  The two-frequency heating technique was studied to increase the beam
intensities of highly charged ions provided by the high-voltage extraction
configuration (HEC) ion source at the National Institute of Radiological
Sciences (NIRS). The observed dependences on microwave power and frequency
suggested that this technique improved plasma stability but it required precise
frequency tuning and more microwave power than was available before 2013.
Recently, a new, high-power (1200 W) wide bandwidth (17.1-18.5 GHz)
travelling-wave-tube amplifier (TWTA) was installed. After some single tests
with klystron and TWT amplifiers the simultaneous injection of the two
microwaves has been successfully realized. The dependence of highly charged
ions (HCI) currents on the superposed microwave power was studied by changing
only the output power of one of the two amplifiers, alternatively. While
operating the klystron on its fixed 18.0 GHz, the frequency of the TWTA was
swept within its full limits (17.1-18.5 GHz), and the effect of this frequency
on the HCI-production rate was examined under several operation conditions. As
an overall result, new beam records of highly charged argon, krypton, and xenon
beams were obtained at the NIRS-HEC ion source by this high-power two-frequency
operation mode.
