Langmuir Probe System Research Articles

RF plasma parameter determination by a Langmuir multipoint double probe array

A multipoint double Langmuir (MDL) probe system, which is exempt from interference, has been designed and assembled to be applied to an RF plasma. The system provides the measurement of fundamental plasma parameters such as density, temperature, plasma potential, etc. on the basis of the Bohm Approximation Theory and the Orbital Movement Limit. Thus, one pair of the MDL system is selected so as to consider the right plasma parameters within the prevailing pressure-power intervals. Both the hardware and software of the system have been applied to the modification of material properties by means of the PIII process.

Electronic system for Langmuir probe measurements

A newly developed Langmuir probe system for measurements of current-voltage (IV) characteristics in the tokamak divertor area is presented and discussed. The system is partially controlled by a computer allowing simultaneous and independent feeding and registration of signals. The system is mounted in the COMPASS tokamak, Institute of Plasma Physics, Academy of Sciences of the Czech Republic.The new electronic circuit boards include also active low-pass filters which smooth the signal before recording by the data acquisition system (DAQ). The signal is thus less noisy and the data processing is much easier. We also designed and built a microcontroller-driven waveform generator with resolution of 1 Ms/s. The power supply is linear and uses a transformer. We avoided the use of a switching power supply because of the noise that it could generate.Examples of measurements of the IV characteristics by divertor probes in the COMPASS tokamak and evaluation of the EEDF are presented.

Plasma parameter analysis of Ag sputter deposition using cathodes with different magnetic flux densities

The effects of the magnetic flux density of the Ag sputter target surface on plasma parameters were investigated using the Langmuir probe system in this work. It was found that the electron energy and electron density near the substrate clearly decreased at a high magnetic flux density. In addition, the difference between plasma potential and floating potential (Vp−Vf) decreased at the high magnetic flux density relative to the potentials at the low magnetic flux density. The changes in plasma parameters could be interpreted as the result of many electrons being trapped in the neighborhood of the target surface (cathode sheath) at the high magnetic flux density; hence, the number of electrons in the space near the substrate is reduced. The Ag thin films exhibited low resistivity at a high magnetic flux density. The reduction in resistivity was attributed to the following factors: the low electron energy and electron density near the substrate; the low kinetic energy of positive argon ions; the low kinetic energy of argon atoms backscattered onto the target surface.

Multichannel Langmuir probe for turbulence study in Heliotron J

New multichannel Langmuir probe system was developed and installed to Heliotron J. The objective of the new probe is to characterize basic turbulence property and the resulting transport in advanced helical configuration. The probe developed here consists of four sets of triple probe and one pin for floating potential measurement. Initial experiments in neutral beam heating plasma were conducted and fluctuation profile of radial and poloidal electric fields and Reynolds stress were estimated. For precise evaluation of the electric fields and Reynolds stress, a technique to compensate radial change of tilt angle between probe array and magnetic surface was proposed and applied to the initial results obtained in edge region of Heliotron J where the complicated magnetic structure exists.

Fast‐Ion Response to Energetic‐Particle‐Driven MHD Activity in Heliotron J

AbstractIn Heliotron J, low magnetic shear configuration, instabilities with frequency chirping in the frequency range of Alfvén eigenmodes have been observed in tangentially injected neutral beam plasmas. These modes are induced by energetic‐particle driven magnetohydrodynamic (MHD) instabilities such as global Alfvén eigenmode or energetic particle mode. A hybrid directional Langmuir probe system has been installed into Heliotron J to investigate the response of fast‐ion fluxes to the MHD modes. A high coherent response of the ion flux to the bursting modes has been observed not only by the co‐directed probe but also by the counter‐directed one. A linear correlation between the response of the co‐directed ion flux and the mode amplitude has been found. The radial profile of the response of the co‐directed ions has decreased with the minor radius and has not been obtained significantly outside last closed flux surface. These results indicate that the fast‐ion response is due to a resonant convective oscillation. The ion flux response of the counter‐directed probe has appeared in the growth phase of the mode burst. Its phase relation is different from that of co‐directed one and magnetic probe located at the Heliotron J vacuum vessel. Two candidates of the detected ion flux of the counter‐directed probe have been discussed (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Design of a multi-needle Langmuir probe system

The main goal of this work was to develop a Langmuir probe instrument for sounding rockets capable of performing high-speed absolute electron density measurements, and thereby be able to detect sub-meter ionospheric plasma density structures. The system comprises four cylindrical probes with a diameter of 0.51 mm and a length of 25 mm, each operated at a different fixed bias voltage in the electron saturation region. The probe diameter was chosen significantly less than the Debye shielding length to avoid complex sheath effects but large enough to ensure a probe area sufficiently large to accurately measure the electron currents drawn by the probes (in the range 1 nA to 1 µA). The crucial feature of the University of Oslo's multi-needle Langmuir probe (m-NLP) is that it is possible to determine the electron density without the need to know the spacecraft potential and the electron temperature Te. The m-NLP instrument covers a density range from ne = 109 m−3 to 1012 m−3, with sampling rates up to 9 kHz. The m-NLP instrument was successfully tested on the ICI-2 (Investigation of Cusp Irregularities) sounding rocket flight from Svalbard on 5 December 2008.

Design of a system of electrostatic probes for the RF negative ion source of the SPIDER experiment

A test facility (SPIDER) is planned to be built in Padova, Italy, in order to test and optimize the RF source of negative ions to be used in the ITER neutral beam injectors. In this paper the Langmuir probe system designed for monitoring the region, where most of the negative ions are expected to be produced, will be presented.

Reciprocating probe measurements of ELM filaments on JET

The ELM filamentary structure in the JET scrape-off layer (SOL) has been investigated using a multi-pin reciprocating Langmuir probe system inserted at the top, low field side of the poloidal cross-section. These measurements clearly demonstrate that both Type I and III edge localized modes (ELMs) are composed of a number of filaments. The filament propagation in the SOL is investigated and the radial velocity estimated using different methods. The large radial velocities observed during Type I ELMs (up to 6 km s−1) imply that filaments have no time to drain significant energy along the field lines, reaching the wall with a reasonable fraction of the pedestal density and temperature. In spite of the time between Type I ELMs being much longer than the ELM duration, most of the particle flux to the main chamber plasma-facing components occurs in the latter period. Furthermore, filaments in Type I ELMs are significantly larger and faster than those observed in Type III ELMs. During Type I ELMs a strong parallel flow (M ∼ 0.4) is measured towards the inner divertor, which increases during filaments. These observations are in agreement with the ballooning nature of the ELM losses.

Research of Plasma Characteristics of the Radio-Frequency Discharge in O2 and Its Mixtures

Electron mean energy and the effects of gas mixture are studied theoretically and experimentally. The electron mean energy in O2 and its mixtures is obtained by solving Boltzmann's equation. The experiments of the Langmuir probe system and spectral analysis are carried out. It is shown that electron temperature goes down with the increasing pressure, narrowing pulse width and the addition of helium and argon. According to the intensity of oxygen atom at 777.19 nm, xenon is more effective in inhibition of O2 decomposition than helium and argon.

Intermittent transport in the JET far-SOL

The intermittent transport in the JET scrape-off layer has been investigated using a multi-pin reciprocating Langmuir probe system. It was found that radial transport in the far-SOL is not continuous but occurs in short bursts observed most often at the beginning of each structure. The magnitude of the parallel ion flux during the filaments quickly decreases with radius, contrary to the cross-field velocity which has a weak radial dependence. Data from the JET boundary plasma have been analyzed to characterize the properties of the intermittent events. Statistical analysis reveals that the filament duration is larger on average in L-mode when compared to that observed during ELMs, while the radial velocity and the density perturbations are smaller.

A Langmuir probe system for high power RF-driven negative ion sources on high potential

A fully automated Langmuir probe system capable of operating simultaneously with beam extraction has been developed and commissioned for the negative hydrogen ion source testbeds at IPP Garching. It allows the measurement of temporal and spatial distributions of the plasma parameters within a single plasma pulse (<5 s). This system can operate even in the presence of multi-harmonic RF interference due to a novel transformer-based RF compensation system. Analysis methods of the probe data are described in the paper along with a discussion of errors. Measurements of the plasma parameters for RF powers (30–80 kW) and source pressures (0.3–0.8 Pa) both in plasma generation region and near the plasma grid have been carried out. The plasma generation region has both a high density (>1018 m−3) and hot (Te > 10 eV) plasma with bi-Maxwellian electron energy distribution at low pressures. The plasma found near the plasma grid is very different being of low density (⩽1017 m−3) and very cold (Te < 2 eV). This plasma is also strongly influenced by the presence of caesium, the potential of the plasma grid, and if an ion beam is extracted from the source. Caesium strongly reduces the plasma potential of the source and enhances the negative ion density near the plasma grid. Extracting an ion beam is observed to reduce the electron density and increase the potential near the plasma grid. Applying a potential greater than the plasma potential to the plasma grid is found to significantly decrease the electron density near the plasma grid.

A Langmuir probe system incorporating the Boyd–Twiddy method for EEDF measurement applied to an inductively coupled plasma source

This paper reports on a modern implementation of a Langmuir probe method that was first proposed in the 1950s. The system directly measures the first and second derivative of the I–V characteristic and determines the EEDF. This probe offers many advantages over existing systems that rely on numerical differentiating and data smoothing techniques. In addition to the EEDF analysis the system also incorporates several theories to extricate the plasma parameters from the ion collection and electron retardation branches of the I–V characteristic.

Bicoherence during confinement transitions in the TJ-II stellarator

Bicoherence was computed for a number of discharges having confinement transitions, either induced by biasing or spontaneous, at the TJ-II stellarator. The bicoherence was computed for a variety of signals obtained using a reciprocating Langmuir probe system, but the main emphasis here is on the analysis of the poloidal electric field (Eθ).During biasing, the auto-bicoherence of Eθ was significant only in a narrow radial range, in contrast to the fluctuation level of Eθ and the linear coherence between Eθ and Er (the radial electric field), which were affected over a very broad radial extension.With spontaneous transitions, significant bicoherence was again detected only in a narrow radial range, associated with the position of the flow shear layer. The observations are consistent with the standard sheared flow model for confinement transitions.The temporal asymmetry A of Eθ was also computed. During biasing, the region of strongly modified asymmetry was located inwards from the bicoherence maximum. This suggests that the detected bicoherence is not merely due to a change in the temporal shape of the signal.

Deposition of BaxSr1–xTiO3 thin Films by Double RF Hollow Cathode Plasma Jet System

AbstractLow‐pressure plasma jet system with two hollow cathodes was used for deposition of Bax Sr1–x TiO3 (BSTO) ferroelectric thin films. Two RF hollow cathode plasma jet guns were pointed to the substrate where BSTO thin films were deposited. High density RF hollow cathode discharge was generated in ceramic nozzles made of SrTiO3 (STO) and BaTiO3 (BTO). Pulse modulated RF power was applied on water‐cooled copper block insulated from the plasma by ceramic and grounded stainless steel shield. In the experiment, the composition of BSTO films was controlled by magnitude of average RF power applied on particular plasma jet guns with STO and BTO nozzles. By this method, BSTO films were deposited on single crystal silicon substrate and on Pt top of Pt/TiO2/SiO2/Si multi‐layer system. X‐ray diffraction proved that BSTO thin films were polycrystalline with perovskite structure and measurement of dielectric properties showed ferroelectric hysteresis loop at room temperature. Time resolved Langmuir probe system and plasma impedance monitor were used for in situ plasma diagnostics and control of deposition conditions. Furthermore, plasma jets were analyzed during the deposition process by spatially resolved optical emission spectroscopy. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

A novel system for the direct measurement of the electron energy distribution function by Langmuir probe in ion sources

To realise a high yield H − (or D −) ion source capable of long pulse operation for the ITER neutral beam heating system, it is vital to understand and optimise the plasma parameters in the ion source. Calculating the electron energy distribution function (EEDF) in a non-Maxwellian plasma using conventional Langmuir probe numerical techniques is prone to error and generally produces poor results. Using a technique developed by Boyd and Twiddy [R.L.F. Boyd, N.D. Twiddy, Proc. R. Soc. A 53 (1959) 250] in the 1950s a new Langmuir probe system has been developed. This paper presents a detailed account of the experimental method, apparatus, software and analysis. The impact of the development is demonstrated by comparing data obtained from the ITER Negative Ion Beam Development Test Stand at CEA Cadarache using this method with similar data obtained with conventional methods.

Langmuir probe system for dusty plasmas under microgravity

This article describes a fully automated 2D-scanning Langmuir probe system for dusty plasmas under microgravity. The design combines necessary features such as random sampling, radio frequency compensation, and a compact mechanical design. The various aspects of the probe implementation and the contamination problem in the dusty plasma environment are discussed and the functionality of the system is demonstrated by measurements performed on parabolic flights.

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.

Measurement of the electron energy distribution function by a Langmuir probe in an ITER-like hydrogen negative ion source

Determining d2I/dV2 from a traditional Langmuir probe trace using numerical techniques is inherently noisy and generally yields poor results. We have developed a Langmuir probe system based on a method first used in the 1950s by Boyd and Twiddy (1959 Proc. R. Soc. A 53 250). The system measures the 2nd derivative directly. This paper presents an account of the experimental method, apparatus and software used along with some preliminary results from the KAMABOKO III source including a comparison with conventional probe methods.

Langmuir probe diagnostics in the IMPF device and comparison with simulations and tracer particle experiments

The plasma parameters in the IMPF (International Microgravity Plasma Facility) device for studying complex plasmas are determined with a 2D-scanning Langmuir probe system, which is suitable for operation on the International Space Station. The probe characteristics in low density, low pressure radio frequency (rf) discharges are evaluated in the framework of the radial-motion theory with corrections for collisions. A critical comparison between experimental values for plasma potential and ion density with SIGLO simulations (Kinema Software) is made, which yields good overall agreement. In particular, the shaping of the plasma profile by different rf power in the plasma centre and in an outer ring is well described. Small amounts of dust particles are used as a novel method to mark the boundary, where ion-drag force and electric field force are balanced. Again close agreement with simulation is found and demonstrates the applicability of the tracer technique as a quantitative diagnostic means.

Analysis of uncompensated Langmuir probe characteristics in radio-frequency discharges revisited

Measurements of the electron temperature, plasma density, and floating and plasma potentials with Langmuir probes in radio-frequency discharges often represent a challenge due to rf oscillations of the plasma potential. These oscillations distort the probe characteristic, resulting in wrong estimates of the plasma parameters. Both active and passive rf compensation methods have previously been used to eliminate rf fluctuation effects on the electron current drawn by an electrostatic probe. These effects on an uncompensated probe have been theoretically and experimentally studied by Garscadden and Emeleus [Proc. Phys. Soc. London 79, 535 (1962)], Boschi and Magistrelli [Nuovo Cimento 29, 487 (1963)], and Crawford [J. Appl. Phys. 34, 1897 (1963)]. They have shown theoretically that, assuming a Maxwellian distribution and sinusoidal plasma-potential oscillation, the electron temperature can be deduced directly from an uncompensated Langmuir probe trace, by taking the natural logarithm of the electron current. It is the purpose of this paper to bring back the attention onto this result, which shows that under certain discharge conditions it is not necessary to build any rf compensation in a Langmuir probe system. Here we present and reference experimental data found on the literature which support this result. Also computational data are presented.