Vladimir G. Geyman

Glow-to-Spark Transitions in a Plasma System for Ignition and Combustion Control

This paper deals with the investigation of a new regime in a plasma ignition and flame control system as applied to air-hydrocarbon mixtures. The system is based on the design of a classical high-current arc plasmatron. Compared with a thermal plasmatron mode, the average discharge current in the described device has been decreased to about 0.1 A. Although average power dissipated in the discharge does not exceed 200 W and average gas temperature at the plasmatron exit for typical regimes of the discharge operation in air is less than 500 K, the device demonstrates reliable ignition and flame stabilization in a wide range of equivalence ratios. Physical mechanism of ignition is associated with the nonsteady-state properties of discharge. At a low current level, the discharge burns in a kind of glow mode, and because of the glow-to-spark transitions, the high-current nanosecond pulses are superimposed on the glow plasma background. Then, the spark discharge initiates the combustion process, which is efficiently sustained in the glow plasma.

Low-Current “Gliding Arc” in an Air Flow

This paper describes the result of the investigation of a gas discharge in a flow of air with electrode geometry typical for the so-called gliding arc. The feature characteristic of the experimental conditions is a rather low level of an average discharge current (of about 0.2 A). The discharge is initiated due to a spark breakdown in the narrow part of the gap. After that, the spark discharge is transformed into a kind of a glow discharge. At the subsequent stage, the plasma column travels in the gap under the effect of a gas flow, and the current is sustained in the regime of a normal glow discharge. The typical cathode-voltage-drop value in this regime is estimated to be about 300 V, an average electric field in the positive column plasma is (700-800) V/cm, and the neutral-particle temperature in the negative glow region is T ≈ 1100 K.

Nonsteady-State Gas-Discharge Processes in Plasmatron for Combustion Sustaining and Hydrocarbon Decomposition

This paper deals with the investigation of nonsteady-state discharge regimes in the plasmatron as applied to air-hydrocarbon mixtures. The electrode system is based on the design of a classical high-current arc plasmatron. Compared with a thermal plasmatron mode, the averaged discharge current in the described device has been decreased to about 0.2 A. Then, the discharge regime can be interpreted as a kind of glow discharge with the random transitions from glow to sparks. Two types of transitions have been observed: completed and non-completed transitions. Completed transition is accompanied by the appearance of a high-conductivity spark channel, and for non-completed transition, a low-conductivity diffuse channel arises. The discharge features have been investigated for plasmatrons with a long-length and with a short-length anode nozzle. The principal features of the nonsteady-state discharge behavior are the same for both designs. However, for the long nozzle, the discharge phenomena mainly proceed inside the anode cavity and for the short nozzle outside the cavity. Use of different designs of the plasmatron anode cavity offers to extend an area of plasmatron applications.

Plasma-Assisted Combustion System Based on Nonsteady-State Gas-Discharge Plasma Torch

This paper describes the experiments with the plasma-assisted combustion system as applied to gaseous hydrocarbons. The system is based on a nonsteady-state gas-discharge plasmatron with a low average current. One of the subjects of the investigations is to elucidate a correlation between the discharge burning regimes in the plasmatron and the properties of the torch flame in the combustion chamber. Depending on the gas-discharge regimes and plasmatron design, the conditions of complete hydrocarbon combustion and partial oxidation have been demonstrated. Aside from that, the data on testing a special power supply intentionally developed for nonsteady discharge powering are presented.

Transient Processes During Formation of a Steady-State Glow Discharge in Air

This paper describes the investigation of an atmospheric-pressure glow discharge in air at a current of 0.05-0.3 A. Before the glow discharge is established, a preliminary nonsteady temporal stage is available in the gap. The principal process, which governs with the nonsteady-state discharge behavior, is the glow-to-spark transition phenomenon. The transition is initiated due to the explosive emission instability in the near-cathode layer of glow-type discharge that results in a microexplosion of the cathode surface and appearing of a spark cathode spot. At a low current, the spot is extinguished, so that the discharge starts burning again in one of the glow modes. After that, a new act of transition occurs and so on. The preliminary nonsteady-state stage ensures two prerequisites. First, an effective gas pressure decreases to a low value. Second, due to microexplosions, conditioning of the cathode surface is provided. Both factors facilitate establishing the normal glow discharge.

High-Current Stages in a Low-Pressure Glow Discharge With Hollow Cathode

This paper presents an interpretation of the dense and superdense glow discharge stages in pseudospark switch geometry. The discharge is treated as a self-organizing system that is able to rearrange itself to provide the current requested by external electric circuit. The principal discharge regions in the glow stages are the hollow-cathode plasma, the positive column plasma, and the double electric layer that separates these plasma regions. A model that allows some quantitative estimates when applied to the hollow-cathode plasma is developed, in which a generalized secondary emission coefficient that considers an external emission current is introduced. The abrupt transition from dense glow stage to superdense glow stage occurs because of microexplosions at the cathode surface and appearing the metal vapor plasma. In terms of the model, this means an abrupt increase in the secondary emission coefficient. The comparison with the experiment is made for discharges in hydrogen and xenon at a current up to several kiloamperes and at a current rise time from several microseconds to hundreds of nanoseconds. The physical reasons for the current quenching effect that manifests itself at a decreased gas pressure are also discussed.

Low-voltage triggering for a pseudospark switch with an auxiliary glow discharge

Different electric circuits for triggering the switch with a trigger unit based on an auxiliary glow discharge are discussed. Most attention is concentrated on the recent experimental results on low voltage triggering of the switch and on the mechanisms for the main discharge initiation under the action of the trigger pulse. Due to modifications in the trigger electric circuit, the switch is triggered starting from a voltage of 50 V at a current level in the trigger circuit of 10 mA. With a voltage of 200-250 V and a trigger current of about 0.25 A, the delay time to triggering does not exceed 200 ns.

Methane Oxidation in a Low-Current Nonsteady-State Plasmatron

This paper describes the results on methane oxidation in the plasma torch of low-current plasmatron at typical air expenditure of 0.1-0.55 g/s and at a flow velocity in a longitudinal direction up to 22 m/s. The discharge in a vortex gas flow burns in a glow regime with the spontaneous transitions from glow to spark. Due to special design of the plasmatron nozzle (with a ring groove at the inner surface of the nozzle), an efficient interaction of the gas flow with plasma column and the reproducible data on chemical gas composition in a combustion chamber are provided. An average discharge current in the plasmatron was varied from 0.05 to 0.2 A, which corresponded to an average power dissipated in the discharge from 60 to 150 W. A heat power due to fuel burning in the plasma torch was at a level of 1 kW. The data on chemical gas composition in the combustion chamber in a wide range of air excess coefficient α had been obtained. For the lean air-to-fuel compositions (that is for α > 1), the lower flammable limit was of α ≈ 3. In a regime of syngas generation, i.e., for the rich air-fuel mixtures, the upper flammable limit was of about α ≈ 0.55. It is demonstrated that both the low and the upper flammable limits depend on the discharge current.

Temporal structure of the fast electron beam generated in the pseudospark discharge with external triggering

This paper deals with the investigation of the fast electron beams that form in the high-current pseudospark discharge at different stages of its development. Three short-duration peak of the beam current at the discharge axis have been revealed. The physical reasons for the appearance of these peaks of the beam are discussed. The main idea of the proposed physical mechanism is that the electrons are accelerated in a double electric layer, which forms between the hollow cathode plasma and the near-anode plasma.

Nanosecond Triggering for Sealed-Off Cold Cathode Thyratrons With a Trigger Unit Based on an Auxiliary Glow Discharge

This paper deals with the investigations of ceramic-metal sealed-off pseudospark switches with an auxiliary glow discharge in the trigger unit. The switches are produced commercially by the Pulsed Technology Ltd., Ryazan, Russia. Attention is concentrated on the switch TPI1-10k/50 (pulsed current-10 kA and anode voltage-up to 50 kV). The schematic design of the devices and the principles of function are described. It is demonstrated that the switch is able to operate with an electric circuit corresponding to the classical hydrogen thyratron and to the thyratron with a grounded grid. In both cases, a delay time to triggering in the vicinity of 100 ns and a nanosecond jitter in the delay time are achievable.