Current Dip Research Articles

Pauli blockade in a few-hole PMOS double quantum dot limited by spin-orbit interaction

We report on the hole compact double quantum dots fabricated using a conventional CMOS technology. We provide the evidence of Pauli spin blockade in the few hole regime that is relevant to spin qubit implementations. A current dip is observed around zero magnetic field, in agreement with the expected behavior for the case of strong spin-orbit. We deduce an intradot spin relaxation rate ≈120 kHz for the first holes, an important step towards a robust hole spin-orbit qubit.

Gas storage and controlling factors in an over-mature marine shale: A case study of the Lower Cambrian Lujiaping shale in the Dabashan arc-like thrust–fold belt, southwestern China

Lower Cambrian units in the upper Yangtze area of China are important strata for shale gas exploration. The Lower Cambrian Lujiaping shale in the Dabashan arc-like thrust–fold belt of the upper Yangtze area is thermally over-mature and has been transformed to slate by epimetamorphism. The Lujiaping shale has also experienced strong structural deformation; its current stratigraphic dip is 35°–80°. Over-maturity and strong deformation are considered unfavorable conditions for gas storage. However, desorption tests using canisters indicate that gas contents in the Lujiaping shale range from 0.05 to 3.39 m3/t (average, 1.12 m3/t); this level of gas presents challenges for conventional theories of shale gas enrichment. To analyze the controls on gas enrichment, we performed petrological, geochemical, and petrophysical experiments on shale samples from the Lujiaping Formation. Results show that the TOC contents are between 1.17% and 6.91%. The porosities of the shales are very low, in the range of 0.68%–3.93%, possibly related to over-maturity. The correlations between petrophysical parameters and the total organic carbon (TOC) content indicate that the organic fraction contribution to micropores and mesopores is highly significant. The Langmuir volumes for the studied shale samples are in the range of 1.07–5.87 m3/t. The gas content and methane adsorption capacity are strongly dependent on the TOC content. Because the studied shales have low porosities and are from shallow depths, their capacity for free gas storage is extremely limited, and the gas is stored mainly in an adsorbed state. The studied shales occur at temperatures of 20–37 °C. The maximum sorption methane capacity occurs at depths of 500–1500 m. Well-developed micropores, high surface area, and strong gas adsorption capacity are important controls on gas enrichment in the Lujiaping shales. The three controls may be related to the inter-particle pores between the organic matter and clay minerals.

Measurements of fast electron beams and soft X-ray emission from plasma-focus experiments

Abstract The paper reports results of the recent experimental studies of pulsed electron beams and soft X-rays in plasma-focus (PF) experiments carried out within a modified PF-360U facility at the NCBJ, Poland. Particular attention was focused on time-resolved measurements of the fast electron beams by means of two different magnetic analyzers, which could record electrons of energy ranging from about 41 keV to about 715 keV in several (6 or 8) measuring channels. For discharges performed with the pure deuterium filling, many strong electron signals were recorded in all the measuring channels. Those signals were well correlated with the first hard X-ray pulse detected by an external scintillation neutron-counter. In some of the analyzer channels, electron spikes (lasting about dozens of nanoseconds) and appearing in different instants after the current peculiarity (so-called current dip) were also recorded. For several discharges, fast ion beams, which were emitted along the z-axis and recorded with nuclear track detectors, were also investigated. Those measurements confirmed a multibeam character of the ion emission. The time-integrated soft X-ray images, which were taken side-on by means of a pinhole camera and sensitive X-ray films, showed the appearance of some filamentary structures and so-called hot spots. The application of small amounts of admixtures of different heavy noble gases, i.e. of argon (4.8% volumetric), krypton (1.6% volumetric), or xenon (0.8% volumetric), decreased intensity of the recorded electron beams, but increased intensity of the soft X-ray emission and showed more distinct and numerous hot spots. The recorded electron spikes have been explained as signals produced by quasi-mono-energetic microbeams emitted from tiny sources (probably plasma diodes), which can be formed near the observed hot spots.

The study of pinch regimes based on radiation-enhanced compression and anomalous resistivity phenomena and their effects on hard x-ray emission in a Mather type dense plasma focus device (SABALAN2)

In this study, by using argon and nitrogen as the filling gases in a Mather type dense plasma focus device at different values of pressure and charging voltage, two different kinds of pinch regimes were observed for each of the gases. The physics of the pinch regimes could be explained by using the two versions of the Lee's computational model which predicted each of the scenarios and clarified their differences between the two gases according to the radiation-enhanced compression and, additionally, predicted the pinch regimes through the anomalous resistivity effect during the pinch time. This was accomplished through the fitting process (simulation) on the current signal. Moreover, the characteristic amplitude and time scales of the anomalous resistances were obtained. The correlations between the features of the plasma current dip and the emitted hard x-ray pulses were observed. The starting time, intensity, duration, and the multiple or single feature of the emitted hard x-ray strongly correlated to the same respective features of the current dip.

Exchange-Induced Spin Blockade in a Two-Electron Double Quantum Dot.

We have experimentally identified the exchange-induced spin blockade in a GaAs double quantum dot. The transport is suppressed only when the eigenstates are well-defined singlet and triplet states, and thus sensitive to dynamic nuclear-spin polarization that causes singlet-triplet mixing. This gives rise to unusual current spectra, such as a sharp current dip and an asymmetric current profile near the triplet resonance of a double quantum dot. Numerical simulations suggest that the current dip is a signature of identical nuclear-spin polarization in the two dots, which is attractive for coherent spin manipulations in a material with nuclear spins.

Plasma Kinetics in Deuterium-Filled Plasma Focus with Step Anode Configuration

A numerical analysis of a plasma focus device with step anode as the central electrode is carried out with Lee Model Code for a stepped central electrode configuration. The snowplow model is employed to describe the dynamics of current sheath between the electrodes using momentum conservation of the swept gases. The discharge current and the tube voltage are determined by circuit equation. Different plasma parameters including plasma temperature and speed of current sheath are simulated for the step anode configuration. The step anode model is compared with the standard electrode configuration to observe the dynamical behaviour of the plasma during the axial and radial phase. The results indicate that the current dip and the spike voltage are slightly higher and an increase in the speed of current sheath at both axial and radial phases are observed. The step anode has shown peak current of 0.4% less than the cylindrical anode. However, the current sheath speed is enhanced at both axial and radial phases up to 20.1% for step anode configuration. The step anode configuration is also contributed to rise the plasma pinch temperature. The new configured anode in Lee's model can carry a new tube parameter for optimizing the plasma focus device for SXR yield and neutron yield based on numerical experiment.

Simulation of electrical discharge in a 3.6 Joule miniature plasma focus device using SIMULINK

A novel technique has been developed and studied in this paper to simulate the electrical discharge circuit of a 3.6 J miniature plasma focus device (PFD) and investigate the effect of inductance variation on voltage spike and current dip. The technique is based on a correlation between the electrical discharge circuit and plasma dynamics in a very small PFD that operates at the energy of 3.6 J. The simulation inputs include the charging voltage, capacitor bank capacitance, current limiter resistance, by-pass resistance as well as the time-dependent inductance and resistance of the plasma sheath which are calculated by assuming the plasma dynamics as transit times in going from one phase to the next. The variations of the most important elements in the circuit (i.e. the constant and breakdown inductances) and their effects on the current dip are studied in PFDs with low and high constant inductance. The model demonstrated for achieving a good pinch in the PFD, although the total inductance of the system should be low; however there is always an optimum inductance which causes an appropriate pinch. Furthermore, the electrical power produced by the pulsed power supply, the mechanical energy as well as the magnetic energy which are transferred into the plasma tube were obtained from simulation. The graph of electrical power demonstrated a high instantaneous increment in the power transferred into the plasma as one of the greatest advantages of the pulsed power supply. The simulation was performed using software tools within the MATLAB/SIMULINK simulation environment.

Effects of geometry and linearly polarized cavity photons on charge and spin currents in a quantum ring with spin-orbit interactions

We calculate the persistent spin current inside a quantum ring as a function of the strength of the Rashba or Dresselhaus spin-orbit interaction. We provide analytical results for the spin current of a one-dimensional (1D) ring of non-interacting electrons for comparison. Furthermore, we calculate the time evolution in the transient regime of a two-dimensional (2D) quantum ring connected to electrically biased semi-infinite leads using a time-convolutionless non-Markovian generalized master equation. In the latter case, the electrons are correlated via the Coulomb interaction and the ring can be embedded in a photon cavity with a single mode of linearly polarized photon field. The electron-electron and electron-photon interactions are described by exact numerical diagonalization. The photon field can be polarized perpendicular or parallel to the charge transport. We find a pronounced charge current dip associated with many-electron level crossings at the Aharonov-Casher phase $\Delta\Phi=\pi$, which can be disguised by linearly polarized light. Qualitative agreement is found for the spin currents of the 1D and 2D ring. Quantatively, however, the spin currents are weaker in the more realistic 2D ring, especially for weak spin-orbit interaction, but can be considerably enhanced with the aid of a linearly polarized electromagnetic field. Specific spin current symmetries relating the Dresselhaus spin-orbit interaction case to the Rashba one are found to hold for the 2D ring in the photon cavity.

Impact of a circularly polarized cavity photon field on the charge and spin flow through an Aharonov–Casher ring

We explore the influence of a circularly polarized cavity photon field on the transport properties of a finite-width ring, in which the electrons are subject to spin–orbit and Coulomb interaction. The quantum ring is embedded in an electromagnetic cavity and described by “exact” numerical diagonalization. We study the case that the cavity photon field is circularly polarized and compare it to the linearly polarized case. The quantum device is moreover coupled to external, electrically biased leads. The time propagation in the transient regime is described by a non-Markovian generalized master equation. We find that the spin polarization and the spin photocurrents of the quantum ring are largest for circularly polarized photon field and destructive Aharonov–Casher (AC) phase interference. The charge current suppression dip due to the destructive AC phase becomes threefold under the circularly polarized photon field as the interaction of the electrons׳ angular momentum and spin angular momentum of light causes many-body level splitting leading to three many-body level crossing locations instead of one. The circular charge current inside the ring, which is induced by the circularly polarized photon field, is found to be suppressed in a much wider range around the destructive AC phase than the lead-device-lead charge current. The charge current can be directed through one of the two ring arms with the help of the circularly polarized photon field, but is superimposed by vortices of smaller scale. Unlike the charge photocurrent, the flow direction of the spin photocurrent is found to be independent of the handedness of the circularly polarized photon field.

Relationship between previous history ofStreptococcus uberisinfection and response to a challenge model

Streptococcus uberis is the most common cause of clinical mastitis at calving in pasture-based dairy cows. Results of experimental inoculations were compared with cows' previous history of infection to help define a model for susceptibility to Str. uberis mastitis. Cows used had either no apparent history of intramammary infection (IMI) by Str. uberis or other major mastitis pathogens throughout their productive lifetime ('apparently uninfected'; AUI), or had a confirmed history of Str. uberis IMI ('historically infected'; HI). Cows were exposed to Str. uberis in sequential steps: dipping of the teat end (DIP; n=53 cows); a teat canal inoculation (TCI; n=33 cows); and, finally, intramammary inoculation challenge (IC; n=7 cows). Only cows that remained free of infection at each step progressed to the next phase. Infection rates were similar between AUI or HI cows following the DIP (9 and 17% respectively), or the TCI (75 and 68% respectively). Physical and biochemical traits of cows were examined. Analysis of traits prior to inoculations implied that HI cows produced more milk fat, while AUI cows tended to have longer teat canals. Analysis of traits for cows that became infected following DIP, implied that there was a positive association with milk fat production and negative association with somatic cell count (SCC), while there was a positive association with the duration of p.m. milking, and negative association with SCC in those cows that became infected following TCI. Only AUI cows became infected following the IC inoculation. Similarity in response to experimental inoculation between the two groups suggests that the current dip or teat canal inoculation (using a 3-mm depth of inoculation) models are not good predictors of natural resistance to Str. uberis. However, a population of cows was identified that remained uninfected after DIP, TCI and IC, and may comprise a resistant phenotype.

Power angle control of grid‐connected doubly fed induction generator wind turbines for fault ride‐through

This study proposes a power angle control strategy of grid-connected doubly fed induction generators (DFIGs) to improve the transient performance during grid fault ride-through (FRT). The proposed power angle control can efficiently restrict the rotor-side current under a limited converter rating, support the terminal voltage of DFIGs and hence improve system stability during the fault period. In this study, the power angle of DFIGs in a multi-machine power system is defined firstly and then the relation between the defined power angle and rotor-side current surge and terminal voltage dip of DFIGs are analysed, respectively. Based on the analysis, control of DFIG power angle is implemented expediently on the existing flux magnitude and angle control (FMAC) scheme of DFIGs as an improved FMAC scheme. In this study, two test examples are presented to demonstrate and validate the effectiveness of improved FMAC with power angle control and to compare it with the conventional PQ dq control and FMAC. Simulation results show that it can significantly enhance the FRT capability of grid-connected DFIGs.

Fault Current Limitation (FCL) and Voltage Dip Improvement Thanks to Distributed Static Series Compensator (DSSC)

This paper presents the comprehensive implementation of Distributed Static Series Compensator (DSSC) to limit the fault currents in power systems. This is the first time that the limitation of fault currents with D-FACTS devices is addressed. DSSC is one of the D-FACTS families whichoperate in a similar manner as Static Synchronous Series Compensator (SSSC) but in smaller size, lower price and more capability. The effectiveness of the DSSC in fault current limitation is investigated through the series voltage effect upon the line. The short circuit current limitation strategy presented here exhibited that besides of the power flow control which is carried out by DSSC; it can also perform this additional function. In the following the potency of the DSSC in reduction of instantaneous voltage dip range during fault current limiting mode is clarified. Furthermore, it is disclosed that with performing more DSSC in the power system, the entire system voltage dip will be improved. In order to validate the claims, computer simulations using PSCAD/EMTDC are exploited.

Current-Step Technique to Enhance Plasma Focus Compression and Neutron Yield

A current-step technique is applied to the plasma focus by modifying the Lee Model code, incorporating a current-step bank to add current to the focus pinch at the time of the current dip. For a 50 kV, 1 MJ, 6 μs rise-time bank, the current-step from a 200 kV, 0.4 MJ, 0.8 μs rise-time bank maintains the pinch current at 2.2 MA, enhances compression by 1.9 and increases the neutron yield from 2.5 × 1012 to 1.03 × 1013. The increase is attributed mainly to the step nature of the current which favorably shifts the end-point of compression; rather than to the scaling in terms of energy or current.

Emission Characteristics of a High-energy Pulsed-ion-beam Produced in a Dense Plasma Focus Device

The characteristics of ion beams produced in a Mather-type plasma focus device have been studied. When a capacitor bank of 41.6 μF was charged to 30 kV, the discharge current reached a peak current of 450 kA, and after the current dip, an ion current density of 8 kA/cm with a pulse width of 60 ns was obtained 110 mm downstream from the top of the anode on the axis. In order to evaluate the angular distribution of the ion current density and the ion energy spectrum, we arrayed the biased ion collectors and the incident angle resolved energy spectrometers simultaneously at nine different angular positions (−80◦, −60◦, −40◦, −20◦, 0◦, 30◦, 50◦, 70◦, and 90◦) on a circumference of 110 mm in radius, centered at the focus. From measurements with the BIC, we found that the angular distribution of the ion current density was axially symmetric and that the ion current density decreased with increasing emission angle. Experimental results for the energy spectrum showed that the proton beam energy was distributed from 0.1 MeV to 1.4 MeV and that ions with energies lower than 300 keV were distributed widely over emission angles ranging from −80◦ to 90◦, with higher-energy ions (>1 MeV) being confined to emission angles of −20◦ ∼ 30◦. In addition, the ion energy and the track density were found to decrease rapidly with increasing emission angle.

Measurement and Processing of Fast Pulsed Discharge Current in Plasma Focus Machines

The fast pulsed electric discharge current drives all physical processes in the plasma focus device; in turn all physical processes in the focus affect the current waveform. Thus the discharge current waveform is the most important indicator of plasma focus performance. This underlies the importance of properly measuring, processing and interpreting the discharge current waveform. This paper reports the measurement of fast pulsed discharge current by the Rogowski coil, in two different modes: the current transformer, “I” mode, and current derivative, “Idot” mode. The processing and interpretation of recorded current waveform to obtain useful information about the physical processes in the plasma focus device are discussed. The current transformer with a large number of turns and a sub-1 Ohm terminator has good high frequency response, necessary for the sharp current dip region when dI/dt exceeds 2 × 1011 A/s. However the signal is “noisy” in the current dip region. Several methods to extract the current dip from the noise are discussed and examples of how low pass filters affect the signals are shown. The dI/dt coil, the Rogowski coil in “Idot” mode, with a few turns terminated by 50-Ohm is also described. Integrating the 1 GSa/s digital waveform does remove the high frequency noise components, yet the extracted waveform shows sharp angular features indicative of the retention of short-time features. This makes the dI/dt coil superior to the current transformer. A 7-turn coil is tested against the Lee Model code and found to be suitable to measure the plasma focus discharge current.

Dynamical nuclear spin polarization induced by electronic current through double quantum dots

We analyse electron-spin relaxation in electronic transport through coherently coupled double quantum dots (DQDs) in the spin blockade regime. In particular, we focus on hyperfine (HF) interaction as the spin-relaxation mechanism. We pay special attention to the effect of the dynamical nuclear spin polarization induced by the electronic current on the nuclear environment. We discuss the behaviour of the electronic current and the induced nuclear spin polarization versus an external magnetic field for different HF coupling intensities and interdot tunnelling strengths. We take into account, for each magnetic field, all HF-mediated spin-relaxation processes coming from different opposite spin level approaches. We find that the current as a function of the external magnetic field shows a peak or a dip and that the transition from a current dip to a current peak behaviour is obtained by decreasing the HF coupling or by increasing the interdot tunnelling strength. We give a physical picture in terms of the interplay between the electrons tunnelling out of the DQD and the spin-flip processes due to the nuclear environment.

Beam stability improvement of the HIMAC synchrotron using a feed-forward system for magnet power supplies

In order to realize a precise dose distribution in heavy-ion cancer therapy, high beam stability is required for the accelerator complex. Owing to load fluctuation caused by the upper ring, which is one of the two rings in HIMAC, current dips of ≈5–10 Hz were observed in the power supply for the bending/quadrupole magnet of the other lower ring. The parameters of the beam stability, such as the spill variation, the beam position, and the size, were adversely affected by the current dips. In order to suppress these current dips, we developed a new feed-forward system in the magnet power supply. We verified the performance of the feed-forward system by measuring the suppression of the current dips. We also performed beam experiments to measure the variation of the horizontal tune and the structure of the beam spill, which is slowly extracted by the resonance method. The experimental result showed that the current dips were successfully reduced by the system to Δ I/ I ∼ 10 −6. It was also confirmed that the horizontal tune and the spill structure could be stabilized by the current dip suppression.

Simulation of Plasma Focus Devices with Hemisphere Electrodes

A magneto-hydrodynamic simulation of a plasma focus device with hemisphere electrodes is constructed. The snowplow model is used with help of the momentum conservation equation to describe the motion of the plasma sheath between the two concentric hemispheres. The model simulates various plasma parameters like plasma temperature and plasma sheath velocity. The circuit equation is used to calculate the discharge current and electrodes voltage across the two hemisphere terminals. A comparison between the cylindrical and spherical devices is built. The results show that the current dip and the spike voltage is expected to be much pronounced in the spherical devices. It is found also that the plasma sheath velocity and temperature in the case of the cylindrical system are higher than that in the spherical one.

Effect of a cathodic knife edge on the PFMA-1 pinch performance

This paper presents some experimental results obtained with the plasma focus PFMA-1 operated at about 20.5 kV, corresponding to a total capacitor bank energy of about 70 kJ, with 4He as working gas and pressure ranging between 50 and 3000 Pa. The aim of the study is the assessment of the differences in pinch performance with and without the adoption of a cathodic knife edge with the role of improving the electrical breakdown and the formation of the initial plasma layer. The pinch performance is estimated with the introduction of a voltage quality factor, which is related to the overvoltage intensity during the pinch, and of a current dip factor, correlated with the sudden total current reduction. More than 1500 shots have been analyzed with a semiautomated numerical special procedure. It is found that the improvement in the pinch efficiency with the use of a cathodic knife edge in these experiments can be even higher than a factor of 2.

Response to “Comment on ‘Pinch current limitation effect in plasma focus’” [Appl. Phys. Lett. 94, 076101 (2009)

The main point of the comment [Appl. Phys. Lett. 94, 076101 (2009)] is that Eq. (2) and consequentially Eq. (3) of the commented paper [Appl. Phys. Lett. 92, 021503 (2008)] require correction. The alternative equation suggested in the comment is derived using Kirchhoff’s voltage rule. The comment consider only the energy distribution in the inductive components and the resultant equation confirms a progressive lowering of the Ipinch/Ipeak ratio as the static inductance L0 is reduced, lowering from 0.87 to 0.31 as L0 is reduced from 100 to 5 nH according to the revised formula corresponding to Eq. (3), compared to 0.63–0.25 according to Eq. (3). This progressive lowering of the ratio Ipinch/Ipeak due to the inductive energy distribution is one of two factors responsible for the pinch current limitation. The other factor is the progressive reduction in the L-C interaction time compared to the current dip duration denoted by δcap in Eq. (2). The comment does not deal with δcap at all; hence, its conclusion based on inductive energy distribution only is not useful, since in the low L0 region when pinch current limitation begins to manifest, δcap becomes more and more the dominant factor. In any case, the results of the paper do not depend on Eqs. (2) and (3), which are used in the paper only for illustrative purposes.