Modelling the Ohmic L-mode Ramp-down Phase of JET Hybrid Pulses Using JETTO with Bohm-Gyro-Bohm Transport
The empirical Bohm-gyro-Bohm (BgB) transport model implemented in the JETTO code is used to predictively simulate the purely Ohmic (OH), L-mode current-ramp-down phase of three JET hybrid pulses, which combine two different ramp rates with two different electron densities (at the beginning of the ramp). The modelling is discussed, namely the strategy to reduce as much as possible the number of free parameters used to benchmark the model predictions against the experimental results. Hence, by keeping the gas puffing rate as measured whilst tuning the line-araverage electron density via the recycling coefficient (which in the modelling is taken at the separatrix instead of the wall), it is shown that the BgB model reproduces well the experimental data, as far as both average quantities (plasma internal inductance, linear-average electron density and volume average electron temperature) and profiles (electron density and temperature) are concerned, with relative errors remaining mostly below 20%. The sensitiveness with respect to the recycling coefficient, the ion effective charge and the energy of neutrals entering the plasma through the separatrix are assessed, being also shown the need for a proper sawtooth model if experimental results are to be reproduced. The strong non-linear coupling in a OH plasma between density, temperature and current (essentially via interplay between the power-balance equation, Joule's heating with a temperaturedependent resistivity and the dependence of BgB transport coefficients on profile gradients) is put in evidence and analyzed in light of modelling results. It is still inferred from the modelling that the real value of the recycling coefficient at the separatrix (basically, the so-called fuelling efficiency times the actual recycling coefficient at the wall) must become close to one in the final stages of the discharges, when the gas puffing is switched off and so recycling comes to be the only source of particles. If the wall recycling remains close to one (as standard for tokamaks), this may indicate that the fuelling efficiency also approaches unity, apparently consistent with the observed fact that the plasma is pushed towards the machine wall at the end of the current ramps.