A 280 GHz single-channel millimeter-wave interferometer system for KSTAR.

Area of Science:

• Plasma physics
• Fusion energy research
• Diagnostic instrumentation

Background:

• Accurate measurement of plasma electron density is crucial for understanding and controlling fusion plasmas.
• Existing diagnostic systems face challenges with deep access and signal integrity within large cryogenic environments like tokamaks.

Purpose of the Study:

• To develop and implement a robust 280 GHz millimeter-wave interferometer system for electron density measurements on the Korea Superconducting Tokamak Advanced Research (KSTAR).
• To enhance the accuracy and reliability of plasma density diagnostics in a challenging tokamak environment.

Main Methods:

• Fabrication and installation of a single-channel horizontal millimeter-wave interferometer system.
• Utilization of a retractable cassette system for deep positioning within the KSTAR cryostat, incorporating a pneumatic vacuum window shutter and a beam focusing module.
• Employing concave mirrors and a carbon inner-wall tile for beam reflection, enhancing received power and reducing phase errors.
• Locating microwave components away from the cryostat with a shielding box and transmitting intermediate frequency signals for phase difference measurement using a multifringe counting phase comparator.

Main Results:

• Successful installation and operation of the 280 GHz interferometer system on KSTAR.
• Demonstrated enhancement of receiving beam power and reduction of phase errors through the beam focusing module.
• Development of a beam path analysis method to deduce effective line-integrated density from measured data.
• Quantification of beam path length errors caused by plasma refraction under various plasma conditions.

Conclusions:

• The developed millimeter-wave interferometer system is effective for accurate plasma electron density measurements on KSTAR.
• The retractable cassette system and beam focusing module significantly improve diagnostic performance in a large cryogenic tokamak.
• The beam path analysis provides a method to correct for refraction effects, leading to more reliable density data.