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Pseudo-spark switch (PSS) characteristics under different operation conditions
The present paper concentrates on the characteristics of the pseudospark switch (PSS) designed in a previous work. The special characteristics of PSS make it a replacement for other high voltage switches such as thyratrons and ordinary high-pressure spark gaps. PSS is characterized by short rise time and small jitter time. The pseudo park chamber consists of two hollow cylindrical electrodes made of a stainless steel material (type 306L) separated by an insulator. The insulator used in our design is a glazed ceramic 70 mm in diameter and 3.5 mm in thickness. A PSS with an anode voltage of 29.2 kV, and a current of 3.6 kA and 11 ns rise time was achieved and used successfully at a repetition rate of about 2.2 kHz. A simple trigger circuit designed, built, and used effectively reaching more than 1.56 kV trigger pulse which is sufficient to ignite the argon gas inside the cathode to cause a breakdown. A non-inductive dummy load is designed to be a new technique to find the accurate value of the PSS inductance. A jitter time of ±10 ns pulses is observed to occur in a reliable manner for more than 6 h of continuous operation. In this research, the important parameters of this switch like rise time, peak current, and anode voltage were studied at various values of charging capacitance. The lifetime of this system is depending on the kind of the electrode material and on the type of insulation material in the main gap of the pseudospark switch.
Reduction of hydrogen content in deuterium plasma with mixed graphite and tungsten divertors in EAST
Reduction of hydrogen content in deuterium-fueled fusion plasmas is important not only to avoid diluting reacting core deuterons but also so as to allow the optimization of hydrogen-minority-heating efficiency in those fusion devices which employ ion-cyclotron-radio-frequency heating systems. In EAST, the amount of hydrogen released from plasma-facing components has been shown to depend strongly on both their composition and their temperature. In this paper, as measured by thermal desorption spectroscopy, the hydrogen inventory in graphite – used in EAST as lower divertor material – has been determined to be >25 times larger than that of tungsten which comprises the upper divertor. This difference in hydrogen inventory is attributed mostly to the intrinsically porous nature of bulk graphite. Thus the main source of hydrogen release into EAST discharges was identified as the graphite tiles used in the lower divertor. The hydrogen content in EAST plasmas were clearly reduced by first employing a high-temperature vacuum baking of all graphite tiles and then renewing a 100–200 μm thick SiC coating before an EAST experimental run campaign. Subsequent active surface conditioning of all wall components with elemental silicon and then with elemental lithium were seem to again reduce the plasma hydrogen content significantly – with lithium proving to be more effective than silicon. Combining these several techniques, H/(H + D) levels as low as ~3% have been achieved in EAST discharges. Additionally, the effects of lithium thickness on H surface implantation and retention has been re-examined semi-quantitatively using data from a previous run campaign. These data suggest that relatively thick Li films coated on the first wall can effectively isolate the rich source of hydrogen stored in the porous bulk of the underlying graphite from the deuterium-fueled plasma so as to minimize hydrogen release. In conclusion, an operational maneuver whereby a diverted plasma is repetitively switched from an upper single null configuration to a lower single null configuration is presented. This switching maneuver has been shown to suppress hydrogen influx into a 35s-long EAST discharge by alternately mitigating the rise in divertor temperature.
Sharpening of the front of the current through a cylindrical foil liner
Results are presented from experiments on the electromagnetic implosion of aluminum foil liners at the MIG generator with a current rise time of ≈80 ns. Plasma with a density of 10{sup 17} cm{sup –3} was preliminarily injected into the liner region by using a set of radial plasma guns. The Lorentz force J × B causes plasma acceleration in the radial direction. Since the magnetic field pressure is inversely proportional to the radius squared, the plasma displacement is maximum near the liner surface. As a result, plasma motion becomes two-dimensional, a gap appears between the plasma and the liner, and the generator current is switched over to the liner. The plasma velocity at the liner surface is close to the local Alfvén velocity, while the time during which the current is switched over to the liner is nearly equal to the ratio of the liner length to the Alfvén velocity. The proposed scheme allows one to decrease the rise time of the current through the liner to several nano-seconds and, as a result, to reduce the initial liner radius and improve the stability of liner implosion.
Frequency and energy characteristics of a Cu – Ne laser at different durations of the leading edge of the excitation pulse
The lasing characteristics of a copper vapour laser in a tube with forced heating, having a length of 50 cm and a diameter of 2 cm, excited by a train of pulses are investigated. Comparative studies of the frequency and energy characteristics of the laser are performed with a leading edge duration of the excitation pulse of ∼25 ns (when the capacitance discharges through a thyratron and a magnetic compression line) and ∼3 and 1 ns (using circuits with a high-speed switch – kivotron). It is shown that a decrease in the leading edge duration gives rise to an increase in the optimal pulse repetition rate up to ∼30 kHz, the generation efficiency up to 3.2 % and the generation power per unit length over 100 W m{sup −1}. The results obtained confirm the concept of limiting the frequency and energy characteristics of a copper vapour laser due to the insufficient rate of energy input into the plasma at high prepulse electron concentrations. (paper)
A High Voltage Fast Switching Power Module for Active Control of Magnetic Fields and Edge Plasma Currents in Fusion Validation Platforms (Final Report)
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PLASMA BREAKDOWN IN SULFUR HEXAFLOURIDE AND AIR MIXTURES FOR HIGH VOLTAGE SWITCH APPLICATIONS
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Uncovering a membrane-distal conformation of KRAS available to recruit RAF to the plasma membrane
The small GTPase KRAS is localized at the plasma membrane where it functions as a molecular switch, coupling extracellular growth factor stimulation to intracellular signaling networks. In this process, KRAS recruits effectors, such as RAF kinase, to the plasma membrane where they are activated by a series of complex molecular steps. Defining the membrane-bound state of KRAS is fundamental to understanding the activation of RAF kinase and in evaluating novel therapeutic opportunities for the inhibition of oncogenic KRAS-mediated signaling. We combined multiple biophysical measurements and computational methodologies to generate a consensus model for authentically processed, membrane-anchored KRAS. In contrast to the two membrane-proximal conformations previously reported, we identify a third significantly populated state using a combination of neutron reflectivity, fast photochemical oxidation of proteins (FPOP), and NMR. In this highly populated state, which we refer to as “membrane-distal” and estimate to comprise ~90% of the ensemble, the G-domain does not directly contact the membrane but is tethered via its C-terminal hypervariable region and carboxymethylated farnesyl moiety, as shown by FPOP. Subsequent interaction of the RAF1 RAS binding domain with KRAS does not significantly change G-domain configurations on the membrane but affects their relative populations. Overall, our results are consistent with a directional fly-casting mechanism for KRAS, in which the membrane-distal state of the G-domain can effectively recruit RAF kinase from the cytoplasm for activation at the membrane.
Underwater plasma breakdown characteristics with respect to highly pressurized drilling applications
Deep earth drilling is a key technique to extract oil, gas, and geothermal heat from the earth. Many complex energy focusing methods have been explored as an alternative approach to reach these resources but most of them require high energy. However, by utilizing short time span liquid plasma discharges, energy focusing can be achieved within traditional drilling systems. These discharges induce a rapid expansion process and a resulting shockwave. It is believed that this focused energy will lower the required cutting force to progress through the rock. Lowering the required cutting force will allow for lower drill bit wear, quicker rate of penetration, and an overall cost savings of the project. Plasma breakdown characteristics at drilling relevant pressures, ranging from 1 to 350 atm, were studied. A resistance–capacitance circuit with an air gapped spark switch was utilized to generate pulsed plasma between the pressurized electrodes. It was found that the required breakdown voltage increases as the pressure increases. It was also found that a plasma channel formation and an associated breakdown may or may not occur between the electrodes at different pressures due to variation in required breakdown voltages. Breakdown time-lag in the dielectric medium (tap water, 780 μS cm –1 ) increased as the pressure was increased, which indicated a higher voltage drop at higher pressures (>100 atm). The plasma generated cavitation bubble with an associated shockwave occurred as pressures were increased. However, the bubble radius and the bubble duration decreased as the pressure was increased. The plasma generated shockwave speeds fall within the expected speed of sound in water. Lastly, preliminary rock cracking tests were performed on granite at high pressures (340 and 272 atm) and it was found that plasma is able to create cracks in the rock.
Coulomb expansion of a thin dust cloud observed experimentally under afterglow plasma conditions
The Coulomb expansion of a thin cloud of charged dust particles was observed experimentally, in a plasma afterglow. This expansion occurs due to mutual repulsion among positively charged dust particles, after electrons and ions have escaped the chamber volume. In the experiment, a two-dimensional cloud of dust particles was initially levitated in a glow-discharge plasma. The power was then switched off to produce afterglow conditions. The subsequent fall of the dust cloud was slowed by reversing the electric force, to an upward direction, allowing an extended observation. At early time, measurements of the Coulomb expansion in the horizontal direction are found to be accurately modeled by the equation of state for a uniformly charged thin disk. Finally, bouncing from the lower electrode was found to be avoided by lowering the impact velocity <100 mm/s.
Plasma Emission, Wave Speeds, and Temperatures of Laser-Supported Plasmas in Air
A study of the generation and subsequent expansion of plasmas using a single beam (532 nm) from the second harmonic generator of a Q-switched Nd:YAG laser is described. The plasmas in air are studied using an intensified charge-coupled device camera over periods of 1000 ns following the laser pulses. The plasma experimental results are interpreted in terms of wave speeds and plasma temperatures. Energy balance equations are considered to estimate the plasma temperatures and the laser-supported radiation (LSR) and the laser-supported detonation (LSD) wave speeds. Within 1000 ns observation time, the circular spectral images turn elliptical because of plasma expansion toward the laser beam. The plasma expansion increases with increasing laser irradiance. A laser irradiance range of [Formula: see text] yields LSR wave speeds between 45 and 170 km/s and LSD wave speeds of 90–130 km/s. The measured wave speeds are compared with the LSR and the LSR wave speed models. The plasma temperatures are between 90,000 and 120,000 K at 15 ns after the laser pulse based on measured emission spectra and are between 640,000 and 760,000 K at 3 ns based on the LSR model.
Simulation of the Plasma Density Evolution during Electron Cyclotron Resonance Heating at the T-10 Tokamak
In ohmically heated (OH) plasma with low recycling, an improved particle confinement (IPC) mode is established during gas puffing. However, after gas puffing is switched off, this mode is retained only for about 100 ms, after which an abrupt phase transition into the low particle confinement (LPC) mode occurs in the entire plasma cross section. During such a transition, energy transport due to heat conduction does not change. The phase transition in OH plasma is similar to the effect of density pump-out from the plasma core, which occurs after electron cyclotron heating (ECH) is switched on. Analysis of the measured plasma pressure profiles in the T-10 tokamak shows that, after gas puffing in the OH mode is switched off, the plasma pressure profile in the IPC stage becomes more peaked and, after the peakedness exceeds a certain critical value, the IPC-LPC transition occurs. Similar processes are also observed during ECH. If the pressure profile is insufficiently peaked during ECH, then the density pump-out effect comes into play only after the critical peakedness of the pressure profile is reached. In the plasma core, the density and pressure profiles are close to the corresponding canonical profiles. This allows one to derive an expression for the particle flux within the canonical profile model and formulate a criterion for the IPC-LPC transition. The time evolution of the plasma density profile during phase transitions was simulated for a number of T-10 shots with ECH and high recycling. The particle transport coefficients in the IPC and LPC phases, as well as the dependences of these coefficients on the ECH power, are determined.
A new method for isolating plasma interactions from those of the laser beam during plasma nitriding
Highlights: • Nitriding of titanium was studied using a laser sustained plasma (LSP) reactor, independent of the laser beam. • Nucleation and growth of faceted, dendritic, rectangular and particulate TiN morphologies was observed. • TiN layers up to 300 μm thick formed within 5 s - one of the most rapid TiN growth rates observed for LSP/laser nitriding. • LSP accessed novel microstructures compared to laser nitriding with or without surface-struck plasm plasma. - Abstract: A new method for separating the interactions of a laser plasma from those of a laser beam with a titanium substrate was developed. A laser-sustained plasma (LSP) was generated by striking the plasma on a titanium plate in a flowing nitrogen atmosphere at ambient pressure and subsequently removing the strike plate. The resultant nitrogen plasma was sustained indefinitely around the laser focal plane until the nitrogen gas flow and/or the laser power were switched off. The laser-sustained nitrogen plasma was then used as a “reactor” to study the effects of a nitrogen plasma, independent of the laser beam, when the plasma was brought into close proximity, (2 to 3 mm), parallel to a titanium substrate. There was no interaction of the parallel laser beam with the substrate. Heat, nitrogen and titanium were exchanged between the nitrogen LSP and substrate resulting in melting, nucleation and growth of faceted and dendritic TiN crystals, rectangular hollow TiN morphologies and particulate TiN deposits. Dense TiN layers up to 300 μm thick formed within 5 s. The results demonstrated that compositionally graded layers of nitrogen-enriched titanium, and titanium nitride could be formed very rapidly on model, commercially-pure titanium substrates, with promise for extension to commercial alloys such as Ti-6Al-4V.
Temperature Distribution Within an Ignition Kernel Initiated by a Laser-Induced Plasma
The sizes of, and temperature distributions within, ignition kernels initiated by a Q-switched neodymium-doped yttrium aluminum garnet laser-induced plasma in an unconfined lean premixed hydrogen-air upward jet flow are investigated. The experiments involved a range of jet velocities and a range of deposited laser energies at a fixed height above the exit along the axis of a burner. The growth of, and the temperature distributions within, the ignition kernels, as affected by the size and the energy distribution of the laser-induced plasma, are monitored with an infrared camera. The initial ignition kernels’ areas are larger with higher laser pulse energies and remain unchanged up to [Formula: see text] and then increase by factors of up to 3 at [Formula: see text]. The change in the kernel area caused by the jet velocities is less than 1.5%. An increase of the bulk velocity by 190% decreases the ignition kernel temperature by 6%. This reduction in the ignition kernel temperatures is because of an increase in energy losses by a factor of 2 and decreases in heat releases by 2% at [Formula: see text] and by 11% at [Formula: see text]. The present contributions are: measurements of and insights into temperature distributions and kernel development rates during the laser-induced plasma ignition process at different deposited energies and flow velocities.
Investigation of local thermodynamic equilibrium of laser induced Al{sub 2}O{sub 3}–TiC plasma in argon by spatially resolved optical emission spectroscopy
Plasma plume of Al{sub 2}O{sub 3}–TiC is generated by third harmonic Q-switched Nd:YAG nanosecond laser. It is characterized using Optical Emission Spectroscopy (OES) at different argon background gas pressures 10, 10{sup 2}, 10{sup 3}, 10{sup 4} and 10{sup 5} Pa. Spatial evolution of excitation and ionic temperatures is deduced from spectral data analysis. Temporal evolution of Ti I emission originated from different energy states is probed. The correlation between the temporal behavior and the spatial temperature evolution are investigated under LTE condition for the possibility to use the temporal profile of Ti I emission as an indicator for LTE validity in the plasma.
CHARACTERIZATION OF PLASMA CONDUCTIVITY BY LASER THOMSON SCATTERING IN A HIGH-VOLTAGE LASER-TRIGGERED SWITCH
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Spatially-periodic microplasmas in complex microchannel networks: wall-plasma interactions and dynamic behavior
Wall-plasma interactions have been observed for spatially-periodic microplasmas generated in 300–700 µm wide channels fabricated in nanoporous alumina. Examination of Ne microplasma discs produced in a standing-wave pattern in Al 2 O 3 channels illustrates the competition between electron production at the sheath-wall interface and loss by recombination in an atmospheric pressure background. Two topologies of the microplasma arrays are observed. For channel widths (d) less than 450 µm, the microplasmas are generally centered in the channel and sustained by electron generation at both plasma sheath/channel wall interfaces, presumably including the release of charge residing in the hexagonal alumina pores. As d is progressively increased from 300 to 450 µm, the microchannel plasma cross-section is gradually transformed from circular to elliptical, and its surface area declines by as much as 50% so as to minimize e - losses to the background gas. Increasing d above ~450 µm abruptly switches the topology to one in which plasmas having a triangular cross-section attach to one of the channel walls in a pattern that alternates along the channel axis. Microfabricating trapezoidal cross-section channels into complex geometries, including the Cornu spiral and intersecting linear arrays, also reveals dynamic behavior in the propagation of microplasmas. For a common spiral structure, observations of plasma expansion show the wavefront propagates over the corrugated surface or within the channel with radial and azimuthal velocities of 3 ± 1 km s -1 and 8 ± 1.5 km s -1 , respectively. Plasma formation is initiated in each ring of the spiral through electron seeding by streamers propagating radially outward at velocities approaching 200 km s -1 . In addition to electrostatic charge-mediated variations in the mean separation between adjacent microplasmas, the time-dependent interference between two 1D microplasma arrays has been observed during plasma expansion. Reproducible ignition of a microplasma ensemble along ridges micromachined into channels near the intersections of two linear arrays has also been realized. The results reported here demonstrate that microchannels with the walls overcoated with any of a variety of materials provide a promising platform for examining in detail the interaction of low temperature plasma with a surface of arbitrary composition and topography.
Thermal Management of Wide-Bandgap Semiconductor Amplifiers Used for Plasma Heating and Control
Princeton Fusion Systems (PFS) has designed, built, and tested a Load Switch printed circuit board (PCB) to demonstrate the capabilities of 2 kV silicon carbide (SiC) cascodes in development by Qorvo towards plasma heating and control applications. Initial tests have been conducted at low power (~100 W) for validation with thermal finite element analysis (FEA) modeling performed by the National Renewable Energy Laboratory (NREL). Comparisons of experimental data with the thermal modeling results, along with considerations for operating in plasma systems, will be discussed.