Abstract
The goals of the National RF Research and Development Program (NRRDP) are to advance the basic understanding of high power radio frequency (RF) in a plasma environment, and to maintain the high-power transmitters and cooling water systems required to support the ITER Ion Cylcotron Range of Frequencies (ICRF) transmission line high power component testing and operation of the Proto-Material Plasma Exposure eXperiment (MPEX) device. The work includes the study of the RF sheath, and development of RF-related diagnostics. These include active and passive spectro¬scopic diagnostics that can provide local and line integrated measurements of RF and Direct Current (DC) electric fields in the edge plasma of fusion devices, as well as measurements of other parameters.
One of these, the Doppler Free Saturation Spectroscopy (DFSS) diagnostic, utilizes a laser beam split into two intersecting, counter-propagating pump and probe beams, whose intersection defines the measurement region. DFSS has demonstrated measurements of atomic spectra with 500 times greater resolution than passive Optical Emission Spectroscopy (OES). This allows Stark effect based electric field measurements to be made with two orders of magnitude high sensitivity than with OES. Measurements have been made using an ORNL electron-cyclotron resonant (ECR) plasma source producing densities up to 1019 m-3, in the vicinity of an RF-biased planar electrode.
In addition, highly accurate measurement of background mag¬netic fields via Zeeman splitting has been demonstrated in the device, and studies are underway to apply the technique to plasma density measurements in order to obtain more precise edge plasma density profiles.
OES has been applied to the measurement of RF electric fields near the plasma facing surface of a lower hybrid launcher on the Alcator C-Mod tokamak. A strong poloidal field component was observed experimentally that was not initially reproduced in a COMSOL full-wave plasma model. It was found that incorporating realistic turbulence-induced density fluctuations produced a polarization angle that more closely matched the experimental measurements. Similar experiments are planned on 'W' Environment in a Steady-state Tokamak (WEST). In addition, a proof-of-principle Proto measurement has been made of D emission modulated by an electron cyclotron heating (ECH) generated electric field in the external plasma on the DIII-D National Fusion Facility (DIII-D), in which harmonics of the ECH frequency are observed that can be related to the electric field magnitude. The technique will be used to improve understanding of beam defocusing, mode conversion, and scattering by turbulence.
Work was initiated to investigate electrostatic coupling and sheath formation in the near field of a RF driving/DC grounded electrode simulating the capacitive coupling between surfaces that have a non-zero RF impedance to ground and the edge plasma. A retarding field energy analyzer was placed downstream of the sheath to measure the ion energies on field lines mapped to the electrode. Initial measurements indicate an increase in ion energies as the RF electrode voltage increased. The practical impact is that higher energies will lead to increased erosion of surfaces magnetically connected to the plasma and thereby introduce impurities into the plasma.
Much of the work at the ORNL Multi-Purpose High-Bay Facility at ORNL, though funded by other programs, depends on high power RF sources maintained by the NRRDP. This includes high power tests of component designs for the ITER Ion Cyclotron Transmission Line and Matching System, and operation of the fully RF powered Prototype Material-Plasma Exposure eXperiment (Proto-MPEX) ITER transmission line component test articles including straight sections, elbows, gas barriers, assembly bellows, a power splitter, and coaxial switch have been tested for power, voltage, and current handling at levels at or above those specified for ITER. Steady state power handling of 6 MW and short pulse voltage handling up to ~ 100 kV have been demonstrated for some components.
Also utilizing NRRDP supported RF and microwave sources, Proto-MPEX has produced deuterium plasma densities up to 8 1019 m-3, surpassing the design specification. Ion heating in the core plasma by the ion cyclotron heating (ICH) system has been demonstrated at high density (3.5 1019 m-3), and overdense electron Bernstein wave (EBW) heating has been achieved, increasing the electron temperature to > 20 eV.
