A potential buildup in front of a magnetized cascaded arc hydrogen plasma source is explored via E x B rotation and plate potential measurements. Plasma rotation approaches thermal speeds with maximum velocities of 10 km/s. The diagnostic for plasma rotation is optical emission spectroscopy on the Balmer-beta line. Asymmetric spectra are observed. A detailed consideration is given on the interpretation of such spectra with a two distribution model. This consideration includes radial dependence of emission determined by Abel inversion of the lateral intensity profile. Spectrum analysis is performed considering Doppler shift, Doppler broadening, Stark broadening, and Stark splitting.
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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
The B2.5-Eunomia code is used to simulate the plasma and neutral species in and around a Pilot-PSI plasma beam. B2.5, part of the SOLPS5.0 code package, is a multi-fluid plasma code for the scrape-off layer. Eunomia is a newly developed non-linear Monte Carlo transport code that solves the neutral equilibrium, given a background plasma. Eunomia is developed to simulate the relevant neutral species in Pilot-PSI and Magnum-PSI, linear devices that study plasma surface interactions in conditions expected in the ITER divertor. Results show the influence of the neutral species on the Pilot-PSI plasma beam. We show that a fluid description for the neutrals is not sufficient and Eunomia is needed to describe Pilot-PSI. The treatment of individual vibrational states of molecular hydrogen as separate species is crucial to match the experiment.
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