Note: Multi-pass Thomson scattering measurement on the TST-2 spherical tokamak.

Area of Science:

• Plasma physics
• Optical diagnostics
• Fusion energy research

Background:

• Thomson scattering (TS) is crucial for plasma diagnostics.
• Multi-pass TS enhances laser energy and signal-to-noise ratio.
• Confining laser pulses in optical cavities is a potential advancement.

Purpose of the Study:

• To develop and implement a coaxial optical cavity for multi-pass Thomson scattering.
• To perform the first TS measurements in tokamak plasmas using this novel cavity.
• To evaluate the effectiveness of the coaxial cavity in enhancing laser energy and measurement accuracy.

Main Methods:

• Development of a coaxial optical cavity for laser pulse confinement.
• Implementation of the cavity in a tokamak for Thomson scattering measurements.
• Analysis of laser energy attenuation and photon gain across multiple passes.

Main Results:

• Achieved a photon number gain of approximately 3x compared to the first pass.
• Observed an energy attenuation of about 30% per round trip within the cavity.
• Demonstrated improved temperature measurement accuracy by accumulating waveforms from the first three passes.

Conclusions:

• The coaxial optical cavity is effective for multi-pass Thomson scattering in tokamak plasmas.
• This technique significantly enhances effective laser energy and signal-to-noise ratio.
• The developed method offers improved accuracy for plasma temperature measurements.