Ultrashort electron beam and volume high-current discharge in air under the atmospheric pressure

Conditions are studied under which an electron beam and a volume discharge with a subnanosecond rise time of a voltage pulse are produced in air under atmospheric pressure. It is shown that the electron beam appears in a gas-filled diode at the front of the voltage pulse in ∼0.5 ns, has a half-intensity duration of ≤0.4 ns and an average electron energy of ∼0.6 of the voltage across the gas-filled diode, and terminates when the voltage across the gap reaches its maximum value. The electron beam with an average electron energy of 60 to 80 keV and a current amplitude of ≥70 A is obtained. It is assumed that the electron beam is formed from electrons produced in the gap due to gas ionization by fast electrons when the intensity of the field between the front of the expanding plasma cloud and the anode reaches its critical value. A nanosecond volume discharge with a specific power input of ≥400 MW/cm³, a density of the discharge current at the anode of up to 3 kA/cm², and specific energy deposition of ∼1 J/cm³ over 3 to 5 ns is created.     

On the mechanism of subnanosecond electron beam formation in gas-filled diodes

Subnanosecond electron beams formed in diodes filled in with a gas at atmospheric pressure and X-rays emitted from nanosecond-discharge plasmas are studied. Both phenomena hold promise for lasing technology. A three-group separation of fast electrons in a gas-filled diode is proposed. It is found that the duration of the beam current in a diode filled with air at atmospheric pressure does not exceed 0.1 ns. It is also shown that the amplitude of the beam current attains maximum with a certain delay after the application of voltage to the discharge gap. A current of ～400 A is detected behind the foil of a diode filled with air at atmospheric pressure. At a subnanosecond duration of the voltage pulse and the diffuse discharge, X-ray radiation is observed from the brightly glowing area of corona discharge. The mean steady-state velocities and energies of fast electrons in nitrogen are calculated. Head-on collisions are shown to control the constancy of the mean velocity of fast electrons for the field strengths E/p < 170 kV/(cm atm). At E/p > 170 kV/(cm atm), the escape of fast electrons takes place. It is particularly the head-on collisions that are decided to be responsible for the emission of X-rays from the bulk.     

Effect of insulating films on the current of an electron beam from gas-filled diodes exposed to voltage pulses with a nanosecond rise time

The parameters of subnanosecond electron beams generated in gas-filled diodes are studied. When the voltage pulse rise time is short (≈1 ns or less) and the electron beam is extracted from the diode through insulating films, the amplitude of the signal measured at the collector depends not on the electron beam current but on the electromagnetic radiation arising in the diode and the capacitive current from the collector. If the electron beam is extracted through thin metallic foils and fine metallic grids, the FWHM of the runaway electron pulsed beam is ≈0.1 ns and its amplitude reaches several hundreds of amperes.     

Two-photon correlations of luminescence at the Bose-Einstein condensation of dipolar excitons

Correlations of the luminescence intensity (the second-order correlation function g ⁽²⁾(τ)), where τ is the delay time between the photons detected in pairs) under the conditions of the Bose-Einstein condensation (BEC) of dipolar excitons has been studied in a temperature range of 0.45–4.2 K. Photoexcited dipolar excitons have been accumulated in a lateral trap in a GaAs/AlGaAs Schottky diode with a 25-nm wide single quantum well with an electric bias applied across the heterolayers. Two-photon correlations have been measured with the use of a two-beam intensity interferometer with a time resolution of }~0.4 ns according to the well-known classical Hanbury-Brown-Twiss scheme. The photon bunching has been observed at the onset of Bose-Einstein condensation manifested by the appearance of a narrow exciton condensate line in the luminescence spectrum at an increase in the optical pumping (the line width near the threshold is ?200 μeV). At the same time, the two-photon correlation function itself obeys the super-Poisson distribution, g ⁽²⁾(τ) > 1, at time scale τ_c ? 1 ns of the system coherence. The photon bunching is absent at a pumping level substantially below the condensation threshold. The effect of bunching also decreases at pumping significantly above the threshold, when the narrow exciton condensate line starts to dominate in the luminescence spectra, and finally disappears with the further increase in the optical excitation. In this region, the distribution of pair photon correlations is a Poisson distribution manifesting the united quantum coherent state of the exciton condensate. Under the same conditions, the first-order spatial correlation function g ⁽¹⁾(r) determined from the interference pattern of the luminescence signals from the spatially separated parts of the condensate at constant pumping remains noticeable at distances of no less than 4 μm. The discovered effect of photon bunching is very sensitive to temperature and decreases by several times with a temperature increase in the range of 0.45–4.2 K. Assuming that the luminescence of the dipolar excitons directly reflects the coherence properties of the gas of interacting excitons, the discovered photon bunching at the onset of condensation, where the fluctuations of the exciton density and, consequently, of the luminescence intensity are most significant, indicates a phase transition in the interacting Bose gas of excitons, which is an independent way of detecting the Bose-Einstein condensation of excitons.     

Circular ion diode with self-magnetic insulation

The results of a study of the generation of a gigawatt-level pulsed ion beam formed by a diode with an explosive-emission potential electrode in self-magnetic insulation mode are presented. The experiments have been performed on the TEMP-4M ion accelerator operating in double-pulse formation mode: the first pulse is negative polarity (300–500 ns, 100–150 kV) and the second is positive (150 ns, 250–300 kV). The ion current density is 20–40 A/cm²; the beam consists of protons and carbon ions. To increase the efficiency of the ion current generation, a circular geometry diode is proposed. It is shown that with the new design, the plasma is effectively formed over the entire working surface of the graphite potential electrode. During ion beam generation, magnetic insulation of the electrons is achieved over the entire length of the diode (B/B _cr ≥ 3). Because of the high drift velocity, the transit time of electrons in the anode-cathode gap is 3–5 ns, whilst the transit time of C⁺ carbon ions exceeds 8 ns. This indicates low efficiency self-magnetic insulation for this geometry of diode. At the same time, it has been observed experimentally that during ion current generation (the second pulse), the electron component of the total current is suppressed by a factor of 4–5. A new mechanism of limiting the electron emission, which explains the decrease in the electron component of the total current in the circular diode with self-magnetic insulation, is proposed.     

快速关断半导体开关工作特性及实验研究北大核心CSCD

介绍了快速关断半导体开关(DSRD)的工作原理,研究了开关内部的物理过程,分析了系统参数对开关输出特性的影响,研究发现:基区材料的击穿阈值越高、载流子饱和漂移速度越大输出电压上升速率越快;基区高的电场击穿阈值或低的掺杂浓度会增加器件关断时间和最大工作电压;考虑各参数的影响,基于高击穿阈值的DSRD是实现快脉冲输出的理想器件;缩短正向泵浦时间可有效抑制预脉冲,当正向泵浦时间小于200 ns时,输出脉冲波形基本不变;为了获得理想的脉冲前沿,反向电流应在达到峰值时完成对注入电荷的抽取。设计了单前级开关的DSRD泵浦电路,研制了基于DSRD的快脉冲产生系统,输出脉冲前沿约4 ns,电压约8 kV,电压上升速率约2 kV/ns,满足FID开关器件对触发电压的要求。     

Edge termination study and fabrication of a 4Hben SiC junction barrier Schottky diode

The 4H-SiC junction barrier Schottky (JBS) diodes terminated by field guard rings and offset field plate are designed, fabricated and characterized. It is shown experimentally that a 3-μm P-type implantation window spacing gives an optimum trade-off between forward drop voltage and leakage current density for these diodes, yielding a specific on-resistance of 8.3 mΩ·cm². A JBS diode with a turn-on voltage of 0.65 V and a reverse current density less than 1 A/cm² under 500 V is fabricated, and the reverse recovery time is tested to be 80 ns, and the peak reverse current is 28.1 mA. Temperature-dependent characteristics are also studied in a temperature range of 75 ℃-200 ℃. The diode shows a stable Schottky barrier height of up to 200 ℃ and a stable operation under a continuous forward current of 100 A/cm².     

Quasi-three-level neodymium vanadate laser operation under polarized diode pumping: theoretical and experimental investigation

We propose a novel technique for pumping neodymium vanadate crystal in ⁴ F _3/2 ?? ⁴ I _9/2 transition with polarized diode light. With a theoretical model on quasi-three-level neodymium vanadate lasers including excited state absorption and energy transfer upconversion effects, the improvement on the laser performance of polarized pumping is evaluated. A maximum output power of 4.8 W in Nd:GdVO₄ 912 nm laser is achieved with the incident pump power of 21.8 W, the maximum output power increases about 85% and the slope efficiency is enhanced to 1.5 times towards the unpolarized pumping under the same condition. This technique is especially suitable for quasi-three-level systems end pumped by high-brightness fiber coupled diode sources associated with short neodymium vanadate crystals.     

On a source of outgoing electrons in a pulsed gas discharge

It has been shown that explosive electron emission is delayed by 10^?10 s with respect to field emission in a pulsed subnanosecond discharge in atmospheric air. A pulse of outgoing electrons is observed for approximately the same time in air. Correspondingly, field emission is a source of these electrons. Owing to the sharp nonlinearity of the emission current density as a function of the electric field j(E), the real duration of the current pulse of the outgoing electrons is equal to about 10^?11 s.     

Electroforming of thin film silicon based homojunction pin diode

The recent observations of bright visible electroluminescence (EL) from electroformed thin film silicon based wide-gap alloys are further clamped down in a simpler structure. For this purpose, a standard quality, ordinary hydrogenated amorphous silicon (a-Si:H) homojunction pin diode was fabricated by plasma enhanced chemical vapor deposition. The fresh diode was characterized by temperature scanned current–voltage (I–V) and constant photocurrent measurements. The energy distribution of density of states within the forbidden gap of the intrinsic a-Si:H layer was determined by space charge limited current and optical absorption spectroscopies. Then the diode was intentionally subjected to a sufficiently high, calibrated electric field leading to its Joule heating assisted rapid crystallization at ambient atmosphere. The fresh and the formed diodes exhibit different I–V and EL characteristics. The current density of the formed diode increases drastically at low voltages while remaining unchanged at high voltages when compared to that of the fresh diode. Parallelly, the room temperature EL intensity under a particular current stress is boosted with electroforming. These interesting phenomena have been discussed in the frame of a self-consistent model.     

Luminescence properties of Li6GdB3O9:Ce crystal fibers upon their excitation in the range of 4d → 4f core transitions

We have investigated UV luminescence with a subnanosecond time resolution of Li₆GdB₃O₉:Ce crystal fibers upon their ultrasoft X-ray selective excitation at 10 and 293 K in the range of 4d → 4f core transitions. We have revealed an intense fast-decaying subnanosecond luminescence component, which is caused by a high local density of electronic excitation and Auger core hole relaxation processes, and modulation of the luminescence excitation spectrum by an absorption band of the 4d–4f photoionization giant resonance in the energy range 135–160 eV.     

高效高稳定高光束质量声光调Q绿光激光器的研究

通过优化双棒串接直腔结构设计,利用大功率LD侧面抽运、声光Q开关、Ⅱ类相位匹配S-KTP内腔倍频获得高效大功率绿色激光输出.当抽运电流为45 A、重复频率为15 kHz时,激光平均功率为132 W,光—光转换效率为132％,脉宽约为120 ns.在输出130 W时,测得1 h功率不稳定度小于05%,光束质量因子M²为67.对高功率抽运情况下激光介质的热透镜效应以及谐振腔稳定运转工作区域也进行了理论分析和实验研究. 关键词： 绿光激光器 腔内倍频 声光调Q LD侧面抽运     

Ultrafast strain-induced current in a GaAs Schottky diode

Picosecond acoustic pulses generated by femtosecond laser excitation of a metal film induce a transient current with subnanosecond rise time in a GaAs/Au Schottky diode. The signal consists of components due to the strain pulse crossing the edge of the depletion layer in the GaAs and also the GaAs/Au interface. A theoretical model is presented for the former and is shown to be in very good agreement with the experiment.     

Pulsed volume discharge with preionization in two-dimensional gasdynamic flow

The interaction between a pulsed volume discharge with preionization by ultraviolet radiation from plasma sheets and a gasdynamic flow with a known density distribution is studied experimentally. The complex quasi-two-dimensional flow that emerges after the diffraction of a plane shock wave by rectangular obstacles in the channel is experimentally studied and numerically simulated. The glow intensity fields for an unsteady gasdynamic flow are imaged for the first time when recording the plasma radiation from a pulsed discharge in the flow. Since the ionization duration is short (150–200 ns), the gas-flow structure does not change and the flow does not heat up in the glow time of the discharge plasma in the flow. Our images are compared with the reciprocal-density fields of the corresponding two-dimensional gas flow. The effects of gasdynamic structures on the discharge plasma redistribution in the flow are analyzed. The energy contribution is localized into low-density zones (vortices, rarefaction waves) and into regions of density jumps and significant density gradients. The discharge current from adjacent regions with low E/N is redistributed into these zones. Breakdown channels are formed along rarefaction waves, vortices, and discontinuity surfaces between high-electron-density regions.     

Theoretical and experimental studies on passively Q-switched KTiOAsO4 optical parametric oscillator

A passively Q-switched intracavity optical parametric oscillator based on KTiOAsO₄ (KTA) crystal is studied theoretically and experimentally. The rate-equation-based theoretical model is established to describe the time evolutions of the population inversion density of the laser crystal, ground-state population density of the saturable absorber, fundamental photon density, signal photon density and the idler photon density. In the experiment, a laser diode-end pumped, passively Q-switched Nd:YAG/KTA IOPO with a Cr⁴⁺:YAG crystal as the saturable absorber is realized to verify this model. The characteristics including the output power, the pulse repetition rate, the pulse width and the beam quality were investigated for this OPO. The experimental results for the output power and the repetition rate agree with the theory well. And both results show that with same pumping level the idler pulse width is shorter than the signal one.     

A Compact YAG/Nd:YAG/Cr:YAG Passively <Emphasis Type="Italic">Q</Emphasis>-Switched Pulse Burst Laser Pumped by 885 nm Laser Diode

We demonstrate a high-repetition-rate, short-pulse-width pulse burst laser from a compact 885 nm laser diode directly pumped by a passively Q-switched YAG/Nd:YAG/Cr:YAG laser. We investigate the output laser characteristics with different output transmissions and spot sizes of the pumping laser and compare these characteristics. After optimization, we achieve a shortest pulse width of 1.4 ns generated by a 1,064 nm pulse burst laser. The single-pulse energy reaches 239 μJ at 86.3 kHz, with a peak power of 117.2 kW.     

Current-voltage analysis of a-Si:H Schottky diodes

Direct current (dc)-voltage (I-V) characteristics of the hydrogenated amorphous silicon (a-Si:H) Schottky diode have been measured at different temperatures under dark and light. From the fourth quadrant of illuminated characteristics, fill factor (FF) values were obtained for each temperature measured (173-297 K). We have found that FF increases very little as the temperature is decreased. The measured data from I-V characteristics has been analyzed in detail. In particular, from dark I-V characteristics obtained, the density of state (DOS) near the Fermi level was determined using a simple model based on the space-charge limited current (SCLC). On the other hand, from the illuminated I-V characteristics, the density of carriers was calculated for each temperature using the analysis of diode equation as known. A comparison of the carrier density and the measured photocurrent as a function of the reverse temperature was also made and a good correspondence was obtained.     

An Electron-Beam Triggered Spark Gap

The triggering of a high-voltage gas-insulated spark gap by an electron (e) beam has been investigated. Rise times of approximately 2.5 ns with subnanosecond jitter (~0.2 ns) have been obtained for 3-cm gaps charged at voltages as low as 50 percent of the self-breakdown voltage (varied up to 0.5 MV). The switch delay (including the e-beam diode) was 52 ns. The triggering e-beam pulse has a duration of 15 ns and a 0-50 percent rise time of 1.5 ns. The e-beam current is 0.5 kA, and the electron energy can be varied in the range from 80 to 145 keV. The working media were N2, mixtures of N2 and A, and N2 and SF6 at pressures of 1-3 atm. Voltage, current, and jitter measurements have been made for a wide range of gap conditions and e-beam parameters. Variations in the character of the discharge have been inferred using streak and open shutter photography. The photographs show that the discharge has a broad cross section and that its character varies for differing polarites and voltages. The effects of varying the e-beam width and the beam energy are discussed.     

Abnormal polarity effect in nanosecond-pulse breakdown of SF6 and nitrogen

The breakdown of gas gaps in an inhomogeneous electric field at subnanosecond and nanosecond voltage pulse rise times are studied, and the famous polarity effect in point-to-plane gaps is investigated. It is shown that at a voltage pulse rise time of ∼0.5 ns, the inversion of polarity effect takes place not only in electronegative gases such as SF₆, but also occurs in electropositive nitrogen. The inversion of polarity effect is related to a delay of electron emission from the plane cathode on arrival of the ionization wave front anode to the cathode. It is found that with a voltage pulse rise time of ∼0.5 ns, the inversion of polarity effect occurs at SF₆ and SF₆–N₂ pressures of 0.25 MPa and lower, and with a voltage pulse rise time of 15 ns, at a SF₆ pressure lower than 0.12 MPa.