Speaker
Prof.
Miklos Porkolab
(MIT)
Description
Phase contrast imaging (PCI) is an internal reference beam interferometric technique which provides a direct image of line integrated electron density fluctuations in a plasma. The method has been used with great success to measure waves and turbulence in magnetically confined high temperature plasmas over the past three decades. The principle of PCI was developed in optics in the 1930s by the Dutch physicist Zernike, leading to the development of phase-contrast microscopy for which he received the Nobel Prize. The technique allows one to detect the variation of the index of refraction of a dielectric medium (such as a plasma) due to the presence of waves or turbulent density fluctuations. The concept relies on the introduction of a phase plate in the light beam path, causing a 90 phase shift in the unscattered portion of the beam relative to the scattered one, and subsequently the expanded laser beam is imaged onto a cryogenically cooled detector array that combines the two beams, thus allowing one to measure wavelengths and correlation lengths of fluctuations. The first time such method was used to measure waves in fusion plasmas is attributed to Henry Weisen who detected Alfvén waves in the TCA tokamak in Lausanne. Other experiments followed in rapid succession by Porkolab and coworkers, measuring not only turbulence in the edge and the core plasmas in Alcator C-Mod and DIII-D but also detecting coherent waves in the ICRF regime with heterodyne techniques that use Acousto-Optic Modulators. Further work included studies of MHD modes, for example Alfvén wave cascades in the presence of reversed shear and energetic ions in ICRF heated plasmas in C-Mod. More recently, the PCI diagnostic has been implemented in the Wendelstein 7-X stellarator at the Max Planck Institute in Greifswald and initial results have been presented very recently. In this talk, the principle of PCI is discussed and examples of measurements will be presented from a wide range of recent and past experiments, demonstrating the versatility of this diagnostic. From practical considerations, typical wave numbers that can be measured with PCI using a 10-50 Watt cw CO2 laser at 10.6 um wavelength are in the range of 1.5 k(cm-1) 30 and frequencies 10 kHz to 2.0 MHz. This covers most of the wave-numbers and frequencies of interest in turbulence studies, including ITG, TEM and some ETG modes. It also includes typical wave numbers of ICRF waves. Finally, we note the recent extension of the PCI diagnostic on the DIII-D tokamak where it was combined with an interferometer method to measure long (0 k(cm-1) 4.0) wavelength fluctuations at frequencies up to 1 MHz.
* Work supported by the US Department of Energy
Primary author
Prof.
Miklos Porkolab
(MIT)
Co-authors
Dr
Alessandro Marinoni
(MIT)
Dr
Chris Rost
(MIT)
Prof.
Eric Edlund
(SUNY, Cortland)
Dr
Evan Davis
(MIT)
