Showing papers in "Review of Scientific Instruments in 1990"

Thermal conductivity measurement from 30 to 750 K: the 3ω method

Abstract: An ac technique for measuring the thermal conductivity of dielectric solids between 30 and 750 K is described This technique, the 3ω method, can be applied to bulk amorphous solids and crystals as well as amorphous films tens of microns thick Errors from black‐body radiation are calculated to be less than 2% even at 1000 K Data for a‐SiO2, Pyrex 7740, and Pyroceram 9606 are compared to results obtained by conventional techniques

Automatic rotating element ellipsometers: Calibration, operation, and real‐time applications

Abstract: In a typical reflection ellipsometry experiment, one characterizes the polarization state change that a polarized light beam undergoes upon reflection from a specular surface. This measurement provides ρ≡rp/rs, where rp and rs are the complex amplitude reflection coefficients of the surface for p‐ and s‐polarized waves. Nearly 15 years have passed since the development of automatic ellipsometers along with the detailed calibration, error analysis, and data reduction procedures to be used with them. More specifically, these powerful instruments permit (1) determination of bulk dielectric functions and nondestructive depth profiling of static multilayered materials through measurements as a function of photon energy and (2) characterization of dynamic surfaces in adverse environments through measurements as a function of time at fixed photon energy. In the 15 intervening years, the major research thrusts in ellipsometry have been the exploitation of these instruments in materials and process characterizatio...

Luminescent barometry in wind tunnels

Abstract: A flexible and relatively inexpensive method and apparatus are described for continuous pressure mapping of aerodynamic surfaces using photoluminescence and imaging techniques. Platinum octaethylporphyrin (PtOEP) has a phosphorescence known to be quenched by oxygen. When dissolved in a silicone matrix, PtOEP may be distributed over a surface as a thin, uniform film. When the film is irradiated with ultraviolet light, the luminescence intensity provides a readily detectable, qualitative surface flow visualization. Moreover, since the luminescence intensity is found to be inversely proportional to the partial pressure of oxygen, a quantitative measure of pressure change may be obtained using a silicon target vidicon or a charge-coupled device video sensor to measure intensity. Luminescent images are captured by a commercial frame buffer board. Images taken in wind tunnels during airflow are ratioed to images taken under ambient 'wind-off' conditions. The resulting intensity ratio information is converted to pressure using calibration curves of I0/I vs p/p0, where I0 is the intensity at ambient pressure p0 and I is the intensity at any other pressure p.

Scanning electron microscopy with polarization analysis (SEMPA)

Abstract: The high spatial resolution imaging of magnetic microstructure has important ramifications for both fundamental studies of magnetism and the technology surrounding the magnetic recording industry. One technique for imaging surface magnetic microstructure on the 10‐nm‐length scale is scanning electron microscopy with polarizationanalysis (SEMPA). This technique employs a scanning electron microscope(SEM) electron optical column to form a medium energy (10–50 keV), small probe ( 1 nA) on a ferromagnetic specimen. Secondary electrons excited in the ferromagnet by the high spatial resolution probe retain their spin‐polarization orientation as they leave the sample surface. The spin polarization of the emitted secondary electrons can be related directly to the local magnetization orientation. A surfacemagnetization map is generated when the spin polarization of the secondary electrons is analyzed as the electron beam is rastered point‐by‐point across the ferromagnet’s surface. In this review article we review the important instrumental components characterizing the SEMPA system. Characteristics of the electron probe forming optics, electron spin‐polarization analyzers with associated transport optics, and signal processing electronics will be described. Emphasis on the fundamental design requirements will be stressed. Data acquisition, storage, and processing, as it applies specifically to SEMPA, will be reviewed. Instrumental artifacts specific to SEMPA will be outlined and techniques for their correction given. Examples of magnetic images at high spatial resolution will be shown.

Atmospheric pollution monitoring using CO2‐laser photoacoustic spectroscopy and other techniques

Abstract: The subject of air pollution monitoring is reviewed. First, we discuss the main conventional techniques currently applied to the detection of gaseous air pollutants, as well as the state of the art of novel detection schemes. We distinguish between nonspectroscopic and spectroscopic methods. Spectroscopic techniques are of primary interest since they offer several advantages, e.g., the simultaneous monitoring of numerous substances. Photoacoustic (PA) spectroscopy represents a promising spectroscopic technique due to its intrinsically high sensitivity, the large dynamic range, and the comparatively simple experimental arrangement. Emphasis is put on detection selectivity which often may restrict the range of applications of the technique for pollution monitoring due to the lack of powerful, continuously tunable IR laser sources. The theoretical aspects of PA spectroscopy with respect to trace gas detection and multicomponent analysis are thus discussed in detail. Some characteristics of a portable PA system built by us are outlined in order to illustrate the usefulness of such systems.

Direct injection supersonic cluster beam source for FT‐ICR studies of clusters

Abstract: A miniaturized pulsed supersonic beam source has been developed using laser vaporization of a computer‐controlled target disk, producing intense beams of cluster ions with excellent repeatability and control. Due to its small size and narrow pulse width, the entire source is adequately pumped by a single 170 l /s turbopump. The resultant vacuum quality permits this source to be attached to a Fourier transform ion cyclotron resonance apparatus (FT‐ICR) such that the supersonic cluster ion beam is directly injected. The result is a powerful but simple FT‐ICR instrument of wide applicability. The new source is suited as well for a variety of other uses such as molecular beam epitaxy.

A parallel plate resonator technique for microwave loss measurements on superconductors

Abstract: A simple method for the evaluation of microwave surface loss of superconductive samples is discussed. The method allows the evaluation of small (∼1 cm×∼1 cm), flat samples over a broad range of temperatures. It can accurately characterize samples with surface resistances as low as 5 μΩ or as high as 1 mΩ at 10 GHz. In addition to high resolution, it has several advantages: sample preparation requirements are minimal; the current distribution within the sample under test is relatively uniform and can be accurately calculated; no other superconducting or normal conducting material is required for the resonator, so no additional corrections for such materials are needed; and the procedure is comparatively rapid and can be performed in the necktube of a liquid‐helium storage Dewar.

A 10‐GHz frequency‐domain fluorometer

Abstract: We describe the design and performance of a 10‐GHz harmonic‐content frequency‐domain fluorometer. The modulated excitation is provided by the harmonic content of a train of ps pulses. High‐speed and/or high‐frequency detection was attained with a triode‐type microchannel plate photomultiplier tube (MCP PMT) from Hamamatsu, R‐2566‐6, with 6 μm channels. To minimize the cost of the electronic components, and to minimize the noise due to these components, the detection circuits consists of two frequency ranges, 10 MHz–2 GHz and 2–10 GHz. The upper frequency limit of 10 GHz is determined by the current MCP PMT, so the usual configuration includes a low‐noise 2–10‐GHz amplifier. This amplifier is easily replaced with a 2–18‐GHz amplifier to allow operation to 18 GHz and the use of faster PMTs, should they become available in the future. Measurement of known optical delays demonstrates the accuracy of the instrument. For instance, a 1.69 ps optical delay was measured as 1.7±0.4 ps from 0.5 to 10 GHz, and 1.7±0.2 ps from 2 to 8 GHz, where the uncertainty indicates the maximum deviation from the expected value. The data were shown to be free of systematic errors by measurements on fluorophores with single exponential decays, with decay times ranging from 61 ps to 1.24 ns. Measurement of anisotropy decays with correlation times of 24 ps are shown and it is predicted that correlation times as short as 1 ps could be measured with this instrument. And finally, the sensitivity of the instrumentation was demonstrated by measurements of the very weak intrinsic tryptophan emission of deoxyhemoglobin, which displays decay times ranging from 2 to 820 ps.

Improved pulsed laser vaporization source for production of intense beams of neutral and ionized clusters

Abstract: We describe an improved laser vaporization source for production of intense and stable cluster beams. Intensity and stability are notably improved compared with standard sources by the introduction of a cavity in which the vaporization takes place before expansion. This source has been successfully used for electric deflection studies of highly collimated cluster beams.

SIMION PC/PS2 electrostatic lens design program

Abstract: SIMION PC/PS2 4.02 is a personal computer program for designing and analyzing charged particle (ions and electrons) lenses, ion transport systems, and various types of mass spectrometers and surface probes that utilize charged particles. The modification of an existing design or the generation of a completely new one is performed interactively with a graphics screen and mouse. Once the geometry has been defined, the operating conditions (electrode voltages and magnetic field configuration) can be quickly changed and the resultant fields viewed in several different 2D and 3D modes. The trajectories of charged particles moving in these fields are calculated utilizing sophisticated ‘‘look‐ahead’’ algorithms that dynamically control the time step to optimize speed and accuracy. A unique graphics display of the electrostatic fields and ion trajectories gives the user an intuitive, easily understood view of the performance characteristics. simion also includes a totally integrated capability for users to easily...

Force microscopy with a bidirectional capacitance sensor

Abstract: A new method for sensing cantilever deflection in the atomic force microscope (AFM), based on capacitance measurement, is described. Parameters governing the design of such an instrument are considered in detail. Two different geometries are compared, wire on plate and an integrated flat plate sensor. The electronic circuitry, providing 6×10−19 F noise in a 0.01–1000 Hz bandwidth, is also described. Implementation of the design ideas into a working AFM in ultrahigh vacuum is demonstrated. This AFM allows simultaneous measurement of cantilever deflection in two orthogonal directions, necessary for our nanotribology studies. The theoretical sensitivity of 5×10−7 F/m is not achieved due to roughness. The bidirectional sensing and imaging capabilities are demonstrated for an Ir tip on cleaved graphite, and a diamond tip on diamond films. The capacitance detection technique is compared and contrasted with other AFM sensors.

Plasma fluctuation measurements in tokamaks using beam‐plasma interactions

Abstract: High‐frequency observations of light emitted from the interactions between plasma ions and injected neutral beam atoms allow the measurement of moderate‐wavelength fluctuations in plasma and impurity ion densities. To detect turbulence in the local plasma ion density, the collisionally excited fluorescence from a neutral beam is measured either separately at several spatial points or with a multichannel imaging detector. Similarly, the role of impurity ion density fluctuations is measured using charge exchange recombination excited transitions emitted by the ion species of interest. This technique can access the relatively unexplored region of long‐wavelength plasma turbulence with k⊥ρi≪1, and hence complements measurements from scattering experiments. Optimization of neutral beam geometry and optical sightlines can result in very good localization and resolution (Δx≤1 cm) in the hot plasma core region. The detectable fluctuation level is determined by photon statistics, atomic excitation processes, and b...

A vertical piezoelectric inertial slider

Abstract: We have developed a linear translation device using piezoelectric‐induced slip‐stick motion. Reproducible single steps of about 30 A, as well as continuous stepping with an overall translation speed of 0.25 mm/s, are routinely realized. The notable feature of this device is that this performance is achieved in the vertical orientation with the translator moving against gravity. This remarkable result is made possible using cycloidal functions instead of sawtooth signals to activate the motion. We have realized a very simple translator which can be used in any orientation with a displacement onset voltage of 15 V. The instrument was successfully tested in the temperature range from 1.6 to 300 K. Since no mechanical connections are required, this design is well suited for many applications, including scanning tunneling microscopy.

Miniaturization techniques for obtaining static pressures comparable to the pressure at the center of the earth: X-ray diffraction at 416 GPa

Abstract: X‐ray diffraction studies on tungsten and molybdenum were performed, using a 4‐μm‐diam x‐ray beam, to very high pressures, with the pressures being obtained from the measured lattice parameters and isothermal equations of state of tungsten and molybdenum deduced from shock data. The bcc structure persists to the highest pressure, 378 GPa in tungsten, and 416 GPa in molybdenum. The static pressures generated and measured here exceed the pressure of 361 GPa at the center of the earth, the first time that this has been achieved and measured with a calibrated pressure scale. The details of the pressure profile at 335 GPa are shown and are of great use in designing anvils for future research. It is noted that the maximum pressures attained by x‐ray diffraction with beveled anvils varies linearly with D−1/2 where D is the diameter of the flat suggesting that, perhaps, even higher pressures are possible with further miniaturization. A scaling law is used to calculate the minimum correction due to the presence of...

An automatic molecular beam microwave Fourier transform spectrometer

Abstract: We present hardware and software details of the pulsed molecular beam microwave Fourier transform (MB‐MWFT) spectrometer used in the Kiel microwave group. We emphasize an automatic scanning facility which greatly increases the efficiency of MB‐MWFT spectroscopy for the measurement of unassigned spectra.

Optical probes for local void fraction measurements: Characterization of performance

Abstract: Optical probes are widely used in two phase gas‐liquid flows, especially in laboratories for experimental research. However, despite all the work previously done on the subject, the performance of these sensors is not yet firmly established. To clarify this point, well controlled experiments dealing with the response of various optical probes during the piercing of an interface are presented. The latency length concept L*, which is the spatial resolution of the interface detection by a given probe, is introduced. The minimum value recorded is about 130 μm. Moreover, this parameter is closely related to the hydrodynamic response time of the sensor, and as such, to the geometric displacement velocity of each interface detected. Hence, the latency length is proposed as an objective basis for the comparison of the different probes. Finally, the relation between the response time and the signal processing techniques is discussed, and it is shown that the shortest length L* must be sought in order to ensure a better accuracy of the local void fraction.

The double‐ring electrodynamic balance for microparticle characterization

Abstract: A simple form of the electrodynamic balance, suitable for a wide range of microparticle measurements, is described and analyzed The ac electrode of the device consists of a pair of parallel rings, and the dc endcaps are either simple disks or they can be eliminated entirely by applying suitable dc bias voltages to the rings The stability characteristics of the device are determined by extension of well‐established stability theory, and experiments are compared with that theory The device is particularly well‐suited for detection of radioactive aerosols, for it has significant advantages over the bihyperboloidal device for radioactivity measurement The detection of radioactivity levels of less than 20 pCi is feasible Coupled with a Raman spectrometer the balance serves as a stable ‘‘platform’’ for the study of the chemistry of microparticles, and both qualitative and quantitative analysis of microdroplet chemistry are demonstrated for binary droplets of 1‐octadecene and 1‐bromoctadecane

Time‐of‐flight scattering and recoiling spectrometer (TOF‐SARS) for surface analysis

Abstract: Low energy (< 10 keV) ion scattering spectrometry [10.1] is becoming increasingly important as a surface analysis technique in three specific areas, i.e., surface elemental analysis [10.2–4], probing surface structure [10.5–16], and studying electronic transition probabilities [10.7,7–19] between ions or atoms and surfaces. This is largely due to the following recent advances: (i) impact collision ion scattering spectrometry [10.6] (ICISS) in which the scattering angle is close to 180°, thus simplifying the scattering geometry and allowing experimental determination of the shadow cone radii, (ii) the use of alkali primary ions [10.9, 10] which have low neutralization probabilities, leading to higher scattered ion fluxes, (iii) time-of-flight (TOF) techniques [10.20–23] with detection of both neutrals and ions in a multichannel mode in order to enhance sensitivity, (iv) scattered ion fractions [10.7,17] to probe the spatial distributions of electrons, and (v) the use of recoiling [10.24, 25] to determine the structure of light adsorbates on surfaces.

High spatial and temporal resolution visible spectroscopy of the plasma edge in DIII‐D

Abstract: In DIII‐D, visible spectroscopic measurements of the He ii 468.6 nm and C vi 529.2 nm Doppler broadened spectral lines, resulting from charge exchange recombination interactions between beam neutral atoms and plasma ions, are performed to determine ion temperatures, and toroidal and poloidal rotation velocities. The diagnostics system comprises 32 viewing chords spanning a typical minor radius of 63 cm across the midplane, of which 16 spatial chords span 11 cm of the plasma edge just within the separatrix. A temporal resolution of 260 μs per time slice can be obtained as a result of using MCP phosphors with short decay times and fast camera readout electronics. Results from this system will be used in radial electric field comparisons with theory at the L–H transition and ion transport analysis.

Repetitive flash x‐ray generator utilizing a simple diode with a new type of energy‐selective function

Abstract: The construction and the fundamental studies of a repetitive flash x‐ray generator having a simple diode with an energy‐selective function are described. This generator consisted of the following components: a constant high‐voltage power supply, a high‐voltage pulser, a repetitive high‐energy impulse switching system, a turbo molecular pump, and a flash x‐ray tube. The circuit of this pulser employed a modified two‐stage surge Marx generator with a capacity during main discharge of 425pF. The x‐ray tube was of the demountable‐diode type which was connected to the turbo molecular pump and consisted of the following major devices: a rod‐shaped anode tip made of tungsten, a disk cathode made of graphite, an aluminum filter, and a tube body made of glass. Two condensers inside of the pulser were charged from 40 to 60 kV, and the output voltage was about 1.9 times the charging voltage. The peak tube voltage was primarily determined by the anode‐cathode (A‐C) space, and the peak tube current was less than 0.6 kA. The peak tube voltage slightly increased when the charging voltage was increased, but the amount of change rate was small. Thus, the maximum photon energy could be easily controlled by varying the A‐C space. The pulse width ranged from 40 to 100 ns, and the x‐ray intensity was less than 1.0 μC/kg at 0.3 m per pulse. The repetitive frequency was less than 50 Hz, and the effective focal spot size was determined by the diameter of the anode tip and ranged from 0.5 to 3.0 mm in diameter.

Vibrating orifice droplet generator for precision optical studies

Abstract: A special purpose vibrating orifice droplet generator is described possessing improved short‐term monodispersity (instantaneous diameter fluctuations of 2×10−5 and a differential drift of 10−5/min) We demonstrate that this is sufficient to allow cw laser excitation of specific morphology‐dependent resonances (MDRs) Improved performance results from (1) the liquid sample being direct pressure fed to the vibrating orifice from a closed pressure reservoir rather than by a conventional gear driven syringe pump, and (2) the vibrating orifice is driven by a periodic square wave voltage source having a frequency which is four orders of magnitude more constant (1 part in 108 per day) than sources normally used A novel variational size spectroscopy is also described that is made possible by programmed frequency ramping of the voltage source driving the vibrating orifice By monitoring elastic scattering during ramped size changes we have been able to infer precise values of droplet index of refraction (to ±0001) and size (to ±3 parts in 104) at any orifice operating frequency by comparison of the resulting experimental pattern to Lorentz–Mie calculations This aerosol source permits the next generation of optical experiments (eg, double resonance, fluorescence rate inhibition and enhancement, cw nonlinear optical, accurate chemical speciation, etc) to be performed on droplets with diameters of 10–80 μm

A system for systematically preparing atom‐probe field‐ion‐microscope specimens for the study of internal interfaces

Abstract: A versatile system has been designed and fabricated to prepare atom‐probe field‐ion‐microscope (APFIM) specimens in a systematic manner, such that internal interfaces can be positioned in the tips of these wire specimens for subsequent analysis of their chemical composition. This system incorporates both beaker electrolytic and zone electrolytic cell configurations, a specially constructed power supply, and a special transmission electron microscope holder for wires. The power supply enables ac electroetching or dc electropolishing in the automated or manual modes. The ac wave forms available are sine (0.002 Hz–200 kHz) or square (10 Hz–20 kHz). Triggering and gating are performed manually or with a pulse generator. The dc output is gated manually to produce a continuous output or with a pulse generator to produce single pulses with widths in the range 50 μs–1 s. A counter indicates the number of periods of voltage applied, and the total charge transferred in the electrolytic cell is integrated in the ran...

Motional Stark effect polarimetry for a current profile diagnostic in DIII‐D

Abstract: Motional Stark effect produces large net linear polarization of Hα emission from neutral beams in tokamaks. Measurement of the polarization direction permits determination of the local magnetic field pitch angle. Design of a single point, spatially scannable, high‐sensitivity polarimeter installed on DIII‐D is described. Excellent signal‐to‐noise ratio with good temporal and spatial resolution was obtained in preliminary measurements of magnetic field pitch angle.

Device for dispersal of micrometer- and submicrometer-sized particles in vacuum

Abstract: A simple, versatile device for dispersing micrometer‐ and submicrometer‐sized particles in vacuum is described. The source allows control of particle size (0.5 μm≤l≤200 μm) and particle flux density up to roughly 107 cm−2 s−1. Several types of microparticles were successfully dispersed.

High resolution time‐of‐flight mass spectrometers. Part III. Reflector design

Abstract: The reflector has a central influence on the properties of a time‐of‐flight mass spectrometer. The mass resolution can be greatly improved using a reflector with homogeneous fields. A reflector with homogeneous fields can also have focusing properties and thus enhance sensitivity of the instrument. In this case it is important to be aware of design limitations. We present a mathematical method of determining these limitations and also give a guideline for mechanical design.

Visible charge exchange spectroscopy at JET

Abstract: Recent developments and results of the JET CXRS diagnostic are reported. The measurements of radial profiles of ion temperatures and densities are based on CXR spectra of fully stripped ions of either carbon or beryllium. Considerable effort has been expended in ensuring consistency between radial profiles of low Z impurity densities and those from other diagnostics. The contributions of the main light impurities are used to reconstruct radial profiles of Zeff which can be compared with Abel‐inverted signals from visible bremsstrahlung or soft x‐ray emission. Active Balmer‐Alpha spectroscopy (ABAS) is being introduced as a diagnostic tool providing data on local magnetic fields, neutral beam densities, and dilution factors. The effects of collision‐energy‐dependent CXR cross sections on observed CXR spectra are calculated. Corrections for the values of deduced ion temperatures, toroidal velocities, and impurity densities are discussed for the case of plasmas with high ion temperatures and high toroidal rotation velocities. Some recent results of the JET 1989 operation illustrating the CXRS diagnostic potential are given.

Instrumentation for the stable operation of laser diodes

Abstract: We have developed a constant current supply and a temperature control circuit that can be used for frequency‐stable operation of laser diodes. These instruments can stabilize laser diode injection current and temperature to better than ±1 μA and ±0.3 mK, respectively, over time periods exceeding 1 h. We have excited the Li(2S↔2P) transition with a red‐light‐emitting laser diode and find that our instrumentation stabilizes the laser frequency to within ±10 MHz over a period exceeding 1 h.

Magnetic field pitch angle diagnostic using the motional Stark effect (invited)

Abstract: The Stark effect has been employed in a novel technique for obtaining the pitch angle profile and q(r) using polarimetry measurements of the Doppler shifted Hα emission from a hydrogen diagnostic neutral beam. As a neutral beam propagates through a plasma, collisions of the beam particles with the background ions and electrons will excite beam atoms, leading to emission of radiation. The motional Stark effect, which arises from the electric field induced in the atom’s rest frame due to the beam motion across the magnetic field (E=Vbeam×B), causes a wavelength splitting of several angstroms and polarization of the emitted radiation. The Δm=±1 transitions, or σ components, from the beam fluorescence are linearly polarized parallel to the direction of the local magnetic field when viewed transverse to the fields. Since the hydrogen beam provides good spatial localization and penetration, the pitch angle can be obtained anywhere in the plasma. A photoelastic modulator (PEM) is used to modulate the linearly polarized light. Depending on the orientation of the PEM, it can measure the sine or cosine of the angle of polarization. Two PEM’s are used to measure both components simultaneously. Results of q(r) for both Ohmic and NBI heated discharges have been obtained in the Princeton Beta Experiment (PBX‐M) tokamak, with an uncertainty of ∼6% for q(0).The Stark effect has been employed in a novel technique for obtaining the pitch angle profile and q(r) using polarimetry measurements of the Doppler shifted Hα emission from a hydrogen diagnostic neutral beam. As a neutral beam propagates through a plasma, collisions of the beam particles with the background ions and electrons will excite beam atoms, leading to emission of radiation. The motional Stark effect, which arises from the electric field induced in the atom’s rest frame due to the beam motion across the magnetic field (E=Vbeam×B), causes a wavelength splitting of several angstroms and polarization of the emitted radiation. The Δm=±1 transitions, or σ components, from the beam fluorescence are linearly polarized parallel to the direction of the local magnetic field when viewed transverse to the fields. Since the hydrogen beam provides good spatial localization and penetration, the pitch angle can be obtained anywhere in the plasma. A photoelastic modulator (PEM) is used to modulate the linearly po...

Efficient generation of multigigawatt rf power by a klystronlike amplifier

Abstract: This article addresses the new development of high‐power rf klystronlike amplifiers using modulated intense relativistic electron beams. Development of these amplifiers follows earlier research in which the interaction between a high‐impedance (120‐Ω) intense relativistic electron beam and a low‐power rf pulse resulted in the generation of coherent bunches of electrons with excellent amplitude and phase stabilities. In the present experiment a low‐impedance (30‐Ω) large‐diameter (13.2‐cm) annular electron beam of power ∼8 GW was modulated using an external rf source (magnetron at 1.3 GHz) of 0.5 MW power. The interaction of the modulated electron beam with a structure generated a 3‐GW rf pulse that was radiated into the atmosphere. The self‐fields of the intense beam provided significant electrostatic insulation against vacuum breakdown at the modulating gaps and at the rf extraction gap.

Constant current corona triode with grid voltage control. Application to polymer foil charging

Abstract: A new version of a corona triode setup, allowing charging dielectric foils with a constant current, was developed. The charging current is kept constant by controlling the grid voltage. We show that the sample surface potential can be inferred from the grid voltage and present results of the uniformity of charge deposition during the charging procedure. Sample charging currents could be varied from 1 to 300 nA/cm2 and the foils could be charged up to ±6 kV. The method was applied to measure the potential buildup on Teflon FEP and PVDF samples and these results are discussed. They showed that the method can be successfully applied to study the polarization buildup of β‐PVDF foils under corona charging.