Plasma current-carrying

The dc plasma jet described for example by Margoshes and Scribner [367] is a current-carrying plasma. This also applies to the disk stabilized arc according to Riemann [352], where the form of the plasma is stabilized by using several disks with radial gas introduction keeping the plasma form stable under different sample loads and the influence of the plasma composition on the plasma properties low. However, the plasma described by Kranz [368] is a transferred plasma. 

At temperatures above 5000 K, the plasma is very viscous, and hence the effective sample introduction into the plasma is difficult. The current carrying plasmas, especially, strongly resist the introduction of aerosol particles because of the cooling effect of the impinging flow. However, the mixing of plasma and sample aerosol with current free plasma jets is easier. 

With the so-called current-free or transferred plasma, the observation zone is situated outside the current-carrying zone. A source such as this can e.g. be realized by the use of a supplementary gas flow directed perpendicular to the direction of the arc current and by the observation zone being in the tail-flame. In this observation zone no current is flowing. This type of plasma reacts significantly on cooling as no power can be delivered to compensate for temperature drops. Therefore, it is fairly insensitive to the addition of easily ionized elements. They do not cause a temperature drop but only shift the ionization equilibrium and give rise to ambipolar diffusion, as discussed previously. 

Particle injection, withdrawal, and heating lead to the emission of bremsstrahlung and synchrotron radiation of an energy much less than that corresponding to the fusion temperature. It is, therefore, lost by the plasma and absorbed in the walls of the vessel, which must be cooled. An additional difficulty is the heat insulation required between the very hot walls of the vacuum vessel ( 1000°C) and the current carrying very cold superconducting coils (at 4.5 K). 

The analytical plasmas are classified according to the method of power transmission to the working gas. There are three dominant types of plasma source in use today (i) Inductively coupled plasmas, ICPs (ii) Direct current plasmas, DCPs (current carrying DC plasmas and current-free DC plasmas) (iii) Microwave plasmas (microwave induced plasmas, MIPs, and capacitively coupled microwave plasmas, CMPs). 

A feature common to all these DC plasma sources is the DC arc discharge. The discharge is stabilized in various ways or transferred away from the arc column to produce a flame-like appearance (plasmajet). DC plasmas are divided into two groups (i) current carrying DC plasmas, and (ii) current free DC plasmas or plasmajets. 

Auroral potential structure Electric structure formed by interaction between field-aligned electric currents and plasma at an altitude of about 10,000 km accelerates the current-carrying electrons that excite or ionize upper atmospheric atoms and molecules. At the same time, it accelerates upward positive ions, such as O, from the ionosphere. 

Ion-neutral reactions in plasmas are studied from the ion current to the wall. The relation between the wall current and the plasma composition is obtained by balancing the total production processes, e.g., electron-impact ionization. Penning reaction, ion-neutral reaction, etc., to the total loss processes, e.g., diffusion, volume recombination, ion-neutral reactions, etc. An example of the balance equation is given by Eq. 5 for an infinite cylinder of radius R, in which electrons are multiplied by electron-impact ionization and lost by diffusion. The example is carried through to obtain the general relation of wall currents to plasma composition. 

The cathode spot, which is essential for the veiy existence of a vacuum arc, is the least understood of all vacuum-arc phenomena. Clearly, it is the source of electron emission for the arc, but it also provides plasma and metal vapor. The cathode spot is a highly efficient electron emitter. The current carried by a single spot depends on the cathode material and may vary from several amperes to a few hundred for most metals. When the arc current exceeds the current that a single spot normally carries, additional spots will form, sometimes by the enlargement or division of the original spot and also by the formation of new spots. Cathode spots do not respond instantly to a demand for an increase in current when the voltage applied across the arc is increased. It would appear that the spots normally operate at maximum current for the available heated emitting area and require a thermal response time of several microseconds to meet a demand for increased current. 

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