Hyeon K. Park

Multichannel far-infrared laser interferometer for electron density measurements on the tokamak fusion test reactor

A ten-channel far-infrared laser interferometer which is routinely used to measure the spatial and temporal behavior of the electron density profile on the tokamak fusion test reactor is described, and representative results are presented. This system has been designed for remote operation in the very hostile environment of a fusion reactor. The possible expansion of the system to include polarimetric measurements is briefly outlined.

Error analysis of a new asymmetric Abel-inversion method

Propagation of the various errors in a new asymmetric Abel‐inversion method [Hyeon K. Park, Plasma Phys. Controlled Fusion 31, 2035 (1989)] is investigated. In order to provide a meaningful error analysis of the inversion process, both the effects of geometrical factors and measuremental errors are studied. The dominant source of uncertainty in the determination of local electron density is due to the spline fitting of the interferometric measurements with uneven spacings. The propagation of these errors is highly spatially localized and heavily damped in this inversion method. The absolute range of error in the local electron density is ±1.5×1012 cm−3.

Survey of Features in Radiative Power Loss Profiles in the Tokamak Fusion Test Reactor

Although the total radiated power in the Tokamak Fusion Test Reactor is often as high as 70% of the heating power, most of the radiation is concentrated near the surface of the plasma, and the interior loss is almost negligible. Fractional radiation loss declines during neutral beam heating. Under most interesting plasma conditions, the radiation profiles are dominated by asymmetrical peaks, which indicate locally intense edge radiation. As the high-density limit is approached, under most conditions, a bright band of radiation (a marfe) appears on the inner side of the plasma column. Marfe location is affected by toroidal field direction, neutral beam direction, and nearness to the high-density limit. Marfes have been observed to drift under the plasma column to the lower outside plasma edge. Marfes naturally develop into detached plasmas. In enhanced confinement discharges (supershots), an unexplained peculiar bright band, distinct from a marfe, appears in the lower outside part of the vacuum vessel, outside of the limiter radius. In high-density pellet-fueled discharges, there is a central peak that shows evidence for inward impurity convection.

Polarization characteristics of wire mesh at 119 μm

Polarization characteristics of the transmitted beam through the various sizes of metal wire grid meshes are examined experimentally at 118.8 microm. When these results were compared to Chens waveguide theory, we obtained excellent agreement. In particular, when the incident beam polarization is neither parallel (TM) nor perpendicular (TE) to the wire grid axis, a linearly polarized incident beam can be made elliptic due to the phase difference between the TE and TM modes.

Disruption Avoidance on TFTR

Disruptions on the Tokamak Fusion Test Reactor (TFTR), especially those occurring at high stored energy, result in lost experimental run time because many discharges are required to regain wall conditions necessary for good plasma performance. A variety of disruption types have been observed on TFTR. These include density-limit disruptions, those caused by a high influx of impurities, those occurring during the current rampdown, those resulting from locked modes, and those occurring at high normalized β (β N = β T αβ T /I p ). A combination of operational experience and limiter development has helped to avoid many potential disruptions. However, the experimental goal of high fusion power production engenders the risk of high-β N disruptions. A system to limit β N by reducing the neutral beam power as a preprogrammed β N limit is reached is now in use to help avoid high-β N disruptions. Operational issues of disruption avoidance, the β N feedback system, the limitations and possible improvements of the system are discussed.

Results from the low-power 60 GHz gyrotron collective Thomson scattering diagnostic on TFTR

A low-power 60 GHz gyrotron collective Thomson scattering diagnostic has been operating on TFTR to test the feasibility of detecting alpha particles when scattering perpendicular to the magnetic field. An enhanced scattered signal is predicted to result from the interaction of the energetic ions with plasma resonances in the lower hybrid frequency range. Millimeter-wave power levels at the plasma were approximately 200 W for typical pulse lengths of 50 ms. Deuterium and possible fusion product ion cyclotron frequencies and their harmonics were observed during neutral beam heating. These spectra are similar to ion cyclotron emission spectra which are detected with radio-frequency probes on TFTR at the plasma edge. Also, ion cyclotron resonance heating fundamental and harmonic fluctuations were observed. However, a signal has not been definitively detected in the lower hybrid frequency range which correlates to alpha particles. Broadband noise was observed during neutral beam heating which is greater than t...

TFTR 60 GHz alpha particle collective Thomson scattering diagnostic

A 60 GHz gyrotron collective Thomson Scattering alpha particle diagnostic has been implemented for the D-T period on TFM. Gyrotron power of 0.1-1 kW in pulses of up to 1 second can be launched in X-mode. Efficient corrugated waveguides are used with antennaes and vacuum windows of the TFTR Microwave Scattering system. A multichannel synchronous detector receiver system and spectrum analyzer acquire the scattered signals. A 200 Megasample/sec digitizer is used to resolve fine structure in the frequency spectrum. By scattering nearly perpendicular to the magnetic field, this experiment will take advantage of an enhancement of the scattered signal which results from the interaction of the alpha particles with plasma resonances in the lower hybrid frequency range. Significant enhancements are expected, which will make these measurements possible with gyrotron power less than 1 kW, while maintaining an acceptable signal to noise ratio. We hope to extract alpha particle density and velocity distribution functions from the data. The D and T fuel densities and temperatures may also be obtainable by measurement of the respective ion cyclotron harmonic frequencies.

New compact and efficient local oscillator optic system for the KSTAR electron cyclotron emission imaging system

Electron cyclotron emission imaging (ECEI) diagnostic on Korean Superconducting Tokamak Advanced Research utilizes quasi-optical heterodyne-detection method to measure 2D (vertical and radial) Te fluctuations from two toroidally separated poloidal cross section of the plasma. A cylindrical lens local oscillator (LO) optics with optical path length (OPL) 2-2.5 m has been used in the current ECEI system to couple the LO source to the 24 vertically aligned array of ECE detectors. For efficient and compact LO optics employing the Powell lens is proposed so that the OPL of the LO source is significantly reduced from ∼2.0 m to 0.4 m with new optics. The coupling efficiency of the LO source is expected to be improved especially at the edge channels. Results from the optical simulation together with the laboratory test of the prototype optics will be discussed in this paper.

Dual far infrared laser diagnostic of magnetized plasmas

A dual far infrared laser has been constructed and its properties have been exploited to probe tokamaklike discharges in the CDX toroidal device. Thermal variation of the difference frequency between the two far infrared cavities is slow, although the cavities lack thermal stabilization, simply because their assembly on the same chassis exposes them to virtually identical temperature changes. The optical arrangement beyond the laser permits conversion within minutes between interferometry and density fluctuation observation, and within an hour between different operating wavelengths. Line average densities of 2 x 10(13) cm(-3) and coherent fluctuations in the neighborhood of 20 kHz have been measured with this diagnostic.

Modeling and diagnostics of micro-plasmas for application to plasma display panels (PDP)

Summary form only given. Reduction of plasma cell size by /spl sim/40% (presently /spl sim/200 /spl mu/m) in length is an essential step in bringing current flat panel television technology to meet true HDTV standards and moderate size (<30) high resolution display terminal in the future. In order to reduce the cell size effectively while maintaining and/or improving the brightness, modeling techniques more sophisticated than the simple fluid model is needed. Such modeling should enable us to test more intricate new physics such as transient modification of the distribution function which can lead to an increase in the population of Xe energy state favorable for UV emission. Kinetic modeling based on particle in cell (PIC) has been known as a powerful tool for such study but one shortcoming in real application is a long computation time. A new algorithm based on kinetic calculation, which can reduce the computational time significantly, is under development. This paper will also address a new technique for a direct measurement of plasma density in a plasma cell comparable to the real PDP cell size. On behalf of this effort, we have constructed a single cell in which plasmas produced across ITO electrodes coated with MgO are studied. The measured electron emission coefficients of MgO will be used to benchmark a new improved kinetic based code. A future plan for simultaneous measurement of plasma parameters and emission coefficients to test the validity of the new algorithm will be discussed.