Emission Measurements of EBW for Development of a Microwave Heating System on the PEGASUS-III Experiment

Development of non-solenoidal heating and current drive (CD) techniques are advantageous to advanced tokamak concepts and critical for spherical tokamak (ST) concepts for compact fusion power plants. PEGASUS-III is an ultra-low A solenoid free ST focused on non-solenoidal plasma startup and sustainment techniques using local helicity injection (LHI), coaxial helicity injection (CHI) and microwave injection. PEGASUS-III will implement a microwave heating system employing a 28 GHz gyrotron for both electron Bernstein wave (EBW) and electron cyclotron (EC) heating. To aid the development of this system, measurement of mode-converted EBW emission will be carried out using a synthetic aperture microwave imaging diagnostic (SAMI), allowing the first exploration of EBW emission measurements from HI generated plasmas. The emission measurements will be used to validate the ongoing work of modeling EC and EBW injection into HI plasmas, optimize the antenna launch angle, and guide initial experiments. The 28 GHz system on PEGASUS-III will allow direct tests and investigation into synergistic improvement of proposed startup and ramp-up scenarios, providing results relevant for larger scale ST and reactor technologies.