1. magnetic confinement - ENEA - Fusione

3. FUSION TECHNOLOGY 71
3.4.2 Development of calculation codes for CIC conductors (EFDA
Task TWO-T400-1/01)
The development of a new calculation code, started in 2000, continued in 2001. The
code includes all the macroscopically relevant physical phenomena characterising a
forced flow cooled cable-in-conduit (CIC) conductor and is being developed by
various universities co-ordinated by ENEA. The code is structured as a set of four
separate software modules, each describing one conductor characteristic.
The modules deal with the EM and thermal-hydraulic behaviour of the conductor,
the EM behaviour of the joints and the mechanical behaviour of the cable. During
2001, all these modules were fully developed and the first tests started. The EM
section was first tested vs. the current distribution code (CUDI) developed by the
University of Twente. (This code is a simplified tool applied in the case of a short
single-stage cable.) Using 36 ad-hoc-fabricated insulated Cu wires cabled as 3x3x4, a
second code test was performed. The self- and mutual inductances of the conductor
were measured and compared successfully with those calculated by the code. Finally,
the code was used to calculate the expected value of the self- and mutual inductance
for a group of strands of the ENEA Stability Experiment Upgrade (SExUp) .
The thermal-hydraulic module was successfully tested vs. the already validated
MITHRANDIR code. Finally, the two electromagnetic and the thermal-hydraulic
sections were coupled together to check the coupling effectiveness.
The final tests of the results from the ENEA SEx are planned for 2002.
3.4 Magnets
3.4.3 New diagnostics for a CIC conductor (EFDA Task TWO-T400-
1/01)
The helium temperature in a CIC conductor is usually measured by means of
resistance sensors glued or soldered onto the external part of the conductor jacket.
This arrangement gives an indirect measurement of the helium temperature, a delay
in the temporal response during fast transients and a very light EM neutrality that
seriously affects the measurement accuracy. One way of overcoming these problems
is to use optical measurements, which can give fast, highly accurate, noise and
magnetic-field-insensitive helium temperature measurements.
This new approach is based on an optical fibre with Bragg gratings that can be photoimprinted
into the fibre. By illuminating the fibre with a broadband source of light,
a narrow band is reflected at the Bragg wavelength. Its dependence on temperature
comes from two effects: the index of refraction and the thermal expansion of glass.
The local temperature at different positions can be measured by imprinting along the
fibre various gratings with different pitches. In the cryogenic temperature range, the
heat expansion coefficient is not monotonous [3.25], and the feasibility of the
measurement still has to be demonstrated. Moreover, a strain effect competes with
the thermal effect by increasing the fibre length, although temperature/strain
discrimination has recently been successfully achieved [3.26].
[3.25] G. H. White et al.,
Phys. Chem. Glasses 6, 3
(1965)
[3.26] M.G. Xu et al.,
Electron. Lett. 30, 1085
(1994)
A cryogenic system was realised to test the optical fibre temperature measurements
by comparing them with measurements from a traditional sensor. Three different
fibres were tested (fig. 3.8): uncoated fibre (i.e., fibre whose external acrylate
protection coating has been removed) in order to have a reference value for the bare
fibre; fibre with its own acrylate coating; fibre whose acrylate coating has been
removed and the sensor coated with zinc by plunging the fibre into liquid Zn. As
shown in figure 3.8, the Zn-coated fibre shows a much larger heat expansion
coefficient than the others.