In this type of configuration electrons that are closes to the positively charged sphere are held at the highest potential and the one further are at lower potenttial.  Because of this the further the electron is from the sphere the easier it is to knock it out and reduce the amount of shielding.  Most of these electrons are knocked out due to the thermal interactions, thus the edge of the shield is at the location where the potential energei is equal to thermal energy KT of the particles.  Thus from here we can define the Debye length, which is the thickness of the shield, as shown below.

Equation (1) can further be simplified in equations (2) and (3), thus we don’t need to keep track of the constants. Final units are given in square brackets [].

(2)

(3)

Now that we have defined the Debye length we can use it to look at the criteria necesary in order to call an ionized gas a plasma. On of the additional plasma parameters is , which is the number of particles present inside the Debye shield.  Equation for this quantity is given in equation (4).

(4)

From here we can gather the three criteria that plasma must satisfy are outlined below:

So from here we can gather the first criteria that the plasma length defined as L must be much greater than the Debye length.  Second criteria states that the number of particles inside the Debye shield must be much greater than one, this one seems like a pretty obvious one.  Third criteria, which will be covered in more detail later, states that the product of (frequency of typical plasma oscillations) and (mean time between colissions with neutral atoms) is greater than one.  All these three criteria will ensure that the gas behaves like plasma rather than the neutral gas.

Currently there are four know states of mater, plasma is one of them.  Most people are familiar with solid, liquid and gaseous states of mater, but very few people know much about plasma.  It is said that about 99% of all matter in universe is in plasma state, yet on Earth we see very little plasma.  This is because the thing that makes mater transition from one state to next is temperature.  In the case of water ice is the solid state and it occurs when temperature is below 0C, liquid water occurs between 0-100C and above 100C is vapor or water’s gas state.  In order to make water into plasma we would have to raise the temperature of the water to thousands of degrees, which is very impractical here on Earth, but on Sun and billions of other stars it’s a completely different story.

What makes plasma unique is that, while for all other states of matter molecules and atoms stay intact, in plasmas the dissociate from each other.  This means that the ions get created.  So if you have some hydrogen atoms floating around when the temperature gets increased they will start to move extremely fast, so fast that when they hit into each other they would loose an electron.  This electron would flow freely in space until is captured by another free proton that’s floating in space.  In plasma this happens constantly and in the end we have a very large number of free protons and electrons zooming around, but we have very little of actual Hydrogen left.  Free protons are also known as ions.  In the case of other elements like Helium the ion would be two protons and 2 neutrons which were part of the nucleus.  In the case of Helium all or some of the electrons could be knocked out, all depending on the temperature.

In general in places, such as stars, where temperatures reach millions of degrees, plasma is fully ionized.  This means that all of the original elements have been broken up into free electrons and ions.  This is not the case for most situations.  In order to figure out how much ionization we have we can use the Saha equation.

(Eq. 1: Saha equation)

In Saha equation we can see that the ratio of (number of ionized electrons) and (number of “neutral”, or un-ionized atoms) depends on the temperature and (ionization energy of the gas).  Here K is the Boltzmann’s constant and T is temperature in Kelvin.

So for now this should be it.  In next installment I will try and describe Debye shielding and plasma parameters necessary for plasma to happen.

]]> http://lazyroot.org/2009/05/03/plasma-what-is-it/feed 0 http://lazyroot.org/2008/11/25/phd-is-a-go http://lazyroot.org/2008/11/25/phd-is-a-go#comments Tue, 25 Nov 2008 14:12:19 +0000 Dejan http://lazyroot.org/?p=82 Went to the graduate advisor’s office today to check-up on my graduate application status and it turns out that she was writting a congratulatory letter to me .  This means I’m officially in the program.  I had to run to physics department and get some signatures, since apparently the physics department has to agree to transfer.  Hopefully all the paperwork will be done soon, otherwise I’ll be stuck with some “late transfer” fee.  All-in-all great news.