The excitation source consists of at least three identical electrodes arranged symmetrically in relation to the axis of the central tube, which supplies an analytical sample, and electrode cooling agent supply and removal systems. The electrodes are mounted in an electrically isolated metal housing so that the electrode tops are placed at the central tube outlet, and their ends are shorted in the power supply point with the microwave connections embedded in the housing on the extension of the electrode longitudinal axis and the connections are coupled with the microwave power source, the length of each electrode is ¼ L, where L is the length of the microwave. Each electrode has a hollowed longitudinal flow chamber for the cooling agent connected with metal side tubes, which supply and remove the cooling agent, while outside tube ends are electrically shorted with the housing.