materials and products in vacuum has never been established yet unlike in atmosphere where corona ionizers are available. Therefore, the authors have been proposing a new static elimination method using a plasma jet as a source of neutralizing charge carriers. In the present paper, the model of the plasma static elimination based on an electrostatic double probe is developed to predict the performance such as the neutralizing current, the static elimination time, the offset potential and the kinetic energy of neutralizing particles. In addition, the experimental verification of the model is presented. It is found that the object potential, regardless of the polarity, decreases linearly with time and approaches to 0 V without overcharging; the speed is much higher than those of conventional corona ionizers. In addition, asymmetry in the elimination time between positive and negative polarities exists. Results suggest that the plasma plays a role of conductor that connects the object to a grounded chamber electrically.

Measuring The Temperature and Density of Secondary Electrons in an Argon Electron-Beam Plasma

The results of temperature and density measurements of secondary electrons in a free jet of argon, activated in an electron beam plasma, carried out using a Langmuir double electrostatic probe. A cold plasmatron prototype with a primary beam energy of 1 keV is used obtain a jet of dense cold plasma with a cross size of approximately 80 mm and parameters with which silicon layers may be deposited with necessary characteristics in a forvacuum pressure range.

Ion velocities in a micro-cathode arc thruster

Ion velocities in the plasma jet generated by the micro-cathode arc thruster are studied by means of time-of-flight method using enhanced ion detection system (EIDS). The EIDS triggers perturbations (spikes) on arc current waveform, and the larger current in the spike generates denser plasma bunches propagating along with the mainstream plasma. The EIDS utilizes double electrostatic probes rather than single probes. The average Ti ion velocity is measured to be around 2×104 m/s without a magnetic field. It was found that the application of a magnetic field does not change ion velocities in the interelectrode region while leads to ion acceleration in the free expanding plasma plume by a factor of about 2. Ion velocities of about 3.5×104 m/s were detected for the magnetic field of about 300 mT at distance of about 100–200 mm from the cathode. It is proposed that plasma is accelerated due to Lorentz force. The average thrust is calculated using the ion velocity measurements and the cathode mass consumption rate, and its increase with the magnetic field is demonstrated.

Continuous Emission Spectrum Measurement for Electron Temperature Determination in Low-Temperature Collisional Plasmas

Continuous emission spectrum measurement is applied for the inconvenient diagnostics of low-temperature collisional plasmas. According to the physical mechanism of continuous emission, a simplified model is presented to analyze the spectrum in low temperature plasma. The validity of this model is discussed in a wide range of discharge parameters, including electron temperature and ionization degree. Through the simplified model, the continuous emission spectrum in a collisional argon internal inductively coupled plasma is experimentally measured to determine the electron temperature distribution for different gas pressures and radio-frequency powers. The inverse Abel transform is also applied for a better spatially resoluted results. Meanwhile, the result of the continuous emission spectrum measurement is compared to that of the electrostatic double probes, which indicates the effectiveness of this method.

Temperature measurements in a laminar plasma jet generated at reduced pressure

Double electrostatic probe method and Boltzmann-plot method are applied to measure the electron temperature and the excitation temperature of a DC laminar argon plasma jet generated at reduced pressure. Attempts are also made to measure the gas temperature by using a pair of WRe-5/26 thermocouple. Experimental results show that the measured excitation temperature at the jet center increases with the vacuum chamber pressure and the arc current, but is appreciably lower than the measured electron temperature, indicating that the plasma jet under study is evidently deviated from the LTE state. The temperature indicated by the thermocouple probe is much lower than the measured excitation temperature and electron temperature. Theoretical analysis shows that the thermocouple method intended for gas temperature measurement contains very large errors due to rarefied-gas heat transfer effects, and thus is unsuitable for this purpose.

Application of Electrostatic Probe in Wool Fabric LTP Finishing

By using a double electron probe the electron density and electron temperature were measured during the low temperature plasma (LTP) treatment of wool fabrics. The dependences of the electron density upon the power, pressure and the plasma-treating time was studied. The effects on the wool fabrics' shrinkage were studied. The results showed that the angle of the double electrostatic probe and the area of the treated fabrics had a strong impact on the density measurement. The felt-proof property of wool fabrics treated with an argon plasma was better than that with a nitrogen plasma. The gas, power and pressure of the LTP treatment all affected the electron density. The electron density increased with the increase in power and pressure. The electron density did not change significantly with treating time, in a certain range of both power and pressure of the LTP treatment.

Fluctuation characteristics of arc voltage and jet flow in a non-transferred dc plasma generated at reduced pressure

A torch with a set of inter-electrode inserts between the cathode and the anode/nozzle with a wide nozzle exit was designed to generate plasma jets at chamber pressures of 500–10 000 Pa. The variation of the arc voltage was examined with the change in working parameters such as gas flow rate and chamber pressure. The fluctuation in the arc voltage was recorded with an oscilloscope, and the plasma jet fluctuation near the torch exit was observed with a high-speed video camera and detected with a double-electrostatic probe. Results show that the 300 Hz wave originated from the tri-phase rectified power supply was always detected under all generating conditions. Helmholtz oscillations over 3000 Hz was detected superposed on the 300 Hz wave at gas flow rates higher than 8.8 slm with a peak to valley amplitude lower than 5% of the average voltage value. No appreciable voltage fluctuation caused by the irregular arc root movement is detected, and mechanisms for the arc voltage and jet flow fluctuations are discussed.

Flame Detection by Electrostatic Double Probe with Parallel Detection and Reference Electrodes

The flame detection characteristics of an electrostatic probe with parallel detection and reference electrodes were investigated on a simple two-dimensional laminar premixed flame. For the large distance between two electrodes, ion current records showed the dependence on the angle of the plane including those two electrodes from the flame surface, that is, the large angle induced double peaks on the ion current record and the decline of maximum peak. On the other hand, for the small distance between two electrodes, in particular less than the thickness of high ion-electron concentration zone, ion current records were independent and constant with respect to the angle. Those features were caused whether both electrodes could be in the flame simultaneously. For the small distance, furthermore, the variation of current peak as functions of passing velocity of the probe through the flale and equivalence ratio was qualitatively same as that for the conventional single electrostatic probe. Consequently, the present probe with small distance between detection and reference electrodes can be applicable to the flame detection.

Diagnostics of pulsed vacuum arc discharges by optical emission spectroscopy and electrostatic double-probe measurements

In this work the study of plasma characteristics by means of two of the most common techniques employed by the scientific community dedicated to the experimental plasma research is presented. The plasma was generated in a vacuum reaction chamber which was filled with hydrogen gas. Inside the chamber, two opposite electrodes were placed: the cathode, which was formed by a target of highly oriented pirolitic carbon and the anode. The electron temperature T e and the electron density n e were measured by using optical emission spectroscopy and electrostatic double probe, obtaining very close values for each case. T e was calculated as approximately 0.7 eV and n e of the order of 10 13 cm −3. The optical emission measurement allows one to identify the substances that are in the plasma like C I, C II, C III, H I and H 2 and some possible reactions. The double-probe technique showed the plasma potential of about 24 V. The characteristic curve of the double probe exhibited oscillational plasma instabilities which could be attributed to the charge density variation or other factors, such as the employed AC signal and the geometric probe.

Characterization of a plasma produced by pulsed arc using an electrostatic double probe

In this work the determination of plasma parameters of a pulsed-arc discharge by using an electrostatic double probe is presented. The system to generate the plasma is composed mainly of a reaction chamber, where the electrodes of different materials (Ti, Zr, Al) were placed in order to identify the variation of the plasma parameters when the cathode material is changed. An automatic system has been implemented for acquiring data starting from the bias of a double electrostatic probe. This system allowed registering complete curves of current-voltage in relatively short times (≈30ms). The electron density ne and electron temperature Te can be extracted from the I-V characteristic curves obtained. The advantage of this technique is its facility to provide the whole quantity of “in situ” information, which can be compared with the theoretical results using numeric methods for the I-V curves simulation. By means of a suitable fit this allowed the determination of the plasma parameters. The values obtained for Te were in the order of 1eV and for ne of about 1013cm−3. A significant variation for ne was not found.

Comparative determination of the electron temperature in Ar- and N2-plasmas with electrostatic probes, optical emission spectroscopy OES and energy dispersive mass spectrometry EDMS

The electron temperature T e as the often most essential parameter for applications of low pressures plasma has been derived from comparative in situ measurements with a single and a double electrostatic probe, and with optical emission spectroscopy OES and energy dispersive mass spectrometry EDMS as remote techniques. Electrodeless rf discharges maintained by electron cyclotron wave resonance ECWR in pure Ar and N 2 in the pressure regime from 10 −2 to 1 Pa have been used as sample plasmas. The evaluation of the OES- and EDMS-signals is described in detail. The pressure dependence of the T e -results derived therefrom is found to compare well with the data from the probe measurements, and with calculations from a charge carrier balance equation. By matching the OES data to the absolute T e -values from the probe measurements, numerical expressions have been obtained by which T e can be quantitatively calculated from the intensity ratios between selected emission lines from the Ar- and the N 2 -plasma. Furthermore, the EDMS-results are also shown to deliver quantitative information about T e .

Measured Exit Plane Properties of a Low-Power Simulated-Hydrazine Arcjet

Introduction I N two previouspaperson the 1-kWhydrazinearcjet, two types of electrostatic probe diagnostics were used at the nozzle exit plane to measure electron density ne , electron temperature Te, ion Mach number, plasma velocity u i , gas temperature Tg , and related quantities. All measurements were performed at 10 A and 50 mg/s propellantmass  ow on a NASA Lewis Research Center design,3 at a speciŽ c power of P= P mD 22:4 MJ/kg. These measurements were then compared to the two-temperaturechemicalnonequilibriumnumerical arcjet model of Megli et al. The quantitiesne , Te, and ion Mach numberwere determinedwith a novel quadrupleelectrostatic probe swept across the exit plane,with the probe axis parallel to the  ow velocity.1;6 The plasma velocity was measured with a double electrostatic probe using a technique similar to current modulation velocimetry (CMV).2;6 Both radial and axial variations of plasma propertieswere measured and compared to the numerical results. This Note presents further results for P= P mD 19:8 MJ/kg and P= P mD 26:0 MJ/kg (Ref. 6). The arcjet operating conditions are given in Table 1. This range of P= P m is representative of the stable operating range of this arcjet on simulated hydrazine (1 mole N2, 2 moles H2). The speciŽ c impulse is interpolatedfromdata acquired elsewhere on a comparable thruster. The quadruple probe combines the triple probe method of Chen and Sekiguchi, Ž rst applied to electric thruster plumes by Tilley et al.,8;9 with the crossed probe theory of Kanal,10 Ž rst applied to a single-speciesplasma by Johnson andMurphree and to a two-species plasma by Burton and Bufton.1 The quadrupleprobe was Ž rst appled to an electric thruster plume by DelMedico and Burton et al.13 The plasma velocity, assumed to equal the ion velocity u i , is determined by a modiŽ cation of a technique Ž rst developed by Pobst et al., in which the arc current is brie y interruptedand the resulting deŽ cit in electron density is convected at the plasma velocity and either detected optically or by a double time-of- ight (TOF) electrostaticprobe. Radial velocity proŽ les generatedby this technique are accurate to §500 m/s and provide excellent agreement with those generated by numericalmodeling.

Investigation of High-Enthalpy Air Plasma Flow with Electrostatic Probes

At the Institute for Space Systems of the University of Stuttgart, four plasma wind tunnels are in operation to investigate the thermochemical behavior of thermal protection systems of various space vehicles during planetary entries To achieve a detailed understanding of the plasma conditions in a plasma wind tunnel and of the erosion mechanisms of thermal protection materials, a specific flight condition on the re-entry trajectory of a winged vehicle was investigated experimentally and numerically in detail during the last few years. Electron temperatures, electron densities, plasma potentials, electron energy functions, and particle velocities of the air plasma flow generated by a magnetoplasmadynamic plasma generator were investigated by cylindrical electrostatic single, double, and triple probes. Plasma velocities were determined with time-of-flight probes, taking advantage of natural fluctuations of the ion current densities in the air plasma flow. Using the ratio of directed and thermal ion velocities related to currents of parallel- and perpendicular-oriented probes according to the Kanal theory, the ion temperature is estimated

Mass spectrometric and electrostatic probe measurements of N2/O2 plasma flow

Plasma wind tunnels are used at the Laboratoire d'Aerothermique at Meudon to simulate the different parameters of the gas surrounding the surface of space vehicles during hypersonic flight in the upper atmosphere. In the present work, mass spectrometric and double electrostatic probe measurements in a stagnation point are performed in N2/O2 plasmas. Axial distributions of electron density and temperature are obtained from the fit between experimental data and Laframboise expression for the double cylindrical probe current. Neutral and ion distributions are measured by mass analysis for different axial positions, and are then compared with a chemical model for air, in order to evaluate the dominant reactions in the plasma flow.

Characterization of a microwave plasma jet containing Ar-CH4 gas mixture

This work is devoted to the study of the expansion of a Ar-CH4 microwave plasma. Diagnostics have been performed within the reactor using both an electrostatic Langmuir double probe and spatially resolved emission spectroscopy. We have proven that the plasma jet results from electromagnetic surface wave propagation (ne≥nec=3.6×1011 cm−3, Te∼23 000–46 000 K) and have shown that injection of methane into the jet decreases ne and Te, and thus limits the striking conditions. Such an expanding plasma finds applications in plasma enhanced chemical vapor deposition technology. It can be used to create simple radicals close to a substrate and thus limits their destruction on the reactor walls.

Plasma parameters in noble-gas narrow-tube and capillary-tube discharge, positive column plasmas

The plasma parameters in noble-gas narrow-tube and capillary-tube plasmas have been studied experimentally over the pressure (p) range from 0.1 to 10 Torr. Tube inner diameters, 2R, of 0.5 and 0.8 mm (capillary) and 3 mm (narrow) were used in the present investigation, together with axially movable anodes and cylindrical electrostatic double probes for the electric field and the electron temperature measurements, respectively, in noble gases, i.e. He, Ne, Ar, Kr and Xe. The results show that the axial electric field Ez/p and plasma density decrease with increasing pR and the atomic weight of noble gases. No significant tube radius effect has been observed for the axial electric field if one analyses results in terms of pR. The electron temperature decreases significantly with increasing pR for He and Ne, and depends slightly on pR for Ar, Xe, and Kr.

Use of a light emitting diode as a current sensor for electrostatic double probes

A light emitting diode has been used as a convenient current sensor for electrostatic double probes under conditions of very high common mode voltages at frequencies around 1 MHz. A simple probe circuit permitted the signal to be transmitted optically to a remote conditioning and detection circuit. A calibration curve was used to correct the nonlinearities in the system.

Mach number deduced from probe measurements in the divertor and boundary layer of DITE

A pair of asymmetric electrostatic double probes has been used in the boundary layer and divertor chamber of DITE. The electrode configuration is such that while one double probe collects ions moving towards the divertor target, the other collects ions moving away from it. The probes are used to measure electron temperature, density and Mach number of the flowing plasma. Although the interpretation of Mach number is model dependent, it is concluded that the Mach number within 6–10 cm from the divertor target ranges from about one to two while in the scrape-off layer it varies from 0.3 to 0.45.

Simultaneous rocket and radar measurements of currents in an auroral arc

A detailed study of electric field, current and conductivities associated with an auroral arc was made in a coordinated rocket and radar experiment in Alaska on March 9, 1978. The payload, designated 29.007 UE, was launched at 1013 p.m. local time. It penetrated the diffuse aurora on the upleg and at apogee traversed field lines connected to a stable auroral arc of 40 kR intensity. Among the instruments carried by the payload were a vector magnetometer, a set of electrostatic double probes and a set of electron and proton spectrometers. Simultaneous electron density and line‐of‐sight velocity measurements were made by the Chatanika radar operating in an elevation scan mode in the magnetic meridian plane. Both the radar and rocket measurements indicated that the zonal electric field was westward and approximately constant across the arc with a magnitude of about 7 mV/m. Small differences between the rocket and radar zonal electric field measurements indicated the presence of upward drifting ions in the region of the arc. The meridional field was large and northward equatorward of the arc, but negligible within the arc. Conductivities computed from measured fluxes of energetic electrons agreed well with the conductivities derived from the radar measurements of electron density. The electric field and conductivity measurements indicated that the zonal currents were eastward equatorward of the arc and westward within the arc. These electrojet currents agreed well with those inferred from the rocket magnetometer data. Better agreement was obtained when a westward neutral wind was added. The westward wind was also consistent with differences between the rocket and radar meridional electric fields. The meridional currents computed from the electric field measurements were northward over the entire region. Since the northward current was enhanced within the arc, a pair of oppositely directed field‐aligned current sheets associated with the arc edges was inferred. However, these current sheets were not detected by the rocket‐borne vector magnetometer. The magnetometer measured a downward current in the diffuse aurora equatorward of a wide region of upward current within which the arc was embedded. The differences in the field‐aligned current patterns inferred from the two methods are attributed either to a time dependent field‐aligned current profile or east‐west current divergence in the region of the Harang discontinuity.