# Presentation: 349378

## The Trip 

*The Trip *
*of *
*One Xenon Atom *
*Through an Ion Engine*

***Simplified Graphic***

**Notes:**
The most significant word in this title is “Simplified”.  If you have read the student texts entitled “Pushing with Plasma, and “Charges“, you know that what is happening in the mixture of gaseous particles in an ion engine chamber is very complex.  

However, in this presentation, we will focus on what a single xenon atom (or ion) “sees” and “does” as it goes through the reactions and processes that provide the ion jet propulsion engine’s thrust. 

## As a Xenon atom moves through an Ion Engine...it is

***As a Xenon atom moves through an Ion Engine...it is***

*1. Ionized*

*2. Attracted*

*3. Accelerated*

*4. Neutralized*

**Notes:**
A xenon atom undergoes four basic processes as it moves through an ion propulsion engine.

{click} A xenon atom is injected into the engine chamber and is ionized by a
{click} collision with an energetic electron. 
{click} An electrically charged grid gradually accelerates the ion 
{click} so that it lines up with a small hole in the grid
{click} The ion passes through the hole in the first grid and is greatly accelerated 
{click} as it approaches a hole in the second electrically-charged grid.
{click} Once the ion passes through the second grid, it is neutralized to form a xenon atom that is left in space. 

We will examine these processes more fully during the presentation.

## Why Xenon?

***• ******it is an inert gas***

***• ******it has a low first****** ******ionization energy***

***• ******its charge-to-mass ratio can provide***
***  ******exhaust speeds of approximately 10******6****** km/h***

***• ******it is not radioactive***

**Notes:**
Why Xenon?

Xenon is a desirable propellant gas for ion propulsion for three reasons.
{click}
First, it is an inert gas, so corrosion reactions with the metal chamber are minimal. 
{click}
Second, Xenon’s first ionization energy, the energy required to remove one electron from a neutral Xenon atom, is 12.1 kJ/mol or 0.125 eV/atom*.   This a low first ionization energy for a inert gas, although it is relatively high when compared with other elements. 
{click}
Third, with an atomic mass of 131.3 g/mol, the charge-to-mass ratio of the Xenon ion is 7.14 x 105 coul/kg, which can provide exhaust speeds of approximately 106 km/h. 

*An electron volt (eV) is the energy a particle of one electronic charge acquires from falling through a potential difference of one volt, which is equal to 1.6 x 10-19 joule.

## Relative Charge Number Line

- Dawn

***Relative Charge Number Line***

*** ******Negative char******ge****** ******str******engths****	**	**	**            ****Positive****** ******charge****** ******strengths***

**Notes:**
The color scheme in these graphics is the same as that used in the tutorials for the Student Activity “Positive and Negative Charges” simulation.  Negatively charged electrons are red, positively charged ions are blue, and neutral atoms are green. 

You can think of the relative charges on the grids, the chamber, the electrons and the xenon ions, as being similar to that of signed numbers on a number line.

In the tutorials, you assigned charge strengths to the particles that you placed in the chamber.   When you changed the strength on a negative particle from 5 to 10, the size of the particle increased.  This also means that you moved to the left on the charge number line.  In other words, 
{click} a particle with a –5 charge is more negative than the positively-charged test bullet, but it is also less negative than 
{click} the particle with –10 charge. 

When you change the strength of a positively-charged particle from 5 to 10, you move to the right on the number line.  So the particle 
{click} with +5 charge is less positive than 
{click} the particle with +10 charge, but more positive than any particle that is to the left of it on the number line.  As we trace the path of a xenon ion through an ion engine, we must think in terms of relative positive and negative charges.  

## Relative Charges in Ion Engine

- Dawn

***Relative Charges in Ion Engine***

*Engine walls *

*< Xenon ion plasma*

*< First electrode grid*

* **<** **Second electrode grid*

**Notes:**
As the xenon plasma moves through the Ion Engine, the charges on parts of the engine that it encounters are in the reverse order of the signed number line, that is, in order of increasing negative charge:

{click} the engine walls are less negative 
{click} than the xenon ion plasma, which is less negative 
{click} than the first electrode grid, which is less negative 
{click} than the second electrode grid.

It is important to remember that some processes in the Ion Engine depend upon the difference in magnitude of the charges on particles, the chamber and grids.

Based on this information can you see how a positive Xenon ion can be attracted to a positive electrode grid?  

## Ionization

***Ionization***

- • *Collision forms a xenon ion and two free electrons*

- • *Electron collides with xenon atom*

***Enlargement of action area***

**Notes:**
When a solar-powered ion engine is running, energetic electrons are emitted from a hollow-tube cathode. These electrons enter the ionization chamber where the positive engine wall attracts them, {click} giving them additional energy to ionize the Xenon atoms.

In the simplified graphic on the right, 
{click} this symbol (ê-) represents an electron with sufficient energy to ionize a Xenon atom.  As it travels toward the engine walls, it undergoes a collision with 
{click} a Xenon atom Xe, to form
{click} a Xenon ion Xe+ and 
{click} another electron, e- ̆. 
{click} This electron e-* is the electron that was involved in the collision, but it is now at a lower kinetic energy than it had prior to the collision. This collision is inelastic, so some kinetic energy is also converted into another form of energy. The electrons present after the collision are attracted to the engine walls. 

The “Pushing with Plasma“ student text describes the many types of collisions that charged particles can undergo while in a mixture of atoms, ions, and electrons.  However, if the Xenon ion is to contribute to the thrust of the ion engine, it must remain a charged particle and instead of moving randomly in the chamber, it must also be aligned to move in the same direction as other Xenon ions toward the rear of the engine.

## Attraction to Positive Grid

***Attraction to Positive Grid***

***Enlargement of action area***

***1000 micrometers (************m***)

**Notes:**
{click}  The function of the two electrically-charged grids at the rear the Dawn ion engine is to move Xenon ions in the same plane and to accelerate them. 
In our enlarged graphic,
{click} the first electrode grid encountered by a Xenon ion is labeled V+ which indicates that it is positively-charged relative to the second electrode grid 
{click} at the rear of the engine, labeled V-.  Remember however, it is more negative than the walls of engine and the
{click} Xenon ions. 

{click} Although the distance between the grids appears to be quite large, the second electrode, V- , is located only about 1000 micrometers (m) (the width of 10 human hairs)  away from the first electrode. 

Since the first grid is slightly negative compared to the Xenon ions, the ions are drawn toward it. The grid moves the ions in the correct plane and increases the chance of the ions going through the grid by increasing the effective surface area of the hole. 
{click} Once near the first grid, the ion ”feels” the great potential difference between the first and second grid and is accelerated to very high speeds.  A “lensing affect” by the electric field on the first grid also helps improve the chances of the ion going through the holes in the grid.

The blue arrow in this graphic indicates that our xenon ion is now a part of a very directionalized stream of ions going through the first grid.  This ion stream is sometimes referred to as a thrust beam or plasma stream. 

{click} Negative electrons that were not attracted to the engine walls can be attracted to the grid and are discharged when they strike it as indicated by the {click} white flash.

{click} Occasionally a xenon ion strikes the grid, where it is 
{click} neutralized and 
{click} reforms a xenon atom.  If it is to contribute to the engine’s thrust it must then be re-ionized by collision with an electron. 

## Acceleration

***Enlargement of action area***

***Acceleration***

**Xe+**

**Xe+**

**Xe**

*Acceleration 35,000 m/s ***

***1/50th of a pound thrust*

**Notes:**
{click}  The Xenon ion stream is accelerated between the first and second grid.

{click}   The second grid is more negative than the first electrode by 1,280 volts, so the grid exerts a strong electrostatic pull on the positively-charged Xenon ion stream, accelerating the ions at a rate of 8.7 x 10^11 m/s/s. 

{click} to a speed of 35,000 m/s as they move between the two grids.  This change in momentum is translated to the spacecraft if the ions pass through a hole in the last grid.   

{click} According to Newton's 3rd law, a reaction force accelerates the grid, which is attached to spacecraft, forward {click} with 1/50th of a pound (92 milliNewtons) of thrust.  This is the amount of force roughly equal to the weight of a piece of printer paper as it rests on your hand.

{click} If an ion doesn’t go through the grid, 

{click} its collision with the grid causes it to accelerate in the opposite direction.  This applies a force that completely negates the force applied to the spacecraft because of the first acceleration.

## Neutralization

***Enlargement of action area***

***Neutralization***

**Notes:**
Remember that electron that was removed from the Xenon atom during the ionization process?  Well, that electron (and others resulting from the ionization of Xenon atoms) has moved from the chamber walls to a cathode toward the rear of the engine.  Shortly after moving through the second grid, but before the beam leaves the engine, the Xenon ion recombines with an electron released from the cathode, thus reforming a neutral Xenon atom. 

This is important since the second grid is very negatively charged.  If the positively-charged ion is not neutralized, it will stop and reverse its direction.  This is even worse than having an ion stopped as it hits the grid since the change in momentum is much greater.  This slows the spacecraft instead of speeding it up.

## Xenon Ions Provide Thrust 

***Xenon Ions Provide Thrust ***

***m****** i****** v****** i*****  ******

***  ************ ******m****** e****** v****** e***

***=***

***750-kg Dawn spacecraft***

***∆******v = 10******-5****** m/s***

**Notes:**
How do these very tiny Xenon ions provide enough thrust to move the
spacecraft? Thrust is the reaction force described by Newton's Third Law when a system accelerates mass in one direction to propel a vehicle in the opposite direction.. 

Mathematically expressed, 
{click} the force of the ions, Fi = m i a i, 
{click} equals 
{click} the force in the opposite direction on the spacecraft engine, Fe = meae.  So, meae = m i a I 

Although the mass of Xenon ions is very small, they provide enough thrust to propel the 
{click} 750-kg Dawn spacecraft in the opposite direction. The positively-charged Xenon ions actually pull the negatively-charged grid toward them with enough force to 
{click} change the spacecraft’s velocity 10-5 m/s. That doesn’t sound like a very great acceleration, but it is enough to propel the Dawn spacecraft on its 6.3 billion kilometer, ten-year trip to Vesta and Ceres.