Testing of ITER-Class ECH Transmission Line Components at the JAEA Radio-Frequency Test Stand

Full text: Testing of prototypical ITER ECH transmission line components at representative ITER conditions has been carried out at the JAEA RF test stand (RFTS). Based on these tests, it appears that the ITER requirement for 2 MW cw operation with high transmission efficiency is achievable. The tests showed that improvement in the polarizer miter bend mirrors is needed and that it is critical to have accurate alignment of the gyrotron output into the waveguide. Progress in both areas is being made. GA components tested in 2008 included polarizer miter bends, waveguides, very low diffraction loss miter bends (LDLMBs) and a DC break. Components to be tested in 2010 include a waveguide switch, waveguide water cooling bars, a sliding waveguide joint and polarizer miter bends with improved grooved mirrors. For the 2008 tests, JAEA measurements and analyses show that the beam entering the test section had an HE11 content of 66%, with the rest of the power in LP11 and other high order modes. JAEA subsequently improved the HE11 mode purity to 93%, which is approaching the ITER requirement of > 95%. Two polarizer miter bends separated by a 2-m length of waveguide were tested. Measurements of the DC break showed that the temperature increase of the outer surface of the ceramic insulator was about 8 deg. C (equivalent to 27 deg. C for 2 MW transmission) - this increase is still less than a safe maximum increase of 50 degC. For pure HE11, the calculated temperature increase is about 10 degC for 2 MW transmission. The polarizer mirrors had losses 1.7 to 2.9 times the theoretical losses at room temperature. This high loss is attributed to a resistive recast layer on the surface of the copper grooves created by the wire-EDM process. The polarizers were returned to GA, their mirror surfaces were re-machined using conventional NC machining, and they will be retested at the JAEA RFTS. The LDLMB tests showed that mirror losses, especially on the downstream LDLMB (0.21%), were close to the theoretical prediction (0.16% at room temperature). Losses in adjacent waveguides were greater than expected for pure HE11 transmission because of higher ohmic and mode conversion losses of the high order modes. A prototype sliding joint waveguide was fabricated and will be tested in 2010 at the JAEA RFTS. Work supported by the USDOE under DE-FC02-04ER54698. (author)