1 - Nuclear Sciences and Applications - IAEA

584 NOTERDAEME et al.
in the start-up phase of the ICRH, made possible long pulses with ICRH alone at high power. These
experiments allowed the investigation of the plasma evolution on previously inaccessible time-scales. For the
first time, ICRH in the second harmonic hydrogen regime (2 Wcn) and repetitive pellet refueling were
successfully combined. The density profiles became peaked (ne(0)/ = 1.75) and the confinement time
increased from 50 to 70 ms. H° and D° neutral beam healing were added to second harmonic heating. D° neutral
injection occasionally resulted in large density increases, with only slight confinement lime improvement.
Minority heating with D° injection produced similar density increases, but with peaked profiles and
confinement time improvement. H modes lasting 0.5 s were also obtained with D° neutrdl injection and D(H)
minority heating. The ICRH power triggered the transition.
1. INTRODUCTION
Between April 1986 and April 1987, ASDEX was modified [1] to allow long pulse
additional heating. The ICRH system was also upgraded : the generators can deliver Pgcn = 2 x
2 MW [2], the two low field side antennas are now water cooled [3], and the voltage stand-off
of transmission line components has been improved [4].
The actual operating space has to date been extended from Pgen = 2.5 MW [5] to 3 MW
for 0.5 s and, at Pgen = 2MW, from a pulse length of 1 s [5] to presently 2 s. The coupled
power (PIC) is estimated to be 0.85 x P Experiments were performed at the second
harmonic frequency of hydrogen (67 MHz) in various H/D mixtures, with and without H° or
DO neutral injection and at the fundamental frequency (33.5 MHz) in D+ plasmas (npj/np = 10
%) with D° injection. The vessel wall was carbonized and the parameters were: R = 1.67 m, a
= 0.4 m, Ip = 380 kA, Bt (at 1.65 m ) = 2.42 T.
In mis paper we report a novel scenario used to couple high power at 2 cH for long
pulses, and for the first time results obtained in combination with repetitive pellet refueling.
The confinement in the second harmonic and minority heating regimes is analysed. Finally H
mode transitions obtained with ICRH + NI are discussed.
2. BRIEF APPLICATION OF NI AT THE BEGINNING OF THE ICRH PULSE
The turn-on of the ICRH is usually accompanied by a density increase, which leads, at
high power, to a disruption. The effect can be partly alleviated by operation of ICRH in
combination with NI. At a given ICRH power, the density increase is smaller with NI, and
the maximum RF power that can be coupled increases. However, combination with NI makes
ICRH specific properties stand out less clearly. Recent experiments have shown that only a
short NI pulse is required at the beginning of the ICRH pulse. At the turn-off of the Nl-pulse,
the density still increases somewhat, but this density increase (also present without ICRH) is
independent of the ICRH power. The reason for the favourable influence of the NI on the
ICRH is still unclear although a number of explanations have been put forward [6-9].
A further interesting observation [1] was that, after the NI is turned off, the antenna resistance
and the reflected power change slowly on a long (Is) time-scale. This could be due to a slowly
changing isotope concentration (the decrease in resistance, with increasing H concentration,
would then indicate that eigenmodes play a role). However, a slow change of boundary
density gradients cannot be excluded.
3. RF HEATING AND PELLET REFUELING
The long ICRH pulse, described above, that can be applied to an OH plasma after an
initial NI pulse, was used to combine ICRH (PjC =1.4 MW; the absorbed power, for ICRH
alone, is about 0.6 of PjC ) and pellet refueling (up to 15 pellets, every 67 ms, velocity 600
m/s, mass = 1 x 10 20 D atoms). In contrast to other attempts to heat a pellet refueled plasma,
we started pellet injection during the RF pulse. The resonance layer was located at r = 15 cm
(with q = 1 at r = 13 cm and q = 2 at about r = 22 cm). Figure 1 presents some of the
experimental traces on which two phases are clearly observed.