Wall-stabilized arc

Figure 4-43. General schematic of a wall-stabilized arc with segmented anode.

Fig. 38a—e. ftinciples of DC plasma torch stabilization a arc plasma torch with stabilization by eddy gaseous flow b arc plasma torch with stabilization by longitudinal gaseous flow c wall stabilized arc d autostabilized arc e external stabilization... 

Such arcs can be obtained by sheathing gas flows or by wall-stabilization, as e.g. described by Riemann [352], Magnetic fields can be used to provide rotating arcs, which have a better precision. Here a magnetic field should be applied transversally to the direction of the arc, and the rotation frequency is a function of the magnetic field strength as well as of the arc current, as described in classical dc arc papers from Todorovic et al. (see e.g. Ref. [353]). 

A non-linear wall-stabilized non-transferred arc is shown in Fig. 4 8. It consists of a cylindrical hollow cathode and coaxial hollow anode located in a water-cooled chamber and separated by an insulator. Gas flow blows the arc column out of the anode opening to heat a downstream material, which is supposed to be treated. In contrast to transferred arcs, the treated material is not supposed to operate as an anode. Magnetic 7x5 forces cause the arc roots to rotate around electrodes (Fig. 4-48), which provides longer electrode lifetime. The generation of electrons on the cathode is provided in this case by field emission. An axisymmetric version of the non-transferred arc, usually referred to as the plasma torch or the arc jet, is illustrated in Fig. 4-49. The arc is generated in a conical gap in the anode and pushed out of this opening by gas flow. The heated gas flow forms a very-high-temperature arc jet, sometimes at supersonic velocities. 

Stabilized d.c. arcs may be obtained by sheathing gas flows or by wall stabilization [213]. Magnetic fields can be u.sed to produce rotating arcs, which have higher precision. 

Double-walled carbon nanotubes (DWNTs), first observed in 1996, constitute a unique family of carbon nanotubes (CNTs). -2 DWNTs occupy a position between the single-walled carbon nanotubes (SWNTs) and the multiwalled carbon nanotubes (MWNTs), as they consist of two concentric cylinders of rolled graphene. DWNTs possess useful electrical and mechanical properties with potential applications. Thus, DWNTs and SWNTs have similar threshold voltages in field electron emission, but the DWNTs exhibit longer lifetimes.3 Unlike SWNTs, which get modified structurally and electronically upon functionalization, chemical functionalization of DWNTs surfaces would lead to novel carbon nanotube materials where the inner tubes are intact. The stability of DWNTs is controlled by the spacing of the inner and outer layers but not by the chirality of the tubes 4 therefore, one obtains a mixture of DWNTs with varying diameters and chirality indices of the inner and outer tubes. DWNTs have been prepared by several techniques, such as arc discharge5 and chemical vapor depo-... 

C Con.sider transient une-diinensioiial heal conduction in a plane wall (hat is to be solved by the explicit method. If both sides of (he wall arc at specified temperatures, express tlie stability criterion for this problem in its simplest form. 

The temperature distribution in a long cylindrical steady-state thermal plasma colunm stabilized by walls in a tube of radius R is described by the Elenbaas-Heller equation, assuming heat transfer across the positive coliunn provided by heat conduction with the coefficient X(T). According to a Maxwell equation, cur IE = 0 and the electric field in a long homogeneous arc colunm is constant across its cross section. Radial distributions... 

Interesting for applications is gliding arc stabilization in the reverse vortex (tornado) flow. This approach is opposite that of the conventional forward-vortex stabilization (Fig. 4-59a), where the swirl generator is placed upstream with respect to discharge and the rotating gas provides the walls with protection from the heat flux (Gutsol, 1997). Reverse... 

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