Generation Of High-current Electron Beams Research Articles

Generation of High-Current Electron Beams in Diodes With Large-Area Explosive Emission Cathodes

This paper reports the investigation of several types of large-area (20–40 cm2) explosive emission cathodes different in shape and emitting surface material. We concentrated on the cathodes with emitters made of dense fine-grained graphite and carbon-epoxy capillary arrays (multicapillary cathodes), but also considered a cathode with copper emitter. We experimented with planar and profiled (Ernst profile) emitters and a graphite ring-type cathode. The velocity of cathode plasma expansion, the stability of operation, and the homogeneity of generated beams are compared for different cathodes. The current density distribution over the beam cross section is examined using the beam imprint on the radiochromic film. The pepper pot method is used to find whether the beam is converging or diverging.

The influence of a change in the electron trajectory in the vacuum-diode anode–cathode gap on the impedance

The analysis of the influence of the electron-trajectory curvature in the anode–cathode gap of a vacuum diode on its impedance during generation of high-current electron beams is performed, and the applicability of the one-dimensional Child–Langmuir formula for the electron-current calculation is evaluated. The results of an experimental study of a flat diode with explosive-emission graphite, copper, and carbon- fabric cathodes and also with a multipointed cathode are presented. These investigations were performed on the TEU-500 accelerator (350–500 kV, 60 ns). A strip diode with a graphite cathode was also studied in the mode of magnetic self-insulation of electrons. The experiments were performed on the TEMP-4М accelerators (150–200 kV, 400–600 ns). The investigation results showed that the satisfactory coincidence of the experimental values of the total current with those calculated from the one-dimensional Child–Langmuir formula (for the working area of the cathode) is observed in diodes not only in the absence of a change in the electron trajectory in the anode–cathode gap but also upon a deflection of the trajectory from the normal to the cathode surface by an angle of <90°. An increase in the electron current owing to a decrease in the anode–cathode gap and an increase in the emission area of the cathode during the cathode-plasma production and also at the expense of the presence of microirregularities on the cathode is properly described by the onedimensional Child-Langmuir formula, if the above factors are taken into account.

Experimental research of different plasma cathodes for generation of high-current electron beams

The results of experimental studies of different types of cathodes—carbon-epoxy rods, carbon-epoxy capillary, edged graphite, and metal-dielectric—under the application of high-voltage pulses with an amplitude of several hundreds of kV and pulse duration of several nanoseconds are presented. The best diode performance was achieved with the edged graphite and carbon-epoxy-based cathodes characterized by uniform and fast (&amp;lt;1 ns) formation of explosive emission plasma spots and quasi-constant diode impedance. This result was achieved for both annular cathodes in a strong magnetic field and planar cathodes of a similar diameter (∼2 cm) with no external magnetic field. The cathodes based on carbon-epoxy rods and carbon-epoxy capillaries operating with an average current density up to 1 kA/cm2 showed insignificant erosion along 106 pulses of the generator and the generated electron beam current showed excellent reproducibility in terms of the amplitude and waveform.

Modeling of high-current devices with explosive electron emission

Based on a detailed analysis of explosive electron emission in high-current electronic devices, we formulate a system of equations that describes the expansion of cathode plasma and the generation of high-current electron beams. The system underlies the numerical algorithm for the hybrid code which enables the simulation of the charged particles' dynamics in high-current vircators with open resonators. Using the Gabor–Morlet transform, we perform a time-frequency analysis of vircator radiation.

Modeling of explosive electron emission and electron beam dynamics in high-current devices

Based on a detailed analysis of explosive electron emission in high-current electronic devices, we formulate a system of equations that describes the expansion of the cathode plasma and the generation of high-current electron beams. The system underlies the numerical algorithm for the hybrid code which enables simulating the charged particles' dynamics in high-current vircators with open resonators. Using the Gabor-Morlet transform, we perform the time-frequency analysis of vircator radiation.

Generation of high-current electron beam in a wide-aperture open discharge

In the present study, it was examined generation of nanosecond-duration electron-beam (EB) pulses by a wide-aperture open discharge burning in helium or in a mixture of helium with nitrogen and water vapor. In the experiments, a discharge cell with coaxial electrode geometry, permitting radial injection of the electron beam into operating lasing medium, was used, with the cathode having radius 2.5 cm and length 12 cm. It was shown possible to achieve an efficient generation of a high-intensity electron beam (EB pulse power ∼250 MW and EB pulse energy up to 4 J) in the kiloampere range of discharge currents (up to 26 kA at ∼12 kV discharge voltage). The current-voltage characteristics of the discharge proved to be independent of the working-gas pressure. The existence of an unstable dynamic state of EB, conditioned by the presence of an uncompensated space charge accumulated in the discharge cell due to the exponential growth of the current in time during discharge initiation and the hyperbolic growth of current density in the direction towards the tube axis, was revealed. The obtained pulsed electron beam was used to excite the self-terminated laser on He 21P10-21S0 transition. The oscillations developing in the discharge cell at high discharge currents put limit to the pumping energy and emissive power of the laser excited with the radially converging electron beam.

Plasma Emission Sources for High-Current Electron Beam Generation

The main results of recent experimental research on the different plasma sources for high-current electron beam generation are presented. Namely, passive plasma sources (velvet-based and multicapillary dielectric cathodes) based on flashover plasma and active plasma sources based on a ferroelectric plasma source (FPS) and an FPS-assisted hollow-anode plasma source are described. Different time- and space-resolved electrical, optical, and spectroscopic diagnostics used in these experiments were applied for the characterization of the plasma source parameters. The main data concerning the plasma parameters (plasma density and temperature, plasma uniformity, and plasma potential) and the main features of these plasma sources (plasma formation, lifetime, and vacuum compatibility) are considered. Also, data concerning electron diode operation and the parameters of the generated electron beam while using these plasma sources are presented.

An intense-current electron beam source with low-level plasma formation

The generation of high-current electron beams is accompanied by strong plasma formation on the cathode surface. The cathode plasma sheath expands towards the anode, which limits the pulse length. In this paper the experiments were performed using a high-voltage pulse generator with 400 kV output voltage and 300 ns pulse duration. We used electrical and optical diagnostics to study the process of plasma formation. This paper presents data on a caesium–iodide (CsI) coated carbon fibre cathode capable of operating with the lack of strong plasma formation. The addition of CsI caused an increase in the voltage pulse duration and reduced the turn-on electric field for electron emission, thus resulting in the fast-rise current. In particular, the CsI coating effectively inhibited the plasma expansion and, as a result, the diode gap remained almost unchanged within approximately 200 ns. These results show that CsI-coated carbon fibre cathodes are promising electron emitters for generating long pulse high-current electron beams.

High-current electron beam generation in a diode with a multicapillary dielectric cathode

Results of high-current electron beam generation in an ∼200kV, ∼250ns diode with a multicapillary dielectric cathode (MCDC) assisted by either velvet-type or ferroelectric plasma sources (FPSs) are presented. Multicapillary cathodes made of cordierite, glass, and quartz glass samples were studied. It was found that the source of electrons is the plasma ejected from capillaries. The plasma parameters inside capillary channels and in the vicinity of the cathode surface were determined during the accelerating pulse using visible range spectroscopy. It was shown that glass multicapillary cathodes are characterized by less surface erosion than the cordierite cathodes. Also, it was found that multicapillary cathodes assisted by a FPS showed longer lifetime and better vacuum compatibility than multicapillary cathodes assisted by a velvet-type igniter. Finally, it was found that quartz glass MCDC assisted by FPS is characterized by almost simultaneous formation of the plasma in a cross-sectional area of the dielectric sample with respect to the beginning of the accelerating pulse. The latter is explained by intense UV radiation which synchronized formation of parallel discharges due to induced secondary electron emission.

Passive and Active Plasma Emission Sources for High-current Electron Beam Generation

In this review we present main results of experimental research of passive and active plasma sources for high-current electron beam generation obtained during the last few years in our Plasma & Pulsed Power Laboratory. We will describe passive plasma sources (ceramic-metal, velvet and carbon fiber cathodes) based on flashover plasma and active plasma sources based on a ferroelectric plasma source (FPS) as well as on an FPS-assisted hollow anode (HA) plasma source. The main data concerning the plasma parameters (plasma density and temperature, plasma uniformity and plasma potential) and the main features of these plasma sources (plasma formation, life time, vacuum compatibility) will be discussed. Also, data concerning electron diode operation and parameters of the generated electron beam while using these plasma sources will be presented.

Optimization of a low-pressure hollow-anode electrical discharge for generation of high-current electron beams

We report results on the optimization of the design of a high-current hollow-anode (HA) electrical discharge electron beam source that is triggered by a ferroelectric surface discharge. For the ferroelectric sample, we used a BaTi solid solution with a large dielectric constant (ε ≈ 1600). Three different electric schemes for ignition and sustainment of the HA discharge were investigated. The studied HA designs allow reliable ignition and sustainment of the discharge with current amplitude of up to 1.2 kA and pulse duration of up to 2 × 10−5 s, with and without gas flooding. It was found that the rise time of the discharge current monotonically decreases from ∼10–7.5 µs with the increase of the background pressure from 4 × 10−5 to 7 × 10−5 Torr. Generation of high-current electron beams was demonstrated under an accelerating voltage of up to 300 kV and ∼400 ns pulse duration. It was shown that the use of an optimal resistor which supplies an auto-bias potential to the HA output grid eliminates almost entirely the plasma pre-filling of the accelerating gap prior to the application of the accelerating pulse. In addition, it was found that within a certain range of time delays (12.5–15.5 µs) of the application of the accelerating pulse with respect to the beginning of the HA discharge, the amplitude of the diode current remains practically unchanged in spite of a considerable decrease in the amplitude of the discharge current.

Investigation of a hollow anode with an incorporated ferroelectric plasma source for generation of high-current electron beams

We report experimental results of operation of a high-current hollow anode (HA) with a BaTi ferroelectric plasma source (FPS) incorporated in it. It is shown that the application of the FPS allows one to significantly decrease the HA surface area, thus providing a compact electron source. Use of this HA as an electron source in a high-voltage diode for generation of high-current electron beams is described as well. It was found that the FPS allows reliable ignition and sustaining of the HA discharge with current amplitude ⩽1.2 kA and pulse duration ⩽2×10−5 s at N2 gas pressure of (1–3)×10−4 Torr. Also, it was found that the operation of the HA is characterized by plasma formation with density of ∼4×1012 cm−3, electron temperature of ∼5 eV, and that the plasma acquires a positive potential of ∼10 V with respect to the anode and of 50–70 V with respect to the autobiased HA output grid. It is shown that the autobiased HA output grid prevents plasma penetration towards the accelerating gap if the grid half-cell size has approximately the same value as the thickness of the double layer formed between the plasma and the grid wires. Generation and characterization of a high-current electron beam with current amplitude of ∼1.2 kA was achieved under an accelerating pulse amplitude ⩽300 kV and ∼400 ns pulse duration.

Generation of high-current electron beams by a hollow cathode with a ferroelectric plasma source

This paper presents results and analysis of an experimental investigation of the operation of a hollow cathode (HC) with an incorporated ferroelectric plasma source (FPS). It was shown that the use of FPS based on a BaTi solid solution allows one to ignite and to sustain a 10 2 -10 3 A HC discharge with duration of 10 �3 -10 �5 s at background pressure of ∼5 × 10 �3 Pa while keeping the HC design with small dimensions. It was found that the development of the HC discharge is accompanied by formation at the surface of the FPS of dense plasma which serves as a powerful (hundreds of kW) pulsed source of current carrying electrons. Parameters of the HC plasma (radial distribution of the plasma density and temperature and plasma potential) for different discharge current amplitudes and two types of FPS are presented. Application of the FPS as an electron source in a diode under an accelerating pulse ≤300 kV and pulse duration ≤400 ns showed that the latter operates in a plasma pre-filled mode with a current amplitude up to 1.6 kA. Parameters of the diode and electron beam for different experimental conditions are presented and discussed.

Electron beam generation in a diode with a gaseous plasma electron source II: Plasma source based on a hollow anode ignited by a hollow-cathode source

We describe the operation of a hollow plasma anode (HPA) with a hollow cathode incorporated in it for the production of preliminary plasma. The operation of the hollow anode was studied with a discharge current amplitude up to 1 kA. It was shown that the gaseous discharge is realized without formation of plasma spots at the anode wall and the output grid. The plasma parameters inside the HPA were measured for different N2 gas pressures and discharge current amplitudes. It was found that the plasma acquires a positive potential with respect to the anode and that the plasma density and temperature are ⩽5×1012 cm−3 and ⩽10 eV, respectively. This plasma source was used as a cathode in a diode for the generation of high-current electron beams under an accelerating-voltage ⩽250 kV and 400 ns pulse duration. The characteristics of the diode and the generated electron beam as a function of the HPA parameters are presented. It was found that at the beginning of the accelerating pulse the diode operates in a plasma prefilled mode, and further in the accelerating pulse the diode current is determined by the emission capability of the HPA. It was shown that this source allows reliable generation of a uniform electron beam with a cross-sectional area of 100 cm2 and a current amplitude up to 1 kA without the formation of explosive plasma at the HPA output grid.

High-current electron beam generation by a metal–ceramic cathode

We present results of the investigation of a metal–ceramic cathode suitable for technological applications. The cathode was tested in electron diodes powered by two different high-voltage generators (500 kV, 50 ns, 300 Ω, ⩽200 Hz and 300 kV, 250 ns, 84 Ω, ⩽5 Hz). The metal–ceramic cathode which was made in a form of a disk was composed of TiO2 ceramics with stainless steel spherical particles uniformly inserted inside. It was shown that already at relatively low accelerating fields (E⩽5 kV/cm) fast cathode plasma formation occurs as a result of surface flashover. This surface flashover is initiated in triple points located in micropores which were formed during the process of cathode preparation due to the different thermal expansion coefficients of the ceramic and the metal. Experimental data concerning the uniformity of the light emission from the cathode surface and inside the anode–cathode gap and divergence of the generated electron beams are presented. Also, it is shown that the uniformity of the generated electron beam depends strongly on the duration of the accelerating pulse.

High-current electron beam generation by a pulsed hollow cathode

In this paper the parameters of a hollow-cathode electron source and the generated electron beam are presented. A gas puff valve is used to provide a sharp pressure gradient between the cathode cavity and the accelerating gap. To produce the plasma inside the hollow cathode we used a pulse forming network (10 kV, 5–10 μs, 5 Ω). Generation of a high-current electron beam was achieved under an accelerating voltage of 20-kV amplitude and 500-ns pulse duration. The parameters of the He gas and the plasma inside the cathode and the accelerating gap were studied by electrical and optical diagnostics for different gas pressures and discharge current amplitudes. It was found that the operation of the hollow cathode is characterized by a negative anode potential fall and that the plasma density and temperature inside the hollow cathode do not exceed 5×1012 cm−3 and 14 eV, respectively. The parameters of the electron beam were studied for different discharge current amplitudes and anode grid cell sizes. It was shown that efficient electron beam generation with a beam current amplitude of up to 300 A is achieved without significant increase of the plasma potential. Space-charge neutralization processes in the accelerating gap explain the excess of the electron current density above its space-charge-limited value, in agreement with the measured potential distribution in the accelerating gap. The results of computer simulations of the hollow-cathode operation are in satisfactory agreement with the obtained experimental data.

Ferroelectric plasma cathode with a control grid

Experimental results of high-current electron beam generation in a planar diode with a ferroelectric plasma cathode and a control grid are presented. The plasma is formed as a result of noncomplete discharge on the surface of a BaTiO3 sample when a driving pulse is applied to its rear electrode. This noncomplete surface discharge leads also to the appearance of fast plasma flows which fill the anode–cathode gap prior to the application of the accelerating electric field. The measured plasma densities and flow velocities allow us to estimate the bias potential of the control grid which is needed for the suppression of the plasma flows. The dependence of the plasma density on the grid potential is presented. Using the control grid, operation of the diode with the ferroelectric plasma cathode is achieved with negligible plasma prefilling of the anode–cathode gap.

Plasma sources for high-current electron beam generation

A review of experimental studies of the operation of cathodes made of metal-ceramic, velvet, corduroy, carbon fibers, carbon fabric, and different types of ferroelectrics is presented. These cathodes operated at electric fields in the range of 5–60 kV/cm that allowed the generation of electron beams with duration of several hundreds of nanoseconds while keeping a quasi-constant diode impedance. All cathodes had the same diameter and were tested in a diode powered by a high-voltage generator (300 kV, 85 Ω, 250 ns, ⩽5 Hz). It was shown that the source of electrons for all the studied cathodes is a plasma which is formed as a result of surface discharges. Different types of electrical and optical diagnostics were used to study the formation and parameters of the plasma, the potential distribution inside the anode–cathode gap, and the uniformity and divergence of the extracted electron beam as a function of the amplitude and rise time of the accelerating pulse. Results of the lifetime of the tested cathodes and their compatibility with vacuum requirements are presented as well.

Electron emission from lead–zirconate–titanate ceramics

We report extensive experimental data on electron emission from lead–zirconate–titanate ferroelectric ceramics. A 1–2 MV/m pulse is applied to a gridded ferroelectric cathode and diode currents of up to 120 A/cm2 are measured across an A–K gap of 5×10−2 m, with the anode at 35 kV. Both the current and the anode voltage pulse duration may extend to several microseconds. The measurements extend previously reported data by nearly two orders of magnitude in the diode voltage and by a factor of more than 3 in the diode spacing. Two major regimes of operation were identified. In the first ∼1 μs the ferroelectric cathode controls the electron flow through the diode. Beyond this time plasma effects dominate the current flow. The results are of importance to the development of novel cathodes for high current electron beam generation.

Generation of electron beams for pumping of the GARPUN wide-aperture excimer laser

A description is given of the design of the GARPUN excimer laser module intended for generation of ultraviolet radiation or wide-aperture amplification of short laser pulses. The main parameters of the electron-beam laser-pumping system are given. A detailed discussion is provided of the task of generation of high-current electron beams with a large cross-sectional area and uniform distribution of the current. An analysis of the losses in the system supplying high-voltage pulses to electron accelerators, and also of the losses in the formation and coupling of electron beams to the laser chamber shows that the attained laser-pumping efficiency of ~16% may be increased by a factor of 1.5–2.