Two-frequency heating technique at the 18 GHz electron cyclotron resonance ion source of the National Institute of Radiological Sciences

TL;DR

This paper demonstrates that two-frequency microwave heating at 18 GHz, using a high-power broadband amplifier, enhances plasma stability and increases highly charged ion beam intensities at the NIRS ion source.

Contribution

The study introduces a successful implementation of simultaneous two-frequency microwave injection with a high-power broadband amplifier, achieving record ion beam intensities.

Findings

Improved plasma stability with two-frequency heating.

Achieved record highly charged ion beam intensities.

Frequency tuning within 17.1-18.5 GHz affects ion production.

Abstract

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.