Enthalpy Probe Technique for Thermal Plasma Diagnostics

The measuring principle and experimental results of the enthalpy probe technique for thermal plasma diagnostics are presented. Its calibration and errors are discussed. Typical results are presented for the system operation in an Ar/H2(5 % H2) plasma arc jet under a reactor chamber pressure of 101.3 kPa. The plasma temperature and velocity profiles are measured. The center temperature and velocity are 6600 K and 850 m/s for plasma power 9 kW at axial location of 17 mm.

Iterative Calculation of Plasma Density from a Cylindrical Probe Characteristic

A novel method is proposed for treating cylindrical probe characteristics to obtain plasma density. The method consists of exponential extrapolation of the transitional part of the I -V curve to the floating potential for the ion saturation current, other than the existing theories which use the ion branch, and an iterative sheath thickness correction procedure for improved accuracy. The method was tested by treating Langmuir probe I-V characteristics obtained from inductively coupled Ar discharges at various pressures, and comparing the present results with those deduced by existing theories. It was shown that the plasma densities obtained by the present method are in good agreement with those calculated by the Allen-Boyd-Reynolds （ABR） theory, suggesting the effectiveness of the proposed method. Without need of manual setting and adjustment of fitting parameters, the method may be suitable for automatic and real time processing of probe characteristics.

Study of plasma parameters of coaxial plasma source using triple Langmuir probe and Faraday cup diagnostics

Coaxial plasma guns are a type of plasma source that produces plasma which propagates radially and axially controlled by the shape of the ground electrode, which has attracted much interest in several applications. In this work, a 120° opening angle of CPG nozzle is used as a plasma gun configuration that operates at the energy of 150 J. The ionization of polyethylene insulator between the electrodes of the gun produces a cloud of hydrogen and carbon plasma.The triple Langmuir probe and Faraday cup are used to measure plasma density and plasma temperature. These methods are used to measure the on-axis and off-axis plasma divergence of the coaxial plasma gun. The peak values of ion densities measured at a distance of 25 mm on-axis from the plasma gun are(1.6±0.5)×10^(19)m^(-3)and(2.8±0.6)×10^(19)m^(-3)for hydrogen and carbon plasma respectively and the peak temperature is 3.02±0.5 eV. The mean propagation velocity of plasma is calculated using the transit times of plasma at different distances from the plasma gun and is found to be 4.54±0.25 cm/μs and 1.81±0.18 cm/μs for hydrogen and carbon plasma respectively. The Debye radius is obtained from the measured experimental data that satisfies the thin sheath approximation. The shot-to-shot stability of plasma parameters facilitates the use of plasma guns in laboratory experiments. These types of plasma sources can be used in many applications like plasma opening switches, plasma devices, and as plasma sources.

Sweep Langmuir Probe and Triple Probe Diagnostics for Transient Plasma Produced by Hypervelocity Impact

Two techniques are applied to diagnose characteristic parameters of plasma created by hypervelocity impact, such as electron temperature and electron density. The first technique is a sweep Langmuir probe （SLP）, which is a new apparatus based on a dual channel circuit that can compensate for stray capacitance and obtain a good synchronicity, so that electrostatic turbulence with a good temporal resolution can be acquired. The second technique is a triple Langmuir probe （TLP）, which is an electrostatic triple Langmuir probe diagnostic system, in which no voltage and frequency sweep is required. This technique allows to measure electron temperature, electron density as a function of time. Moreover, the triple Langmuir probe diagnostic system allows the direct display of electron temperature and semidirect display of electron density by an appropriate display system, the system permits us to eliminate almost all data processing procedures. SLP and TLP were applied to obtain fluctuations of the characteristic parameters of plasma generated by hypervelocity impact. As an example of their application to time-dependent plasma measure- ment, the electron temperature and electron density of plasmas were acquired in hypervelocity impact experiments. Characteristic parameters of plasma generated by hypervelocity impact were compared by the two kinds of diagnostic techniques mentioned above.

Preparation of a beam emission spectroscopy diagnostic based on a Lyman-alpha line to diagnose core and edge-plasma density fluctuation on the HL-2A tokamak

In this article,the design of a Lyman-alpha-based beam emission spectroscopy(LAB)diagnostic on the HL-2A tokamak has been proposed for the first time.The purpose of this novel diagnostic is to measure density fluctuations of tokamak plasma.The light-collection system of LAB,which consists of the first mirror and two groups of coaxial double-mirror telescopes,can realize a twosegmented viewing field ofρ=0–0.2 andρ=0.75–1,which is optimized to measure plasma density fluctuation,not only in the edge transport barrier region but also in the internal transport barrier region,to investigate the underlying physics of turbulence in tokamaks.Spectrometers are developed to separate out the Doppler-shifted target line(122.03 and 122.17 nm)from the background Lyman-alpha line(121.53 nm).Here,30 Core-LAB channels and 30 Edge-LAB channels are under development on the HL-2A tokamak.It has high radial spatial resolutions of about 2.7 mm and 3.3 mm for the core and edge channels,respectively.Taking the high light intensity of this Lyman-alpha line into account,temporal resolution of 200 k Hz can be ensured by broad bandwidth amplifiers.This high spatio-temporal resolution makes LAB a potential keen tool to experimentally investigate tokamak plasma physics.

Online plasma diagnostics of a laser-produced plasma

In this study,we report a laser interferometry experiment for the online-diagnosing of a laserproduced plasma.The laser pulses generating the plasma are ultra-fast（30 femtoseconds）,ultraintense（tens of Terawatt） and are focused on a helium gas jet to generate relativistic electron beams via the laser wakefield acceleration（LWFA） mechanism.A probe laser beam（λ=800 nm） which is split-off the main beam is used to cross the plasma at the time of arrival of the main pulse,allowing online plasma density diagnostics.The interferometer setup is based on the No Marski method in which we used a Fresnel bi-prism where the probe beam interferes with itself after crossing the plasma medium.A high-dynamic range CCD camera is used to record the interference patterns.Based upon the Abel inversion technique,we obtained a 3D density distribution of the plasma density.

Plasma density measurement and downstream etching of silicon and silicon oxide in Ar/NF3 mixture remote plasma source

In this study,plasma density measurements were performed near the plume region of the remote plasma source (RPS) in Ar/NF3 gas mixtures using a microwave cutoff probe.The measured plasma density is in the range of 1010-1011 cm-3 in the discharge conditions with RPS powers of 2-4 kW and gas pressures of 0.87-4 Torr.The plasma density decreased with increasing gas pressures and RPS powers under various Ar/NF3 mixing ratios.This decrease in the plasma density measured at the fixed measurement position (plume region) can be understood by the reduction of the electron energy relaxation length with increases in the gas pressures and mixing ratio of NF3/(Ar/NF3).We also performed downstream etching of silicon and silicon oxide films in this system.The etch rate of the silicon films significantly increases while the silicon oxide is slightly etched with the gas pressures and powers.It was also found that the etch rate strongly depends on the wafer position on the processing chamber electrode,and that the etch selectivity reached 96-131 in the discharge conditions of RF powers (3730-4180 W) and gas pressures (3.6-4 Torr).

Volumetric Langmuir probe mapping of a transient pulsed plasma thruster plume

The triple Langmuir probe enables measurements of the transient plasma parameters over time at a point of interest.We demonstrate how these measurements can be easily combined to obtain a visualization of the overall plasma behavior of a pulsed plasma thruster.Through this,it is possible to identify features in the expansion of the plasma such as the canting angle of the plume.We also identified the early arrival of a negatively canted low-density plasma plume.The 2D profiles also reveal data that would otherwise be obscured by other planes in optical measurements.

Improving the uniformity of RF-plasma density by a humped variable-gap spiral antenna

This paper develops a humped spiral antenna of top inductively coupled plasma with variable gap. Comparing with planar spiral antennae, it investigates the performance of humped spiral antennae in the calculated electromagnetic configurations and experimental results. It finds that the humped antenna has the improved uniformity of plasma density in the radial direction and the decreased electron temperature in the top inductively coupled plasma. By experimental and theoretical analyses, the plasma performance in the case of humped antennae is considered to be the combined results of the uniform electromagnetic configurations and the depressed capacitively coupling effect.

Experimental and Simulational Studies on the Theoretical Model of the Plasma Absorption Probe

Plasma absorption probe （PAP） was developed for measuring the electron density in plasmas processing based on the surface-wave characteristics. In order to diagnose the plasma with lower density and higher pressure, a sensitive PAP was also developed. Both types of PAP were analyzed theoretically under the quasi-static approximation, which is highly problematic when a conductor exists in the resonance region of the probe. For this reason, a theoretical model for the PAP is presented in this paper. The model is derived from the electromagnetic wave equation. Its principle is then verified via experiments and numerical simulations. Both experimental and numerical results show that the electromagnetic theoretical model is valid as compared with the quasi-static model. Consequently, a new type of PAP, named as the electromagnetic PAP, is thus proposed for the measurement of electron density.

Automated electron temperature fitting of Langmuir probeⅠ-Ⅴtrace in plasmas with multiple Maxwellian EEDFs

An algorithm for automated fitting of the effective electron temperature from a planar Langmuir probe I-V trace taken in a plasma with multiple Maxwellian electron populations is developed through MATLAB coding.The code automatically finds a fitting range suitable for analyzing the temperatures of each of the electron populations.The algorithm is used to analyzeⅠ-Ⅴtraces from both the Institute of Plasma Physics Chinese Academy of Sciences's Diagnostic Test Source device and a similar multi-dipole chamber at the University of Wisconsin-Madison.Ⅰ-Ⅴtraces reconstructed from the parameters fitted by the algorithm not only agree with the measured I-V trace but also reveal physical properties consistent with those found in previous studies.Cylindrical probe traces are also analyzed with the algorithm and it is shown that the major source of error in such attempts is the disruption of the inflection point due to both decreased signal-to-noise ratio and greater sheath expansion.It is thus recommended to use planar probes with radii much greater than the plasma Debye length when signal-to-noise ratio is poor.

Plume diagnostics of BUSTLab microwave electrothermal thruster using Langmuir and Faraday probes

This study presents the Langmuir and Faraday probe measurements conducted to determine the plume characteristics of the BUSTLab microwave electrothermal thruster(MET).The thruster,designed to operate at 2.45 GHz frequency,is run with helium,argon and nitrogen gases as the propellant.For the measurements,the propellant volume flow rate and the delivered microwave power levels are varied.Experiments with nitrogen gas revealed certain operation regimes where a very luminous plume is observed.With the use of in-house-built Langmuir probes and a Faraday probe with guard ring,thruster plume electron temperature,plasma density and ion current density values are measured,and the results are presented.The measurements show that MET thruster plume effects on spacecraft will likely be similar to those of the arcjet plume.It is observed that the measured plume ion flux levels are very low for the high volume flow rates used for the operation of this thruster.

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.

Correlation ofⅢ/Ⅴsemiconductor etch results with physical parameters of high-density reactive plasmas excited by electron cyclotron resonance

Reactive ion etching is the interaction of reactive plasmas with surfaces. To obtain a detailed understanding of this process, significant properties of reactive composite low-pressure plasmas driven by electron cyclotron resonance（ECR） were investigated and compared with the radial uniformity of the etch rate. The determination of the electronic properties of chlorine-and hydrogen-containing plasmas enabled the understanding of the pressure-dependent behavior of the plasma density and provided better insights into the electronic parameters of reactive etch gases. From the electrical evaluation of I（V） characteristics obtained using a Langmuir probe,plasmas of different compositions were investigated. The standard method of Druyvesteyn to derive the electron energy distribution functions by the second derivative of the I（V）characteristics was replaced by a mathematical model which has been evolved to be more robust against noise, mainly, because the first derivative of the I（V） characteristics is used. Special attention was given to the power of the energy dependence in the exponent. In particular, for plasmas that are generated by ECR with EM modes, the existence of Maxwellian distribution functions is not to be taken as a self-evident fact, but the bi-Maxwellian distribution was proven for Ar-and Kr-stabilized plasmas. In addition to the electron temperature, the global uniform discharge model has been shown to be useful for calculating the neutral gas temperature. To what extent the invasive method of using a Langmuir probe could be replaced with the noninvasive optical method of emission spectroscopy, particularly actinometry, was investigated,and the resulting data exhibited the same relative behavior as the Langmuir data. The correlation with etchrate data reveals the large chemical part of the removal process—most striking when the data is compared with etching in pure argon. Although the relative amount of the radial variation of plasma density and etch rate is approximately ？5%, the etch rate shows a slightly concave shape in contrast to the plasma density.

Ion Density Distribution in an Inductively Coupled Plasma Chamber

The ion density distribution in the reaction chamber was diagnosed by a Langmuir probe. The rules of the ion density distribution were obtained under the pressures of 9 Pa, 13 Pa, 27 Pa and 53 Pa in the reaction chamber, different radio-frequency powers and different positions. The result indicates that the ion density decreases as the pressure increases, and increases as the power decreases. The ion density of axial position z =0 achieves 5.8×1010 on the center of coil under the power of 200 w and pressure of 9 Pa in the reaction chamber.

Diagnosis of the Argon Plasma in a PECVD Coating Machine

A Langmuir probe plasma diagnostic system was developed to measure the plasma parameters in a PECVD vacuum coating machine. The plasma was a capacitively coupled plasma （CCP） driven by a radio-frequency （RF） power supply. To avoid the disturbance of radio-frequency field on the Langmuir probe measurement, a passive compensation method was applied. This method allowed the ＇dc＇ component to be passed and measured in the probe circuit. It was found that the electron temperature in the range from 2.7 eV to 6.4 eV decreased with the increase in RF power. The measured plasma density ranged from 8×10^16 m^-3 to 0.85×10^15 m^-3 and increased with the increase in RF power.

A Study of Diamond-Like Carbon Thin Films Prepared by Microwave Plasma Chemical Vapour Deposition

In order to deposit good films, we need to study the uniformity of plasma density and the plasma density under different gas pressures and powers. The plasma density was diagnosed by a Langmuir probe. The optical emission spectroscopy （OES） of CH4 and H2 discharge was obtained with raster spectroscopy, with characteristic peaks of H and CH achieved. Diamond-like carbon films were achieved based on the study of plasma density and OES and characterized by atomic force microscope （AFM）, X-ray diffraction instrument （XRD）, Raman spectroscope and profiler.

Plasma diagnosis of tetrahedral amorphous carbon films by filtered cathodic vacuum arc deposition

Filtered cathodic vacuum arc(FCVA)deposition is regarded as an important technique for the synthesis of tetrahedral amorphous carbon(ta-C)films due to its high ionization rate,high deposition rate and effective filtration of macroparticles.Probing the plasma characteristics of arc discharge contributes to understanding the deposition mechanism of ta-C films on a microscopic level.This work focuses on the plasma diagnosis of an FCVA discharge using a Langmuir dualprobe system with a discrete Fourier transform smoothing method.During the ta-C film deposition,the arc current of graphite cathodes and deposition pressure vary from 30 to 90 A and from 0.3 to 0.9 Pa,respectively.The plasma density increases with arc current but decreases with pressure.The carbon plasma density generated by the arc discharge is around the order of10^(10)cm^(-3).The electron temperature varies in the range of 2-3.5 eV.As the number of cathodic arc sources and the current of the focused magnetic coil increase,the plasma density increases.The ratio of the intensity of the D-Raman peak and G-Raman peak(I_(D)/I_(G))of the ta-C films increases with increasing plasma density,resulting in a decrease in film hardness.It is indicated that the mechanical properties of ta-C films depend not only on the ion energy but also on the carbon plasma density.

Experimental study on parameters of dust plasma in SiH_4 /C_2H_4 /Ar discharges

Measurements of dust plasma parameters were carried out in the discharges of （SiH4/C2H4/Ar） mixtures. Dust particles were formed in the capacitively coupled radio-frequency discharge of these reactive mixtures in a cylindrical chamber. Langmuir probe was employed for diagnosing and measuring the important plasma parameters such as electron density and electron temperature. The results showed that the electron density dropped, and in contrast the electron temperature rose when the dust particles formed. The curves of the electron density and temperature versus the RF power and pressure were presented and analysed. Further, it was found that the wriations of electron temperature and the size of dust void with the RF power followed the similar trends. These trends might be useful for understanding more about the characteristics of dusty voids.

Investigation of self-generated magnetic field and dynamics of a pulsed plasma flow

Due to the growing interest in studying the compression and disruption of the plasma filament in magnetic fusion devices and Z-pinches, this work may be important for new developments in the field of controlled thermonuclear fusion. Recently, on a coaxial plasma accelerator, we managed to obtain the relatively long-lived(~300 μs) plasma filaments with its self-magnetic field. This was achieved after modification of the experimental setup by using high-capacitive and lowinductive energy storage capacitor banks, as well as electrical cables with low reactive impedance. Furthermore, we were able to avoid the reverse reflection of the plasma flux from the end of the plasma accelerator by installing a special plasma-absorbing target. Thus, these constructive changes of the experimental setup allowed us to investigate the physical properties of the plasma filament by using the comprehensive diagnostics including Rogowski coil,magnetic probes, and Faraday cup. As a result, such important plasma parameters as density of ions and temperature of electrons in plasma flux, time dependent plasma filament’s azimuthal magnetic field were measured in discharge gap and at a distance of 23.5 cm from the tip of the cathode. In addition, the current oscillograms and Ⅰ–Ⅴ characteristics of the plasma accelerator were obtained. In the experiments, we also observed the charge separation during the acceleration of plasma flow via oscillograms of electron and ion beam currents.