Technical developments at the KIT gyrotron test facility

Abstract

Parasitic beam tunnel oscillations have been discovered on all gyrotrons currently being developed and tested at KIT (1MW/140 GHz tube for W7-X, 2 MW/170 GHz coaxial gyrotron and 105-143 GHz frequency step tunable gyrotron). In the case of the W7-X gyrotrons these problems have been observed only on some of the series production tubes. A modified beam tunnel design, technologically close to the old design, has successfully been tested on both the coaxial and f-step-tunable gyrotrons and has subsequently been implemented on one of the W7-X series-production-tubes presently undergoing factory acceptance tests in Karlsruhe.The ECRH test loads at KIT are operated under normal atmospheric conditions. Several loads have eventually failed in 1 MW long pulse experiments and KIT has therefore started to design its own loads. The first KIT-load is based on a fixed conical mirror and a cylindrical aluminum absorber coated with a lossy material. The new load was very economical to manufacture and has so far successfully been used during the acceptance tests of two 1-MW CW gyrotrons. Nevertheless a new load based on (uncoated) stainless steel absorbers is being developed as a backup solution for the ongoing high priority gyrotron testing.A superconducting magnet capable of rapid field changes between 4.15 and 5.67 Tesla for frequency steptunable gyrotrons has been procured, has achieved its specified maximum (static) field of 7.2 Tesla and has also demonstrated its capability of rapid field-changes. Final acceptance however has been delayed pending rectification of problems with the magnetic field alignment.

Recent achievements on tests of series gyrotrons for W7-X and planned extension at the KIT gyrotron test facility

“…The problems with (backward-wave) parasitic beam tunnel oscillation on several (TH1507) W7-X series production gyrotrons [1], [2] have been overcome with a corrugated beam tunnel design, as shown in Fig.1 and 2. Series production gyrotrons no.…”

Study of a 105 GHz short-pulse dummy load for the electron cyclotron resonance heating system with the quasi-optical method

“…Considering that the size of the dummy load can reach a few meters, which is much longer than the wavelength, a quasi-optical model has been constructed. In this quasi-optical model, the incident wave is considered as a Gaussian beam [4,9]. The analytical process of the dummy load is based on the quasi-optical formulas introduced in section 2.…”

Plasma Spraying of a Microwave Absorber Coating for an RF Dummy Load

“…The development of gyrotrons for W7-X as well as ITER has induced some more efforts to develop absorber coatings, especially for testbed dummy loads, as published in [5]. The paper describes the basic concept for a dummy load design with a mixed aluminum titanium oxide absorption coating.…”

“…Figure 1 shows the general concept we used for the design of the load: It is based on a symmetric cylindrical structure, where the microwave beam is axially injected through an input port from one side, scattered by a conical mirror on the opposite site and from there distributed to the coated cylindrical surface by multiple reflections. Tapering of the absorbing ceramic layer on the cylinder allows an adaption and balancing of the power density on the structure to avoid hotspots, especially in the case of the first impact of the scattered microwave beam [5,10]. This requirement implies that the thickness of the layer increases from the mirror side to the input area according to the decreasing microwave power density.…”

“…Adding up the losses of the rays gives the power per area. Since none of the peaks exceeds 125 W/cm², overheating problems of the thermally well conducting water-cooled copper-chrome structure are not expected [5]. The calculated back-reflected microwave power leaving the load has a value of 2.4% and is therefore easily manageable.…”