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Francesco Grilli
Priv.-Doz. Dr. Francesco Grilli
Group leader AC losses in high temperature Superconductors

phone: +49 721 608 - 28528

Email: francesco grilliObi5∂kit edu

Prof. Dr.-Ing. Mathias Noe
Prof. Dr.-Ing. Mathias Noe

Research Activity

The group, created in May 2010 as Helmholtz-University Young Investigator Group in the Research Field “Energy”, focuses on the experimental characterization and the numerical modeling of high-temperature superconductor tapes and components for power applications. Here below are some highlights of our recent research activity.


Tape characterization:
Angular dependence of Ic on the magnetic field

Measurement of the angular dependence of the critical current Ic of HTS tapes at 77 K, varying the amplitude of the applied field and its orientation with respect to the sample.

From experimental data we can extract the Jc(B, θ) dependence to be used as input of numerical calculations.


Tape characterization:
High-speed fluorescent thermal imaging of quench in HTS tapes

A thin layer of fluorescent coating is applied on the surface HTS tapes that has a known temperature-dependant light emission around 77 K. Temperature fluctuations in a superconductor, due to joule heating, then cause changes in the emitted light, which consequently allow calculating the temperatures. By using a high-speed camera, transient events, such as the quench of superconductors, can be observed and the temperature distribution can be visualized over a larger surface at high temporal resolution.

AC loss measurements:
Magnetization and transport AC losses

Measurement of magnetization AC losses with the calibration-free technique, which does not require to calculate calibration factors, as typically happens with methods based on pick-up coils. Fields up to 100 mT can be reached in the present set-up.

Measurement of transport AC losses of a variety of HTS devices, ranging from individual HTS tapes to complex coil and cable assemblies.

Figure: a lab-scale setup of the Ampacity cable


Numerical modeling:
Determination of effective Ic of HTS devices
Magnetic flux dynamics in HTS

A newly developed model allows us to calculate the critical current of HTS devices taking into account self-field effects and complex angular dependences of Jc. Figure: critical current of pancake coils assembled from a Roebel cable as a function of the separation between the turns.
Time-dependent electromagnetic models allow us to calculate current and field distributions in HTS devices and to estimate their AC losses. The model has been recently extended to take thermal effects into account as well. Figure: geometry, mesh and current density distribution in a 3-D model of a Roebel cable. 3-D Finite-Element-Model