PURPOSE: To block the counterflow electron by providing an electrode independent in D.C. and operating with the potential higher than that at the high frequency coupling section between the high frequency coupling section and a current collecting section nearest to said section.

CONSTITUTION: An electrode 109 is provided between a high frequency coupling section 104 and a current collecting electrode 105, where the potential is brought higher than that of said section 104 by a power source 110 and operated. Consequently the potential of the electrode 109 will form a barrier against the counterflow electron thereby the counter-flow electron is caught by the electrode 109. Here the electrode 109 is separated from the high frequency coupling secton 104 thereby the design freedom of the power withstandability is high.