Experimental measurement of microwave broadening by fluctuations on the DIII-D tokamak

High gain, disruption free operation in tokamak fusion reactors requires the use of localized heating and current drive. Microwave power is the ideal tool for this purpose. Microwave power from gyrotrons is injected in focused, steerable beams which are absorbed by a thin layer of electrons at a resonant position in the tokamak’s magnetic field. Heretofore, simulations and experimental benchmarking of deposition suggested a continuous, traceable propagation of this power from the edge to the deposition region. Recent work in analytic theory[1] and simulation[2] suggests that an untreated interaction between density fluctuations inherent to the tokamak edge will lead to scattering of the RF beam, broadening its deposition in the plasma. This work details the first experimentally derived scaling of fluctuational broadening. In a set of perturbative heat transport experiments on the DIII-D tokamak, deposition broadening has been studied. Electron temperature (Te) measurements from a 500 kHz, 48-channel ECE radiometer are Fourier analyzed and used to calculate a deposition-dependent heat flux generated by perturbative heating.

Consistency of the electron heat flux with a transport model is used to evaluate the power deposition profile. A diffusive(∝ ∇Te) and convective(∝ Te) transport solution is linearized and compared with energy conservation-derived fluxes. Fitting agreement between these two forms of heat flux can be used to evaluate the quality of ECH deposition profiles, and a χ2 minimization finds a significant broadening is needed over base 1D equilibrium ray tracing calculations from TORAY-GA. This work presents transport results and a validation of this transport fitting against past studies. The method is applied to a range of DIII-D discharges and finds a broadening of deposition which scales linearly with edge density fluctuation level. Simulation results from full wave[3] and beam tracing codes[4] which treat fluctuation effects can be evaluated through comparison with these experimental results.

As part of this work, upgrades have been undertaken to improve the radiometer system, with 8 tunable high-resolution channels based on Yttrium Iron Garnet spheres. Their narrow bandpass and flexible positioningincrease the resolution of measurements. Cross-calibration through density cutoff[5] has become an important application for the DIII-D ECE system, yielding a publication. Heat deposition measurements made as part of this work have already yielded one paper[6], with two journal papers, one experimental and one based on 3D full wave simulations[3], currently in review for publication in Nuclear Fusion.