Plasmas: Debye length

An important characteristic of plasma is that the free charges move in response to an electric field or charge, so as to neutralize or decrease its effect. Reduced to its smaUest components, the plasma electrons shield positive ionic charges from the rest of the plasma. The Debye length, given by the foUowing ... 

The spectral distribution of light scattered from a plasma depends on its wavelength Xq, the electron Debye length and the scattering... 

In addn, for an ionized gas to be called a plasma, it must have an equal number of pos and neg charges for, by definition, a plasma has no net charge. Regions termed "sheaths , having large (net charges) do develop at the plasma boundaries. Such sheaths are to the plasma what the surface is to a solid or liquid, and their thickness is of the order of the "Debye length ... 

Note The Debye Length is the distance over which the thermal energy of the particles causes major differences in the pos and neg charge densities, hence the max distance within a plasma that a particular charged (either pos or neg) particle can be "seen . The Debye length is not constant. 

PLASMA (Particle). 1 An assembly of ions, electrons, neutral atoms and molecules in which the motion of the particles is dominated by electromagnetic interactions. This condition occurs when the macroscopic electrostatic shielding distance (Debye length) is small compared to the dimensions of the plasma. Because of the large electrostatic potentials... 

The different regions are delineated by the Debye length (d), the mean-free path (X), and the Larmor radius rL). In a plasma, there can be many mean-free paths, since there are many different types of particles (different neutral species, electrons and ions). Of primary interest are the mean-free paths for collisions between electrons and heavy particles (Xe) and ions and heavy particles (Xd). 

The Debye length (d) in a plasma is an indication of the distance a strong electric field can extend from a surface into a plasma. It is given by... 

Although our primary goal is to produce sufficient positrons for antihydrogen production, it is useful to examine the plasma that will be formed and to see if trapped positrons may also be useful for plasma studies. A single component plasma can be confined within a trap. As with two component plasmas, the Debye length... 

In our initial experiments with trapped positrons, if we achieve densities of order n = 108/cm3 with N = 107 positrons in the trap at T = 4.2 K, we will clearly have a plasma in the sense of Eqs. (2) and (4). The Debye length will be much smaller than the dimension of the positron cloud. Correlations will be important since T 0.3. This may make it possible, for example, to study particle transport for a plasma with n 3D 1. To produce a classical plasma for study, the positrons can easily be heated using... 

To study the self-image effect on the interaction between two plates, we will develop a linear theory with respect to the plasma parameter with kt1 = (SncoPBe2 / e)1/2, where k-1 is the Debye length, and cq is the electrolyte concentration in solution. According to this approximation, all the higher distribution functions (gapy, etc.) are represented via the correlation function as (see, for example, [19])... 

The motive diagram for the potential energy of electrons in an ignited mode thermionic converter has a more complicated shape, as shown at the top of Figure 4. The presence of positive ions in the plasma creates a minimum in the electron motive inside the interelectrode gap. There are narrow collisionless sheaths (the order of a Debye length in thickness) at both the emitter and collector edges of the plasma. 

There is no direct indication where glow exists according to and Hq, that is, Tq and can be measured both in dark space and in glow. The calculated Debye length decreases nearly linearly with the distance from the cathode covering the dark space and luminous gas phase, i.e., the value alone does not indicate where is plasma. 

Figure 10. Comparison of charge distributions for different types of boundary conditions for the same bulk plasma parameters. The Debye length is rn/o = 10 for (1) and (la), and rD/a = 2 for (2) and (2a). Dashed and solid lines relate to the BC (I) and (II), respectively.

To sample the plasma in as undisturbed a state as possible, the diameter of the sampling orifice must be large compared with the Debye length in the plasma, which is a measure of the distance of the electric field of an ion as seen by the electrons and is given by ... 

Electroneutrality in the bulk plasma If one is not interested in resolving length scales of the order of the Debye length, the electroneutrality constraint in the bulk plasma is applicable. 

This is an excellent assumption for the plasmas of interest since the Debye length is exceedingly small (10s of pm) compared to the reactor dimensions. Of course, the electroneutrality constraint can t be applied in the sheath, where the Poisson equation... 

Numerically, the Child law sheath can be of the order of 100 Debye lengths in conditions of typical low-pressure discharges applied for surface treatment. More details regarding sheaths, including collisional sheaths, sheaths in electronegative gases, radioftequency plasma sheaths, and pulsed potential sheaths can be found, in particular, in the book of Lieberman and Lichtenberg (1994). 

Neutral species within a plasma undergo diffusive and convective transport identical to that in a gas, however, charged species have an additional restriction as a result of the conservation of charge neutrality over distances which are greater than the Debye length of the plasma (of the order of 100 pm). Therefore, in a plasma which contains an equal number of positive ions and electrons, their transport is coupled through the coulombic interactions forcing them to have an equal diffusivity called the ambipolar diffusivity, DA3). 

---