Study On The Stability And Electron Neutralization Mechanism Of Self-magnetically High-intensity Pulsed Ion Diode

In recent years,the application value of high-current pulsed ion beam(HIPIB)technology in the field of materials engineering has received extensive attention.Compared with other high-power pulse technologies,HIPIB has the advantages of high energy deposition efficiency,larger irradiation area,and deeper modified layer.Improving the stability of its output is the key to popularizing this technology in engineering applications,including energy distribution,stability of focus position and energy density,and beam density fluctuations.Only by meeting the requirements of high repeatability can the HIPIB technology be widely used in the engineering field.Based on the HIPIB technology,this paper uses a new structure of metal grid diodes to study the effects of diode gap,grid material,grid transmittance,mesh size and center hole size on the total energy,energy density,and beam of the focused ion beam.The influence law of current density and stability establishes the corresponding relationship between ion beam electron neutralization degree(neutral beam proportion)and ion beam focusing characteristic(divergence angle).The experimental results show that under the condition of the best plasma extraction time of 450 ns,the diode gap of 10-9-12 mm is the best,and the aluminum grid cathode and the stainless steel grid cathode both increase the focus energy density of the ion beam.From 4.3J/cm~2 without grid to 6.4 J/cm~2 and 6.0 J/cm~2,the energy density of aluminum grid is higher than that of stainless steel grid.The reason is that the secondary electron yield of different materials is different.The focusing effect is also different.The ion beam composition of the aluminum grid cathode has changed from the original H~+,C~+to C~+and Al~+,while the stainless steel grid cathode is still H~+,C~+,and the fluctuation of energy density and beam current density is lower than that of the aluminum grid cathode.The transmittance of the cathode grid has a great influence on the output characteristics of the ion beam.The total energy decreases as the transmittance decreases.Both the focal energy density and the neutral beam ratio show an overall upward trend as the transmittance decreases.The lowest 0.38 stainless steel grid increases the focal energy density to 12.4 J/cm~2.In a cathode grid structure with the same transmittance and different mesh sizes,the total energy is roughly the same,and the energy density and the proportion of neutral beams are both higher for small holes than large holes.The grid cathode structure reduces the stability of the ion beam.When the transmittance is 0.48,the fluctuation is the smallest,and the coefficient of variation(CV value)of the focus position fluctuation is about 10%,which is closer to that of the non-grid.In the experiment of opening the center hole of the stainless steel grid,the 20 mm square hole has the most significant increase in the energy density.The energy density of the 10 mm hole and the non-opening hole are close,but the fluctuation is reduced by 15%.Opening a 20 mm central square hole has improved the stability of the ion beam,and the coefficient of variation has been reduced to varying degrees.The CV value of a stainless steel grid with a transmittance of 0.48is 4.6%in the horizontal position and 7.6%in the vertical position.It is lower than 5.6%and14.1%of gridless,which not only improves the focus energy density,but also improves the stability.The analysis of the ion beam divergence found that the focus energy density and the proportion of neutral beams change with the transmittance and the divergence changes with the transmittance tend to be consistent.The smaller the divergence,the better the focusing effect,and the greater the focus energy density.High,the higher the proportion of neutral beam.The focusing effect of the grid cathode makes the ion beam focus position move forward,and the point 13 cm from the anode surface becomes the actual focus,and the total energy and the focus energy density both reach the maximum.After 13 cm,HIPIB will gradually diverge as it continues to transmit,and energy will gradually be lost,so the total energy and energy density are in a downward trend.The analysis of the HIPIB electron flow density of different transmittance grid cathodes shows that the total amount of electrons with a transmittance of 0.38-0.78 is roughly the same on both sides and low in the middle.Experiments with the same transmittance and different mesh sizes show that the mesh size is between 0.35-1.5,and the total power is a process of first decreasing and then increasing.Taking into account the influence of grid transmittance on the electron current density,the law of the proportion of neutral beam and the change of focal energy density with transmittance and the law of the total amount of electrons generated tend to be consistent,which can explain the neutralization and focusing of the ion beam.The improvement of the effect mainly depends on the secondary electron yield of the ion beam hitting the grid.