The Designs of High Efficiency Launcher of Quasi-Optical Mode Converter for High Power Gyrotrons

Abstract

A high efficiency launcher of quasi-optical (QO) mode converters for high power gyrotrons have been designed and tested. A helical cut launcher radiates the RF power via its straight cut onto the first phase correcting mirror. The launchers have been optimized for the TE 31,8 mode at 170 GHz and TE 22,6 mode at 110 GHz by numerically optimizing a launcher surface. The helical cut of the launcher has been optimized by taking the taper angle into account. Further more, the amplitude of the surface perturbation have been optimized for improved focusing in order to reduce the diffraction losses at the helical cut. Low power measurement shows a good agreement with the design. High efficiency characteristics of the design have also been calculated on the assumption of frequency downshift due to the thermal expansion of the cavity and stepwise frequency tuning by changing the operating mode. Besides, the possibility of high efficiency launcher for higher mode is discussed, and these results give the prospect to high efficiency long pulse gyrotrons.

Progress of high-power and long-pulse ECRF system development in JT-60

“…As mentioned above, the DCB is significantly heated by the stray radiation generated by the large amount of diffraction loss when an old-type (single helix) internal mode converter (typically 8-10%) in the gyrotron is used. The recent development of mode converters [28][29][30] enables us to reduce the diffraction loss to typically 2-4%; this is one of the keys to achieve long-pulse operations [13]. We re-designed the mode converter of the 110 GHz gyrotron [29] using the Surf3d/Lot code [28]; the diffraction loss was improved from 6.8% to 3.5% in the calculations.…”

Long pulse operation of 170GHz ITER gyrotron by beam current control

“…The new mode converter with high mode conversion efficiency has been already fabricated, and the high performance has been also demonstrated in a low-power RF test [17]. Therefore, when the mode converter is installed accurately in the long pulse gyrotron, the total mode conversion efficiency will be improved from 87% (measured value of present gyrotron) to 97.5% (calculated value of improved gyrotron).…”

Cold testing of quasi-optical mode converters using a generator for non-rotating high-order gyrotron modes

“…[11][12][13] The mode converter is critical: it converts a higher-order mode from the interaction circuit into a Gaussian-like beam, which effectively transmits RF power through free space. [14][15][16] Typically, a quasi-optical mode converter (QOMC) comprises an open-ended, non-symmetrical waveguide launcher and a series of quasi-parabolic mirrors, which are situated directly after the gyrotron's interaction circuit. 17 A QOMC must be tested before being positioned in the vacuum tube, owing to the following issues: First, there may be inaccuracies in the location and alignment of mirrors, which must be resolved in air before the QOMC is placed in vacuum.…”