Local Plasma Potential Research Articles

Ion Energy Distributions in a Permanent-Magnet-Expanded Plasma Containing an Electric Double Layer

Ion energy distribution functions are measured along the axis in a solenoid-free plasma expanded by permanent magnets (PMs). The source diameter, the working argon gas pressure, and the 13.56-MHz radio-frequency power are chosen as 6.5 cm, 0.35 mtorr, and 250 W, respectively. The divergent magnetic field of about 100 G in the source tube and of about a few gauss in the diffusion chamber is provided by the PMs. The contour image of the measured ion energy distributions clearly shows the electrostatic ion acceleration in the region where the local plasma potential rapidly drops over a few centimeters. The result also indicates a loss of the beam ions and an increase of the bulk ions in the diffusion chamber due to the charge exchange process between the beam ions and argon neutrals.

The temporal evolution in plasma potential during laser photo-detachment used to diagnose electronegative plasma

A floating emissive probe is applied in conjunction with pulsed laser photo-detachment of O− ions to enable measurement of the dynamic evolution in a plasma potential resulting from the presence of photoelectrons in a 13.56 MHz inductive radio-frequency oxygen discharge. The emissive probe emits thermionic electrons, allowing it to reach a saturation potential which is characterized as the local space potential of the plasma. After the photo-detachment pulse, the local space plasma potential in the illuminated region shoots up to a higher positive value and then relaxes to equilibrium in microsecond time scales. Using the relaxation time of the space potential, the negative ion temperature of O− is estimated over a 10–50 mTorr range and is found to be in the 0.19–0.03 eV range. The negative ion temperature measured by this method is found to be lower than that calculated from the time evolution in electron density resulting from photo-detachment which is independently measured using a resonance hairpin probe.

Influence of H− velocity on H− extraction probability from a negative ion source

We investigate influence of H(-) initial transport direction and kinetic energy on H(-) extraction probability with three-dimensional Monte Carlo calculation. As a result, lower energy H(-) ions are strongly trapped by the electrical potential structure, so that initial condition of H(-) transport direction is cancelled by alignment of the electric field; thus, it has lower influence for H(-) extraction probability. Besides, the potential hill induced by the beam extraction voltage more effectively enhances H(-) extraction probability for the lower energy H(-) ions. The correlation between the magnitude of the local plasma potential near the extraction region and the mean velocity of H(-) ions in the region should determine the H(-) extraction probability from the ion source.

The SMART-1 Spacecraft Potential Investigations

The SMART-1 mission was successfully completed on September 3, 2006, with a controlled crash on the visible side of the Moon. The primary electric-propulsion (EP) system included a nonredundant SNECMA PPS-1350-G Hall effect thruster. The thruster discharge was electrically floating with respect to the spacecraft ground. The cathode floating potential ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">U</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CRP</sub> ) is an important thruster parameter, giving information on cathode performance and life expectancy. During ground testing and steady-state operation, <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">U</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CRP</sub> was approximately constant and negative. However, in space, it was observed that the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">U</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CRP</sub> was slightly positive and was periodically varying in time by a few volts. The capability to compare to ground-test data and check the cathode good health and life potential was largely impaired. Fortunately, SMART-1 included a plasma diagnostic package (electric-propulsion diagnostic package) with a Langmuir probe and a retarding-potential analyzer, both positioned on the same plane as the plasma thruster and provided data on the plasma potential as well as the energy of the charge-exchange ions. The short- and long-term correlations of these data with the cathode reference potential (CRP) were performed. It was concluded that the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">U</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CRP</sub> is evolving with respect to the plume potential near the cathode, and this potential difference is shown to be constant over the mission. Plume-potential distribution is adapting itself relative to the local plasma potential. The spacecraft potential with respect to the plasma potential is defined in steady state when the electron and ion currents entering and leaving the spacecraft are balanced. This balance depends primarily on the solar-array electrical configuration and orientation and thruster operating conditions. When the local plasma is chosen as a reference, the absolute cathode potential is found to be the same as the one on the ground. This paper confirms the good health of the cathodes. Contrary to a spacecraft without EP, the SMART-1 spacecraft potential varies within a limited range and is comparatively close to zero during electric-thruster operation. In addition, <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">U</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CRP</sub> can be used to derive the spacecraft potential throughout the mission. All the various cyclic and long-term effects influencing the spacecraft potential on SMART-1 have been identified and characterized across the mission lifetime. In particular, a strong correlation between the solar-array rotation and the variation of the spacecraft potential has been established.

Diagnosing pure-electron plasmas with internal particle flux probes

Techniques for measuring local plasma potential, density, and temperature of pure-electron plasmas using emissive and Langmuir probes are described. The plasma potential is measured as the least negative potential at which a hot tungsten filament emits electrons. Temperature is measured, as is commonly done in quasineutral plasmas, through the interpretation of a Langmuir probe current-voltage characteristic. Due to the lack of ion-saturation current, the density must also be measured through the interpretation of this characteristic thereby greatly complicating the measurement. Measurements are further complicated by low densities, low cross field transport rates, and large flows typical of pure-electron plasmas. This article describes the use of these techniques on pure-electron plasmas in the Columbia Non-neutral Torus (CNT) stellarator. Measured values for present baseline experimental parameters in CNT are phi(p)=-200+/-2 V, T(e)=4+/-1 eV, and n(e) on the order of 10(12) m(-3) in the interior.

Experimental Evidence of a Double Layer in a Large Volume Helicon Reactor

The self-consistently generated current-free electric double layer (DL) is shown to scale up with the source tube diameter and appears not to be affected by rf driving frequency and changes in reactor geometry. This Letter presents the first simultaneous measurements of local plasma potential and beam energy as a function of axial position. The DL is shown to be no more than 5 mm thick (20 D lengths) and positioned just downstream of the maximum in the magnetic field gradient. Furthermore, its position relative to the magnetic field is observed to be invariant as the magnetic field is translated axially. Measurements of the potential drop across the DL are presented for pressures down to 0.09 mTorr and the DL strength (phiDL/T(e)) is determined to be between 5 and 7.

Electron-beam-generated plasmas for materials processing

The results of investigations aimed at characterizing pulsed, electron-beam-produced plasmas for use in materials processing applications are discussed. In situ diagnostics of the bulk plasma and at the plasma/surface interface are reported for plasmas produced in Ar, N 2, and mixtures thereof. Langmuir probes were employed to determine the local electron temperature, plasma density, and plasma potential within the plasma, while ion energy analysis and mass spectrometry were used to interrogate the ion flux at an electrode located adjacent to the plasma. The results illustrate the unique capabilities of electron-beam-produced plasmas and the various parameters available to optimize operating conditions for applications such as nitriding, etching, and thin film deposition.

Laboratory evidence of a supersonic ion beam generated by a current-free “helicon” double-layer

An electric double-layer is generated near the open end of a high-density low pressure helicon sustained radio frequency (13.56 MHz) plasma source which expands into a diffusion chamber. Ion energy distribution functions measured with a retarding field energy analyzer placed in the diffusion chamber with its aperture facing the double-layer show the presence of a low energy peak (∼29 V) around the local plasma potential and a high energy peak (∼47 V) corresponding to a supersonic ion beam (∼2.1cs). At an axial distance 12 cm downstream of the double-layer, the beam density is 14% of the local density at that position and the ion energy gain is approximately 70% of the potential drop of the double-layer. The ion beam is observed from the center out to a radius corresponding to that of the plasma source tube (−6.8 cm⩽r⩽+6.8 cm) and is not greatly affected by the expanding magnetic field. A depression in the total ion flux just downstream of the double-layer—previously measured on the main z-axis of the reactor—is also present across the chamber diameter. Evidence of an electron beam near the closed end of the source tube, generated via “backwards” acceleration through the double-layer, has been observed on a Langmuir probe trace.

Floating Potentials in the Vicinity of Biased Flush-Mounted Probes

An array of surface-mounted probe triplets in the Alcator C-Mod divertor region [1, 2] demonstrates the influence of a biased probe tip on local plasma potential. For the experiments described here, each poloidally aligned triplet contains one probe which measures the local I-V characteristic with a bias sweep and two passive probes which sample the floating potential. The floating potential at the two passive tips varies as a function of the bias at the swept tip. This dependence is shown to vary according to the local plasma parameters. Variations in both the amplitude and width of the floating potential disturbance on electron temperature and density are resolved and compared with different mechanisms of cross-field current transport.

Increasing the space-charge limit and other effects of cesium seeding in hydrogen negative ion sources

The role of cesium seeding in increasing the negative ion current in volume sources is described. By a reduction in the local plasma potential the current of extracted electrons is vastly reduced. As a result, cesium increases the fraction of the transverse space-charge limit available to the ions by as much as a factor of 3. In addition, cesium can increase the total space-charge limit by injection of Cs+ into the presheath—a newly recognized phenomenon consistent with experimental measurements and determined from application of a Double–Vlasov model for negative ion extraction.

Measurement of Space Potential Fluctuations During MHD Activities in a Toroidal Helical Plasma with a Heavy Ion Beam Probe

A heavy ion beam probe (HIBP) is a unique diagnostic instrument which can measure local plasma potential and its fluctuations. However, in the presence of MHD activities, acceleration or deceleration of both primary and secondary beams are caused by the fluctuating vector potential ∂ A /∂ t along the beam trajectories (path integral effect). Such effect possibly induces a contamination to space potential measurement. We have evaluated those effects during a bursting MHD instability in the Compact Helical System (CHS), which is considered to be an m / n =2/1 interchange instability. In this procedure, the vector potential profile is estimated both theoretically and experimentally. Contribution from path effect is found to be less than several percent to the measured space potential fluctuation. Then space potential fluctuation is found to be localized around q =2 magnetic surface, which is consistent with a model for the interchange mode.

Measurements of broadband fluctuations and plasma potential with the 2 MeV heavy ion beam probe on the TEXT-U Tokamak (oral)

Broadband density fluctuations and plasma potential profiles are measured with a 2 MeV heavy ion beam probe (HIBP) diagnostic system on the TEXT-U tokamak. Measurements are performed in various locations including the edge and the core, as well as the high and low field side regions of the plasma. Results show a poloidal asymmetry in the core density fluctuation power spectra in which a distinct mode in the frequency range 70–170 kHz is present in the low field side region only. Furthermore, the HIBP provides a direct measurement of the local plasma potential from which the electric field profile across the plasma is inferred. Results showing the poloidal asymmetry as well as the potential and electric field profiles are presented.

Scrape-off layer broadening by the E × B convective cell induced by non-axisymmetric divertor biasing

The effective divertor heat-load relaxation using E × B induced convective cells in the SOL is studied. The E × B convective cells in the SOL are generated by the toroidally asymmetric divertor biasing, which can control the local plasma potential in the SOL. The preliminary experiment is done in the JFT-2M tokamak with poloidal divertor. The helical SOL current flows along the magnetic field between the locally biased inboard plate and grounded outer plates, thereby modifying plasma potential in the vicinity of the current flows. The poloidal distribution of the plasma potential in the SOL measured by Langmuir probe array shows that the localized potential structure is generated by the non-axisymmetric divertor biasing with the poloidal electric field reaching up to 1.5 kV/m is generated locally. In addition to the generation of a localized poloidal electric field, the modification of the heat flux profile on the divertor plate is also observed.

A method for measuring large changes in the payload voltage of rockets and satellites

We present a method for measuring large changes in the electrical potential of a spacecraft. It is known that a spacecraft in the ionosphere can obtain voltages as large as a few kilovolts. Spacecraft charging can occur naturally in the auroral regions due to high-energy streaming electrons or during the operation of active experiments, such as ion/electron beams or electromagnetic tethers. Charging and discharging events are often impulsive in nature and a method fast enough to track these potential changes could be extremely useful. For this purpose we have designed the Naval Research Laboratory Floating Probe (FP). The FP consists of a metallic sphere containing a high-impedance amplifier and a capacitive divider network for scaling large voltages to the range that typical solid state circuits can handle. Operation is similar to other floating probes; when placed in the surrounding ionosphere via a long deployable boom, the sphere quickly attains a voltage that approximates its local plasma potential. By measuring the difference between the sphere potential and the spacecraft-body potential it is possible, under certain circumstances, to determine the payload potential relative to the surrounding region. The probe can be used for either positive or negative polarity measurements, however, the ‘‘worst case’’ time response is associated with negative charging since the probe must collect ions to reach the local plasma potential. We tested the FP in a large vacuum chamber and followed this with a successful flight on a sounding rocket (SPEAR III). We will discuss the FP design, construction, theory of operation, and some problems and limitations encountered during the testing of the instrument.

Plasma potential measurements on text-upgrade with a 2 MeV heavy ion beam probe (abstract)a)

The plasma potential is measured in TEXT-upgrade tokamak by injection and detection of high energy heavy ions (thallium and cesium with a single charge) using a 2 MeV accelerator and a parallel plate energy analyzer. The change in beam energy, as it crosses the plasma, gives the local plasma potential at the measurement volume. Recent results of high energy beam operations are presented.

Cylindrical anode double layers ('firerods') produced in a uniform magnetic field

The effect of an external magnetic field on the evolution of cylindrical, luminous anode double layers ('firerods') from spherical 'fireballs' has been investigated in weakly ionized argon or xenon discharge plasmas. The 'fireballs' are initially produced from an additional discharge which forms when a disc electrode (3-5 cm diameter) is biased, relative to the local plasma potential, to a voltage larger than the ionization potential of the neutral gas. If a magnetic field of sufficient strength is applied in the direction normal to the disc, the fireball is transformed into an elongated magnetic field aligned luminous object which we refer to as the 'firerod'. The dependence of the firerod length on magnetic field strength and neutral gas pressure has been studied. The formation of the firerods requires that the ions be weakly magnetized; an upper limit on the magnetic field strength also exists, beyond which they cannot be maintained. The firerods have been analysed in terms of a simple model which attempts to account for the observed dependence of their length on magnetic field and neutral gas pressure. The results of this study are relevant to the so called 'plasma contactors' which are used to control the potential of spacecraft in low Earth orbit.

Ion currents to cylindrical Langmuir probes in RF plasmas

The positive ion current collected by cylindrical Langmuir probes has been measured in an RF argon plasma. The current-voltage characteristics were measured using an RF compensation technique whereby the probe was forced to follow the RF fluctuation in the local plasma potential. The I-V curves from probes of different radii and material show an ion current which is always greater than that predicted by the orbital motion theory and that agrees well with the radial motion theory. This is because the ions travel to the probe from a distance which is limited by the vessel geometry or by the collisional mean free path. The role of the possible collisions, ion-ion and ion-neutral, in the ion trajectories is analysed. It is suggested that the criterion for the application of the orbital motion theory should be tested whenever this theory is used for the determination of the charged particle density.

Ion emission to actively control the floating potential of a spacecraft

Charging of the outer surface of a spacecraft in orbit has occasionally a severe impact on the science return from the instruments. The ISEE and GEOS spacecraft have shown that in their orbits potentials of, respectively, up to +50 and ≥ +8 V are not uncommon and make it practically impossible to measure the very cold component of the ambient plasma. They also have interfering effects on the electirc field measurements. With active neutralization of the electrostatic surface charge more accurate measurements are feasible. We report on the development of an active emission system for future magnetosPheric missions. Emitting energetic particles (3–5 keV) and simultaneously measuring the actual surface potential by an electric field instrument we aim at clamping the spacecraft potential to 1–2 V above the local plasma potential for most of the expected plasma conditions. Negative potentials (eclipses) will not be controlled as the payload will be inoperative. Our method requires that all surfaces be conductive.

Pressure and plasma effects on the properties of magnetron sputtered carbon films

Thin, amorphous films of carbon have been deposited by conventional and hollow cathode enhanced magnetron sputtering of a pyrolytic graphite target. Films in the range of 15 to 200 nm were deposited over a pressure range or 50 to 0.1 mTorr on fused silica, sapphire, and Si substrates at 20 °C. Langmuir probe measurements of the local electron temperature, density, and plasma potential were made at several locations in the plasma as well as the substrate location as a function of pressure in both the conventional mode (50–1 mTorr) and the hollow cathode enhanced magnetron mode (1–0.1 mTorr). Several film properties were found to vary markedly over the pressure range investigated. For example, electrical resistivity was reduced from about 1×106 Ω cm at 50-mTorr sputtering pressure to a value of 0.01 Ω cm at 0.1-mTorr pressure. The optical band gap was reduced over the same pressure range from 2.1 to 0.4 eV. The index of refraction increased from n=2 at 50 mTorr to n=2.7 at 0.1 mTorr. The electron temperature of the plasma at the substrate location changed from a value of less than 1 eV at the higher pressure to over 8 eV at the lowest pressure, and the electron density increased 1 to 2 orders of magnitude in the same range. The results suggest that the local energy imparted to the growing film from the plasma environment can dramatically change the properties of the carbon films.

Long-term behaviour of photo-electron emission from the electric field double probe sensors on GEOS-2

A double probe instrument with a probe separation of 42 m was used aboard the geostationary spacecraft GEOS-2 to measure the ambient electric field. The probes contained built-in pre-amplifiers and were current biased in order to clamp their surface potential near to the local plasma potential while the spacecraft surfaces floated at a more positive potential. Depending on the plasma density, the potential difference ranges between 1 V (dense plasma, N e > 10 cm −3) and more than 10 V (tenuous plasma, N e < 3 cm −3). In the case of the more tenuous plasma, photoelectrons emitted from the probes tend to be picked up by the more positive potential of the wire booms. The net current of this electron flow depends on the spin orientation of the wire booms with respect to the Sun. It occurrs in the data as a spurious signal and was already earlier found to be a function of the spacecraft potential. With data from quiet magnetospheric conditions ( K p ⩽ 2) the long-term variation of the spurious signal was investigated. It turned out to be constant during the initial 3 y of the mission. Afterwards, as from 1981, it became smaller and did so for the following 2 y. In 1983, the end of the 5-y period of available data, it had vanished. The cause of the drop is suggested to be a consequence of the partial removal of the conductive surface layer of the wire booms by sputter effects. Expressions were derived that yield more accurate correction of the data. A valuable by-product of this work is the statistical relation of spacecraft potential and total electron flux.