Study On Laser-driven Pulsed Magnetic Fields And Its Application In Laboratory Astrophysics

As an important tool in modern scientific research,magnetic field is very significant to many research areas,including plasma physics,astrophysics,material science,and atomic and molecular physics.“Every time the magnetic field range has been enlarged,fundamental discoveries have been made that could not be predicted beforehand”,such as the quantum Hall effect and the fractional quantum Hall effect,for which the Nobel Prize in Physics was won in 1985 and 1998.A pulsed magnetic field,which has higher strength and easier technology,has attracted much more attention than the static one since the beginning of the 19 th century.At present,a pulsed magnetic field with hundreds of Tesla is usually destructive one as a result of the high forces on its own structure.In recent years,a laser-driven pulsed magnetic field with the advantage of high strength and convenience has developed rapidly.In this thesis,we proposed a simple scheme to produce strong pulsed magnetic fields,and invested the characteristics of them generated with different laser pulses.Also,the laser-driven pulsed magnetic fields have been used in our laboratory astrophysics experiments.First,we proposed a simple scheme to produce strong magnetic fields due to cold electron flow in an open-ended coil heated by high power laser pulses on ShenguangII(SG-II)high power laser facilities in Shanghai Institute of Optics and Fine Mechanics,Chinese Academy of Sciences.The magnetic field strength at the coil center is measured to be 205 T.The generation mechanism of the magnetic field is straightforward and the coil is easy to be fabricated.To verify the current source,the magnetic field induced by the open-ended coil is directly measured by ultrafast proton radiography on Xingguang-III(XG-III)laser facilities in Laser Fusion Research Center,Chinese Academy of Engineering Physics.From the proton deflection,we can infer that it is the cold electron current excites the pulsed magnetic fields.Also,we measured the magnetic field strength and the energy conversion efficiency by adjusting the laser intensity and the wavelength .We found that the magnetic field strength is proportional to 2.And at a similar 2,the conversion efficiency is higher with a longer wavelength as a result of plasma instabilities.Second,we performed a similar experiment on 20 TW laser facilities in Institute of Physics,The Chinese Academy of Sciences,for higher repetition and shorter duration.The rise time of the pulsed magnetic field is ~20.8 ps,and the duration is ~77.6 ps.To evaluate the temporal evolution of such an ultrafast magnetic field,we proposed an improved Faraday-rotation measurement,which employed a chirped pulse as the probe beam.This measurement with a high temporal resolution is not vulnerable to electromagnetic noise,and it allows us to record the whole temporal evolution in single shot.We confirm that it is still the cold electron return current excites the magnetic field by distinguishing the rotation of the polarization.Third,since the heating efficiency is very high with a short-pulsed laser,a relativistic picosecond laser at VULCAN laser facilities,in Rutherford Appleton Laboratory,was employed to irradiate a solenoid target for a higher energy conversion efficiency from laser energy to magnetic fields.The magnetic field strength at the coil center was measured to be 40 T,while the energy conversion efficiency is 35%,which is the largest one up to now.And it proves that such a strong pulsed magnetic field can be used to adjust the ejection of suprathermal hot electrons.In the end,we employed 8 laser beams to irradiate an Omega-shaped target to generate strong poloidal magnetic fields on SG-II laser facilities.The surface of the coil target was ionized,and the plasmas moved forward the coil center.A bipolar jet with a high aspect ratio 1:11 was formed.Based on both calculations and MHD simulations,we conclude that the magnetic fields played an important role in the formation and collimation process.This result is very significant for the astrophysical jet study.In conclusion,the pulsed magnetic fields due to cold electron return current were studied with various laser parameters.We also summarized the different characteristics: the strength of the magnetic fields is very high with a nanosecond laser;the duration is ultrafast with a femtosecond laser;the energy conversion efficiency is very high with a picosecond laser.To measure the ultrafast magnetic field,we proposed an improved Faraday-rotation measurement with a high temporal resolution,which employs a chirped pulse as the probe beam.In the end,we utilized such a strong magnetic field in laboratory astrophysics experiments,and get a highly collimated magnetized jet.With the development of the laser-driven magnetic fields,they are expected to be used in more laser-plasma researches.