Faraday Cup Array Research Articles

Measurement system for ion beam profiles using fixed parallel wires and Faraday cup array

The radial profiles of ion current density are measured using a combination of two techniques—fixed parallel wires and a Faraday cup array—in a ring cusp ion source. These measurements are carried out for Ar+ ion beam current in the range of 20–90 mA and energy in the range of 850–1650 eV. Current density profiles near the extraction plane of the ion source are obtained by Abel inversion of the profiles of current measured using the parallel wires. In addition, the Faraday cup array provides radial profiles of ion current density at two different locations along the beam axis. A numerical model is developed and utilized for estimating the beam divergence and focal length from the profiles of ion current density measured using the two techniques. A fixed parallel wire array is established as an effective method for monitoring the ion beam profiles close to the ion source's extraction plane when the ion beam is utilized for other applications. The performance of the two measurement techniques is compared by estimating the ion beam current from the measurement of ion current density with the beam current measured from the power supply.

Effects of axial magnetic field in a magnetic multipole line cusp ion source

Experiments have been performed to study the effects of axial magnetic field on plasma and ion beam parameters in a magnetic multipole line cusp ion source. Studies performed on ring cusp and Kaufmann type ion sources suggest that the axial component of magnetic field may help in improving the ion extraction current at a low discharge power. The line cusps ion sources have been used since long to produced uniform beams, however, they lack axial component of the magnetic field. These ion sources generally suffer from low efficiency possibly due the absence of axial magnetic field. In this work, an additional magnetic coil is added at various axial positions between the back-plate and the plasma grid of the multipole line cusp ion source. We have investigated the effects of axial magnetic field on the discharge efficiency and source parameters like beam current and uniformity in the multipole line cusp ion source. The beam profiles are obtained using an eleven-channel faraday cup array to estimate the effects of the axial magnetic field on beam uniformity and divergence. Initial studies suggest a reduction of beam divergence with increasing axial magnetic field. A significant rise in the beam current and the discharge current is also observed when the axial magnetic field is increased. Particle trajectory simulation using the CST-Studio suite is utilised in understanding the role of confinement of primary electrons behind the improved performance of the ion source.

Characterization of a Multicusp Ion Source With Two-Grid Extraction System for Studying Extraction and Transport of Ion Beam

A low-energy multipole line cusp ion source with two-grid extraction system has been developed and characterized for studying the role of the charge exchange processes on ion beam extraction, transport, and neutralization. The ion source is operated for extraction of Ar <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> ion beam of current 50–240 mA at energies 850–1650 eV. The characterization of the ion beam is carried out at various axial positions along a 200–815-mm beam path with a background pressure of 4– <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$20\times10$ </tex-math></inline-formula> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−5</sup> Torr. An 11-channel Faraday cup array is used to measure the radial profiles of ion current density and angular beam divergence at these positions. The beam divergences are found to be in the range of 6°–15° for beam perveance of 0.50– <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3.75\times10$ </tex-math></inline-formula> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−9</sup> AV <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3/2</sup> . The attenuation of ion beam current along the beam path is observed due to the charge exchange neutralization of the energetic ions in the presence of high background pressure. Since ion current density along the beam path can be reduced by both the charge exchange processes and beam divergence, the accounting of beam particles, i.e., energetic ions and neutrals, at larger distances is difficult by the measurements of ion current density using the Faraday cup alone. The flux of energetic particles (energetic ions and fast neutrals) is therefore obtained by a thrust measurements on a beam target fixed at a distance of 800 mm from the ion source. The neutralization of the ion beam obtained by the emission of electrons from a hot filament neutralizer is also investigated in the presence of the charge exchange processes. Space-charge neutralization is provided initially by the secondary electrons emitted from the vessel walls or by the charge exchange processes even without the neutralizer operating at the desired temperature. However, the current neutralization is achieved only by the electrons emitted from the hot filament neutralizer operating in a space-charge-limited mode.

Increasing ion and fusion yield in a dense plasma focus by combination of pre-ionization and heavy ion gas admixture

The results of an experimental study of pre-ionization and heavy gas introduced into driven gas in a plasma focus device are reported. To achieve this purpose, we made use of two methods: first, the pre-ionization method by applying the shunt resistor and second, the admixture of heavy ions. We applied the different shunt resistors and found the optimum amount to be 200 MΩ at an optimum pressure of 0.5 Torr. Ion yield that was measured by array of Faraday cups and the energy of fast ions that was calculated by using the time-of-flight method were raised up to 22% and 45%, and the impurity caused by anode's erosion was reduced approximately by 67% in comparison to when there was no pre-ionization. Also, we have used the admixture of 5% argon ions with nitrogen (working gas) to improve the ion yield up to 45% in comparison with pure nitrogen. Finally, for the first time, we have utilized the combination of these methods together and have, consequently, reached the maximum ion yield and fusion yield. With this new method, ion yield raised up to 70% greater than that of the previous condition, i.e., without pre-ionization and heavy ion admixture.

Rapid uranium-series age screening of carbonates by laser ablation mass spectrometry

Uranium-series dating is a critical tool in quaternary geochronology, including paleoclimate work, archaeology and geomorphology. Laser ablation (LA) methods are not as precise as most isotope dilution methods, but can be used to generate calendar ages rapidly, expanding the range of dating tools that can be applied to late Pleistocene carbonates. Here, existing LA methods are revisited for corals (cold- and warm-water) and speleothems spanning the last 343 thousand years (ka). Measurement of the required isotopes (238U, 234U, 230Th and 232Th) is achieved by coupling a laser system to a multi-collector inductively-coupled-plasma mass spectrometer (MC-ICPMS) using a combination of a single central ion counter and an array of Faraday cups. Each sample analysis lasts for ∼4.3 min, and fifty samples can be measured in 12 h with an automated set up, after a day of sample preparation. The use of different standard materials and laser systems had no significant effect on method accuracy. Uncertainty on the measured (230Th/238U) activity ratios ranges from 5.4% to 7.6% for (230Th/238U) ratios equal to 0.7 and 0.1 respectively. Much of this uncertainty can be attributed to the heterogeneity of the standard material (230Th/238U) at the length scale of LA. A homogeneous standard material may therefore improve measurement uncertainty but is not a requirement for age-screening studies. The initial (234U/238U) of coral samples can be determined within ∼20‰, making it useful as a first indicator of open-system behaviour. For cold-water corals, success in determination of (232Th/238U) – which can affect final age accuracy – by LA depended strongly on sample heterogeneity. Age uncertainties (2 sigma) ranged from <0.8 ka at 0–10 ka, ∼1.5 ka at 20 ka to ∼15 ka at 125 ka. Thus, we have demonstrated that U-series dating by LA-MC-ICPMS can be usefully applied to a range of carbonate materials as a straightforward age-screening technique.

Basic research for development of the beam profile monitor based on a Faraday cup array system

The basic design used to develop a beam profile monitor based on a Faraday cup array (FCA), which has the advantages of high robustness, reliability, and long-term stability, along with the ability to measure the ion current over a wide dynamic range, was developed. The total system is divided into three parts: i.e., a faraday cup, measuring electronics, and a display program part. The FCA was considered to consist of a collimator, suppressor, insulator frame, and 64 (8 × 8 array) tiny Faraday cups (FC). An electronic circuit using a multiplexer was applied to effectively address many signal lines and the printed circuit board (PCB) was designed to be divided into three parts, i.e., an electrode PCB (ELEC PCB), capacitance PCB (CAP PCB), and control PCB (CON PCB).

TIMS measurements of full range of natural Ca isotopes with internally consistent fractionation correction

This study presents static measurements of the Ca isotopic composition of standard reference materials SRM 915 a/b on a Triton Plus™ thermal ionization mass spectrometer with a specially developed Faraday cup array allowing simultaneous measurement of 40Ca and 48Ca. The total amount of Ca in all analyses was kept <1μg. With this setup the measurement uncertainties were 0.06‰ for 40Ca/44Ca and 0.12‰ for 48Ca/40Ca. Measuring all isotopes simultaneously better allows to test the internal consistency of different Ca isotope abundances reported in the literature. The exponential law was observed to correct incompletely instrumental mass fractionation. An improved fractionation correction based on the exponential law is proposed. It changes the 40Ca/44Ca ratio of SRM 915a (corrected relative to 42Ca/44Ca=0.31221; 48Ca/44Ca=0.08871) from 47.1635±0.0028 to 47.1649±0.0047. The measurements of SRM 915b were performed with different analytical conditions (runs were prolonged till complete filament load depletion). Even if the 40Ca/44Ca ratio of SRM 915b, when corrected with the simple exponential law, appears different (47.1532±0.0038) from that of SRM 915a, it becomes coincident (47.1613±0.0028) when corrected with a second-order refinement. This supports the use of the improved exponential law to obtain internally consistent Ca isotope ratio for natural samples.

Self-confined plasma in a magneto-plasma compressor and the influence of an externally imposed magnetic field

The spatial profile of a magnetically self-confined plasma formed by a magneto-plasma compressor is measured in radial and axial direction with an array of Faraday cups, and radiation with a wavelength as short as extreme ultraviolet is detected. A Gaussian distribution approximates the radial profile of the ion current density. Along the axis, the ion current density remains approximately constant up to 2.25 inner anode radii from the electrodes end, before diverging. The plasma stream is further compressed by the addition of an external magnetic field. The external magnetic field is measured and is close to the self-induced magnetic field. The measurements of the profile show an increase in the ion current density after the inlet into the external magnetic field. The ion current density at 3.25 inner anode radii from the end of the electrodes doubles, and the width of the plasma stream decreases. Estimations show that the increase of the ion current density is also due to an increase of around 25% of the mean ion charge, hence to a temperature increase of around 50% enhancing photon emission at shorter wavelength. A design guideline is presented to dimensionalize the amplitude of the external magnetic field depending on the wanted temperature increase in the plasma stream.

Angular distribution of energetic argon ions emitted by a 90 kJ Filippov-type plasma focus

Characteristics of the energetic argon ions emitted by a 90 kJ Filippov-type plasma focus are studied by employing an array of Faraday cups. The Faraday cups are designed to minimize the secondary electron emission effects on their response. Angular distribution of the ions is measured, and the results indicate a highly anisotropic emission with a dip at the device axis and a local maximum at the angle of 7° with respect to the axis. It has been argued that this kind of anisotropic emission may be related to the surfatron acceleration mechanism and shown that this behavior is independent of the working gas pressure. It has been also demonstrated that this mechanism is responsible for the generation of MeV ions. Measuring the total ion number at different working gas pressures gives an optimum pressure of 0.3 Torr. In addition, the energy spectrum of ions is measured by taking into account of the ambient gas effects on the energy and charge of the ions. The current neutralization effect of electrons trapped in the ion beam as well as the effect of conducting boundaries surrounding the beam, on the detected signals are investigated.

微型法拉第筒阵列束流均匀性测量

微型法拉第筒阵列束流均匀性测量

A method of measuring the incidence angle of intense electron beam

In recent years, a great many of effect data obtained from the high current pulsed electron beam play an important role in the studying of X-ray thermal-mechanical effects. Energy deposition profile is the criterion to measure the equivalence of thermal-mechanical effects between high-current electron beam and X-rays. To adjust the energy deposition profiles to improve the equivalence of the simulations of X-ray and thermal-mechanical effect, the intense electron beam energy deposition profile measurement should be studied. Two-dimensional distribution measurement which is an important part of the energy deposition profile is to obtain a two-dimension (r, θ) incidence angle distribution. A new method of measuring the incidence angle based on small Faraday cup array covered with aluminum films, called modified multi-layer stacking, is presented in this paper. With the help of the filtered Faraday cups, the transmission fraction of the electron beam confined at a specific position and time is stored. Two-dimension incidence angle distribution on the anode target that changes over the working time is obtained with these transmission fractions by computer calculation. The result indicates that the two-dimension incidence angle distribution has a close relationship with the pinch of the beam. The electrons tend to move vertically to the equipotential line when the diode is under Child-Langmuir flow, then they hit the target in a small angle range (E×B drift, the trajectory of the electrons becomes a slanted helix with pitch changing. The incidence angle then increases to about 60° from small angle.

Development and first experimental tests of Faraday cup array

A new type of Faraday cup, capable of detecting high energy charged particles produced in a high intensity laser-matter interaction environment, has recently been developed and demonstrated as a real-time detector based on the time-of-flight technique. An array of these Faraday cups was designed and constructed to cover different observation angles with respect to the target normal direction. Thus, it allows reconstruction of the spatial distribution of ion current density in the subcritical plasma region and the ability to visualise its time evolution through time-of-flight measurements, which cannot be achieved with standard laser optical interferometry. This is a unique method for two-dimensional visualisation of ion currents from laser-generated plasmas. A technical description of the new type of Faraday cup is introduced along with an ad hoc data analysis procedure. Experimental results obtained during campaigns at the Petawatt High-Energy Laser for Heavy Ion Experiments (GSI, Darmstadt) and at the Prague Asterix Laser System (AS CR) are presented. Advantages and limitations of the used diagnostic system are discussed.

Testing of a Micro Faraday Cup Array for Ion Detection in SIMS

A micro-Faraday array detector (row of miniature 32 thin film strip electrodes wire bonded to an on-chip integrated capacitance transimpedance amplifier) was tested for use in a multiple collector secondary ion mass spectrometry. The detector was mounted on a standard IMS3f SIMS instrument in place of a microchannel plate. The measurements were performed by using a silicon sample bombarded by either or Cs+ primary ions with or without D2O flooding and detecting secondary ions of Si±, SiH±, O-, SiD−, OD−. A parallel detection of near masses of 29Si and 28SiH or 18O and 16OD was demonstrated at a sensitivity level ∼230 counts/s.

Spatially resolved measurements of electron cyclotron resonance ion source beam profile characteristics

Simulations predict that the concentric rings and the triangular structures in the profiles of strongly focused ion beams that are found in different experiments should be dominated by ion species with the same or at least similar m/q-ratio. To verify these theoretical predictions we have tuned our ECR ion source to deliver a beam consisting of multiple ion species whose particular m/q-depending focusing ranges from weakly focused to overfocused. We then recorded spatially resolved charge-state distributions of the beam profile at characteristic positions in the plane perpendicular to the beam line. The results validate theoretical predictions and are summarized in this paper. To achieve the required beam profile characteristics we moved the extraction along the beam line to achieve stronger focusing than by only changing the extraction voltage. To fit the regions of interest of the beam profile into the transmission area of the sector magnet, we steered the beam by moving the extraction in the plane perpendicular to the beam axis. The results of both investigations, beam focusing and beam steering by using a 3D-movable extraction, are also reported in this paper. A brief overview of the new beam monitor extensively used during these measurements, the Faraday cup array, is also given.

Isotope ratio analysis of actinides, fission products, and geolocators by high-efficiency multi-collector thermal ionization mass spectrometry

A ThermoFisher “Triton” multi-collector thermal ionization mass spectrometer (MC-TIMS) was evaluated for trace and ultra-trace level isotope ratio analysis of actinides (uranium, plutonium, and americium), fission products and geolocators (strontium, cesium, and neodymium). Total efficiencies (atoms loaded to ions detected) of up to 0.5–2% for U, Pu, and Am, and 1–30% for Sr, Cs, and Nd can be reported employing resin bead load techniques onto flat ribbon Re filaments or resin beads loaded into a millimeter-sized cavity drilled into a Re rod. This results in detection limits of <0.1 fg (10 4 atoms to 10 5 atoms) for 239–242+244Pu, 233+236U, 241–243Am, 89,90Sr, and 134,135,137Cs, and ≤1 pg for natural Nd isotopes (limited by the chemical processing blank) using a secondary electron multiplier (SEM) or multiple-ion counters (MICs). Relative standard deviations (RSD) as small as 0.1% and abundance sensitivities of 1 × 10 6 or better using a SEM are reported here. Precisions of RSD ≈0.01–0.001% using a multi-collector Faraday cup array can be achieved at sub-nanogram concentrations for strontium and neodymium and are suitable to gain crucial geolocation information. The analytical protocols reported herein are of particular value for nuclear forensic and nuclear safeguard applications.

In-situ analysis of Pb isotope ratios using laser ablation MC-ICP-MS: Controls on precision and accuracy and comparison between Faraday cup and ion counting systems

This contribution compares in-situlaser ablation multicollector ICP-MS (LA-MC-ICP-MS) measurements of Pb isotope composition using parallel Faraday cup and ion counter detector array to those made using solely Faraday cups. In the former configuration ion counting is used for the low abundance isotope 204Pb as well as 200Hg and 202Hg isotopes required for correction of isobaric interference of 204Hg. Accurate measurement of the differences in gain between ion counters and Faraday cups, required for determination of 208Pb/204Pb, 207Pb/204Pb and 206Pb/204Pb ratios using ion counting, can be made by analysis of standard glasses to within ±0.3% (2s). For analyses made using Faraday cups or combined Faraday cup and ion counting detector arrays mass bias is effectively controlled by external normalization to 208Pb/206Pb ratios measured in the same standard glasses. Provided that He sweep gas flow rates are kept constant and that adequate time is allowed for stabilization after changes in He flow, mass bias can be controlled to within ±0.05% (2s). The precision of measurements of Pb isotope ratios is strongly dependent on ion beam intensities. Internal precision of ≤ 0.02% (2se) on 208Pb/206Pb and 207Pb/206Pb ratios requires ∑Pb ion beam intensity of > ∼30–50 mV. At beam intensities ∼200 mV, equivalent to ∼100 000 counts per second (cps) 204Pb. Correction for 204Hg interference does not substantially contribute to uncertainty as long as 204Hg comprises less than ∼20% of the total 204 ion beam.

JET new diagnostic capability on the route to ITER

The JET scientific programme is directed towards the development of ITER relevant scenarios. In support of this, significant effort has been made to develop diagnostics to better characterise the power deposition on the plasma facing components, to investigate in more detail the radiation losses particularly in the divertor region and to better detect Magneto Hydrodynamic Modes (MHD) instabilities and their effects on fast ion confinement. A new wide-angle infrared camera provides for the first time the opportunity to perform infrared thermography in the JET main chamber, even during fast events like ELMs and disruptions. A completely new bolometric system, with better spatial resolution particularly in the divertor, is now used to investigate the total radiation losses and their influence on the ELM behaviour. A new set of microwave waveguides has improved by 20 dB the signal to noise ratio of the JET X-mode reflectometers, that are now routinely used to detect MHD instabilities and in particular to localise the location of Alfvén Eigenmodes. This improved diagnostic capability to monitor MHD instabilities is complemented by two new diagnostics to detect lost fast particles. Both the new scintillator probe and a poloidal array of Faraday cups have already shown clear correlations between MHD activity and ion losses at the edge.

Microfabrication of fine-pitch high aspect ratio Faraday cup arrays in silicon

Dense one-dimensional arrays of high aspect ratio Faraday cups have been microfabricated for application as position sensitive charged particle detectors. The method developed to fabricate these Faraday cup arrays utilizes deep reactive ion etching (DRIE) of low resistivity silicon, conformal deposition of an insulator and metal, and a novel method for patterning the cup-to-fanout interconnect to allow for high cup density. Linear arrays of 64 cups have been fabricated with cups ranging in width from 15 to 45 μm. Cup depth-to-width aspect ratios in excess 8:1 can be realized with the method. The minimum spacing between cups demonstrated here was 5 μm. Linear fill factors of the Faraday cups ranged from 50% to 90%, depending on the cup geometry and pitch. The capacitance and leakage current measurements on microfabricated Faraday cups will be discussed along with an electrical model and predictions of the detector performance.

Observation and modelling of fast ion loss in JET and ASDEX Upgrade

The confinement of fast particles is of crucial importance for the success of future burning plasma experiments. On JET, the confinement of ion cyclotron resonant frequency (ICRF) accelerated fast hydrogen ions with energies exceeding 5 MeV has been measured using the characteristic γ-rays emitted through their inelastic scattering with carbon impurities, 12C(p,p'γ)12C. Recent experiments have shown a significant decrease in this γ-ray emission (by a factor of 2) during so-called tornado mode activity (core-localized toroidal Alfvén eigenmodes (TAEs) within the q = 1 surface) in sawtoothing plasmas. This is indicative of a significant loss or extensive re-distribution of these (>5 MeV) particles from the plasma core. In this paper, mechanisms responsible for the radial transport and loss of these fast ions are investigated and identified using the HAGIS code, which describes the interaction of the fast ions and the TAE observed. The calculations show that the overlap of wave-particle resonances in phase-space leads to an enhanced radial transport and loss. On both JET and ASDEX Upgrade, new fast ion loss detectors have been installed to further investigate the loss of such particles. On JET, fast ion loss detectors based around an array of Faraday cups and a scintillator probe have been installed as part of a suite of diagnostic enhancements. On ASDEX Upgrade, a new fast ion loss detector has been mounted on the mid-plane manipulator allowing high resolution measurements in pitch angle, energy and time. This has enabled the direct observation of fast ion losses during various magnetohydrodynamics (MHD) phenomena to be studied in detail. Edge localised mode (ELM) induced fast ion losses have been directly observed along with the enhancement of fast ion losses from specific areas of phase-space in the presence of neoclassical tearing modes (NTMs) and TAEs.

Recombination effects during the laser-produced plasma expansion

The precise characterization of plasmas generated by laser irradiation is needed for the development of ion sources. As the characteristic parameters of the expanding plasma vary with both the distance and the time, an experimental study of their evolution is appropriate for a deeper knowledge of the plasma. The purpose of this work is to study the same characteristics of laser plasma produced by ablation of a pure Cu target as a function of the distance from the target along the propagation axis of the plasma plume. We irradiated the target by a KrF laser and a lens of 15 cm focal length. As the diagnostic system, a small Faraday cup array and an axial Faraday cup were utilized to study the spatial variation in the total charge carried by plasma ions. Charge loss during the plasma expansion was observed due to the recombination of charged species, which occurs within a critical distance, relatively close to the target, where the plasma density is high enough. The critical distance was determined for different laser fluences; beyond the critical distance, the collisions among plasma particles are negligible and the ion charge remains frozen. It was observed that the critical distance increases as the laser fluence increases.