Achievement of Reactor-Relevant Performance in Negative Triangularity Shape in the DIII-D Tokamak

Plasma discharges with a negative triangularity (δ=-0.4) shape have been created in the DIII-D tokamak with a significant normalized beta (β_{N}=2.7) and confinement characteristic of the high confinement mode (H_{98y2}=1.2) despite the absence of an edge pressure pedestal and no edge localized modes (ELMs). These inner-wall-limited plasmas have a similar global performance as a positive triangularity (δ=+0.4) ELMing H-mode discharge with the same plasma current, elongation and cross sectional area. For cases both of dominant electron cyclotron heating with T_{e}/T_{i}>1 and dominant neutral beam injection heating with T_{e}/T_{i}=1, turbulent fluctuations over radii 0.5<ρ<0.9 were reduced by 10-50% in the negative triangularity shape compared to the matching positive triangularity shape, depending on the radius and conditions.

Variable location channels to improve efficiency and precision for direct ∇T e measurements and high spatial resolution T e -profile measurements using electron cyclotron emission

Electron cyclotron emission (ECE) diagnostics that use variable location channels based on yttrium iron garnet (YIG) bandpass filters improve the precision and the efficiency of measurements of electron temperature (T _e ) profiles and fluctuations (δT _e ). These variable frequency filters were substituted for fixed frequency filters in the intermediate frequency section to achieve the required higher resolution over a target radial range, just before the experiment. Here, we present the proof-of-principle for high temporal resolution measurement of the electron temperature gradient, via real-time slewing of a YIG filter for relocation of an ECE channel during a long pulse. The key component is the application of YIG tunable filters with their narrow bandwidth and capability for a high slew rate of their center frequency. This application permits fast relocation of the ECE channels for direct measurement of the gradient and close spacing of channels to investigate the magnetic island's dynamic behavior.

A spectroscopic electric field vector imaging diagnostic for electron cyclotron heating systems

An experimental measurement of the wave electric field vector provides important data that can be used to directly compare against 3D full-wave simulations. This direct comparison yields the fastest approach toward identifying missing physics in computational models and providing a high fidelity validation platform. In this paper, we present a diagnostic that is capable of imaging the Electron Cyclotron (EC) wave electric field vector by acquiring filtered images of polarized D _β spectral satellites. The diagnostic is designed to have a spatial and temporal resolution on the order of 100 μm and 100 μs, respectively. The diagnostic purpose is to provide experimental data for the direct validation of full-wave codes used to predict EC beam propagation and absorption and to provide real-time monitoring of EC waves.

Design and first plasma measurements of the ITER-ECE prototype radiometer

On ITER, second harmonic optically thick electron cyclotron emission (ECE) in the range of 220-340 GHz will supply the electron temperature (T_e). To investigate the requirements and capabilities prescribed for the ITER system, a prototype radiometer covering this frequency range has been developed by Virginia Diodes, Inc. The first plasma measurements with this instrument have been carried out on the DIII-D tokamak, with lab bench tests and measurements of third through fifth harmonic ECE from high T_e plasmas. At DIII-D the instrument shares the transmission line of the Michelson interferometer and can simultaneously acquire data. Comparison of the ECE radiation temperature from the absolutely calibrated Michelson and the prototype receiver shows that the ITER radiometer provides accurate measurements of the millimeter radiation across the instrument band.

Improved cross-calibration of Thomson scattering and electron cyclotron emission with ECH on DIII-D

Thomson scattering produces n_e profiles from measurement of scattered laser beam intensity. Rayleigh scattering provides a first calibration of the relation n_e ∝ I_TS, which depends on many factors (e.g., laser alignment and power, optics, and measurement systems). On DIII-D, the n_e calibration is adjusted against an absolute n_e from the density-driven cutoff of the 48 channel 2nd harmonic X-mode electron cyclotron emission system. This method has been used to calibrate Thomson n_e from the edge to near the core (r/a > 0.15). Application of core electron cyclotron heating improves the quality of cutoff and depth of its penetration into the core, and also changes underlying MHD activity, minimizing crashes which confound calibration. Less fueling is needed as "ECH pump-out" generates a plasma ready to take up gas. On removal of gyrotron power, cutoff penetrates into the core as channels fall successively and smoothly into cutoff.