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Theory and Modeling of High-power Gyrotrons

Principal Investigator(s): 

Abstract: 

With support of the Office of Fusion Energy of the Department of Energy, this work provides the development of the theory and numerical codes for supporting design of high average power, millimeter-wave gyrotrons required for experiments on electron cyclotron resonance plasma heating and current drive as well as suppression of plasma instabilities in large-scale controlled fusion reactors within the framework of the magnetic fusion energy program. The work consists of three tasks:

*Task A* deals with studying physical limitations imposed on the millimeter-wave power available from a single gyrotron in long-pulse or continuous-wave regimes imposed by simultaneous excitation of several high-order modes. Defining the ultimate high-order mode available for efficient single-mode operation should help to more accurately estimate the maximum output power from a single gyrotron and, therefore, make the cost of a whole gyrotron system for large-scale plasma installations less expensive.

*Task B* deals with developing a more accurate theory and code describing the excitation of backward waves in beam tunnels of high-power gyrotrons. Avoiding this sort of parasitic excitation will allow developers to increase the orbital-to-axial velocity ratio in electron beams and, hence, to increase the gyrotron interaction efficiency.

*Task C* deals with generalization of the non-stationary, self-consistent code MAGY to the case of millimeter-wave circuits with azimuthally non-uniform walls.  A new version of the code will also be valid for the case when the electron transit time through the interaction space is of the same order as the cavity decay time, while present versions of the code are valid only for cases of small transit times. These actions are important in view of the recently discovered effect of the after-cavity interaction in gyrotron launchers. The surface of such launchers is often made azimuthally corrugated for improving the efficiency of conversion of gyrotron operating modes into Gaussian wave beams.

figure 1

 

This figure shows the results of theoretical studies of gyrotron operation at a very high-order mode TE31,18 in a high-power MW-class 170 GHz gyrotron for ITER-class tokamaks. Numerical simulations of excitation of a quintet of modes are performed with the use of the self-consistent, non-stationary code MAGY.  This code was developed by our group under the guidance of Prof. T. M.  Antonsen.