A Microwave System for Solenoid-free Startup Studies on Pegasus-III

Pegasus-III is a solenoid-free, low aspect ratio ST focused on studying non-solenoidal tokamak startup techniques. It will be equipped with helicity injection (HI) systems and a 28 GHz gyrotron-based system for electron Bernstein wave (EBW) and electron cyclotron (EC) heating and current drive. Microwave injection can provide heating during HI to reduce losses from resistive dissipation. Also, microwave heating and current drive techniques are candidates for non-inductive plasma initiation, ramp-up and sustainment on their own. STs are well-suited for EBW heating and current drive because a large volume of plasma is overdense. During low-density startup, ST plasmas can remain accessible to EC waves for heating and current drive. To aid in the design of this microwave system, a synthetic aperture microwave imaging diagnostic will be used for the first exploration of EBW emission measurements from HI generated plasmas. This diagnostic will be used to identify effective operational windows for EBWs. Modeling using the GENRAY and CQL3D suite of codes is being performed to assess the viability of these EBW and EC methods. A combination of scenario development and modeling will be used to inform system requirements and launcher design.