Achieving fusion energy will require a comprehensive understanding of turbulence and instabilities in high temperature magnetically confined plasma. This research program develops and operates a suite of spectroscopic instruments to observe the detailed dynamics of plasma turbulence and instabilities at the DIII-D National Fusion Facility, one of the most sophisticated and well-diagnosed fusion experiments in the world.
This collaborative program has developed several advanced diagnostic systems to measure turbulence and other instabilities. A high performance density fluctuation diagnostic system, Beam Emission Spectroscopy (BES), provides 2D fluctuation imaging measurements at high spatial (~1 cm) and high time resolution (up to 1 microsecond) by utilizing high-efficiency optical components (optical fibers, specialized light filters, lenses), customized electronics and light detectors. An ion temperature and rotation velocity fluctuation diagnostic system, Ultra-Fast Charge Exchange Recombination Spectroscopy (UF-CHERS), has also been deployed at DIII-D and has been upgraded with a much improved detector system. We are also developing a charge-exchange-emission-imaging (CXI) system to achieve higher spatial resolution (~3 mm) fluctuation measurements in the important boundary zone by observing emission from charge exchange reactions between carbon ions and neutral beam atoms. CXI will measure small scale pedestal instabilities and sharp edge gradients. These spectroscopic systems probe the nature, characteristics and scaling properties of turbulence in DIII-D plasmas.
2D BES at NSTX-U
UW has developed and deployed a high-performance Beam Emission Spectroscopy (BES) diagnostic system on NSTX-U to observe the plasma fluctuations associated with turbulence and instabilities. These instabilities include ion temperature gradient-driven turbulence, trapped-electron modes, microtearing modes and kinetic ballooning modes that exist at the ion gyroradius scale (~1 cm in NSTX-U plasmas); meso-scale instabilities at low-order rational surfaces; and Alfven instabilities that drive the loss of energetic particles. The recently expanded BES diagnostic features 64 channels in a 2D viewing geometry to image turbulence and measure radial and poloidal characteristics, flows, and wavenumber spectra.
The 2D BES system developed and implemented by UW will facilitate measuring and understanding the turbulence and instabilities in high-performance NSTX-U plasmas. The upgraded features of NSTX-U include higher magnetic field (1 T), higher current (2 MA), long pulse operation (5 seconds), and additional tangential beam injection for toroidal rotation and off-axis current drive. NSTX-U will probe low collisionality regimes projected to exhibit improved confinement. The high-normalized pressure and tangential beam will modify the safety factor profile that enables steady-state advanced scenarios and strongly impacts turbulence and turbulent transport. Macroscopic stability, transport, turbulence and current drive are tightly coupled in these high-performance scenarios. The distinct physics of the edge and pedestal region is a topic of special emphasis due to unresolved questions of the LH transition, steep gradients in pressure and flows, and tight coupling to core performance. The proposed research program contributes to these scientific areas.
2D BES at HL-2A
BES feasibility study for W7-X
Turbulent transport in W7-X will be a complex interaction of ITG/TEM instabilities, about 50 degrees of freedom for 3D shaping, connection length variation, magnetic curvature and wells, ion-root and electron-root radial electric field solutions for ambipolarity, and weak damping of zonal flows. To investigate the physics of turbulence and transport in W7-X, the University of Wisconsin-Madison is conducting a feasibility study for 2D multi-field turbulence measurements with a fluctuation Beam Emission Spectroscopy (FBES) diagnostic system. The feasibility study will assess the diagnostic and technical considerations for 2D measurements of density and flow field fluctuations, but also state-of-the-art techniques for ion temperature, electric potential, impurity, and magnetic field fluctuations. The feasibility study will also address technical solutions for high throughput optical measurements in a superconducting, long-pulse device with high heat loads on plasma-facing components.
Edge ML and real-time BES analysis for control and prediction
This multi-institution project will develop a hierarchy of Artificial Intelligence(AI)/Deep Learning(DL)/Machine Learning (ML) techniques for real-time fusion plasma prediction and control. Facets of the project are to 1) enable real-time analysis of high resolution diagnostics, 2a) label and predict proximity to instability limits, 2b) produce real-time control-relevant predictions of plasma evolution that are difficult to obtain from physics simulations alone, and 3) manipulate experimental actuators in real-time. The UW BES group will develop and deploy “edge ML” resources for the real-time analysis and featurization of 2D BES data at DIII-D. The central plasma control system will integrate real-time signals from BES edge-ML calculations for real-time prediction and control.