INVESTIGATIONS ON MEGAWATT CLASS SUB-THz WAVE COAXIAL CAVITY GYROTRON OSCILLATORS

Abstract

Energy requirement of the world is increasing at an alarming rate due to fast economic growth and increased urbanization. The thermonuclear fusion promises to deliver highly efficient, clean and sustainable energy for future energy demands. For the fusion reaction to take place, the temperature inside the reactor must be increased to 150 millionKelvin. In the controlled fusion experiments, electron cyclotron resonance heating and current drive (ECRH&CD) is considered to be an effective way of heating confined plasmas. The required high power millimeter wave radiations are provided by the gyrotron oscillators. Currently in International Thermonuclear Fusion Reactor (ITER), twenty four 1MWconventional cavity gyrotrons operating at 170 GHz are planned for installation at the ECRH system. Next generation fusion reactors like DEMOnstaration (DEMO) tokamak are planned to prove the commercial feasibility of the fusion energy. In ITER, it is experienced that electron cyclotron current drive (ECCD) efficiency is lower than other current drives (CDs) like ion cyclotron CD, neutral beam CD, and lower hybrid CD. In a future commercially attractive tokamaks, for achieving stable operation condition for several hours, the ECRH system should be optimized to provide higher ECCD efficiency. Simulation studies on the ECRH&CD system of a DEMO tokamak demand gyrotrons operating at the frequencies above 200 GHz for an optimum ECCD efficiency. Multifrequency operation of these gyrotrons provides flexibility to the ECRH system for other applications like plasma startup, bulk heating and in the upgraded versions of other existing tokamaks. Furthermore, fast frequency tunability of these gyrotrons in steps of 2-3 GHz would be advantageous for stabilization of the neo-classical tearing modes (NTM) by using nonsteerable, fixed launchers. Introduction to thermonuclear fusion and a brief overview of different classes of gyro-devices are discussed in Chapter 1 of the thesis. The major challenge in the development of megawatt class gyrotrons operating at higher frequencies is to keep control over the ohmic wall loading under allowable cooling limit i.e. 2kW=cm2. To reduce the ohmic wall loading at high power operation, oversized cavities are used as an interaction cavity in gyrotrons. However, conventional overmoded cavities lead to the problem of mode competition and voltage depression. Mode competition can be controlled by using a coaxial cavity as the interaction circuit. Insert in the coaxial cavity also reduces the voltage depression of the electron beam and thus increases the limiting current. Consequently, the operational beam current of the coaxial cavity gyrotrons can be increased to a higher value and therefore can be operated at higher power levels compared to conventional cavity gyrotrons. Basic interaction mechanism of gyrotrons and major sub-assemblies of the high power gyrotrons are presented in detail in Chapter 2. Detailed literature review on the development of high power gyrotrons for i plasma heating applications are also presented in this chapter. Feasibility studies of a 2MW, dual frequency (220/251.5 GHz) coaxial cavity gyrotron for the probable application in the next generation fusion tokamaks are carried out in Chapter 3. The desired cavity mode pair is chosen by considering the technical constraints for multifrequency operation and geometry of the interaction circuit is optimized for the targeted output power with maximum interaction efficiency. In this design study, a coaxial cavity with triangular corrugations on the insert is used as the interaction circuit. Triangular shaped corrugations on the insert are reported as having the advantage of reducing the localized heating on the insert walls compared to the rectangular or wedge shaped corrugations. RF behavior studies are performed with time dependent multi-mode calculations for both the operating frequencies. Further, startup studies are also performed with beam space charge neutralization using realistic non-uniform magnetic field profile. Design studies of the electron gun and output system of the proposed dual frequency (220/251.5 GHz) coaxial cavity gyrotron are carried out in Chapter 4. The magnetron injection gun and the magnetic guidance system are designed for required beam parameters determined through RF behavior studies. Output system design studies include physical design of a common nonlinear taper (NLT), a quasi-optical launcher (QOL), and a single disk RF window to support dual regime operation. The NLT is designed with maximum transmission efficiency and minimum mode conversion of the desired mode pair at their respective operating frequencies. The dimpled wall launcher is designed such that it supports, the conversion of the both cavity modes into Gaussian-like mode and the single disk RF window is designed for the effective extraction of high power output RF beam at the desired operating frequencies. Further, design studies are carried out for this proposed dual regime gyrotron to explore the possibility at the third operating frequency of 283 GHz in Chapter 5. The operational cavity mode for this operating frequency (283 GHz) is selected such that the same physical structure of the gyrotron that has already been designed for the dual regime operation would be used for this operation also. RF behavior studies are performed for this 283 GHz operation, and the startup calculations are also carried out. In addition, design studies of the electron gun and the output system are performed to confirm whether these components support the additional operating frequency of 283 GHz. The design studies of a triple frequency (170/204/236 GHz) coaxial cavity gyrotron are carried out in Chapter 6 that is suitable for plasma heating application in the commercial fusion DEMO tokamak class prototype reactor. To reduce the spatial and maintenance requirements of the heating systems in the DEMO tokamak, the output power of the proposed gyrotron is targeted at 3MW for all the three operating frequencies. In addition to the physical and technical constraints of the multifrequency operation, cavity modes ii for this gyrotron are chosen with the mode spread angle of 72 0:5 to support a dual beam output system. RF behavior studies predict the possibility of obtaining 3MW at the desired operating frequencies in the proposed gyrotron. The geometry of the magnetron injection gun is optimized for generating required electron beam determined through RF behavior studies to support this multifrequency operation. Startup calculations are also carried out with beam space charge neutralization to ensure the 3MW continuous wave operation at all the three operating frequencies. In the practical development of coaxial cavity gyrotrons, axes of the outer cavity, coaxial insert, and the electron beam must align together. Any misalignment in the axes of the insert and the outer cavity would change the electromagnetic field structure in the coaxial interaction cavity. Field analysis of the coaxial cavity with misaligned triangular corrugated insert is carried out using surface impedance model (SIM) and full wave analysis (SHM) in Chapter 7. Effect of insert misalignment on the eigenvalue and oscillating frequency of the cavity modes in the interaction cavity of coaxial cavity gyrotron are studied. Finally, Chapter 8 summaries the contribution of this thesis in the development of coaxial cavity gyrotrons for the application in the ECRH system of the experimental tokamaks with the future scope of the present work. As an outline, in this thesis feasibility studies of the high power, multifrequency coaxial cavity gyrotrons for the probable applications in the next generation fusion tokamaks are carried out. Suitable physical design studies of the major sub-assemblies of gyrotrons supporting the multifrequency operation are performed. In addition, the field analysis of a triangular corrugated coaxial cavity with a misaligned inner rod is also carried out using both SIM and SHM methods.