Experimental activity and highlights of 2000
The second part of the year experienced a shutdown in order
to install some new systems: boronization, the second LH and Ion Bernstein Wave
(IBW) antennas. However, the IBW antenna could not be installed because it was
not possible to obtain the vacuum window and the waveguide gold coating on time.
An important outcome of this period was the successful utilisation
of a new remote handling tool to substitute the broken tiles of the toroidal
limiter. This new remote arm is able to remove and/or install one of the toroidal
limiter sectors of an adjacent port. In other words, all the one of the twelve
toroidal limiter sectors can be replaced by merely opening four equatorial ports.
The visual inspection of the vacuum vessel, routinely performed after venting, showed four broken tiles in the toroidal limiter, located in sectors 2,8 and 12 (2 tiles). Since the disassembly procedure of the ECRH antenna located in port 12 is time consuming, the remote handling arm was located in port 1 and used to extract the toroidal limiter sector 12, following a fully remote procedure. The broken tiles were then replaced in the vacuum laboratory and one of them was sent off to the supplier for analysis.
The main highlights of the FTU experimental programme are as
Electron transport studies
Critical gradient theories have been tested on data produced with off- and on-axis Electron Cyclotron Resonance Heating (ECRH) either in the current ramp-up or in the plateau phase of the discharge, with and without pellet injection. Electron temperature profile consistency, showing stiffness or rigidity of the temperature gradients, has been well documented. Thermal profiles appear to be locally rigid with off-axis heating, similar to the results obtained in Asdex Upgrade (AUG) with a data base extending to high and low densities. In addition, rigidity disappears when heating is central, i.e. when the magnetic shear is low. In that case, some heat pinch is required in order to simulate temperature profiles. The possible role of shear in Te stiffness is. illustrated by the fact that improved confinement is achieved with ECRH in the current
ramp-up phase, thus leading to very high central electron temperatures. Central transport is similar in the high-power phase and the Ohmic phase, in a way that suggests the existence of an Internal Transport Barrier (ITB). ECRH-LHCD synergy Analysis of synergy at high magnetic field (fig. 1) (down-shifted resonance) is done by using simulation tools both for transport and wave-damping. In particular, ECRH absorption by Lower Hybrid Current Drive (LHCD) induced fast electrons (about 50%) is well reproduced. Such an analysis will allow further experiments at higher power to be planned. Preparation for the upper-shifted ECRH scenarios is also underway.
Fig. 1 - Thomson Te(r) profile at different times: OH (red), LH (0.6 MW) only (yellow), LH+ECRH (0.6+0.35 MW: green; 0.6 + 0.7 MW: blue)
Synergy analysis of plasmas where ECRH was applied
to LHCD driven plasma target with low or reversed shear at 5.3T (resonance on-axis)
has focused on transport analysis. As in electron transport studies, analysis
has indicated that the criteria for gradTe rigidity were
broken in these discharges, thus confirming the possibility of the existence
of an electron ITB.
These studies are in line with the more general definition of the existence of an ITB based on gradTe rigidity (JET, AUG, DIIID). These results are important for a theoretical model of the physics mechanisms that would allow a transition to a transport barrier to take place (effects of shear flow and magnetic shear on ITG and ETG turbulence).
Tearing mode stabilisation with ECRH
The 2.1 tearing mode can be fully suppressed in FTU with localised ECRH (fig. 2). Although tearing mode stabilisation dynamics might be different for neo-classical tearing modes (NTMs), the stabilisation procedure is the same. Therefore, one of the foci of the subsequent analysis was to study the impact of FTU data on the stabilisation of NTMs on a machine such as ITER. In particular, it has been found that radial localisation with the deposition width of the island is essential. This could have some bearing on the design of EC wave systems for NTM stabilisation in ITER. It has also been found that coupled modes can affect the stabilisation process. CW operation is sufficient for stabilising the modes, but some power saving can, in principle, be achieved through heating synchronized with island rotation. This is particularly true if the modes are blocked and the island no longer rotates. An automatic detection and tracking suppression system is being prepared to define the real advantage of using a synchronous system. Such a system is based on the identification of the absorption radius required by ECRH power modulation, Mirnov coils and ECE emission. Subsequently, several ECRH beams could be directed at the island by feedback on ECE signals. If the island rotates faster than the mirrors can be adjusted, stabilisation can be tested by using fast switching of the beams.
Fig. 2 - Shot #18004 and #18015 are compared. In #18004, absorption is at rabs/a=0.46, and stabilization fails. Top to bottom: Mirnov signals, Te, ECE at centre and half radius, central line density, global energy increase. Core confinement improves with TM stabilization
Steady high performance plasmas with pellet injection
MHD studies have focused on mechanisms allowing sawteeth to
be stabilised, in particular for pellet injection both in the plasma current
ramp-up phase with or without ECRH. The aim of these studies is not merely to
understand the physics, but mainly to discover conditions allowing plasma confinement
to be optimised. Domains where stabilisation occurs have been
identified. The main effect is linked to pellet central fuelling. It is important that pellet ablation takes place within or close to the q=1 surface. In particular, the role of heavy impurities (such as molybdenum, which can result in radiative collapse) on sawteeth has been documented. The critical factors allowing heavy impurity accumulation to be avoided are as follows: high plasma density, peaked temperature profiles and plasmas with low edge temperature, likely to minimise sputtering. It was found that operation at high field and high current is beneficial in the obtaining of high performance plasmas with pellets (fig. 3).
Fig. 3 - Time traces of lineaverage density, neutron yield and central electron temperature for FTU pulse #18598, with five pellets being injected during the 1.2 MA current plateau at B=8T (edge safety factor q=3.3)
An analysis of m=1 mode and snake activity has also been performed.
Snakes seem to be located at the O point of a magnetic island, a point that
is displaced with respect to the inversion radius. The high level of impurities
radiated in the snake permits an accurate reconstruction by using x-ray emission,
thus allowing interesting studies to be made on rotation frequency and coupling
to other modes.
When the density increase is not too great, access to the turbulence spectrum is possible through the FTU reflectometer (operated in co-operation with the Kurchatov Institute). After pellet injection, a simultaneous disappearance of the low frequency part of the spectrum as well as of the quasi-coherent feature is achieved. The analysis confirms the link between low frequency and quasi-coherent mode, as observed in T-10. Reduction of turbulence is also compatible with the formation of an improved confinement zone in the plasma core.
Transport analysis has confirmed that the energy confinement time is significantly higher than the ITER89P L-mode scaling law. An interesting aspect of transport is that the core total radiation decreases for pellet-fuelled discharge at high plasma current. This could be due to the remaining m=1 activity acting as a cleaning process, but a careful analysis of impurity
transport also suggests the presence of an outward pinch process. Analysis is expected to confirm this process and utilise it for further optimisation of these scenarios.