Experimental Study On Magnetic Flux Avalanche Behavior Of Superconductors

When the non-ideal type-? superconductor in the superconducting state is placed in the external magnetic field,the external magnetic field enters the superconductor in the form of quantized flux lines under the action of Lorentz force,and interacts with the pinning force(obstruction)generated by the internal defects of the superconductor to form the non-uniform distribution of the flux vortex structure.When the driving force is equal to the pinning force,the structure has reached a metastable equilibrium state(i.e.critical state).Once the environment variables(i.e.,magnetic field,temperature,and current)are changed,the local vortex in the superconductor will break away from the pinning center and start to move,and then generate energy dissipation,which leads to the rising of local temperature in the superconductor.Finally resulting in the decrease of the pinning energy and destruction of the critical state of the flux vortex distribution,this physical processes are collectively referred to as flux avalanche.The existing experimental results show that the position of the avalanche is random and difficult to realize the artificial control.The velocity of the avalanche process is very fast,and the maximum velocity can reach 180Km/s.The time course is very short,where the time course of the whole process is less than 100ns.Once the superconducting material occurs flux avalanche,the electromagnetic characteristics of the corresponding superconducting device will be destroyed disastrously in a short time,which will lead to security reduction of the device,and they are difficult to achieve the expected function.The existing experimental research on flux avalanche mostly adopts the induction method of the magnetic field change,which has the advantages of low experimental cost and convenient operation,but it is difficult to directly determine the power threshold value triggering flux avalanche of superconducting materials,and the threshold energy is the core indicator of structural safety design of superconducting device.This Ph.D.thesis focuses on the problems of the flux avalanche in superconducting,such as random location,high velocity,short time history and difficulty to determining the threshold energy of the avalanche.Taking the most promising YBCO superconductor as an example.The random position problem of the flux avalanche of superconductors is solved by controlling the position of the flux avalacnhe with pulsed laser,and the threshold power of flux avalanche of this material is directly given.A multi-exposure magneto-optical imaging system based on single pulse was designedto obtain the velocity distribution characteristics of the flux avalanche process of YBCO superconducting materials.Based on the laser-induced experimental model,the sensitive dependence on initial conditions(SDIC)of the flux avalanche was studied,and the chaotic characteristics of the laser-induced avalanche were found.At the same time,through the analysis of the avalanche path,a new magnetic flux avalanche mechanism driven by electromagnetic force is proposed to give a qualitative explanation of this phenomenon.Finally,the suppression methods of the avalanche process are preliminarily explored.The main contents are as follows:(1)An experimental mode of laser induced flux avalanche is proposed.Combined with the classical magneto-optical observation system,the controllable research of avalanche position by pulsed laser is realized.The energy loss of laser pulsed through the visual low temperature Dewar window and LaAlO₃ substrate was determined,and then the power threshold of laser pulsed induced flux avalanche was determined.The relationship between the threshold value and magnetic field,temperature and laser spot position was systematically studied.A theoretical model is established to describe the power threshold of flux avalanche and temperature,and the calculated results are in good agreement with the experimental results.In addition,the effects of the instantaneous power of the pulsed laser,the magnetic field,the temperature and the position of the laser point on the avalanche dendratic is studied,and the effects of four different conditions on the magnetic flux,the avalanche area,the magnetic field and the current density before and after the avalanche is given.(2)A multi-exposure optical path based on a single pulse laser is designed to realize the continuous high-speed measurement.This method has advantages of full field,maximum energy utilization close to 100% and adjustable exposure time.For the pulsed laser with a pulse width of 600ps,the minimum time resolution of this system is 766.7ps,and with the improvement of the performance of the pulsed laser,the time resolution also can be further improved.With the existing optical devices,the maximum speed of the system can reach3.84×10~7 m/s,which solves the problem of increasing the speed and decreasing the measurement resolution faced by the traditional high-speed camera.Based on this method,we have established a multi-exposure magneto-optical microscopy system to study the velocity characteristics of magnetic flux avalanche of YBCO superconducting thin films,and have realized the experimental observation of the time evolution process of flux avalanche on superconducting materials,and found the avalanche velocity decreased with time.(3)The sensitive dependence on initial conditions in the flux avalanche is studied experimentally.Based on the multi-exposure magneto-optical microscopy system,we first compared the multi-exposure magneto-optical images of magnetic flux avalanche obtained from the same experimental conditions(pulse energy will exist a certain small fluctuations).It is found that the avalanche paths under the same environmental variables cannot completely coincide,which indicates that the magnetic flux avalanche of superconducting materials have the sensitive dependence on initial conditions.Through then the image processing,the avalanche path has positive Lyapunov exponent,which indicates that the magnetic flux avalanche has chaotic characteristics.Finally,the time evolution of the avalanche shows that the laser-induced flux avalanche always starts at the edge of the superconductor,and a new mechanism driven by the electromagnetic force is proposed to provide a qualitative explanation for this phenomenon.(4)The method of deposition of Ag metal layer on the surface of superconducting material to suppress the laser-induced flux avalanche was explored.It is found that the thickness of Ag layer and deposition temperature have significant effects on the inhibition of magnetic flux avalanche.Under the experimental conditions in this paper,the flux avalanche is significantly inhibited when the thickness of Ag layer is 200nm.When the deposition temperature of Ag layer is 100?,the avalanche inhibition effect is the best.