1 - Nuclear Sciences and Applications - IAEA

588 NOTERDAEME et al.
with the time at which the sawteeth start to lock to the pellet period. There are also similarities
to transitions from the SOC (saturated ohmic confinement) to the IOC (improved ohmic
confinement) regime [13]. The transition could be consistently observed. The liming of the
transition is related to the programming of the gas valve. An earlier closing of the gas valve
leads to an earlier transition.
4. HEATING AND CONFINEMENT
4.1. Second harmonic heating
4.1.1. Maximum heating power
The maximum coupled RF power (PJC = 2.5 MW, 0.5s) was reached in combination
with 2.05 MW H° injection leading to a total deposited heating power of 3.8 MW, almost
equally divided between the two heating methods. Table I.a. compares plasma parameters of
these L mode discharges without and with RF during stationary periods of 0.4 s duration.
The profiles are similar in both phases and the confinement agrees with results
previously reported [5], showing that the new divertor does not affect the confinement of
ICRH L-discharges. The neutron rate increases by a factor of 4 with RF, of which a factor of
1.7 is due to the density increase. When the RF is applied the plasma rotation decreases by
30%. The increase of Zeff with RF is almost equally divided between oxygen, carbon and
iron. The concentrations or iron and titanium increase by a factor 2 and 5-10 respectively (the
antenna Faraday screen is coated with TiC and the side protection plates of one antenna are
made of titanium).
4.1.2. Comparison of harmonic heating combined with Ho and Do injection
We compared ICRH combined with H° and D° injection fired successively in the same
discharge and separated by a time gap of 100 ms. In a first series (SI), an injection power
scan (H° and D°) was performed, and the two phases are labelled S1H° and S1D°
respectively. Unusual and reproducible density behaviour was observed in S1D°, as illustrated
in Fig. 3.a. It must be noted that the density decreases in the middle of the stationary phase,
before the heating ends and without any external change. Only ten ohmic D+ shots later and
with the same machine parameters, we made an extended RF power scan S2 (Pic= 0 to 2 MW)
with PHo=0.85 MW (S2H) and PDo=l.l MW (S2DO), in which, however, the density
remained constant (Fig. 3.b). The results of S1 and S2 are compared in Table I.b. Although
the density and temperature peaking factors are very similar, the S1D° density profiles are
slightly more triangular in the centre and steeper at the plasma edge, which may be related to
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FIG. 3. Time evolution of the line averaged density (he), loop voltage (VJ, and plasma energy given
by the diamagnetic loop (WJ for Prf = 0.85 MW, PDo = 1.1 MW and P,c = 1.4 MW. (a) Series
SI: density increase in phase SID 0 ; (b) series S2: constant density.
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