Preionization Source Research Articles

Comparison of round and knife-edge-like cathodes on gas-puff implosions.

Preionization is believed to play an important role on the implosion of gas-puff Z pinches. Some experiments have used an external preionization source, e.g., UV light or electron beam. In contrast, other experiments rely completely on over voltage breakdown by the own generator's voltage pulse. However, this approach lacks shot-to-shot reproducibility since self-breakdown is mainly a stochastic process. In this work, we performed a systematic study on self-breakdown using two different cathode geometries: (i) a smooth, round cathode to provide a homogeneous electric field, (ii) a sharp, knife-edge-like geometry to enhance the electric field locally and eventually electron emission. The experiments were carried out on the Llampudken current generator, which provides a current pulse of ∼400kA amplitude and 200ns rise time (10%-90%). We implemented gated XUV imaging, filtered diodes and time-integrated x-ray imaging to obtain information about the implosion as well as the stagnation phase for the two cathode geometries. We found that erosion of the knife-edge cathode might be a serious problem, and we had to replace it every 15 shots. On the other hand, the round cathode lasted for the whole series of experiments. We also measured a more reproducible and larger peak current for the knife cathode. From the photo-conductive detectors we observed that even if the round cathode might present shots with higher x-ray yield compared to the knife cathode, dispersion is almost twice as large. Moreover, after a statistic analysis, it is demonstrated that the dispersion in the yield is due solely to differences imposed by the cathodes and not to variations in the driver, as no correlation was found between them. We found that in order to fit the experimental data with the snowplow model, only ∼60% of the total mass is compressed in the knife cathode while ∼20% for the round one, highlighting the importance of the cathode and preionization. Therefore, we conclude that the use of the knife cathode increases the reproducibility of the experiment in comparison with the round cathode.

Plume Performance of Electrodeless Plasmoid Electromagnetic Propulsion

Electrodeless plasmoid electromagnetic propulsion was recently proposed for robotic and crewed space missions. This electrodeless construction has the potential to solve the existing limitations, such as duration and low efficiency. The rotating magnetic field (RMF) excitation and Lorentz Force acceleration mechanism of the plasmoid has the advantage of producing a highly variable thrust and a discrete impulse. In this article, the plume performance of the proposed propulsion primer was presented for the first time and evaluated by varying the RMF input power, bias magnetic field, and flow rate, while the power of the preionization source and discharge propellant are held constant. The RMF excitation and Lorentz Force acceleration mechanism of the plasmoid were confirmed by the plume performance and discharge characteristics. The stable discharge in Ar, N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , and Xe demonstrated the potential application of this electrodeless technology in future multipropellant high-power propulsion applications. The phase angle of 90° in the RMF coil was essential for the higher performance of the plasma plume. The propulsion thrust and specific impulse were estimated to be 16.1 mN and 1600 N <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\cdot $ </tex-math></inline-formula> s, respectively, at 1.2 kW from the initial test conducted using the Langmuir probe, assuming a complete penetration of RMF in the plasma.

Interactions between atmospheric pressure plasmas and metallic catalyst particles in packed bed reactors

Atmospheric-pressure plasmas sustained in packed bed reactors (PBRs) are being investigated for chemical conversion of gases and pollution control. Metallic catalysts added to the surfaces of the dielectric beads of PBRs can increase the energy efficiency and selectivity of chemical processes by reducing operating temperature and providing additional reaction pathways. In this paper, results from a computational investigation of plasma surface interactions between micron-scale metallic catalysts and humid-air plasmas in PBRs are discussed. We found that high plasma density regions form in the proximity of the metallic catalysts. These higher-density plasma regions were confirmed experimentally using ICCD imaging. The intense plasmas result from geometrical electric field enhancement and redistribution of charges within the conductive particles, leading to further enhancement. The high electric field at the triple points of the catalysts can produce electric field emission of electrons, which provides a pre-ionization source or additional source of electrons. These regions of high electric field and sources of electrons guide discharges towards the catalysts and increases fluxes of excited species, ions, electrons and photons to their surfaces. These fluxes are focused primarily at the triple points between the metal, dielectric and gas. As a result, the catalyst is locally heated, which could lead to further increased rates of thermocatalytic reactions on the surface. Surface roughness of the metal inclusions can lead to additional electric field enhancement, which changes the character of the discharges in the vicinity of the catalysts while reducing breakdown voltage.

Start-Up Studies of GLAST-III Spherical Tokamak in the Presence of Poloidal Field

GLAST-III (glass spherical tokamak) is a small device having an aspect ratio of two (R/a = 2) with major radius R = 20 cm and minor radius a = 10 cm. Experiments are performed to examine the effect of increasing the poloidal field (PF) produced by poloidal coils as well as to study the effect of the inductance of PF coils on the plasma current formation. To investigate the GLAST-III plasma, several diagnostic tools such as Rogowski coil, loop voltage, photodiode, Optical emission spectroscopy, high-speed camera, and Langmuir probe are used. After optimizing the working pressure, the first experiment is performed using toroidal field coils and central solenoid along with radio frequency (RF) pre-ionization source. A plasma current of 1 kA is achieved for 1.2 ms. In later experiments, the effect of the poloidal magnetic field is also included. It is observed that with increasing the PF, the plasma current increases, attain a maximum value (5 kA) and then after a critical value of the applied poloidal magnetic field, the plasma current starts decreasing. Similarly, with the inclusion of inductor in series with the PF coils, the plasma current value is reduced and its pulse duration is increased. Both optical emission and Langmuir probe diagnostics results show a similar trend of the plasma current.

Observation of Streamer Coronas Preceding the Formation of an Apokampic Discharge

It is observed that the formation of an apokamp is preceded by the appearance of streamer coronas starting from the channel of a repetitively pulsed discharge. It is shown that the streamer coronas under experimental conditions alleviate the development of an apokamp, acting as a preionization source.

Stable current sheath dynamics caused by preionization source in a small plasma focus device using He as an operating gas

ABSTRACTPinching of the current sheath in the plasma focus (PF) device is an important phase to transfer the stored energy to the high density short lived plasma for the production of a group of useful radiations. The current sheath formation during the initial breakdown of the gas is a critical process in the PF operation. In this regard, the understanding of the current sheath dynamics is of prime importance. In this paper, we present the effects of alpha particle induced preionizaion by U238 in 3.3 kJ He operated PF device on the dynamics of current sheath, focus amplitude and axial rundown velocity by employing Rogowski coil and High voltage probe as main diagnostic tools. The formation of parabolic axisymmetric sheath is confirmed by Rogowski coil and high voltage probe signals which also reveals the variation in focusing time as well as amplitude during collapse phase by using the preionization source. Time resolved X-ray diagnostics are also carried out using PIN diodes covered with Al and Mg filters of 12.5 and 25 µm which provide an important information about an early onset of X-ray generation. Time integrated X-ray imaging is also carried out to estimate X-ray production zone in the PF device indicating enlarged and symmetric images with preionization.

Development of microwave pre-ionization source for GLAST tokamak

A microwave pre-ionization source is designed, fabricated and installed on Glass Spherical Tokamak (GLAST) to reduce the high-loop voltage requirement during the plasma startup. In GLAST II, limited space is available at the in-bore side, which restricts the size and heating capability of central solenoid. Therefore, the central solenoid alone cannot produce the required loop voltage for plasma breakdown. Electron Cyclotron Resonance Heating (ECRH) can be very useful to overcome this limitation. In this work, an economical microwave source is fabricated using magnetron of 2.45±0.02GHz, obtained from a household microwave oven. Pulsed-mode operation of the magnetron is achieved through certain modifications in the half-wave circuit of domestic microwave oven. A new technique is proposed to enhance the microwave power, where an additional electromagnet is introduced around the magnetron cavity that confines the fast moving electrons. This modified microwave source is sufficient to achieve the breakdown in GLAST II with improved plasma current of 5kA.

Mixing gasdynamic laser with nonequilibrium arc excitation

A mixing gasdynamic laser with nonuniform arc excitation is investigated using a model setup. Tentative analysis of the results indicates the appropriateness of using plasmatrons to improve the efficiency of mixing gasdynamic lasers by making carbon dioxide molecules vibrationally more nonuniform. In addition, a plasmatron serves as a preionization source both for a fast-flow gas-discharge laser and for a gasdynamic laser with combined pumping.

Battery-powered pulsed high density inductively coupled plasma source for pre-ionization in laboratory astrophysics experiments.

An electrically floating radiofrequency (RF) pre-ionization plasma source has been developed to enable neutral gas breakdown at lower pressures and to access new experimental regimes in the Caltech laboratory astrophysics experiments. The source uses a customized 13.56 MHz class D RF power amplifier that is powered by AA batteries, allowing it to safely float at 3-6 kV with the electrodes of the high voltage pulsed power experiments. The amplifier, which is capable of 3 kW output power in pulsed (<1 ms) operation, couples electrical energy to the plasma through an antenna external to the 1.1 cm radius discharge tube. By comparing the predictions of a global equilibrium discharge model with the measured scalings of plasma density with RF power input and axial magnetic field strength, we demonstrate that inductive coupling (rather than capacitive coupling or wave damping) is the dominant energy transfer mechanism. Peak ion densities exceeding 5 × 10(19) m(-3) in argon gas at 30 mTorr have been achieved with and without a background field. Installation of the pre-ionization source on a magnetohydrodynamically driven jet experiment reduced the breakdown time and jitter and allowed for the creation of hotter, faster argon plasma jets than was previously possible.

The effect of working gas impurities on plasma jets

Air intrusion reduced the purity of working gas inside the tube for plasma jet, and thereby, affected the discharge dynamics. In this paper, the effect of using working gas with different purity level (helium purity 99.99999%, 99.99%, 99.9%, and 99%) on photoionization and the chemical reactivity of plasma jet were studied using a 2 dimensional plasma jet model. Photoionization of air species acted as a source of pre-ionization in front of the ionization region, which facilitated the transition from localized discharge to streamers inside the tube. The density of reactive species inside the tube was found to increase with the concentration of working gas impurities. For the highest purity helium (99.99999%), despite a low photoionization rate and the distance between the photoionization region and ionization region inside the tube, by increasing the applied voltage and decreasing the distance between the electrode and nozzle, plasma jets were formed

Estimation of Electron Temperature on Glass Spherical Tokamak (GLAST)

Glass Spherical Tokamak (GLAST) is a small spherical tokamak indigenously developed in Pakistan with an insulating vacuum vessel. A commercially available 2.45 GHz magnetron is used as pre-ionization source for plasma current startup. Different diagnostic systems like Rogowski coils, magnetic probes, flux loops, Langmuir probe, fast imaging and emission spectroscopy are installed on the device. The plasma temperature inside of GLAST, at the time of maxima of plasma current, is estimated by taking into account the Spitzer resistivity calculations with some experimentally determined plasma parameters. The plasma resistance is calculated by using Ohm's law with plasma current and loop voltage as experimentally determined inputs. The plasma resistivity is then determined by using length and area of the plasma column. Finally, the average plasma electron temperature is predicted to be 12.65eV for taking neon (Ne) as a working gas.

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.

The formation of diffuse discharge by short-front nanosecond voltage pulses and the modification of dielectrics in this discharge

The dynamics of diffuse discharge formation under the action of nanosecond voltage pulses with short fronts (below 1 ns) in the absence of a source of additional preionization and the influence of a dielectric film on this process have been studied. It is established that the diffuse discharge is induced by the avalanche multiplication of charge initiated by high-energy electrons and then maintained due to secondary breakdowns propagating via ionized gas channels. If a dielectric film (polyethylene, Lavsan, etc.) is placed on the anode, then multiply repeated discharge will lead to surface and bulk modification of the film material. Discharge-treated polyethylene film exhibits a change in the optical absorption spectrum in the near-IR range.

Development of a high energy pulsed plasma simulator for the study of liquid lithium trenches

To simulate detrimental events in a tokamak and provide a test-stand for a liquid-lithium infused trench (LiMIT) device [1], a pulsed plasma source utilizing a theta pinch in conjunction with a coaxial plasma accelerator has been developed. The plasma is characterized using a triple Langmuir probe, optical methods, and a calorimeter. Clear advantages have been observed with the application of a coaxial plasma accelerator as a pre-ionization source. The experimental results of the plasma gun in conjunction with the existing theta pinch show a significant improvement from the previous energy deposition by a factor of 14 or higher, resulting in a maximum energy and heat flux of 0.065±0.002MJ/m2 and 0.43±0.01GW/m2. A few ways to further increase the plasma heat flux for LiMIT experiments are discussed.

Reduction of plasma density in the Helicity Injected Torus with Steady Inductance experiment by using a helicon pre-ionization source

A helicon based pre-ionization source has been developed and installed on the Helicity Injected Torus with Steady Inductance (HIT-SI) spheromak. The source initiates plasma breakdown by injecting impurity-free, unmagnetized plasma into the HIT-SI confinement volume. Typical helium spheromaks have electron density reduced from (2-3) × 10(19) m(-3) to 1 × 10(19) m(-3). Deuterium spheromak formation is possible with density as low as 2 × 10(18) m(-3). The source also enables HIT-SI to be operated with only one helicity injector at injector frequencies above 14.5 kHz. A theory explaining the physical mechanism driving the reduction of breakdown density is presented.

Order of Magnitude Enhancement in Axial Run-Down Velocity of a Current Sheath and the Focus Duration of a PF Device with Preionization

The effects of the neon gas pressure and preionization on the current sheath dynamics in a low energy (3.3 kJ) Mather type plasma focus (PF) are investigated. The formation and dynamics of the current sheath are monitored by using a Rogowski coil and a high voltage probe. The experiment is performed to explore the evolution of the plasma sheath and to estimate the range of its velocity during the run-down phases. The Rogowski coil and high voltage probe signals reveal an axisymmetric parabolic current sheath, which propagates down the co-axial tube, and a longer confinement time during the collapse phase with a preionization source.

Effect of preionization on the axial run-down velocity, focus amplitude and current sheath formation in 3.3 kJ small He plasma

The sheath dynamics is an important parameter in dense plasma focus. In this paper, the effect of preionization on sheath dynamics, pinching and its filamentary structure is studied by measuring the axial run-down velocity and its confinement time. A discharge is produced in a 3.3 kJ pure He plasma. The Rogowski and high-voltage signals reveal the formation of an axisymmetric parabolic current sheath, high focusing amplitude during collapse phase with preionization source. The working pressure effect on sheath dynamics and spatial evolution is also studied. The enlarge and symmetric pinhole images with preionization support and conform with the results obtained.

Effect of preionization on the dynamics of current sheath in a small plasma focus device

In the dense plasma focus (PF) device, the dynamics of the current sheath is an important parameter for good focusing or pinching. In this paper, we present the effect of preionization on current sheath dynamics, its pinching evidence and details of its filamentary structure by measuring the axial run-down velocity, confinement time and focus amplitude in a miniature PF device using a Rogowskicoil and high-voltage (HV) probe. For this purpose, the discharge is produced in deuterium gas with deposited energies lying in the range of 3.3 kJ. The Rogowski coil and HV probe signals reveal the formation of an axisymmetric parabolic current sheath propagating down the coaxial tube, and longer confinement time of it during the collapse phase and increase in focus amplitude with preionization source. The axial run-down velocity of the current sheath is increased from 2.1 to 3.2 cm/μ s. The focus duration is from 45 to 72 ns and the focus amplitude is from 1.8, which increases to 2.9 with preionization at the optimum pressure. The effect of the working pressure on the current sheath dynamics and its spatial evolution are also studied.

Atmospheric Pressure Cold Argon/Oxygen Plasma Jet Assisted by Preionization of Syringe Needle Electrode

An atmospheric pressure nonequilibrium argon/oxygen plasma jet assisted by the preionization of syringe needle electrode discharge is reported. With the syringe needle plasma as its pre-ionization source, the hybrid barrier-jet was shown to generate uniform discharge with a lower breakdown voltage and a relatively low gas temperature varying from 390 K to 440 K, even when the vol.% oxygen in argon was up to 6%. Utilizing the actinometry method, the concentration of atomic oxygen was estimated to be about in an orders of magnitude of 1017 cm−3. The argon/oxygen plasma jet was then employed to clean out heat transfer oil, with a maximum cleaning rate of 0.1 mm/s achieved.

Effect of preionization on soft x-ray emission and plasma dynamics in a small plasma focus system

A conventional plasma focus device is operated in a volumetric mixture ratio of Ne+He (50:50) to produce a hot and dense plasma without and with preionization which emits radiation in the soft x-ray region. A ring of depleted uranium is employed as a preionization source. Quantrad silicon PIN diodes and a multipinhole camera with suitable filters are used for x-ray analysis. X-ray emission in 4π geometry is measured as a function of Ne+He mixture filling pressure. The experimental results show that preionization not only enhances the yield of the soft x-rays but also increases the focus duration and shot to shot reproducibility of plasma focus. The x-ray images reveal that preionization makes the focus filament symmetric and also enhances its volume.