Faraday Cup Array Research Articles

Design of lost alpha particle diagnostics for JET

In a future magnetic fusion reactor alpha particles will be utlilized for plasma heating. In order to achieve a high efficiency of this process, the aim has to be a good confinement of alpha particles. Therefore, direct measurement of alpha particle losses is of particular interest. Two diagnostics are being prepared for the JET Tokamak that are targeting on exactly this subject: a scintillator probe and a set of Faraday cups [A. Lioure, A. Kaye, A. Murari, J. Sanchez, T. Todd, C. Damiani, J. Pamela, JET-EFDA Contributors, The JET-Enhanced Performance Program: more heating power and diagnostic capabilities in preparation for ITER, Fusion Eng. Des. 74 (2005) 141] . These systems are capable of measuring ICRH tail ions and charged fusion products. The scintillator probe aims to allow the detection of particles with a pitch angle between 30° and 86° (5% resolution) and a gyroradius between 20 and 140 mm (15% resolution). The Faraday cup array will detect the current of fast ions at multiple poloidal locations, with a dynamic range of 1 nA/cm 2 to 100 μA/cm 2 at a temporal resolution of 1 ms. For 3.5 MeV α-particles the energy binning of the foil detector will be 15–50% of the full energy depending on the geometry of the individual collector. The experience in operating both diagnostics in a high temperature and high radiation environment will give valuable information in preparation for the design of similar diagnostics for future fusion devices. This paper covers the design and engineering of both diagnostics together with their envisaged performance.

Design and construction of a fast ion loss Faraday cup array diagnostic for Joint European Torus

A thin foil Faraday cup array is being built to measure the loss of 3.5 MeV alpha particles and MeV ion cyclotron heating tail ions on Joint European Torus. It will consist of nine detectors spread over five different poloidal locations and three radial positions. They will measure the poloidal distribution and radial scrape off of the losses. The detectors will be comprised of four layers of thin (2.5 μm) Ni foil, giving some resolution of the lost particle energy distribution as different ranges of energies will stop in different layers of the detector. One detector will utilize eight thinner (1.0 μm) foils to obtain a better-resolved energy distribution. These detectors will accept particles incident up to 45° from the normal to the foils.

Characterization of a focal plane camera fitted to a Mattauch-Herzog geometry mass spectrograph. 2. Use with an inductively coupled plasma.

A novel charge-sensitive detector array, termed the focal plane camera (FPC), has been coupled to a Mattauch-Herzog mass spectrograph (MHMS) with an inductively coupled plasma ionization source. The FPC employs an array of gold Faraday cups, each with its own charge-integrating circuit that allows the simultaneous detection of several m/z ratios. The ion-sampling interface of the MHMS has been redesigned to provide better heat transfer away from the sampler and skimmer cones and to reduce the negative effects of turbulent gas flows around the plasma. The instrument has produced limits of detection in the tens to hundreds of parts per quadrillion regime and isotope ratio accuracy and precision of 5% error and 0.007% RSD, respectively. Limits of detection with the FPC are comparable to those obtained with a single-channel secondary electron multiplier (SEM). However, the isotope ratio accuracy and precision are better with the FPC than when the SEM is employed. The dynamic range has been shown to be linear over 7 orders of magnitude.

Direct measurement of the ion current density distribution of broad beam ion sources

A Faraday cup array with 256 measuring points has been developed and tested to measure the ion current density distribution of broad beam ion sources. The ion beam profiler consists of a measuring head and an evaluation unit. The signals are fed into a computer via an RS 232 interface and then visualised with a comfortable software. There is good qualitative correspondence between the etch profiles on silicon wafers and the measured ion current density distribution. Measurements that have been made at various ion sources clearly show that the ion current density distribution on the target surface varies greatly and essentially depends on the operational parameters and the type of source. With the ion beam profiler it is possible to find the operational parameters which ensure the technologically necessary ion current density distribution, in a very short time.

Reduction of edge emission in electron beam diodes

This paper presents measurements of the enhanced current density along the edges of a large area electron beam as well as successful techniques that eliminated this edge effect/beam halo. The beam current is measured with a Faraday cup array at the anode, and the spatial, time-integrated current density is obtained with radiachromic film. Particle-in-cell simulations support the experimental results. Experiments and simulations show that recessing the cathode reduces the electric field at the edge and eliminates the edge effect. However, the cathode recess structure itself emits under long-term repetitive operation. In contrast, using a floating, metallic, electric field shaper that is electrically insulated from the cathode eliminates the beam halo and mitigates electron emission from its surface during repetitive operation.

Characterization of a focal plane camera fitted to a Mattauch-Herzog geometry mass spectrograph. 1. Use with a glow-discharge source.

A Mattauch-Herzog geometry mass spectrograph (MHMS) has been equipped with a novel array detector, the focal plane camera (FPC). The FPC consists of an array of gold Faraday cups, each coupled to its own integrator, with interrogation of the integrators performed by a multiplexer. The initial coupling of this instrument with a pin-type glow discharge source has provided limits of detection in the single to hundreds of nanograms per gram regime; isotope ratio accuracy and precision better than 5% error and 0.2% RSD, respectively; and a linear dynamic range of at least 6 orders of magnitude. A current weakness of the FPC is its pixel size, which limits both sensitivity and baseline resolution (to R = 130). The minimum data acquisition time for multiple images at present is 1 ms/image, with a dead time of 3.2 ms between images, which will limit the ability of the FPC to monitor extremely short transient signals.

Micromachined Faraday cup array using deep reactive ion etching

A micromachined Faraday cup array (MFCA) for position sensitive ion detection has been developed using a deep reactive ion etching (DRIE) process. Linear, closely spaced arrays of 64, 128, and 256 cups have been produced with pitches of 250 /spl mu/m and 150 /spl mu/m. Low leakage MOS capacitors formed into DRIE trenches form effective ion collection traps with stable and electrostatically isolated capacitances. These closely spaced arrays of Faraday cups enable a new generation of compact mass spectrometers with true multi-channel detection capability. Since all of the incident ion flux is continuously intercepted by the array, no ion flux is lost as in scanning systems, and the overall sensitivity of the mass spectrometer is drastically improved by a factor approximately equal to the number of cups in the array. The MFCA is thus an ideal component for miniaturized mass spectrometers, ion beam profiling, and chemical analyzers which must work with very small sample volumes or high throughputs.

Volume and surface photoemission from a niobium target by polarized ultraviolet radiation

This work reports on the experimental results of a Nb photocathode stimulated by two different lasers, of wavelengths 308 and 222 nm. A fast current detector, Rogowski coil, recorded the high output currents, while the beam angular distributions were measured by an array of small Faraday cups placed along the propagation axis of the beam. With p-polarized light, we found quantum efficiencies (QEs) of 1.6×10−5 and 1.0×10−6 for λ=222 nm and λ=308 nm, respectively. The efficiency of the photocathode was measured to be higher for s-polarized laser beams than for the p-polarized ones. They resulted in QEs of 5.3×10−5 and 2.1×10−6 for λ=222 nm and λ=308 nm, respectively. From the electron-beam angular divergence values, we determined the upper limit emittance. The lowest upper limit normalized emittance value found was 7 (π mm mrad), achieved by the s-polarized radiation of 308 nm. In both cases the s-polarized light generated electron beams of lower emittance.

Scalable large volume ECR plasma generation for low pressure applications

Our paper describes a modular large volume electron cyclotron resonance plasma source ( f=2.45 GHz) which consists of several coaxial microwave guides each with a λ/4 antenna. Alternatively permanent magnets or electromagnets generate a static magnetic field of 87.5 mT in a distance of about 2 cm from the chamber wall. A Langmuir probe provides for spatially resolved information on electron temperature, charge carrier density (6×10 9–1.5×10 10 cm 3) and the plasma potential at operating pressures between 3×10 −4 mbar and 1×10 −2 mbar. Simultaneous operation of two or more microwave couplings is demonstrated to conform to a principle of microwave and plasma superposition (H. Nihei, Jpn. J. Appl. Phys. 33 (1994) 2718). The resulting ion current density distribution is investigated using a 81 Faraday cup array (450 mm×350 mm). Ion current density (up to 1 mA cm −2) and the homogeneity across the hole area depend on the number of microwave couplings, their local arrangement, especially the distance, the pressure, and the microwave power supplied. We demonstrate the capability of the arrangement during homogeneous large area (50 cm×50 cm) etching of platinum and achieve an etch rate in Ar + sputtering of 30 nm min −1 with a homogeneity of 10% absolute across the 72 samples.

Guiding and confinement of a laser produced plasma by a curved magnetic field

An axial magnetic field with a maximum strength of 2.2 kG was used to confine and guide a laser produced carbon plasma around a 30 deg bend. The plasma was produced by a 25 ns KrF laser pulse at a peak intensity of 1×109 W cm−2. An array of Faraday cups positioned at the exit of the guide field was used to determine that approximately 20% of the original plasma was confined by the magnetic field and guided along a 15 cm trajectory to a 5 cm2 area.

Charge collection thermal ion mass spectrometry of thorium

The determination of thorium isotope ratios using a new technique suitable for low intensity ion beams has resulted in significant improvement in analytical precision. A capacitor was substituted in place of the usual high-value resistor at the feedback loop of an electrometer. Charge collected in a Faraday cup accumulates on the capacitor, resulting in a time-varying voltage proportional to the ion current. Electrometer noise levels are reduced by a factor of up to 500, and immunity to temperature fluctuations is improved by a factor of more than 10. Charge collection thermal ion mass spectrometry bridges the gap between the measurement range for resistive feedback electrometers and electron multiplier devices. Standard design Faraday cup arrays and electrometers equipped with capacitive feedback elements can be used for simultaneous data collection in multiple channels. Maximum working range, for unit mass separation in adjacent Faraday cups, is uncompromised. The charge collection method was tested with thorium ion beams. Filaments loaded with 15 pg of 230Th produce intensities up to 6 × 1 −14 A over 1 h. The ionization efficiency is 4% and analytical errors for a single run are better than ±0.6‰; these results are superior to those reported for other techniques. Charge collection mass spectrometry will be most useful in cases where large ion beams cannot easily be obtained or where the concentration of the element of interest in geological materials is very low.

Large-diameter plasma profile monitoring system using Faraday cup and Langmuir probe arrays

Details of a personal computer-based data acquisition system, which allows monitoring of large-diameter plasma source ion current profiles and Langmuir probe data, using an interchangeable rotatable, Faraday cup array and Langmuir probe array respectively are described. The arrays span the plasma diameter and can be rotated under computer control. Software-selectable monitor modes allow investigation of the angular and temporal dependence of ion current profiles, basic plasma parameters and their associated non-uniformities. Isolated aluminium plates can be fixed to the respective arrays to investigate the intrusive influence of a substrate holder and substrate bias on plasma parameters. Angular dependences is of ion current non-uniformity values of a large-diameter permanent magnet electron cyclotron resonance source are reported to demonstrate the benefits of the system over a single-probe/cup system.

Ion flows from laser-irradiated microcylinders

Hollow aluminium microcylinders were irradiated internally through a circular aperture located in one wall using a pulsed ruby laser beam. Plasma formation and ion flows were monitored using a Nomarski interferometer and an array of Faraday cups. A collimation of ion energy along the cylinder axis was observed.

Ion flows from laser-irradiated microcylinders

Hollow aluminium microcylinders were irradiated internally through a circular aperture located in one wall using a pulsed ruby laser beam. Plasma formation and ion flows were monitored using a Nomarski interferometer and an array of Faraday cups. A collimation of ion energy along the cylinder axis was observed.

Dual ion beam deposition of oxide, nitride, and carbide films

Compound films such as oxides, nitrides, and carbides have been prepared in a dual ion beam (DIB) deposition system, which contains an ionized cluster beam (ICB) source and an ionized gas beam (IGB) source. The characteristic of the DIB system and the properties of compound films such as SiOx, TiNx, and C were investigated. The ion current density distribution was measured at acceleration voltages from 0 to 10 kV by an array of Faraday cups at the substrate position. Uniform ion current distributions over 5-in. diam substrates have been obtained for ICB of Au, Cu, and Si and IGB of CH4, N2, and O2. The composition of SiOx films prepared by DIB approaches stoichiometric SiO2 with increasing oxygen ion current density. The crystalline structure of TiNx films changes gradually from TiN to Ti2N as the atomic flux ratio of (N/Ti) decreases. The Knoop hardness values of TiN and C films prepared at 200 °C are over 1700 and 3000 kg/mm2, respectively. Therefore the advantages of using the DIB deposition system for low-temperature compound film growth include control over film composition and crystalline structure with excellent physical properties. Ion irradiation during film growth by both metal and gas beams enhances chemical reaction kinetics on the substrate.

Diagnostic system for measurement of particle balance in TMX-U

Several diagnostics measure the particle sources and losses in the Tandem Mirror Experiment-Upgrade (TMX-U) plasma. An absolutely calibrated high-speed (0.5 ms per frame) filtered (6561 Å) video camera measures the total ionization source as a function of radius. An axial view of the plasma automatically integrates the axial variations within the depth of field of the system. Another camera, viewing the plasma radially, measures the axial source variations near the deuterium fueling source. Axial ion losses are measured by an array of Faraday cups that are equipped with grids for repelling electrons and are mounted at each end of the experiment. Unequal ion and electron (nonambipolar) radial losses are inferred from net current measurements on an array of grounded plates at each end. Any differences between the measured particle losses and sources may be attributed to ambipolar radial losses and/or azimuthal asymmetries in the particle-loss profiles. Methods of system calibration, along with details of computer data acquisition and processing of this relatively large set of data, are also presented.

TARA diagnostic set

The TARA Tandem Mirror Experiment has recently begun operation. The set of diagnostics available at this time is discussed. The following diagnostics are now in use: diamagnetic loops, a multichord microwave interferometer, Langmuir and emissive probes, pick-up loops, and secondary-emission detectors. End-loss diagnostics include net current detector arrays, Faraday cup arrays, swept particle analyzer arrays, and calorimetry. Light-emission measurements are made in the visible and VUV regions. A multichord fiber-optic array for plasma position detection is also used. In addition, a three-channel charge exchange analyzer, a hard x-ray system, and fast pressure gauges are available.

Beam characteristics of laser-excited high-emission cathodes

Recently, a new method was developed to generate high current density pulsed electron beams by Q-switched laser irradiation of cesiated targets. A series of diagnostic experiments is now being performed to examine the characteristics of these beams. With a Marx generator providing electric fields of up to 180 kV/cm between the cathode and annular anode, a Faraday cup array positioned 10 cm downstream of the anode measures the spatial distribution of the electron beam in this field-free region. although self-induced vacuum breakdown will by itself generate an electron beam, its intensity is found to substantially increase when the laser pulse is coupled to the Marx discharge. Further increases are observed with cesium coverage of the cathode. For example, preliminary results have shown that laser irradiation of a bare tungsten cathode surface enhances the magnitude of the electron beam current near the axis by approximately 70% in comparison to that observed from self-breakdown alone. This increase is suspected to be due to surface impurities on the cathode. With a thin layer of cesium deposited on the cathode an averaged additional increase of 40% in the electron beam intensity near the axis was noted. Although quantitatively the currents are limited by space-charge effects, these results show qualitatively that laser irradiation and cesium coverage of cathodes substantially raises the intensities of the generated electron beams.

High frequency Faraday cup array. (USA)

High frequency Faraday cup array. (USA)

High-frequency Faraday cup array

A high frequency response Faraday cup array has been developed to measure the beam profile and high frequency wave behavior of the Auto-Resonant Accelerator electron beam.