Wendelstein 7-AS was the first modular stellarator device to test some basic elements of stellarator optimization: a reduced Shafranov shift and improved stability properties resulted in β-values up to 3.4% (at 0.9 T). This operational limit was determined by power balance and impurity radiation without noticeable degradation of stability or a violent collapse. The partial reduction of neoclassical transport could be verified in agreement with calculations indicating the feasibility of the concept of drift optimization. A full neoclassical optimization, in particular a minimization of the bootstrap current was beyond the scope of this project. A variety of non-ohmic heating and current drive scenarios by ICRH, NBI and in particular, ECRH were tested and compared

Basic principles of heterodyne techniques are introduced and the various components of a heterodyne system are summarized. Special applications in ECE, interferometry and reflectometry are discussed after introducing the diagnostic principles. Realized systems as described in the literature are briefly outlined. Ordering principles are radiometer types in the case of ECE, mixing scheme and generation and stabilization of local oscillator and intermediate frequency signals in the case of interferometry and reflectometry. Special techniques and their impact on the performance of the diagnostic instruments are discussed. Contents Introduction 1694 Components of the heterodyne receiver 1695 2.1 Heterodyne detection scheme 1696 2.1.1 Mixers 1697 2.1.2 Local oscillators 1705 2.1.3 IF chain 1706 2.2 Sensitivity 1707 2.3 Antennas and waveguides 1709 Radiometry of electron cyclotron emission 1711 3.1 Principles of ECE diagnostics 1711 3.1.1 Frequency and intensity of the emission 1711 3.

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