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Post-acceleration of protons in helical coil targets driven by intense, ultrashort laser pulses can enhance ion energy by utilizing the transient current from the targets' self-discharge. The acceleration length of protons can exceed a few millimeters, and the acceleration gradient is of the order of GeV/m. How to ensure the synchronization between the accelerating electric field and the protons is a crucial problem for efficient post-acceleration. In this paper, we study how the electric field mismatch induced by current dispersion affects the synchronous acceleration of protons. We propose a scheme using a two-stage helical coil to control the current dispersion. With optimized parameters, the energy gain of protons is increased by four times. Proton energy is expected to reach 45 MeV using a hundreds-of-terawatts laser, or more than 100 MeV using a petawatt laser, by controlling the current dispersion. [ABSTRACT FROM AUTHOR]

A comprehensive study of drain current dispersion effects in $\beta $ -Ga2O3 field-effect transistors (FETs) has been done using dc and pulsed measurements. Both the virtual gate effect in the gate–drain access region and mobile traps under the gate are the most plausible explanations for the experimentally observed pulsed current dispersion and high-temperature threshold voltage shift, respectively. Unpassivated devices show significant current dispersion between dc and pulsed ${I}$ – ${V}$ response in gate lag measurements characterized by time constants in the range of 400–690 $\mu \text{s}$ both at room temperature and higher temperatures. The increasing time constant with temperature suggests a complex electron capture and emission process in the virtual gate. The reactive ion etching step during the device fabrication is most likely responsible for introducing the traps. The effect of traps can be minimized by using surface passivation layers, in this case, silicon nitride, which shows significant improvement in the current dispersion. This work demonstrates the detrimental effect that the traps can have on the current dispersion, which significantly limits the high-frequency operation of the device. [ABSTRACT FROM AUTHOR]

The current distribution of the grounding electrode in a high-voltage direct current (HVDC) transmission system affects the state of power equipment in its vicinity, which depends on the soil resistivity and shape of the grounding electrode. In this paper, current distribution in the vicinity of an ±800 kV grounding electrode is investigated by simulation and experiments. Firstly, the model to calculate the current distribution with two typical frozen soils is set up, and simulation models and experimental platforms are established; meanwhile, the finite element method (FEM) is used to calculate the current and potential dispersion of linear, cross-shaped, and ring-shaped grounding electrodes in the simulation models. After obtaining the lab current data from the simulation, an innovative method based on a "drainage wire" with a Hall sensor is proposed to measure the current in an experimental setup. The results show that current and potential distribution characteristics are related to the shape of the grounding electrode and soil resistivity. Meanwhile, the current measurement scheme can measure the current in soil with a lower error. This article concludes that these two typical models can reduce the complexity of frozen soil analysis, and the measurement scheme can accurately monitor the current to reduce the damage to the surrounding power equipment. [ABSTRACT FROM AUTHOR]

The current distribution of the grounding electrode in a high-voltage direct current (HVDC) transmission system affects the state of power equipment in its vicinity, which depends on the soil resistivity and shape of the grounding electrode. In this paper, current distribution in the vicinity of an ±800 kV grounding electrode is investigated by simulation and experiments. Firstly, the model to calculate the current distribution with two typical frozen soils is set up, and simulation models and experimental platforms are established; meanwhile, the finite element method (FEM) is used to calculate the current and potential dispersion of linear, cross-shaped, and ring-shaped grounding electrodes in the simulation models. After obtaining the lab current data from the simulation, an innovative method based on a “drainage wire” with a Hall sensor is proposed to measure the current in an experimental setup. The results show that current and potential distribution characteristics are related to the shape of the grounding electrode and soil resistivity. Meanwhile, the current measurement scheme can measure the current in soil with a lower error. This article concludes that these two typical models can reduce the complexity of frozen soil analysis, and the measurement scheme can accurately monitor the current to reduce the damage to the surrounding power equipment.

V-gate GaN high-electron-mobility transistors (HEMTs) are fabricated and investigated systematically. A V-shaped recess geometry is obtained using an improved Si3N4 recess etching technology. Compared with standard HEMTs, the fabricated V-gate HEMTs exhibit a 17% higher peak extrinsic transconductance due to a narrowed gate foot. Moreover, both the gate leakage and current dispersion are dramatically suppressed simultaneously, although a slight degradation of frequency response is observed. Based on a two-dimensional electric field simulation using Silvaco "ATLAS" for both standard HEMTs and V-gate HEMTs, the relaxation in peak electric field at the gate edge is identified as the predominant factor leading to the superior performance of V-gate HEMTs. [ABSTRACT FROM AUTHOR]

• DC, output-admittance, transient characteristics of AlGaN/GaN HEMT (L G = 0.25 µm) are simulated. • The transient responses of another set of HEMTs with LG = 0.5 µm are simulated. • Surface donors at E C − 0.5 eV and buffer traps at E C − 0.47 eV are considered in TCAD model. • Device regions (surface and/or buffer) responsible for trapping induced deteriorations in HEMT characteristics are identified. This paper describes device simulation studies of surface and buffer trapping effects on static I-V , output-admittance (Y 22), and transient characteristics of AlGaN/GaN HEMTs. The TCAD simulation model considering surface donors at E C − 0.5 eV and buffer traps at E C − 0.47 eV have been used to quantitatively reproduce the measured DC, Y 22 frequency dispersion, gate-lag (GL) and drain-lag (DL) transients of AlGaN/GaN HEMT with 0.25 µm gate length. Moreover, simulated GL and DL transient responses of AlGaN/GaN HEMT with a longer gate length (0.5 µm) are validated with the reported experimental results. The impact of barrier trap at E C − 0.45 eV on the HEMT properties is also explored. It is shown that by matching simulation results with experimental data, it is possible to identify the trap (surface or buffer) responsible for a particular trapping induced degradation as well as its concentration and capture cross-section. [ABSTRACT FROM AUTHOR]