The input port is connected to the high power gyrotron source and the output port is connected to a dummy load to absorb any power not coupled into the ring. To tune the ring, the overall length is adjusted with short bellows in two sides or by moving the plate in one of the miter bends slightly outward.

The effective power gain or ratio of electric field in the ring to the input field level can be shown to be

E_ring/E_input = 1/[(C)^0.5 - α (C-1)^0.5] = (P_ring/P_input)^0.5                 (17)

where C is the directional coupling power ratio and α is the waveguide loss for a single pass including mode conversion loss, resistive loss and any loss in a component or window under test. For a given total loss level in the ring, a particular value of C gives the maximum possible ring gain. If α is small, C_optbecomes large and the ring gain increases rapidly. Since the oversized waveguides used in this work have very low losses, the ring loss will be dominated by miter bend mode conversion loss and the window loss. If the mode conversion loss is too large, the mode that power is converted into can also build up in the ring and will cause a small level of interference with the primary mode at certain frequencies where both modes are resonant. Fortunately, for increasing frequency and waveguide diameter, the miter bend loss decreases significantly as indicated in the Table 5. The predicted optimum performance for a quasi-optical resonant ring with TE₀₁ and HE₁₁ modes in two waveguide diameters is shown in Table 5. These estimates include a 1% window loss.

The resonant ring was constructed and first tested at low power in the ORNL Fusion Energy Division microwave development laboratory. This laboratory is well equipped with a wide variety of test equipment and oversized waveguide components available for past fusion heating experiments, making development much easier and faster. The test resonant ring was fabricated using 63.5 mm (2.5") smooth-wall copper waveguide and existing miter bends used for the TE₀₁ mode. A directional coupler was formed using a cross miter bend with a perforated plate inserted between. The coupling value was controlled by the plate thickness, perforation hole size and hole density. Since round holes were used, the polarization of the TE₀₁ mode was accurately transmitted through the perforated plate. Two bellows sections were used to demonstrate frequency tunability. Power was fed into the input port using a TE₀₁ mode transducer from standard rectangular waveguide (WR-15 for the 50-75 GHz band) and a gradual diameter taper to large diameter for high mode purity. A TE01 mode purity of 99% was generated by this apparatus. For low power testing purposes, a 50-75 GHz sweep oscillator and a scalar network analyzer were used to generate and monitor the test signals.

Table 5. Predicted ring gains with 1% window loss included

Actual monitoring of signal levels in the resonant ring was difficult, since a mode-selective sampling coupler is required. For these tests, a perforated screen was used for one of the miter bend reflectors that radiated a small portion of the TE₀₁ pattern (slightly distorted by the bend) outside the waveguide where a small sampling horn was placed at a peak in the radiation pattern. This arrangement gave a reasonably accurate and directional sample of the TE₀₁ wave inside the ring with good mode selectivity. The signal level and corresponding ring gain can be calibrated relative to an open end waveguide when the pickup horn is placed at the same distance away from one of the miter bends with the reflector plate removed to spoil the ring resonance.

The specific directional coupler perforated plates that were used gave an optimum gain in the 50-54 GHz range. In this range, the C value was determined to be 5-10 dB and the ring loss (based on theoretical predictions for miter bend mode conversion loss) totaled ~17%. The theoretical ring power gain was 6 and the measured ring gain was 6-8 which was excellent agreement. With a suitably sized hole array, the ring gain can be optimized for any frequency range of interest.

The physical path length of the test ring configuration was measured to be 1.94 m, which gave a resonant frequency spacing of ~160 MHz. Measured plots of ring gain vs. frequency were nearly identical with the exception of the null region between peaks filled in at 10-15 dB below the peaks due to the non-resonant mode converted power.

Comparisons between the model and experiment for the proof-of-principle resonant ring test were very favorable and indicated that the concept performs properly and provides effective power gain as desired. Predictions by the model imply that at higher frequency or larger waveguide diameter, the ring performs even better. Next, a high power HE₁₁ mode version of the ring was constructed for testing high power at 53 GHz using an installed and operational cw gyrotron system at ORNL.

A HE11 mode launcher was modified for lab tests of the HE11 resonant ring, low power tests of the HE11 resonant ring were performed. The resonant ring worked well at low power and measurements agree with theory. Tuning of the ring was practical and simple. These tests demonstrated a greater than factor of 10 power gain. At low power a ring gain of over 20 was demonstrated near 53 GHz using a perforated plate coupler. Frequency tunability by shifting one miter bend plate was also demonstrated using perforated plates for the directional couplers. These were adequate for medium pulse length tests; for longer pulse tests water cooled grids were added to the directional couplers.

Second order effects were investigated. These included reflections from a window, a miter bend, the dummy load, or another component under test that could have de-tuned ring gain. It is also possible that the double miter bend mode re-conversion can enhance performance. A lack of mode purity of the input power was also investigated.

The 53 GHz gyrotron system at ORNL provided the 200kW a high mode purity HE11 mode to drive the resonant ring unit as shown in Fig 19. An oversized waveguide configuration was designed for the high power resonant ring tests, and a series of mode converters and mounting fixtures were fabricated.

Figure 19. The waveguide configuration for high power window tests using the resonant ring.

A TE₀₁-TM₁₁ mode converter for high power tests was fabricated, and its mode purity was found to be >98%. The remaining two components were the TM₁₁ to HE₁₁ mode converter and the HE₁₁ corrugated diameter uptaper. The TM11-HE11 high power transition was modeled, and these two components were designed and fabricated.

To convert from the TM₁₁ smooth-wall waveguide to the HE₁₁ corrugated waveguide, a gradual corrugation depth taper was required. The taper length was carefully controlled to produce the exact amount of mode conversion to generate a high purity HE₁₁ mode. The HE₁₁ mode can be decomposed into smooth wall modes in the following ratios: TE₁₁ 82%, TM₁₁ 14%, TE₁₂ 0.6%, TM₁₂ 2%, and small amounts of higher modes. The TM₁₁-HE₁₁ mode converter was designed with a waveguide diameter step cascade code developed previously to analyze circular waveguide structures consisting of a series of smooth wall waveguide sections. A large number of higher order, non-propagating modes were included to properly model the small diameter changes at each corrugation. The resulting mode converter design had ~155 corrugations of linearly varying depth and a short phase correcting section at the output. Mode purity was estimated to be >98%. The converter had an input and output diameter of 38.3 mm and a length of 305 mm.