Fast visible imaging and edge turbulence analysis in QUEST

Area of Science:

• Plasma Physics
• Fusion Energy Research
• Advanced Imaging Techniques

Background:

• Understanding plasma edge turbulence is crucial for magnetic confinement fusion.
• Spherical tokamaks present unique challenges for plasma control and diagnostics.

Purpose of the Study:

• To characterize edge turbulence in spherical tokamak plasmas using a novel fast visible imaging system.
• To investigate the impact of vertical magnetic field topology on edge turbulence dynamics.

Main Methods:

• Installation and utilization of a high-speed complementary metal-oxide semiconductor (CMOS) camera system on the QUEST spherical tokamak.
• Acquisition of visible imaging data at high frame rates (up to 775 kfps).
• Systematic variation of vertical magnetic field (B(z)) topology using poloidal field coils in electron cyclotron resonance heating (ECRH) driven slab plasma.

Main Results:

• Edge turbulence fluctuation amplitude was maximal under shallow poloidal field (PF) well conditions.
• Distinct coherent modes along the z-direction were observed in steep density gradient regions.
• The observed coherent modes intensified with increasing poloidal field mirror ratio.

Conclusions:

• The fast visible imaging system effectively visualizes edge turbulence in spherical tokamak plasmas.
• Vertical magnetic field topology significantly influences edge turbulence, with shallow PF wells promoting higher fluctuation amplitudes.
• Coherent structures in the edge plasma are linked to magnetic field geometry and density gradients.