Development of Electron Bernstein Wave Research in MAST

Burning plasma spherical tokamaks (STs) rely on off-axis current drive (CD) and nonsolenoid start-up techniques. Electron Bernstein waves (EBWs) may provide efficient off-axis heating and CD in high-density ST plasmas. EBWs may also be used in the plasma start-up phase because EBW absorption and CD efficiency remain high even in relatively cold plasmas. EBW studies on the Mega Ampere Spherical Tokamak (MAST) can be subdivided into four separate subjects: thermal electron cyclotron emission observations from overdense plasmas, EBW modeling, proof-of-principle EBW heating experiments with the existing 60-GHz gyrotrons, and EBW assisted plasma start-up at 28 GHz. These studies are also aimed at determining the potential for a high-power EBW system for heating and CD in MAST. The optimum choice of frequency and launch configuration is a key issue for future applications in MAST. This paper describes diagnostics, modeling tools, and high-power radio frequency systems developed specifically for EBW research in MAST. The experimental methodology employed in proof-of-principle EBW heating experiments along with experimental results is discussed in detail. EBW heating via the ordinary-extraordinary-Bernstein (O-X-B) mode conversion has clearly been observed for the first time in an ST.