Debye sheath

The Debye sheath is the transition from a plasma to a solid surface.

Debye sheath theory explains the basic behavior of Langmuir probes, but is not complete.

The quantitative physics of the Debye sheath is determined by four phenomena:

Thus an ion matrix Debye sheath which is depleted of electrons forms around it.

In the theory of the Debye sheath there is a related but not identical condition also known as the Bohm criterion.

The Bohm criterion requires the ions to enter the Debye sheath at the sound speed.

Double layers are conceptually related to the concept of a 'sheath' (see Debye sheath).

The spacing between probes must be larger than the Debye length of the plasma to prevent an overlapping Debye sheath.

Debye sheath - The non-neutral layer, several Debye lengths thick, where a plasma contacts a material surface.

As the potential drop in the Debye sheath is reduced, the more energetic electrons are able to overcome the potential barrier of the electrostatic sheath.

Its scale is the ion Larmor radius at the sound speed, which is normally between the scales of the Debye sheath and the pre-sheath.

If an electrode is inserted into a plasma, its potential will generally lie considerably below the plasma potential due to what is termed a Debye sheath.

A Debye sheath arises in a plasma because the electrons usually have a temperature on the order of magnitude or greater than that of the ions and are much lighter.

One method of confining plasmas is by using the properties of the Debye sheath, a near-surface layer in plasmas similar to the Double layer in other fluids.

The physics is essentially that of a Langmuir probe that draws no net current, including formation of a Debye sheath with a thickness of a few times the Debye length.

The Bohm criterion for ions entering the magnetic sheath applies to the motion along the field, while at the entrance to the Debye sheath it applies to the motion normal to the surface.

The beginning of Langmuir probe theory is the I-V characteristic of the Debye sheath, that is, the current density flowing to a surface in a plasma as a function of the voltage drop across the sheath.

It was first used to give the space-charge-limited current in a vacuum diode with electrode spacing d. It can also be inverted to give the thickness of the Debye sheath as a function of the voltage drop by setting :

For example, if the Debye sheath length on a slotted quartz part is at least half the width of the slot, the sheath will close off the slot and confine the plasma, while still permitting uncharged particles to pass through the slot.

The Debye sheath (also electrostatic sheath) is a layer in a plasma which has a greater density of positive ions, and hence an overall excess positive charge, that balances an opposite negative charge on the surface of a material with which it is in contact.

Looking at it from the other side, any measured I-V characteristic can be interpreted as a hot plasma, where most of the voltage is dropped in the Debye sheath, or as a cold plasma, where most of the voltage is dropped in the bulk plasma.