Long-lived explosive-emission cathode for high-power microwave radiation generators

The operation of cold explosive-emission cathodes having a current density of ∼10⁴ A/cm², fabricated using various materials, was investigated under a large number of switching cycles. The cathode voltage was ∼500 kV, the maximum current ∼5 kA, and the pulse duration ∼20 ns. It is shown that when the number of switchings is small (⩽10³ pulses), cathodes having similar geometry exhibit similar emission properties. For most of the materials studied, as the number of switching cycles increases (⩾10³ pulses), the current rise time increases (as far as the pulse duration) and the maximum vacuum diode current decreases. When a graphite cathode was used, the maximum current remained unchanged up to 10⁸ switching cycles. The mass removed from the cathode was determined for various materials. The results were used to achieve continuous operation of a relativistic 3 cm backward-wave tube having an output power of 350–400MW and an almost constant power level during 10⁸ pulses at a repetition frequency of 100–150 Hz. Pis’ma Zh. Tekh. Fiz. 25, 84–94 (November 26, 1999)     

Dense plasma formation on the surface of a ferroelectric cathode

An enhanced electron emission mode of the ferroelectric plasma cathode operation is reported. The enhanced emission is achieved due to the generation of dense plasma (10¹⁹-10²⁰ m⁻³). This plasma is formed by a flashover which is initiated by charged particles. These particles are attracted to the ferroelectric surface by a driving electric field and are released during its decay. Generation of an electron beam with current amplitude ?2.5 kA is demonstrated in a diode under an accelerating voltage of 150-300 kV and pulse duration of 300 ns.     

High-current-density subnanosecond electron beams formed in a gas-filled diode at low pressures

The formation of electron beams in a gas diode filled with various gases at low and medium pressures under the action of nanosecond voltage pulses has been studied. It is shown that subnanosecond pulses of the beam current in helium, hydrogen, neon, nitrogen, argon, methane, sulfur hexafluoride, krypton, and xenon can be obtained both at atmospheric pressure and at a pressure of several units or dozens of Torr. In particular, a beam current density above 2 kA/cm² behind the foil at a pulse duration (FWHM) of 250 ps has been obtained in helium-filled diode. On the passage from the regime of ultrashort avalanche electron beam formation to the vacuum diode regime, the beam current pulse amplitude decreases, while both the beam pulse duration (FWHM) and the pulse front width increase.     

Electron emission from La-Doped lead zirconate stannate titanate antiferroelectric ceramic under fast electric field pulses

High current density pulsed-electron emission is observed from a lead zirconate stannate titanate lanthanum-doped (PLZST) antiferroelectric ceramic disc on application of positive or negative triggering voltage pulses. Electron-emission pulse with a peak current density 1,400 A/cm² and a full-width at half-maximum (FWHM) duration of 560 ns was recorded in the presence of a 3.5 kV dc extraction voltage. It is higher than the various earlier results obtained using lead zirconate titanate ferroelectric ceramic. Self emission of electrons with a current density of 1.3 A/cm² and the FWHM duration of about 100 ns were also observed. Strong electrons emission was the co-effect of surface plasma and noncompensated charges at the surface of the antiferroelectric. Field-induced local phase transition in the vicinity close to triple junction results in primary electron emission from these areas. These primary emission electrons ignited surface plasma and then led to the strong emission.     

Binary semiconductor In2Te3 for the application of phase-change memory device

Nonvolatile phase-change memory devices with 500 nm contact hole based on In₂Te₃ were successfully fabricated by using focused ion beam, pulsed laser deposition, and dc magnetic sputtering techniques. In₂Te₃ films were characterized by using differential thermal analysis, X-ray diffraction, and UV–vis diffuse absorption spectroscopy, respectively. The devices can be switched between high and low resistance states repeatedly with the programmed voltage pulses. The reset operation (crystalline to amorphous) was done by the voltage pulse with a magnitude of 3.5 V and a duration of 30 ns, and the set operation (amorphous to crystalline) was done by the voltage pulse with a magnitude of 1.4 V and a duration of 100 ns. A dynamic resistance switching ratio (OFF/ON ratio) of 3.2 × 10³ has been obtained.     

High-current electron sources based on gaseous discharges

We review the operation of a ferroelectric plasma source (FPS) with enhanced plasma density and of high-current hollow cathode (HC) and hollow anode (HA) discharges. Different schemes (arc sources, magnetron and FPS) were used for ignition and sustaining the HC and HA discharges with current amplitude ?4 kA.These discharges are characterized by a positive anode potential with respect to the hollow electrode walls and the plasma density and temperature inside the hollow electrode cavity reach ∼3×10¹⁹m⁻³ and ?12 eV, respectively. It was shown that the incorporation of the FPS in the HA and HC enables one to develop a compact high-current electron source. The characteristics of an electron diode with FPS, HA and HC were studied under an accelerating voltage ?250 kV and ∼400 ns pulse duration. It was shown that these sources allow the generation of an electron beam with a cross-sectional area of 0.01 m² and amplitude up to 1-2 kA.     

Switching of pulses with a power of 500 MW and subnanosecond rise front based on open discharge

The switching characteristics of open discharge in the plane geometry that occurs in a cell with opposed electron beams are studied. Switching time of 400 ps at voltage of 20 kV is obtained in helium. Current amplitude of 28 kA at current rise rate of 3.7 × 10¹³ A/s is achieved. The mechanism of the transition of the switcher to the high-conductivity state, which is based on photoemission induced by the resonance radiation of fast atoms, is considered.     

Transient characteristics of reversely switched dynistors in the submicrosecond pulse range

Transient injection processes in reversely switched dynistors (RSDs) operating in a submicrosecond pulse range have been numerically simulated and the output current and voltage characteristics have been calculated. It is shown that, provided a proper choice of parameters of the silicon device structure and external chains, RSDs can be used to switch current pulses with amplitudes up to several dozen kA/cm², total durations of several hundred nanoseconds, and leading edge widths within 50–100 ns. The voltage decreases from the initial level by an order of magnitude already in 15–20 ns, and attains a steady value of 10–25 V during the leading edge time.     

Design of Power Supplies for the Pulsed High Magnetic Field Facility at HUST

Two types of pulsed power supply, a modular 12 MJ/25 kV capacitor bank and a 100 MVA flywheel pulsed generator, are under construction for the pulsed high magnetic field facility at the Huazhong University of Science and Technology (HUST) in Wuhan, China. The capacitor bank consists of 11 independent 1 MJ modules with a short circuit current of 40 kA each and 2 independent 0.5 MJ modules for 50 kA each. The bank is used to energize coils for magnetic fields in the 50–80 T range with pulse duration from 15 to 200 ms. The pulsed flywheel-alternator is used to energize a 50 T/100 ms long-pulse magnet via two 12-pulse power converter modules. Each converter module is designed to operate in the 95 to 66 Hz frequency operation range of the generator and can provide a no-load voltage of 4.6 kV and a full-load voltage of 3.4 kV at the rated current of 20 kA. In this paper the design of these two types of power supply is presented.     

Switching from negative to positive nonlinear absorption in p type 0.5 at% Sn doped GaSe semiconductor crystal

The nonlinear absorption properties of p type 0.5 at% Sn doped GaSe crystal have been studied by using open-aperture Z-scan technique under 1064 nm wavelength and 4 ns or 65 ps pulse duration. A switching from negative nonlinear absorption (saturated absorption) to positive nonlinear absorption (two photon absorption) has been observed as the input laser irradiance increases from 0.049 GW/cm² to 0.106 GW/cm². The nonlinear absorption coefficient increases monotonically with the increase of pulse duration from 65 ps to 4 ns.     

Switching high-voltage pulses with subnanosecond fronts by an open-discharge pulse shaper

A new principle of switching high-voltage pulses with leading edge widths below 1 ns has been studied. The pulse shaper is based on an open-discharge device. It is demonstrated that, in a coaxial geometry, a minimum switching time as short as ～0.65 ns can be achieved on an active load of R _L = 50 Ω at a voltage of U = 20 kV. The shaper is capable of operating at repetition frequencies up to 30 kHz.     

Development of a 300-kV Marx generator and its application to drive a relativistic electron beam

We have indigenously developed a twenty-stage vertical structure type Marx generator. At a matched load of 90–100 Ω, for 25 kV DC charging, an output voltage pulse of 230 kV, and duration 150 ns is obtained. This voltage pulse is applied to a relativistic electron beam (REB) planar diode. For a cathodeanode gap of 7.5 mm, an REB having beam voltage 160kV and duration 150ns is obtained. Brass as well as aluminum explosive electron emission-type cathodes have been used     

An electron source with a multiarc plasma emitter for obtaining submillisecond pulsed megawatt beams

An electron source with a plasma emitter based on an arc-discharge system with six cathodes and a common cylindrical hollow anode is described. Upon synchronous initiation of vacuum-arc discharges, the space of the hollow anode is filled by dense low-temperature plasma, the emission boundary of which is stabilized by a fine-structure metal grid with a 150-cm² area. The arc-current amplitude for each cathode amounts to 100–300 A. Under the action of a constant accelerating voltage applied between the plasma emitter and grounded accelerating electrode combined with the drift tube, electrons are extracted from plasma and accelerated. At a working pressure of 0.04 Pa, an electron beam with a maximum current amplitude of 1 kA has been obtained at an initial accelerating voltage of 80 kV and pulse duration (FWHM) of 100 μs, which has been transported in a longitudinal magnetic field of 0.035 T over a distance of 80 cm.     

Parameters of silicon carbide diode avalanche shapers for the picosecond range

Parameters of ultrafast avalanche switching of high-voltage diode structures based on 4H-SiC have been estimated theoretically. The calculation was carried out using the analytical theory of the impact ionization wave of the TRAPATT type, which makes it possible to determine the main characteristics of a wave for arbitrary dependences of the impact ionization coefficients and carrier drift velocity on electric field. It is shown that, for a high-voltage (1–10 kV) 4H-SiC structure, the time of switching from the blocking to the conducting state is ~10 ps, which is an order of magnitude shorter than that for a Si structure with the same stationary breakdown voltage, and the concentration of the electron-hole plasma created by the wave is two orders of magnitude higher. Picosecond switching times can be reached for 4H-SiC structures with a stationary breakdown voltage exceeding 10 kV.     

Plasma cathode for a broad-beam electron accelerator

We propose a grid-stabilized plasma cathode based on a slit-contracted low-pressure glow discharge with hollow anode. The area of the plasma cathode is one order of magnitude higher than that in systems where electrons are extracted immediately from plasma in the cathode cavity. Conditions for the discharge initiation, the current switching to the hollow anode, and the obtaining of uniform emission from the plasma cathode are determined. At an accelerating voltage of 160 kV, an electron beam with a 1000 × 180 mm cross section, a total current of several amperes, and a current pulse duration of up to 10⁻³ s was obtained. The plasma cathode operates under technical vacuum conditions (air, 0.1 Pa) and ensures high stability and reproducibility of the beam current pulses.     

Fatigue,retention and switching properties of PLZT(x/30/70) thin films with various La concentrations

We investigated the fatigue, retention and switching properties of PLZT(x/30/70) thin films with various La concentrations. By applying 10⁹ square pulse switching cycles with a voltage of ±5 V to study the fatigue properties of the film, we found that the decrease of the initial polarization is improved from 64% to 40% as the La concentration is increased from 0 mol% to 10 mol%. The retention properties are also greatly improved as the decrease of the initial polarization decrease is reduced from 47% to 9% after 10⁵ s. The switching time is decreased from 0.8 μs to 0.55 μs as the La concentration is increased. While the dielectric constant of the PLZT thin films increases from 450 to 600 as the La concentration is increased, the dielectric loss and leakage current density measured at 100 kV/cm decrease from 0.075 to 0.025 and from 5.83 × 10⁻⁷ to 1.38 × 10⁻⁷ A/cm², respectively. By analyzing the hysteresis loops of the PLZT thin film measured at 175 kV/cm, we found that the remnant polarization and coercive electric field decrease from 20.8 μC/cm² to 10.5 μC/cm² and from 54.48 kV/cm to 32.12 kV/cm, respectively, as the La concentration is increased.     

Annular structures formed in a beam of ions during their collective acceleration in a system with dielectric anode

We have studied the collective acceleration of protons and deuterons in an electron beam emitted from plasma formed at the surface of a dielectric anode insert. The experiments were performed with a pulsed electron accelerator operating at an accelerating voltage up to 1 MV, current amplitude up to 40 kA, and pulse duration of 50 ns. Reduction of the accelerating voltage pulse front width and optimization of the diode unit and drift region ensured the formation of several annular structures in the electron beam. As a result, up to 50% of the radioactivity induced in a copper target was concentrated in a ring with 4.5-cm diameter and 0.2-cm width. The formation of high energy density in these circular traces and the appearance of an axial component of the self-generated magnetic field of the electron beam are related with the increasing efficiency of acceleration of the most intense group of ions.     

Wide-aperture plasma jet source based on low-voltage spark discharge with ferroelectric electrode

Low-voltage vacuum spark discharge initiated at a storage capacitor voltage of 75–600 V using a metal grid cathode situated on the front surface of a polarized ferroelectric (FE) electrode has been experimentally studied. The discharge was initiated when a control voltage pulse with an amplitude of 1 kV and a duration of 100 ns at only negative polarity was applied to the rear FE surface (irrespective of the direction of its polarization vector). Optical measurements showed that the emitting surface area on the cathode increases approximately in proportion to the discharge voltage. According to the collector measurements, the ion plasma flux has slow and fast components, the velocities of which remain almost constant when the discharge current amplitude varies in a wide interval.     

High-current electron beam in a hollow-cathode gas discharge at elevated pressures (∼100 Torr)

A discharge with plasma filling a flat-bottom cavity of depth δ in the cathode, partly closed by a dielectric plate with a hole (determining the aperture of the discharge between the cavity bottom and the anode), has been studied. In a discharge cell of type 1 with δ = 0.5 mm and a hole diameter of 22 mm, a pulsed electron beam was obtained with a duration of t _EB = 700 ns and a beam current j _EB approximately 10 times greater than that (j _AD) of the equivalent anomalous discharge (at fixed discharge voltage U and gas pressure p _He = 3.5 Torr). An electric field with the direction opposite to the field of applied voltage appeared at the cathode that was related to a space charge formed at the cathode plasma boundary, which could not follow a rapid drop of voltage across the discharge gap. In a discharge cell of type 2 with δ = 0.5 mm and a narrow slit (S = 0.1 × 5 cm²) in the dielectric plate, a pulsed electron beam was obtained with a duration of t _EB = 2 ns and a beam current of j _EB = 0.7 kA/cm² (j _EB/j _AD = 1.5) at U = 4.2 kV and p _He = 50 Torr.     

High Current Injection into Dynamic p–n Homojunction in Polymer Light‐Emitting Electrochemical Cells

Ions and electrons in blends of polymer–electrolyte can work in ensemble to operate light‐emitting electrochemical cells (LECs), in which the unique features of in situ formed p–n homojunctions offer efficient charge injection and transport. However, electrochemical features give rise to significant stability and speed issues due to limited electrochemical stability and low ion mobility, resulting in low brightness and a slow response of LECs. Here, these issues are overcome by the separate control of ionic and electronic charges, using a simple driving pulse superimposed on a small base voltage; ions with slow response are rearranged by a constant base voltage, while a high‐voltage pulse, superimposed upon the base, injects electrons/holes which have fast response, with minimal effect on the ions. This scheme successfully injects an extremely high current density of > 2 kA cm^?2 with a balanced electron/hole ratio, at a high‐speed response time of ≈ 50 ns; both properties demonstrate advantages of LECs in making polymers brighter. An in situ electron spin resonance measurement on the LECs further revealed that this impressive performance is due to the highly doped polymers, whose spin density reached 7 × 10¹⁹ spins cm^?3, and an ordered polymer structure in the active layer blend.