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

magnetic confinement (cont’d.)
progress report
2010
017
Experimental observation of beta–induced Alfvén Eigenmodes.
FTU activities in 2010 were focused on the observation of long
(>200 ms) beta–induced Alfvén eigenmodes (BAEs), which are
high frequency oscillations (30–70 kHz) in tokamak plasmas,
identified in the spectrum of the Alfvénic modes as discrete
eigenmodes located in the low frequency gap of the Alfvén
continuum produced by the geodesic curvature and finite beta
effect. BAEs have been first identified as waves excited by
circulating fast ions, however they were also observed in ohmic
plasmas with the simultaneous presence of large magnetic islands
[1.8]. BAE oscillations from 30 to 50 KHz have been observed in
FTU plasmas with the simultaneous occurrence of tearing modes
(characterized by frequencies from 1 to 5 KHz). BAEs have
intensities two order of magnitude less than tearing modes, and
are characterized by two main lines, which merge in a single line
when the island oscillation frequency becomes very low. Mode
analysis for tearing mode shows that poloidal and toroidal mode
numbers are (m, n)=(–2,–1). On the other side, the higher BAE
frequency is characterized by (m, n)=(–2,–1), i.e., propagates in
the same direction as the island, while the lower BAE frequency
has (m, n)=(2,1), i.e., propagates in the opposite direction. The
difference in frequency of the two BAE lines is exactly twice the
fundamental frequency of the tearing mode, and the frequency
difference can be explained by the doppler shift due to island
rotation.
In the following figure 1.11 the comparison between the
theoretical prediction [1.9] of BAE frequency as a function of
magnetic island amplitude δB r
/B p
and the experimental
observations is shown. During the first part of the island growth,
BAE frequency increases roughly linearly, in good agreement
with theoretical predictions; on the other hand, a discrepancy is
found for δB r
/B p
>2 ×10 –3 where BAE frequency remains rather
constant. Therefore, we can state that, as expected, the
perturbative theory used gives consistent results only for
sufficiently low magnetic island amplitudes.
BAE frequency (kHZ)
55
45
Observed frequency (N 23184)
Observed frequency (N 26644)
Observed frequency (N 25877)
Predicted frequency (N 23184)
Predicted frequency (N 26644)
Predicted frequency (N 25877)
35
0.5 1.5 2.5 3.5 4.5 ×10 -3
δBr(rs)/Bp(rs)
Figure 1.11 – BAE frequency as a function of
magnetic island amplitude δB r /B p (curves
are the theoretical predictions and markers
are the experimental points)
(V)
T e (keV)
n/s
Counts
Energy (MeV)
1.5
0.5
0.5
0
4
2
0
10 11
10 10
10 3
10 2
8
4
Experimental V I
Collisional threshold
NE213
BF 3
a)
b)
c)
Total γ - counts
#22472
0
0 0.4 0.8 1.2
Time (s)
d)
e)
Electron cyclotron
Runaway electrons in FTU. Runaway electrons constitute a
well–known phenomenon in tokamaks: due to the decrease in the
Coulomb collision frequency with energy, electrons with energy
larger than some critical value are continuously accelerated by
the toroidal electric field. Calculations including relativistic effects
indicate that below a critical or threshold electric field,
E R
=(n e
e 3 lnΛ)/(4πε 0
m e
c 2 ) absolutely no runaway electrons are
produced [1.10]. Experiments carried out in FTU show that
Figure 1.12 – Runaway suppression in
discharge 22472: a) loop voltage; b) electron
temperature (the ECRH time window is
represented by the shaded area); c)
comparison between neutrons (BF 3 ) and
gamma & neutron detectors (NE213): when
the two signals coincide runaways are
suppressed; d) total counts and e) maximum
measured energy from γ–ray NaI
spectrometer
synchrotron radiation losses should also be taken into account, since they lead to an increase in the critical
electric field. These results are important when making predictions on runaway generation and mitigation
during disruptions in next–step devices such as ITER.
Suppression of runaway electrons has been achieved in FTU (fig. 1.12) by application of ECRH) during the
flat–top phase of the discharge, at electric field values substantially larger (by a factor ∼2) than the predicted
collisional threshold field, E R
∼0.09 n e
(10 20 m –3 ). The parameters used in the experiments are the following:
I p
=0.3–0.5 MA, B t
=4–6 T, central line averaged density n e
=4–8×10 19 m –3 and electron cyclotron (140 GHz)
input power P ECRH
= 0.3–1 MW. The interpretation of these observations, based on calculations including the
electron synchrotron (radiation) term (fig 1.13) leads to the conclusion that the radiation losses are responsible