1 - Nuclear Sciences and Applications - IAEA

IAEA-CN-50/A-VII-l 373
4. SIMULATION OF JET RESULTS WITH PREDICTIVE TRANSPORT CODES
We shall present results obtained with the two transport models
proposed at JET with reference to results from other models. The
RL model [7,12] produces confinement degradation when VTg exceeds a
critical gradient. In the PT model [12,16] confinement degradation
is due to the non-linear dependence of the heat fluxes on the
pressure gradient. In both models Xi = Xe is assumed.
Uncertainties (± 40%) remain in the values of the numerical
constant in the expressions of xe [7,12], determined empirically
and kept fixed for all simulations. Notice that the corresponding
variations of the values of xe> Xi and TR are lower (= ± 20%), as a
result of the non-linear dependence of the models on gradients of
the temperature profiles.
Results of Computer Simulations
Satisfactory results with both models have been obtained when
simulating ohmic discharges, L-mode discharges, "monster" sawteeth
and ICRH off-axis heating cases. They produce resilient Te
profiles in agreement with experiments, without needing ad hoc
modifications, as is the case for models based on drift wave
microinstabilities. The time evolution of global quantities and
temperature profiles following any modification of the input power
is computed correctly by these models. On the other hand, the
"profile consistency" (PC) model in [14] causes a delayed plasma
response especially when power is suddenly switched off. Such a
delay is due to the dependence xe "= J P net e x dx in the PC model,
P being the net power input profile to the electrons. It is
expected that other PC-related models involving a similar
dependence of xe (and possibly x±) on tne power deposition [15],
will exhibit similar shortcomings, while being satisfactory for
quasi-steady-state conditions.
The main uncertainties with both RL and PT models refer to the
plasma core, and show up when attempting to simulate the central
temperature evolution during sawteeth, including "monsters". It is
difficult, however, to establish whether these uncertainties
reflect an intrinsic deficiency of the models or are related to an
inadequate evaluation of quantities such as local shear, nonthermal
component of Vp, local power deposition. Similar problems
appear with most theoretical transport models [13].
As a more demanding test for the models, we performed complete
simulations of H-mode discharges at I = 2,3 and 4 MA, starting from
the ohmic phase. Notice that JET H-modes are non-stationary. The
time evolution of ne has been input from the experimental data
base, while the formation of an edge thermal barrier during the
H-mode has been simulated by transiently increasing the boundary
temperatures Tea, in agreement with a systematic trend in