Simple Circuit for Obtaining the Electron Energy Distribution in a Plasma

We describe a very simple technique for obtaining an instantaneous display of the electron energy distribution in a plasma. We electronically doubly differentiate the current-voltage characteristic of the Langmuir probe, following the method of Druyvesteyn. In our technique, we use operational amplifiers for differentiation. Our new contributions are: First, we have discovered where the noise is produced that formerly made operational amplifiers unsatisfactory; second, we have demonstrated that this noise can be eliminated; and third, we have produced simple techniques for cross checking our double-differentiating system to demonstrate that the noise-elimination process does not appreciably distort the desired information.