High Current Mode Research Articles

Spectroscopic study of a long high-electron-density argon plasma column generated at atmospheric pressure

A stable plasma column is generated in a quartz tube using a pair of hollow electrodes driven by a sinusoidal power supply of 45 kHz at atmospheric pressure in argon. Two distinct operating modes (low-current and high-current modes) are identified through observing its discharge phenomena, measuring its electrical characteristics, and determining the gas temperatures by spectroscopic diagnosis of Q branch of UV OH spectrum. The electron density in the high-current mode is diagnosed by Stark broadening and is found to be two orders higher than that in low-current mode.

Experimental Investigation of Anode Activities in High-Current Vacuum Arcs

It is well known that the melting of electrodes (mainly anode melting) in vacuum arc can increase the metal vapor density around current zero and even lead to interruption failure. In order to clarify the anode activities and their influence on arc appearance and interruption capacity, series experiments of cup-shaped axial magnetic field copper electrodes were conducted. Obvious anode melting was detected; the liquid copper flowed on the contact plate of anode and formed a clockwise swirl flow. The appearance of anode melting is likely to correlate to the transition of arc mode from high-current diffuse mode to high-current diffuse column mode. The melting of anode was severer than cathode and was influenced by the distribution of cathode spots. Various kinds of copper particles at macroscopic level can be seen in arc column. Even at the interruption limit, the majority of melted copper of anode sputtered out of gap in form of liquid droplets or was pressed into the cup of anode, the copper vapor evaporated into arc column only accounted for a few portion and no obvious anode jets was found due to large plasma pressure in arc column.

Emission characteristics of planar field electron emitters containing carbon nanotubes operating in the high current density mode

Planar field electron emitters containing carbon nanotubes that demonstrate stable emission currents with densities about 1A/cm2 have been fabricated using the chemical vapor deposition method. The analysis of field emission characteristics based on Fowler–Nordheim theory allowed one to calculate not only the field amplification coefficient β, but also the total emission area A of all the emitting nanotubes. Using the value of A and an estimate of the emission area of an individual nanotube fulfilled in this work, the approximate number of emitting nanotubes N before and after the flow of the high density emission current, has been calculated. It has been found that the concentration of the emitting nanotubes N in such field emitters after the high density emission current flow was about 105cm−2. Besides this, the location of light radiating nanotubes heated by high density emission currents has been investigated. This has revealed the regions of the emitter surface that gave the main contribution to the electron emission.

Recent operation status of Japan Proton Accelerator Research Complex H− ion source

A cesium-free H(-) ion source driven with a LaB(6) filament is being operated at the Japan Proton Accelerator Research Complex for approximately three years without any serious trouble. In the beam commissioning or supply runs, the ion source has been operated in two different modes such as low current mode of 5 mA and high current mode of 30 mA. The total interruption time during the runs due to the ion source failure is approximately 50 h, which correspond to the ion source availability of 99%. After a long-term operation, the surface of the filament and the plasma electrode become discolored with dark partially. The result of surface analysis with field emission scanning electron microscope showed that most of the dark material is formed with boron. At the beam test performed in the interval of the run, we demonstrated that the H(-) current increased by miniaturizing the LaB(6) filament.

High bandwidth current mode amplifier for envelope modulated RF amplifiers

A high bandwidth discrete current mode amplifier capable of driving large voltages into low impedances is presented. A differential current mirror output stage ensures stability at high frequencies. Small signal bandwidth is 24 MHz and large signal bandwidth extends into the megahertz region. When amplifying the AC component of a 3 MHz 3rd generation partnership project (3GPP) long term evolution (LTE) envelope signal, an efficiency of 19.1% is achieved at 18.8 V peak-to-peak output across a 15 Ω load.

Enhancement of light emission from silicon nanocrystals by post-O2-annealing process

This study investigated the effects of the post-O2-annealing process on light emission from silicon nanocrystals. Using pulsed laser ablation in an O2-filled atmosphere, the SiOx (x<2) thin layers were synthesized prior to O2-annealing. We observed a strong photoluminescence from the silicon nanocrystals embedded in the annealed layers at 500 °C, which depended on the post-O2-annealing time. We also found that the interface property between the silicon nanocrystal and stoichiometric SiO2 matrix is critical for efficient light emission. The electroluminescence spectra in the visible range were obtained above the on-current density of 0.7 A/cm2 under several forward-bias voltages. The blueshift in the electroluminescence was attributed to the band-filling effect under high current injection mode. The external emission efficiency was estimated to be about 0.3% by using the band-filling model.

Comparison of Vacuum Arc Behaviors for Slot-Type Axial Magnetic Field Contacts With and Without Iron Plates

In order to investigate the influence of ferromagnetic-material-enhanced axial magnetic field (AMF) on high-current vacuum arc behaviors for a slot-type AMF contact, we experimentally compared the following vacuum arc behaviors between slot-type AMF contact with iron plates and that without iron plates, which include diffuse mode interval before current zero, arc modes at current peaks, and arc energy. The contact diameters were 48, 58, and 66 mm, and the contact gaps were 8 and 14 mm, respectively. The AMF contacts were fixed in a demountable vacuum chamber, and the vacuum arcs were initiated by a trigger electrode. The experimental current peak values ranged from 2.5 to 21.8 kA. The experimental results show that, with a high contact diameter of 66 mm, a stronger AMF contributes to keep the diffuse mode interval before current zero longer, and it contributes to keep the vacuum arcs at current peaks in diffuse modes until higher current values. However, with the low contact diameter of 48 mm and contact gap of 14 mm, the enhanced AMF constricts the vacuum arcs. Based on the results, an AMF window is suggested. Within the AMF window, the vacuum arcs are considered to keep longer diffuse mode interval before current zero, and the arcs have higher transition current value to high-current modes. The arc energy decreases with the enhanced AMF, significantly in the high-current range. Furthermore, it is suggested to apply iron plates in the high diameter slot-type AMF contacts. However, it does not recommend applying iron plates in the low contact diameter contacts.

Cathode Current Density Distributions in High Power Impulse and Direct Current Magnetron Sputtering Modes

AbstractDuring the operation of high power impulse magnetron sputtering discharges, peak currents in excess of 1 000 A may be observed, leading to large instantaneous power levels. To investigate this method of operation, a series of planar probes have been constructed that allow for the spatial and temporal variations of current across the surface of a cathode to be assessed. These measurements provide information on the flux of charged particles to and from the cathode whilst it is being sputtered. Under operating conditions that led to a peak current of 140 A, measured current densities varied spatially from ∼0.1 to 1.5 A · cm−2. By the use of a simple model, the measured current densities were used to predict the sputtering rate of the cathode, which allowed for the erosion profile and target lifetime to be estimated. The results suggest that, with regard to sputtering, operating in HiPIMS mode may be less energy efficient than operating in DC mode, but may achieve a higher target utilisation fraction.

A Generalized Criterion of Transition to the Diffuse Column Vacuum Arc

The high-current diffuse column arc in vacuum consists of a bright central column of plasma, with some number of individual cathode spots burning on the remainder of the contact surface. The arc voltage imposed by the main column alters the plasma expansion and current flow of single jets burning alongside the column. Stable existence of the displaced single jets drives the transition into the high-current diffuse mode. The new model assumes that the displaced single jets are confined along the magnetic field, which is a combination of a self-magnetic field and an axial magnetic field. Azimuthal self-magnetic field leads to increase in the effective length of the jet path between two contacts. The longer effective length leads to higher critical voltage below which single jets can stably exist without a source of additional plasma. This produces conditions favorable to single-jet stability. A generalized criterion for single-jet existence is presented.

Technical description of the CENBG nanobeam line

Two years ago, the CENBG has commissioned the AIFIRA (Application Interdisciplinaire des Faisceaux d’Ions en Aquitaine) facility for the development of an interdisciplinary research program based on a 3.5 MV Singletron ™ accelerator (HVEE, The Netherlands). In addition to the existing beam lines, this facility is being equipped with a high demagnification focused beam line allowing the focusing of protons, deuterons and alpha particles down to a sub-micrometer resolution. This so-called “nanobeam line”, based on a long working distance doublet–triplet of Oxford Microbeams Ltd., OM-50 ™ quadrupoles, is at its final stage of development. The chosen layout of the beam line has been computed in details using the GEANT4 simulation toolkit. In the simulations, experimental measurements of the beam emittance at the entrance slits have been used to obtain more realistic beam distributions and intensities along the full beam line. According to these simulations, a beam resolution of about 300 nm in high current mode and below 100 nm in STIM mode is expected. The components of the beam line have been mounted at the 0° output of the Singletron ™ switching magnet and the fine alignment will be performed using the ion beam in the coming weeks. In the present paper, all the major components of the CENBG nanobeam line are described in details.

Performance of the Sumy nuclear microprobe with the integrated probe-forming system

The integrated probe-forming system for the Sumy nuclear microprobe comprises two doublets of magnetic quadrupole lenses of new design. Each doublet has been made from a single piece of soft iron by electro-discharge machining. This paper describes the performance tests of the microprobe including the beam scanning control and data acquisition systems. In the first runs a spot size of about 2 μm (FWHM) was obtained for the high beam current mode (up to 200 pA) by scanning a conventional copper grid with 1000 meshes per inch. Simulated beam optics parameters are compared with the experimental data.

Spatial distribution of average charge state and deposition rate in high power impulse magnetron sputtering of copper

The spatial distribution of copper ions and atoms in high power impulse magnetron sputtering discharges was determined by (i) measuring the ion current to electrostatic probes and (ii) measuring the film thickness by profilometry. A set of electrostatic and collection probes were placed at different angular positions and distances from the target surface. The angular distribution of the deposition rate and the average charge state of the copper species (including ions and neutrals) were deduced.The discharge showed a distinct transition to a high current mode dominated by copper self-sputtering when the applied voltage exceeded the threshold of 535 V. For a lower voltage, the deposition rate was very low and the average charge state was found to be less than 0.4. For higher voltage (and average power), the absolute deposition rates were much higher, but they were smaller than the corresponding direct current (dc) rates if normalized to the same average power. At the high voltage level, the spatial distribution of the average charge state showed some similarities to the distribution of the magnetic field, suggesting that the generation and motion of copper ions is affected by magnetized electrons. At higher voltage, the average charge state increases with the distance from the target and locally may exceed unity, indicating the presence of significant amounts of doubly charged copper ions.

Lateral Current Confinement Determines Silicon Avalanche Transistor Operation in Short-Pulsing Mode

The transient in a Si bipolar junction transistor was investigated in high-current short-pulsing ( 2 ns) mode both experimentally and numerically. A comparison of measured and simulated waveforms clearly showed that only a small fraction of the perimeter of the emitter-base interface (in the lateral direction) takes part in the switching transient when a capacitor of relatively small value (80 pF) is discharged across the transistor to obtain a current pulse of a few nanoseconds in duration. A good agreement was found between measurements and simulations in the 2-D numerical model when the effective operating perimeter was used as a parameter in the model. The results allowed reliable analyses of the thermal regime to be performed. Possible reasons for the significant current confinement in short-pulsing mode and relatively homogeneous transistor switching with longer current pulses are discussed, and a mechanism of fast lateral turn-on spread is assumed. One conclusion of practical importance is that a short-pulsing relatively high-current mode could not be realized without current confinement in the lateral direction.

A postproportional gas-amplification mode in cylindrical counters

A high-current gas-amplification mode in cylindrical detectors is considered as a time sequence of three amplification modes: a proportional mode, a mode of limited proportionality, and a self-quenched streamer (SQS) mode. The probability of a change from the limited proportionality to the SQS mode depends on the degree of ionization of the primary particle, the used gaseous mixture, and the anode diameter. The proposed gas-amplification mechanism complies with the main experimental results. Examples of promising applications of detectors in SQS modes are considered.

Comparison of Vacuum Arc Behaviors Between Axial-Magnetic-Field Contacts

We compared vacuum arc behaviors of three slot-type axial-magnetic-field (AMF) contacts at two fixed contact gaps using high-speed digital photography and arc-voltage measurements. Solid angle is subtended by anode at the center of cathode or the ratio of the anode diameter to the gap length (D/g). With coupled influence of the AMF, we observed that with the D/g ratio 48 mm/14 mm, the arc kept in low current modes until higher current compared with the D/g ratio 48 mm/8 mm case. With the D/g ratio 58 mm/14 mm, the arc-mode transitions with the increase of current were similar as the D/g ratio 58 mm/8 mm case. However, with the D/g ratio 66 mm/14 mm, the arc became high current modes at lower current compared with the D/g ratio 66 mm/8 mm case. Correspondingly, with the D/g ratio 48 mm/14 mm, arc voltages at current peaks were close to the D/g ratio 48 mm/8 mm case, whereas arc voltages at current peaks with the D/g ratio 58 mm/14 mm were close to that of the 58 mm/8 mm case. However, with a contact diameter of 66 mm, the arc voltages with the D/g ratio 66 mm/14 mm were higher than the 66 mm/8 mm case. The arcs with the D/g ratio 66 mm/8 mm were more diffuse than the 48 mm/8 mm case. Moreover, the arcs with the D/g ratio 66 mm/14 mm were also more diffuse than the 48 mm/14 mm case when the current peak was 17.8 kA and above. The relationship between interrupting capacity of vacuum interrupters with the slot-type AMF contacts could be expressed as I=ktimes(D/g)+b, where k and b are constants.

Transport property study of MgO–GaAs(001) contacts for spin injection devices

The electrical properties of Au∕MgO∕n-GaAs(001) tunnel structures have been investigated with capacitance-voltage and current-voltage measurements at room temperature with various MgO thicknesses between 0.5 and 6.0nm. For an oxide thickness higher than 2nm and for low bias voltages, the voltage essentially drops across the oxide and the structure progressively enters the high-current mode of operation with increasing reverse bias voltage, the property sought in spin injection devices. In this mode, we demonstrate that a large amount of charge accumulates at the MgO∕GaAs interface in interface traps located in the semiconductor band gap.

Lattice and beam optics design for suppression of CSR-induced emittance growth at the KEK-ERL test facility

The lattice and beam optics of the arc section of the KEK-ERL test facility, having an energy of 200 MeV, were optimized to efficiently suppress emittance growth based on a simulation using a particle-tracking method taking coherent synchrotron radiation effects into account. The lattice optimization in the arc section was performed under two conditions: a high-current mode with a bunch charge of 76.9 pC without bunch compression, and a short-bunch mode with bunch compression, producing a final bunch length of around 0.1 ps. The simulation results showed that, in the high-current mode, emittance growth was efficiently suppressed by keeping a root-mean-square (rms) bunch length of 1 ps at a bunch charge of 76.9 pC, and in the short-bunch mode, emittance growth was kept within permissible limits with a maximum allowable bunch charge of 23.1 pC at an rms bunch length of 0.1 ps.

F-18 production with the TOP linac injector

ENEA and ISS (Italian National Institute of Health), are collaborating to develop a dedicated proton medical accelerator, TOP (Oncological Therapy with Protons) linac, consisting of a sequence of three pulsed linear accelerators. The 7 MeV injector can be used in three operating modes: Protontherapy and Radiobiology Mode—injecting low current proton beam into the TOP linac accelerating sections; Radioisotope Mode—generating an intense proton beam (8–10 mA, 50–100 μs, 30–100 Hz) to produce the positron-emitting radionuclide F18 for PET analyses. In the high current mode, at the exit of the injector the beam is guided through a magnetic quadrupoles channel to a target composed by a thin chamber (0.5 mm thick and 1 in. diameter) containing water enriched with O18. Production yield as well as total activity similar to these achieved with higher energy cyclotrons have been obtained. Environmental doses measured give indications on the shielding required for operation under current radioprotection regulations. Improvements are foreseen to optimize the production yield, the useful beam current and to better characterize gamma and neutron dose rates in the different operational modes.

New method of high brightness ion extraction based on bias electrode

It is well known that beam brightness can be enhanced with smaller extraction aperture in plasma ion sources. Reducing aperture size to micrometer scale, however, decreases beam currents significantly since the extraction field cannot be penetrated into the plasma source. In this article, a novel extraction method based on bias electrode is proposed and demonstrated by observing the existence of high current mode with 100μm aperture diameter. Ion current with a positively biased electrode is increased by an order of magnitude from that of the nonbiased case. Locally converging electrons near positively biased electrode are thought to be the main cause of the high current mode.

MPDスラスタ内における電熱・電磁加速モード間の遷移に関する研究

It is widely known that a self-field MPD thruster changes from an electrothermal acceleration mode to an electromagnetic one as the discharge current is increased. Such transitional behavior is numerically studied with argon propellant using a two-dimensional TVD Lax-Friedrich scheme incorporating multiple ionization processes. As the discharge current J is increased from 4 to 10kA under a mass flow rate of 0.8g/s, a steep rise of the voltage, which indicates the transition, occurs between low and high current modes of operation around J=7kA, which is equivalent to the Alfven’s critical ionization current. The steep voltage rise is attributed to the increase in the work of the Lorentz force around the cathode. The radial distributions of the heavy particles and electrons are examined and compared with the past experimental results. Additionally, in the electromagnetic mode, the ratio of the ionization energy to the input power decreases with increasing input power, and the maximum ratio of the kinetic energy to the input power is 0.57. This high thrust efficiency can be attributed to the idealized model, in which the effects of the sheath as well as the heat loss to the electrodes are neglected.