Diagnostics for Helical Systems

Abstract

Diagnostics for helical systems are reviewed. A helical system is expected to be a disruption-free steady-state reactor since no plasma current is required. Different types of medium-sized helical systems such as CHS1, W7-AS2, Heliotron E3 and J4, H-1NF Heliac5 and HSX have been developed and constructed. Recently, a large helical device (LHD) has been constructed in Japan and another large one (W7-X8) is under construction in Germany. These two devices are fully equipped with superconducting magnets, modern diagnostics and high power heating systems. The plasma parameters are expected to be comparable to large tokamaks. While most plasma diagnostics are similar to those for tokamaks, there are also specific features: (1) measurement of vacuum magnetic flux surfaces; (2) intrinsic 3-D structure including divertor; (3) intensive diagnostics for the electric field; (4) diagnostics to verify device design elements; (5) high density diagnostics; (6) steady state operation. W7-X optimization principles call for sensitive current diagnostics (Pfirsch-Schluter and bootstrap currents) and tomographic means to verify the reduced Shafranov shift as well as fast particle confinement diagnostics11. Steady state or long pulse operation fulfills the demands of correlation diagnostics as Te fluctuation measurements, it allows for coherent averaging and it enables scans of various diagnostics. Examples are: i) polarization scans of ECE, ii) wavelength scans of spectrometers, and iii) spatial scans of several diagnostics. The special features of diagnostics for helical systems are described in the following chapters.