In the GD source, the anode is normally held at ground potential and a potential is applied to the sample. This power supplied to the sample may be RF or DC.
RF (radio frequency)
For RF, the inner surface of the sample reaches a negative dc bias potential. This dc potential arises from the combination of two factors:
- the area of the sample surface inside the plasma is much smaller than the area of the inside of the anode, making it easier, because of charge density (i.e. electrons repel each other), for electrons to flow to the anode rather than to the sample; and
- the large mass of the argon atom, means the positive argon ions cannot follow the rapid changes in rf potential and so travel in the opposite direction to the average electron flow.
The DC bias potential in RF sources is generally a little lower than the equivalent DC voltage in DC sources. The difficulty of measuring the RF currents means that most RF sources are controlled by the applied RF power, which typically varies from 5 W to 80 W.
To see how to measure RF currents, click here, then click on Search and enter: Development of special hardware for RF-GD-OES.
DC (direct current)
If DC, the applied voltage is typically varied between 400-1200 V and the DC current produced typically varies between 20-120 mA.
First published on the web: 15 May 2000.
Author: Richard Payling