Eclipse laser photodetachment method for avoiding probe surface ablation in negative ion measurement

Direct laser irradiation to an electrostatic probe disturbs the negative ion signals of laser photodetachment (LPD) in helium–hydrogen plasmas, particularly when the electron density is considerably high. In order to analyse the characteristics and production mechanism of ablation signals, laser irradiation experiments were performed on a tungsten surface using helium plasmas. To prevent direct laser irradiation to the electrostatic probe tip, a thin wire is installed in the laser channel to form a laser shadow for the probe tip. The ablation signal disappears when the probe tip is fully eclipsed by the laser shadow. The influence of a screening object on the signal intensity of LPD is investigated by experiments and model calculations. It is confirmed that the LPD signal intensity is accurately estimated when some conditions are satisfied, even when a part of the collection region of photodetached electrons is overlapped by the shadow. It is an ablation-free LPD method that can be used to evaluate the negative ion density at a sufficient laser power, even when the ablation disturbs the LPD signals in the conventional LPD technique.