Prime pagine RA2010FUS:Copia di Layout 1 - ENEA - Fusione

040
progress report
2010
2
Pulse # 73744
a)
6
S(μW/sr nm) Ipla (MA)
1
0
0.4
0.2
0
0
4
Time (s)
Figure 1.49 – After a disruption, in the absence of plasma, the
extra light seen by the Thomson scattering detectors is thought
to be due to the interaction between the laser pulses and dust
particles later released by the wall. a) plasma current, b)
average dust signal of the 4th spectral channel of every core
spectrometer
b)
8
P thr (MW)
4
2
0
0
H e conc scan shots
P thr, scal 08
D references
40 80
H e concentration (%)
Figure 1.50 – Power threshold (P thr ) obtained with NBI
as function of helium concentration for both He and D
reference discharges at 1.7 MA/1.8 T. All the
configurations had the same low δ≈0.25 shape, and
densities variations 2.3–2.8×10 19 m –3
Dust
Radiation has been observed by the high resolution Thomson scattering (HRTS) system after plasma
terminations, which reveals the presence of impurities along the beam path released by the wall during the
disruptive power quench, known as dust [1.92]. This radiation has been carefully analyzed and, due to its
spectral features, seems not to be due to pure scattering but to a more complicated laser–matter interaction.
The intensity is compatible with the expected dimension of dust grains and the temporal decay shows two
timescales (fig. 1.49). Averaging the values of the peaks in the signal traces over all spectrometers, one can have
a rough estimate of the dust content. The dust lasts for about 1.5 seconds after the disruption. A preliminary
distribution of the signals has been obtained and compared with a compound Poisson distribution.
L to H mode transition
Data taken during the 2009 JET helium campaign have been fully analyzed and presented at the 2010 EPS
Conference [1.93]. L–H in helium has been observed as a transition to Type III ELMs with a small
confinement improvement associated with a small edge pedestal. Helium concentration varied from 1 to 87%
and was found to have little impact on the power threshold (fig. 1.50). This is in line with recent ASDEX–U
studies, but in contrast with JET 2001 results, which showed that He plasmas had a 40% higher threshold than
D equivalents. However, it should be noted that the 2001 comparison was performed at lower density
(∼2.0×10 19 m –3 ) than the more recent data base (∼2.3–2.8×10 19 m –3 ). If a slope inversion of the density
dependence were at play, the 2009 and the 2001 results might be reconciled. Unfortunately, helium data at low
density are not available in the 2009 database. Although physics understanding still remains to be improved
and further experiments will have to be carried out, recent results seem to encourage the helium choice for the
non–nuclear phase of ITER.
Safety factor profile determination
Profiles of the safety factor are determined by the motional stark effect (MSE) diagnostic combined with
equilibrium reconstruction. The positions of low–order rational surfaces q=m/n (from 2/1 to 4/3, m and n
being the poloidal and toroidal mode numbers, respectively) have been systematically compared with the ones
inferred from the frequency of MHD modes. The connection between mode frequency and rational surface
location depends on the velocity of the magnetic island produced by the mode at the corresponding rational
surface. A previous assumption of island propagation at the toroidal velocity of the carbon impurity resulted
to be inconsistent. Much better agreement was found by assuming that the island rotates with the deuterium
ions fluid; the latter was calculated by radial force balance from carbon toroidal rotation and ion temperature
profiles neglecting poloidal rotation. If neoclassical carbon poloidal rotation is included in the force balance,