Phase Contrast Imaging Methods to Study Waves and Turbulence in High Temperature Tokamak Plasmas

Abstract

Summary form only given. Phase contrast imaging diagnostic (PCI) is an internal reference beam interferometric technique which has been used successfully in high temperature tokamak plasma experiments to image line integrated plasma density fluctuations. The PCI technique utilizes a 18 deep grooved "phase plate" which is inserted into an expanded beam path, and with the aid of a detector array one is able to measure wavelengths and correlation lengths of fluctuations propagating perpendicular to the laser beam. In the Alcator C-Mod and DIII-D tokamak PCI experiments, a CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> laser beam is used to probe low frequency (f = 1 MHz) instabilities and in addition, in C-Mod high power launched ICRF waves (80 MHz) are also monitored. The fluctuations studied in the past in Alcator C-Mod include the so-called "quasi-coherent mode" (a ballooning mode localized to the edge pedestal), semi-coherent TAE-like modes, including Alfven wave cascades, low frequency turbulence, and high power launched ICRF waves. The ICRF waves are detected by a heterodyne technique using optical modulation of the laser beam. The ICRF wave propagation studies have confirmed mode conversion into kinetic ion cyclotron waves (the shear wave branch) and electrostatic ion Bernstein waves. In DIII-D, PCI has been used to study low frequency turbulence during L to H mode transition, ELMs, and coherent edge modes during the quiescent H-mode. Signatures of zonal flows have also been observed in past experiments. While most of the past studies were limited to wavelengths equal or longer than the ion gyro-radius (k <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</sub> = 1, f = 1 MHz ), new upgrades to the electronics and optics will allow detection of wavelengths and frequencies in the electron gyro-radius regimes (k <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</sub> = 1, f = 20 MHz). This new capability will allow us to study the electron temperature gradient modes and the trapped electron mode, both being candidates for determining electron transport in magnetically confined plasmas. While spatial localization of long wavelength modes along the PCI laser beam is usually not possible, in the short wavelength regimes in a sheared magnetic field localization can be achieved by using a rotating masking plate in conjunction with the phase plate