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BES & Tangential Thomson scattering

105 R-0

Beam Emission Spectroscopy

Beam Emission Spectroscopy, or BES, measures plasma density fluctuations at high spatial resolution in the core and edge 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 believed 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 phenomenon.

Tangential Thomson scattering

Thomson is a spectral diagnostic used at DIII-D to obtain plasma temperatures and densities at various locations within the plasma throughout the course of a shot. * the diagnostic is considered non-intrusive, (ie it does not affect or interfere with the plasma). * basic concept involves shooting laser beams (photons) into plasma and monitoring reaction of (scattering) electrons.* reactions can be measured in wavelengths. * analysis of these reactions give us information to derive characteristics of the plasma such as temperature and density. * the DIII-D thomson system uses 8 Nd:YAG lasers. * 7 core lasers * 1 divertor laser * 1 horizontal laser * there are currently three laser entry points: 1. the primary original path (also known as the core laser path) passes vertically through the outer region of the plasma (r=2meters). 2. the second path installed passes vertically through the divertor region (r=1.5 meters). 3. the third most recently installed (1/99) laser path passes through the central region of the plasma (central or tangential thomson project) and enters horizontally. * the laser firing sequence is arbitrary and programmable with microsecond time accuracy. * the lasers can be "flashed" or "lased". * up to 1 joule of energy which can vary from laser to laser and pulse to pulse. * each laser can operate up to 20Hz. * 8 lasers firing at 20Hz for an 8 second shot produces 1200 pulses. * a DIII-D shot typically lasts between 6 to 8 seconds. * measurements are made at 44 possible points in space in the plasma at various times throughout a shot. with multiplexing additional measurements can be made. * 3 collection optics (lens ports) attached to fiber optics focus on specific points in plasma space where a specific laser pulse passes by and causes scattering. certain optics are focused on the core region and others on the divertor region. * 44 polyboxes act as filters for 44 measurement points in plasma space, analog amplifiers are attached at the end of the filters. * Each polybox has 7 detectors and is associated with a single point in space within the vessel. A polybox looks at light scattered from one or more lasers. * there are two basic types of thomson shots * calibration test shots * plasma shots