Neutron yield studies in JET H-modes

The measured D-D neutron rate of NBI heated JET baseline and hybrid H-modes in Deuterium is found to be between approximately 50% and 100% of the neutron rate expected from the TRANSP code, depending on plasma parameters. A number of candidate explanations, such as fuel dilution, errors in beam penetration and effectively available beam power have been excluded. As the neutron rate in JET is dominated by beam-plasma interactions, the 'neutron deficit' may be caused by a yet unidentified form of fast particle redistribution. Modelling, which assumes fast particle transport to be responsible for the deficit, indicates that such redistribution would have to happen at time scales faster than the slowing down time and the energy confinement time. Sawteeth and ELMs are found to make no significant contribution to the deficit. There is also no obvious correlation with MHD activity measured using magnetic probes at the tokamak vessel walls. Modelling of fast particle orbits in the 3D fields of NTM's shows that realistically sized islands can contribute only a few % to the deficit

Density profile peaking in JET H-mode plasmas: experiments versus linear gyrokinetic predictions

As an independent complement to previous studies (Weisen et al 2005 Nucl. Fusion 45 L1-4, Weisen et al 2006 Plasma Phys. Control. Fusion 48 A457-66, Angioni et al 2007 Nucl. Fusion 47 1326-35), density peaking in the JET tokamak was investigated on the dataset, comprising virtually all H-mode experiments performed in 2006-2007. Unlike previous studies, this work focuses on low collisionality data as most representative of reactor conditions. The study confirms that collisionality is the most important parameter governing density peaking in H-mode, followed by the NBI particle flux and/or the T-i/T-e temperature ratio. For the first time in JET a modest, albeit significant dependence of peaking on internal inductance, or magnetic shear is seen. The experimental behaviour is compared with an extensive database of linear gyrokinetic calculations using the GS2 code. The predictions from GS2 simulations based on the highest linear growth rate mode are in good agreement with experimental observations. They are also corroborated by initial results from the non-linear code GYRO

Energy Confinement in Shaped TCV Plasmas with Electron Cyclotron Heating

The effects of plasma shape on confinement and sawtooth stability are studied for positive and negative discharge triangularity and for different elongations with 1.5 MW centrally deposited ECH powe

Preliminary Confinement Studies during ECRH in TCV

Within the range of plasma shapes and plasma currents investigated, the electron confinement time, Tau_E increases with density, elongation and negative triangularity (-0.4<delta<+0.4), similar to Ohmic heating (in these low density discharges). In addition, TauEe increases with q_a up to q_a~5 after which it decreases. There is little dependence of TauEe on the heating location provided it is inside the q= I surface. As the heating location is moved outside the q=l surface, TauEe decreases. This may be the explanation of the observed decrease in TauEe at high q_a. The power-induced degradation exponent found is generally as expected: alpaha_P = -0.5

EFFECT OF LOCALISED ELECTRON CYCLOTRON HEATING ON ENERGY CONFINEMENT AND MHD IN TCV

Within the range of plasma shapes and plasma currents investigated, the electron confinement time, tau_Ee, increases with safety factor, density and negative triangularity similar to the Ohmic heating case. There is little dependence of tau_Ee on the heating location provided power deposition occurs inside the q=1 surface; as power deposition moves out of the inversion surface, tau_Ee decreases. The power-induced energy confinement degradation exponent (tau_Ee~PaP) is as usual: alpha_P ~-0.5. As a general trend, central relaxations decrease in amplitude with increasing qa, P_EC, or negative delta, in a situation where the confinement time increases