Normalized spatio-temporal plots of the electron impact excitation rate from the ground state of helium into He-I 1s3s 3S1 obtained experimentally for N = 1 (ﬁrst row) and N = 4 (second row) and for diﬀerent concentrations of the O2 admixture (columns) at ﬁxed z = −15 mm. The base frequency is f0 = 13.56 MHz and the peak-to-peak value of the driving voltage waveform is kept constant at 500 V. The powered electrode is located at x = 0, while the grounded electrode is at x = 1.

Experimental data are marked as exp

Calculated data are marked as calc

x [t/TRf] , y[mm]

(Figure8a8c): Exp Exc. Rate for O2-admixtures of 0.05%, 0.2% and 0.5% respectively with N=1

(Figure8d-8f): Exp Exc. Rate for O2-admixtures of 0.05%, 0.2% and 0.5% respectively with N=4

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