If a magnetic ﬁeld above a certain critical ﬁeld (Bc1) is imposed on a type II superconductor, it will start to penetrate in the form of magnetic ﬂux quanta, or vortices, resulting in a meta-stable non-equilibrium critical state. The critical state can collapse due to thermo-magnetic instability in the superconductor, resulting in a sudden large scale redistribution of vortices. This might cause the superconductor to heat up to above the critical temperature, at least locally, where sudden resistivity can have catastrophic consequences.
In this thesis we have studied thermo-magnetic instability in superconductor thin-ﬁlms, in a wide range of ﬁelds and temperatures, by automating the experimental setup and developing algorithms for graphical recognition of ﬂux avalanches, and determination of threshold ﬁelds for the formation of these avalanches.
When constructing a phase diagram of the threshold ﬁelds as a functionof temperature, we found a hysteretic behaviour of the upper thresholds for increasing and decreasing magnetic ﬁelds. An explanation is given based on a theoretical model of the correlation between the threshold ﬁeld and the critical current density. We also found that sufficiently large primary avalanches have the ability to move independently of the shielding current inside of the ﬂux front. At decreasing ﬁelds we observed secondary avalanches starting at the center d-line. Heat from a primary avalanche reduce the pinningforce, which in some locations will let the Lorentz force overcome the pinning and move vortices. This create a local instability that can sometimes trigger a secondary avalanche.