In the Joint European Torus the ablation of injected pellets produces a striking resonance effect when the pellets reach surfaces with q values 1 and j. Subsequently, structures with mode numbers m = l,/i = 1 and m =3,« = 2 are observed with the soft-x-ray cameras for up to 2 s as compact snakelike perturbations. These structures, which persist through several sawtooth collapses, give information on the radii of the q = 1 and q = j surfaces and the ^-profile evolution. The observations can be explained by the formation of magnetic islands.

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Mcasurements from the JET ncutron profile monitor BTC analyzed tomographically to deduce thc 2-D spsatial distribution of ncutron emissivity during NBI heating both beforc and after sawtooth crashcs. In a discharge where the global neutron emission decreases slightly i o 5/6 ofihe pre-crash rate of nearly ION6 n s -I , the iixial emissivity drops much more IO only 1/6 of ihe pre-crash value. The sources of neutron emissivity arc analyzcd with an analytic Fokkrr-Planck ionnulalion. The main change in neutron emissivity is due I o fast ion rcdistribution duringa sawtooth crash and a decrease in beam-beam-produced emissivity. The deduced Cast ion redislrihulion is consistent with uthcrobservaiions uf sawtooth behaviour in JET.

The axial wavelengths and azimuthal mode structures of a cylindrical helicon plasma source have been investigated over a broad range of input conditions (pressure 1-16 µbar, field 0-450 G, rf power 0-2500 W @ 13.56 MHz) in an argon plasma. Several distinct modes of operation, separated by discontinuous jumps, have been identified: an electrostatic mode, an m = 0 wave mode and three m = 1 helicon wave modes. Two different types of mode jump have also been identified. A cavity mode transition has been identified in which a mode jump occurs when the wavelength of standing waves governed by the length of the plasma vessel leads to a node in the wave field at the centre of the antenna. A radial mode transition has also been identified in which the favourable condition that the axial wavelength should be equal to twice the length of the antenna for efficient coupling of the rf power is maintained at high powers by the plasma making a transition from the first to the second radial mode. A procedure based on these observations is suggested for predicting where mode transitions are likely to occur within the parameter space of a helicon source.

Giant edge localized modes (ELMs) play an important role in the transport of plasma at the edge of tokamaks with a divertor. The loss of plasma, by mechanisms that are not well understood, causes very high power loading on parts of the wall structure. The flows of particles and energy have been determined on a rapid time-scale for the first time in JET. The soft X ray measurements have a complex structure and show a rapid onset of the instability. They prove that some plasma is lost outside the divertor and that recycling occurs to well within the separatrix. The implications of the results for the ELM instability mechanism and the design of tokamaks are discussed.

An experimental and theoretical investigation of a Helicon plasma source has been completed as a precursor to the development of an automatic system for controlling the operation of such sources. The number density in an argon plasma has been observed over a parameter space defined by the input variables of filling pressure (1–100 μbar), magnetic field (0–450 G) and rf power (0–1250 W). Four distinct modes of operation have been identified, namely: electrostatic, inductive, Helicon and high pressure Helicon modes. A global model of the source has been modified to include the effect of radial confinement by the axial field. Comparisons with the experimental results show that the model generally can predict the plasma density to within 15%–20% for both the inductive and Helicon modes. It is found that the ratio of the ion cyclotron radius compared with the radius of the plasma is a useful measure of the amount of confinement. An alternative measure based on the product of the ion cyclotron frequency and the collision time for momentum transfer does not lead to satisfactory agreement between the global model and the experimental observations. The modified global model appears to describe the behavior of the plasma well enough to allow the development of a conventional control system that can be used within each of the modes. The model, however, is unable to predict the locations of the mode jumps within the parameter space.

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