Development of microwave pre-ionization source for GLAST tokamak

Abstract

A microwave pre-ionization source is designed, fabricated and installed on Glass Spherical Tokamak (GLAST) to reduce the high-loop voltage requirement during the plasma startup. In GLAST II, limited space is available at the in-bore side, which restricts the size and heating capability of central solenoid. Therefore, the central solenoid alone cannot produce the required loop voltage for plasma breakdown. Electron Cyclotron Resonance Heating (ECRH) can be very useful to overcome this limitation. In this work, an economical microwave source is fabricated using magnetron of 2.45±0.02GHz, obtained from a household microwave oven. Pulsed-mode operation of the magnetron is achieved through certain modifications in the half-wave circuit of domestic microwave oven. A new technique is proposed to enhance the microwave power, where an additional electromagnet is introduced around the magnetron cavity that confines the fast moving electrons. This modified microwave source is sufficient to achieve the breakdown in GLAST II with improved plasma current of 5kA.