H.W. Moos

Grazing incidence time‐resolving spectrograph for magnetic fusion plasma diagnostics

A time‐resolving grazing incidence spectrograph based on an image intensified photodiode array detector has been constructed and has demonstrated its utility as a fusion plasma diagnostic. The total wavelength coverage is 15–360 A with 0.7 A resolution (line profile FWHM); portions of this range may be observed during a single plasma discharge with a bandwidth of ∼40 A when the detector is centered at 40 A and ∼80 A with it centered at 200 A. Integration times from 5.4 to 13.1 ms are available when the entire photodiode array is read out; integration times as short as 1 ms can be obtained for a few lines of interest through the readout of selected photodiodes. The spectrograph has been radiometrically calibrated over the 60‐ to 360‐A range at the NBS SURF II electron storage ring and is currently in operation on the Princeton Large Torus (PLT) tokamak.

Scanning polarimeter for measurement of the poloidal magnetic field in a tokamak

The internal magnetic field in a magnetically confined plasma may be deduced from the analysis of circular polarization of spectral lines emitted by the plasma. The theory of the measurement and a detailed design of a polarimeter constructed to measure the poloidal field profile in the Texas Experimental Tokamak (TEXT) are presented. The instrument measures the difference between left‐hand and right‐hand circularly polarized line profiles, a quantity directly proportional to the magnetic field component in the direction of observation. The high throughput of the Fabry–Perot interferometer employed in this design, combined with efficient light‐collecting optics and lock‐in detection of the polarization signal, allows measurement of the fractional circular polarization of the magnetic dipole line Ti xvii 3834 A with an accuracy on the order of 10−3. The line‐of‐sight averaged poloidal field is determined with uncertainty as small as 50 G. The line emission used in the present measurement is not well localiz...

Changes in impurity radiation during ICRF heating of PLT tokamak plasmas

This paper presents the results of a study of the increase in impurity line radiation in the 15-360 A region during ICRF heating of PLT plasmas, with emphasis on metallic impurity (Ti and Fe) behaviour. Central titanium and iron densities are given for a variety of ICRF heating experiments; total central metallic impurity concentrations of up to about 0.3% of ne(0) are observed at the 2.0 MW RF power level. This study shows that the power radiated by these elements is a small (about 10%) fraction of the total input power to the plasma for the present heating efficiency at 2.0 MW RF power. The impurity line brightnesses scale approximately linearly with RF power up to 2.8 MW. The antenna Faraday shields are shown to be the primary source of metallic impurities during both ICRF heating and Ohmic heating only. The impurity content of discharges heated using a single half-turn antenna and a pair of centre-fed antennas (having the same total surface area but half the poloidal extent of the half-turn antenna) is the same at a relatively low RF power of 350 kW, indicating that the impurity influx does not depend on the poloidal length of the antennas (or that the plasma interacts only with a localized area on the Faraday shields).

EUV impurity study of the Alcator tokamak

The intensity of resonance line radiation from oxygen, nitrogen, carbon and molybdenum impurities has been measured in the high-field (80 kG), high-density (6 × 1014 cm−3) discharges of the Alcator Tokamak, using a 0.4-m normal-incidence monochromator (300–1300 A) with its line of sight fixed along a major radius. Total light-impurity concentrations of a few tenths of a percent have been estimated by using both a simple model and a computer code which included Pfirsch-Schluter impurity diffusion. The resulting values of Zeff, including the contributions due to both the light impurities and molybdenum, were close to one. The power lost through the impurity line radiation from the lower ionization states accounted for ~10% of the total Ohmic input power at high densities.

Impurity transport during the H-mode in DIII-D

In H-mode plasmas in DIII-D, large modulations in spectroscopically measured impurity densities have been observed during shots with giant edge localized modes (ELMs). These spectral modulations have been analysed with the MIST impurity transport code. This analysis indicates that impurities are alternately flowing towards the plasma centre and then away from it. This alternating flow is correlated with ELM produced changes in the electron density. The electron density oscillations are extreme, causing the density profile to switch from hollow (just before an ELM) to centrally peaked (just after an ELM). Neoclassical convection, dependent on ion density gradients, causes impurities to concentrate most heavily where the electron density is largest and can explain the modulating impurity behaviour. Anomalous diffusion, D 1.0 ? 104 cm2/s, reduces the degree of impurity peaking. As the plasma current increases, the increase in hollowness of electron density profiles can account for the observed decrease in central impurity accumulation. Transport of cobalt, injected by laser ablation, has also been studied; cobalt transport variations are consistent with the ELM induced changes seen in intrinsic impurity transport. The transport results may be consistent with neoclassical impurity convective fluxes and suggest that impurity accumulation in tokamaks will occur unless the electron density profile is flat or particle confinement is low.

Impurity profiles for H-mode discharges in DIII-D

Impurity concentration profiles have been determined for H-mode discharges in the DIII-D tokamak from measured ne, Te, Zeff and radiated emissivity profiles. The central impurity levels in DIII-D high current H-modes, as modelled using this technique, remain below those seen in L-modes (fractional nickel concentrations ≤0.02%) throughout the neutral beam heating pulse. In contrast to some other experiments (ASDEX [1], JET [2], JFT-2M [3]), the H-mode does not terminate because of excessive radiation in DIII-D discharges heated with co-injected neutral beams. For increasing plasma current, the global impurity concentrations decrease and the profiles become more dominated by edge radiation. H-modes as obtained with electron cyclotron heating and co-injected neutral beams at similar heating powers also have low impurity levels, but the impurity distribution is significantly more hollow in the case of neutral beam heating.

Space‐ and time‐resolving spectrograph for high‐temperature plasma diagnostics

Construction of an EUV (60 –370 A) space‐ and time‐resolving, grazing incidence spectrograph (STRS) is described. The simultaneous spectral coverage of the instrument ranges from 40 to 75 A, depending on the wavelength region. The spectral resolution is about 1 A. The spatial resolution, accomplished by using the pinhole camera effect and the inherent astigmatism of a concave grating in grazing incidence, is about 2 cm, with a total field of view of 60 cm at a distance of 2 m from the plasma. The detector consists of a 75‐mm MCP image intensifier optically coupled to three CCD area array detectors. Time resolution of up to 2 ms is achieved with high‐speed readout electronics. A PDP 11/73 minicomputer controls the spectrograph and collects and reduces 3.0 Mbyte of data per shot.

Construction, calibration, and application of a compact spectrophotometer for EUV (300-2500-Å) plasma diagnostics

A 400-mm normal incidence concave grating spectrophotometer, specifically designed for plasma diagnostics, is described. The wavelength drive, in which the grating is translated as well as rotated, is discussed in detail; the wavelength linearity of the sine drive and methods of improving it are analyzed. The instrument can be used in any orientation, is portable under vacuum, and quite rugged. The construction techniques utilized produce a high quality vacuum making the instrument compatible with both high purity plasma devices and synchrotron radiation sources. The photometric sensitivity calibration was found to be very stable during extended use on high temperature plasma devices. The applications of the instrument to diagnose plasmas in two tokamaks and a mirror device are described. A facility used for photometric calibration of extreme ultraviolet (lambda > 300-A) spectrophotometers against NBS standard diodes is described. The instrumental calibration obtained using this facility was checked by using synchrotron radiation from SURF II; very good agreement was observed.

Magnetic‐field measurement based on Zeeman effect in lines excited by charge exchange recombination

It is suggested that the poloidal magnetic field in a tokamak may be deduced from the analysis of circular polarization of light impurity ion lines excited by charge‐exchange recombination. Visible or near‐ultraviolet transitions in hydrogenlike oxygen and carbon are considered. Estimations of the linewidths and the magnitude of the Zeeman effect show that, with appropriate polarization analyzer and the experiment geometry, the poloidal component of the magnetic field can, in principle, be measured. Uncertainties of the model used to calculate the polarized line profiles do not affect the interpretation of the measurement in a fundamental way and may be accounted for experimentally by calibrating the polarization effect against the components of the toroidal field. In principle, the analysis of only one transition may provide information about the whole poloidal field profile.

Spatial imaging in the soft x-ray region (20-304 Å) utilizing the astigmatism of a grazing incidence concave grating

Soft x-ray (20–304-A) astigmatic line shapes were measured in order to evaluate the spatial imaging properties of a Rowland mounted concave grating in grazing incidence. The practicability of coarse 1-D spatial imaging in the soft x-ray region is demonstrated. Spatial resolution equivalent to ∼4 cm at a source distance of 2 m can be achieved with practical parameters (e.g., sensitivity and time resolution) for a fusion diagnostic spectrograph. The results are compared to computer-generated ray tracings and found to be in good agreement. The ray tracing program which models the grazing incidence optics is discussed.