Biased Langmuir Probe Research Articles

Current density limitation during disruptions due to plasma-sheaths

The presented experimental study realized in the COMPASS tokamak demonstrates, for the first time, that the current density that flows from the plasma into the vacuum vessel during disruptions is limited by the ion particle flux. Such a limitation shows that, at least in COMPASS, the sheath that forms between the plasma and the first wall dominates the halo current flow. This observation is achieved by measuring simultaneously the ion saturation current with negatively biased Langmuir probes and the halo current with grounded probes to the vacuum vessel. These comparative measurements, which were never performed during disruptions in other machines, directly confirm that the halo current density remains below the ion particle flux in COMPASS. The study also shows, using Mirnov coils measurement, that the total electric current entering the wall grows with the plasma current while the current density obtained by Langmuir probes remains unaffected. This, together with the current density limitation, leads to a novel finding that the halo current width increases with the pre-disruptive plasma current, which limits the local forces. The new findings reported here could also provide potential constraints on the modeling of disruption-induced loads on future reactor scale tokamaks and motivation for further experiments on existing devices.

Wake formation behind Langmuir probes in ionospheric plasmas

• A significant wake is formed behind thin cylindrical Langmuir probes. • Thin Langmuir probes ought to be at least ∼15 Debye lengths apart to not interfere. • An external magnetic field enhances and skews the wake behind a thin probe. This paper presents a simulation study of the wake formation behind a Langmuir probe thinner than the Debye Length in the space environments such as the ionosphere’s F region. We find that the wakes formed in plasma density and electric potential behind the positively biased probe can extend up to 15 Debye lengths in the subsonic plasma flow. Higher electric bias and flow velocity can further enhance the plasma wake perturbations. With an external magnetic field parallel to the object’s axis, the plasma wake becomes asymmetric and more extensive than for the unmagnetized case. The wing structures in the electron and ion densities are also observed along the background magnetic field in the case of subsonic plasma flow. The quantitative results in this paper may provide a practical reference for data processing and the future design of the Langmuir probe instrument flying in the F region on such as sounding rockets and low Earth orbit (LEO) satellite missions. In particular, since the Langmuir probe instrument may comprise two or more biased Debye-scale Langmuir probes, the plasma wake formed behind a single probe may influence the measurements from other instruments onboard.

Electronegative plasma diagnostic by laser photo-detachment combined with negatively biased Langmuir probe

We propose a new technique for diagnosing negative ion properties using Langmuir probe assisted pulsed laser photo-detachment. While the classical technique uses a laser pulse to convert negative ions into electron-atom pairs and a positively biased Langmuir probe tracking the change of electron saturation current, the proposed method uses a negatively biased Langmuir probe to track the temporal evolution of positive ion current. The negative bias aims to avoid the parasitic electron current inherent to probe tip surface ablation. In this work, we show through analytical and numerical approaches that, by knowing electron temperature and performing photo-detachment at two different laser wavelengths, it is possible to deduce plasma electronegativity (ratio of negative ion to electron densities) α, and anisothermicity (ratio of electron to negative ion temperatures) γ−. We present an analytical model that links the change in the collected positive ion current to plasma electronegativity and anisothermicity. Particle-In-Cell simulation is used as a numerical experiment covering a wide range of α and γ− to test the new analysis technique. The new technique is sensitive to α in the range 0.5 < α < 10 and yields γ− for large α, where negative ion flux affects the probe sheath behavior, typically α > 1.

Development of a novel wafer-probe for in situ measurements of thin film properties

We report a new development of a diagnostic technique, referred to as the wafer probe, which enables us to qualitatively monitor the plasma-induced changes in thin film dielectrics, in-situ and in real time. The wafer probe is an adaptation of the well-established ion flux probe technique, also known as RF biased or pulse biased planar Langmuir probe. This technique utilises the top surface of a tile cut from a multi-layer wafer as the probing area. This technique was successfully used to characterise different plasma conditions and monitor the plasma-induced changes in the thin layers of the tile, e.g. a porous organosilicate-glass low-κ dielectrics. The wafer probe was used to monitor the different effects of an argon and a hydrogen plasma on low-κ dielectrics, as well as to monitor the etch rate and endpoint of an Ar/SF6 plasma.

Improved optogalvanic detection with voltage biased Langmuir probes

Optogalvanic detectors show great potential for infrared spectroscopy, especially in cavity enhanced techniques where they, in contrast to ordinary absorption detectors, can perform intracavity measurements. This enables them to utilize the signal-to-noise ratio improvement gained from the extended effective path length inside an optical cavity, without losing signal strength due to the limited amount of light exiting through the rear mirror. However, if optogalvanic detectors are to become truly competitive, their intrinsic sensitivity and stability has to be improved. This, in turn, requires a better understanding of the mechanisms behind the generation of the optogalvanic signal. The study presented here focuses on an optogalvanic detector based on a miniaturized stripline split-ring resonator plasma source equipped with Langmuir probes for detecting the optogalvanic signal. In particular, the effect of applying a constant bias voltage to one of the probes is investigated, both with respect to the sensitivity and stability, and to the mechanism behind the generation of the signal. Experiments with different bias voltages at different pressures and gas composition have been conducted. In particular, two different gas compositions (pure CO2 and 0.25% CO2 in 99.75% N2) at six different pressures (100 Pa to 600 Pa) have been studied. It has been shown that probe biasing effectively improves the performance of the detector, by increasing the amplitude of the signal linearly over one order of magnitude, and the stability by about 40% compared with previous studies. Furthermore, it has been shown that relatively straightforward plasma theory can be applied to interpret the mechanism behind the generation of the signal, although additional mechanisms, such as rovibrational excitation from electron-molecule collisions, become apparent in CO2 plasmas with electron energies in the 1–6 eV range. With the achieved performance improvement and the more solid theoretical framework presented here, stripline split-ring resonator optogalvanic detectors can evolve into a compact, inexpensive, and easy-to-operate alternative for future infrared spectrometers.

Ion flux enhancements and oscillations in spatially confined laser produced aluminum plasmas

Ion signals from laser produced plasmas (LPPs) generated inside aluminum rectangular cavities at a fixed depth d = 2 mm and varying width, x = 1.0, 1.6, and 2.75 mm were obtained by spatially varying the position of a negatively biased Langmuir probe. Damped oscillatory features superimposed on Maxwellian distributed ion signals were observed. Depending on the distance of the probe from the target surface, three to twelve fold enhancements in peak ion density were observed via confinement of the LPP, generated within rectangular cavities of varying width which constrained the plasma plume to near one dimensional expansion in the vertical plane. The effects of lateral spatial confinement on the expansion velocity of the LPP plume front, the temperature, density and expansion velocity of ions, enhancement of ion flux, and ion energy distribution were recorded. The periodic behavior of ion signals was analyzed and found to be related to the electron plasma frequency and electron-ion collision frequency. The effects of confinement and enhancement of various ion parameters and expansion velocities of the LPP ion plume are explained on the basis of shock wave theory.

Impact of laser wavelength on the emission of electrons and ions from thin gold films during femtosecond laser ablation

This experimental study supports computational evidence that the excitation of d-band electrons have a significant impact on the material response of gold to an ultrashort laser pulse. A voltage biased Langmuir probe was used to detect the electron and positive ion emission from a 20nm thick Au film at 343nm, 515nm and 1030nm laser wavelengths. The laser wavelengths used provided photon energies above and below the interband transition threshold (ITT) for d-band excitation in gold (1.9eV). Variations in optical properties of the Au film at different laser wavelengths were accounted for to ensure that comparisons were made at equal values of absorbed fluences. Data showed a significantly higher number of electrons emitted over a longer period when the energy of the incident photons exceeded the ITT. The electronic current generated at the probe indicated that the first electrons detected were primarily emitted via multiphoton photoemission. Electrons were also detected at later times when the photon energy exceeded the ITT and this was attributed to thermionic emission. The current generated from the positive ion emission was significantly lower than for electron emission, but indicated the formation of a plasma at a threshold fluence that increases with decreasing photon energy.

Magnetic dipole discharges. II. Cathode and anode spot discharges and probe diagnostics

The high current regime of a magnetron-type discharge has been investigated. The discharge uses a permanent magnet as a cold cathode which emits secondary electrons while the chamber wall or a grounded electrode serves as the anode. As the discharge voltage is increased, the magnet develops cathode spots, which are short duration arcs that provide copious electrons to increase the discharge current dramatically. Short (1 μs), high current (200 A) and high voltage (750 V) discharge pulses are produced in a relaxation instability between the plasma and a charging capacitor. Spots are also observed on a negatively biased plane Langmuir probe. The probe current pulses are as large as those on the magnet, implying that the high discharge current does not depend on the cathode surface area but on the properties of the spots. The fast current pulses produce large inductive voltages, which can reverse the electrical polarity of the magnet and temporarily operate it as an anode. The discharge current may also oscillate at the frequency determined by the charging capacitor and the discharge circuit inductance. Each half cycle of high-current current pulses exhibits a fast (≃10 ns) current rise when a spot is formed. It induces high frequency (10–100 MHz) transients and ringing oscillations in probes and current circuits. Most probes behave like unmatched antennas for the electromagnetic pulses of spot discharges. Examples are shown to distinguish the source of oscillations and some rf characteristics of Langmuir probes.

Divertor ‘death-ray’ explained: An artifact of a Langmuir probe operating at negative bias in a high-recycling divertor

Abstract The divertor ‘death-ray’, enhanced plasma pressure near the outer strike-point relative to ‘upstream’ values, was thought to correspond to axisymmetric increased divertor heat flux. Recent measurements on Alcator C-Mod show that the ‘death-ray’ is localized to biased Langmuir probes. Heat fluxes deduced from plasma-sheath theory and surface thermocouples agree in sheath-limited and moderate-recycling regimes. They diverge in high-recycling and detached regimes; surface thermocouples measure reduced heat flux while a ‘death-ray’ appears on Langmuir probes. The ‘death-ray’ is caused by the probe’s negative bias affecting the local flux tube. With the bias, electron heat flux to the probe surface is reduced. Thus, the local electron temperature is raised, enhancing neutral ionization and increasing the ion flux to the probe. The plasma fluid code UEDGE is used to simulate and reproduce many of the features of this integrated biased probe/divertor system.

A comparative study of dc biased RF impedance probes and Langmuir probes for the measurement of processing plasma parameters

In this work, we compare measurements of electron density (ne), plasma potential (φp), electron temperature (Te), and electron energy distribution function (EEDF or f(E)) made with a dc biased RF probe and a Langmuir probe. The measurements show good agreement between the two probes across an order of magnitude in plasma density.

Nonlinear instability dynamics in a high-density, high-beta plasma

Entrainment and periodic pulling of an ion acoustic instability have been observed in the power spectra of a low-pressure high-beta plasma. The observed nonlinear phenomena can be modeled by using the van der Pol equation with a forcing term. Experimental results of the nonlinear processes are presented. Ion density fluctuations are detected on a negatively biased Langmuir probe for magnetic fields and input powers above 30 G and 900 W at 7.2 MHz respectively, and gas pressure below 1.5 mTorr. This low-frequency instability is observed in the central plasma blue core (argon II emission) and can be controlled by amplitude modulation of the radio frequency input power at frequencies close to the instability frequency.

Ion‐Neutral Collision Effects in Langmuir Probe Theory

AbstractIn weakly collisional sheaths, potential wells and barriers could appear. These can cause the presence of repulsed and trapped ions in the region surrounding a negatively biased Langmuir probe, invalidating the orbital motion limited (OML) theory. In this work, a Particle‐In‐Cell/Test‐Particle Monte Carlo (PIC/TPMC) model of Ar plasma at medium gas pressure has been developed in order to show the effect of ion‐neutral collisions. In fact, the results of the present simulation for different discharge pressures demonstrate the complex structure of electric potential around the probe and the failure of analytical theories. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Langmuir probe differential measurement technique in inductively coupled RF plasmas

A differential measurement technique has been proposed in order to reduce noise level and stray capacitance leakage usually affecting Langmuir probe data. The technique employs two identically designed and biased Langmuir probes, connected to an instrumentation amplifier. Both probes are immersed in plasma of approximately the same space potential, one of them being plasma current collecting probe, and the second one being isolated from plasma and serving as a pick-up probe, detecting leakage currents from parasitic capacitive coupling and noise. Avoiding averaging of probe current data is the main advantage of the proposed differential technique. Experiments in the plasma expansion region of inductively driven RF source are shown to achieve lower electron temperature and higher electron density as measured by conventional single Langmuir probe. Obtaining more sharpness of the "knee" on the characteristic, thus lowering the uncertainty in plasma potential is another true merit of the differential Langmuir probe technique.

Ion orbits in a cylindrical Langmuir probe

It has been suggested that in weakly collisional sheaths, potential wells and barriers could appear due to ion-neutral momentum and charge transfer collisions. These can cause the presence of repulsed and trapped ions in the region surrounding a negatively biased Langmuir probe, invalidating the commonly used orbital-motion-limited theory of ion current. This is still an open question concerning also the charging and shielding of dust grains, and at present, no fully self-consistent treatment exists. For this reason, a particle-in-cell/test-particle Monte Carlo simulation of the dynamics of an argon plasma in the region surrounding an attracting cylindrical probe at medium gas pressure has been developed. The results of the present simulation for different probe potentials and discharge pressures demonstrate the complex structure of electric potential around the probe and the failure of collisionless theories.

Ion sheath evolution and current characteristics for a trench immersed in a titanium cathodic arc discharge

Evolution of an ion sheath and the ion current generated by a pulse voltage application are discussed for a trench-shaped target immersed in a titanium cathodic arc discharge plasma with a current of 80 A DC at a nitrogen pressure of 8 Pa. The ion sheath evolution inside the trench with a width of 30 mm and a depth of 20 mm is experimentally observed. Considering a secondary electron emission coefficient, the observed fits the theoretical result by Lieberman. From an ion sheath evolution observed experimentally, the plasma density is estimated to be about 10 10 cm −3 using Lieberman's theory. The obtained plasma density fits well with a measured value. The ion sheath has a sharp edge at the initial stage, but the sharp edge gradually disappears due to merging of the sheath edge evolving outside the trench, which is confirmed by the current through positively biased Langmuir probe.

Disturbance of a Langmuir Probe at the Steady-State Sheath Boundary in a Drifting Plasma

The disturbance resulting from the presence of a positively biased Langmuir probe at an equilibrium steady-state ion sheath boundary in the drifting plasma is simulated by the two-dimensional particle-in-cell (PIC) method in rectangular coordinates. The positive potential of the probe was found to shield electrons on the side of the probe away from the sample surface from the negative potential of the sample surface and thereby cause an increase in the electron current drawn by the probe. The drifting ions on the same side of the probe are slowed by it and thereby raising the ion density in front of the probe. The electron currents of the probe are calculated at different probe positions from the sample surface. It is found that the sheath boundary as defined by the probe current is wider than the unperturbed boundary. The widening of the sheath boundary agrees well with experimental sheath boundary measurements.

Plasma measurements in a 100 W cylindrical Hall thruster

Conventional annular Hall thrusters become inefficient when scaled to low power. Their lifetime decreases significantly due to the channel wall erosion. Cylindrical Hall thrusters, which have lower surface-to-volume ratio and, thus, seem to be more promising for scaling down, exhibit performance comparable with conventional annular Hall thrusters of the similar size. Plasma potential, ion density, and electron temperature profiles were measured inside the 2.6 cm cylindrical Hall thruster with the use of stationary and slow movable emissive and biased Langmuir probes. Potential drop in the 2.6 cm cylindrical Hall thruster is localized mainly in the cylindrical part of the channel and in the plume, which suggests that the thruster should suffer lower erosion of the channel walls due to fast ion bombardment. Plasma density has a maximum of about (2.6–3.8)×1012 cm−3 at the thruster axis. At the discharge voltage of 300 V, the maximum electron temperature is about 21 eV, which is not enough to produce multiple ionization in the accelerated flux of Xe+ ions.

Dynamics of high-voltage pulsed cylindrical sheath

A semi-analytic model of collisionless sheath in plasma immersion ion implantation (PI/sup 3/) for a step bias with finite rise time in cylindrical geometry has been developed. The model takes into account the formation of the presheath, predicts the sheath overshooting and its further recovery to the steady-state Child value. To verify the model, a sheath dynamics has been investigated experimentally on a 30-kV PI/sup 3/ setup using the positively biased Langmuir probe technique. The measurements have been carried out in argon and helium plasmas of inductively coupled and hot cathode plasma sources. The model predictions agreed well with the experimental results.

Coherent structures in crossed-field closed-drift Hall discharges

The structure of a magnetized, collisionless crossed-field coaxial Hall discharge is investigated using low-impedance, negatively biased Langmuir probes. We present images depicting frequency-position renderings and cross-correlation plots, which identify coherent and random fluctuations in plasma density. These fluctuations are found to depend on the discharge operating conditions, and are believed to greatly affect cross-field electron transport.

Propagation of a disturbance created by a probe in electron collection

Experiments in the edge plasma of TEXT-U (the Texas Experimental Tokamak-Upgrade) [K. W. Gentle, Nucl. Technol. Fusion 1, 479 (1981)] have used a positively biased Langmuir probe in conjunction with two triple probes to examine the “wake” or disturbance in the plasma caused by the electron current collected by the driven probe. Spatial measurements of the disturbed volume have been made in the radial and poloidal directions at two points along the disturbed flux tube, at 12 meters and 28 meters away from the driven probe. Both non-time dependent and time-dependent experiments are used to determine the dispersion of the driven wave. A classical collisional two-fluid model of electron collection by a biased surface, which accounts for electric potential perturbations and the associated currents only, successfully predicts many of the qualitative features of the disturbance. The magnitudes of the collision rates, however, have to be increased beyond the best estimates in order to achieve quantitative agreement, indicating the possible involvement of anomalous cross-field current transport processes.