A multi-chord, two-color interferometer using Hilbert transform phase detection for measuring electron density in...

Area of Science:

• Plasma Physics
• Fusion Energy Research
• Advanced Diagnostics

Background:

• Sustained spheromak devices like HIT-SI3 and HIT-SIU require precise plasma density measurements for operational control and research.
• Traditional interferometry methods can be susceptible to environmental noise, such as vibrations, affecting data accuracy.
• Accurate electron density (n_e) measurements are crucial for understanding plasma behavior and confinement in fusion devices.

Purpose of the Study:

• To design and implement a robust, four-chord CO2/He-Ne heterodyne interferometer for measuring line-averaged plasma density.
• To overcome vibration-induced errors common in vacuum-chamber-mounted optical systems.
• To achieve high-resolution electron density measurements within the 10¹⁸-10²⁰ m^-3 range for spheromak devices.

Main Methods:

• Development of a two-color (CO2/He-Ne) heterodyne interferometer system.
• Utilizing a four-chord configuration for multi-point, line-averaged density measurements.
• Implementing a system designed to mitigate vibration effects from the vacuum chamber mounting.

Main Results:

• Successful elimination of vibration-induced errors through the two-color heterodyne approach.
• Resolution of electron densities (n_e) across the full operating range of 10¹⁸-10²⁰ m^-3.
• Achieved an integrated error of 4.7 × 10¹⁷ m^-2 in measurements.
• Presented data showing time evolution of n_e and j_ϕ/n_e during high toroidal current plasma discharges.

Conclusions:

• The developed CO2/He-Ne heterodyne interferometer is a reliable diagnostic for measuring plasma density in sustained spheromak devices.
• The two-color system effectively suppresses vibration noise, enhancing measurement accuracy.
• The diagnostic provides valuable data for studying plasma dynamics and optimizing fusion device performance.