Subnanosecond electron beams formed in a gas-filled diode at high pressures

Subnanosecond electron beams can be formed in gas-filled diodes at high pressures (up to 6 and 4 bar in helium and nitrogen, respectively). In a diode filled with air at atmospheric pressure, a beam current amplitude above 240 A was obtained at a pulse duration (FWHM) of ～0.2 s and a beam current density of ～40 A/cm².     

Ultrashort electron beams generated on the flat part of a voltage pulse in nitrogen and helium

In the case of voltage pulses with a small amplitude, an ultrashort avalanche electron beam (UAEB) in a diode filled with nitrogen or helium is generated on a flat part of the pulse. UAEBs obtained at a voltage of 25 kV have a full width at half maximum (FWHM) of about 200 ps and are delayed relative to the voltage pulse front by a time reaching tens of nanoseconds. Waveforms of the electron beam current pulse with several peaks of subnanosecond duration have been observed. At elevated pressures in a gas-filled diode, the voltage across the gap decreases by 10–20% during the electron beam generation.     

Dynamics of subnanosecond electron beam formation in gas-filled and vacuum diodes

The dynamic characteristics of a subnanosecond pulsed electron beam formation in the accelerating gap of a gas-filled or evacuated diode have been studied at a time resolution ～10^?11 s. In the air-filled gap, the electron beam pulse with a current amplitude of several amperes is formed up to about one hundred picoseconds earlier than the analogous pulse under vacuum conditions, and the measured pulse duration (～10^?10 s) is close to the electron flight time across a diode gap in the continuous acceleration regime. It is shown that a nanosecond prepulse plays an important role by initiating the emission of electrons that are subsequently accelerated by the high-voltage pulse with a subnanosecond front.     

Effective regimes of runaway electron beam generation in helium,hydrogen, and nitrogen

Runaway electron beam parameters and current-voltage characteristics of discharge in helium, hydrogen, and nitrogen at pressures in the range of several Torr to several hundred Torr have been studied. It is found that the maximum amplitudes of supershort avalanche electron beams (SAEBs) with a pulse full width at half maximum (FWHM) of ∼100 ps are achieved in helium, hydrogen, and nitrogen at a pressure of ∼60, ∼30, and ∼10 Torr, respectively. It is shown that, as the gas pressure is increased in the indicated range, the breakdown voltage of the gas-filled gap decreases, which leads to a decrease in the SAEB current amplitude. At pressures of helium within 20–60 Torr, hydrogen within 10–30 Torr, and nitrogen within 3–10 Torr, the regime of the runaway electron beam generation changes and, by varying the pressure in the gas-filled diode in the indicated intervals, it is possible to smoothly control the current pulse duration (FWHM) from ∼100 to ∼500 ps, while the beam current amplitude increases by a factor of 1.5–3.     

Generation of subnanosecond electron beams in air at atmospheric pressure

Optimum conditions for the generation of runaway electron beams with maximum current amplitudes and densities in nanosecond pulsed discharges in air at atmospheric pressure are determined. A supershort avalanche electron beam (SAEB) with a current amplitude of ∼30 A, a current density of ∼20 A/cm², and a pulse full width at half maximum (FWHM) of ∼100 ps has been observed behind the output foil of an air-filled diode. It is shown that the position of the SAEB current maximum relative to the voltage pulse front exhibits a time shift that varies when the small-size collector is moved over the foil surface.     

The temporal structure of a runaway electron beam generated in air at atmospheric pressure

Supershort avalanche electron beams (SAEBs) generated in air at atmospheric pressure have been studied with picosecond time resolution. It is established that an SAEB has a complicated structure that depends on the interelectrode gap width and cathode design. In a gas-filled diode with a small gap width, an SAEB current pulse with a full width at half maximum (FWHM) of ??25 ps has been observed behind a collimator with a hole diameter of 1 mm. As the gap width is increased or decreased relative to the optimum value that corresponds to the maximum beam current, the SAEB current pulse shape changes and pulses with two peaks are more likely detected. The two-peak SAEB current pulse shape is retained behind aluminum foil with a thickness of 60 and 110 ??m.     

Subnanosecond electron beams formed in a gas-filled diode

We have studied the conditions for the formation of a pulsed beam of runaway electrons in a diode filled with air at atmospheric pressure, whereby the current and voltage pulses in the system were measured with a subnanosecond time resolution. It is experimentally demonstrated for the first time that the electron beam appears on the leading front of the voltage pulse at a relatively small voltage on the discharge gap. At atmospheric pressure, a full width at half maximum of the current pulse does not exceed 0.3 ns.     

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.     

Protecting the anode foil of a high-current electron accelerator against arc-discharge damage

The mechanism of anode foil damage during the extraction of a high-power pulsed electron beam from a high-current diode has been experimentally studied on a TEU-500 electron accelerator [1]. It is established that the breakage of the anode foil is caused by the appearance of cathode spots on its surface, the intense electron emission from these spots during positive voltage pulses (postpulses following the main negative pulse of accelerating voltage), and the formation of arc discharge in the interelectrode gap. The improvement of diode matching to the pulse-forming line of the accelerator and the use of an auxiliary electrode (anode) forming additional vacuum discharge gap (crowbar) with the cathode practically excludes the anode foil breakage by arc discharge and significantly increases the working life of the foil (up to ∼10⁵ electron beam pulses).     

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.     

The effect of applied voltage on the formation of a subnanosecond electron beam in a gas-filled diode

We have studied the effect of the magnitude of the voltage applied to a gas-filled diode on the formation of a subnanosecond pulsed electron beam at atmospheric pressure. It is theoretically demonstrated that an increase in the interelectrode voltage leads to a decrease in the charge transferred by the beam. This may result in a decrease in the amplitude of the beam current at a pulse duration below the time resolution of the detection system.     

Electron beam formation in helium at elevated pressures

The formation of a beam of runaway electrons in a diode filled with helium at a pressure from 0.1 to 760 Torr was studied under conditions of a pulsed ≈4 ns) high ≈200 kV) voltage applied to the discharge gap. Both theoretical results and experimental data indicate that the electron beam is generated both at a large strength of the electric field, when the fraction of runaway electrons is large, and in a field of low strength, where intensive electron multiplication takes place. In the latter case, a high current can be obtained despite a small fraction of runaway electrons relative to their total number. The electron beams obtained in the helium-filled diode had a current amplitude of up to 140 A (corresponding to a current density above 10 A/cm²) at an electron energy of ～150 keV.

     

Pulsed cathodoluminescence of diamond,calcite, spodumene,and fluorite under the action of subnanosecond electron beam

Amplitude and temporal characteristics of pulsed cathodoluminescence (PCL) of diamond (natural and synthetic), calcite, spodumene, and fluorite have been studied at a temporal resolution of ∼0.3 ns. The PCL was generated by electron beam pulses with a full width at half maximum (FWHM) of 0.1, 0.25, and 0.65 ns. The PCL spectra have been measured for the emission induced by 0.1- and 0.25-ns pulses at a beam current density of ∼90 A/cm².     

Effect of nitrogen pressure on the energy of runaway electrons generated in a gas diode

The energy spectra of runaway electrons generated in a gas diode under the action of voltage pulses with a front width of ∼300 ps and amplitude of ∼140 kV have been studied using a time-of-flight spectrometer at nitrogen pressures in a range of 0.1–760 Torr. The delay of runaway electron beam pulse relative to the driving voltage pulse has been determined. The electron energy depends in a complicated manner on the nitrogen pressure in the gas diode and on the cathode geometry. A minimum breakdown voltage for a gap between tubular cathode and flat anode has been observed at a nitrogen pressure of ∼100 Torr. A decrease in the nitrogen pressure below 100 Torr leads to an increase in the maximum of voltage drop on the gap and the energy of the main fraction of electrons.     

Utilizing asymmetric laser pulses for the generation of high-quality wakefield-accelerated electron beams

Employing temporally asymmetric laser pulses in the interaction with plasma has been recently proposed for controlling the pointing angle of an electron beam produced by a laser wakefield acceleration at low plasma density and moderate laser intensity. In this paper, results on the electron beam parameters for both symmetric and asymmetric laser pulses are presented. These results show that the highest-quality (well-pointed, well-collimated and bright) electron beams are generated in the current regime only using asymmetric laser pulses, which are longer than the plasma wave’s acceleration period, τ>λ_p/2c. The interaction between the laser pulse and the accelerated electron beam in the first plasma-wave period is extracted from the experimental results and observed in preliminary two-dimensional particle-in-cell simulation.     

Subnanosecond 1-GW pulsed 38-GHz radiation source

The regime of excitation of subnanosecond high-power microwave pulses has been studied in a Cherenkov device with an extended periodic slow-wave structure, using an electron beam from a compact pulsed high-current electron accelerator (290 keV, 2.3 kA, 1 ns). Conditions are established for which the power conversion coefficient can reach up to 1.5 at an output pulse power of 1.2 GW and a pulse duration of 200 ps.     

Generation of subnanosecond 10-GHz pulses in high peak and high average power mode

The regime of excitation of microwave pulses in a 10-GHz range at a pulse duration of 0.8 ns and a peak power of ～2 GW has been studied in a relativistic backward wave oscillator with an extended periodic slow-wave system. A pulsed electron accelerator generating high-current electron beams (3 GW, ～600 keV, ～5 kA, 7 ns) at a repetition rate of 700 Hz and a pulse train width of 1 s has been developed based on a high-voltage generator with inductive energy storage, a semiconductor current interrupter, and a pulse-sharpening hydrogen-filled discharge gap. Optimization of the regime of the field-particle interaction allowed an average microwave power of 2.5 kW to be obtained at a transport magnetic field strength reduced below the cyclotron resonance value.     

High-power source of 200–350 nm spontaneous emission excited by unipolar current pulses

Spectral, energy, and temporal characteristics of pulse discharge in xenon have been experimentally studied. Upon passing from an oscillatory regime to unipolar pulses of discharge current, the power of emission in the wavelength interval 200–350 nm increases, while the emission pulse full width at half maximum (FWHM) decreases. A quartz pulsed discharge lamp excited by a generator based on high-current high-voltage diodes radiates in the 200–350 nm interval at a peak radiant intensity above 65 kW/sr and at a pulse FWHM ∼ 2 μs.     

Mechanism of electron current suppression in self-magnetically-insulated ion diode

The process of pulsed ion beam generation at a gigawatt output power level by a diode with an explosive-emission potential electrode operating in a self-magnetic-insulation regime has been studied. It is shown that a plasma is effectively formed in the diode and the condition of magnetic cutoff of electrons is satisfied along the entire diode length during ion beam generation. However, because of a high drift velocity, the residence time of electrons and protons in the anode-cathode gap is the same, 3–5 ns, while that for C⁺ carbon ions is greater than 8 ns. This is indicative of a low efficiency of self-magnetic insulation in the given diode. At the same time, it is experimentally established that, during generation of the ion current, the electron component of the total current in stripe diodes of both planar and focusing geometry is suppressed by a factor of 1.5–2. A new mechanism of electron emission suppression is proposed that explains the observed decrease in the electron component of the total current in self-magnetically-insulated ion diodes.     

Compact, high-gain pulsed dye amplifier for weak cw sources

A compact two-cuvette dye amplifier capable of pulse amplifying weak cw visible sources by factors in excess of 10(9) has been demonstrated. Seeded with a 300-μW single-frequency He-Ne laser and without the need for a Faraday isolator, the preamplifier yields >40-μJ pulses of 4.5-ns duration and 1.4× transform-limited linewidths around 140 MHz. Subsequent power amplification yields 4-ns FWHM, 2-mJ pulses with excellent pulse-to-pulse stability and linewidths around 170 MHz. Scaling of the operational envelope and extension to pulse amplification of cw single-mode tunable diode lasers is discussed.