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.     

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.     

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     

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.     

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.     

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.     

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).     

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.     

Flash X-ray radiography source based on plasma-filled rod pinch diode

Results of experiments on the formation of a high-power focused electron beam in a plasma-filled rod pinch diode driven by a high-current MIG generator (maximum voltage, 1.3 MV; impedance, 0.65 Ω) are presented. The proposed diode with a sharpened 1.5-mm-thick tungsten rod anode provides an X-ray source for flash radiography with a size of ∼1 mm, which is capable of producing a radiation dose of 2.4 rad per pulse at a distance of 1 m. The results of comparative experiments with and without plasma injection into the diode are presented.     

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.     

Voltage-tuned relativistic backward wave oscillator

A relativistic backward wave oscillator for the 10-GHz range with the oscillation frequency tuned within about 5% by changing the accelerating voltage from 600 to 350 kV has been developed. Discrete variations in the voltage and the corresponding frequency tuning from pulse to pulse is rapidly performed by changing the anode-cathode distance in the vacuum diode without breaking vacuum in the working volume. During this, the electron beam power remains almost constant, while the output microwave power varies within 0.4–0.8 GW. The introduction of a dielectric cylinder into the accelerating gap provides a smooth voltage drop from 600 to 350 kV with the corresponding frequency tuning during a 20-ns pulse.     

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.     

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.     

High repetition rate pulsed X-ray source employing supershort avalanche electron beams

The formation of a volume discharge in an open gas diode with coaxial electrodes was accompanied by hard X-ray emission. The conditions of supershort avalanche electron beam formation are retained at a pulse repetition rate up to 1.5 kHz.     

Experimental investigation of graphite explosive-emission cathodes operating in a periodic pulse regime

We have studied the electron emission from graphite cathodes under the action of voltage pulses with an amplitude of up to 300 kV, a pulse duration of 10^?9 s, and a pulse repetition frequency of 1–3.5 kHz. The magnetically insulated electron beam had a peak power of up to 600 MW at an average power of 1–3 kW. The dynamics of emission current delay was studied in relation to the charge transferred by the beam and to the state of the cathode surface (studied by scanning electron microscopy). It is established that smoothening of the microrelief leads to degradation of the cathode emissivity, which can be compensated by increasing the pulse repetition rate above a certain critical level.     

Matching a double forming line to explosive-emission diode

The regime of operation of an explosive-emission diode is affected by a matching transformer between this diode and a double forming line of a high-current electron accelerator. Preliminary forced demagnetization of the transformer core makes the shape of the voltage pulse applied to the cathode close to the optimum, corresponding to a decrease in the resistance of the anode-cathode gap related to expansion of the explosive-emission plasma. In addition, matching of the double forming line to the explosive-emission diode significantly decreases the amplitude of parasitic prepulses and increases (to 90–92%) the fraction of energy supplied to the diode during the main current pulse.     

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.

     

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.     

Volume x-ray emission in gas diodes at atmospheric pressure

The generation of x-rays and high-energy electron beams in gas diodes filled with air and nitrogen at atmospheric pressure has been studied by experimental and theoretical methods. It is established that soft x-ray radiation is not only generated in the region of dense discharge, but is predominantly emitted from a weak-current discharge region. For a high-energy electron beam formation in the gap, the role of the voltage pulse front is not less important than that of the voltage amplitude; the electric field strength at the cathode has an optimum value for the electron beam formation.     

Picosecond-controlled switching of high-voltage gas discharge

It is experimentally demonstrated that a nitrogen-filled discharge gap of a high-voltage oscillator can be switched by an electron beam with a time spread no exceeding ～25 ps relative to the accelerating voltage pulse front. The regime of high-precision control is obtained in the case of a homogeneous potential distribution in the gap, at a microsecond-long voltage buildup to a level of ～90% of the self-induced breakdown. The period of induced conductivity in the discharge gap corresponds to the beam current pulse duration.