RFQ electrodes for use as an acceleration tube of a high energy ion implanter, capable of accelerating an ion beam of large current without divergence are arranged, with respect to a low resonance frequency of substantially 33 MHz suitable for heavy ions such as B, P, and As, such that a radius R1 of a beam passage spacing surrounded by four RFQ electrodes is 5 mm to 9 mm, a curvature R2 in a direction perpendicular to an axis of a crest portion of respective crest and trough portions on surfaces of the electrodes in a beam propagation direction is 5 mm to 9 mm, and a height H from a peak of the crest portion to a bottom surface is set so that H/R1 is 4 to 6. When the height H of the electrodes is reduced, while shunt impedance is increased and power efficiency is improved, a cooling ability becomes insufficient due to the fact that a cross section of a coolant channel cannot be increased, and a problem is presented that oscillation of electrodes is likely to occur due to insufficient mechanical strength. However, by adopting the above arrangement, an optimum configuration of the RFQ electrodes is obtained, in which power efficiency is high, cooling efficiency is superior, a mechanical strength is sufficient, and beam acceptance is large.
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