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If the power lost by impurity radiation from the low-temperature boundary region of a confined plasma exceeds the power transported from the central region then temperature equilibrium is impossible and the temperature profile will collapse. This situation is studied on the assumption of coronal equilibrium and the results are used to predict the permissible impurity concentration at the edge of a tokamak reactor. The effect of neoclassical impurity transport is studied analytically and a 1D tokamak diffusion code is used to illustrate the detailed collapse of the temperature profile when the plasma density in a radiation-cooled tokamak is increased. Finally, the results are used to predict the limiting density of a radiation-cooled tokamak; this predicted density limit is compared with the density limits observed in current tokamak experiments.

A sensitive He-Ne interferometer with passive beam power stabilization for lowdensity pulsedplasma measurements Rev.The characteristics of the He-Ne gas laser used in a new simple interferometric technique have been studied experimentally and theoretically. The interferometer has two novel features: first, the intensity of the laser itself is used to detect the fringes and second, because the intensities of the 0.63-", (red) and 3.39-", (infrared) laser beams are coupled, interference in the infrared can be detected by a simple photomultiplier monitoring the red beam.The system does not respond instantaneously to changes in the optical path length; experimental measurements show that when the red beam is used to follow interference in the infrared, the maximum detectable response is limited to about 3 X 10 6 fringes per second. Discussion of the frequency response and the crosscoupling between the two wavelengths leads to the conclusion that the frequency response is limited by the red channel only.Experimental details of the interferometer are described, including the application of a mUltipass system which, with some loss in spatial resolution, increases the sensitivity of the interferometer by at least a factor of 20.