Combined theoretical and experimentalwork has resulted in thecreation of a paradigm which has allowedsemi-qusntitative understandingof the edge confinementimprovement that occursin the H-mode. Shear in the E × B flowof the fluctuations in theplasma edge can lead to decorrelation ofthefluctuations, decreaae!d radialcorr._ lation lengthsand reduced turbulenttransport.Changes in the radialelectric field, the densityfluctuations and the edge transportconsistent with shear stabilization of turbulencehave been seen ha severaltokamaks, The purpose of thispaper isto discuss the most recentdata in the light of the basicparadigm of electric field shear stabilization and to critically cx)mparethe experimentalresults with various theories.

Recent upgrades to many of the diagnostic systems on DIII-D [J.L. Luxon, Nucl.

A full-scale 120m path length ITER toroidal interferometer and polarimeter (TIP) prototype, including an active feedback alignment system, has been constructed and undergone initial testing at General Atomics. In the TIP prototype, two-color interferometry is carried out at 10.59 μm and 5.22μm using a CO 2 and quantum cascade laser (QCL) respectively while a separate polarimetry measurement of the plasma induced Faraday effect is made at 10.59μm. The polarimeter system uses co-linear right and left-hand circularly polarized beams upshifted by 40 and 44 MHz acoustooptic cells respectively, to generate the necessary beat signal for heterodyne phase detection, while interferometry measurements are carried out at both 40 MHz and 44 MHz for the CO 2 laser and 40 MHz for the QCL. The high-resolution phase information is obtained using an all-digital FPGA based phase demodulation scheme and precision clock source. The TIP prototype is equipped with a piezo tip/tilt stage active feedback alignment system responsible for minimizing noise in the measurement and keeping the TIP diagnostic aligned indefinitely on its 120 m beam path including as the ITER vessel is brought from ambient to operating temperatures. The prototype beam path incorporates translation stages to simulate ITER motion through a bake cycle as well as other sources of motion or misalignment. Even in the presence of significant motion, the TIP prototype is able to meet ITER's density measurement requirements over 1000s shot durations with demonstrated phase resolution of 0.06°and 1.5°for the polarimeter and vibration compensated interferometer respectively. TIP vibration compensated interferometer measurements of a plasma have also been made in a pulsed radio frequency device and show a line-integrated density resolution of d = ńL 3.5 10 17 m −2 .

A fiber optic, heterodyne, two-color interferometer utilizing wavelength division multiplexing (WDM) technology has been developed for measuring electron density in plasmas. Vibration compensation is accomplished via common path 1.31 and 1.55 µm distributed feedback (DFB) laser interferometers. All beam combining, splitting, frequency modulation and collimation is accomplished by shared single-mode fiber optic components. Measurements of an argon radio-frequency generated plasma with electron densities of 10 20 m −3 show effective vibration compensation and typical line density resolution of approximately 2 × 10 19 m −2 .