Plasma Potential Measurements Research Articles

Recommended Practice for Use of Emissive Probes in Electric Propulsion Testing

This article provides recommended methods for building, operating, and taking plasma potential measurements from electron-emitting probes in electric propulsion devices, including Hall thrusters, gridded ion engines, and others. The two major techniques, the floating point technique and the inflection point technique, are described in detail as well as calibration and error-reduction methods. The major heating methods are described as well as the various considerations for emissive probe construction. Special considerations for electric propulsion plasmas are addressed, including high-energy densities, ion flows, magnetic fields, and potential fluctuations. Recommendations for probe design and operation are provided.

Improved inflection point method of emissive probe for accurate measurement of plasma potential

The potential of the inflection point of emissive probe characteristics (Vip) in the limit of zero emission is considered to be an accurate indication of the plasma potential. Previous method for this limit is linearly extrapolating the change of Vip with respect to the electron emission current Iemis to the limit Iemis≈0, which may result in inaccurate results since evidences showed that Vip does not change well linearly with Iemis. The authors found in this study that, instead of Iemis, Vip changes linearly with the probe heating current (Iht) which is a function of the probe temperature (Tp), and the phenomenon is reasonably interpreted by the space charge voltage increasing linearly with Tp. An improved inflection point method of emissive probe, which utilizes linear extrapolating the Vip−Iht relation to the heating current for initial emission, is proposed for accurate measurement of plasma potential.

Double capacitive probe for the measurement of plasma potential in tokamak

Capacitive probe with two pins that can measure floating potential of plasma in the tokamak is described. Probe is compared with a Langmuir probe. Small probes size and short connections allow to achieve better performance.

A comparative study of sheath potential profile measurements with laser-heated and current-heated emissive probes.

Emissive Langmuir probe is one of the most efficient diagnostic tools available for plasma potential measurements. Extensive studies have been carried out in designing different kinds of conventional (electrically heated) emissive probes (CEPs) to estimate the plasma potential. Laser heated emissive probe (LHEP) has been developed with certain advantages over the conventional probes such as low evaporation rate of the probe material, high lifetime, and high emission levels. Most importantly, the LHEP uses laser to heat the probe-tip and does not require electric current to heat the probe-tip like in CEP. The heating current in CEP substantially affects the plasma potential measurements, especially in the regions of plasma where high electric and magnetic field gradients are present. In this paper, we studied the plasma potential structures in sheath-presheath region using both LHEP and CEP in an unmagnetized dc-filament discharge plasma. Measurements of sheath spatial potential profile using laser heated emissive probe are compared with those obtained using conventional emissive probe.

A hybrid model for low pressure inductively coupled plasmas combining a fluid model for electrons with a plasma-potential-dependent energy distribution and a fluid-Monte Carlo model for ions

A hybrid plasma model is utilized for the simulation of inductively coupled plasmas (ICPs). It consists of a plasma fluid model coupling fluid with Maxwell’s equations and a Monte Carlo (MC) particle tracing model utilized for the calculation of the ion mobility in high electrostatic fields (sheaths). The model is applied to low pressure Argon plasma in the gaseous electronics conference (GEC) reference cell. Following measurements of electron energy distribution function (EEDF) in low pressure ICPs, a three-temperature EEDF is considered; it is formulated with a generalized equation and depends on the local plasma potential. The use of a predefined formula for the EEDF entails a low computational cost: All parameters affected by the EEDF are calculated as functions of the plasma potential and the mean electron energy once and before the solution of the model. The model results are validated by a comparison with spatially resolved (on axial and radial distance) measurements of electron density, electron temperature, and plasma potential. Both the calculation of the ion mobility by the MC model and the consideration of the three-temperature EEDF are critical for the accuracy of the model results. The very good agreement of the model results with the measurements and the low computational cost in combination with the flexibility of the code utilized for the numerical solution manifest the potential of the hybrid plasma model for the simulation of low pressure ICPs.

Possibility of plasma position detection by means of peripheral plasma potential measurements in tokamak

In the current paper a method of plasma position measurement, alternative to magnetic diagnostics, is proposed. The method is based on determination of the last closed flux surface (LCFS) location by detecting the maximum of electrostatic potential profile. The experiments were carried out on the TUMAN- 3M tokamak in a series of similar ohmic heating shots. The method proved effective in LCFS localization; it allowed calculation of corrections for magnetic diagnostics, used routinely for plasma position detection. This method can be used in limiter and divertor tokamaks for independent check and clarification of electromagnetic plasma position diagnostics.

Existence of a virtual cathode close to a strongly electron emissive wall in low density plasmas

The interaction between an electron emissive wall, electrically biased in a plasma, is revisited through a simple fluid model. We search for realistic conditions of the existence of a non-monotonic plasma potential profile with a virtual cathode as it is observed in several experiments. We mainly focus our attention on thermionic emission related to the operation of emissive probes for plasma diagnostics, although most conclusions also apply to other electron emission processes. An extended Bohm criterion is derived involving the ratio between the two different electron densities at the potential minimum and at the background plasma. The model allows a phase-diagram analysis, which confirms the existence of the non-monotonic potential profiles with a virtual cathode. This analysis shows that the formation of the potential well critically depends on the emitted electron current and on the velocity at the sheath edge of cold ions flowing from the bulk plasma. As a consequence, a threshold value of the governing parameter is required, in accordance to the physical nature of the electron emission process. The latter is a threshold wall temperature in the case of thermionic electrons. Experimental evidence supports our numerical calculations of this threshold temperature. Besides this, the potential well becomes deeper with increasing electron emission, retaining a fraction of the released current which limits the extent of the bulk plasma perturbation. This noninvasive property would explain the reliable measurements of plasma potential by using the floating potential method of emissive probes operating in the so-called strong emission regime.

Direct measurements of classical and enhanced gradient-aligned cross-field ion flows in a helicon plasma source using laser-induced fluorescence

Direct laser induced fluorescence measurements are shown of cross-field ion flows normal to an absorbing boundary that is aligned parallel to the axial magnetic field in a helicon plasma. We show Langmuir and emissive probe measurements of local density and plasma potential in the same region, as well as floating probe spectra near the boundary. With these measurements, we investigate the influence of ion-neutral collisionality on radial ion transport by varying the ratio of the ion gyro-radius, ρi, to the ion-neutral collision length, λ, over the range 0.34 ≤ ρiλ−1 ≤ 1.60. Classical drift-diffusion transport along density and potential gradients is sufficient to describe flow profiles for most cases. For two parameter regimes (ρiλ−1 = 0.65 and 0.44), low-frequency electrostatic fluctuations (f < 10 kHz) and enhanced cross-field bulk ion flow to the boundary are observed.

Low energy highly charged ion beam facility at Inter University Accelerator Centre: Measurement of the plasma potential and ion energy distributions

A deceleration lens coupled to one of the beam lines of the electron cyclotron resonance based low energy beam facility at Inter University Accelerator Centre is reported. This system is capable of delivering low energy (2.5 eV/q–1 keV/q) highly charged ion beams. The presence of plasma potential hinders the measurements of low energies (<50 eV), therefore, plasma potential measurements have been undertaken using a retarding plate analyzer in unison with the deceleration assembly. The distributions of the ion energies have been obtained and the effect of different source parameters on these distributions is studied.

Helical modulation of the electrostatic plasma potential due to edge magnetic islands induced by resonant magnetic perturbation fields at TEXTOR

The electrostatic response of the edge plasma to a magnetic island induced by resonant magnetic perturbations to the plasma edge of the circular limiter tokamak TEXTOR is analyzed. Measurements of plasma potential are interpreted by simulations with the Hamiltonian guiding center code Orbit. We find a strong correlation between the magnetic field topology and the poloidal modulation of the measured plasma potential. The ion and electron drifts yield a predominantly electron driven radial diffusion when approaching the island X-point while ion diffusivities are generally an order of magnitude smaller. This causes a strong radial electric field structure pointing outward from the island O-point. The good agreement found between measured and modeled plasma potential connected to the enhanced radial particle diffusivities supports that a magnetic island in the edge of a tokamak plasma can act as convective cell. We show in detail that the particular, non-ambipolar drifts of electrons and ions in a 3D magnetic topology account for these effects. An analytical model for the plasma potential is implemented in the code Orbit, and analyses of ion and electron radial diffusion show that both ion- and electron-dominated transport regimes can exist, which are known as ion and electron root solutions in stellarators. This finding and comparison with reversed field pinch studies and stellarator literature suggest that the role of magnetic islands as convective cells and hence as major radial particle transport drivers could be a generic mechanism in 3D plasma boundary layers.

Electron density dependence of impedance probe plasma potential measurements

In earlier works, we used spheres of various sizes as impedance probes in demonstrating a method of determining plasma potential, φp, when the probe radius is much larger than the Debye length, λD. The basis of the method in those works [Walker et al., Phys. Plasmas 13, 032108 (2006); ibid. 15, 123506 (2008); ibid. 17, 113503 (2010)] relies on applying a small amplitude signal of fixed frequency to a probe in a plasma and, through network analyzer-based measurements, determining the complex reflection coefficient, Γ, for varying probe bias, Vb. The frequency range of the applied signal is restricted to avoid sheath resonant effects and ion contributions such that ωpi ≪ ω ≪ ωpe, where ωpi is the ion plasma frequency and ωpe is the electron plasma frequency. For a given frequency and applied bias, both Re(Zac) and Im(Zac) are available from Γ. When Re(Zac) is plotted versus Vb, a minimum predicted by theory occurs at φp [Walker et al., Phys. Plasmas 17, 113503 (2010)]. In addition, Im(Zac) appears at, or very near, a maximum at φp. As ne decreases and the sheath expands, the minimum becomes harder to discern. The purpose of this work is to demonstrate that when using network analyzer-based measurements, Γ itself and Im(Zac) and their derivatives are useful as accompanying indicators to Re(Zac) in these difficult cases. We note the difficulties encountered by the most commonly used plasma diagnostic, the Langmuir probe. Spherical probe data is mainly used in this work, although we present limited data for a cylinder and a disk. To demonstrate the effect of lowered density as a function of probe geometry, we compare the cylinder and disk using only the indicator Re(Zac).

Measurement of Plasma Potential using Deceleration technique

SynopsisWe report a novel technique to measure the plasma potential. In this technique, we extract an ion beam into the chamber with few keV of energy and then decelerate it using series of electrodes till the deceleration potential nullifies the extraction potential and measure the retaining energy of the beam using retarding plate analyzer (RPA) to determine the plasma potential.

Extracted Current, Bias Voltage, and Ion Production of Cathodic Hollow-Cathode-Driven Plasma Contactors

Plasma properties on extracted electron current–bias voltage characteristics are presented for three selected plasma contactor configurations: a hollow-cathode-only concept, a motive discharge chamber concept, and a passive discharge chamber concept. Measurements were used to demonstrate how one could achieve low impedance performance without being affected by space plasma properties or by consuming significant propellant mass and power. A one-dimensional model was applied to describe the plasma expansion process that occurs downstream of a cathodic contactor. The model matched well with experimental trends and indicated that the plasma ion production rate within and nearby the plasma contactor dominated the emission-bias behavior of the devices. High ion production rate at a given total mass flow resulted in high propellant utilization and low discharge loss. However, plasma potential measurements showed that an anode sheath limited the maximum propellant unitization to less than and led to a foldback in discharge loss curves. The anode sheath and foldback resulted in poor agreement with zero-dimensional energy balance models that do not model this process. Finally, it was found that the passive discharge chamber concept resulted in the lowest energy cost and a larger clamping extracted current compared with the hollow-cathode-only concept.

Comparative measurements of plasma potential with ball-pen and Langmuir probe in low-temperature magnetized plasma

The ball-pen probe (BPP) is used for direct plasma potential measurements in magnetized plasma. The probe can adjust the ratio of the electron and ion saturation currents Isat−/Isat+ to be close to one and therefore its I-V characteristic becomes nearly symmetric. If this is achieved, the floating potential of the BPP is close to the plasma potential. Because of its rather simple construction, it offers an attractive probe for measurements in magnetized plasma. Comparative measurements of plasma potential by BPPs of different dimensions as well as one Langmuir probe (LP) in an argon discharge plasma of a cylindrical magnetron were performed at various experimental conditions. An additional comparison by an emissive probe was also performed. All these types of probes provide similar values of plasma potential in a wide range of plasma parameters. Our results for three different BPP dimensions indicate that the BPP can be operated in a cylindrical magnetron DC argon discharge if the value of the ratio of the magnetic field and neutral gas pressure, B/p, is greater than approximately 10 mT/Pa.

Profile measurements in the plasma edge of mega amp spherical tokamak using a ball pen probe.

The ball pen probe (BPP) technique is used successfully to make profile measurements of plasma potential, electron temperature, and radial electric field on the Mega Amp Spherical Tokamak. The potential profile measured by the BPP is shown to significantly differ from the floating potential both in polarity and profile shape. By combining the BPP potential and the floating potential, the electron temperature can be measured, which is compared with the Thomson scattering (TS) diagnostic. Excellent agreement between the two diagnostics is obtained when secondary electron emission is accounted for in the floating potential. From the BPP profile, an estimate of the radial electric field is extracted which is shown to be of the order ∼1 kV/m and increases with plasma current. Corrections to the BPP measurement, constrained by the TS comparison, introduce uncertainty into the ER measurements. The uncertainty is most significant in the electric field well inside the separatrix. The electric field is used to estimate toroidal and poloidal rotation velocities from E × B motion. This paper further demonstrates the ability of the ball pen probe to make valuable and important measurements in the boundary plasma of a tokamak.

Development of a High-Frequency Emissive Probe System for Plasma Potential Measurements in a Hall Thruster

A heated emissive probe is a smart diagnostic tool for acquisition of the plasma potential in the discharge and in the plume of a Hall thruster. However, measurements of high-frequency oscillations of the potential require a specific electrical circuit with a low <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$RC$ </tex-math></inline-formula> time constant. We developed and tested an emissive probe system with a cutoff frequency of 1 MHz. Details about the low-pass filter design as well as first experimental outcomes obtained in the plume of a 200 W permanent magnet Hall thruster are given in this paper.

Laser Heated Emissive Probes Design and Development under National Fusion Program and Potential Measurement

This work addresses the challenges in the design, construction and use of laser heated emissive probes (LHEPs). The choice of a suitable material for the probe plays an important role. We describe the basic theory of a laser heated emissive probe, along with an analysis of the materials suitable for the probe. Furthermore, we present a comparison of the plasma potential measurements carried out with a conventional filament emissive probe and an LHEP.

A set-up for a biased electrode experiment in ADITYA Tokamak

An experimental set-up to investigate the effect of a biased electrode introduced in the edge region on ADITYA tokamak discharges is presented. A specially designed double-bellow mechanical assembly is fabricated for controlling the electrode location as well as its exposed length inside the plasma. The cylindrical molybdenum electrode is powered by a capacitor-bank based pulsed power supply (PPS) using a semiconductor controlled rectifier (SCR) as a switch with forced commutation. A Langmuir probe array for radial profile measurements of plasma potential and density is fabricated and installed. Standard results of improvement of global confinement have been obtained using a biased electrode. In addition to that, in this paper we show for the first time that the same biasing system can be used to avoid disruptions through stabilisation of magnetohydrodynamic (MHD) modes. Real time disruption control experiments have also been carried out by triggering the bias-voltage on the electrode automatically when the Mirnov probe signal exceeds a preset threshold value using a uniquely designed electronic comparator circuit. Most of the results related to the improved confinement and disruption mitigation are obtained in case of the electrode tip being kept at ~3 cm inside the last closed flux surface (LCFS) with an exposed length of ~20 mm in typical discharges of ADITYA tokamak.

Test ion transport in a collisional, field-reversed configuration

Diffusion of test-ions in a flux-coil generated, collisional, field-reversed configuration is measured via time-resolved tomographic reconstruction of Ar+ optical emission in the predominantly nitrogen plasma. Azimuthal test ion diffusion across magnetic field lines is found to be classical during the stable period of the discharge. Test ion radial confinement is enhanced by a radial electric field, reducing the observed outward radial transport rate below predictions based solely on classical cross-field diffusion rates. Test ion diffusion is ∼500 m2 s−1 during the stable period of the discharge. The electric field inferred from plasma potential measurements and from equilibrium calculations is consistent with the observed reduction in argon transport.

Direct Plasma Potential Measurements by Ball‐Pen Probe and Self‐Emitting Langmuir Probe on COMPASS and ASDEX Upgrade

AbstractExperimental investigations of the plasma potential by means of a ball‐pen probe and self‐emitting Langmuir probe technique were performed on the tokamaks COMPASS and ASDEX Upgrade. A spontaneous transition between the cold and self‐emitting Langmuir probe has been observed during the deep reciprocation in the vicinity or even inside the last closed flux surface (LCFS) of both tokamaks. These measurements of the plasma potential allow us to compare two independent probe techniques based on heating and non‐heating processes. The observed different conditions of the spontaneous transition can be ascribed to different physical mechanisms. Nevertheless, the comparison of the power spectra of ball‐pen probe and self‐emitting Langmuir probe indicates very good agreement in all measurements and therefore these two probe techniques are interchangeable. (© 2014 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)