Electron Temperature Measurement by Floating Probe Method Using AC Voltage

This study presents a novel floating probe method to measure electron temperatures using a hollow cathode-type discharge tube. The proposed method detects a shift in the floating potential when an AC voltage is applied to a probe through an intermediary blocking capacitor.The shift in the floating potential is described as a function of the electron temperature and the applied AC voltage. The floating probe method is simpler than the Langmuir probe method because it does not require the measurement of volt-ampere characteristics. As the input AC voltage increases, the electron temperature converges. The electron temperature measured using the floating probe method with an applied sinusoidal voltage shows a value close to the first(tail)electron temperature in the range of the floating potential.     

Comparative Study of Plasma Parameters in Magnetic Pole Enhanced Inductively Coupled Argon Plasmas

Langmuir probe measurements of radio frequency (RF) magnetic pole enhanced inductively coupled (MaPE-ICP) argon plasma were accomplished to obtain the electron number densities and electron temperatures. The measurements were carried out with a fixed RF frequency of 13.56 MHz in a pressure range of 7.5 mTorr to 75 mTorr at an applied RF power of 10 W and 100 W. These results are compared with a global (volume average) model. The results show good agreement between theoretical and experimental measurements. The electron number density shows an increasing trend with both RF power and pressure while the electron temperature shows decreasing trend as the pressure increases. The difference in the plasma potential and floating potential as a function of electron temperature measured from the electrical probe and that obtained theoretically shows a linear relation with a small difference in the coefficient of proportionality. The intensity of the emission line at 750.4 nm due to 2p 1 → 1s 2 (Paschen’s notation) transition closely follows the variation of n e with RF power and filling gas pressure. Measured electron energy probability function (EEPF) shows that electron occupation changes mostly in the high-energy tail, which highlights close similarity of 750.4 nm argon line to n e .     

A comparison of emissive and cold floating probe techniques for electric potential measurements in rf inductive discharge

A cold floating probe method was compared with the emissive floating probe method in terms of a low-pressure radio-frequency inductive discharge. The dependences of difference between the plasma potential and the floating potential on the electron temperature 1–8 eV, plasma density 10⁹ –10¹² cm⁻³ and magnetic field 100–650 G were obtained. It was demonstrated that the difference between the potentials that obtained by these two methods can differ significantly from the expected value of 5.2 kT_e/e for argon.     

Measurements of plasma parameters in a hollow electrode AC glow discharge in helium

Measurements of the plasma parameters of coaxial gridded hollow electrode alternating current(AC)discharge helium plasma were carried out using an improved probe diagnostic technology.The measurements were performed under well-defined discharge conditions(chamber geometry,input power,AC power frequency,and external electrical characteristics).The problems encountered in describing the characteristics of AC discharge in many probe diagnostic methods were addressed by using an improved probe diagnostics design.This design can also be applied to the measurement of plasma parameters in many kinds of plasma sources in which the probe potential fluctuates with the discharge current.Several parameters of the hollow electrode AC helium discharge plasma were measured,including the plasma density,electron temperature,plasma density profiles,and changes in plasma density at different input power values and helium pressures.The characteristics of the coaxial gridded hollow electrode plasma determined by the experiments are suitable for comparison with plasma simulations,and for use in many applications of hollow cathode plasma.     

Design and Fabrication of a Movable Langmuir Probe for Plasma Edge Parameters Measurement in the IR-T1 Tokamak

For the edge plasma parameters measurement, a movable Langmuir probe is fabricated and installed on the IR-T1 tokamak. The set-up consists of two sets of single Langmuir probes with tungsten tip movable in the radial direction. Edge plasma parameters including electron temperature, Ion density, floating potential and the corresponding radial changes are measured. Using two-point correlation technique clearly reveal that in the SOL region the poloidal propagation of floating potential fluctuation is in the direction of ion diamagnetic drift and in the edge it propagates in the electron diamagnetic drift direction.     

Steady and oscillatory plasma properties in the near-field plume of a hollow cathode

Hollow cathodes serve as electron sources in Hall thrusters,ion thrusters and other electric propulsion systems.One of the vital problems in their application is the cathode erosion.However,the basic erosion mechanism and the source of high-energy ions cause of erosion are not fully understood.In this paper,both potential measurements and simulation analyses were performed to explain the formation of high-energy ions.A high-speed camera,a single Langmuir probe and a floating emissive probe were used to determine the steady and oscillatory plasma properties in the near-field plume of a hollow cathode.The temporal structure,electron temperature,electron density,and both static and oscillation of plasma potentials of the plume have been obtained by the diagnostics mentioned above.The experimental results show that there exists a potential hill (about 30 V) and also severe potential oscillations in the near-plume region.Moreover,a simple 2D particle-in-cell model was used to analyze the energy transition between the potential hill and/or its oscillations and the ions.The simulation results show that the energy of ions gained from the static potential background is about 20 eV,but it could reach to 60 eV when the plasma oscillates.     

Fast radial scanning probe system on KTX

A fast radial scanning probe system was constructed for the Keda Torus eXperiment(KTX) to measure the profiles of boundary plasma parameters such as floating potential, electron density,temperature, transport fluxes, etc. The scanning probe system is driven by slow and fast motion mechanisms, corresponding to the stand-by movement of a stepping motor and the fast scanning movement of a high-torque servo-motor, respectively. In fast scanning, the scanner drives the probe radially up to 20 cm at a maximum velocity of 4.0 m s~(-1). A noncontact magnetic grating ruler with a high spatial resolution of 5 μm is used for the displacement measurement. New scanning probe can reach the center of plasmas rapidly. The comparison of plasma floating potential profiles obtained by a fixed radial rake probe and the single scanning probe suggests that the high-speed scanning probe system is reliable for measuring edge plasma parameter profiles on the KTX device.     

Electrical and optical measurements in the early hydrogen discharge of GLAST-Ⅲ

This work presents the first electrical and optical measurements of the initial phase of hydrogen discharge in the upgraded spherical tokamak GLAST-III, initiated with electron cyclotron heating(ECH). Diagnostic measurements provide insights into expected and unexpected physics issues related to the initial phase of discharge. A triple Langmuir probe(TLP) has been developed to measure time series of the floating potential, plasma electron temperature and number density over the entire discharge, allowing monitoring of the two phases of the discharge: the ECH pre-ionization phase following by the plasma current formation phase. A TLP has the ability to give time-resolved measurements of the floating potential(V_(float)), electron temperature(T_e) and ion saturation current(I_(sat)∝n_e√kT_e).sat e eThe evolution of the ECH-assisted pre-ionization and subsequent plasma current phases in one shot are well envisioned by the probe. Intense fluctuations in the plasma current phase advocate for efficient equilibrium and feedback control systems. Moreover, the emergence of some strong impurity lines in the emission spectrum, even after only a few shots, suggests a crucial need for improvements in the base vacuum level. A noticeable change in the shape of the temporal profiles of the floating potential, electron temperature, ion saturation current(I_(sat)) and light emission has been observed with changing hydrogen fill pressure and vertical magnetic field.     

Electrical and optical measurements in the early hydrogen discharge of GLAST-III

This work presents the first electrical and optical measurements of the initial phase of hydrogen discharge in the upgraded spherical tokamak GLAST-III, initiated with electron cyclotron heating (ECH). Diagnostic measurements provide insights into expected and unexpected physics issues related to the initial phase of discharge. A triple Langmuir probe (TLP) has been developed to measure time series of the floating potential, plasma electron temperature and number density over the entire discharge, allowing monitoring of the two phases of the discharge: the ECH pre-ionization phase following by the plasma current formation phase. A TLP has the ability to give time-resolved measurements of the floating potential (V_float), electron temperature (T_e) and ion saturation current (I_sat∝ n_e√kT_e). The evolution of the ECH-assisted pre-ionization and subsequent plasma current phases in one shot are well envisioned by the probe. Intense fluctuations in the plasma current phase advocate for efficient equilibrium and feedback control systems. Moreover, the emergence of some strong impurity lines in the emission spectrum, even after only a few shots, suggests a crucial need for improvements in the base vacuum level. A noticeable change in the shape of the temporal profiles of the floating potential, electron temperature, ion saturation current (I_sat) and light emission has been observed with changing hydrogen fill pressure and vertical magnetic field.     

Simulation of Electrical Discharge Initiated by a Nanometer-Sized Probe in Atmospheric Conditions

In this paper,a two-dimensional nanometer scale tip-plate discharge model has been employed to study nanoscale electrical discharge in atmospheric conditions.The field strength distributions in a nanometer scale tip-to-plate electrode arrangement were calculated using the finite element analysis(FEA) method,and the influences of applied voltage amplitude and frequency as well as gas gap distance on the variation of efective discharge range(EDR) on the plate were also investigated and discussed.The simulation results show that the probe with a wide tip will cause a larger efective discharge range on the plate;the field strength in the gap is notably higher than that induced by the sharp tip probe;the efective discharge range will increase linearly with the rise of excitation voltage,and decrease nonlinearly with the rise of gap length.In addition,probe dimension,especially the width/height ratio,afects the efective discharge range in diferent manners.With the width/height ratio rising from 1:1 to 1:10,the efective discharge range will maintain stable when the excitation voltage is around 50 V.This will increase when the excitation voltage gets higher and decrease as the excitation voltage gets lower.Furthermore,when the gap length is 5 nm and the excitation voltage is below 20 V,the diameter of EDR in our simulation is about 150 nm,which is consistent with the experiment results reported by other research groups.Our work provides a preliminary understanding of nanometer scale discharges and establishes a predictive structure-behavior relationship.     

Assessment of an Ultrasonic Sensor and a Capacitance Probe for Measurement of Two-phase Mixture Level

We perform a comparison of two-phase mixture levels measured by an ultrasonic sensor and a two-wire type capacitance probe with visual data under the same experimental conditions. A series of experiments are performed with various combinations of airflow and initial water level using a test vessel with a height of 2 m and an inner diameter of 0.3 m under atmospheric pressure and room temperature. The ultrasonic sensor measures the two-phase mixture level with a maximum error of 1.77% with respect to the visual data. The capacitance probe severely under-predicts the level data in the high void fraction region. The cause of the error is identified as the change of the dielectric constant as the void fraction changes when the probe is applied to the measurement of the two-phase mixture levels. A correction method for the capacitance probe is proposed by correcting the change of the dielectric constant of the two-phase mixture. The correction method for the capacitance probe produces a r.m.s. error of 5.4%.     

Plasma characteristics of direct current enhanced cylindrical inductively coupled plasma source

Experimental results of a direct current enhanced inductively coupled plasma (DCE-ICP) source which consists of a typical cylindrical ICP source and a plate-to-grid DC electrode are reported.With the use of this new source,the plasma characteristic parameters,namely,electron density,electron temperature and plasma uniformity,are measured by Langmuir floating double probe.It is found that DC discharge enhances the electron density and decreases the electron temperature,dramatically.Moreover,the plasma uniformity is obviously improved with the operation of DC and radio frequency (RF) hybrid discharge.Furthermore,the nonlinear enhancement effect of electron density with DC + RF hybrid discharge is confirmed.The presented observation indicates that the DCE-ICP source provides an effective method to obtain high-density uniform plasma,which is desirable for practical industrial applications.     

Spatial Evolution Study of EEDFs and Plasma Parameters in RF Stochastic Regime by Langmuir Probe

An RF compensated cylindrical Langmuir probe system has been developed and used to characterize an RF capacitive two temperature plasma discharge in a stochastic mode. The novelty of the work presented here is the use of the driven electrode （cathode） without ground shield. Measurements of the electron energy distribution function （EEDF） and plasma parameters were achieved under the following conditions： 50 W of RF power and 5× 10-2 mbar of argon pressure. The probe measurements are performed at 3 cm above the electrode and the probe was shifted radially （r direction） from the center （r = 0 cm） of the inter-electrodes region towards the chamber wall （R = 10.75 cm）. The results show that the EEDF is bi-Maxwellian and its shape remains the same through the scanned region. The farther the probe from the central region, the lower the EEDF maximum. The plasma density is observed to decrease according to a Gaussian profile along the radial direction and falls to 50% of its maximum when close to the cathode edge （r = 5.5 cm）. At the same time the effective electron temperature remains constant for r〈4 cm and increases for r≥4 cm. The high-temperature and low-temperature electrons＇ densities and temperatures are also discussed in the article.     

Automatic emissive probe apparatus for accurate plasma and vacuum space potential measurements

We have developed an automatic emissive probe apparatus based on the improved inflection point method of the emissive probe for accurate measurements of both plasma potential and vacuum space potential.The apparatus consists of a computer controlled data acquisition card,a working circuit composed by a biasing unit and a heating unit,as well as an emissive probe.With the set parameters of the probe scanning bias,the probe heating current and the fitting range,the apparatus can automatically execute the improved inflection point method and give the measured result.The validity of the automatic emissive probe apparatus is demonstrated in a test measurement of vacuum potential distribution between two parallel plates,showing an excellent accuracy of 0.1 V.Plasma potential was also measured,exhibiting high efficiency and convenient use of the apparatus for space potential measurements.     

Design of a multi-voltage probe system for ICRF antenna coupling resistance measurement on EAST

A multi-voltage probe array system is designed to measure the coupling resistance of an ion cyclotron resonance frequency antenna. In the process of the antenna coupling resistance data extraction, the minimization algorithm, the original Levenberg–Marquardt algorithm, is replaced by the Broyden–Fletcher–Goldfarb–Shanno algorithm to achieve more stable and accurate results. Moreover, a simple model of the multi-voltage probe array was applied to simulate the performance of the Kalman filter, and to optimize the distance and position of the probes and probe number to mitigate the influence of the system noise on the rebuilt results. During the EAST experiment in 2019, a four-voltage probe array was applied to measure the coupling resistance of line 6 during high confined mode discharge. The measurement results by the multi- voltage probe array system and the voltage/current probe pair show a good agreement.     

Measurement of Ion Parameters by Ion Sensitive Probe in ECR Plasma

Ion parameters in electron cyclotron resonance (ECR) microwave plasma were measured by ion sensitive probe and were compared with the electron parameters obtained by double Langmuir probe. The effects of gas pressure and microwave power on the ion temperature and density were analyzed. The spatial distribution of the ion parameters was also investigated by the ion sensitive probes with a tunable radial depth installed on different probe windows along the chamber axis. Results showed that the ion density measured by the ion sensitive probe was in good agreement with the electron density measured by the double Langmuir probe. The influence of gas pressure on the ion parameters was stronger than that of microwave power. With the increase in working pressure, the ion temperature decreased monotonously with a decreasing rate larger than that at higher pressure. The ion density first increased to a peak (42.3?1010cm-3) at 1 Pa and then decreased. The ion temperature and density increased little with the increase in the microwave power from 400 W to 800 W. The plasma far away from the resonant point is found to be radially uniform.     

平行极板间等离子体RF共振特性研究

为探索用于ＡＶＬＩＳ的更为有效的离子引出方法,对等离子的ＲＦ共振离子出引方法进行了理论研究,用ＰＩＣ方法对其了模拟。结果表明：在弱磁场下可产生鞘层－等离子体振荡,其振荡频率与理论一致;外加与振荡频率王牟的交流电压后将发生共振,其振荡幅度大幅增加,电流加大,共振在等离子体中产生的最大电位幅可高于外加电压幅度几倍;与离子引出方法中传统的平行板电场相比,ＲＦ共振法使得出出的离子流加大,离子引出时间显著     

Characteristic Analysis of AC Plasma Arc in Water

An experimental system of AC arc discharge in water was designed with pole-pole electrodes and a peak voltage of 1500 V and a test circuit was set up using virtual instrument technology. The mechanism of an AC plasma arc generated in water was analyzed. The voltage- current characteristic of the AC plasma arc was obtained from the waveform. The temperature characteristic was tested with a spectrum diagnosis system, and the effect of different electrode materials on the striking voltage and peak current was analyzed. The results show that when a power supply of 6 KW is applied on electrodes with a gap of 2 mm in water, the striking voltage is from 900 to 1300 V, the arc voltage is from 40 to 100 V, the arc current is from 2 to 7 A, and the zero rest period is from 1 to 2 ms. In addition, the arc voltage and current are different for electrodes in aluminum, copper and stainless steel. The arc voltage is lower and the current is higher for an aluminum electrode than those for copper and stainless steel ones. The highest temperature of the arc is 7643 K.     

Monopole Antenna Probe for Density Measurements in Cold Plasmas

A simple diagnostic tool for density measurements in plasma with a certain spatial resolution is proposed in the this paper. It uses the emission characteristics of monopole antenna to determine the dielectic property of plasmaε=1-ƒ_p²/ƒ², withƒ_pthe electron plasma frequency related to plasma density. We immersed a monopole antenna probe into plasma and introduced a microwave signal via a network analyzer. When the emitted power is maximized, the reflected power is minimized and there occurs a resonance. Sinceε can be derived from the resonant frequency, this is actually a method to measure the absolute electron density. Validated by a comparison with the amended Langmuir double-probe method, the monopole antenna probe is valuable. In addition, it is free from the difficulties, such as fluctuation in plasma potential.     

Comparison between fluctuation of floating potential gradient and velocity of blob structure on HL-2A tokamak

Several results based on the Langmuir probes' data on the HL-2A tokamak are presented. The blob structures' radial and poloidal drift velocities, estimated by the gradient of floating potential and by time delay evaluation, are compared in different line-averaged density and electron cyclotron resonance heating conditions. A positive correlation is observed in the comparison between blobs' radial velocity estimated by the two methods mentioned above, regardless of the situation differences mentioned above. Correlation is also observed in the comparison between the blobs' poloidal velocity estimated by the two methods in different situations, while a shift due to the different line-averaged density is observed. These results imply that the radial gradient of floating potential may have some value as a reference during data analysis in low-parameter discharge.