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

038
progress report
2010
non–integrability, due to the presence of two or more degrees of freedom, may lead to Hamiltonian chaos.
These aspects have been studied by producing phase portrait plots, Poincare maps, and computing numerically
the Lyapounov exponents, thus elucidating some interesting features of the LH propagation in the external
plasma.
The propagation and absorption of lower hybrid waves in H–mode plasma with very sharp
density gradient at the pedestal
Flat density profiles in tokamak plasmas are characterized by a density rise at the plasma periphery, which may
prevent LH waves from penetrating deeply into the center. The reasons may be the deviation of the ray
trajectories toward the plasma edge or the lack of validity of the linear optics approximation (WKB). In this
work the wave equations has been solved in full wave limit, in order to analyze whether outward reflections
take place. The results indicate that, unless the density rise is a very sharp jump, the reflection is negligible and
the linear WKB wave equations are a valid tool to describe the wave ray tracing. These trajectories result to
be deviated across the density rise, but their deviation depends on the central density value and scarcely on the
edge gradient strength.
Tritium minority heating with mode conversion of fast waves
A new ion–heating scenario in tokamak plasmas is proposed, based on cyclotron damping of IBWs by tritium
minority at the first ion cyclotron harmonic (i.e. ω=2Ω cT
). The IBWs are coupled by mode conversion of fast
magneto–sonic waves (FMW) in a D–H(T) (tritium minority in hydrogen-deuterium) plasma. The mode
conversion layer is located near the centre of the plasma column as well as the resonant layer of the tritium
minority. A possible scenario for the JET tokamak [1.84], based on the present idea, has been analyzed by
means of the numerical codes toroidal ion cyclotron (TORIC) & steady state Fokker Planck quasi linear
(SSFPQL) [1.85,1.86]. As a result, Tritium ions are accelerated up to energies close to the peak value of the
DT cross–section and a significant increase in Q has been found [1.87].
∼
ν z
2000
2.0
1000
0
u
0
1.0
2
4
x
6
8 0.0
Figure 1.46 – Behavior of the vertical velocity ν ∼ z as a
function of the dimensionless radial x and vertical u
coordinates
Stellar winds or matter–jet seeds from disk
plasma configuration
The profile of a thin disk configuration was studied
[1.88] as described by an axisymmetric ideal MHD
steady equilibrium (in collaboration with the University
of Rome “La Sapienza” and ICRANet). The disk was
considered as a differentially rotating system dominated
by the Keplerian term, but allowing for a non–zero radial
and vertical matter flux. In this scheme, the steady state
allows for the existence of local peaks for the vertical
velocity ν ∼ z
of the plasma particles (fig. 1.46), though it
prevents the radial matter accretion rate from taking
place. This ideal MHD scheme is therefore unable to
solve the angular momentum–transport problem, but it is
suggested that it provides for a mechanism for the
generation of stellar winds or matter–jet seeds. A
visco–resistive scenario has also been set up [1.89], which generalizes previous two–dimensional analyses by
reconciling the ideal MHD coupling of the vertical and the radial equilibria within the disk with the standard
mechanism of the angular momentum transport, relying on dissipative properties of the plasma configuration.
1.4 JET Collaboration
In October 2009, JET operations were suspended to start the installation of the new ITER like wall (ILW)
using beryllium and tungsten as plasma facing components (PFC). The related shut–down is still continuing
and the first plasma is now foreseen for the beginning of August 2011.