David S. Tax

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.

Mode conversion losses in ITER transmission lines

Mode conversion in miter bends and polarizers is the main contributor of loss in the transmission lines (TLs) for the ITER 170 GHz ECH system, which should transport one megawatt of power with the smallest possible loss. Previous loss estimates assumed that the power in the TL was carried by a pure HE11 mode; however, in practice, there is significant power in higher order modes (HOMs). It is shown that the mode conversion loss of the power in an HE11 mode at a miter bend is greatly altered by the presence of even a small proportion of HOMs in the TL, and is a strong function of both the magnitude of the HOM and its phase relative to that of the HE11 mode. The resulting total loss in the ITER transmission lines is expected to be very different from the loss previously predicted using single mode theory.

Experimental Study of the Start-Up Scenario of a 1.5-MW, 110-GHz Gyrotron

We present experimental results of the modes excited during the voltage rise of a 1.5-MW, 110-GHz gyrotron operating in the TE22,6,1 forward-wave mode. Results were obtained by two different experimental techniques: measurements with a time-gated heterodyne receiver and measurements during the flat-top portion of the voltage pulse with a sequence of increasing voltages. Two operating points were selected: a high-efficiency 1.2-MW power-level point at 4.38 T and a highly stable 600-kW point at 4.45 T. In the former case, the TE21,6,3 and TE21,6,4 backward-wave modes far from cutoff were excited during the voltage rise of the pulse before the desired TE22,6,1 operating mode was excited; in the latter case, the excitation of a TE22,6,2 backward-wave mode dominated the voltage rise before eventually exciting the desired operating mode. Analysis of the microwave output beam spatial pattern and the frequency and power levels recorded indicate that these modes are indeed excited within the cavity. Single-mode MAGY simulations provide further evidence that such modes can exist in the gyrotron during the voltage rise. Knowledge of the modes excited during start-up is important for achieving high efficiency and avoiding power at unwanted 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.

Experimental Results for a Pulsed 110/124.5-GHz Megawatt Gyrotron

We report experimental results on a two frequency gyrotron, operating in the TE22,6 mode at 110 GHz and the TE24,7 mode at 124.5 GHz. The gyrotron uses the same electron gun as a previous single frequency 110-GHz gyrotron, with a new cavity and internal mode converter designed for optimized performance at the two frequencies. For a 98 kV, 42-A electron beam operating in 3-μs pulses, an output power of 1.25 MW was obtained at 110 GHz (30% efficiency) and 1.0 MW at 124.5 GHz (24% efficiency). The highest power obtained was 1.4 MW with a 96 kV, 45-A beam (32% efficiency) at 110 GHz. In both modes, mode competition was minimal around the high-power operating point and operation was extremely stable. The output power and efficiency in the TE24,7 mode were limited by the electron beam quality. At both frequencies, excellent Gaussian beam content was found: 1) 99% for the TE22,6 mode and 2) 97% for the TE24,7 mode. Both output beams had waist radii of 2.65 cm, in very good agreement with theory.

Low power testing of losses in components for the ITER ECH transmission lines

The transmission lines (TLs) for ITER electron cyclotron heating require extremely low losses, since the 24 MW of power generated by the 170 GHz gyrotrons should be delivered to the plasma with an efficiency of 83% or more. We are developing cold-test techniques to precisely measure the losses in the TL components for ITER, before installation. Experimental results on the measurement of the loss in some TL systems with coherent and incoherent techniques are presented and compared. We report a preliminary measurement of loss in the prototype TL components for the ITER 170 GHz transmission line.

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

High power gyrotrons utilize high order TE cavity modes that must be converted to a Gaussian beam that is matched to a corrugated transmission line that supports the HE11 mode for low-loss, efficient transmission to their loads. In the standard configuration, the corrugated waveguide is external to the gyrotron. We report hot-tests results of a novel design that converts the TE mode to the HE11 mode inside the gyrotron.

Experimental research on A 1.5 MW, 110 GHz gyrotron

Summary form only given. Megawatt gyrotrons have a variety of applications including electron cyclotron heating (ECH) of plasmas in fusion devices. High efficiency operation is necessary to minimize the prime power required and to ensure good reliability of the device over its lifetime. To achieve high power and efficiency at the required frequencies, the gyrotrons must utilize large, overmoded cavities that face severe mode competition. It is important to understand the start-up scenario of the gyrotron, that is the sequence of modes excited during the voltage rise, since, under such conditions, small changes in operational parameters can affect the final operating state of the gyrotron. The start-up scenario had previously been examined using the numerical code MAGY and showed that higher-order modes would be excited and different start-up scenarios could yield a different final operating state [1,2].

Experimental results of the start-up scenario for a 1.5 MW, 110 GHz pulsed gyrotron

We present experimental results on our 1.5 MW, 110 GHz, TE22,6 mode gyrotron, operating with 3 µs pulses. A new internal mode converter (IMC) was recently installed, and it performed well, providing a highly Gaussian output beam1. Our latest studies have focused on the operation of the gyrotron during the rise of the voltage pulse. The power and frequency are measured as a function of voltage in two different ways. First, a heterodyne receiver system that can be gated over various time intervals of the pulse allows for frequencies to be identified during the rise of the voltage pulse to its peak value of 96 kV. Alternatively, the peak voltage can be varied and the power and frequency measured during the flat top of the voltage pulse can be recorded. Around the nominal operating voltage of 96 kV, a small shift in frequency vs. voltage was observed, being slightly higher at lower voltages. As the voltage was further decreased, the frequency shift became more pronounced, and eventually the cavity mode switched to a lower frequency mode. This lower frequency mode varied from ∼109 – 107.5 GHz as the voltage varied from 50 to 70 kV at a main magnetic field of 4.38 T (where the gyrotron has its highest output power). The mode may be a higher order axial TE21,6 BWO mode. We also observed examples of mode switching without a large instantaneous shift in frequency, which is suspected to be the switch from one axial mode number to another. In these cases, the rate of the frequency shift changes suddenly, being larger for the modes with higher axial mode numbers, though the frequency remains continuous.

Mode excitation during the voltage rise in megawatt gyrotrons

To achieve high power and efficiency, megawatt gyrotrons must utilize large, overmoded cavities that face severe mode competition. Under these conditions, small changes in operational parameters can alter the sequence in which modes are excited during the voltage rise, affecting the final operating state of the gyrotron. We present experimental results from our 1.5 MW, 110 GHz gyrotron operating in the TE22,6 mode at 96 kV and 42 A. While codes such as MAGY predict that higher frequency modes, such as the TE23,6 mode at 113 GHz, should be generated during the voltage rise, we instead measured lower frequencies, near 108 GHz. The measured frequencies indicate that these modes, if excited in the cavity, must be backward waves in the TE21,6 mode with higher-order axial field structures.