Speaker
Description
High power gyrotrons are normally axially symmetric, which applies to both the electron optics and the microwave optics from the electron gun up to the end of the cavity and this is crucial for the most efficient interaction between the electron beam and the desired mode. On the other hand, due to the symmetry, undesired modes can also be excited, which are in competition with the main mode and can reduce the gyrotron performance. There are two areas in the gyrotron where this is the case. In the so-called beam tunnel between the anode and the cavity, the highly unstable electron beam must propagate through a range of different magnetic field strengths without exciting any electromagnetic modes, also known as the beam tunnel instability. In the actual interaction zone, the cavity modes with nearby frequencies and field maxima (caustic) compete with the desired operating mode. In high-power gyrotrons, both components have already many eigenmodes that could be excited. On the other hand, further increasing the performance of the gyrotrons requires even larger cavities and beam tunnels and thus resulting in an even denser spectrum of eigenmodes. Increasing the gyrotron frequency also increases the number of possible parasitic modes.
We will present the applications of the so-called Schroeder diffuser structures [1] in the beam tunnel and in the gyrotron cavity. These structures are random phase gratings that scatter a single wave vector into a broad wave vector spectrum already at a single reflection. They are also broadband and lead to a randomization of the wave field in the resonator, where the radial electric field maxima of the parasitic modes can be reduced and their ohmic wall losses are increased. Such structures were built into the existing beam tunnel of the short-pulse prototype of the TH1507U gyrotron [2] and tested at KIT. The results will be presented.
The possibility of using Schroeder diffuser structures in the cavity was also investigated. Numerical simulations show that it is possible to suppress all modes with an azimuthal mode number different from the desired mode. The results and consequences are discussed.
References
[1] M.R.Schroeder,” Number Theory in Science and Communication”, Springer 1984. Sec. Enl. Edit. 1990
[2] Z. Ioannidis, et.al., Generation of 1.5MW-140GHz pulses with the modular pre-prototype gyrotron for W7-X, IEEE Electron Device Letters, 42, 6, June 2021, DOI: 10.1109/LED.2021.3073221
