In the aperture limiter shadow of the T-10 tokamak, the temporal evolution of plasma density and electron temperature during single discharges is studied by using a Langmuir double probe. It is observed that the plasma of the scrape-off layer (SOL) passes through a stationary phase when the Ohmic heat input Uloop Ip also does, suggesting that the stationarity of the SOL is correlated with the stationarity of the discharge power balance. During this stationary phase the plasma density at the aperture limiter edge was found to be approximately proportional to the square of the line averaged core plasma density. – It is shown in the paper that this quadratic density scaling can be easily understood within the framework of a stationary particle balance model of the recycling process, i.e. the interaction of plasma and neutral particles of the working gas in the edge region. Quadratic density scaling is expected to hold generally if the plasma density is high enough to prevent the neutrals from reaching the centre. For high densities, on the other hand, there is a density limit defined by the requirement that the discharge should supply enough power to sustain recycling.

Tracks of unipolar arcs found on parts of the T-10 tokamak (shield and pipe section of the siirface analysis station IVASA, edge of the movable limiter) are analyzed.I t is the aim of this paper to explain the observed deviations of the arc motion from the first order retrograde direction. It is shown thnt the drift of the arc which occurs if the magnetic field is inclined to the cathode surface, and which was studied in detail by ROBSOX [ 11, causes the deviations. To this end, arc trncks on three differently shaped electrodes (cone, cylinder, sphere) are calculated theoretically and compared with the data from T-10. Good agreement is obtained if the drift angle is approximately equal to one half of the inclination angle of the mngnetic field. The present investigation is consistent with the model of retrograde arc motion of ROBSOS and VON ENQEL [2, 31.

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In the peripheral region of the plasma, radial transport is due to both closed and open orbits. However, owing to the presence of open orbits, the question of the resulting fluxes on the periphery requires special consideration.Both the distribution and the direction of the tracks suggest the existence of an m = 1, n = 1 plasma mode lasting for at least 1 ms. A simple model is proposed to match the observed structures and to yield information about the plasma column.

A b s t r a c t . Recycling of neutral particles plays an essential role for stationary confinement of the plasma column in a tokamak. With a simple nonlinear particle balance model we showed that under certain conditions the edge plasma density depends approximately quadratically on the central plasma density, in accord with experimental observation [3, 41.This model is extended here to the case of a highly localized recycling zone (e.g. the limiter edge), and various limiting conditions to be imposed on the central plasma density are derived. These compare favourably with experimentally observed "critical" situations, such as the sharp drop of the power-density flus incident on the limiter edge in tokamak T-10 for central plama densities >6.lO19 m-3 [7]. It is concluded that (i) the response of the plasma core t o a spatially localized recycling zone may be an interesting (ii) ordinary limiters can be protected above a certain density threshold by the self-shielding effect that deserves further study, action of the locally recycling plasma. I n h a l t s u b e r s i c h t . Das Recycling von Neutralteilchen spielt eine wichtige Rolle bei der stationaren Halterung des Plasmas in einem Tokamak. I m Rahmen eines einfachen nichtlinearen Modells der Teilchenbilanz haben wir gezeigt, daB unter bestimmten Bedingungen die Dichte des Randplasmas etwa quadratisch von der zentralen Plasmadichte abhlngt, in Ubereinstimmung mit experimentellen Beobachtungen [3,4]. Dieses Modell wird hier erweitert auf den Fall einer stark lokalisierten Recycling-Zone (2. B. die Limiterkante) und verschiedene sich daraus ergebende Grenzbedingnngen fur die zentrale Plasmadichte werden abgeleitet.Der Vergleich mit experimentell beobachteten ,,kritischen" Situationen, wie dem scharfen Abfall der die Limiterkante belastenden EnergiefluBdichte am Tokamak T-10 fur zentrale Plasmadichten >5 . 1019 m-3 171. ergibt gute Ubereinstinimung. Die SchluBfolgerungen sind:1. Die Reaktion des Zentralplasmas auf eine raumlich lokalisierte Recycling-Zone konnte ein interessanter Effekt sein, der naher untersucht nerden sollte.' 3. Gewohnliche Limiter konnen oberhalb eines Dichteschwellmertes infolge des lokalen Recyclings vom heil3en Plasma abgeschirmt werden.