Abstract
The upgraded millimeter-wave interferometer with the frequency of 70 GHz is installed on a large-scaled negative ion source. Measurable line-averaged electron density is from 2 × 1015 to 3 × 1018 m-3 in front of the plasma grid. Several improvements such as the change to shorter wavelength probing with low noise, the installation of special ordered horn antenna, the signal modulation for a high accuracy digital phase detection, the insertion of insulator, and so on, are carried out for the measurement during the beam extraction by applying high voltage. The line-averaged electron density is successfully measured and it is found that it increases linearly with the arc power and drops suddenly at the beam extraction.
Highlights
The understanding of the dynamics of the negative ion and electron during the beam extraction in the negative ion source is important for the neutral beam injection (NBI) heating in magnetically confined fusion plasma and for high-energy particle accelerator development
A 39 GHz millimeter-wave interferometer was temporarily installed in the test stand of 1/3-scaled negative ion source for the LHD-NBI system, in order to test the possibility of electron density measurement near the plasma grid (PG)
The SSB modulator, which is driven by a 17.5 MHz direct digital synthesizer (DDS), shifts the frequency of the reference signal for heterodyne detection
Summary
The understanding of the dynamics of the negative ion and electron during the beam extraction in the negative ion source is important for the neutral beam injection (NBI) heating in magnetically confined fusion plasma and for high-energy particle accelerator development. The negative ion current density has been increased by the several intensive efforts[1,2,3,4,5], and it will become the order of 100 MW in ITER In such high power operation, the accelerated electrons might damage the beam line components due to heavy heat load. A 39 GHz millimeter-wave interferometer was temporarily installed in the test stand of 1/3-scaled negative ion source for the LHD-NBI system, in order to test the possibility of electron density measurement near the plasma grid (PG). This interferometer successfully measured the electron density in the pure hydrogen in the plasma arc chamber without the beam extraction voltage[6].
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