An improved laser photo-detachment diagnostic for negative ion density measurement

Abstract A photo-detachment Langmuir probe is a crucial tool because it gives a point measurement of negative ion density. The detection circuitry of a photo-detachment diagnostic with nanosecond laser pulses is critical for the accuracy of the results. Applying the electromagnetic theory to the design of the photo-detachment system has allowed it to stabilize its frequency response up to ~445 MHz, providing a significantly higher time resolution than in a common photo-detachment circuit setup. A systematic design rule is given in this paper to standardize the proper circuit. The new standard allows comparison between laboratories without concern for electronic parameter differences. The high-time resolution result shows three different peaks in the photo-detached electron current. This paper identified that the first peak is the most correlated to negative-ion density information, and the second and third peaks are related to background electrons interacting with build-up potential.