This process is known as secondary electron emission. Whichever species (ions or atoms) strike the cathode, collisions within the cathode redistribute this energy resulting in electrons ejected from the cathode. These neutral atoms then strike the cathode. Ions strike the more numerous neutral gas atoms, transferring a portion of their energy to them. The primary mechanism, however, is less direct. This happens partially through the ions striking the cathode directly. Some of the ions' kinetic energy is transferred to the cathode. As long as the potential is maintained, a population of ions and electrons remains. The initial population of ions and electrons collides with other atoms, exciting or ionizing them. The positive ions are driven towards the cathode by the electric potential, and the electrons are driven towards the anode by the same potential. A small fraction of the population of atoms within the cell is initially ionized through random processes, such as thermal collisions between atoms or by gamma rays. An electric potential of several hundred volts is applied between the two electrodes. The cell is typically filled with neon, but other gases can also be used. A low pressure is used to increase the mean free path for a fixed electric field, a longer mean free path allows a charged particle to gain more energy before colliding with another particle. In its simplest form, it consists of two electrodes in a cell held at low pressure (0.1–10 torr about 1/10000th to 1/100th of atmospheric pressure). The simplest type of glow discharge is a direct-current glow discharge. If the current is increased still further, other factors come into play and an arc discharge begins. When the current is increased above the level where the entire cathode surface is involved, the discharge is known as an abnormal glow. As the current is increased, more of the cathode surface is involved in the glow. The glow discharge starts as a normal glow. When a glow discharge develops, the electric field is considerably modified by the presence of positive ions the field is concentrated near the cathode. When the electric field increases enough to cause ionization, the Townsend discharge starts. īelow the breakdown voltage there is little to no glow and the electric field is uniform. In an arc discharge, electrons leave the cathode by thermionic emission and field emission, and the gas is ionized by thermal means.For example, the average electron may cause dozens of ionizing collisions via the Townsend avalanche the resulting positive ions head toward the cathode, and a fraction of those that cause collisions with the cathode will dislodge an electron by secondary emission. In a glow discharge, the carrier generation process reaches a point where the average electron leaving the cathode allows another electron to leave the cathode. At higher voltages across the anode and cathode, the freed carriers can gain enough energy so that additional carriers are freed during collisions the process is a Townsend avalanche or multiplication.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |