Clarence E. Thomas

A swept two‐frequency microwave reflectometer for edge density profile measurements on TFTR

The edge density profile can play a significant role in determining the plasma confinement and the coupling of the ion cyclotron resonance frequency (ICRF) heating power to the plasma. To experimentally measure the edge density profile in the Tokamak Fusion Test Facility (TFTR), a two‐frequency microwave reflectometer is being built. This reflectometer will operate in a swept two‐frequency configuration between 91 and 118 GHz using the extraordinary mode. The frequency separation between the two microwave signals will be held constant while the signals are swept across the frequency band. By measuring the differential phase delay between these two signals, the density profile can be reconstructed. Two‐frequency profile reflectometry is discussed and results of modeling of this type of reflectometer measurement for TFTR are shown. Finally, the design of the TFTR edge profile reflectometer microwave system is described.

High speed digital holography for density and fluctuation measurements (invited)

The state of the art in electro-optics has advanced to the point where digital holographic acquisition of wavefronts is now possible. Holographic wavefront acquisition provides the phase of the wavefront at every measurement point. This can be done with accuracy on the order of a thousandth of a wavelength, given that there is sufficient care in the design of the system. At wave frequencies which are much greater than the plasma frequency, the plasma index of refraction is linearly proportional to the electron density and wavelength, and the measurement of the phase of a wavefront passing through the plasma gives the chord-integrated density directly for all points measured on the wavefront. High-speed infrared cameras (up to ∼40,000 fps at ∼64×4 pixels) with resolutions up to 640×512 pixels suitable for use with a CO(2) laser are readily available, if expensive.

Microwave reflectometry for edge density profile measurements on TFTR

A new type of reflectometer not previously used on tokamaks has been installed on TFTR to measure edge electron density profiles. This reflectometer gives unambiguous edge profiles despite large edge density fluctuations and the presence of strong auxiliary heating. The effects of the large density fluctuations in the edge gradient region are overcome by reducing the multiplicity of fringes and eliminating phase excursions produced by scattering from density fluctuations at the reflecting layer. This is accomplished by using differential-reflectometry: the difference phase between two probing signals reflecting from cut-off layers separated by a distance much less than the correlation length of the density fluctuations is used. This system probes the TFTR plasma using the extraordinary mode (X-mode) with two signals swept from 90-118 GHz while maintaining a fixed difference frequency of 125 MHz between these signals. It has been used to obtain density profiles in the range of 1*1011 to 3*1013 cm-3 in high-field (4.5-4.9 T) full size (R0=2.62 m, a=0.96 m) TFTR plasmas. The reflectometer launcher is located in an ion cyclotron range of frequencies (ICRF) antenna and views the plasma through a small penetration in the centre of the Faraday shield. Initial measurements demonstrated that this technique is an effective way to measure the electron density profile in the edge gradient region.

Digital holography for in situ real-time measurement of plasma-facing-component erosion.

In situ, real time measurement of net plasma-facing-component (PFC) erosion/deposition in a real plasma device is challenging due to the need for good spatial and temporal resolution, sufficient sensitivity, and immunity to fringe-jump errors. Design of a high-sensitivity, potentially high-speed, dual-wavelength CO2 laser digital holography system (nominally immune to fringe jumps) for PFC erosion measurement is discussed.

High-speed digital holography for neutral gas and electron density imaging

An instrument was developed using digital holographic reconstruction of the wavefront from a CO2 laser imaged on a high-speed commercial IR camera. An acousto-optic modulator is used to generate 1-25 μs pulses from a continuous-wave CO2 laser, both to limit the average power at the detector and also to freeze motion from sub-interframe time scales. Extensive effort was made to characterize and eliminate noise from vibrations and second-surface reflections. Mismatch of the reference and object beam curvature initially contributed substantially to vibrational noise, but was mitigated through careful positioning of identical imaging lenses. Vibrational mode amplitudes were successfully reduced to ≲1 nm for frequencies ≳50 Hz, and the inter-frame noise across the 128 × 128 pixel window which is typically used is ≲2.5 nm. To demonstrate the capabilities of the system, a piezo-electric valve and a reducing-expanding nozzle were used to generate a super-sonic gas jet which was imaged with high spatial resolution (better than 0.8 lp/mm) at high speed. Abel inversions were performed on the phase images to produce 2-D images of localized gas density. This system could also be used for high spatial and temporal resolution measurements of plasma electron density or surface deformations.

Effect on antenna structure of high‐power rf during plasma operation

ICH antenna structure response during high‐power RF operation on the TORE SUPRA tokamak is described. The condition of the antenna after prolonged use is discussed. (AIP)

Design of a digital holography system for PFC erosion measurements on Proto-MPEX

A project has been started at ORNL to develop a dual-wavelength digital holography system for plasma facing component erosion measurements on prototype material plasma exposure experiment. Such a system will allow in situ real-time measurements of component erosion. Initially the system will be developed with one laser, and first experimental laboratory measurements will be made with the single laser system. In the second year of development, a second CO2 laser will be added and measurements with the dual wavelength system will begin. Adding the second wavelength allows measurements at a much longer synthetic wavelength.

Ion cyclotron heating experiments and plans for the Advanced Toroidal Facility (ATF)

In Advanced Toroidal Facility (ATF) Ion Cyclotron Resonant Frequency (ICRF) experiments to data, a single‐strap tunable antenna with carbon limiters has been used at power densities up to 13.6 MW/m2 (900 kW). Hydrogen minority heating experiments at 14.4 MHz were performed in deuterium plasmas formed initially with both second‐harmonic electron cyclotron heating (ECH) and helium neutral beam injection (NBI). No ICRF heating was observed in the low‐density ECH target plasmas. Two distinct NBI plus ICRF operating modes are described. The moderate heating of these discharges is probably limited by impurities sputtered from the vessel walls and/or poorly confined orbits for the heated particles. A two‐strap, phased antenna, designed to reduce impurities and capable of both minority and direct electron heating is described. A second antenna design for optimizing the coupling to well confined ion orbits, additional ICRF heating schemes, and methods for reducing the RF edge interactions in a stellarator geometry...

Possible integrated diagnostic set for ion cyclotron heating for the tokamak physics experiment (TPX) (abstract)a)

Heating and current drive with ICRF is one of the major missions of TPX and is crucial to its success. In contrast to the integrated nature of core diagnostic programs, diagnostics that measure ICRF propagation and edge interactions are often assigned lower priority, have not been included in the base diagnositics set, or were included as upgrades. This can result in an incomplete and unoptimized set of measurements. Thus it is important that an integrated set of diagnostics (engineered along with the antenna design), capable of fully diagnosing the interaction, propagation, and absorption of the ICRF be available for TPX. The parameters of interest include: coupling of the ICRF antenna to the plasma, launched spectrum, wave propagation, edge plasma interactions, electron heating, and current drive, and fast ion power deposition and losses. The diagnostic set should be designed so it can be upgraded for control of loading and spectrum by providing feedback information for adjusting phase, power level, fue...

TORE SUPRA fast reciprocating radio frequency probea)

A fast reciprocating ion cyclotron range of frequencies (ICRF) probe was installed and operated on TORE SUPRA during 1992/1993. The body of the probe was originally used on the ATF experiment at Oak Ridge National Laboratory. The probe was adapted for use on TORE SUPRA, and mounted on one of the two fast reciprocating probe mounts. The probe consists of two orthogonal single‐turn wire loops, mounted so that one loop senses toroidal rf magnetic fields and the other senses poloidal rf magnetic fields. The probe began operation in June, 1993. The probe active area is approximately 5 cm long by 2 cm, and the reciprocating mount has a slow stroke (5 cm/s) of 30 cm and a fast stroke (1.5 m/s) of about 10 cm. The probe was operated at distances from the plasma edge ranging from 30 to −5 cm (i.e., inside the last closed flux surface). The probe design, electronics, calibration, data acquisition, and data processing are discussed. First data from the probe are presented as a function of ICRF power, distance from t...