Design of an electron cyclotron emission diagnostics suite for COMPASS Upgrade tokamak.

COMPASS Upgrade is a medium size and high field tokamak that is capable of addressing key challenges for reactor grade tokamaks, including power exhaust and advanced confinement scenarios. Electron cyclotron emission will be available among the first diagnostics to provide measurements of high spatial and temporal resolution of electron temperature profiles and electron temperature fluctuation profiles through a radial view. A separate oblique view at 12° from normal will be utilized to study non-thermal electrons. Both the radial and oblique views are envisioned to be located in a wide-angle midplane port, which has dimensions that enable simultaneous hosting of the front-end of their quasi-optical (QO) designs. Each QO design will have an in situ hot calibration source in the front-end to provide standalone and calibrated Te (R,t) measurements. The conceptual design for each QO system, the Gaussian beam analysis, and the details of the diagnostic channels are presented.

Fast modulating electron cyclotron emission (FMECE) diagnostic for tokamaks.

Utilizing variable-frequency channels, e.g., yttrium iron garnet (YIG) bandpass filters, in the intermediate frequency (IF) section of an electron cyclotron emission (ECE) radiometer facilitates flexibility in the volume viewed by the ECE channels as well as high resolution electron temperature and temperature fluctuation measurements in tokamaks. Fast modulating electron cyclotron emission (FMECE), a stand-alone IF section with eight channels, is a novel application of YIG filters for real-time electron temperature gradient and gradient scale length measurements. Key to FMECE is a simultaneous input/output data acquisition unit, as well as a modified type of YIG filters, which is capable of fast switching of their center (set) frequencies with a frequency slew rate of 600 µs/GHz. A new FMECE has been implemented and tested on the DIII-D tokamak, demonstrating its capability in real-time gradient measurements. The data presented here shows that FMECE can identify flattening in the electron temperature profile; the latter can be used as a sensor for real time monitoring and control of plasma instabilities. Implementation and application are planned for the EAST tokamak.

Upgrade of the ECE diagnostic on EAST.

The electron cyclotron emission (ECE) diagnostic on the experimental advanced superconducting tokamak (EAST) was upgraded recently to provide electron temperature profile measurement with wider radial coverage and better precision. The lower limit of the ECE detection frequency band was extended from 104 GHz to 97 GHz by adding a new 8-channel heterodyne radiometer, which ensures capability for the measurement of the second harmonic ECE with toroidal magnetic field down to 1.75 T. Also, the existing 32-channel heterodyne radiometer has been upgraded, with the frequency interval for the lower frequency range up to 120 GHz reduced from 2 GHz to 1 GHz by introducing eight channels in the intermediate frequency part. In addition, a plan is presented to incorporate tunable yttrium iron garnet filters into the existing heterodyne radiometer to obtain detailed measurements of the electron temperature gradient scale length as well as finer spatial pinpointing of magnetohydrodynamic modes. Examples from DIII-D are provided where similar ECE diagnostic allowed precise measurement of the center and width of neoclassical tearing modes.

High resolution ion Doppler spectroscopy at Prairie View Rotamak.

A fast ion Doppler spectroscopy (IDS) diagnostic system is installed on the Prairie View Rotamak to measure ion temperature and plasma flow. The diagnostic employs a single channel photomultiplier tube and a Jarrell-Ash 50 monochromator with a diffraction grating line density of 1180 lines/mm, which allows for first order spectra of 200-600 nm. The motorized gear of the monochromator allows spectral resolution of 0.01 nm. Equal IDS measurements are observed for various impurity emission lines of which carbon lines exhibit stronger intensities. Furthermore, the diagnostics is examined in an experiment where plasma experiences sudden disruption and quick recovery. In this case, the IDS measurements show ~130% increase in ion temperature. Flow measurements are shown to be consistent with plasma rotation.

Measurements of neutral helium density in helicon plasmas.

Laser-induced-fluorescence (LIF) is used to measure the density of helium atoms in a helicon plasma source. For a pump wavelength of 587.725 nm (vacuum) and laser injection along the magnetic field, the LIF signal exhibits a signal decrease at the Doppler shifted central wavelength. The drop in signal results from the finite optical depth of the plasma and the magnitude of the decrease is proportional to the density of excited state neutral atoms. Using Langmuir probe measurements of plasma density and electron temperature and a collisional-radiative model, the absolute ground state neutral density is calculated from the optical depth measurements. Optimal plasma performance, i.e., the largest neutral depletion on the axis of the system, is observed for antenna frequencies of 13.0 and 13.5 MHz and magnetic field strengths of 550-600 G.