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Pellet enhanced performance (PEP) has been observed in a number of JET discharges at various plasma conditions, in both L and H modes, with the H multiplier (the confinement enhancement factor over the Goldston confinement time) covering the range from 1 to 4, and with plasma currents from 1 MA to 4.1 MA. Most of the PEP plasmas have been created by refuelling with pellets of 4 mm diameter injected at 1.2 km/s. PEPs show an improved central confinement with an effective heat conductivity reduced by factors of approximately 2-5 relative to otherwise comparable discharges. This is possibly related to the inverted shear in the plasma core due to the large local bootstrap current density. The limitations in the PEP performance seem to be set by at least two mechanisms: impurity behaviour, MHD activity or a combination of both. In certain discharges, MHD modes seem to be able to check the often observed impurity accumulation. Too much MHD mode activity, however, easily destroys the enhanced confinement of the PEP discharge. The stability of the ballooning modes has been studied and the PEP plasma core is found to be in the second stability region against ballooning modes or close to marginal stability. In a number of discharges complex high (m,n) modes have been observed with the soft X-ray cameras. The behaviour of the low (m,n) MHD modes can only be understood by considering the detailed evolution of the inverted q profile, which exists in a given discharge

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A comparatively fast, time-dependent code for the interpretation of neutron emission rates of neutral-beam-heated tokamak plasmas which is based on a Fokker-Planck model for injected ions is presented. The code includes a package for calculating line-integrated timedependent neuuon spectra which can be readily compared with measured spectra. Timedependent interpretation calculations of dilution ratios noln,, for several JET discharges which cover a wide range of plasma conditions are presented. The results for the inferred dilution ratios are shown to be in good agreement with the experimental dah. By using the measured and calculated neutron spectra the deuteron temperaNre can be inferred. Combining the calculations for the deuteron tempentures and the dilution ratios, aconsistent set of these plasma parameters cm be obtained. Results from this method are confirmed by data ofindependent measurements.

Non-thermal He II spectra for discharges with helium beam fuelling are analysed. Simulated spectra are used to study the effects of plasma temperature, plasma density and Z eff on observed charge-exchange (CX) spectra. Differences in modelling the non-thermal velocity distribution function with a numerical Fokker-Planck code or alternatively using analytical expressions are investigated. The intensities and spectral shapes of both active, localized CX spectra and competing, non-localized, passive electron-impact excitation components are simulated and compared with observations. The 'plume' contributions of electron-impact excited He +, * particles are found to be quite appreciable and uncertainties in the plume calculation lead to non-negligible errors in the extraction of the active signal from the total spectrum. However, for experimental conditions with magnetic field configurations minimizing the plume effect good agreement can be found between fast-particle densities derived from the numerical calculations and the experimental observations. Significant problems in deriving absolute He 2+ densities are encountered when a helium beam also acts as a CX diagnostic beam. For the case of dominant passive emission components, simulated fast spectral intensities for the core lines of sight agree within a factor of two with experimental data.