Commissioning a microwave based Collective Thomson Scattering (CTS) Diagnostic on ASDEX-Upgrade

Intr oduction: The fast ion collective Thomson scattering (CTS) diagnostic installed on ASDEX Upgrade (CTS-AUG) uses mm-waves generated by the newly installed 1 MW dual frequency gyrotron as probing radiation at 105 GHz ( 1). The CTS-AUG makes use of one of the ECRH launching systems (gyrotron, transmission line and antenna) as a receiver. The scattered radiation will be intersected by a moveable mirror in the transmission line and redirected to the CTS receiver. It measures back-scattered radiation with a heterodyne receiver having 50 channels to resolve the 1-D ve locity distribution of the confined fast ions. The physics feasibility of such a microwave base d system has already been described in Refs 1 and 3. The presentation will briefly review the commissioning activities carried out to date which will implicitly describe the prerequi sites of a microwave based CTS system. Receiver tests: The details of the detection system of the CTS on AUG are described in Ref 4. The receiver is equipped with 3 sets of IF amplifier pairs (70 - 80 dB amplification for each frequency band) that need to be robust against gain compression. Gain compression is the reduction in differential or sl ope gain caused by nonlinearity of the transfer characteristic of the amplifying device. This nonlinearity ma y be caused by heat due to power dissipation, or by overdriving the active device beyond its linear region. Using a programmable IF source, tests have been carried out to study the IF amplifier characteristics. In situ tr ansmission line alignment and beam quality: Careful design, construction, alignment and quality assurance of the of the CTS transmission section and its coupling to the ECRH transmission line are important not only to achieve low loss transmission, but also to provide good spatial localisation of the measurement and accurate definition of the location of the movable measurement volume and resolved velocity direction. The first