Elizabeth J. Kowalski

Loss Estimate for ITER ECH Transmission Line Including Multimode Propagation

Abstract The ITER electron cyclotron heating (ECH) transmission lines (TLs) are 63.5-mm-diam corrugated waveguides that will each carry 1 MW of power at 170 GHz. The TL is defined here as the corrugated waveguide system connecting the gyrotron mirror optics unit (MOU) to the entrance of the ECH launcher and includes miter bends and other corrugated waveguide components. The losses on the ITER TL have been calculated for four possible cases corresponding to having HE11 mode purity at the input of the TL of 100, 97, 90, and 80%. The losses due to coupling, ohmic, and mode conversion loss are evaluated in detail using a numerical code and analytical approaches. Estimates of the calorimetric loss on the line show that the output power is reduced by about 5, ±1% because of ohmic loss in each of the four cases. Estimates of the mode conversion loss show that the fraction of output power in the HE11 mode is ~3% smaller than the fraction of input power in the HE11 mode. High output mode purity therefore can be achieved only with significantly higher input mode purity. Combining both ohmic and mode conversion loss, the efficiency of the TL from the gyrotron MOU to the ECH launcher can be roughly estimated in theory as 92% times the fraction of input power in the HE11 mode.

Simple Correctors for Elimination of High-Order Modes in Corrugated Waveguide Transmission Lines

When using overmoded corrugated waveguide transmission lines for high power applications, it is necessary to control the mode content of the system. Ideally, overmoded corrugated transmission lines operate in the fundamental HE11 mode and provide low losses for long distances. Unwanted higher order modes (HOMs), particularly LP11 and HE12, are often excited in the experimental systems due to practical misalignments in the transmission line system. This paper discusses how the unwanted modes propagate along with the fundamental mode in the transmission line system by formulating an equation that relates the center of power offset and angle of propagation of a beam (for the HE11 and LP11 modes) or the waist size and phase front radius of curvature of a beam (for the HE11 and HE12 modes). By introducing two miter bend correctors into the transmission system-miter bends that have slightly angled or ellipsoidal mirrors-the HOMs can be precisely manipulated in the system. This technique can be used to eliminate small quantities of unwanted modes, thereby creating a nearly pure fundamental mode beam with minimal losses. Examples of these applications are calculated and show the theoretical conversion of up to 10% HOM content into the fundamental HE11 mode with minimal losses.

An Overmoded W-Band Coupled-Cavity TWT

A 94-GHz overmoded traveling-wave tube (TWT) has been designed, fabricated, and successfully tested. The TWT operates in the rectangular TM31 mode of the cavity, while lower order modes are suppressed using selectively placed strips of lossy dielectric. The 87-cavity TWT circuit was directly machined from Glidcop, a dispersion-hardened copper. The TWT was tested in a 0.25 T solenoidal magnetic field in 3-μs pulses. Operating at a voltage of 30.6 kV with 250 mA of collector current, the TWT was zero-drive stable and achieved 21 ± 2 dB linear device gain with 27-W peak output power. Taking into account 3 dB of loss in both the input and output coupling circuits, the gain of the TWT circuit itself is 27 ± 2 dB with 55 W of saturated circuit output power. Using the 3-D particle-in-cell code CST Particle Studio, the linear circuit gain was estimated to be 28 dB and the saturated output power 100 W, in good agreement with the experimental results. The measured bandwidth of 30 MHz was significantly smaller than the predicted value of 250 MHz. The overmoded TWT is a promising approach to high-power TWT operation at W-band and to the extension of the TWT to terahertz frequencies.

Design and test of an internal coupler to corrugated waveguide for high power gyrotrons

Gyrotrons produce power in high-order TE modes that are converted to a Gaussian beam inside the tube and then matched to the HE11 mode in an external waveguide. A design for coupling to the HE11 mode inside the gyrotron has been developed. Test results show high HE11 output mode purity.

A 94 GHz overmoded coupled cavity TWT experiment

A 94 GHz overmoded coupled-cavity Traveling Wave Tube (TWT) has been designed with 30 dB of gain and 300 W peak power output. An experiment is being fabricated to test the overmoded TWT. The device operates in the TM31 mode with minimal losses, and a lossy AlN dielectric loading is selectively placed in the structure to suppress unwanted modes in the cavities. A cold test of the overmoded coupled-cavities was machined in Glidcop and agrees well with simulation. The TWT experiment is currently under operation, with a 2.5 kG solenoid magnet, 2 microsecond pulse modulator, and in-house designed 30 kV Pierce electron gun. An initial beam test of the 30kV, 320 mA Pierce electron gun shows good agreement with Michelle simulations, and further results are forthcoming.

Over-moded W-band Traveling Wave Tube design

The design and cold test of an over-moded 94 GHz Coupled-Cavity Traveling Wave Tube (TWT) with operation in the rectangular TM31 mode is discussed. Lower order modes are suppressed with dielectric loading. Simulations with CST Particle Studio show 31 dB of gain with 300 W peak output power. Cold test results for a 9-cavity structure agree well with HFSS simulations.

Overmoded W-band traveling wave tube (TWT) design and test

We report the design and experimental test of a W-band overmoded traveling wave tube amplifier. The TWT circuit is designed to operate in the TM31 mode at 94 GHz and utilizes dielectric loading to suppress unwanted lower order modes. 3-D CST PIC simulations at 31 kV, 310 mA predicted 32 dB of gain and 300 W peak output power. Testing of the overmoded TWT found zero-drive stable operation with full suppression of unwanted modes. Operating at 94.26 GHz, 27 dB of circuit gain (21 dB of device gain) and 55 W peak circuit output power (25 W peak device output power) were measured at 31 kV with 250 mA current on the collector. Simulations predicted 28 dB of circuit gain, in good agreement with experiments.

94 GHz overmoded TWT experiment

Summary form only given. A 94 GHz overmoded travelling wave tube (TWT) amplifier has been designed and experimentally implemented. The overmoded coupled cavity TWT operates in the TM31 mode at 94 GHz. Lower order modes that may oscillate at lower frequencies in the cavity are suppressed using selectively placed lossy dielectric loading. The AlN dielectric loading has a negligible affect on the transmission of the TM31 mode. Operation in a higher order mode allows for a larger cavity at 94 GHz than the equivalent fundamental mode structure, permitting a larger beam tunnel and a higher current to propagate through the TWT. These advantages lead to an increase in the gain and peak power of the TWT. The overmoded TWT is designed for pulsed operation with a 30 kV, 320 mA Pierce electron gun and 2.5 kG solenoid magnet. The design has been simulated in 3-D CST Particle Studio and predicts 32 dB of gain and 300 W peak power. The design and implementation of the overmoded cavity has been verified with several cold test structures manufactured with standard CNC milling using Glidcop, a copper alloy with a small quantity of alumina. Low power S-measurements show good agreement with theory (HFSS) for the 94 GHz TM31 mode and TM41 mode at approximately 112 GHz. In addition, AlN composite dielectric loading with a loss tangent of 0.25 at 90 GHz was used to induce losses in the unwanted modes. The AlN dielectric, when placed around the cavity structure induced large losses in the unwanted modes and minimal losses in the TM31 mode. The electron gun has been activated and a beam test in underway to verify operation of the experimental set-up for the TWT. The Pierce electron gun was designed in Michelle for use with the experiment and 2.5 kG solenoid magnet. Initial experimental results for electron gun operation agree well with theory and show very good beam transmission to the collector. Once the operation of the electron gun has been verified, the overmoded TWT will be installed in the experimental set-up and stable operation at 94 GHz will be verified.

PPPS-2013: Over-moded W-band traveling wave tube (TWT) amplifier

A 94 GHz over-moded coupled-cavity TWT has been designed and cold test structures of the over-moded cavities analyzed. An experimental beam test is underway, with implementation of the TWT to follow.

P3-3: Measurement of loss in high power 170 GHz gyrotron transmission lines

Over-moded corrugated cylindrical waveguides are used for high power gyrotron radiation transmission applications, such as ITER, because of their low loss characteristics at millimeter wavelengths. Measurements on such over-moded systems are fraught with inaccuracies due to higher order modes. We report a new technique to accurately measure the inherent loss of an over-moded device in the desired operation mode. This technique is demonstrated successfully on low-power measurements at 170GHz in 63.5 mm diameter corrugated ITER transmission lines. We find the miter bend loss to be 0.022 ±0.008 dB (0.5 ±0.2 %), in good agreement with theory.