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.     

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.     

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.     

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.     

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 source and acceleration regime of a picosecond electron beam in a gas-filled diode with inhomogeneous field

It is experimentally demonstrated that, upon the application of a subnanosecond high-voltage pulse to the gap of a diode filled with air at atmospheric pressure, a bunch of runaway electrons is formed in a sharply inhomogeneous electric field near the cathode. The bunch duration does not exceed 50 ps, which is shorter than the electron flight time through the interelectrode gap in the continuous acceleration regime. This duration remained unchanged when the gap width was varied between 6 and 26 mm. The electron energy in the picosecond electron beam, as determined from the time-of-flight measurements in the drift channel behind the anode foil of the diode, agree with the results of numerical calculations of the electron acceleration dynamics in the vacuum diode approximation.     

Demonstrating gain in a dielectric Cherenkov maser with a rod slow-wave system

We report the first results of experiments that demonstrate the amplification of megawatt nanosecond microwave pulses in a Cherenkov maser with a dielectric rod and moderately relativistic annular electric beam generated in a compact linear induction accelerator module. The input signal was generated by a resonant microwave compressor operating in a 3-cm wavelength range. A maximum gain of ∼12.5 dB and a maximum output power of ∼16 MW for a pulse duration of ∼4 ns at a frequency of 9.388 GHz were obtained with a quartz rod. The dependence of the gain on the compressor power was determined for various values of the accelerating voltage and beam current.     

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.     

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.     

Fabricating a silicon nanowire by using the proximity effect in electron beam lithography for investigation of the Coulomb blockade effect

We present an approach to fabricate a silicon nanowire relying on the proximity effect in electron beam lithography with a low acceleration voltage system by designing the exposure patterns with a rhombus sandwiched between two symmetric wedges. The reproducibility is investigated by changing the number of rhombuses. A device with a silicon nanowire is constructed on a highly doped silicon-on-insulator wafer to measure the electronic transport characteristics. Significant nonlinear behavior of current-voltage curves is observed at up to 150 K. The dependence of current on the drain voltage and back-gate voltage shows Coulomb blockade oscillations at 5.4 K, revealing a Coulomb island naturally formed in the nanowire. The mechanism of formation of the Coulomb island is discussed.     

Design and performance of a Tesla transformer type relativistic electron beam generator

A relativistic electron beam generator driven by an air core Tesla transformer is described. The Tesla transformer circuit analysis is outlined and computational results are presented for the case when the coaxial water line has finite resistance. The transformer has a coupling coefficient of 0·56 and a step-up ratio of 25. The Tesla transformer can provide 800 kV at the peak of the second half cycle of the secondary output voltage and has been tested up to 600 kV. A 100–200 keV, 15–20 kA electron beam having 150 ns pulse width has been obtained. The beam generator described is being used for the beam injection into a toroidal devicebeta.     

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.     

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.     

Inverse free electron lasers and laser wakefield acceleration driven by CO2 lasers

The staged electron laser acceleration (STELLA) experiment demonstrated staging between two laser-driven devices, high trapping efficiency of microbunches within the accelerating field and narrow energy spread during laser acceleration. These are important for practical laser-driven accelerators. STELLA used inverse free electron lasers, which were chosen primarily for convenience. Nevertheless, the STELLA approach can be applied to other laser acceleration methods, in particular, laser-driven plasma accelerators. STELLA is now conducting experiments on laser wakefield acceleration (LWFA). Two novel LWFA approaches are being investigated. In the first one, called pseudo-resonant LWFA, a laser pulse enters a low-density plasma where nonlinear laser/plasma interactions cause the laser pulse shape to steepen, thereby creating strong wakefields. A witness e-beam pulse probes the wakefields. The second one, called seeded self-modulated LWFA, involves sending a seed e-beam pulse into the plasma to initiate wakefield formation. These wakefields are amplified by a laser pulse following shortly after the seed pulse. A second e-beam pulse (witness) follows the seed pulse to probe the wakefields. These LWFA experiments will also be the first ones driven by a CO(2) laser beam.     

Dynamics of plasma and ion flux in a vacuum neutron tube

Results of the numerical simulation of the formation of the ion beam in the accelerating gap of a vacuum neutron tube are presented. Calculations are performed with the KARAT code in a two-dimensional nonstationary formulation for plasma formed in arc discharge and inflowing into an accelerating gap with the given time dependences of parameters (density, expansion velocity). The small duration of the vacuum arc leads to a considerable change of parameters of inflowing plasma during the accelerating pulse. Two geometries are considered: the conventional and sectioned diode, in which the total voltage is divided between the anode, intermediate electrode, and cathode.     

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.     

Generation of chaotic microwave pulses in a ring system based on a klystron power amplifier and a nonlinear delay line on magnetostatic waves

The autonomous generation of stationary chaotic microwave pulse trains in a self-oscillating ring system with a multicavity klystron power amplifier operating in a small-signal regime and a wideband non-linear delay line on surface magnetostatic waves has been experimentally studied. It is established that the characteristics of generated chaotic microwave pulses can be controlled by varying the electron beam current and accelerating voltage in the klystron.     

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.     

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.     

Experimental investigation of the efficiency of a high-voltage glow discharge as a source of a beam of run-away electrons

To determine the energetic efficiency of the formation of the run-away electron beam in a highvoltage glow discharge, we measure the temperature of the electron beam generator. According to the measurements, the energetic efficiency (the part of the input discharge power carried away by the beam) was 50–70% at the generator supply voltage of 4.4 kV and it increased up to ~85% at ~8 kV. The measurements were performed for the discharge in helium with copper, steel, molybdenum, and lanthanum hexaboride cathodes.