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.

Presheath formation and area design limit satellite-based Langmuir probes

In this article,the effect of the finite conductive surface area of a satellite on the use of satellite-based Langmuir probes is reviewed in light of the basic theory of asymmetric double Langmuir probes(ADLPs).Recent theoretical and experimental studies have discussed electron sheath/presheath formation and the electron Bohm criterion along with their implications for satellite-based Langmuir probes.The effects predicted by the latest theory of the electron Bohm criterion were not experimentally observed and the experimental results remain supportive of a critical area ratio(A_(L)/A_(S))_(crit)=(m_(i)/(2.3m_(e)))^(1/2)between the probe area A_(S)and the satellite area A_L as conventionally believed.A satellite-based Langmuir probe must satisfy this criterion to physically act as a single Langmuir probe.However,experimental investigations also found that high-energy electrons adversely affect(A_(L)/A_(S))_(crit)and a Langmuir probe's signal quality by giving additional electron current to A_(L).Based on these results,a number of limitations of the maximum probe area are derived when designing satellite-based Langmuir probes,with consideration of both the aim of the satellite and the plasma where the satellite-based probe works.These proposed measures are expected to only partially alleviate the effect of the inadequate satellite surface area on the application of satellite-based Langmuir probes.Using a larger satellite to carry a Langmuir probe remains the most viable means to obtain precise space plasma parameters.

Fast Diagnosis of Transient Plasma by Langmuir Probe

A method for the fast measurement of electron temperature and density with temporal resolution in transient plasma has been implemented by Langmuir probe. The diagnostic system consists of a single Lang- muir probe driven by a high frequency sinusoidal voltage. The current and voltage spectrum on the probe were detected synchronously by an oscilloscope with sampling rate being at least 5 times higher than the frequency of sweep voltage. The system has been used to diagnose the transient plasma generated by hypervelocity-impact of LY12 aluminum projectile into LY12 aluminum target.

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.

The Langmuir Probe onboard CSES: data inversion analysis method and first results

The Langmuir Probe(LAP), onboard the China Seismo-Electromagnetic Satellite(CSES), has been designed for in situ measurements of bulk parameters of the ionosphere plasma, the first Chinese application of in-situ measurement technology in the field of space exploration. The two main parameters measured by LAP are electron density and temperature. In this paper, a brief description of the LAP and its work mode are provided. Based on characteristics of the LAP, and assuming an ideal plasma environment, we introduce in detail a method used to invert the I-V curve; the data products that can be accessed by users are shown. Based on the LAP data available, this paper reports that events such as earthquakes and magnetic storms are preceded and followed by obvious abnormal changes. We suggest that LAP could provide a valuable data set for studies of space weather, seismic events, and the ionospheric environment.

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.

Radial and Angular Resolved Langmuir Probe Diagnosis of a Pulsed Vacuum Arc Plasma

Vacuum arc ion sources are known for delivering high currents of ion beams in many technological applications. There is a great need in the present ion accelerator injection research for a titanium vacuum arc source to produce high-ionized plasma, in which its parameter is extremely important to match the extractors geometry and the extraction voltage. In this paper, the radial and angular distributions of the titanium cathodic vacuum arc plasma parameters are measured by a cylindrical Langmuir probe and analyzed by the Druyvesteyn method from the I-V curves. The electron density ne is about 10^（17）m^（-3） and the effective electron temperature Tefr is in the range of 6.12-11.11 eV in the free expansion cup before the ion extraction. The measured distribution of ne over the expansion cross-section is non-uniform and axially unsymmetrical with its form similar to the Gaussian distribution, and most of the plasma is concentrated into an area whose radius is smaller than 5 mm. Teff has a nearly uniform distribution over the expansion cross-section during the discharge. The results of the plasma parameters＇ non-uniformity encourage the researchers to make some optimization designs to improve the parameter distributions, and then to facilitate ion extraction.

Measurement of Zonal Flows in a Tokamak Using Langmuir Probe Array

Zonal flows have been measured with several novel Langmuir probe array on the HT-7 tokamak since 2002. The forked probe and its improved generation, the farmer forked probe were designed based on the original idea of the Reynolds stress triple tips array. In order to measure the radial shearing rate of zonal flows, the pyramid probe was also designed. This paper addresses the technical aspects regarding this new application. Since the zonal flows dynamics is now widely believed crucial to plasma confinement physics, an important application field of Langmuir probe was opened up.

A 57-Pin Langmuir Probe Array for Two-Dimensional Diagnosis in the Toroidal KT-5D Steady-State Plasma

In this article, the design idea of a 57-pin Langmuir probe array is presented, together with some experimental results. The manufacture technique of the probe array is discussed. Two-dimensional distributions of the plasma density, the floating potential and their fluctuations in the poloidal cross section are obtained. The blobs, which move radially and poloidally, cause great particle and energy transport. The charge neutralization by the poloidal movement of particles greatly decreases the vertical electric field in the helimak plasma. This leads to a confinement time which is two orders of magnitude larger than the theoretical prediction.

Optimization of Langmuir Probe System Parameters in a Stationary Laboratory Plasma

In order to obtain creditable data an applicable method to optimize parameters of the Langmuir probes and circuits in a stationary laboratory device is investigated and an experimental criterion of the probe dimension is developed. To obtain the electron temperature and density the Electron Energy Distribution Function （EEDF） approach with less computing time and more accurate results is applied, instead of the conventional slope approach. Moreover the influence of the vessel wall materials on the plasma density is discussed briefly, indicating that the dielectric wall is helpful to enhancing the electron density.

Diagnostics of Parameters by Optical Emission Spectroscopy and Langmuir Probe in Mixtures (SiH_4/C_2H_4/Ar) Radio-Frequency Discharge

Optical emission spectroscopy and Langmuir Probe diagnostics were incorporated into the experiment, in which dust particles were formed in-situ by using reactive mixture gases （SiHa/C2H4/Ar） in a radio-frequency （RF） discharge plasma. The excitation temperature was first calculated by combining several optical emission spectra of argon lines and using a Boltzmann distribution to fit the experimental data, then the excitation temperature as functions of both gas pressure and RF power in SiH4/C2Ha/Ar discharges for different discharge conditions were obtained. Correspondingly, based on the measurement of the electron temperature by a Langmuir probe, the excitation temperature was compared with the electron temperature, and some discussions were presented. Finally the emission intensities of spectral lines of Si 390.6 ran, Si2＋ 380.6 nm and C＋ 426.7 nm were measured and presented as functions of pressure, RF power and flow rate of SiH4/C2H4.

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.

Upgrade of an integrated Langmuir probe system on the closed divertor target plates in the HL-2A tokamak

A newly designed divertor Langmuir probe diagnostic system has been installed in a rare closed divertor of the HL-2A tokamak and steadily operated for the study of divertor physics involved edge-localized mode mitigation,detachment and redistribution of heat flux,etc.Two sets of probe arrays including 274 probe tips were placed at two ports(approximately 180°separated toroidally),and the spatial and temporal resolutions of this measurement system could reach6 mm and 1μs,respectively.A novel design of the ceramic isolation ring can ensure reliable electrical insulation property between the graphite tip and the copper substrate plate where plasma impurities and the dust are deposited into the gaps for a long experimental time.Meanwhile,the condition monitoring and mode conversion between single and triple probe of the probe system could be conveniently implemented via a remote-control station.The preliminary experimental result shows that the divertor Langmuir probe system is capable of measuring the high spatiotemporal parameters involved the plasma density,electron temperature,particle flux as well as heat flux during the ELMy H-mode discharges.

Machine learning combined with Langmuir probe measurements for diagnosis of dusty plasma of a positive column

This paper reports the use of machine learning to enhance the diagnosis of a dusty plasma.Dust in a plasma has a large impact on the properties of the plasma.According to a probe diagnostic experiment on a dust-free plasma combined with machine learning,an experiment on a dusty plasma is designed and carried out.Using a specific experimental device,dusty plasma with a stable and controllable dust particle density is generated.A Langmuir probe is used to measure the electron density and electron temperature under different pressures,discharge currents,and dust particle densities.The diagnostic result is processed through a machine learning algorithm,and the error of the predicted results under different pressures and discharge currents is analyzed,from which the law of the machine learning results changing with the pressure and discharge current is obtained.Finally,the results are compared with theoretical simulations to further analyze the properties of the electron density and temperature of the dusty plasma.

Fast-sweeping Langmuir probes:what happens to theⅠ–Ⅴtrace when sweeping frequency is higher than the ion plasma frequency?

Limited particle transit time is one of several limiting factors which determine the maximum temporal resolution of a Langmuir probe.In this work,we have revisited the known fast sweep Langmuir probe techniques in a uniform,quiescent multi-dipole confined hot cathode discharge with two operation scenarios:one in which the probe sweeping frequency fsweepis much lower than the ion plasma frequency fpi,another one where fsweepis much greater than fpi,respectively.This allows investigation into the effect of limited ion-motion on I–V traces.Serious distortions of I–V traces at high frequencies,previously claimed to be an ion-motion limitation effect,were not found unless shunt resistance is sufficiently high,despite a f;/f;ratio of~3.On the other hand,evidences of sheath capacitance on the I–V traces have been observed.Distortions of I–V traces qualitatively agree with predictions of sheath capacitance response to the sweeping voltage.Additionally,techniques in fast sweep Langmuir probe are briefly discussed.The comparison between the high-speed dual Langmuir probe(HDLP)and the single probe setup shows that the capacitive response can be removed via subtracting a leakage current for the single probe setup almost as effectively as using the HDLP setup,but the HDLP setup does remain advantageous in its facilitation of better recovery of weak current signal commonly in low density plasma.

Characteristics of Langmuir Probe in Low Temperature,Weakly Magnetized Plasmas

The systematic Langmuir probe measurements for a weakly magnetized plasma have been carried out in the Linear Magnetized Plasma Device for different magnetic fields. By comparing the ion current density of probes with different sizes, the sheath thickness can be evaluated. It is found that while the ratio of cylindrical probe's dimension to ion Larmor radius is not more than 2, the model of probe for non-magnetized plasma is still applicable.

Analysis of Langmuir Probe Characteristics for Measurement of Plasma Parameters in RF Discharge Plasmas

A simple method for measuring RF plasma parameters by means of a DC-biased Langmuir probe is developed. The object of this paper is to ensure the reliability of this method by using the other methods with different principles. First, Langmuir probe current response on RF voltage superimposed to DC biased probe was examined in DC plasmas. Next, probe current response of DC biased probe in RF plasmas was studied and compared with the first experiment. The results were confirmed by using an emissive prove method, an ion acoustic wave method, and a square pulse response method. The method using a simple Langmuir probe is useful and convenient for measuring electron temperature , electron density , time-averaged space potential , and amplitude of space potential oscillation in RF plasmas with a frequency of the order of .

Axial profiles of argon helicon plasma by optical emission spectroscope and Langmuir probe

We present the axial profiles of argon helicon plasma measured by a local optical emission spectroscope(OES) and Langmuir RF-compensated probe. The results show that the emission intensity of the argon atom lines(750 nm, 811 nm) is proportional to the plasma density determined by the Langmuir probe. The axial profile of helicon plasma depends on the discharge mode which changes with the RF power. Excited by helical antenna, the axial distribution of plasma density is similar to that of the external magnetic field in the capacitive coupled mode(E-mode). As the discharge mode changes into the inductively coupled mode(H-mode), the axial distribution of plasma density in the downstream can still be similar to that of the external magnetic field, but becomes more uniform in the upstream. When the discharge entered wave coupled mode(W-mode), the plasma becomes nearly uniform along the axis, showing a completely different profile from the magnetic field. The W-mode is expected to be a mixed pattern of helicon(H) and Trivelpiece-Gould(TG) waves.

Design and first result of combined Langmuir-magnetic probe on J-TEXT tokamak

In order to measure boundary electrostatic and magnetic fluctuations simultaneously,a combined Langmuir-magnetic probe(CLMP)has been designed and built on joint-Texas experimental tokamak.The probe consists of 8 graphite probe pins and a 3D magnetic probe,driven by a mechanical pneumatic device.By means of simulation,the shielding effect of the graphite sleeve on the magnetic fluctuation signal is explored,and the influence of the eddy current was reduced by cutting the graphite sleeve.In the experiment,it has been verified that the mutual inductance of electromagnetic signals can be ignored,and a 70–90 k Hz electromagnetic mode is observed around the last closed magnetic surface.The establishment of CLMP provides data for the exploration of the coupling of electrostatic and magnetic fluctuations.

Combined Langmuir-magnetic probe measurements of type-Ⅰ ELMy filaments in the EAST tokamak

Detailed investigations on the filamentary structures associated with the type-I edge-localized modes（ELMs） should be helpful for protecting the materials of a plasma-facing wall on a future large device.Related experiments have been carefully conducted in the Experimental Advanced Superconducting Tokamak（EAST） using combined Langmuir-magnetic probes.The experimental results indicate that the radially outward velocity of type-I ELMy filaments can be up to 1.7 kms^（-1） in the far scrape-off layer（SOL） region.It is remarkable that the electron temperature of these filaments is detected to be ~50eV,corresponding to a fraction of 1/6 to the temperature near the pedestal top,while the density ~3×10^（19）m^（-3） of these filaments could be approximate to the line-averaged density.In addition,associated magnetic fluctuations have been clearly observed at the same time,which show good agreement with the density perturbations.A localized current on the order of ~100kA could be estimated within the filaments.