DESIGN STUDIES ON MULTI-FREQUENCY OPERATION OF TRIANGULAR CORRUGATED COAXIAL CAVITY GYROTRON AND INSERT MISALIGNMENT

Abstract

Gyrotron oscillators (gyrotrons) are energy sources that are capable of generating millimeter wave with powers as high as several megawatts. They are popularly employed in Electron Cyclotron Resonance Heating (ECRH) and Current Drive (ECCD) of magnetically enclosed plasma in fusion research and studies. They are used in experimental tokamaks for diverse purposes including start up, stability, control and diagnosis of plasma. Gyrotrons are also used in DNP=NMR-spectroscopy; in the field of medicine, generation of soft X-Rays, ceramic sintering in material processing and in active denial system for defence=military requirements. A brief introduction to gyrotron oscillators is done in Chapter 1. The various applications are discussed in detail and the motivation for the project is also mentioned.The principle of working of gyrotrons is discussed. The various structural sections such as the magnetron injection gun (MIG), interaction cavity and RF extraction are also studied. Two main setbacks affecting the high power operation of gyrotrons are voltage depression and wall loading while high frequency operation is hindered by mode competition. The development of the RF interaction cavity in order to overcome these phenomena has been reviewed in Chapter 2. Incorporating an open ended coaxial insert to a conventional RF interaction cavity will reduce the problem of mode competition as well as voltage depression. Reducing the wall loading while reducing the mode competition is a trade-off and hence the radius of the coaxial insert is a vital parameter in the design of coaxial cavity gyrotrons. The mode competition can also be controlled by providing an axial taper for the coaxial insert which provides an extra parameter to control the Q-factor of the various competing modes inside the cavity without affecting the operating mode. The losses on the coaxial conductor can be reduced by providing axial corrugations on the coaxial rod. Triangular corrugated coaxial insert has lower losses in comparison to rectangular or wedge shaped corrugated coaxial insert owing to the fact that localized heating at the bottom of the corrugations can be avoided by the virtue of the triangular shape of the corrugations. Detailed review of rectangular and triangular corrugated coaxial cavity structures are done in Chapter 2. The proposed commercial thermonuclear fusion tokamak, DEMOnstration power plant (DEMO) is expected to employ megawatt class gyrotrons working at elevated frequency values, to cater the requirement of high efficiency in heating and current drive systems for plasma handling. Current research activities around the world are driven by the motive of developing sub-terahertz wave gyrotrons at megawatt class power levels with multifrequency operational capabilities. This is mainly because, employing a multi-frequency i gyrotron can greatly improve the adaptability of an ECRH&CD system to operate at specific and contrasting frequencies and function in diverse experimental setups without a large increase in construction costs. The RF behavior studies of a 220=251.5 GHz, 2 MW, coaxial cavity gyrotron housing triangular corrugated inner rod, extended to a third operating frequency at 283 GHz are done in Chapter 3. Magnetron Injection Guns (MIGs) are used in gyrotron systems to produce the hollow electron beam with the optimal beam parameters for the satisfactory power generation at the required frequency. The electron beam radius and the velocity ratio are crucial for the performance of the gyrotron system. The third frequency operation of the coaxial triode MIG of the dual frequency regime (220=251.5 GHz, 2 MW) is studied in Chapter 4. This already designed MIG is electrically optimized for the 283 GHz operation and the results are detailed. Subsequently, the start-up behaviour analysis considering the beam neutralization of 283 GHz operation is also carried out presented in Chapter 4. During the practical development of coaxial cavity gyrotrons, two problems are faced namely; the misalignment of the electron beam axis with respect to the outer cavity axis and the misalignment of the coaxial insert axis with respect to the outer cavity axis. The mathematical analysis of a coaxial cavity gyrotron with a misaligned triangular corrugated inner rod using Surface Impedance Method (SIM) and the effect of the eccentricity on the oscillating frequency of modes in the triangular corrugated coaxial cavity are studied in Chapter 5. The full wave analysis of such a misaligned system is also carried out using Space Harmonics Method (SHM) approach and is presented in Chapter 5. As an outline, this dissertation includes the design studies of the already proposed dual freq (220/251.5 GHz) 2 MW triangular corrugated coaxial cavity gyrotron extended to the third frequency operation at 283 GHz and the corresponding electrical optimization of the input system to support this 283 GHz operation. The field analysis of a triangularcorrugated coaxial RF interaction cavity with a misaligned insert is also carried out using both SIM and SHM methods.