Pulsed Strong Magnetic Field Generator And Its Application In Nanosecond Laser Ablation

The study of magnetized laser plasma evolution has attracted much attention in recent years.Externally applied magnetic fields have the potential to substantially improve key performance in laser fusion and reduce the parameter requirements for ignition.Laser plasma behavior under strong magnetic fields can reproduce astrophysical and space physics phenomena such as collisionless excitations,protostellar jets,and magnetic reconnection in laboratory.In industrial applications,magnetic fields can modulate the physical processes in technologies such as laser cutting,laser welding,and pulsed laser deposition.Pulsed magnetic field generators are the key equipment for conducting magnetized laser plasma research.Compared with other generation methods,the strong magnetic field generation method based on pulsed power technology has special advantages in that it can provide magnetic field strengths of 10 T or even more than 100 T in the cubic centimeter scale space with peak durations of more than one hundred nanoseconds,which can meet the requirements of most laser plasma experiments in terms of magnetic field area,magnetic field strength and duration.However,the field environment of different laser devices varies greatly,and matching pulsed strong magnetic field equipment needs to be studied according to the actual target environment and experimental requirements.We have customized a set of pulsed strong magnetic field equipment for Shanghai Shenguang-II laser device(SGII).The transmission line and the adjustment mechanism of the pulsed strong magnetic field equipment were specially designed for the small target chamber of SGII and the limited space in and around the target chamber.The transmission line with a small radial size can be installed in any flange of the target chamber with high flexibility and unrestricted magnetic field orientation.The soft connection between the main body of the pulsed strong magnetic field device and the target chamber eliminates the need for the flange of the target chamber to bear the weight.The adjustment mechanism is located outside the target chamber,so that the position of the magnetic field coil in vacuum can be precisely adjusted in real time.The small solid angle occupied by the transmission line and the coil assembly inside the target chamber provides sufficient space for the laser channel and the diagnostic field of view,which is conducive to the experimental arrangement.The magnetic field device generates a driving current with rising edge of 280 ns,a flat top width of 200 ns,and a peak value of 40 kA in a 12 mm diameter double-turn magnetic field coil during a 40kV high-voltage discharge,resulting in a pulsed magnetic field of 8.7 T.In order to obtain a higher intensity magnetic field,we have upgraded the former pulsed strong magnetic field equipment.The new equipment features a modular,scalable capacitor bank structure that allows easy parallel connection of multiple modules.A single capacitor bank module supports fast plugging of up to 24 100-nF capacitors and the storage is up to 1.92 kJ at 40 kV charging,six times to the original equipment.Instead of using bulky stainless steel cylinders to seal the capacitor banks,gas switches and Rogowski coil and filling it with high voltage gas for insulation,this modular design has been redesigned with a connection structure that allows for atmospheric operation and single-person operation.To reduce electromagnetic interference and discharge delay jitter,we replaced the electrically triggered gas switch with a laser-triggered gas switch.The upgrade of the energy bank also relaxed the requirements for load inductance,allowing the flexible selection of different magnetic field coils according to experimental needs without significantly changing the discharge parameters.Using 12 mm diameter double-turn and three-turn magnetic field coils,strong magnetic fields of 9.6 T and 12.6 T can be generated at a charging voltage of 30 kV,respectively,with peak duration and rise time exceeding 1 μs.Using inductively coupled coils,the magnetic field strength can be further increased to 32 T at 35 kV discharge,with peak duration and rise time as long as 3μs,which greatly broadens the parameter range of strong magnetic field.Past studies have shown that the applied magnetic field can modulate the processes of laser cutting,laser welding and laser-induced breakdown spectroscopy,but it has not been found that the applied magnetic field can increase the amount of plasma generated by laser ablation.In our experiments,we found for the first time that a 9 T quasi-steadystate strong magnetic field can significantly enhance the plasma produced by laser ablation,with a nearly fourfold increase in the number of ions,a more isotropic angular distribution of ion flux and kinetic energy,and a significant reduction in the kinetic energy of the ions.By measuring the self-luminous images of the plasma and the morphology of the ablation craters on the target surface,it is found that the enhanced plasma and more uniform angular distribution come from the confinement of the laser plasma by the externally magnetic field.The transverse magnetic field constrains the laser plasma expansion,converting the kinetic energy of the plasma to thermal energy and increasing the internal energy share and the temperature of plasma,so that the hightemperature plasma confined near the target surface can ablate the target by the electron heat conduction process,producing more plsama.The magnetic field confinement inhibits the directional flow of plasma,and the plasma is smoothed more fully laterally,making the angular distribution of ion emission more uniform.The laser-ablation-based pulsed laser deposition coating technique naturally suffers from serious defects such as large droplets contamination and directional ion jet.We found for the first time in our experiments that a periodically oscillating pulsed strong magnetic field can substantially improve the pulsed laser deposition coating technique:significant elimination of large droplets contamination,an order of magnitude increase in the number of plasma generated,a more uniform angular distribution and a significant reduction in the share of energetic ions.By measuring the spatial and temporal evolution of plasma self-emission images and large droplet jets,it was found that the 105 V/m induced electric field excited by the periodically oscillating pulsed strong magnetic field removes large particle droplets by ablating the hightemperature gas near the target surface and generates a large amount of low-energy plasma with a more uniform angular distribution.