Triple Probe Method Research Articles

Plasma parameters of argon and argon/molecular gas mixture plasmas produced in microwave discharge

Abstract The experimental results and their analysis are presented concerning electron density and temperature of the microwave discharge plasma (power range 100–500 W) produced in Ar gas and Ar–N 2 and Ar–N 2 –NO gas mixture at the total pressure of 0.5 Torr. The triple probe method was primarily used in order to set-up a direct and on-time technique to measure electron temperature in complex plasma. The results were compared with those obtained using a cylindrical probe mounted in a circuit with reference electrode. Each probe was made of tungsten wires of 0.5 mm diameter and 3 mm in length each. When plasma contains only one maxwellian group of electrons, within the experimental errors, both the probes systems get comparable value of plasma parameters. When the characteristic of cylindrical probe shows two groups of thermal electrons within the microwave plasma the triple probe may show always results concerning one group but of which temperature depends on the ratio of the densities of those two groups of electrons.

309 (N_2+CO_2)混合ガスアフターグロープラズマの電子温度および電子密度測定(OS3-2 オーガナイズドセッション《熱・流体・エネルギー変換工学の進展》)

Radiative characteristic of N_2-CO_2 mixture afterglow plasma generated by microwave discharge changes by mixture ratio of N_2 and CO_2 gases. In order to investigate the cause of the change, the electron temperature and electron density were measured using triple probe method. The triple probe was inserted in the afterglow plasma and was set at 10mm intervals at the point of 40 to 100mm from the downstream end of the cavity resonator. From the measured probe voltage and current, the electron temperature and density were calculated. The profiles of the electron temperature and density were changed by the mixture ratio of N_2 and CO_2 gases. For the CO_2 mixture ratio of larger than 6%, the electron temperature and density decreased steadily along with the flow, however, for the CO_2 mixture ratio of less than 4%, increased temporarily in the lower region of 50 to 90mm.

Practical solutions for reliable triple probe measurements in magnetized plasmas

The triple probe method to obtain local, time-resolved measurements of density, electron temperature and plasma potential is investigated in detail. The difficulties in obtaining reliable measurements with this technique are discussed and overcome. These include phase delay errors, ion sheath expansion and limited bandwidth due to stray capacitance to ground. In particular, a relatively simple electronic circuit is described to strongly reduce stray capacitance. Measurements with the triple probe are presented in a plasma characterized by interchange-driven turbulence in the TORPEX device. The measured time-averaged and time-dependent, conditionally averaged parameters are cross-checked with other Langmuir probe based techniques, and show good agreement. Triple probe measurements show that electron temperature fluctuations are sufficiently large, such that the identification of plasma potential fluctuations with fluctuations of the floating potential is not a good approximation. Over a large radial region, the time-averaged fluctuation-induced particle flux can, however, be deduced from floating potential only. This is because the phase shift between density and electron temperature is close to zero there and temperature fluctuations do not give rise to a net radial particle transport.

Improvement of thermal probe method for the plasma diagnostics

We have proposed to determine the ion temperature from the heat flux data obtained with the thermal probe. In this paper, we made a prototype of the thermal probe and tested it with the glow discharge plasma. In order to use thermal probe in high temperature plasma, rapid measurement method without bias voltage scanning is preferable. Using the sheath particle/heat flux formula, well-known triple probe method is expanded to include the ion heat flux measurement. It is shown that there exists the possibility to measure ion temperature directly.

Application of Plane Probe to Weakly Ionized Plasmas.

Probe characteristics are numerically obtained in a wide range of electron temperatures and densities for the collision-dominated plasma sheath in which the fundamental Langmuir theory of the electrostatic probe fails. The asymptotic theory of Cohen for continuum conservation equations is applied to the ellipsoidal probe, and to the plane probe as the limiting case. It is found that, for the collision-dominated plasma sheath, significant errors in measured electron temperature are expected if the fundamental Langmuir theory is used to evaluate the temperature from the measured probe current and voltage. For quick evaluation of the electron temperature in such plasmas, the bound triple-probe method is proposed, and numerical results are summarized in a graphical form for practical use.

Adiabatic electron thermal pressure fluctuations in tokamak plasmas.

Electron thermal pressure fluctuations measured in the edge plasma of the Texas Experimental Tokamak Upgrade are a fundamental component of plasma turbulence on both sides of the velocity shear layer. The ratio of specific heats, estimated from fluctuations in electron temperature and electron number density measured simultaneously at the same electrode, indicates that observed fluctuations are adiabatic. The observations are made by means of a novel Langmuir probe technique, the time domain triple-probe method, which concurrently measures multiple plasma properties at each of two electrodes with the temporal and the spatial resolution required to estimate thermodynamic properties in a turbulent plasma.

Process control of plasma polymerization in a large industrial reactor

Application of the triple-probe method for volume-resolved monitoring of electron density and electron temperature during industrial-scale polymerization plasma process was tested. As a model process, simultaneous deposition of barrier film on the inner wall of six bottles using an asymmetrical r.f. discharge was investigated. The reproducibility of the process was better than 94%. The plasma density differed between particular bottles up to a factor of 2, and this difference increased with pressure. The barrier properties of the film increased with the product of the ion current and square root of the electron temperature

Diagnostics of the diamond depositing inductively coupled plasma by electrostatic probes and optical emission spectroscopy

Electrostatic probe methods and optical emission spectroscopy have been used for diagnostics of the low-pressure inductively coupled radio frequency plasma at pressures between 5 and 80 mTorr under real environments of diamond deposition. The single, triple, and emissive probe methods were used alone or combined to obtain electron temperature (Te) and electron density (ne). The single or triple probe method combined with the emissive probe method allowed the determination of ne with high reliability as confirmed by cross-checking. Comparison with the grown deposits suggested the existence of a critical ne value of around 2×1010 cm−3, which was responsible for the drastic decrease in diamond growth rate observed below 10 mTorr and the resulting no growth at around 5 mTorr. The energy distribution of excitation levels of the H atom Balmer series was almost constant and the distribution equilibrium was not established except the case of 5 mTorr, which was ascribed to the difference in the excitation process of H atoms.

トリプルプローブによる細線放電アークプラズマの測定

トリプルプローブによる細線放電アークプラズマの測定

Time-domain triple-probe measurement of edge plasma turbulence on the Texas Experimental Tokamak Upgrade

The time-domain triple-probe method (TDTP) switches Langmuir probe states at high speeds to gather triple-probe information thereby avoiding errors which result when implementing the triple-probe method with spatially separated single state probes in an inhomogeneous plasma. Successful implementation of the TDTP method requires a switching frequency greater than the turbulence bandwidth so that aliased power does not degrade time interpolated estimates of triple-probe states. Additionally, transients between states, largely determined by the ratio of ion saturation current to probe capacitance, must be short enough to establish a probe–plasma equilibrium. These requirements have been met in the edge plasma and parts of the scrape-off layer on the Texas Experimental Tokamak Upgrade. Temperature fluctuations have been measured at two poloidally separated probes. Cross-phase and coherence spectra consistent with those of floating potential fluctuations and ion saturation current fluctuations are measured.

Parameter Change of Magneto-Plasma-Dynamic Arc-Jet Plasma on a Flow-Facing Plate

Electron temperature and density of plasma flow, which was obtained from a magneto-plasma-dynamic (MPD) arc jet, were measured by the triple-probe method. In order to apply the MPD arc jet to thin-film processing, the changes in the plasma parameters on a processing substrate were investigated.

Fast reciprocating probe system for local scrape-off layer measurements in front of the lower hybrid launcher on JT-60U

A fast reciprocating probe system with a long drive shaft was incorporated into a multi-junction lower hybrid (LH) wave launcher on JT-60U in order to investigate an improved coupling mechanism of the radio frequency wave to the core plasma. The system has been operated reliably over a horizontal scan of 25 cm in 1.5 s using a compact pneumatic cylinder drive and springs. A double probe measurement provided the scrape-off layer plasma profile between the last closed flux surface and the first wall with the spatial resolution of 1−2 mm measured with a laser displacement gauge. The profiles of the electron density ne and temperature Te were in good agreement with those obtained with a triple probe method. During the LH wave injection with good coupling to the core plasma, an increase in the local Te was observed in front of the LH launcher mouth. The local ne was (7−10)×1016 m−3, consistent values needed for the good coupling.

The time-domain triple-probe method

A new Langmuir-probe technique based on the triple-probe method is being developed to provide simultaneous measurement of plasma temperature, potential, and density with the temporal and spatial resolution required to accurately characterize plasma turbulence. When the conventional triple-probe method is used in an inhomogeneous plasma, local differences in the plasma measured at each probe introduce significant error in the estimation of turbulence parameters. The time-domain triple-probe method (TDTP) uses high-speed switching of Langmuir-probe potential, rather than spatially separated probes, to gather the triple-probe information thus avoiding these errors. Analysis indicates that plasma response times and recent electronics technology meet the requirements to implement the TDTP method. Data reduction techniques of TDTP data are to include linear and higher-order correlation analysis to estimate fluctuation induced particle and thermal transport, and relationships between temperature, density, and potential fluctuations.

The adverse effect of perpendicular ion drift flow on cylindrical triple probe electron temperature measurements

The cylindrical triple probe method is an attractive technique for measuring electron temperatures (Te) and electron number densities (ne) in a variety of plasmas sources. In practice, however, the cylindrical triple probe can be sensitive to sources of error that affect all Langmuir probe techniques. In particular, the presence of an ion drift velocity component that is perpendicular to the probe axis has been known to result in erroneous measurements of ne. Less obvious, however, is that ion flow perpendicular to the probe has a significant effect on the indicated Te. The purpose of this note is to make researchers aware of such an effect and to demonstrate a technique which can mitigate it. The approach taken to investigate this phenomenon was to make Te measurements in the plume of a 20 kW magnetoplasmadynamic thruster with the probe oriented at several angles with respect to the local ion flow.

The use of a symmetrical triple probe for direct reading of plasma parameters in a planar PCVD reactor

Measurements are presented of electron temperature k T e and density N e in a planar electrode system with an rf excited plasma in N 2 gas, as obtained using triple probe method for instantaneous display of medium density plasma parameters. The results are compared with other electrostatic probe methods. We find that the symmetrical triple probe method is in accord with other probe methods in measuring plasma parameters and it has the advantages of simplicity, convenience and rapid response. Thus, the symmetrical triple probe method is suitable for plasma parameter measurements in a PCVD reactor.

A new scheme for Langmuir probe measurement of transport and electron temperature fluctuations

A new scheme to extend the triple Langmuir probe technique for the measurement of electron temperature fluctuations and the fluctuation-driven transport has been developed. The extension is aimed at reducing the phase delay error introduced by finite probe tip separations in standard triple-probe method. The modified triple-probe scheme provides a more reliable measurement of the temperature fluctuations for a proper interpretation of the density and potential fluctuations and the transport measurement from Langmuir probe data. New results on fluctuations have been obtained from Phaedrus-T and TEXT-U tokamaks.

Probe measurements in the REPUTE-1 reversed field pinch

A four-channel triple-probe and magnetic-probe array, a complex probe which consists of three types of probes and two eight-channel magnetic-probe (poloidal and toroidal) arrays, has been installed in the revised field pinch University of Tokyo experiment (REPUTE-1) reversed field pinch (RFP) device. Mean and fluctuation parts of plasma parameters, including the three components of magnetic fields, three components of electric fields, electron density, electron temperature, and space potential, are measured in 0.5a≲r≤a region. The triple-probe method and the electric field measurement are described in detail, and effects due to the fast electrons, etc., are discussed. Some experimental examples obtained in the REPUTE-1 RFP plasma are given, and the detailed results will be published elsewhere.

Instantaneous triple-probe method for the direct display of plasma parameters in a spacecraft condition

It is proposed that the instantaneous triple-probe method can be applied to the direct display of parameters; particularly in the case of probe application during measurements carried out from spacecraft. The theory of the triple-probe method has been extended for the very high speed flowing collisionless plasma condition, i.e. in which the spacecraft speed U ≈ the most probable electron thermal velocity 〈 v e 〉 > the most probable ion thermal velocity 〈 v i 〉. This condition is particularly important both in the case of re-entry of spacecraft and in the solar wind plasma. Experiments have been conducted in a free molecular beam plasma chamber for S i = U 〈v i〉 > 10 . The experimental results and their relationship to the appropriate theory are presented.

The instantaneous triple-probe method for the direct display of plasma parameters in a low density continuum plasma

An exact theory is developed for the use of the instantaneous triple-probe method in the continuum Laplace limit. The theory applies to probes of arbitrary shape, including the commonly used spherical and finite cylindrical probe configurations. The theory yields the same form for the probe characteristic for l probe shapes. The theory is also extended to deal with the effects of the presence of negative ions. An experimental confirmation of the theory has been carried out both in glow discharge and discharge afterglow plasmas.

Studies of the Effect of Ion Current on Instantaneous Triple-Probe Measurements

A dimensionless parameter η is suggested to express the variation of ion current with probe potential in a probe characteristics which can be used to evaluate the influence of ion current on the ``instantaneous'' measurement of plasma parameters by means of triple-probe technique. Theoretical values of η for a cylindrical probe are obtained as a function of dimensionless parameter ξe(=rp/he), where rp is probe radius and he is the electron Debye length, both from classical sheath model and the more modern unified probe theory. η is small (<0.01) when ξe is large (>100), increases as ξe and mi (the ion mass) decreases and has a saturation value approximately equal to 0.1–0.2 at small ξe(≲1). The experimental η agrees with the theoretical η within a factor of 2. The possible errors associated with the direct-display system of plasma parameters using the triple-probe method by assuming η=0 are discussed in connection with η, and the results show that the maximum errors are within 40% for electron temperature determination and within a factor of 3 for electron density determination, respectively.