J.Shafii, J.N. Talmadge, R.J. Vernon, HSX team HSX Plasma Laboratory, University of Wisconsin-Madison T. S. Bigelow, ORNL K.M.

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1 J.Shafii, J.N. Talmadge, R.J. Vernon, HSX team HSX Plasma Laboratory, University of Wisconsin-Madison T. S. Bigelow, ORNL K.M. Likin, Fusion Division, CIEMAT

2 Outline Abstract HSX ECH system Introduction Transmission line layout Summary Test system layout Pictures of the mode converters, the bends, and the test setup Test results of the mode converters, the bends, and the corrugated taper Mirror Conclusion

3 Abstract A mode converting transmission line has been developed for a 200-kW 28-GHz gyrotron used for electron cyclotron resonance heating (ECRH) in the HSX. The gyrotron produces 86% of its output power in the TE02 mode in a 2.5@ diameter smooth-wall waveguide along with some spurious TE01 and TE03 modes. To obtain a Gaussian-like beam at the output of the line, for efficient plasma heating, the HE11 mode was generated in a circular corrugated waveguide using the following mode conversion sequence: TE02-to-TE01-to-TE11-to- HE11. There are also two waveguide bends with corrugated wall used in the line: a TE02-mode bend with cosine curvature profile and an HE11-mode bend with hyperbolic secant curvature profile. In addition, a linear smooth-wall down taper near the gyrotron and a corrugated-wall up taper near the vacuum vessel are used to change the waveguide size in the line. All components were fabricated and tested at low power level by measuring their far field radiation pattern. For testing, 28 GHz millimeter waves were generated by a 100 mw Gunn oscillator in a fundamental rectangular waveguide. Nonlinear TE01 and TE11 uptapers were designed and fabricated and used in conjunction with mode transitions to generate TE01 and TE11 modes in the oversized waveguide. The experimental results show at least 95% efficiency for all components, except for the TE01-to-TE11 mode converter which has only 85% efficiency. We have traced the problem to an error in previously published coupling coefficients that were used for the design of the TE01-to-TE11. A design based on the correct coupling coefficients can eliminate this problem. For the HE11 bend, we verified experimentally that if the input field polarization is not oriented parallel or normal to the plane of the bend, the output field is not linearly polarized. For instance, the output field is elliptically polarized with an ellipticity of 2 if the input field is polarized 45 degrees respect to the plane of the bend. An ellipsoidal mirror is being fabricated and will be installed inside the vacuum vessel to redirect and focus the beam on the center of plasma. We will present the test results for the transmission line components. *This work was supported by the USDOE grant DE-FG02-93ER54222.

4 HSX ECH System Goal is to heat electrons and test improved neoclassical confinement at low collisionality in a quasi-helically symmetric stellarator. 28 GHz gyrotron used to produce and heat the plasma at 0.5 or 1 T 100 ms pulse length; 200 kw maximum output With B = 1 T, 100 kw absorbed power get T eo 1 kev Tube and Socket /Tank and Magnet Assembly courtesy of ORNL Beam and gun magnet supplies and HV supply (20 A, 90kV) courtesy of LLNL HV supply capable of driving three tubes for 50 ms

5 Introduction The Gyrotron output is in the TE 02 mode with 200 kw maximum power at 28 GHz. The TE 02 mode produces an undesirable hollow conical radiation pattern with zero power along the waveguide axis. The linearly polarized Gaussian-like HE 11 mode radiated from an open-ended, circumferentially corrugated circular waveguide has well defined polarization and direction with low side lobe level. The HE 11 mode can be generated by the following mode conversion sequence: TE 02 -to-te 01 -to-te 11 -to-he 11 The transmission line is to carry the microwave power from the gyrotron to the plasma vessel with very low ohmic loss and with high HE 11 output mode purity. The waveguide components are all overmoded in order to increase their powerhandling capability and reduce ohmic wall losses.

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7 HSX and the transmission line The length of the line is approximately 40 feet.

8 Summary The gyrotron s power is mainly in the TE02 mode in a circular waveguide 2.5 inches in diameter. This waveguide size is tapered down to 1.875" diameter near the gyrotron to reduce the length of the mode converters and the bends. The three mode converters TE02-to-TE01, TE01-to-TE11, and TE11-to-HE11 and the two corrugated waveguide bends for the TE02 mode and the HE11 mode have been fabricated. To reduce the antenna beam width, a corrugated waveguide uptaper is fabricated to increase the waveguide diameter to 2.5 inches near HSX. A window assembly has been designed and fabricated. The window is made of fused quartz. A focusing mirror antenna is needed to redirect and focus the beam on the center of plasma. An ellipsoidal antenna has been fabricated. A test system has been setup to test the various components by measuring the radiation patterns output form the components. By comparing the measured patterns with the theoretical patterns, we can estimate within 2% the efficiency of these components.

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11 TE 02 -to-te 01 and TE 11 -to-he 11 Mode Converters

12 TE 02 -to-te 01 and TE 11 -to-he 11 Mode Converters

13 Radiation Pattern From the Output of the TE01-to-TE02 Mode Converter intensity (db) angle (degrees) Gyrotron output is in TE02 mode. The TE02-to-TE01 mode converter is tested by inputting TE01 from thete01 uptaper and measuring the TE02 output. theory - TE02 mode experiment

14 0 experimental radiation patterns from the TE11-to-HE11 mode converter intensity (db) E-plane H-plane angle (degrees) The TE11-to-HE11 mode conveter is tested by inputting TE11 from TE11 uptaper and measuring the HE11 output. The mode converter has broadened the TE11 pattern and reduced the side lobe level in the E plane such that the output beam is axisymmetric Gaussian-like beam.

15 The Two Blocks of the Three-Cycle TE 01 -to-te 11 Mode Converter (48 long)

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17 Radiation Pattern From the Output of the TE01-to-TE11 Mode Converter in the H plane intensity (db) experiment theory - TE11 mode angle (degrees) The maximum field is shifted. We have traced the problem to an error in previously published coupling coefficients that were used in the design. Reanalysis of this particular mode converter indicates that there is about 4% TE21 and 1.5% TE01 modes at the output. These two spurious modes cause the shifting of the TE11 pattern.

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19 Power of the Main Modes Along the Fabricated Three-Cycle TE01-to-TE11 Mode Converter TE01 TE11 power correct coupling coefficients Thumm's coupling coefficients TE distance (m) The mode converter was originally designed using the Thumm's coupling coefficients. Reanaly of this mode converter with the correct coupling coefficients indicates that the output TE11 mod purity is lower and there are some spurious modes such as TE01 and TE21 modes at the outp The presence of these spurious modes causes the shifting of the measured pattern in the H pal

20 90-Degree TE 02 and HE 11 Corrugated Bends TE 02 bend has cosine curvature variation HE 11 bend has hyperbolic secant curvature variation

21 E-plane radiation patterns for the HE11 bend polarization of the input HE11 mode is in the plane of the bend intensity (db) experimental setup: TE11-to-HE11 -- HE11 bend -- HE11 uptaper co-polarized theory - HE11 mode cross-polarized angle (degrees) The HE11 bend is tested by inputting HE11 from output of TE11-to-HE11 mode converter and measuring the HE11 output. Good agreement exists between the measured and theoretical patterns. The low cross-polarized field indicates that the output field is linearly polarized

22 0 H-plane radiation patterns for the HE11 bend polarization of the input HE11 mode is normal to the plane of the bend -5 intensity (db) experimental setup: TE11-to-HE11 -- HE11 bend -- HE11 uptaper co-polarized theory - HE11 mode cross-polarized angle (degrees) The HE11 bend is tested by inputting HE11 from output of TE11-to-HE11 mode converter and measuring the HE11 output. The low cross-polarized field indicates that the output field is linearly polarized.

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24 Low-Power Test Setup For the TE02 Bend The receiving horn is located at a distance of about 4D 2 / λ (85 cm) from the radiating aperture.

25 radiation patterns from the TE02 bend intensity (db) experiment theory - TE02 mode angle (degrees) The TE02 bend is tested by inputting TE02 from output of the TE01-to-TE02 mode converter and measuring the TE02 output. Good agreement exists between the measured and theoretical TE02 patterns.

26 0 experimental radiation patterns from the HE11 uptaper input diameter = 1.875" ; output diameter = 2.5" intensity (db) E-plane H-plane angle (degrees) The HE11 uptaper is tested by inputting HE11 from the output of the TE11-to-HE11 mode converter and measuring the HE11 output. The output pattern is axisymmetric Gaussian-like beam with a narrower beamwidth with respect to the HE11 mode output from 1.875" size waveguide.

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29 Conclusion There is a good agreement between the measured and theoretical patterns for all components. There is also a good agreement for the TE01-to-TE11 mode converter patterns in the E-plane. The radiation pattern in the H plane is shifted about 2 to 3 degrees from the center axis. We have traced the problem in the H plane to an error in previously published coupling coefficients that were used in the design. (H. Kumric and M. Thumm, Optimized overmoded TE01-to-TM11 mode converters for high-power millimeter wave applications at 70 and 140 GHz, Int. J. of Infrared and Millimeter Waves, Vol. 7, No. 10, 1986) The coupling coefficients in Thumm s paper is not correct for m 2 (m is the azimuthal index of waveguide modes). It is correct only for m = 1. A new 5-cycle TE01-to-TE11 mode converter, with TE11 output mode purity higher than 98%, has been designed based on the correct coupling coefficients. The entire transmission line including the mirror will be tested at low power once the installation of the line has been completed. There might also be a need to use a mode converter near the gyrotron to increase the TE02 output mode purity of the gyrotron.