TRIAC Progress Report - KEK

Fig. 2-29. Photos of the pre-buncher; outside view (Left panel) and inside view (Right panel).
The pre-buncher, as shown in Fig. 2-29, has two gaps at both ends of a single drift
tube respectively applied by a RF-wave synthesized with three harmonics (from the first
to the third), which are combined together into a pseudo saw-tooth wave-form for beam
bunching. By adjusting the length of the drift tube to be 140/360 in units of βλ
according to the beam-bunching frequency, where β is the velocity of the beam (2 keV/
nucleon at the TRIAC) normalized by the light velocity and λ is the wavelength of the
applied RF-waves for beam bunching, the pseudo saw-tooth wave could be properly
applied to the beam passing through the two gaps. In this way, the frequency of the
bunched beam can be varied between 2 and 4 MHz (i.e. variable time interval from 250
ns to 500 ns).
The beam test of the pre-buncher at a 2-MHz operation was performed using 16 O 4+
and 12 C 3+ beams supplied by KEKCB. The time structures of the beam observed with
buncher-on and off are compared in Fig. 2-30. At the buncher-off operation was
indentified 13 bunches of the beam by the acceleration frequency of 26-MHz SCRFQ
during the time interval of the frequency of 2-MHz (500 ns) beam-bunching (blue line
in Fig. 2-30). When the pre-buncher was on, a clear bunch structure (bunched into the
middle of 13 bunches with an interval of 500 ns) was observed at a target position 10 m
away from the exit of the TRIAC (red line in Fig. 2-30), although there exist a small
fraction of the beam failed to be bunched (i.e. observed as a side bunch instead of the
main, central bunch).
The bunching efficiency, defined as the intensity ratio of a bunch of beam (central
bunch in Fig. 2-30 when the pre-buncher is on) to the sum of 13 bunches with
buncher-off, was found to be 42 %. Incomplete beam bunching, partly observed as a
remainder of the 26-MHz operation of SCRFQ in Fig. 2-30 with buncher-on, is due to
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