Filling Gas Pressure Research Articles

Engineering Model for Simulation of Debris Cloud Propagation Inside Gas-Filled Pressure Vessels

Space debris causes a severe hazard to the International Space Station. Because of the possibility for the pressure vessels onboard space station to fail catastrophically being impacted these stored energy devices play a central role in determining the survivability of space station when subject to space debris impact. Recent studies of pressure vessel reaction to hypervelocity impact showed that an interaction between traveling debris cloud and gas medium inside the pressure vessel has a dramatic influence on a vessel rear wall deformation and failure. The purpose of this work is to determine the parameters of fragments that travel across the vessel diameter and act on the vessel rear wall, as well as parameters of a shock wave generated in the process of interaction between the debris cloud and gas. For evaluation of debris and gas parameters near the leading edge of the cloud the approach suggested by Nigmatulin is used. According to this approach the adiabatic two-phase flow is considered. The cloud-gas interaction is described by the set of nonlinear hyperbolic partial differential equations. This system is solved numerically by MacCormack method. The model proposed enables us to simulate both the fragments velocities and pressure amplitude of the shock wave. This engineering model and the code developed can be used for both the pressure vessel designing and analysis of its survivability.

Direct drive target survival during injection in an inertial fusion energy power plant

In inertial fusion energy (IFE) power plant designs, the fuel is a spherical layer of frozen DT contained in a target that is injected at high velocity into the reaction chamber. For direct drive, typically laser beams converge at the centre of the chamber (CC) to compress and heat the target to fusion conditions. To obtain the maximum energy yield from the fusion reaction, the frozen DT layer must be at about 18.5 K and the target must maintain a high degree of spherical symmetry and surface smoothness when it reaches the CC. During its transit in the chamber the cryogenic target is heated by radiation from the hot chamber wall. The target is also heated by convection as it passes through the rarefied fill-gas used to control chamber wall damage by x-rays and debris from the target explosion. This article addresses the temperature limits at the target surface beyond which target uniformity may be damaged. It concentrates on direct drive targets because fuel warm up during injection is not currently thought to be an issue for present indirect drive designs and chamber concepts. Detailed results of parametric radiative and convective heating calculations are presented for direct-drive targets during injection into a dry-wall reaction chamber. The baseline approach to target survival utilizes highly reflective targets along with a substantially lower chamber wall temperature and fill-gas pressure than previously assumed. Recently developed high-Z material coatings with high heat reflectivity are discussed and characterized. The article also presents alternate target protection methods that could be developed if targets with inherent survival features cannot be obtained within a reasonable time span.

Direct plasmadynamic conversion of plasma thermal power to electricity

The generation of electrical energy using direct plasmadynamic conversion (PDC) is studied experimentally for small-scale, chemically-assisted plasmas (CA-plasma) for the first time. Glow discharge and microwave-generated plasma sources are operated at power levels on the order of a few to 50 W in the discharge case and up to 12.83 W/cm/sup 3/ in the microwave case. Extracted power approaching 1/4 W has been achieved as a demonstration. It is envisioned that such a system may be readily scaled to a few hundred Watts to several tens of kilowatts output power for microdistributed commercial applications (e.g., household, automotive, light industry, and space based power). Three-quarter inch long by 0.040-in diameter cylindrical PDC electrodes have been tested in a 10-50 W direct current, glow discharge plasma device with He or Ar as the working gas at 0.3-3.0 torr. The PDC anode was magnetized in the range of 0-700 G with a 1.5-in water cooled Helmholtz electromagnet. Open circuit voltages up to 6.5 V were obtained across the PDC electrodes at 1 torr He and 350-G field. The collector voltage was shown to be a function of applied magnetic field strength B and peaking at about 300 G. A variety of resistive loads were connected across the PDC electrodes, extracting continuous electrical power up to 0.44 mW. The power/load curve peaks at 0.44 mW for a 20 k/spl Omega/ load indicating the impedance matching condition with the plasma source. The most severe limitation to collector output performance is shown to be plasma conductivity. Collector power drops sharply with increasing neutral gas fill pressure in the glow discharge chamber at constant discharge current indicating that electron collisions with neutral gas atoms are responsible for the reduction in conductivity. Scale-up to higher power has been achieved with the use of a microwave plasma generator. A 0.75-in long by 0.094-in diameter PDC anode was magnetized to /spl sim/140 G resulting in open circuit PDC voltages in excess of 11.5 V for He plasmas at /spl sim/0.75-1 torr and 50 sccm flow. Due to higher conductivity, load matching was now obtained at /spl sim/600 /spl Omega/. Langmuir probe results indicate good agreement between the conductivity change and the electron to neutral density ratio scale-up. For this source and electrode configuration, PDC power as high as /spl sim/200 mW was demonstrated in He at 0.75 torr for a microwave input power density of /spl sim/8.55 W/cm/sup 3/. Considering an electron mean-free path as the scale for collector probe influence in the plasma, the peak extracted power density is /spl sim/1.61 W/cm/sup 3/, corresponding to a volumetric conversion efficiency of /spl sim/18.8%.

Determination of HID electrode falls in a model lamp III: Results and comparison with theory

Electrodes of pure and doped tungsten for high intensity dischargelamps are investigated in a special model lamp. Our intention isto obtain experimental data to verify models describing thenear-electrode region. For the DC case, we have two totallyindependent methods to determine the cathode fall separately fromthe anode fall. The first method is based on the spatiallyresolved measurement of the cathode surface temperature and atreatment of the cathodic power balance. The second method usesLangmuir-probe measurements to determine the plasma potential atknown distances from the cathode.The comparison of both methods shows good agreement which justifiesthe assumptions and simplifications introduced. Parameter studiesyield a strong dependency of the cathode fall on the cathodediameter and the material. The cathode length and the fill gaspressure seem to have only minor influence. Comparing the resultsof our experiment with outcomes of models by several authors, thevalues for the cathode fall show good agreement, too. However,concerning the total power loss of the cathode, differences betweenexperiment and theory were found.

Experimental Investigations of x-ray Emission in a New Filippov-Type Plasma Focus

We present an experimental study of the soft and hard x-ray performance characteristics of neon and neon-krypton mixture (Pkr up to 0.2 torr) plasma produced by a new Filippov-type plasma focus device named “Dena”, with 90 kJ (25 kV, 288 µF) operating at pressures in the range 0.3–1.5 torr. In this investigation, the soft and hard x-ray intensities are measured using a filtered PIN diode, a pinhole camera with a Be filter (10 µm thickness), a plastic and NaI scintillators. Using pure neon as the filling gas, and a charging voltage of 16 kV with 37 kJ stored energy, the maximum intensities of the soft and hard x-rays were found to be about 5 V/shot and 2.2 V/shot, respectively. These measurements were obtained at an optimum operating pressure in the range of 0.5–0.9 torr (for the soft x-ray) and about 0.8 torr (for the hard x-ray). The relationships between the optimum neon pressure range and the time of spike appearances on the discharge current signal were experimentally studied. The results indicate that, the x-ray emission is more sensitive to the time of spike appearances than the neon filling gas pressure. By using a krypton admixture to the neon filling gas (PNe = 1 torr), the most pronounced effect is observed for the hard x-ray yield (up to eight times of the initial value) at the optimum krypton pressure of about 0.1 torr. The measurements of maximum x-ray intensity, performed with krypton admixture, show that as the pressure of krypton admixture increases, the optimum discharge voltages fall to a value depending on the quantity of krypton admixtures used. This effect was generally an intensity decreasing matter for the soft x-ray.

Numerical simulation of non-perforating impacts on shielded gas-filled pressure vessels

In order to calibrate the output of hydrocode simulations of hypervelocity impacts on shielded gas-filled pressure vessels, Light Gas Gun impact experiments were performed. In a first step, tests were performed on so-called equivalent Whipple shield (EWS) configurations having basically the same set-up as the shielded pressure vessels (i.e. bumper thickness and - material, stand-off and backwall plate thickness and -material). Purpose was the determination of the impact conditions that lead to penetration into the backwall plate but not perforation of it or leakage through the impacted area. In a second step, impact tests on the corresponding shielded pressure vessels were performed with the same test conditions as the EWS. The purpose of the tests was the investigation whether leakage occurs when the vessel's front wall is not perforated, but just cratered. The test conditions lead to no leakage in all tests. The most important measured damage parameter was the crater depth of the deepest crater in the vessel's front wall/the backwall plate of the EWS, respectively. Hydrocode simulations were then performed to assess the capability of the numerical tool to correctly predict the damage on the impacted vessel surface. Normal impacts of aluminium spheres against shielded vessels were simulated using AUTODYN-2D, including and evaluating the effect of the static stress induced in the vessel walls by the inner pressure. Particular attention was focused on the exact determination of the maximum crater depth caused by the debris cloud impact on the vessel wall/the backwall plate of the EWS, respectively. Bumper and projectile were represented by SPH particles, the vessel shell was represented by a Lagrange grid. The results showed a very good agreement with the measured crater depths of the experiments.

Diagnostics of plasma channel for HIF transport

An alternate technique for heavy ion final transport, from the driver to the target, is by the use of the self-standing Z-pinched plasma channel. Experiments conducted at the Lawrence Berkeley National Laboratory have produced 40 cm long stable plasma channels with a peak discharge current of 55 kA in a 7 Torr nitrogen gas fill. These channels are produced using a double pulse discharge scheme, namely, a pre-pulse discharge and a main capacitor bank discharge. It is postulated that the channel's insensitivity to MHD instabilities within the time scale relevant to beam transport is due to the wall effect the pre-pulse discharge creates. This is accomplished by leaving a gas density depression on the channel's axis after hydrodynamic expansion. Since the pre-pulse discharge creates the initial conditions for the main bank Z-pinch, it is critical to understand how to control and engineer the pre-pulse. Here we present some of the results of ongoing experiments geared to understand the underlying physics of the LBNL Z-pinch plasma channel. Schlieren and phase contrast measurements show the radial propagation of a shock wave during the pre-pulse discharge and suggest indirectly the evidence of the on axis gas density depression, that is believed to be < 1 10 of the original gas fill pressure. For the main bank Z-pinch, interferometry show an integrated electron line density of 1.6×10 17 cm −2 for a 15 kV discharge on axis. These measurements coupled with Faraday rotation measurements will indicate ultimately the current density distribution in the channel. This data will be used to benchmark simulation codes.

Comparison of H−/D− measurement in the magnetically filtered region of the large negative ion source

Characteristics of D− production are experimentally compared with that of H− production, both in extracted negative ion beam and in negative ion density in the extraction region, which is the magnetically filtered region in front of the plasma grid. Extracted D− beam current reduces to 1/2–1/3 in the case of H− beam and electron current extracted simultaneously with D− beam increases more than twice as the case of H2 discharge at the same filling gas pressure. Variation of electron density in the extraction region is similar to extracted electron current. Small reduction of D− density in the extraction region is observed, compared to H− density at the same position, but the ratio of reduction differs from the case of extracted ion beam. When the filling gas pressure is raised, D− density and beam current almost linearly increase, similar to the case of H−.

Current sheath dynamics and X-ray emission studies from sequential dense plasma focus device

A conventional dense plasma focus (DPF) device shows one or two compression phases. In the present paper, we report on a sequential DPF device with modified central electrode design to obtain more than two compression phases (i.e., multiple focusing). The sequential focusing was optimized by taking six different electrode designs for different filling gas pressures of argon. The optimization was inferred on the basis of intensity of spikes of voltage probe signals. The optimized central electrode design has then been used to study current sheath dynamics and X-ray emission using nitrogen laser shadowgraphy and diode X-ray spectrometer, respectively. Shadowgraphs show the breaking of current sheath during first focus as one part of it goes into radial collapse phase, and the other remains in axial acceleration phase. The one that remains in axial phase moves axially ahead in comparison to the other part of the current sheath. A bubble formation is observed after first focus phase. Shadowgraphs also show the formation of weak off-axis second focus. Finally, an on-axis third radial collapse is observed shadowgraphically (X-ray signals depict a multispike structure indicating hereby a sequential X-ray bursts from the sequential DPF device). The plasma electron temperatures have also been estimated using these X-ray signals.

Analysis of the fracture of gas-filled pressure vessels under hypervelocity impact

The first part of the study summarizes the results of hypervelocity impact tests on unshielded thin-walled cylindrical pressure vessels made of aluminium alloy. Impact damages ranged from simple front wall perforation with no further damages inside the vessel to catastrophic bursting into many pieces. Two types of catastrophic bursting were observed: front side and rear side failure. Front side failure was initiated at the rim of the front side impact hole. The second part of the study analyzes the physical mechanisms that are involved in the hypervelocity impact process on pressure vessels and simulates the experimental results presented in the first part. In order to treat this problem, the fragment cloud that was generated from the front side impact was assumed to be completely ablated and decelerated in the gas, which is in correspondance with experimental observations for pressures exceeding a few atmospheres. In the presented model the impact event was divided in different stages, that are treated separately. These are: perforation of the front wall and generation of a strong gas shock wave, propagation and damping of the gas shock wave, impact of the gas shock wave on the rear wall and reflection thereof, propagation of the gas shock wave to the front side and interaction with it. Models for the treatment of front and rear side failure mechanisms are presented. The validity of the presented analysis was proven by simulation of the experimental results obtained in the first part of the paper.

Electrostatic measurement of plasma plume characteristics in pulsed laser evaporated carbon

A triple Langmuir probe measurement has been implemented to investigate plasma plume character in low fluence (∼3.0 J/cm2) pulsed laser evaporation (PLE) discharges and has been found to be an extremely valuable tool. Absolute plasma plume density estimates are found to reside in the range 1.0×1013–2.0×1014 cm−3 for vacuum pulses. A simple heavy particle streaming model for vacuum pulses allows estimates of the plume ionization fraction of ∼10%. This is consistent with typical deposition inventory suggesting that high kinetic energy ions may play an important role in diamond-like carbon (DLC) film deposition. Electron temperature inferred from the electrostatic probe is found to consistently reside in the range 0.5–3.0 eV, and appears to be uninfluenced by operating conditions and large variations in Ar and N2 fill gas pressure. Consistent with strong plume ion and neutral particle coupling to the background fill, constancy of Te suggests expulsion of background gas by the energetic plume. The leading edge ion plume speed is measured via temporal displacement of spatially separated probe signals on consecutive PLE pulses. Flow speeds as high as 5.0×104 m/s are observed, corresponding to ∼156 eV in C+. The ion flow speed is found to be a strongly decreasing function of fill pressure from an average high of ∼126 eV in vacuum to ∼0.24 eV at 600 mTorr N2. Raman scattering spectroscopy indicates DLC film quality also degrades with fill pressure suggesting the importance of high ion kinetic energy in producing good quality films, consistent with earlier work demonstrating the importance of energetic particles. Optical emission indicates an increase in C2 molecular light intensity with fill gas pressure implying a reduced, if any, role of these species in DLC production. Ion current signal anomalies are often seen during high pressure pulses. It is suggested that this may indicate the formation of high mass carbon clusters during plume evolution in the presence of background gas. Mass diffusivity estimates, based on density decay, suggest the presence of C2+ under these conditions. Demonstration and control of such cluster formation may provide method(s) for controlling novel advanced materials properties.

Experimental Investigation of CO2 Laser Assisted by Silent Discharge

A new type of CO2 laser tube has been designed by using a plasma tube having rectangular cross section and is excited by both axial DC glow discharge and transverse high frequency (HF) silent discharge. In this paper, the principle of the design is described briefly and its working characteristics, including of breakdown, V-A, laser power and efficiency, are studied experimentally and compared with those of CO2 laser excited by DC glow discharge only. Under the action of transverse HF silent discharge, the laser power, the laser efficiency, the gas-filled pressure and the fraction of N2 in this laser are increased significantly. In contrast, the breakdown voltage and the discharge voltage of this laser tube are decreased.

Electrostatic Measurement of Plasma Plume Characteristics in Pulse Laser Ablated Carbon

ABSTRACTA triple Langmuir probe measurement has been implemented to investigate plasma plume character in low fluence (∼ 3.0 J/cm2) pulsed laser evaporation (PLE) discharges and has been found to be an extremely valuable tool. Absolute plasma plume density estimates are found to reside in the range 1.0 × 1013–2.0 × 1014 cm−3 for vacuum pulses. A simple heavy particle streaming model for vacuum pulses allows estimates of the plume ionization fraction of ∼ 10%. This is consistent with typical deposition inventory suggesting that high kinetic energy ions play an important role in DLC film deposition. Electron temperature is found to consistently reside in the range 0.5-3.0 eV, and appears to be uninfluenced by operating conditions and large variations in Ar and N2 fill gas pressure. Consistent with strong plume ion and neutral particle coupling to the background fill, constancy of Te suggests expulsion of background gas by the energetic plume. The leading edge ion plume speed is measured via temporal displacement of spatially separated probe signals on consecutive PLE pulses. Flow speeds as high as 5.0 × 104 m/s are observed, corresponding to ∼ 156 eV in C+. The ion flow speed is found to be a strongly decreasing function of fill pressure from an average high of ∼ 126 eV in vacuum to ∼ 0.24 eV at 600 mTorr N2. Raman scattering spectroscopy indicates DLC film quality also degrades with fill pressure suggesting the importance of high ion kinetic energy in producing good quality films. Optical emission indicates an increase in C2 molecular light intensity with fill gas pressure implying a reduced, if any, role of these species in DLC production. Ion current signal anomalies in high pressure pulses indicate the formation of high mass carbon clusters during plume evolution in the presence of background gas. Mass diffusivity estimates, based on density decay, suggest the presence of under these conditions. Demonstration and control of such cluster formation may provide method(s) for controlling novel advanced materials properties.

Emittance and brightness measurement of a high voltage pseudospark electron beam

This is a report of the emittance and brightness measurement of an electron beam produced in a pseudospark discharge device driven by a pulse line accelerator. A ten-gap pseudospark device was operated at 200 kV, in a nitrogen gas fill pressure of 15 Pa. The typical value of emittance was measured to be 47 mn mrad about 5 cm downstream of the anode plane. The dependence of the beam current, HWHM emittance, the normalized emittance, and the normalized brightness on the axial distance from the anode were obtained. The highest brightness is about 2.7 × 1012A/(mrad)2 near the anode, and is still higher than 1010A/(mrad)2, 160 mm downstream of the anode. Such a high quality electron beam can be used for Raman free electron laser, X ray laser producing, and high power microwave.

Correlation Study of Ion, Electron and X-ray Emission fromArgon Focus Plasma

In a low energy argon plasma focus energized by a 32µF, 15kVsingle capacitor, ion, electron and X-ray emission are studied. At alow pressure of 0.25mbar, the X-ray emission region in broad and aconsiderable amount of X-rays originate from the anode surface. Withincreasing filling gas pressure, the X-ray emission zone squeezes topinch filament at the axis. The intensities of the X-ray, electron andion beams signals are found to be correlated mutually as well as withthe high voltage probe signal intensity. The average energy of the Arion beam is found to be filling pressure dependent, and is about2.7MeV at 0.25mbar Ar increasing to 4.8MeV at higherpressure.

Acoustic inspection of inertial confinement fusion targets

Acoustic cavity resonances were used to examine the interior surface geometry and gas fill pressures of various prototype inertial confinement fusion targets. A representative target consists of a millimeter-sized spherical beryllium shell filled to high pressure with deuterium/tritium (DT) gas at room temperature. Below the triple point of DT the gas forms a solid and redistributes itself symmetrically within the shell through a process known as beta-layering. The thickness of the solid DT layer, and the symmetry and smoothness of each surface must adhere to strict specifications for ignition to occur. Sound-speed measurements at room temperature provide the gas fill pressure and thus the solid DT layer thickness upon cooldown. These measurements rely upon an accurate equation of state. Degenerate-mode splitting of the cavity resonances provides interior surface geometry information. This technique was applied to a variety of deuterium and helium filled shells at pressures up to 356 atm. Several of these shells were manufactured with known interior surface perturbations. Measured mode splitting is compared with theory and the utility of the technique for cryogenic targets is examined.

An Intelligent Plasma/Ion Source Controller

Our current plasma source is a low-pressure hot filament arc whose essential features are illustrated schematically in Figure 1. The is able to control the ionizing (primary) electron emission from the filament cathode by adjusting Vf. The primary electrons are accelerated and the discharge is sustained by the variable discharge voltage Vd. (A human adjusts Vf and Vd by turning some knobs.) The gas fill pressure is controllable by a servo actuated needle valve. The operator (either human or mechanical) is able to monitor the source operation by measuring the voltage Vf and Vd as well as the corresponding filament and discharge currents If and Id and the neutral gas pressure (obtainable from thermocouple or ionization type vacuum gauges). (A human reads these values off meters.) The also monitors the real-time performance or output of the source by measuring the plasma density, Ne, and (electron) plasma temperature, Te. The plasma density and temperature themselves are obtained by a computer interfaced Langmuir probe. The details of that circuitry and its operation are described elsewhere (El Jeaid, 1988). In the present device the plasma volume is fixed and is equal to the source chamber volume. In some more advanced experiment additional (independent) variables may be intro

Study of X-ray emission of dense plasma focus device in the presence of external magnetic field

The X-ray emission from a 3.3 kJ dense plasma focus device filled with argon in the presence of an external axial magnetic field has been quantitatively measured for the first time with the help of a diode X-ray spectrometer. The X-ray energy and electron temperature estimated from signals are found to be lower with application of the external axial magnetic field. The variation of the electron temperature with pressure shows a maximum at 80 Pa and a decrease both with increase or decrease of the filling gas pressure.

A study of the rise time in a microstrip gas proportional counter for the development of an X-ray polarimeter

The signal rise times in a microstrip gas proportional counter were investigated as part of the development of an X-ray polarimeter. With a prototype counter, the characteristics of the rise time distributions of pulses from a MSGC (microstrip gas proportional counter) were investigated as a function of fill-gas pressure, energy, gas gain, and reduced field. For lower pressure and X-rays with higher energy, it was observed that the rise time of signals from the MSGC is slower. We confirmed from these results that the size of electron clouds produced by photoabsorption affects the rise time from the MSGC. Because electron clouds produced by photoelectric absorption extend in the direction of the electric vector of incident polarized X-rays, we also investigated the dependence of rise time on the direction of X-ray polarization. When the prototype counter was irradiated with polarized X-rays with electric vector parallel or perpendicular to the MS-plate, a difference of 21 nsec between the peak channels of the rise time distributions was observed.

Debris cloud ablation in gas-filled pressure vessels

The responses of unshielded gas-filled pressure vessels subjected to hypervelocity impact (HVI) are discussed. Similarities and differences between gas-filled pressure vessels and Whipple shields are used to analyze the hypervelocity impact event, specifically the impact of the debris cloud on the rearwall of the vessel. Experimental results demonstrate the effects of important pressure vessel parameters and impact conditions. Damage patterns and mechanisms leading to catastrophic rupture are discussed. Ablation of the debris cloud and pressure vessel wall stress are shown to be important phenomena governing the amount of overall pressure vessel damage. Tests also indicate that ablation is enhanced in the presence of oxygen, and is therefore more extensive in air than in nitrogen under similar test conditions.