Compression of the Arc

on compressing the plasma arc welding - cutting, with clear illustrations.

Despite the variety of designs of arc plasma torches, the principle of operation of the same and is based on the forced cooling and contraction of the arc column is a liquid or, more likely, the gas flow. There are two types of plasma torches: the arc of direct action (Fig. la) and the arc of the indirect effect (Fig. lb). In both types of plasma torches is one of the electrodes attached to the rod with the tip of its refractory (in most cases made of tungsten). Below we consider the most common type of current plasma torches, in which the electrode is the cathode.

Schematic diagrams of plasma torches
as well - with the arc of direct action and the stabilization of the axial-tion of the plasma jet, b - with the arc, and the indirect effect of vortex stabilization of the plasma jet, 1 - electric-rod, 2 - the gas chamber, 3 - adhesive pad and 4 - nozzle, 5 - workpiece 6 - Power Supply

The electrode rod is mounted in a cylindrical chamber 2, ending a copper-tipped nozzle 4 with an opening coaxial with the rod. Electrode and nozzle is usually made ​​of copper is electrically isolated from each other gasket 3 and cooled by running water. In the arc plasma torches with the indirect effect is the anode nozzle. The arc between the electrode and excited by a nozzle (usually breakdown spark gap high-frequency discharge) passes through the nozzle under the pressure of working gas fed into the chamber. The anode arc spot moves along the inner wall of the nozzle channel, and the column is rigidly stabilized on the axis of the electrode and nozzle. Part of the working gas passing through the arc column, heated, ionized and out of the plasma torch nozzle in the form of a plasma jet. The outer layer of gas, washing the arc column, remains relatively cool and makes electrical and thermal insulation between the plasma flow and the channel of the nozzle, preventing the nozzle from being destroyed. In addition, the hard outer layer of the gas cools the arc column, resulting in a cross section of the column is reduced and the current density and temperature increase. At the same time by reducing the column diameter increases the compressive force on the arc of its own magnetic field. Thus, the thermal contraction of the plasma torch (thermal pinch effect) causes enhancement of the magnetic compression (magnetic pinch effect). The current density in the arc plasma torches up to 100 A/mm2, ie, an order of magnitude higher current density of a free arc. The temperature reaches a few tens of thousands of degrees. When the nozzle exit the plasma flow widens slightly, ie, the nozzle is like a diaphragm, tighten the plasma column. This leads to the axial pressure gradient, the intrinsic magnetic field of the arc, which increases the velocity of the plasma jet from the nozzle to values ​​exceeding the speed of sound.

In the arc plasma torches with the direct action of the anode is the workpiece, the nozzle is also electrically neutral and is used to compress and stabilize the arc column . The principle of compression of the arc column is the same as in the arc plasma torches with the indirect effect. In contrast to the arc plasma torches with indirect plasma jet flowing out of the plasma torch with an arc of direct action, combined with the arc column, and therefore has a higher temperature and heat capacity. Direct excitation of the arc between the electrode and the workpiece through a narrow channel difficult to carry out the nozzle. Therefore, initially normally energized auxiliary arc between the electrode and nozzle, fed mostly by the same source through a current limiting resistance R, and then as soon as it touches the flame products, automatically illuminates the main arc between the electrode and the workpiece, and support for a stable arc burning core is disabled.

In the arc plasma torches with indirect heat from the arc to the workpiece is transferred to a stream of plasma heated by the arc column. This type of plasma torches are used mainly for processing non-conducting material (spraying, sferoidezatsiya, heating, chemical synthesis, etc.). In the arc plasma torches with the direct action of the workpiece is introduced additional share of heat due to the electron current. The efficiency of such plasma torches is much higher and therefore they are useful for cutting, welding, welding and other metalworking. In both types of gas torches is the stabilization of the axial and swirl. When the axial stabilization (Fig. a) the gas passes along the cathode, cools it, and exits through the nozzle. Figure b gas enters the chamber through the tangential openings and moves in a spiral arc column washing vortex flow. Often, for welding and cutting torches are used with double or combined gas stream. These torches have two nozzles. Gas supplied into the inner nozzle, can be called primary, and in the outer - the secondary. Primary and secondary gases may be different in purpose, composition and flow rate. When cutting the primary (usually inert) gas acts as a protection from the environment of the tungsten cathode, a secondary (usually an active molecular) gas is a plasma-forming, cutting. When welding gas supplied to the outer nozzle facilitates compression of the plasma jet, formed by the primary gas, and protects the environment from the effect of heat-affected zone of welded metal.

See also:
Plasma Welding
Plasma Welding Introduction
Plasma Welding Technique
Microplasma Welding
Gases for plasma processing of materials
Separation of plasma jet cutting
Compression of the arc
The energy properties of the plasma arc
Rationalization of plasma welding
Plasma welding and spraying
The plasma melting and remelting
Plasmatron. Requirements for plasmatron
Plasmatron. Schemes, classification
Classification by type of electrode plasma torches
Classification of torches by the nature of the current
Structure of the plasmatron basic units