1 - Nuclear Sciences and Applications - IAEA

52 JET TEAM
fl 4
2 3 4
Average density(n)(x10"rrr 3 )
C* H-mode
C- L-mode
Zeff=(2+2/n)
Fig. 10 Effective ion charge Zeff versus average density for JET discharges. H-modes are identified
separately. Line is the function Zp//=2 + 2/«l9 used later in predicting performance.
Impurity accumulation is seen in some H-modes [9] . Accumulation is also seen
in peaked density profile, pellet produced discharges. Curiously, accumulation
is not seen in monster sawtooth cases despite the absence of sawteeth. Normal
limiter bounded discharges with sawteeth do not show accumulation.
Radiation profiles are generally hollow so that radiation does not play any
significant role in the energy balance in the plasma interior. The primary effect
of impurities is to deplete the density of hydrogen isotopes in the core. The ratio
of deuteron density to electron density is reduced by the presence of impurity
ions with charge Z, by, nc/n^iZi-Z^/iZ-X). With Z;=7, intermediate
between carbon and oxygen, and with Ze//between 2 and 6, nD/ne is in the range
0.8-0.15.
This range of values is confirmed by neutron yield measurements. The highest
values of nD/ne are for pellet-fuelled cases, intermediate are H-modes and lowest
are low density, high temperature limiter discharges.
6. Bootstrap Current
A significant bootstrap current driven by the pressure gradient" 0 ' 201 is seen most
clearly in H-mode discharges. Fig. 11 shows the time-dependence of the surface
voltage which transiently reverses sign. The calculated voltages with and without
the bootstrap current are shown. Only the case including the effect gives sign
reversal and agreement with the experiment. The TRANSP code is used which