Measurement of local electric fields in high power ion diodes with resonantly excited probe beam atoms

An atomic probe beam diagnostic for local electric field measurements across the accelerating gap of the KALIF applied-B
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high power proton diode is under development. It is based on the injection of a collimated pencil-like beam of Li-atoms produced by laser-evaporation from a LiF-target. To achieve the desired sensitivity down to a few hundred kV/cm the atoms are resonantly excited in a two-step process with two broadband dye lasers to the 3d-level. The emitted 3d-2p resonance line splits into five components (of which two coincide) in the diode electric field. The local electric field is inferred from the degree of Stark splitting of these lines. Evaluating the m=0,1 components, fields between 200 kV/cm and 1 MV/ cm can be determined, while the m = 2 component is suitable for fields between 1 and 4 MV / cm. Test measurements at the KALIF-diode have demonstrated that adequate spectral line intensities can be achieved without affecting the diode operation by too much Li-vapour. However, the fluorescence line vanishes as soon as voltage occurs in the diode. Possible causes of this line evanescence are discussed.