1
Status of the IRIS superconducting line test station
and 1 GW rated Green Superconducting line design
and procurement
S. Maffezzoli Felis, M. Statera, A. Chiuchiolo, D. D’Agostino, L. Rossi, C. Santini, A. Musso, G. Angeli, M.
Bocchi
Abstract—IRIS (Innovative Research Infrastructure on applied
Superconductivity) is a major project to build a research infrastructure in applied superconductivity, recently approved in
Italy and led by INFN. The design and delivery of a Green
Superconducting Line (GSL) for power transmission and the
construction and commissioning of a test station for GSLs testing
are two key deliverables of the project. The test station is under
construction at the University of Salerno. It foresees the main
building, and a outdoor space for 130 m long cable lodging.
Into the building there will be the current and voltage power
supplies (40 kA and 50 kV, respectively), a 500W@20K Herefrigerator, monitor and control systems and diagnostics for
GSLs testing. The facility will be kept open to external institutions
and companies engaged in the same research field. The GSL will
be 130m long and designed to carry 40 kA at 20K, isolated for 25
kV operation, i.e. 1 GW of power, with almost zero dissipation.
The GSL will be used for the commissioning of the test station.
In this paper we will present the line configuration. We updated
the design of the line from a four conductors (two phases and two
spares for redundancy) inside the same cryostat configuration to
a three conductors (two phases and a spear) each inside its own
cryostat, in order to have a better insulation between them in
case of failure of one phase. Details and preliminary analysis of
the updated line design will be presented.
TABLE I
S UMMARY OF GSL M AIN C HARACTERISTICS
Superconductor
Voltage
Operating temperature
Line length
Expected losses
Bending radius
Inner pressure
M gB2
25 kV
20 K
130 m
1.5 W/m @ 20 K
2.2 m
10 bar
expertise in superconductivity for accelerator applications by
enhancing existing facilities with advanced instrumentation.
As part of the IRIS project, an innovative demonstrator
known as the Green Superconducting Line (GSL), a 1 GW
power transmission line, is being developed. This line is
scheduled for production by the end of 2025 by ASG in Genoa.
A render of the line is shown in Figure 1.
The GSL will be tested at a new cable testing facility
in Salerno that will be available for use by companies and
research institutes for at least ten years.
This article presents the updates on the preliminary analysis
conducted on the Green Superconducting Line due to the new
design, and of the new facility in Salerno1 .
I. I NTRODUCTION
The Italian Minister for University and Research has recently provided funding for the Innovative Research Infrastructure on applied Superconductivity (IRIS) initiative in Italy [1].
This initiative includes a collaboration among existing laboratories from various institutes, such as INFN (which leads
the project with four laboratories in Frascati, Genoa, Milan,
and Salerno), CNR (SPIN institutes in Genoa, Naples, and
Salerno), and five universities: Genoa, Milan, Naples, Salento,
and Salerno. The primary goal of IRIS is to strengthen Italy’s
II. G REEN S UPERCONDUCTING L INE
The Green Superconducting Line (GSL) is a 130-meter
direct current (DC) power transmission line designed to transport 1 GW of power at a medium voltage of 25 kV and
1 This work is part of the project IR0000003 - IRIS supported by the
NextGeneration EU-funded Italian National Recovery and Resilience Plan
with the Decree of the Ministry of University and Research number 124
(21/06/2022) for the Mission 4 - Component 2 - Investment 3.1.
S. Maffezzoli Felis is with the Laboratory of Acceleration and Applied
Superconductivity, National Institute for Nuclear Physics (INFN), Milan
20054, Italy and also with the Sapienza University of Rome, Rome 00185,
Italy.
Marco Statera and C. Santini are with the Laboratory of Acceleration and
Applied Superconductivity, National Institute for Nuclear Physics (INFN),
Milan 20054, Italy.
L. Rossi is with the Laboratory of Acceleration and Applied Superconductivity, National Institute for Nuclear Physics (INFN), Milan 20054, Italy and
also with the Department of Physics, University of Milan, Milan 20122, Italy.
A. Chiuchiolo and D. D’Agostino are with National Institute for Nuclear
Physics (INFN), Napoli 80126, Italy and University Of Salerno, Fisciano
84084, Italy.
A. Musso, G. Angeli and M. Bocchi are with RSE S.p.A, Milano 20134,
Italy
Fig. 1. Portrayal of the superconducting line.
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Fig. 4. Induction magnetic field for an operating line with opposite currents
in the two cables.
Fig. 2. Cross-section of the whole superconducting line, with the two layered
cryostat.
Fig. 5. Induction magnetic field for an operating line with opposite currents
in the two cables for the 3D analysis.
Fig. 3. Portrayal of the cable cross-section. The copper cores (reddish discs)
will be made of several copper wires.
a high current of 40 kA. The line will utilize magnesium
diboride (MgB2 ) as its superconducting material, operating
at a temperature of 20 K, with 10 bar helium gas used for
cooling (Table I).
A. Cable Configuration
Since the start of the project, we have changed the line
configuration from a four conductor line in a cryostat to a
line consisting of three different cables, each with a single
conductor in a single cryostat [2], [3]. Each cable is made
up of 9 petals spiralling around a copper core. Each petal
contains a copper core around which are twisted 14 strands
of MgB2 with a diameter of 1.33 mm. The copper cores are
made up of many filaments. Petals will have a twist pitch of
tpp = 800 mm and strands of tps = 250 mm. The cryostats
will be made by two concentric corrugated pipes with vacuum
and MLI in between. The internal pipe has a radius of around
65 mm and the external one around 150 mm. In picture 2 and
3 a truthful cross-section for a single cable and for the internal
conductor are presented.
magnetic analyses were performed to determine the peak
field on the strands, as already done for the previous configurations. Additionally, a mechanical analysis was conducted to
assess the stress and strain induced on the strands by bending
the entire cable. These analyses are crucial for evaluating
potential reductions in the strands’ critical current. Furthermore, an ongoing study on quench behaviour incorporates
these analyses, along with a thermal analysis.
B. Magnetic analysis
The magnetic field was calculated using a FEM simulation
in COMSOL, both in the case of a single cable, to assess the
case where the line is tested at the Salerno test station, and
in the case of two adjacent cables, to assess the case of an
operating line.
In the case of a single cable, a peak field of approximately
0.58 T was observed, both in the 2D and 3D analysis. In
the case of an operating line, where the current in the two
cables flows in opposite directions, a peak field of approximately 0.63 T was observed in the two-dimensional analysis
(figure 4), while in the three-dimensional analysis, a peak
field of approximately 0.71 T was observed (figure 5). The
discrepancy between the two cable configurations in two and
three dimensions can be attributed to the challenging process
of meshing the rather complex geometry. For this reason, in
3
Fig. 7. Study of strain for the bending of the whole conductor, in COMSOL.
The results are comparable with the ones obtained manually.
Fig. 6. Critical current as function of B field and for different temperature (in
green the working bulk temperature of 20 K). The load lines are shown and
the working points for the single and double cable configurations are marked.
the three-dimensional case, the strand sections are designed
as polygonal rather than circular, which may result in a
higher concentration of current and, consequently, a stronger
magnetic field in the corners. Nevertheless, although the 3D
case may be distorted due to the geometry, we have utilised
it as a basis for computing the critical current margins in a
more conservative manner.
C. Critical current and margins
Measures of critical current on different wires, at different
temperatures and in different external magnetic fields were
performed by ASG during the wire creation process. The
average behaviours of the critical current is shown in figure
6. Since the measured data do not cover the full range of
magnetic field values for all temperatures, the critical current
for the magnetic field values corresponding to the single and
double cable configurations at 15 K and the single cable
configuration at 20 K have been extrapolated by extending
the data set in MATLAB. Therefore, these extrapolated values
should be considered as reference values only. Nevertheless,
they have been used to model the dependence of the critical
current on temperature.
With a working current of I = 317.14 A we have a margin
on the load line of 47.42% and 41.35% for the single cable
and the two cable configurations respectively.
The working bulk temperature of 20 K was selected to be
compatible with a prospective cooling of the line with liquid
hydrogen, resulting in temperature margins of 8.37 K for the
single cable and 7.61 K for the two-cable line at the working
current.
Fig. 8. Temperature and pressure of He gas as function of position x along
the line, varying the mass flow rate Q (kg/s).
is aligned to what is expected to have a maximum reduction
of 5% of the critical current [4], [5].
In order to calculate the total maximum strain experienced
by the superconducting cable, a step-by-step process is followed, whereby the individual contributions from the various
mechanical deformations are combined. Initially, the strain ϵs
induced by the twisting of a strand around its copper core,
which is maintained in a straight position throughout the
process, is calculated (ϵs ≃ 0.13%). Subsequently, the strain
ϵp resulting from the twisting of the entire petal structure,
composed of the twisted strand and the core, around its
central copper core, is determined (ϵp ≃ 0.06%). Finally, the
strain ϵc resulting from the bending of the entire conductor
is incorporated into the total strain calculation (ϵc ≃ 0.03%).
This bending occurs when the cable is subjected to a radius
of curvature of 2.2 metres, which represents the winding on
the transport spool. By aggregating the strains from these
three discrete processes—strand twisting, petal twisting, and
conductor bending—a comprehensive understanding of the
total mechanical strain on the cable can be achieved.
The manual computation was verified with a COMSOL
simulation for the last bending that gives a maximum strain
of ϵc ≃ 0.03%, compatible with the manual computed one
(figure 7).
D. Mechanical analysis
E. Thermal analysis
A preliminary paper-and-pen calculation for the total strain
ϵ on the strand was carried out, giving us ϵ = 0.22 % which
Thermal analysis of the helium flow inside the cryostat is
being performed by RSE S.p.A.
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Fig. 9. Plan of the new test facility in Salerno. On the left the main building
where will be installed the cryogenic system and the power supply apparatus
(current and voltage). From the bottom part of the building the tested cable
(red line), starting from the terminal, will go on the external ground turning
back in to the building, into another terminal, after 130m.
A preliminary analysis of the helium gas flow shows that
at the required flow rate of Q = 22 g/s the temperature rise
is within the required operational limits. Moreover, variations
in flow rate were considered in order to assess the potential
impact of the lack of detailed specifications, such as the
corrugation of the final cryostat, on the results. As can be seen
in figure 8 for the required Q the temperature rise is about of
1.5 K (from the inlet temperature of 18 K) and a pressure
drop of around 2%. Varying the value of Q until Q = 31 g/s
(these values are comparable with values find in literature for
corrugated cryostats) the rising in temperature is even lower
(∼ 1 K), but it is higher the pressure drop (∼ 7%). These
variations are still in the required 20 K operational limit.
F. Line diagnostic
The diagnostic system for the line is currently undergoing
evaluation. However, a modified version has been adopted in
comparison to the one presented in reference [3]. The primary
modifications include the removal of the sensor situated around
the junction (due to the presence of technical challenges) and
the addition of pressure gauges at both extremities of the line.
A significant challenge in the diagnostic system is the
accurate reading of sensors installed on the line, given that the
operating voltage is 50 kV. This requires the use of electrically
isolated interfaces that connect the reading instruments to the
sensors on the line, such as electro-optical transducers. It is
imperative that this aspect be considered during the design of
the test station in Salerno.
III. S ALERNO TEST FACILITY
Construction of the new test facility began in June 2024
at the University of Salerno, Fisciano Campus. The initial
conceptual layout, which included an outdoor trench for GSL
positioning [1], has been improved to allow easier cable laying
and greater flexibility for future applications with varying
cable lengths. A dedicated technical room at ground level will
house the assembly of the HTS terminals, their connection
to the copper busbars and the LN2 supply system. At the
same level, the GSL flexible cryostat will be connected to
the HTS terminals and extended outdoors, while the busbars
will connect the terminals to the power converter located in
the underground interlocked area.
The main components of the cryogenic plant, such as the
500 W refrigerator and the He gas tanks, are currently being
manufactured, while the design of the He transfer lines and
the LN2 supply systems for the power lines are underway. The
41.6 kA/10 V power converter for the GSL, which is currently
being procured, will have a modular layout and will be located
on a dedicated platform isolated from the ground at 50 kV.
The mechanical design is currently undergoing a final preproduction review to optimise dimensions, field assembly and
busbar interface. In parallel, to support the GSL test plan up
to 50 kV, a fully equipped medium-high voltage laboratory
will be established, including equipment such as a pulse
generator, a 50 kV power supply and a readout system for
GSL monitoring and diagnostics.
IV. C ONCLUSION
Since the inception of the project, modifications have been
made to the initial design for the IRIS Green Superconducting
Line, with the objective of enhancing performance. For the
new and last configuration we’ve looked at key technical
aspects like the magnetic field distribution and mechanical
strain on the superconducting cables to make sure the line
can operate reliably under the required conditions. Similarly,
thermal analyses are being conducted to ensure the line’s
reliability.
At the same time, work is also going ahead on the test
facility in Salerno. The main parts, like the cryogenic systems
and power converters, are already being made. Once it’s
finished, this facility will be a key place for testing the GSL in
realistic conditions, including the full 50 kV medium-voltage
environment. It’ll be really useful to be able to simulate what
the line will be like in use and gather data on how well it
performs. This will help us to make sure the design is as good
as it can be. As the project moves forward, we’ll be focusing
on finalising the mechanical design of the cable system. We’ll
be looking in particular at ways of optimising the layout to
make installation easier and to ensure it lasts a long time. We’ll
also be testing the system at medium voltage.
ACKNOWLEDGMENTS
The authors thank the PNRR IRIS project of INFN funded
by the Italian Ministry of Research for the financial support.
Artificial intelligence softwares ChatGPT and DeepL were
employed for the purposes of sentence editing and grammar
enhancement.
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