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In this paper, we report the results of measurements of the intensities anddelays of super-radiance decays from excited helium atoms at multiplewavelengths. The experiment was performed using extreme ultraviolet radiationproduced by the free electron laser at the SPring-8 Compact SASE Source testaccelerator facility as an excitation source. We observed super-radianttransitions on the $1s3p \to 1s2s$ ($\lambda=$502 nm), $1s3d \to 1s2p$($\lambda=$668 nm), and $1s3s \to 1s2p$ ($\lambda=$728 nm) transitions. Thepulse energy of each transition and its delay time were measured as a functionof the target helium gas density. Several interesting features of the data,some of which appear to contradict with the predictions of the simple two-levelsuper-radiance theory, are pointed out.

Simultaneous measurements of super-radiance at multiple wavelengths from helium excited states: (I) Experiment