Effect of Probe Surface Ablation on Laser Photodetachment Signals

Abstract

An electric probe exposed to laser irradiation during negative ion density measurement by applying the laser photodetachment technique may suffer ablation even though the laser power is not significantly high. In this paper, the ablation effects on laser photodetachment signals were examined in a linear plasma device using plasmas of both helium and a mixture of helium and hydrogen. It was found that there was a specific threshold laser power necessary to cause the ablation, and that it depended on the electron density. The ablation signals were also detected using a negatively biased probe. A method for monitoring the ablation phenomena is then presented. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Diagnostics of Recombining Plasmas in Divertor Simulator MAP-II

“…Ablation phenomena can be detected by a negatively biased probe since ions are also produced through the ionizations of neutrals whereas no ions are produced through the PD process. We have revealed that the ablation occurs from the loss of the thermal balance between the adsorbing/implanting and desorbing/released contaminants [35]. Formation of helium blisters may be an enhancement factor.…”

“…In the application of the laser photodetachment (LPD) method to divertor plasmas, we have identifi ed three diffi culties: 1) an anomaly in the probe characteristics [4][5][6], 2) ablation of the probe surface adsorbates which are ionized due to the electron impact processes [35], and 3) the effect of the magnetic fi eld. The item 1) disturbs the T e measurement which is necessary for the determination of n e in Eq.…”

Experimental study of negative ion profiles in H2-MAR plasmas in divertor simulator MAP-II

“…We have noticed two major problems in the application of LPD to high-density recombining plasmas, namely the ablation phenomena of the probe surface adsorbates [9] and an anomaly that appears in the single probe characteristics.…”

“…Although the ablation can be detected by means of a negatively biased probe [9], we have developed an ablation-free technique, called Eclipse-LPD [10], as shown in Fig. 1.…”

Effect of cross-field transport on H− density profile in magnetized plasmas: Comparison between measurement and simulation

“…23 When the probe tip surface is damaged by the formation of helium bubbles and holes 24 or contaminated by the impurities such as hydrocarbon, the ablation of the probe surface may lead to the overestimation of the negative ion density in the conventional LPD method. 25,26 In the present study, a screening object ͑a black-colored object with a dimension of ϳ2 ϫ 5 cm͒ was placed outside the vacuum chamber, as shown in Fig. 1͑a͒; this resulted in a slab geometry configuration in the eclipse LPD method, 27 in which the laser pulses do not irradiate the probe tip surface directly.…”