The electron cyclotron absorption diagnostic at the Rijnhuizen tokamak project

A new 20-channel electron cyclotron absorption diagnostic has been developed at the Rijnhuizen tokamak project. It is the first time the electron pressure profile in a tokamak plasma can be measured directly with a time resolution of 1 ms. The diagnostic measures simultaneously the emission and absorption of the second harmonic electron cyclotron frequencies. Microwaves are injected from the high field side and detected at the low field side in the equatorial midplane. The transmitted power is measured after a single pass through the plasma column. The absorption measurements are complicated by nonresonant losses: refraction of the injected microwaves (losses up to 100%), and scattering of microwaves by density fluctuations (losses 2%–3%). A fast algorithm has been developed to obtain a quantitative measure for these nonresonant losses. This calculation method is based on a parametrization of the experimental data. Combining the electron cyclotron absorption (ECA) measurements and the parametrization provides a reliable tool for determining the optical depth, the electron temperature, and the electron pressure. A good agreement was found between pressure and temperature profiles, measured with ECA and other diagnostics.