Abstract-High power, high frequency microwave radiation can be transmitted with very low loss in oversized corrugated metallic waveguide. We derive a linearly polarized (LPmn) mode basis set for these waveguides for the special case of quarter wavelength depth corrugations. We also show the relationship between the LPmn modes and the conventional modes (HEmn, EHmn, TE0n, TM0n) of the corrugated guide. The loss in a gap or equivalent miter bend in the waveguide is calculated for single mode and multi-mode propagation on the line. In the latter case, it is shown that modes of the same symmetry interfere with one another, causing enhanced or reduced loss, depending on the relative phase of the modes. If two modes with azimuthal (m) indeces that differ by one propagate in the waveguide, the resultant centroid and the tilt angle of radiation at the guide end are shown to be related through a constant of the motion. These results should be useful in describing the propagation of high power, linearly polarized radiation in these overmoded, corrugated waveguides.
The Electron Cyclotron (EC) system for the ITER tokamak is designed to inject ≥20 MW RF power into the plasma for Heating and Current Drive (H&CD) applications. The EC system consists of up to 26 gyrotrons (between 1 to 2 MW each), the associated power supplies, 24 transmission lines and 5 launchers. The EC system has a diverse range of applications including central heating and current drive, current profile tailoring and control of plasma magneto-hydrodynamic (MHD) instabilities such as the sawtooth and neoclassical tearing modes (NTMs). This diverse range of applications requires the launchers to be capable of depositing the EC power across nearly the entire plasma cross section. This is achieved by two types of antennas: an equatorial port launcher (capable of injecting up to 20 MW from the plasma axis to mid-radius) and four upper port launchers providing access from inside of mid radius to near the plasma edge. The equatorial launcher design is optimized for central heating, current drive and profile tailoring, while the upper launcher should provide a very focused and peaked current density profile to control the plasma instabilities.The overall EC system has been modified during the past three years taking into account the issues identified in the ITER design review from 2007 and 2008 as well as integrating new technologies. This paper will review the principal objectives of the EC system, modifications made during the past two years and how the design is compliant with the principal objectives.
The ITER ECH Transmission Lines (TLs) are 63.5 mm diameter corrugated waveguides that will each carry 1 MW of power at 170 GHz with a transmission efficiency that should exceed 83%. The losses on the ITER TL have been calculated for four possible cases corresponding to having HE 11 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 % due to Ohmic loss in each of the four cases. Estimates of the mode conversion loss show that the fraction of output power in the HE 11 mode is about 3% smaller than the fraction of input power in the HE 11 mode. High output mode purity therefore can only be achieved with significantly higher input mode purity. Combining both Ohmic and mode conversion loss, for 1 MW of power generated by the gyrotron, the output power in the HE 11 mode at the end of the ITER TL can be roughly estimated in theory as 920 kW times the fraction of input power in the HE 11 mode.2
The magnetic geometry of the Prototype Material Plasma Exposure eXperiment (Proto-MPEX) was recently modified to enable more effective utilization of 28 GHz microwave auxiliary power, specifically: (1) to heat plasma electrons in the radial core of the device and (2) to deliver the heated plasma to the target plate of the device. To achieve this goal, the microwave launcher geometry and placement were significantly re-engineered, guided by previous experimental results and computational modeling. The core electron temperature in the launcher region is observed to increase from 3 eV to 11 eV with 30 kW of auxiliary power after the improvements, and an increase from 3 eV to 6 eV is concurrently measured in the target region (∼1 m from the launcher) at electron density above O-mode cutoff. Radially resolved measurements in the launcher region exhibit a strong dependence on the magnetic geometry. The results of a magnetic field scan reinforce the effectiveness of the intended O-X-B mode conversion scenario that is currently planned for microwave heating of the Material Plasma Exposure eXperiment (MPEX).
Plasma-facing materials in the divertor of a magnetic fusion reactor have to tolerate steady state plasma heat fluxes in the range of 10 MW/m2 for ∼107 s, in addition to fusion neutron fluences, which can damage the plasma-facing materials to high displacements per atom (dpa) of ∼50 dpa. Materials solutions needed for the plasma-facing components are yet to be developed and tested. The material plasma exposure experiment (MPEX) is a newly proposed steady state linear plasma device designed to deliver the necessary plasma heat flux to a target for testing, including the capability to expose a priori neutron-damaged material samples to those plasmas. The requirements of the plasma source needed to deliver the required heat flux are being developed on the Proto-MPEX device which is a linear high-intensity radio-frequency (RF) plasma source that combines a high-density helicon plasma generator with electron- and ion-heating sections. The device is being used to study the physics of heating overdense plasmas in a linear configuration. The helicon plasma is operated at 13.56 MHz with RF power levels up to 120 kW. Microwaves at 28 GHz (∼30 kW) are coupled to the electrons in the overdense helicon plasma via electron Bernstein waves and ion cyclotron heating at 7–9 MHz (∼30 kW) is via a magnetic beach approach. High plasma densities >6 × 1019/m3 have been produced in deuterium, with electron temperatures that can range from 2 to >10 eV. Operation with on-axis magnetic field strengths between 0.6 and 1.4 T is typical. The plasma heat flux delivered to a target can be >10 MW/m2, depending on the operating conditions. An initial plasma material interaction experiment with a thin tungsten target exposed to this high heat flux in a predominantly helium plasma showed helium bubble formation near the surface, with no indication of source impurity contamination on the target.
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