## Beam Emission Spectroscopy (BES)

University of Wisconsin/DIII-D

Beam Emission Spectroscopy (BES) measures localized, long-wavelength $(k_{\perp}\rho_{I} < 1)$ density fluctuations at high spatial resolution in the core, pedestal, edge and Scrape-Off-Layer regions of DIII-D plasmas in an effort to characterize plasma turbulence and resulting turbulent-driven transport. Fluctuations in the plasma density, temperature, and electrostatic potential are widely recognized to result in the anomalously high cross-field particle and energy diffusion observed in nearly all magnetically confined fusion plasmas. An understanding of the underlying physical mechanisms that give rise to such fluctuations and transport may lead to accurate prediction of their effects in large reactor-scale devices and potentially methods to mitigate this.

BES is used to measured such turbulence parameters as:

• Density Fluctuation Spectra and Amplitudes
• Radial & Poloidal Correlation Lengths
• Decorrelation Times
• Wavenumber Spectra, $S(k_{r},k_{\theta})$
• Equilibrium Poloidal Turbulence Flows
• Fluctuating Poloidal Flows (as pertaining to zonal flows and Geodesic Acoustic Modes)
• 2D Images and Movies of Turbulence
• 2D time-resolved turbulence velocity mapping
• Mode-mode coupling
• Linear growth rates
• Internal Energy Transfer

The localized density fluctuation measurements are also useful for diagnosing Energetic-Particle-Driven Modes (e.g., TAE, RSAE, BAAE, e-GAM), MHD (NTM), pedestal and ELM structures, as well as scrape-off-layer phenomena.