Externally applied magnetic fields have been used to induce major disruptions in the Tokoloshe tokamak. The investigations have included ‘resonant’ and ‘non-resonant’ external field configurations, using any one of three different stellarator-like windings with ℓ = 1, 2 and 3. In cylindrical geometry, these coils produce magnetic islands with dominant modes m/n = 1/1, 2/1 and 3/1, respectively. In the case where the Mirnov oscillations in the plasma are weak, i.e. where the existing tearing mode islands are much smaller than the islands of the same helicity produced by external coils, a major disruption of the plasma occurs when the coil current exceeds a certain critical value. The time to disruption τd decreases with increasing coil current, consistent with a simple model in which a critical perturbing radial field must be exceeded for each coil to induce a disruption. The characteristic time to reach this critical field is similar for each of the three coils and is approximately the time needed for resistive field penetration to the q = 2 surface. No clear precursor is seen until a rapidly growing locked m = 2 mode becomes significant just before disruption. Simultaneous measurements of B̃θ and B̃r show that this is a tearing-like mode, with the locking orientation dependent on coil current direction. The growth time, τg = 200 μs, is essentially independent of τd, with τg << τd, typically. Comparisons with magnetic field geometry calculations show that the observed disruption limits are consistent with tearing mode destabilization precipitated by, and phase locked with, overlapping or nearly overlapping 2/1 and 3/1 coil islands, where the latter island also overlaps the limiter. This holds for each of the coils and for both ‘resonant’ and ‘non-resonant’ cases. The experimental results are also compared with the case of strong Mirnov oscillations. Here the Mirnov signal can be suppressed when the coil current is sufficiently high. This is clearly due to mode locking (∂/∂t → 0) rather than to mode suppression (B̃θ → 0).
A small colony (1 00-000) of Little Red Flying-foxes Pleropus scapulatusis usually resident at Mataranka Hot Springs in the semi-arid zone of the Northern Territory of Australia between October and February. In 1994-95 this changed markedly. More than 2W 000 individuals were present and most did not depart until July. The colony generated a powerful smell and caused significant damage to the vegetation surrounding the Spring, resulting in conflict with tourist use of the area. Many methods were em~loved in anemDts to shin animals. but none was successful. The situation is an extreme examp eof ine pro0 ems caused oy Aussa Ian Preropus ana ntgn lgnts a genera ack 01 ntormal on an0 enect~ve management tecnnlq~es lor the genus an0 ds nab~tats nforrnahon on tne sex rat0 and oreealng conolt on of ternares at tne colony 1 s presented
Some properties of perpendicular collisionless shocks are investigated, using a model in which the ion orbits in the shock are assumed to be determined by the average electric and magnetic fields in the shock. These fields are modelled, with the jump in magnetic field across the shock being determined by the conservation relations, and the potential jump determined self-consistently within the model, using the fact that the mean ion velocity downstream of the shock is determined by the conservation relations. Extensive numerical calculations of ion orbits show that effective ion heating can occur in the absence of any dissipative process, with the energy residing in non-Maxwellian velocity distributions in the downstream regions. Results on this and on a number of other features of shock waves, agree well with experiments.
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