Hollow Cathode Phase Research Articles

Characterization of Pseudospark Discharge-Based Multigap Plasma Cathode Electron Source for the Generation of Short Pulsed Energetic Electron Beam

Pseudospark (PS) discharge-based devices are known as excellent source for the generation of high current density and energetic self-focused electron beam in the hollow cathode (HC) phase. In this article, short pulsed ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\!\!\!&lt; 100$ </tex-math></inline-formula> ns) and high energetic (~20 keV) electron beam has been generated and propagated (up to ~60 mm) in the drift region without using any external guiding magnetic field from the four-gap configuration of PS discharge-based plasma cathode electron (PD-PCE) source. The particle-in-cell (PIC) simulation code OOPIC Pro has been employed, whose results have shown the strong dependence of beam propagation into the drift region on the penetration and distribution of potential lines. The combined experimental and simulation investigations have also been carried for the multigap (four-gap) with wide range of external storage capacitor (40 pF–18 nF) and operating voltages (5–35 kV). The circuit parameter controls the appearance of HC phase for the energetic (50%–70% applied voltage) electron beam and conductive phase for the high current (~10 A to ~0.5 kA) electron beam. The potential distribution has clearly indicated that the electron beam with higher applied voltages can propagate more focused in the drift region. The investigations have evidently shown the generation of low energy and high current to high energy and low current electron beams suitable to cover the potential applications in the field of extreme ultraviolet (EUV)/soft X-ray radiation generation, surface modification, and microwave-terahertz radiation generation.

Investigations on High-Current-Density Sheet-Beam Driven V-Band Slow Wave Oscillator With a Hollow Cathode

In this article, the design approach for a ${V}$ -band miniaturized oscillator with a pseudospark-based high-current-density sheet electron beam source is presented. The hollow cathode electron gun has been designed and developed. In the experiment, the ~10-A current in the hollow-cathode phase has been measured. The high-frequency structure comprised of a truncated sine waveguide slow wave structure (TSW SWS) with wideband-matched output couplers has been designed, optimized, and simulated. The simulations show good transmission characteristics of the structure. The operation of a backward-wave oscillator with the designed TSW SWS driven by a 10-A, 40-kV sheet electron beam has been investigated by using 3-D particle-in-cell simulations. The maximal output power of 70 kW has been attained in the simulation. Frequency tuning within the 10-GHz frequency band has been demonstrated.

Study on the electron emission process of the pseudospark during the hollow cathode phase under nanosecond pulsed voltage

Pseudospark discharge, discovered in 1979, has remarkable parameters that may make it useful in various applications. This paper explores the influence of the cathode aperture on the breakdown characteristics of pseudospark discharge. In addition, the discharge process in both the main gap and the cathode aperture has been researched by a high-speed framing camera. Results revealed that the influence of the structure of the hollow cathode on the breakdown voltage characteristics lies in the structure of the cathode aperture. Besides, the delay of the electron emission process has been calculated based on the explosive electron emission model. In the end, a possible reason for the great discharge current and high increase rate of the discharge current has been discussed.

A multiple gap plasma cathode electron gun and its electron beam analysis in self and trigger breakdown modes.

In the present paper, a pseudospark discharge based multiple gap plasma cathode electron gun is reported which has been operated separately in self and trigger breakdown modes using two different gases, namely, argon and hydrogen. The beam current and beam energy have been analyzed using a concentric ring diagnostic arrangement. Two distinct electron beams are clearly seen with hollow cathode and conductive phases. The hollow cathode phase has been observed for ∼50 ns where the obtained electron beam is having low beam current density and high energy. While in conductive phase it is high current density and low energy electron beam. It is inferred that in the hollow cathode phase the beam energy is more for the self breakdown case whereas the current density is more for the trigger breakdown case. The tailor made operation of the hollow cathode phase electron beam can play an important role in microwave generation. Up to 30% variation in the electron beam energy has been achieved keeping the same gas and by varying the breakdown mode operations. Also, up to 32% variation in the beam current density has been achieved for the trigger breakdown mode at optimized trigger position by varying the gas type.

Generation and application of pseudospark-sourced electron beams

A pseudospark (PS) discharge has been shown to be a promising source of high brightness, high intensity electron beam pulses. A PS-sourced electron beam has two phases, an initial hollow cathode phase (HCP) beam followed by a conductive phase (CP) beam. In our experiments, a 22 kV, 50 A HCP beam of brightness 109–10 A m−2 rad−2 followed by a 200 V, 200 A CP beam of brightness 1011–12 A m−2 rad−2 were measured. Experiments have been conducted with the application of a HCP beam in a Cherenkov interaction with no input seed wave and with post-acceleration of the CP beam. In this paper, a new Cherenkov interaction experiment with an input seed wave from a 20 kW, 35 GHz pulsed magnetron has been designed using the same PS HCP beam. Simulation results of the interaction will be presented and further PS electron beam applications will be discussed.

Cherenkov interaction and post-acceleration experiments of high brightness electron beams from a pseudospark discharge

A pseudospark-sourced electron beam has two phases, an initial hollow cathode phase (HCP) beam followed by a conductive phase (CP) beam. The beam brightness was measured by a field-free collimator to be 10 9 and 10 11 A m −2 rad −2 for HCP beam and CP beam, respectively. The initial HCP beam from an eight-gap pseudospark discharge was applied in a Cherenkov interaction between the electron beam and the TM 01 mode of a 60-cm long alumina-lined waveguide. While the CP beam from a three-gap pseudospark discharge chamber was propagated and post-accelerated from about 200 V to more than 40 kV.

Pseudospark Experiments: Cherenkov Interaction and Electron Beam Post-Acceleration

Pseudospark (PS) discharge experiments to generate high-brightness electron beams have been carried out at the Strathclyde University, Glasgow, U.K. The PS-sourced electron beam has two phases, an initial 22-kV, 50-A hollow-cathode phase (HCP) beam of brightness 10/sup 9-10/ Am/sup -2/ rad/sup -2/ followed by a 200-V, 200-A conductive phase (CP) beam of brightness 10/sup 11-12/ Am/sup -2/ rad/sup -2/. The initial HCP beam from an eight-gap PS discharge was applied for the first time in a Cherenkov interaction between the electron beam and the TM/sub 01/ mode of a 60-cm long alumina-lined waveguide. A gain of 29/spl plusmn/3 dB was measured and an output power of 2/spl plusmn/0.2 kW in the frequency range 25.5-28.6 GHz. Another experiment was focused on the study of the propagation and post-acceleration of the CP beam from a three-gap PS discharge chamber. The beam was successfully accelerated from about 200 V to more than 40 kV.

Propagation and post-acceleration of a pseudospark-sourced electron beam

Propagation and post-acceleration of a pseudospark-sourced electron beam from a three-gap pseudospark discharge chamber were studied in recent experiments. The pseudospark produced an electron beam of two phases, an initial 22 kV, 50 A hollow cathode phase beam of brightness 109−10 Am−2 rad−2 followed by a 200 V, 200 A conductive phase (CP) beam of brightness 1011−12 Am−2 rad−2. The aim of these experiments was to post accelerate the lower-voltage, higher-current CP beam using an acceleration unit driven by a 40 kV, 125 ns voltage pulse produced by a cable Blumlein. The experiments were realized by attaching an acceleration unit to the downstream side of the anode of the discharge chamber. Both the pseudospark discharge and the cable Blumlein were triggered to ensure time correlation between initiation of the pseudospark discharge and post-acceleration of the beam.

Triggering a radial multichannel pseudospark switch using electrons emitted from material with high dielectric constant

The electron emission from a field emission trigger unit was matched to the requirements for the simultaneous ignition of all channels in a radial multichannel pseudospark switch for a large parameter range of the pseudospark discharge. In such trigger unit the electrons are extracted from the surface of a high-ε material by field emission after applying a pulsed voltage. Electron emission occurs in a pressure range from 1–50 Pa, where the pseudospark discharge is operated. The access to low working gas pressures (&amp;lt;10 Pa) using such trigger unit allows for achieving high hold-off voltages. The ignition of the pseudospark discharge was investigated by means of high speed photography and by a parallel observation of the electron beam of a single discharge channel, which occurs in hollow-cathode phase.

A hybrid hydrodynamic-like model for pseudospark discharge

A hybrid hydrodynamic-like formalism is presented for the study of the inception of breakdown and development of plasma bubble in the hollow cathode phase of a pseudospark switch discharge. It allows one to obtain the spatio-temporal evolution of the electronic and ionic densities, velocities, and energies in fast transient situations in low pressure discharges. This hybrid formalism allows the study of situations where the mean free path is not low compared to the spatial variations of the macroscopic parameters, and/or situations for which the transport energy (respectively, the transport velocity) is of the same order of magnitude as the thermal energy (respectively, the thermal velocity), as in the case of pseudospark discharge. The model is self-consistent and its formalism stands on a particular representation of the anisotropic nonlocal distribution function for electrons as having a beam component. The nonlocal electronic velocity is computed as a moment of the distribution function. The transport equations for mass and energy for electrons and ions are solved by the flux-corrected transport (FCT) method where the source terms are calculated from the nonlocal distribution function. The electric field is calculated by the Poisson's equation. The discharge physics is modeled in both main gap and hollow cathode simultaneously to permit a correct coupling of the charged particles, photons, and field between the two zones. Results show that the transport velocity and ionization growth for the case of highly anisotropic nonlocal distribution function is largely higher than that based on a pure hydrodynamic nearly Maxwellian distribution function. The energetic and high density positive ions impacting the inner walls and the back of the cathode hole participate in a composite effect yielding a cathode surface phenomena of super dense secondary emission of electrons which enhances the plasma bubble inception inside the cathode hole. We propose a criterion for switch operation in the pseudospark mechanism based on the formation of the plasma bubble, inside the cathode hole, known as the hollow cathode phase, as a transition phase between the breakdown phase and the conduction phase.

The bottleneck in pseudospark discharges

Physical mechanisms during early phases of pseudospark discharges are still under discussion. Experiments at low rates of current rise (dI/dt&amp;lt;109 A/s) were performed to investigate these phenomena, using filtered fast shutter photography of the discharge plasma in the visible wavelength region. Additionally temporally resolved measurements of line emission of the filling gas (hydrogen and helium) and cathode material (copper) were performed. These experiments revealed a discharge phase where the cathode surface involved in the discharge is restricted mainly to the inside of the cathode aperture. The voltage drop during that phase is about 200–400 V and the current density exceeds 1 kA/cm2. A possible explanation of the discharge mechanisms during that discharge phase is self-sputtering of cathode material from the cathode surface inside the cathode aperture. The described mechanisms might be fundamental for the transition from the transient hollow cathode phase to the high current phase in pseudospark discharges.

Study of field-induced explosive electron emission mechanisms of pseudospark superemission cathodes

Various mechanisms of electron emission, including the field, field-enhanced thermionic, and explosive electron emissions from pseudospark cathodes, are discussed and compared. The mechanism of the field-induced explosive electron emission due to microstructure on the cathode surface is considered to be more likely the pseudospark superemissive mechanism. A high-mean electric field up to 3-5 MV/cm on the cathode surface in the end of hollow cathode phase is enough to initiate the mechanism. The cathode spot initiation delay time (<10 ns) and explosive emission threshold current (/spl sim/10/sup 8/ A/cm/sup 2/) prior to the high current conducting phase are given by solving the initial value problem of the one-dimensional heat conduction equation, thus explaining the existing experimental data of the pseudospark cathode superemission. In the case of multigap discharge, the above mechanism occurs on nearly all cathode and interelectrode surfaces. Experimental evidence in single- and multigap pseudospark discharges supports the suggested explanation.

Predicted emittance and brightness of the pseudospark electron-beam

The emittance and brightness of the electron beam generated during the hollow cathode phase of pseudospark operation are calculated using the two-dimensional hybrid fluid-particle model previously developed to study the time and space development of the plasma in a pseudospark discharge. Two distinct energy components exist in the electron beam; a high-energy component with an energy equivalent to the full discharge voltage and another, broad, low-energy component. In the 100 ns following breakdown and for the conditions of the calculations, the emittance of the high energy component decreases by an order of magnitude and the brightness of the high energy component reaches almost 10/sup 10/ A/m/sup 2/ rad/sup 2/. This work demonstrates the feasibility of using the model to guide the optimization of the pseudospark electron beam properties and shows that the optimum beam properties are achieved after the plasma has filled the hollow cathode and begun to expand radially in the main gap. >

Self-sustained self-sputtering: a possible mechanism for the superdense glow phase of a pseudospark

Self-sustained self-sputtering occurring during high current pseudospark operation (/spl ap/10/sup 4/ A/cm/sup 2/, I>10/sup 3/ A) is shown to be a possible mechanism for the superdense glow. The mean-free-path for ionization of cathode material sputtered in the low-current hollow-cathode phase can be shorter than the cathode-anode gap distance, and ionized atoms can return to the cathode surface, self-sputtering with a yield greater than one. The self-sputtered cathode atoms become ionized in the beam of electrons accelerated in the cathode sheath. A large fraction of the discharge current at the cathode surface can be carried uniformly over the surface by ions and a very high electron emission density is not required to maintain the high current. >

Electron-beam generation during the hollow cathode phase of pseudospark discharges

From calculations using a two-dimensional, hybrid fluid-particle model, the mechanism for the production of the high-current electron beam observed during the hollow cathode phase of pseudospark discharges is identified. The predicted beam properties are in good qualitative agreement with experiments; the onset of the beam current is abrupt and the current in the beam is some 10%–30% of the discharge current. Two distinct components appear in the energy spectrum of the beam. A sizable fraction of the current is carried by electrons with an energy corresponding to the voltage applied to the discharge. A distinct low-energy beam is also predicted with a spread in energy of several 100s of eV.