Filling Gas Pressure Research Articles

Simulation of the motional Stark effect diagnostic gas-filled torus calibration

Many motional Stark effect diagnostics around the world make use of a calibration procedure in which the observed neutral beam is injected into a gas-filled torus with known vacuum fields. The instrument is calibrated by reconciling measured angles with vacuum magnetic reconstructions through a range of pitch angles. This in situ gas-filled torus calibration most closely approximates the working conditions of the diagnostic and includes effects such as beam and viewing geometries, beam voltages, Faraday and stress induced birefringence (in most cases) of the transmissive optics, as well as the polarimeter response. However, secondary neutrals, produced after ionization then reneutralization of a beam neutral, have been found to contaminate measured angles by emitting Balmer alpha with similar Doppler shifts and Stark polarizations as beam neutrals, but with different polarization angles. Simulation results that show spectral and angle behavior versus calibration parameters such as fill gas pressure will be presented. Data from NSTX and C-Mod will be compared to simulations results.

Simulation study of ion effect in the ILC electron damping ring

Ion effect has been recognized as one of the very high priorities of the R&D for the damping rings of the International Linear Collider (ILC) due to multi-bunch operation of extremely low emittance and very high intensity beam. In this paper, we study the ion density near the beam analytically considering the mini-train fill pattern case. The result shows that the gap between the bunch trains can reduce the ion density significantly. Meanwhile, a simulation code is employed to study the fast ion instability in the ILC electron damping ring. Various fill patterns and gas pressures in the ring are investigated. Feedback system is also applied in the simulation.

Hydrogen plasma diagnosis in penning ion source by optical emission spectroscopy

Penning-type discharge plasma ion source is a kind of powerful discharge device applied widely in many fields. In this paper, optical emission spectroscopy (OES) measurement is carried out to diagnose the parameters of hydrogen plasma in a miniature penning-type discharge ion source. The relative intensity of neutral atomic hydrogen emission spectral lines is used to evaluate the electron temperature of hydrogen plasma. The experiment results indicate that the relative intensity of Balmer-α, Balmer-β and Balmer-γ lines exhibits significant dependence on the filling gas pressure and the electron temperature is about 1.5–3 eV in the range of discharge current 30–180 μA when the anode voltage is fixed at 1.4 kV.

Generation of high energy, 30 fs pulses at 527 nm by hollow-fiber compression technique

The compression of 300-fs-long, chirp-free laser pulses at 527 nm down to 30 fs is reported. The laser pulses, originated from a frequency-doubled, mode-locked Nd:glass laser, were compressed by a 0.7-m-long, 150-microm-bore-diameter, argon-filled hollow fiber, and a pair of SF10 prisms with a final energy of 160 microJ. These are the shortest, high energy pulses ever produced by direct pulse compression at the central wavelength of 527 nm. The spectral broadening of the pulses propagating inside the hollow fiber was experimentally examined for various filling-gas pressures and input pulse energies. The spectral width of the pulses was broadened up to 25 nm, and 27 nm for argon- and krypton-filled hollow fiber, respectively, at a gas pressure lower than 2 bar. The physical limitations of the hollow-fiber pulse compression technique applied in the visible range are also studied.

Investigation into Damage of Aluminum Gas-Filled Pressure Vessels under Hypervelocity Impact

Impacts of meteoroids and space debris on pressure vessels can have detrimental consequences for any mission. Depending on the parameters of the impacting particle and the characteristic of the vessel, the damages can range from relatively uncritical craters in the vessel’s surface to the catastrophic bursting of vessels, which besides the loss of vessel may result in severe secondary damages to surrounding components. In order to investigate failure mechanisms of thin-walled aluminum pressure vessels under hypervelocity impact of space debris, a non-powder two-stage light gas gun was used to launch Al-sphere projectiles impacting on unshielded and shielded vessels. Damage patterns and mechanisms leading to catastrophic rupture are discussed. Experimental results indicate that the impact kinetic energy of the projectile and the gas pressure in the vessel have an important effect on the damage modes of the vessel. On the one hand, high pressure gas can lead to a vessel blast. On the other hand, high pressure gas can mitigate the impact of the debris cloud on the rear wall of the vessel. Catastrophic rupture of unshielded gas-filled vessels can be avoided when the impact energy is less than a certain limit value. When the bumper is perforated, damage of shielded pressure vessel might be fatal for vessels with high gas pressure.

Neon soft x-ray emission studies from the UNU-ICTP plasma focus operated with longer than optimal anode length

The UNU-ICTP plasma focus with a significantly longer than conventional anode can still be a reasonably good neon soft x-ray (SXR) source. The highest average neon SXR yield of 3.3 J was achieved at 3 mbar. The time difference between the two first peaks of the voltage probe signal at the radial collapse phase was found to be inversely related to the SXR yield, i.e. the smaller the time difference, the higher the yield and vice versa. The estimation of average current sheath speeds using the shadowgraphic method coupled with laser and focus peak timing signals showed that the average axial rundown speed is similar to the one obtained for the optimal anode length but the average radial compression speed is decreased significantly. The range of pressure for a good neon SXR yield, however, has become much narrower, making efficient plasma focus operation a very sensitive function of the filling gas pressure for longer than the optimal anode length.

Study on breakdown characteristics of a low-pressure dielectric barrier discharge

AbstractThe breakdown characteristics of a low-frequency dielectric barrier discharge (DBD) at low pressure are investigated experimentally in He, Ne and Ar. The current waveform of this DBD, a series of pulses, is caused by the results of the electron avalanches under the action of the applied field and the quenching effect of the wall-charge field on the avalanches. Taking into consideration the diffusion loss of the charged particles in the breakdown processes, the experimental investigation and the theoretical analysis of its breakdown characteristics indicate that its breakdown voltage is higher than the calculation by the Paschen law and depends on the filled-gas pressure and the distance between the electrodes, instead of their product. The ion-induced secondary electron emission coefficient and the mean electron energy at the moment of breakdown can also be determined approximately by measuring the breakdown characteristics of the discharge tubes with different distances between the electrodes and combining the theoretical deductions. The experimental results and the theoretical analysis of the breakdown characteristics of this DBD are discussed.

The boundary layers of ac-arcs at HID-electrodes: phase resolved electrical measurements and optical observations

Arcs are operated in noble gases with sinusoidal and switched dc currents of different frequencies in a model lamp between tungsten electrodes with properties appropriate for high intensity discharge lamps. The sum of the cathode and anode fall, called electrode sheath voltage ESV(t) = uc(t) + ua(t), is determined by a variation of the arc length and the anode fall ua(t) by probe measurements along the arc axis and an extrapolation procedure. It is found that ua(t) adopts low, approximately constant values within a half cycle. Therefore the time variation of the ESV reflects that of uc. It was observed by high speed photography that a high value of uc after current zero crossing (CZC) may initiate a transition of the initially diffuse mode of cathodic arc attachment into a spot mode. It is accompanied by a breakdown of uc causing the formation of a voltage peak starting at current zero crossing. It is a prominent example of the so-called commutation peak (CP). By an adjustment of the operation conditions to a low uc the cathodic arc attachment remains diffuse during the whole half-cycle. It is also indicated by high speed photography that the arc constriction in front of the anode is relieved with increasing operation frequency. It is illustrated by measurements that commutation peaks may be removed by an increase in the arc current, a decrease in the electrode cooling, a reduction of the filling gas pressure and an increase in the operation frequency. Moreover it is demonstrated, that the formation of a CP is sensitively dependent on the electrode surface structure.

Radio-Frequency-Driven Atmospheric-Pressure Plasmas in Contact with Liquid Water

Atmospheric-pressure plasmas were produced on the surface of liquid water by radio-frequency (RF) power supply. A quartz chamber with an off-center inner electrode was used to generate discharges with a wide range of filling gas pressures. When the chamber was filled with fuel gas and liquid water and a sufficient RF power was supplied to the system, plasmas were observed to be produced around the inner electrode as well as on the water surface. Optical emission spectroscopy (OES) identified the generation of oxidative hydroxyl (OH) radicals and decoloration of methylene blue in the solution demonstrated oxidative capacity of water plasmas generated in this system.

Measured dependence of nuclear burn region size on implosion parameters in inertial confinement fusion experiments

Radial profiles of nuclear burn in directly driven, inertial-confinement-fusion implosions have been systematically studied for the first time using a proton emission imaging system sensitive to energetic 14.7MeV protons from the fusion of deuterium (D) and 3-helium (He3) at the OMEGA laser facility [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)]. Experimental parameters that were varied include capsule size, shell composition and thickness, gas fill pressure, and laser energy. Clear relationships have been identified between changes in a number of these parameters and changes in the size of the burn region, which we characterize here by the median “burn radius” Rburn containing half of the total DHe3 reactions. Different laser and capsule parameters resulted in burn radii varying from 20to80μm. For example, reducing the DHe3 fill pressure from 18to3.6atm in capsules with 20μm thick CH shells resulted in Rburn changing from 31to25μm; this reduction is attributed to increased fuel-shell mix for the more unstable 3.6atm implosions rather than to increased convergence, because total areal density did not change very much. Fuel-shell-interface radii estimated from hard (4–5keV) x-ray images of some of the same implosions were observed to closely track the burn radii. Burn radii from one-dimensional (1D) simulations agree fairly well with measurements for glass-shell capsules, but are systematically smaller than measurements for CH-shell capsules. A search for possible sources of systematic measurement error that could account for this discrepancy has been unsuccessful. Possible physical sources of discrepancies are mix, hydrodynamic instabilities, and/or preheat not included in the 1D code. Since measured burn-region sizes indicate where fusion actually occurs as a consequence of all the complicated processes that affect capsule implosion dynamics, it provides exacting tests of simulations.

Plasma Production by Electron Cyclotron Heating on the Internal Coil Device Mini-RT

The internal coil device Mini-RT (Ring Trap) has been constructed in order to study the extremely high beta plasma confinement predicted in two fluid relaxation theory. Plasma is produced by electron cyclotron heating with a 2.45 GHz, 2.8 kW microwave. Plasma experiments were carried out for two conditions: with a mechanically supported coil, and with a magnetically levitated one. In the case in which the internal coil was mechanically supported, plasma densities up to ne=7×1016 m-3 were produced with a filling gas pressure of pn=4×10-2 Pa, and plasma could not be available at a gas pressure less than 10-2 Pa due to the loss of high energy electrons by the supporting structure. It has been clearly demonstrated that the levitation of the internal coil, which was followed by the removal of the supporting structure from the plasma confinement region, improves plasma parameters markedly. The levitation of the internal coil enables plasma production at a lower filling pressure ( pn=1.5×10-3 Pa). It also enables over-dense plasma production (ne=1.5×1017 m-3, while cutoff density is 7.6×1016 m-3 for 2.45 GHz microwave). Since the calculation shows that the efficiency of mode conversion from fast X-mode wave to electron Bernstein wave (FX–SX–B conversion) is sufficiently high (∼87%) in the Mini-RT device, the over-dense plasma production is thought to be caused by heating with electron Bernstein wave (EBW).

Modelling of a nitrogen x-ray laser pumped by capillary discharge

Abstract We performed computer modelling of a fast electrical discharge in a nitrogen-filled alumina capillary in order to discover discharge system parameters that may lead to efficient recombination pumping of soft X-ray laser with active medium created by H-like nitrogen ions. The space-time dependences of pinch plasma quantities were found by means of a one-dimensional MHD code. Time dependences of populations of all ionisation states and populations of selected energy levels of lithium-, helium- and hydrogen-like nitrogen ions were evaluated using the FLY code as a post-processor. The population inversion was found at the quantum transition corresponding to the Balmer α-line of N6+ ions and the resulting gain factor was evaluated for different capillary radii, initial pressure, electric current peaks and periods. A gain factor of 1 cm−1 spanning the time interval of 1 ns was found for an optimised arrangement with capillary radius 1.5 mm, peak current 50 kA, quarter period 40 ns and filling gas pressure 0.5 kPa. It is pointed out that even higher values of the gain factor may be achieved with thinner capillaries and shorter current pulses, e.g. a gain factor of 6 cm−1 is achieved if the capillary radius is 0.5 mm, peak current 56 kA, quarter period 15 ns, and filling nitrogen pressure 3.9 kPa.

Probe diagnosis of electron temperature and electron energy distribution in low-pressure dielectric barrier discharge

The double-probe method is used to diagnose electron temperature and electron energy distribution in low-pressure dielectric barrier discharge (DBD). In this paper, the principle and the setup of diagnosis are described and the experimental results are discussed briefly. The results demonstrate that its electron temperature decreases with the increasing gas-filled pressure and is higher than that in dc discharge under the same discharge condition. It is also evident from experiment that its electron energy distribution is evidently deviated from Maxwellian. There are electron groups of high energy and low energy in the DBD, and the electron group of high energy decreases gradually with the increasing pressure.

Influence of electrode, buffer gas and control gear on metal halide lamp performance

In this paper the influence of electrode composition, buffer gas fill pressure and control gear on the performance of metal halide lamps is investigated. It is shown that pure tungsten electrodes improve lumen maintenance and reduce voltage rise over lamp life. An optimum buffer gas fill pressure condition is discovered which allows for reduced electrode erosion during lamp starting as well as under normal operating conditions. Use of electronic control gear is shown to improve the performance of metal halide lamps.

Development of Multi Faraday Cup Assembly for Ion Beam Measurements from a Low Energy Plasma Focus Device

A multiple Faraday cup assembly has been developed for measuring pulsed ion beam of a low energy plasma focus device. The Faraday cups operating in biased ion collector mode have nanosecond response and these have been used to determine the energy spectrum and flux of fast nitrogen ion beam emerging out of the pinched plasma column. The design feature that makes our Faraday cups unique is that they can register ion energy of higher kinetic value (∼hundreds of keV) as well as lower kinetic value (∼keV). It has been possible to register the ion energy upto a lower kinetic energy threshold of ∼5 keV which is a value much lower than that obtained in any previous works. The correlation of the ion beam flux with filling gas pressure is also reported. Angular distribution of ion measurement reveals a highly anisotropic emission indicating an ion dip at the electrode axis.

Ion beam emission in a low energy plasma focus device operating with methane

An investigation of ion beam emission from a low energy plasma focus (PF) device operating with methane is reported. Graphite collectors, operating in the bias ion collector mode, are used to estimate the energy spectrum and ion flux along the PF axis, using the time-of-flight technique. The ion beam signals are time correlated with the emission of soft x-ray pulses from the pinched focus plasma. The correlation of ion beam intensity with filling gas pressure indicates that the beam emission is maximized at the optimum pressure for focus formation at peak current. Ion beam energy correlations for operation in methane indicate that the dominant charge states in carbon ions are C+4 and C+5. The estimated maximum ion energy for H+, C+4 and C+5 are in the range of 200–400 keV, 400–600 keV and 900–1100 keV, respectively, whereas their densities are maximum for the energy range 60–100 keV, 150–250 keV and 350–450 keV, respectively. These results suggest that the ion beams are emitted from a high density, high temperature, short lived focus plasma, at a time which appears to precede the emission of soft x-ray pulses. The properties of the carbon ion beams are discussed in the context of potential applications in materials science.

Optimization of the high pressure operation regime for enhanced neutron yield in a plasma focus device

The average total neutron yield is measured, using an indium foil activation detector, at various combinations of filling gas pressures (including the higher pressure operation regime) of deuterium, capacitor bank charging voltages, anode lengths and insulator sleeve lengths to optimize the neutron yield from the NX2 Plasma Focus device. A remarkable six-fold increase in the average maximum total neutron yield, to a record value of (7 ± 1) × 108 neutrons per shot, compared to the similar energy UNU-ICTP Plasma Focus device is achieved for deuterium at a relatively much higher filling gas pressure of 20 mbar. The average peak neutron energy for the axial direction (0°), radial direction (90°) and backward direction (180°) is estimated to be 2.89 ± 0.25 MeV, 2.49 ± 0.20 MeV and 2.11 ± 0.12 MeV, respectively. The average forward to radial neutron yield anisotropy is found to be 1.46 ± 0.28. The neutron energy and anisotropy measurements suggest that the neutron production mechanism may be predominantly beam target.

Effect of insulator sleeve length on soft x-ray emission from a neon-filled plasma focus device

Insulator sleeves of different lengths were used at the closed end of the coaxial electrode assembly of the 3.3 kJ United Nation University-International Centre for Theoretical Physics (UNU-ICTP) plasma focus device to investigate soft x-ray (SXR) (900–1550 eV) yield for different filling gas pressures of neon. The time and space resolved SXR emission characteristics were investigated using a multi-channel diode x-ray spectrometer and a CCD based pinhole imaging camera. At pressures below 6 mbar, the average SXR yield was also found to increase with increasing sleeve length from 19 to 22 mm. A further increase in insulator sleeve length to 25 mm, however, decreased the SXR yield. At pressures above 6 mbar, the SXR yields were almost the same for insulator sleeves of different lengths. The UNU-ICTP plasma focus device is found to be best optimized for neon SXR production with a 22 mm insulator sleeve at 4 mbar filling gas pressure, with the highest average yield of 8.2 J per shot, for which the conversion efficiency is about 0.25%. For this sleeve length, even the average yield at 2 or 6 mbar pressure was comparable with the maximum yield of a 25 mm sleeve.

Comparative study of soft x-ray emission characteristics in a low energy dense plasma focus device

An investigation on the soft x rays emitted in a 2.2 kJ Mather-type dense plasma focus device using a multichannel diode spectrometer and a simple pinhole camera is reported. Emitted x rays associated with different shapes (hollow, solid, and hemispherical) of anode and in hydrogen/nitrogen gas medium are compared. The structure of x-ray emitting sites as well as x-ray yields were found to be strongly influenced by the shape of the anode and the filling gas pressure. The maximum yield of 2.2 J into 4π sr was obtained in the case of hemispherical anode in hydrogen gas medium. The x-ray pinhole images of the collapsed plasma with the hemispherical anode indicated spot-like structure having 500–800 μm in diameter. On the contrary, other anode shapes showed columnar pinched structure of 8–10 mm in length and 1–2 mm in diameter. Results indicated that an appropriate design of the anode could enhance the x-ray yield by more than tenfold in a conventional low energy dense plasma focus device.

Electrode material transport and re-deposition in high-intensity arc discharge lamps

A two-dimensional model of sintered electrode activator (in particular, barium) transport in the arc discharge lamp tube volume has been developed, based on a numerical solution of the non-stationary diffusion equation. The dependence of the barium atom flow density at the electrode rod tip on fill gas pressure and electrode dimensions has been calculated. It has been found that the main mechanism of activator transport to the rod tip is evaporation from the sintered body and diffusion through the gas. Evolution of tungsten rod emission characteristics caused by deposition of the evaporated material on its surface has been studied experimentally.