S.J. Diem

Plasma source development for fusion-relevant material testing

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 i...

Analysis of the ITER Low Field Side Reflectometer Transmission Line System

A critical issue in the design of the ITER low field side reflectometer is the transmission line (TL) system. A TL connects each launcher to a diagnostic instrument. Each TL will typically consist of ∼42 m of corrugated waveguide and up to ten miter bends. Important issues for the performance of the TL system are mode conversion and reflections. Minimizing these issues are critical to minimizing standing waves and phase errors. The performance of TL system is analyzed and recommendations are given.

Optimization studies of the ITER low field side reflectometer

Microwave reflectometry will be used on ITER to measure the electron density profile, density fluctuations due to MHD/turbulence, edge localized mode (ELM) density transients, and as an L-H transition monitor. The ITER low field side reflectometer system will measure both core and edge quantities using multiple antenna arrays spanning frequency ranges of 15-155 GHz for the O-mode system and 55-220 GHz for the X-mode system. Optimization studies using the GENRAY ray-tracing code have been done for edge and core measurements. The reflectometer launchers will utilize the HE11 mode launched from circular corrugated waveguide. The launched beams are assumed to be Gaussian with a beam waist diameter of 0.643 times the waveguide diameter. Optimum launcher size and placement are investigated by computing the antenna coupling between launchers, assuming the launched and received beams have a Gaussian beam pattern.

ECH/EBW heating of Proto-MPEX plasmas

Summary form only given. The Prototype Materials Plasma Exposure Experiment (Proto-MPEX) is a linear, high-intensity RF plasma source1 that requires plasma electron heating in overdense conditions to provide target parameters in the density and temperature range needed for plasma facing material studies. In Proto-MPEX, a dense helicon plasma is produced by ~100 kW 13.56 MHz RF power and is further heated by 28 GHz microwaves via Electron Bernstein Waves (EBW). Plasma conditions are well diagnosed utilizing several probes and thermal and optical emission diagnostics.

A 28 GHz 200 kW generation and launching system for ECH/EBW on Proto-MPEX at ORNL

Oak Ridge National Laboratory is developing advanced material-plasma exposure capabilities to test plasma-facing materials for fusion reactors in a realistic edge plasma environment1. A novel RF heating system is being developed for the Prototype Material-Plasma Exposure Exposure eXperiment (Proto-MPEX). Both ion and electron heating with RF power of magnetically confined linear plasma will be utilized. Microwave power generated by a 200 kW cw 28 GHz gyrotron is used for electron cyclotron heating and RF power at 7-13 MHZ provides plasma formation with helicon waves and ion cyclotron heating. The gyrotron and associated HVDC power supply are from an earlier experiment and have been setup with a new waveguide system and launcher. A TE02 to TE01 to HE11 mode converter system launches power from the gyrotron into 15 m long 88.9 mm corrugated waveguide with 2 miter bends and a mirror launcher in a central plasma chamber on Proto-MPEX. The power is beamed toward a resonance zone at the center of the plasma. At higher density above cutoff at 28 GHz (9×1018 m3), it is expected that Electron Bernstein mode conversion will take place and the power will be able to propagate to the center and be absorbed. Several plasma diagnostics are being developed to diagnose the plasma conditions at samples and also be useful in optimizing the EBW launch angle and optimization of the density profile. Details of measurements on the waveguide and launching system and initial high power tests will be presented.

INVESTIGATION OF EBW THERMAL EMISSION AND MODE CONVERSION PHYSICS IN H-MODE PLASMAS ON NSTX

High β plasmas in the National Spherical Torus Experiment (NSTX) operate in the overdense regime, allowing the electron Bernstein wave (EBW) to propagate and be strongly absorbed/emitted at the electron cyclotron resonances. As such, EBWs may provide local electron heating and current drive. For these applications, efficient coupling between the EBWs and electromagnetic waves outside the plasma is needed. Thermal EBW emission (EBE) measurements, via oblique B-X-O double mode conversion, have been used to determine the EBW transmission efficiency for a wide range of plasma conditions on NSTX. Initial EBE measurements in H-mode plasmas exhibited strong emission before the L-H transition, but the emission rapidly decayed after the transition. EBE simulations show that collisional damping of the EBW prior to the mode conversion (MC) layer can significantly reduce the measured EBE for Te < 20 eV, explaining the observations. Lithium evaporation was used to reduce EBE collisional damping near the MC layer. As a result, the measured B-X-O transmission efficiency increased from < 10% (no Li) to 60% (with Li), consistent with EBE simulations.

MODELING RESULTS FOR PROPOSED NSTX 28 GHZ ECH/EBWH SYSTEM

A 28 GHz electron cyclotron heating (ECH) and electron Bernstein wave heating (EBWH) system has been proposed for installation on the National Spherical Torus Experiment (NSTX). A 350 kW gyrotron connected to a fixed horn antenna is proposed for ECH-assisted solenoid-free plasma startup. Modeling predicts strong first pass on-axis EC absorption, even for low electron temperature, Te ~ 20 eV, Coaxial Helicity Injection (CHI) startup plasmas. ECH will heat the CHI plasma to Te ~ 300 eV, providing a suitable target plasma for 30 MHz high-harmonic fast wave heating. A second gyrotron and steered O-X-B mirror launcher is proposed for EBWH experiments. Radiometric measurements of thermal EBW emission detected via B-X-O coupling on NSTX support implementation of the proposed system. 80% B-X-O coupling efficiency was measured in L-mode plasmas and 60% B-X-O coupling efficiency was recently measured in H-mode plasmas conditioned with evaporated lithium. Modeling predicts local on-axis EBW heating and current drive using 28 GHz power in β ~ 20% NSTX plasmas should be possible, with current drive efficiencies ~ 40 kA/MW.

Electron Bernstein Wave Research on the National Spherical Torus Experiment

Off-axis electron Bernstein wave current drive (EBWCD) may be critical for sustaining noninductive high-beta National Spherical Torus Experiment (NSTX) plasmas. Numerical modeling results predict that the {approx}100 kA of off-axis current needed to stabilize a solenoid-free high-beta NSTX plasma could be generated via Ohkawa current drive with 3 MW of 28 GHz EBW power. In addition, synergy between EBWCD and bootstrap current may result in a 10% enhancement in current-drive efficiency with 4 MW of EBW power. Recent dual-polarization EBW radiometry measurements on NSTX confirm that efficient coupling to EBWs can be readily accomplished by launching elliptically polarized electromagnetic waves oblique to the confining magnetic field, in agreement with numerical modeling. Plans are being developed for implementing a 1 MW, 28 GHz proof-of-principle EBWCD system on NSTX to test the EBW coupling, heating and current-drive physics at high radio-frequency power densities.