2024 – Exploring the staircase to heaven: Probing the formation of layered long range order

Exploring the staircase to heaven: Probing the formation of layered long range order

2024 Research Campaign, Frontier Science

Purpose of Experiment

The purpose of this half-day experiment is to elucidate the physics of long-range, layered structures, called staircases. Layering is observed in turbulent mixing processes in stars, oceans, atmospheres, and magnetically confined plasmas. In a tokamak plasma, layering is manifested by the formation of an 𝐸× 𝐡 staircase – a sequence of turbulent mixing zones interspersed by shear layer mini-barriers. Here 𝐸 is the plasma radial electric field and 𝐡 is the confining toroidal magnetic field. Staircase step size defines the effective transport mixing length, and larger mixing zones can lead to the increased radial transport of particles and heat compared to common scaling laws, such as gyro-Bohm scaling. Data collected from these experiments will yield a unique set of dynamical observations which reveal staircase physics and challenge staircase models. This experiment is a part of the Frontier science program. The research effort will be a collaborative venture involving the DIII-D team members and KSTAR team members.

Experimental Approach

Previous core staircase experiments on tokamaks have focused primarily on observation and identification of such structures. This experiment probes dynamics and response. We propose to elucidate the staircase dynamics in the core of L-mode plasma by a series of perturbation experiments, exploiting modulated Electron Cyclotron Heating, small pellets injection, Neutral Beam Injection torque scan, and plasma current ramp. We will look for staircases as periodic radial structures of zonal shear layers, i.e. radial modulation of turbulence poloidal velocity, using advanced fluctuation diagnostics such as Beam Emission Spectroscopy (BES), Doppler Back Scattering (DBS), Fast Sweep Reflectometry, and Electron Cyclotron Emission Imaging (ECEI). Periodic velocity shear layers are expected to modulate the radial profile of turbulence intensity and correlation length, so extensive cross-correlation analysis and velocimetry of BES data will be performed to identify these layers and study their dynamical behavior.