Capillary Plasma Source Research Articles

One-Body Capillary Plasma Source for Plasma Accelerator Research at e-LABs

We report on the development of a compact, gas-filled capillary plasma source for plasma accelerator applications. The one-body sapphire capillary was created through a diamond machining technique, which enabled a straightforward and efficient manufacturing process. The effectiveness of the capillary as a plasma acceleration source was investigated through laser wakefield acceleration experiments with a helium-filled gas cell, resulting in the production of stable electron beams of 200 MeV. Discharge capillary plasma was generated using a pulsed, high-voltage system for potential use as an active plasma lens. A peak current of 140 A, corresponding to a focusing gradient of 97 T/m, was observed at a voltage of 10 kV. These results demonstrate the potential utility of the developed capillary plasma source in plasma accelerator research using electron beams from a photocathode gun.

Wake fields effects in dielectric capillary

Plasma wake-field acceleration experiments are performed at the SPARC_LAB test facility by using a gas-filled capillary plasma source composed of a dielectric capillary. The electron can reach GeV energy in a few centimeters, with an accelerating gradient orders of magnitude larger than provided by conventional techniques. In this acceleration scheme, wake fields produced by passing electron beams through dielectric structures can determine a strong beam instability that represents an important hurdle towards the capability to focus high-current electron beams in the transverse plane. For these reasons, the estimation of the transverse wake-field amplitudes assumes a fundamental role in the implementation of the plasma wake-field acceleration. In this work, it presented a study to investigate which parameters affect the wake-field formation inside a cylindrical dielectric structure, both the capillary dimensions and the beam parameters, and it is introduced a quantitative evaluation of the longitudinal and transverse electric fields.

Investigation of Geometry and Current Density Effects in a Pulsed Electrothermal Plasma Source Using a 2-D Simulation Model

Electrothermal (ET) plasma discharges have application to mass acceleration technologies relevant to military ballistics and magnetic confinement fusion reactor operation. ET plasma discharges are initiated in capillary geometries by passing large currents (order of tens of kiloamperes) along the capillary axis. A partially ionized plasma then forms and radiates heat to the capillary walls inducing ablation. Ablated particles enter the capillary plasma source and cause a pressure surge that can propel pellets to velocities exceeding 2 km/s. These devices present several advantages over other mass accelerator technologies due to their simple design and ability to achieve high projectile launch frequencies. In order to investigate the operation of ET plasma discharges in more detail than previously possible, a 2-D, multifluid model has been developed to simulate the plasma-fluid dynamics that develop in these devices during operation. In this paper, the 2-D simulation model is used to investigate the effect of source geometry and current density on discharge characteristics. Peak pressure and electric field magnitudes for pulsed discharge operation are shown to scale well with theoretical and empirical scaling laws for steady-state discharge operation. The pulse shape of the source internal pressure is shown to change significantly with increasing source radius. The behavior of other plasma parameters is investigated. In addition, observations of the departure from the ablation-controlled arc regime are presented. This analysis suggests that, for the current pulse length investigated, source radii higher than 4 mm require significantly more current density in order to produce sufficient ablation to stabilize the plasma discharge.

Electron density measurement in gas discharge plasmas by optical and acoustic methods

Plasma density represents a very important parameter for both laser wakefield and plasma wakefield acceleration, which use a gas-filled capillary plasma source. Several techniques can be used to measure the plasma density within a capillary discharge, which are mainly based on optical diagnostic methods, as for example the well-known spectroscopic method using the Stark broadening effect. In this work, we introduce a preliminary study on an alternative way to detect the plasma density, based on the shock waves produced by gas discharge in a capillary. Firstly, the measurements of the acoustic spectral content relative to the laser-induced plasmas by a solid target allowed us to understand the main properties of the acoustic waves produced during this kind of plasma generation; afterwards, we have extended such acoustic technique to the capillary plasma source in order to calibrate it by comparison with the stark broadening method.

Hyper Jet Focused-Injection Tapered Capillary Plasma Source Concept

The concept of tapered capillary is introduced as a method of injecting focused plasma jet into the breach of bulk-loaded electrothermal-chemical (ETC) launcher. Tapered capillaries will operate in the confined controlled arc regime, and they can provide the advantage of producing focused hyper plasma jets useful for ignition of ETC systems at controlled plasma temperatures and pressures. The concept is also useful for focused implantation of plasma ions on substrates with desired patterns. Such tapered capillaries can inject plasmas at peak kinetic temperatures in the range of 14000-21000 K (~1.2-1.8 eV) and considerable jet pressures of 10-70 MPa for an input discharge current of 20-30 kA over a short pulselength of 100-150 μs. A tapered geometry made of Lexan polycarbonate with 16.8-mm inlet and 8.8-mm outlet radii produces a plasma jet with a plasma temperature of 14253 K (1.23 eV) at a peak bulk velocity of 4.027 km/s with a 10.38-MPa exit pressure, and a total ablated mass of 10.56 mg. Reducing the tapering to 12.6-mm inlet and 6.6-mm outlet radii produces a plasma jet with a plasma temperature of 16382-K (1.411 eV) at a bulk velocity of 4.34 km/s and a total of 23.27-mg ablated mass. Further reduction in the tapering to 8.4-mm inlet and 4.4-mm outlet radii produces a plasma jet with a plasma temperature of 20954 K (1.8 eV) at a bulk velocity of 4.89 km/s with a 70.78-MPa exit pressure, and a total ablated mass of 15.09 mg. The preliminary study shows that the narrow tapering angle produces higher pressure, temperature, velocity, and more ablated mass. Radiant heat flux at the taper exit varies from 2.2 GW/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for the wider tapers to 6.24 GW/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for the narrower ones.

Development of a Gas-Fed Plasma Source for Pulsed High-Density Plasma/Material Interaction Studies

A gas-fed capillary plasma source has been developed to study plasma-surface interactions under pulsed high pressure arc conditions, without the use of an exploding fuse wire or ablative liner. A nonintrusive preionization source has been developed to break down relatively large interelectrode gaps at low charge voltages of 2-6 kV. The preionization source comprises a nonequilibrium surface streamer discharge that forms a conducting channel through which the main thermal arc discharge is initiated. The arc electron temperature and number density are estimated to be T e ~ 1-2 eV and n e ~ 10 23 m -3 . Silicon and sapphire samples were exposed to the arc plasma and revealed deposition of electrode and wall materials. Substitution of Elkonite 50W3 for brass electrodes reduced plasma contamination to acceptable levels. The plasma-material interactions were examined and quantified using scanning electron microscopy and energy dispersive X-ray spectroscopy.

Calculation of Plasma Radiation in Electrothermal-Chemical Launcher

A numerical model of radiation has been adopted for electrothermal-chemical (ETC) launcher, in which Monte Carlo method and statistical physics are employed to simulate the process of a capillary plasma source in an ETC launcher. The effect on propellant grains with different average absorption coefficients is discussed. The plasma-propellant interaction is also discussed when combined with a thermal model. Results show that the strong instantaneous radiation is responsible for the transmission of energy to the propellant grains leading to ignition. The efficiency of energy absorption in the propellant bed always maintains a high level. Radiant energy caused by plasma is concentrated around the plasma injector. And the “hot zone” efficiency is mainly affected by the properties of propellant grains within a small field around the plasma injector.

Pulsed microwave discharge in a capillary filled with atmospheric-pressure gas

A pulsed microwave coaxial capillary plasma source generating a thin plasma filament along the capillary axis in an atmospheric-pressure argon flow is described. The dynamics of filament formation is studied, and the parameters of the gas and plasma in the contraction region are determined. A physical model of discharge formation and propagation is proposed. The model is based on the assumption that, under the conditions in which the electric fields is substantially below the threshold value, the discharge operates in a specific form known as a self-sustained-non-self-sustained (SNS) microwave discharge.

Energy Skin Effect of Propellant Particles in Electrothermal–Chemical Launcher

A numerical model of radiation has been employed by using a Monte Carlo method and statistical physics to simulate the process of a capillary plasma source for an electrothermal-chemical launcher. The effect of plasma on propellants with different physical parameters is discussed. The plasma-propellant interaction when the radiation model is combined with a thermal model is also discussed. Results show that radiant energy only causes a small field around the plasma injector in the propellant bed. The responses of energy flux and propellant particles to radiation are both on the order of picoseconds. The strong instantaneous radiation is not only the dominant effect of energy transport in hot plasma but is also responsible for the transmission of energy to the propellant particles leading to ignition. This energy skin effect on the propellant-particle surface appears to be the main cause of plasma ignition.

Discharge Characteristics of a Gas-Filled Capillary Plasma for Laser Wakefield Acceleration

A gas-filled capillary plasma source for potential use in conjunction with laser wakefield acceleration is developed, and its discharge characteristics are presented here. In the capillary, helium or hydrogen gas (~150 torr in pressure) is injected, and a pulsed high voltage (~25 kV) is applied for electrical discharge and plasma production. The discharge properties of the helium and hydrogen gases are compared. The plasma densities inside the capillary hole are measured with a Mach-Zehnder interferometer and are obtained separately by a simple analysis using the diffusion equation and measured current profiles.

Numerical Parametric Study of the Capillary Plasma Source for Electrothermal–Chemical Guns

This work expands upon previous research into developing a numerical model of a capillary plasma source for application to electrothermal chemical guns. The capillary model is comprised of a kinetic model simulating polyethylene ablation coupled to a one dimensional time-dependent hydrodynamic model to determine the capillary outflow parameters. As input the simulation requires the capillary internal radius, length, and applied electrical current. Previous work found that best agreement with experimentally measured ablated mass could be achieved with the application of a backflux parameter, representing the percentage of the ablated material that was allowed to return to the polyethylene surface and form a film. Using the model, a parameter study is conducted to determine the effects that varying the peak electrical current, internal capillary radius and backflux parameter have on the plasma temperature, peak outflow pressure, and total ablated mass. A sinusoidal electrical current is assumed in the present work, with the peak current and discharge time taken over a range typically encountered in electrothermal chemical gun application. Analysis of the dependence of total ablated mass and peak capillary pressure on the deposited electrical energy yield insight into non- linearities in the capillary energy equation.

Ablation Study in a Capillary Sustained Discharge

Electrothermal-chemical ignition systems have been demonstrated in gun systems to provide desirable characteristics including reproducible shorter ignition delays. We present a combined theoretical and experimental study of the capillary discharge with an aim to develop a capillary plasma source with efficient energy conversion. The major emphasis in the present capillary discharge model is the ablation phenomenon. Consideration is given to different characteristic subregions near the ablated surface: namely, a space-charge sheath, a Knudsen layer, and a hydrodynamic layer. A kinetic approach is used to determine the parameters at the interface between the kinetic Knudsen layer and the hydrodynamic layer. Coupling the solution of the nonequilibrium Knudsen layer with the hydrodynamic layer provides a self-consistent solution for the ablation rate. According to the model predictions, the peak electron temperature is about 1.4 eV, the polyethylene surface temperature is about 700 K, and the pressure is about 10 MPa in the case of a 0.6 kJ discharge. In parallel, a parametric experimental study of the capillary ablation process is conducted. The ablation rates are measured for capillary tubes made of polyethylene and Teflon. Both experimental measurements and simulations indicate that the ablated mass increases with the peak discharge current and that a smaller diameter capillary yields a larger ablated mass. It is found that model predictions agree well with experimental measurements