Multi-channel Doppler backscattering measurements in the C-2 field reversed configuration

Area of Science:

• Plasma Physics
• Fusion Energy Research
• Diagnostic Techniques

Background:

• Understanding plasma dynamics is crucial for achieving controlled fusion.
• Field-Reversed Configurations (FRCs) are a promising approach to magnetic confinement fusion.
• Advanced diagnostics are needed to characterize FRC plasmas.

Purpose of the Study:

• To introduce and validate a versatile heterodyne Doppler Backscattering (DBS) system.
• To measure density fluctuation levels and toroidal E × B flow velocity in the C-2 FRC.
• To provide detailed plasma profiles across the FRC core and scrape-off layer.

Main Methods:

• Utilized a heterodyne Doppler Backscattering (DBS) system with six tunable frequencies (26-90 GHz).
• Employed monostatic beam optics with a quasi-optical beam combiner and focusing mirror.
• Achieved Gaussian beam spot sizes of 3-5.5 cm at the X/O-mode cutoff for plasma densities of 0.8 × 10^13 to 1 × 10^14 cm^-3.

Main Results:

• Successfully measured density fluctuation levels in the wavenumber range kρs ≤ 50.
• Quantified the toroidal E × B flow velocity within the C-2 FRC.
• Demonstrated system access to the FRC core, including the field null, and the scrape-off layer.

Conclusions:

• The developed DBS system is a versatile and effective diagnostic for FRC plasmas.
• The measurements provide critical data for understanding plasma confinement and stability in FRCs.
• This diagnostic capability advances research towards practical fusion energy.