1. magnetic confinement - ENEA - Fusione

1. MAGNETIC CONFINEMENT
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1.1.1 Introduction
1.1 Tokamak Physics
The main goal of the Frascati Tokamak Upgrade (FTU) scientific programme is to
investigate transport, stability and radiofrequency physics issues at ITER-like
plasma densities and magnetic field values.
In 2001, the FTU lower hybrid system came close to delivering its maximum allowable
(~2.4 MW) power. Up to 2.2 MW were injected into the plasma, with a level of 2.0 MW
routinely achieved. During the 2001 experimental campaign it was, therefore, possible
to study both current-drive and internal transport-barrier formation in high-density
scenarios. In fact, collisional ion heating was observed at densities around 10 20 m -3 .
Electron cyclotron resonance heating was used to explore heat transport and was
combined with the injection of lower hybrid waves for synergy studies. Modulated
electron cyclotron heating was also applied for the transport studies, which focussed on
the issue of profile stiffness and the specific role of collisionality. As proposed at the 2001
Frascati workshop (see below) on the FTU programme, a campaign is being planned to
compare FTU results with those of other devices, such as ASDEX-Upgrade and Tore
Supra.
Steady pellet enhanced performance modes were extensively studied. Careful timing
of the pellet sequence allowed a high degree of reproducibility to be achieved for
these high-performance discharges. The experiments gave interesting information
about the role of sawteeth with respect to impurity accumulation. A compromise
seems possible, where the sawteeth are slowed down enough to achieve improved
confinement, but can still prevent impurity accumulation and also produce an
outward pinch that reduces central radiation. Results from preliminary experiments
performed with pellets plus lower hybrid indicated that good radiofrequency
coupling is possible at high field and density.
Installation of the new boronisation system allowed a significant decrease in
impurity contamination and radiated fraction. Consequently, it was possible to start
studies on the radiative improved mode in the last part of the 2001 campaign.
The new diagnostic for fast x-ray imaging, developed at Frascati, was installed on the
National Spherical Tokamak Experiment. The device is based on a micro-pattern gas
detector with a gas electron multiplier amplifier.
A workshop was held at Frascati on 22-23 November 2001 to discuss the mediumterm
FTU programme and to increase the participation of European laboratories in
the machine. The workshop was organised in plenary and parallel brainstorming
sessions, with each session chaired by an external participant. Forty people were
from ENEA Frascati and the Consiglio Nazionale di Ricerca Milan and about thirty
came from other labs (European, USA, Japanese and Russian). The discussions
focused on areas where FTU can provide unique results. Several interesting
proposals were made and are presently being considered for implementation.
The contribution of ENEA to the C4 Joint European Torus campaign in 2001 amounted to
about 1 ppy and was focussed on the activities of Task Forces S2 and H. Plasma
configurations with internal transport barriers of long duration (11s≈30τ E ≈τ R , with τ E the
energy confinement time and τ R the resistive diffusion time) were produced, thanks to the
current profile control now possible with the lower hybrid system.
1.1.2 Experimental results
Lower hybrid current drive and heating experiments at high density
The lower hybrid (LH) radiofrequency (rf) system in FTU (six gyrotrons,