The high-k poloidal scattering system for NSTX-U.

Area of Science:

• Plasma physics
• Fusion energy research
• Wave phenomena in plasmas

Background:

• High-k electron density fluctuations are critical for understanding turbulent transport in fusion devices.
• Previous diagnostic limitations on NSTX-U restricted measurements to lower wavenumbers.
• The National Spherical Torus eXperiment Upgrade (NSTX-U) requires advanced diagnostics to study plasma behavior.

Purpose of the Study:

• To develop and implement an advanced 8-channel, 693 GHz poloidal far-infrared (FIR) scattering system for NSTX-U.
• To extend the measurement capability of poloidal wavenumbers significantly beyond previous limits.
• To investigate high-k electron density fluctuations across a wide spatial range within the plasma.

Main Methods:

• Utilizing an 8-channel, 693 GHz poloidal FIR scattering system.
• Employing large aperture optics for radiation collection at 8 simultaneous scattering angles (2° to 15°).
• Implementing steerable launch optics and remotely translatable receiver optics (5 axes) to control the scattering volume location and orientation.

Main Results:

• The new system replaces a 5-channel, 280 GHz toroidal system.
• Reduced wavelength in the poloidal system minimizes refraction effects.
• Measurement of poloidal wavenumbers is extended from 7 cm^-1 to over 40 cm^-1.
• The scattering volume can be positioned from r/a = 0.1 to the pedestal region (r/a ∼ 0.99).

Conclusions:

• The developed poloidal FIR scattering system significantly enhances the study of high-k electron density fluctuations on NSTX-U.
• The extended wavenumber and spatial coverage provide unprecedented insights into plasma turbulence.
• This advancement is crucial for improving confinement and achieving sustained fusion reactions.