Millimetre-wave aspects of the FOM fusion free electron maser

Abstract

The FOM Fusion Free Electron Maser (FEM) is a tuneable source designed to deliver millimetre wave radiation in the frequency range of 130 to 260 GHz at high output power (1 megawatt during 0.1 second) and high efficiency (50%). High power millimetre wave sources play an important role in present and future fusion (tokamak) reactors such as ITER. The world demand for energy is increasing and will continue to increase for many decades, primarily because both the world population and the average energy consumption per capita are growing. The largest growth of energy demand is expected to occur in the emerging economies, with China as a prominent example. By the end of the century the world energy consumption is expected to have reached 3 to 5 times the present level. To meet this challenge of finding clean and ‘unlimited’ supplies of energy, which should be available in an economic way, all energy options have to be (re)considered. Application of fusion energy, as used in the sun, appears as one of the most attractive long-term options because of the widespread distribution of its abundant fuel supplies, at low cost, and because of its inherent safety. In fusion applications mm-wave sources are used for start-up, heating and optimizing of the plasma. Many megawatts of continuous mm-wave (CW) power at frequencies of 140 to 300 GHz are needed. Furthermore, rapid frequency tuneability and high overall system efficiency of the mm-wave sources are important requirements, too. When the FEM-project was started, in 1991, the state of the art mm-wave sources, such as gyrotrons, did not meet all these requirements simultaneously (and still do not as of today). As a possible solution the FEM was developed at Rijnhuizen. First the design of the heart of FEM, the undulator waveguide, is discussed. From a number of possible waveguides, the most suitable type is selected and the losses are calculated using an accurate field representation. Based on these fields the dispersion diagram is given. The reflection and out-coupling system are treated in a similar way. Finally, the wideband transmission line and window design are described. The setup and results of the low-power tests are given, not only for the separate cavity sections, but also for the total system including the transmission line. Here losses are measured at a number of frequencies of the system, which are also the basis for improvements of the FEM. Several mm-wave diagnostic systems are described which were developed and used at the FEM to measure the relative power and frequency spectrum versus time, the spatial power distribution and the absolute power. The results of the high power measurements on the FEM, using the above mentioned diagnostics are given. A comparison is made with the various theoretical models developed. Furthermore, possible causes are given for a number of problem issues in the long pulse setup like higher output power and longer pulses, which could not be solved due to termination of the FEM project. Finally suggestions are given for a follow-up of the FEM. One of the most important issues, the heat loading on several components, is treated extensively. An outline is given of the improvements needed for CW operation. The concepts developed in this thesis could benefit other groups developing, or working with, high-power mm-wave devices or transmission line systems.