Cost-Effective Microwave Assisted ECR Heating Using Combination of Quasi-Locked Low-Power Magnetrons on GLAST-III

Glass spherical tokamak-III (GLAST-III) is a small spherical tokamak ( $R =20$ cm, $a =10$ cm, and $A =2$ ) developed at Pakistan Tokamak Plasma Research Institute (PTPRI). To assist the plasma discharge and generate considerable plasma current in GLAST-III, high-power microwave sources can indeed play an important role in noninductive startups of plasma discharges such as electron cyclotron resonance heating (ECRH). A power-enhanced pulsed microwave source (2.45 GHz and 1.6 kW) has previously been developed and tested at PTPRI. However, higher powers are required to increase the generated charge density for effective pre-ionization. Therefore, a microwave pre-ionization source (up to 20 kW at 2.45 GHz) is installed on GLAST-III to reduce the high-loop voltage requirement during the plasma start-up. ECRH has been effectively utilized for this purpose. In this article, an economical high-power microwave source is fabricated by coupling low-power magnetrons in a waveguide. We employed Walton multiplier circuits for the first time to operate two magnetrons coupled for power addition. The coupling of two magnetrons due to constructive interference is demonstrated successfully during continuous as well as pulsed operation. The scheme can be made more effective in future studies by injection locking of magnetrons. These innovative low-cost schemes can play a pivotal role in carrying out microwave studies on spherical tokamaks such as GLAST-III.