Chuanren Wu

Multistage depressed collector conceptual design for thin magnetically confined electron beams

The requirement of higher efficiency in high power microwave devices, such as traveling wave tubes and gyrotrons, guides scientific research to more advanced types of collector systems. First, a conceptual design approach of a multistage depressed collector for a sheet electron beam confined by a magnetic field is presented. The sorting of the electron trajectories, according to their initial kinetic energy, is based on the E × B drift concept. The optimization of the geometrical parameters is based on the analytical equations under several general assumptions. The analysis predicts very high levels of efficiency. Then, a design approach for the application of this type of collector to a gyrotron cylindrical hollow electron beam is also presented with very high levels of efficiency more than 80%.

An Improved Broadband Boundary Condition for the RF Field in Gyrotron Interaction Modeling

Gyrotrons are the only RF sources providing significantly high output powers at continuous-wave operation in the sub-terahertz frequency range. A prerequisite for a proper study of gyrotron behavior is the accurate time-dependent self-consistent simulation of the interaction between the electron beam and the RF wave inside the interaction region (cavity). That requires an accurate description of the RF fields at the boundaries of the interaction region. In this work, an improved broadband boundary condition is proposed. Based on polynomial series expansion of the load impedance and of the wavenumber, this boundary condition not only features a significantly improved broadband matching, but also allows a customizable frequency-dependent reflection coefficient at the boundaries. The boundary condition has a general formulation, i.e., it does not rely on a special numerical method. For the case of perfect matching, the new formulation is validated through comparing the numerically calculated reflection coefficients with those obtained by the state-of-the-art matched gyrotron broadband boundary condition, whereas the validation in the case of a given frequency-dependent reflection is done by comparing the numerically calculated reflection with the prescribed one. Although this concept is initially intended for the simulation of gyrotron cavities, it can be easily extended to any other open-cavity resonators, for which proper definition of the boundary conditions is required.

3D Simulation of a realistic Multistage Depressed Collector for high-power fusion gyrotrons

Multistage Depressed Collectors (MDCs) have been successfully applied in many kinds of vacuum tubes; however, there are only few discussions about applicable MDCs for high-power gyrotrons, regarding the hollow small-orbit electron beam and relatively strong stray magnetic field in the collector region. A novel concept has been recently proposed, where the electron beam is transformed into one or more sheet beams and sorted by the E×B drift. This contribution shows a 3D simulation result of a realistic seven-stage depressed collector model based on the above mentioned concept. With ordinary optimizations, a collector efficiency of 86 % has been achieved in this simulation.

Multistage depressed collector design approach for gyrotron

A design approach for a multistage depressed collector system for gyrotron is presented here. The new concept based on two steps: (i) the transformation of the cylindrical hollow beam to sheet beam and (ii) the sorting of the spent beam electrons on the electrodes, based on ExB approach, in order to achieve the optimal efficiency. The simulation results predict very high levels of efficiency (more than 80%) and this idea can open up new perspectives in gyrotron technology.

Preliminary studies on Multistage Depressed Collectors for fusion gyrotrons

Fusion gyrotrons are high-power microwave sources which are capable to generate RF power in the MW range at millimeter wave and sub-THz frequencies. Today, typically around 35 % electronic efficiency is achieved in the energy transfer (interaction) from the electron beam to the electromagnetic field. Using a single-stage depressed collector (SDC), the overall efficiency of a gyrotron can be increased to around 50 % as planned for ITER. To achieve higher total efficiencies, so-called Multi-stage Depressed Collectors (MDC) need to be introduced. MDCs offer the possibility to sort the electrons by their energy more accurately and to guide the electrons to electrodes on proper voltages. However, the application of the MDC concept to gyrotron is nontrivial. That is due to the small-orbit hollow electron beam, the large variation in energy of the spent beam at the collector entrance and the existence of a relatively strong magnetic field at the collector region, which dominantly constrains and guides the electron beam. According to Buschs theorem, if both electric and magnetic field components are axially symmetric, the cyclotron motion of the electrons cannot be spatially separated. For this case, a new approach for MDC, based on a spatial trajectory modulation caused by non-adiabatic transition is proposed. A design proposal for a two-stage collector is presented which shows 73 % collector efficiency which is already close to the theoretical limit for a two-stage collector. Besides, the MDC has the advantage that the collector wall loading is sufficiently low, thus electron beam sweeping is not necessary.

From W7-X towards ITER and beyond: Status and progress in EU fusion gyrotron developments

In Europe, significant progress in gyrotron research, development and manufacturing has been made in 2014, starting from the successful continuation of the 1 MW, 140 GHz gyrotron production for the stellarator Wendelstein 7-X (W7-X) at Greifswald, Germany and the accelerated development of the EU 1 MW, 170 GHz conventional cavity gyrotron for the ITER tokamak at Cadarache, France. Based on that, a physical design activity was started which shall lead to a dual frequency gyrotron for TCV, Lausanne, Switzerland. Within the European fusion development consortium (EUROfusion), advanced gyrotron research and development has started towards a future gyrotron design which shall fulfil the needs of DEMO, the nuclear fusion demonstration power plant that will follow ITER. Within that research and development, the development of advanced design tools, components, and proper test environment is progressing as well. A comprehensive view over the status and prospects of the different development lines shall be presented.

Separation of thermal expansion and beam charge neutralization effects in high power 140 GHz CW gyrotrons

Summary form only given. During the first few seconds of longer pulses in high-power gyrotrons, the main mode frequency decreases over a few hundred MHz, until a stable operation is reached. This is due to the combination of two effects, namely the thermo-mechanical expansion of the cavity and a shift in the effective electron energy through ionization of residual gas in the tube. Most investigations focus on the quasi-stationary “short-pulse” range of the first few milliseconds, and the long-pulse stationary range after both tuning effects have settled. As the fusion gyrotron development is clearly headed towards higher output power and higher operating frequency, the corridors for operation parameters narrows with the desired cavity mode [1]. Furthermore, modern fusion reactors tend to demand gyrotron power modulation, which will cause operation in the mentioned non-stationary transition regime. In this work, the characterization and time constant separation of the named effects is attempted for the 1 MW, 140 GHz W7-X gyrotron. From the experimental side, this is done with the help of a transient spectrum measurement system [2], which with some modifications is capable of recording continuously in the transient pulse range. The measured time-dependent frequencies are used for the modeling of the two effects, which is supported through parameter studies using the self-consistent gyrotron simulation code EURIDICE. In addition, previous theoretical descriptions of the ionization processes in the device [3] were modified for the application to modern high power gyrotrons, and brought into context with the experimental and modeling data.

Studies on boundary conditions for gyrotron interaction modeling

Summary form only given. In gyrotron interaction simulations, radiation boundary conditions are used for the electromagnetic field in the open-ended resonator. Usually, the boundaries are considered matched, i.e. no power is reflected back into the resonator. In the vast majority of cases, a boundary condition matched only at one single frequency and having low reflection in a narrow band around this frequency is used. This is done on the grounds that the gyrotron radiation is basically monochromatic. However, a broadband matched boundary condition has also been proposed1, which should be employed in the special cases where the gyrotron output has a broader frequency spectrum. Such cases include dynamic after-cavity interaction2 and the appearance of side-bands. Following the broadband concept1, a boundary condition has been formulated in a new way, compatible with the code-package EURIDICE. Two alternatives for the numerical implementation are discussed and tested. In addition, the behavior of both the single-frequency and the broadband boundary condition is studied analytically and compared with the behavior of the numerical implementation. Finally, the broadband boundary condition is applied in cases where the gyrotron radiation exhibits a wider frequency range.