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Playing with Electrons to Produce Useful Cold Plasma

Zdenko Machala
Mechanical Engineering
Stanford University
November 2002

Plasma is a gas composed of atoms and molecules with free electrons and ions which interact together. My experimental research in plasma physics is focused on generating cold air plasma rich with fast electrons. Since I create this plasma in normal atmosphere, it can provide many uses. Examples include cleaning polluted air, decontaminating medical instruments, or shielding aircraft from radar.

We are all familiar with the three basic states of matter - solids, liquids and gases. I create and study the properties of the fourth state of matter, plasma (gas plasma, not blood plasma). Plasma is formed when a gas composed of electrically neutral molecules and atoms is exposed to a high temperature, or strong electric field. Negatively charged electrons are pulled out of atoms or molecules, leaving positively charged ions behind. The negative electrons and positive ions in plasma interact through electric forces, so they feel and influence each other. While plasma has properties of a gas (i.e. it can be squeezed and expanded) it is also electrically conductive, like a metal.

The goal of my research is to produce plasma in normal air which has two essential properties: (1) relatively low temperature that ranges from room temperature to 2000 °C, and (2) a lot of electrons. This is a challenging task because usual plasma in normal air is either very hot (5000 °C or more) or does not contain enough electrons. Cold plasma rich on electrons that I produce can be used for treatment of polluted air since the electrons break the molecules of the pollutant. Such plasma is also applicable to sterilize medical instruments because they kill bacteria efficiently and cheaply, without using chemicals. It can also be applied for an electromagnetic wave shielding of aircraft, which would make it invisible to radar.

I achieve cold plasma by forcing electrons to move quickly, while indirectly forcing all other particles to move more slowly. Electrons are smaller and much lighter than all other particles; we can imagine electrons like grains of rice and the other particles like pool balls. I force electrons - tiny rice grains - to move quickly by accelerating them in a strong electric field. Fast electrons are responsible for breaking molecules, emitted light and almost all processes occurring in cold plasma. At the same time, I keep the temperature of the big particles - the pool balls - low, which makes them move more slowly. As an example, imagine a scalpel that is placed into this plasma to be sterilized. It could withstand fast rice grains hitting its surface, but the fast pool balls would completely damage it.

The typical way to produce plasma is using an electric discharge. My experiments focus on producing cold plasma with many fast electrons using two types of discharges - DC (direct current) and microwave discharge.

Everyone is familiar with lightning, a big natural discharge. My DC discharge is a small laboratory lightning. I apply a high voltage of several thousand volts between metal electrodes across an air gap of a few centimeters. If the voltage is high enough, the air breaks down and a shiny plasma forms. By setting the electric voltage and current, the shape and material of electrodes, and the air flow across the discharge, I am able to control the plasma temperature, shape and other properties. To make cold electron-rich plasma, I have to continuously keep a balance between the current, the voltage and the speed of air flow. In microwave discharge, I use a microwave generator which is similar to a microwave found in a kitchen. A strong microwave field enclosed in a small cavity creates plasma. Sometimes I use both methods simultaneously to get more electrons in the plasma.

The main objective of my research is to study the fundamental processes occurring in cold air plasma. I find answers to the question of how to produce and control cold, electron-rich plasma, and how to make it do things we want. While this research is relatively basic, it is driven by an industrial demand for new techniques to clean polluted gas, decontaminate medical instruments, and shield aircraft.