Comparison between controlled non-adiabatic and E×B concepts for gyrotron multistage depressed collectors

Abstract

IntroductionGyrotrons are used as RF sources for ECH systems in nuclear fusion experiments. To obtain sufficient energy-gain factor, the future DEMOnstration fusion power plant demands highly efficient gyrotrons. The interaction between electron beam and microwave in a fusion gyrotron may have in the optimal case approximately 35 % efficiency (ηint). A large fraction of the input energy still remains in the spent electron beam. Up to ηcol ≈ 60 % of the spent beam energy can be recovered by a Single-stage Depressed Collector (SDC), which leads to an overall gyrotron efficiency of about 50 % according towhere ηRF ≈ 90 % is assumed to be the ratio between the output and the generated RF power. To achieve 60 % of gyrotron overall efficiency, the collector efficiency ηcol is required to be higher than 74 %. For this purpose, Multistage Depressed Collectors (MDC) should be introduced. In an MDC the spent electron beam will be sorted. Proper deceleration voltages (depression voltages) will be applied to each energy stage. Moreover, in order to reduce the thermal loading on the electrodes, the electron beam have to be eventually spread or swept on a large area. Designing an MDC for fusion gyrotrons is nontrivial. Three major reasons are listed here. Firstly, fusion gyrotrons works under high magnetic field, which has a dependence on the cyclotron frequency as ߱ ୡ = ݁ ‫ܤ‬ ߛ ݉ , with ߛ =

Coaxial multistage depressed collector design for high power gyrotrons based on E×B concept

“…This second approach has some advantages, such as a small influence of secondary electrons, insensitivities to the energy distribution, stray magnetic field and misalignments, as it is extensively discussed in Ref. 12 .…”

“…The design of an MDC system for high power and high frequency gyrotrons is not trivial due to the fact that the spent electron beam is confined by a relatively strong magnetic field in the order of 50 mT in the collector region 12 . The sorting of the spent beam electrons according to their initial kinetic energy is essential for the increase of the collector efficiency.…”

“…The thermal loading on the collector wall should be in an acceptable range. The separation of the spent beam electrons of a high power gyrotron based on the E×B drift concept was proposed in 2008 5 and is considered as the most promising method [12][13][14] . Since then, three different types of MDC designs have been suggested for annular gyrotron electron beams based on the E×B drift.…”

European research activities towards a future DEMO gyrotron

“…An overall efficiency of higher than 60% requires the use of advanced multi-stage depressed collectors (MDCs). Two concepts are under investigation [18] at KIT. In the first one, the gyrotron magnetic field is unwound utilizing wellcontrolled non-adiabatic transitions, whereas in the second one, the electrons are sorted by an E × B drift [19][20][21][22].…”

Mechanical Design of the Short Pulse E×B Drift Two-Stage Depressed Collector Prototype for High Power Gyrotron

“…The collector efficiency can be significantly increased with a higher number of depression potentials as applied in a multistage depressed collector (MDC) due to the reduced remaining energy of the decelerated spent beam electrons. However, simple axisymmetric MDC concepts are not suitable for separation of the spent beam electrons in high magnetic field flux gyrotrons due to the strong magnetic field in the collector region [1,2]. To overcome the strong confinement of the beam electrons to the magnetic field lines, a different concept is investigated.…”