Ion Saturation Current Research Articles

Plume Characteristics of a Quasi-Steady Magnetoplasmadynamic Arcjet

The plasma plume of a 1-MW-class quasi-steady magnetoplasmadynamic (MPD) arcjet is studied to determine the plume structure and plasma plume fluctuations in a downstream plume region (250-1250 mm away from the MPD arcjet). By using a double probe and a high-electron-temperature ( ~ 5 eV) and high-number-density (~8 ×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">19</sup> m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> ) region, so-called “cathode jets” are found along the central axis of plasma plume close to the MPD arcjet. Moreover, the plasma plume radial profile is constant in a downstream plume region (≥ 750 mm from the MPD arcjet). The power spectrum density (PSD) of the ion saturation current is proportional to 1/f <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1.6</sup> (f: frequency) for f >; 80 kHz, and in this frequency range, PSD decreased with distance from the MPD arcjet. In contrast, in the frequency region <; 80 kHz, the PSD-f curve is at a constant value. Although a peak in the discharge voltage is attributed to the generalized lower hybrid drift instability (GLHDI), this instability is not found in the plasma plume near the MPD arcjet. The influence of the GLHDI is limited only to the discharge chamber, and fluctuations caused by the instability are random in the downstream region.

Revisiting plasma hysteresis with an electronically compensated Langmuir probe

The measurement of electron temperature in plasma by Langmuir probes, using ramped bias voltage, is seriously affected by the capacitive current of capacitance of the cable between the probe tip and data acquisition system. In earlier works a dummy cable was used to balance the capacitive currents. Under these conditions, the measured capacitive current was kept less than a few mA. Such probes are suitable for measurements in plasma where measured ion saturation current is of the order of hundreds of mA. This paper reports that controlled balancing of capacitive current can be minimized to less than 20 μA, allowing plasma measurements to be done with ion saturation current of the order of hundreds of μA. The electron temperature measurement made by using probe compensation technique becomes independent of sweep frequency. A correction of ≤45% is observed in measured electron temperature values when compared with uncompensated probe. This also enhances accuracy in the measurement of fluctuation in electron temperature as δT(pk-pk) changes by ~30%. The developed technique with swept rate ≤100 kHz is found accurate enough to measure both the electron temperature and its fluctuating counterpart. This shows its usefulness in measuring accurately the temperature fluctuations because of electron temperature gradient in large volume plasma device plasma with frequency ordering ≤50 kHz.

Electron Temperature Measurement in a Premixed Flat Flame Using the Double Probe Method

AbstractThe electron temperatures Te were measured using a double probe in a premixed methane flame produced by a calibration burner according to Hartung et al. The experiment was performed at atmospheric pressure. In contrast to other authors, we have managed to find typical nonlinearities corresponding to the retarding electron current region and to calculate electron temperatures using a suitable fit on the basis of the measured characteristics. A Pt‐Rh thermocouple was used to measure temperatures Th corresponding to “heavy” species. Our results indicate that the flame plasma can be considered to be weakly non‐isothermic — Te = (2400–4000) K, Th = (1400–1600) K. On the basis of measurement of the saturated ion current, the number density of the charged particles was estimated at (0.3–3.8) · 1017 m‐3. The trends in Te and Th in dependence on the positions of the probes and thermocouple in the flame differ substantially; this fact has not yet been explained (© 2012 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Radiofrequency antenna for suppression of parasitic discharges in a helicon plasma thruster experiment

A radiofrequency (rf) antenna for helicon plasma thruster experiments is developed and tested using a permanent magnets helicon plasma source immersed in a vacuum chamber. A magnetic nozzle is provided by permanent magnets arrays and an argon plasma is produced by a 13.56 MHz radiofrequency helicon-wave or inductively-coupled discharge. A parasitic discharge outside the source tube is successfully suppressed by covering the rf antenna with a ceramic ring and a grounded shield; a decrease in the ion saturation current of a Langmuir probe located outside the source tube is observed and the ion saturation current on axis increases simultaneously, compared with the case of a standard uncovered rf antenna. It is also demonstrated that the covered antenna can yield stable operation of the source.

Modeling of electron temperature and DC bias voltage in an inductively-coupled Cl2/Ar plasma using neural network

We proposed a novel neural network (NN) model for the inductively-coupled Cl2/Ar plasma (ICP). Plasma experiments were performed in a planar inductively coupled plasma reactor. The plasma parameters and composition were determined by a combination of plasma diagnostics carried out with a double Langmuir probe (Plasmart DLP2000). The input parameters of the plasma model were gas mixing ratio, source power, and bias power, which were varied within the ranges of 0–100% Cl2/Ar, 500–800W, and 50–300W, respectively. The voltage–current curves were treated to obtain the electron temperature (Te), DC bias voltage, and ion saturation current (Ji) by using the software supplied by the equipment manufacturer. For the outputs and the input parameters, we developed a back propagation neural network (BPNN) model and extracted its features. Then, we designed a pre-processor using the features to optimize the BPNN model. The experimental results showed that the proposed method was very effective for the optimization of the BPNN model with respect to model precision and learning speed.

Turbulent transport in a toroidal magnetized plasma

Turbulent plasma transport due to low-frequency electrostatic fluctuations in a toroidal plasma is studied experimentally. The data are obtained in a magnetized toroidal plasma with no toroidal transform. The plasma is generated by a discharge from a hot electron emitting filament and diagnosed by conventional Langmuir probes measuring densities by electron or ion saturation currents and floating potentials. We present results for the statistical properties of the fluctuating radial transport caused by low-frequency electrostatic turbulence in the device. The turbulent plasma flux is identified as the product of the fluctuating density and the E × B/B2-velocity. Even though the probability densities of the fluctuating electric fields and plasma densities are close to Gaussians, we find strongly intermittent features in the flux signal obtained as the product of these two fluctuating quantities. A conditional statistical analysis gives insight in detail of the turbulent transport. The intermittency studies are extended by analyzing the excess statistics, i.e. the average duration of time intervals in the flux signal spent above a given reference level. We find that this analysis offers a very effective measure for intermittency effects. In our case, the signal is characterized by an excess of temporally narrow, large amplitude bursts, when compared with an equivalent Gaussian random signal.

Design and Manufacturing of the Electrode Biasing System for the Tokamak

An electrode biasing system was designed, constructed, and installed on the IR-T1 tokamak, and then, the positive voltage applied to an electrode inserted inside the tokamak limiter and the plasma current, poloidal and radial components of the magnetic fields, loop voltage, diamagnetic flux, and ion saturation currents in the absence and presence of the biased electrode were measured. Results of measurements of biasing effects on the plasma equilibrium behavior and edge plasma rotation are compared and discussed.

High frequency operation of a hot filament cathode for a magnetized plasma ion source

A tungsten filament cathode has been operated with an ac heating current to excite a plasma in a linear magnetic field. Both the discharge current and the ion saturation current in plasma near the extraction hole of the ion source exhibited fluctuations. The discharge current fluctuated with the amplitude less than 2% of the average, and the frequency two times the frequency of the heating current. Fluctuation amplitude of the ion saturation current was about 10% of the average, while the frequency was the same as that of the heating current. The ac operation has prolonged the lifetime of a hot filament cathode by about 50%.

A close-coupling multi-antenna type radio frequency driven ion source

A newly close coupling multi-antenna type radio frequency driven ion source is tested for the purpose of essentially improving plasma coupling on the basis of our old type ion source, which reuses a NNBI (negative ion source for neutral beam injection) ion source used in 1∕5th scale of the Large Helical Device NNBI. The ion source and the antenna structure are described, and the efficient plasma production in terms of the positive ion saturation current (the current density) is studied. The source is made of a metal-walled plasma chamber which is desirable from the point of view of the structural toughness for fusion and industrial application, etc. At around 160 kW of rf input power, the ion saturation current density successfully reaches the 5 A∕cm(2) level with a gas pressure of 0.6-2 Pa in hydrogen for 10 ms pulse duration. The rf power efficiency of the plasma production with a close coupling configuration of the antenna is improved substantially compared to that with the previous antenna unit in the old type ion source. The power efficiency is assessed as competing with that of other types of sources.

Shunting arc plasma source for pure carbon ion beam

A plasma source is developed using a coaxial shunting arc plasma gun to extract a pure carbon ion beam. The pure carbon ion beam is a new type of deposition system for diamond and other carbon materials. Our plasma device generates pure carbon plasma from solid-state carbon material without using a hydrocarbon gas such as methane gas, and the plasma does not contain any hydrogen. The ion saturation current of the discharge measured by a double probe is about 0.2 mA∕mm(2) at the peak of the pulse.

Characteristic of a triple-cathode vacuum arc plasma source

In order to generate a better ion beam, a triple-cathode vacuum arc plasma source has been developed. Three plasma generators in the vacuum arc plasma source are equally located on a circle. Each generator initiated by means of a high-voltage breakdown between the cathode and the anode could be operated separately or simultaneously. The arc plasma expands from the cathode spot region in vacuum. In order to study the behaviors of expanding plasma plume generated in the vacuum arc plasma source, a Langmuir probe array is employed to measure the saturated ion current of the vacuum arc plasma source. The time-dependence profiles of the saturated current density of the triple vacuum arc plasma source operated separately and simultaneously are given. Furthermore, the plasma characteristic of this vacuum arc plasma source is also presented in the paper.

Blob/Hole Generation in the Divertor Leg of the Large Helical Device

We have analyzed ion saturation current fluctuation measured by a fast scanning Langmuir probe (FSP) in edge region of the Large Helical Device (LHD). Positive and negative spikes of the ion saturation current were observed in the private region and on the divertor leg, respectively. It was found that the boundary position between these regions corresponds to the low-field side (LFS) edge of the divertor leg where the gradient of the ion saturation current profile was the maximum. Such a positional relationship resembles that near the separatrix in the LFS in tokamaks, where blobs and holes are generated. Statistical analysis indicates similar fluctuation characteristics among different magnetic devices.

Analysis of the temperature influence on Langmuir probe measurements on the basis of gyrofluid simulations

The influence of the temperature and its fluctuations on the ion saturation current and the floating potential, which are typical quantities measured by Langmuir probes in the turbulent edge region of fusion plasmas, is analysed by global nonlinear gyrofluid simulations for two exemplary parameter regimes. The numerical simulation facilitates a direct access to densities, temperatures and the plasma potential at different radial positions around the separatrix. This allows a comparison between raw data and the calculated ion saturation current and floating potential within the simulation. Calculations of the fluctuation-induced radial particle flux and its statistical properties reveal significant differences to the actual values at all radial positions of the simulation domain, if the floating potential and the temperature-averaged density inferred from the ion saturation current is used.

Application of electrostatic Langmuir probe to atmospheric arc plasmas producing nanostructures

The temporal evolution of a high pressure He arc producing nanotubes was considered and the Langmuir probe technique was applied for plasma parameter measurements. Two modes of arc were observed: cathodic arc where discharge is supported by erosion of cathode material and anodic arc which is supported by ablation of the anode packed with carbon and metallic catalysts in which carbon nanotubes are synthesized. Voltage-current (V-I) characteristics of single probes were measured and unusually low ratio of saturation current on positively biased probe to that on negatively biased of about 1–4 was observed. This effect was explained by increase of measured current at the negatively biased probe above the level of ion saturation current due to secondary electron emission from the probe surface. Since utilization of standard collisionless approach to determine plasma parameters from the measured V-I characteristic is not correct, the electron saturation current was used to estimate the plasma density.

Experimental Study of the Laser Ablation Plasma Flow From the Liquid Ga–In Target

The use of recoil momentum from a laser ablation flare is found to be one of the promising ways for development of low-propulsion thrusters for launching of microsatellites and nanosatellites. This paper suggests using a liquid Ga-In alloy as the ablated body, instead of a solid-state one. A Nd:YAG laser with the output of 0.3 mJ and the pulse duration of 2.7 ns (full width at half maximum) was used for ablation. Charge-resolved ion energy distributions of the laser-ablation plasma were studied by means of an energy-mass analyzer EQP Hiden Analytical. Angular characteristics of plasma flows were retrieved from a measurement of ion saturation currents on a set of collectors placed at different axial and azimuthal angles relative to the target surface normal. It was found that the thrust from the liquid-metal target remains very stable during a long operation time, while the lifetime of the target is limited only by the working body supply.

Combinatorial analyses of plasma–polymer interactions

Plasma-process analyses were demonstrated on the basis of combinatorial method, in which process examinations with continuous variations of plasma-process conditions can be carried out on a substrate holder with a graded distribution of process parameters. Controllability of density gradation in the combinatorial plasma process analyzer together with combinatorial analyses of etching characteristics and surface chemical bonding states of polyethylene terephthalate (PET) films exposed to argon–oxygen mixture plasmas. The etching properties and chemical bonding states (phenyl group) have been investigated for the PET films exposed to argon–oxygen mixture plasmas. The etching-depth data obtained from three independent batches of the experiments showed universal and almost linear dependence with increasing product of (ion saturation current)×(exposure time), which is in proportion to ion dose. The chemical bonding state analyses of phenyl group showed insignificant degradation with increasing ion dose.

Electrical Characterization of Inductively Coupled Plasma Reactor Excited by RF (13.56MHz)

In the surface treatment by plasma, the saturation ion current is the macroscopic parameter which can give us information on the ionic density in plasma. For this, we followed the evolution of the saturation ion current versus the pressure and power. Our experimental setup consists of spherical plasma reactor linked with inductively source argon plasma and probe to measure ionic current. The obtained experimental results enabled us to highlight the effect of pressure and power on saturation ion current. We found in this study that the saturation ion current increases with pressure and presents an optimum depending on the power.

Electrostatic wave heating and possible formation of self-generated high electric fields in a magnetized plasma

A plasma reactor operates at the Laboratori Nazionali del Sud of INFN, Catania, and it has been used as a test-bench for the investigation of innovative mechanisms of plasma ignition based on electrostatic waves (ES-W), obtained via the inner plasma EM-to-ES wave conversion. Evidences of Bernstein wave (BW) generation will be shown. The Langmuir probe measurements have revealed a strong increase of the ion saturation current, where the BW are generated or absorbed, this being a signature of possible high energy ion flows. The results are interpreted through the Bernstein wave heating theory, which predicts the formation of high speed rotating layers of the plasma (a dense plasma ring is in fact observed). High intensity inner plasma self-generated electric fields (on the order of several tens of kV/cm) come out by our calculations.

DOUBLE AND TRIPLE LANGMUIR PROBES MEASUREMENTS IN INDUCTIVELY COUPLED NITROGEN PLASMA

The double and triple Langmuir probe diagnostic systems with their necessary driving circuits are developed successfully for the characterization of laboratory built low pressure inductively coupled nitrogen plasma, generated by 13.56MHz radio frequency (RF) power supply along with an automatic impedance matching network. Using the DC properties of these two probes, the discharge plasma parameters like ion saturation current (Iio), electron temperature (kTe) and electron number density (ne) are measured at the input RF power ranging from 250 to 400W and fllling gas pressures ranging from 0.3 to 0.6mbar. An increasing trend is observed in kTe and ne with the increase of input RF power at a flxed fllling gas pressure of 0.3mbar, while a decreasing trend is observed in kTe and ne with the increase of fllling gas pressure at a flxed input RF power of 250W.

準定常MPDアークジェットプルームの非定常特性

The plasma plume of a 1-MW-class quasi-steady magnetoplasmadynamic (MPD) arcjet is studied to reveal the plume plasma fluctuations in a downstream plume region (150–900mm away from the MPD arcjet). By using a double probe and a time-of-flight method, the averaged velocity distribution along the centerline of the plasma plume is found to be almost constant (∼38km/s), and the average ion saturation current is inversely proportional to the square of the distance from the MPD arcjet. The power spectrum density (PSD) of the ion saturation current is proportional to 1/f 1.6 (f : frequency) in 50–200kHz range and it is proportional to 1/f 2.6 above 200kHz. However, fluctuations are dominant in the lower frequency region of 50kHz, where PSD vs. f curve is flat. The spectrum analysis of the fluctuations shows that the MPD arcjet plume in the downstream region is isotropic turbulent flow.