R. D. Petrasso

X-ray imaging studies of Alcator-A radiation and internal disruptions

Soft-X-ray data collected with an imaging system on the Alcator-A tokamak are used to study characteristics of quasi-equilibrium plasma radiation, and a method of using soft-X-ray data to determine the electron-temperature-profile change due to internal disruptions is described. It is found that, for a series of discharges characterized by increasing plasma density, X-radiation from the plasma core becomes more like hydrogen bremsstrahlung. For the highest-density discharges studied, local-X-ray enhancement factors are approximately 1, within the central half of the plasma column. Finally, the change in the electron temperature profile due to internal disruptions which occurred during a high-density discharge is determined.

Do changes in coronal emission structure imply magnetic reconnection

We examine three major possible interpretations of observed reconfigurations of coronal X-ray and XUV emitting structures on a scale comparable to the size of the structures themselves. One possibility is that little change in the large-scale magnetic field configuration is associated with the change in emission. The other two possibilities are processes by which the magnetic field structure can change.We demonstrate that large changes in visibility in X-rays or XUV lines can be associated with relatively minor changes in the coronal magnetic field by showing the behavior of magnetic interconnections between individual active regions in a complex of activity observed by the S-054 X-ray spectrographic telescope on Skylab. While the large-scale interconnections are continuously present for at least several days, individual loops in these structures are visible for only relatively short times (≲1 day).The two theoretical possibilities which we discuss are ‘frozen-in’ motion of the fields, and field line reconnection. We emphasize that reconnection occurs in regions much smaller than telescopic resolution. Because there are no measurements of the magnetic field in the corona in projection against the disk, existing observations are generally not sufficient to show in detail howmuch reconnection has occurred.

The quantitative properties of three soft X-ray flare kernels observed with the AS&E X-ray telescope on Skylab

The physical parameters for the kernels of three solar X-ray flare events have been deduced using photographic data from the S-054 X-ray telescope on Skylab as the primary data source and 1–8 and 8–20 Å fluxes from Solrad 9 as the secondary data source. The kernels had diameters of ∼5–7″ and in two cases electron densities at least as high as 3 × 1011 cm−3. The lifetimes of the kernels were 5–10 min. The presence of thermal conduction during the decay phases is used to argue: (1) that kernels are entire, not small portions of, coronal loop structures, and (2) that flare heating must continue during the decay phase.We suggest a simple geometric model to explain the role of kernels in flares in which kernels are identified with emerging flux regions. The flare is triggered at the neutral sheet between the EFR and a larger loop structure. We associate the X-ray kernels with Hα kernels, which previously associated (incorrectly, we believe) with the nonthermal impulsive phases of flares.

Comparison of the 9.1cm and soft X-ray emission from an active region

Thermal bremsstrahlung from the X-ray observed plasma accounts for most of the observed 9.1 cm emission from McMath 12336, an old, spotless active region, on June 2, 1973. This implies that only a small fraction of the emission measure within the active region is in the range around 106 K and below.

Instrument for synchronizing plasma diagnostic measurements with tokamak internal disruptions

We describe an instrument which detects internal disruptions in real time, providing one pulse at each disruption and another pulse at a selectable phase of the internal disruption cycle (as determined by the interval between the two preceding disruptions). By appropriate gating, this instrument allows diagnostics requiring integration times long compared to the internal disruption period to be used in the study of plasma parameter changes produced by internal disruptions. It also permits selection of the phase of the internal disruption cycle at which diagnostics with short sampling times are used. Several features of the instrument are discussed in light of its performance in firing the Thomson scattering laser on the Alcator‐C tokamak.