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

Pegasus-III is a solenoid-free, low-aspect-ratio spherical tokamak (ST) focused on studying nonsolenoidal plasma initiation techniques. It will be equipped with multiple 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. Microwave heating and current drive techniques are also candidates for noninductive plasma initiation, ramp up, and sustainment on their own. STs are well-suited for EBW heating and current drive because a large volume of the 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 is being fielded 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 code has been performed to assess the viability of these EBW mode conversion and EC methods. Modeling results indicate that both methods can be successfully used in Pegasus-III plasmas.