Damage limits of superconducting magnet components due to beam impact and exposition to high temperature

The main goal of the thesis was to investigate the damage limits of superconducting (sc) magnet components Nb-Ti and Nb3Sn due to the interaction of the strands with the beam and due to the exposition to high temperatures. The energy stored in the LHC requires a complex machine protection system to shield the equipment from damage — adapted to the losses of the beam happening at various timescales and varying in energy absorbed. The most dangerous are ultra-fast failures or single turn failures that happen within < 3 LHC turns (270 μs) [1] and can be caused by the beam injection or beam extraction or missing the beam-beam kick after dump of one beam. Protection against ultra-fast losses is based on passive absorber elements. These passive instruments subjected to interactions with the high-energy proton beam produce particle showers that might initiate a quench or cause a damage in the nearby sc magnets. With an increase of beam intensity in HL-LHC, a peak energy density can raise up to 100 J/cm3 (100 K for D1 magnet) in sc magnets [2, 3]. Thus, an improved knowledge of the damage mechanisms and limits of sc magnet components is not only crucial for the nowadays’ LHC, but also in the scope of the future increase in beam brightness. In chapter 1 an introduction into the field of sc magnets was made, that contains the description of magnets in question, the nature of superconductivity with the emphasis on the operational implications and the review of the literature regarding damage mechanisms. To study the damage mechanisms an experimental roadmap consisting of several experiments on the polyimide insulation of sc magnets and on sc multifilamentary strands with varying timeframe of heating. A brief summary of the experimental roadmap is given in chapter 2 to help the reader understand the overall scope of the works. The thesis focuses on the experimental study on the effects of Joule heating on sc strands within millisecond timeframe. The experimental setup was designed to measure the degradation of the critical current in Nb-Ti and Nb3Sn strands due to seconds-long heating using a fast capacitor discharge. Sc strands were heated to peak temperatures ranging from 651 K to 1264 K. The preparation and the course of the experiment as well as the in-situ measurements are included in chapter 4. The results — the degradation of the superconducting properties of the strands were studied with magnetization measurements and microscopic analysis (chapters 5 and 6). The comparison to the literature data and to the results of the experiment with the proton beam was made.