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.     

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.     

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.     

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.     

Frequency dependence of electron temperature in hollow cathode-type discharge as measured by several different floating probe methods

We measured electron temperatures through a hollow cathode-type discharge tube using several different floating probe methods.This method detected a shift in the floating potential when an AC voltage was 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.In this study,the effects of the frequency and waveform on the electron temperatures were systematically investigated.The electron temperature measured when using the floating probe method with applied sinusoidal and triangular voltages was lower than that measured with an applied rectangular voltage.The value in the high frequency range was close to that of the tail electron temperature.     

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.     

New apparatus for performing perturbed-angular-correlation spectroscopy on epitaxially grown III–V surfaces

We have developed a new apparatus for investigating the surface properties of epitaxially grown III–V semiconductor crystals using perturbed-angular-correlation (PAC) spectroscopy. These properties include the chemical-bond symmetry for surface adatoms, adatom surface diffusion, and incorporation of adatoms into the bulk crystal. To measure the properties, we have combined the techniques of PAC spectroscopy and molecular-beam epitaxy (MBE). Specifically, to perform these experiments, we have developed an ultra-high vacuum system to perform MBE crystal growth, surface doping operations, and in situ PAC measurements. Using this system, we can introduce trace quantities of radioactive probe atoms, which are used to make the PAC measurements, at any point in the MBE-growth process. To date, we have used this unique apparatus to obtain results for several different gallium arsenide and indium arsenide surface morphologies.     

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 .     

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.     

Experimental study of sheath potential coefficient in the J-TEXT tokamak

Sheath potential coefficient α is a key parameter, which is used to estimate plasma potential(V_p)for edge plasma physics study. Recently, a series of experiments has been carried out under hydrogen plasmas in the J-TEXT tokamak with swept probe, which is employed for current–voltage(I–V) characteristic measurement. Electron temperature is evaluated from I–V curve by three-parameter fitting method, and the electron energy probability function shows that electron distribution is Maxwellian both outside and inside of last closed flux surface(LCFS). Plasma potential is obtained by crossing point between I–V exponential fitting curve and electron saturation current extrapolating line, which is in good agreement with first derivative probe technique. The α coefficient profile in the vicinity of the LCFS is obtained, which is in the range of 2.1–3, and decreases from outside to inside of LCFS.     

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.     

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%.     

Heavy Ion Beam Probe Measurement in Turbulence Diagnostic Simulator

A numerical measurement module simulating a heavy ion beam probe was developed, and numerical measurements of electrostatic potential and density fluctuations are carried out for 3-D turbulent data generated by a global simulation of drift-interchange mode turbulence in helical plasmas. The deviation between measured and local values is estimated. It is found that the characteristic structures can be detected in spite of the screening e®ect due to the ¯nite spatial resolution.     

Critical Heat Flux and Near-Wall Boiling Behaviors in Saturated and Subcooled Pool Boiling on Vertical and Inclined Surfaces

The mechanism of the critical heat flux (CHF) where the departure from nucleate boiling (DNB)-type boiling transition takes place has not been fully elucidated. In this paper, we examine the trigger mechanism of the CHF for saturated and subcooled pool boiling on vertical and inclined surfaces based on measurements of the liquid-vapor behaviors near heating surfaces by using a conductance probe. The angle of inclination was varied from 90° (vertical) to 170° (facing almost horizontally downwards). The probe signals and the void fraction distributions showed that a liquid layer remains beneath the vapor masses moving upward along the heating surface at high heat fluxes near the CHF. The thickness of the liquid layer was determined from the location where the probe signals corresponding to the vapor masses disappeared. The thickness of the liquid layer formed on the vertical surface increased with increasing degree of subcooling, which may be the cause of the increases in CHF with increasing degree of subcooling. The measurements of saturated boiling on the inclined surface confirmed that the orientation of the heating surface greatly affects the period it takes for vapor masses to pass, but it negligibly affects the liquid layer thickness. This suggests that the decrease in CHF with increasing angle of inclination is primarily caused by the lengthening of the duration of vapor mass passage.     

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.     

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.     

Gate valve and shutter control system of the fusion experiment Wendelstein 7-X

The plasma vessel of the fusion experiment Wendelstein 7-X (W7-X) is a plasma vessel covering a plasma volume of about 30 m³. The vacuum conditions for plasma experiments inside the plasma vessel are supposed to be in a range of 1 × 10⁻⁸ mbar (ultra high vacuum conditions) after evacuation and conditioning. The 254 ports of the plasma vessel allow an external access to the inner space of the plasma vessel. Ports for heating and diagnostic systems are equipped with gate valves or with shutters. The vacuum gate valves are used as a controllable mechanical and a vacuum disconnection point between diagnostics and heating systems on the port side and the inner plasma vessel on the other side. The shutters are responsible for an optical and thermal protection for port windows or installed equipments inside the ports. After an overview of the main requirements for the control of the huge number of gate valves and shutters for the operational phases 1 and 2 of W7-X the design and realization of a centralized control system for controlling and observing all shutters and the majority of gate valves of the machine Wendelstein 7-X will be introduced and discussed.     

复方分子探针及其显像

为克服［¹⁸F］FDG PET、联合用分子探针显像和多靶向偶合分子探针显像的局限性,本文首次提出“复方分子探针及其显像”概念模式。复方分子探针制备方法包括分次合成配制法、一锅单次合成法和同时平行合成法,其中一锅单次合成法最为简便实用,较有前景。复方分子探针具有固定配比、可单次生产配制、药效稳定可控,以及改善用药依从性和使用方便等优势。复方分子探针的研发间接解决了多靶向偶合分子探针复杂合成方面的难题,为创制下一代新型分子探针提供了简单、实用和易推广新模式。复方分子探针显像解决了多模式分子探针同时显像的重要科学问题,为进一步开展复方多模式分子探针显像奠定了基础。联合双探针单次给药同时显像、联合双探针双模式显像和术中联合多探针显像导向治疗,在精准医学方面显示巨大应用潜力,为进一步发展和转化应用复方分子探针显像奠定了基础。本文对“复方分子探针及其显像”的理论依据、概念和原理、重要性、制备方法及其潜在应用价值进行了阐述。     

Vibrations of probe used for the defect detection of helical heating tubes in a fast breeder reactor: Part 1. Experimental results by using mock-up

This paper deals with the vibration and the sensor signal noise of the eddy current testing (ECT) probe used for a defect detection of helical heating tubes in the fast breeder reactor “Monju”, developed in Japan. ECT probes are used for the detection of defects in a heating tube. The heating tube is composed of a helical tube and a straight tube because of their advantages of thermal efficiency and saving space. No vibrations of the ECT probe have been generated in usual straight heating tubes. However, vibrations of the ECT probe in the helical tube cause some noise and decrease the sensitivity of the ECT probe. The experiment was performed using a mock-up, and the noise characteristics of an ECT sensor mounted in an ECT probe were examined. The experimental results showed that the sensor signal noise during the insertion process of the ECT probe was higher than that of the return process, and vibrations of the insertion process had a certain emerging frequency. Attaching the long and flexible guide probe to the top of the ECT probe was an effective countermeasure against sensor signal noise.