Simulation And Experimental Study Of Negative Hydrogen Ion Penning Source For Medical Cyclotron

The built-in cold-cathode Penning-type negative hydrogen ion source is used to supply negative hydrogen ion to 11MeV medical cyclotron. Compared with other types of ion source, this ion source has such advantages:compact, no need of complex ion inject structure, more purity beam, lower beam emittance, easier to extraction from the cyclotron. But, in the other way, the advantages above bring such problems different discharge condition, big gas flow, no heat producer for cathodes. As what is encountered by now in use, ion source's lifetime is too low to make the cyclotron a better accelerator. In order to improve the lifetime of the ion source, when the ion beam extracted is enough, it needs more work to do in engineering and physics.Through the ion source's vacuum condition and other parameters, aware of the pressure in the source is in the order of 4 Pa. Then according to the pressure in the source to estimate the characteristic parameters of plasma. Referring to laboratory conditions, get the discharge feature scale and time, reasoning that which physical processes will make what effect to the actual discharge. And also get the information that the magnetic fluid equations can be used to describe this ion source. According to the magnetic fluid equations and ionization reaction equations, the computer model of ion source discharge is formed, in this model electrons comply with the magnetic fluid equations and ions obey the momentum equation. In the computer simulation, the distribution of charges, the happen of reactions, the effect by the change of macroscopic parameter are analyzed. On the basis of the above theoretical work, the change of impedance characteristics is studied in the experimental way. In this way, the working characteristics of the source is studied, then combine theory and simulation work do more in-depth analysis of the discharge, understanding the specific discharge of the source, and gives an optimized design scheme for the source.The study shows that the electron in the source is far away from the best electron temperature of the ionization which produce negative hydrogen ion when the ion source is in normal operation. In this state, the increase in arc flow can effectively increase the cathode electron emission flow, therefore the electron number density in anode cylinder and increase, low energy electron number density therefore also increases, so the negative hydrogen ion concentration get improved. Although increasing arc current enhance the yield of the ion source, but due to the bombardment of large quantities of ion on the central of cathode can significantly reduce the cathode life. So we need to change the structure of ion source in order to reduce the ion source discharge impedance. According to the simulation results. It is found that the ionization highest region is near the ends of the anode cylinder, and there is lower reaction efficient near the center area of the anode tube, thus obtained the way to reduce the resistivity of the ion source:shorten the length of the anode cylinder. On the basis of cathode erosion and simulation of cathode surface charge bombardment, there cathode improvement program is raised:through the improvement of the structure, reduce the ion bombardment power per unit area; use bombardment tolerant, high temperature tolerant cathode material.The creative work of this dissertation is:combined mounts of physical process with the actual, through the strict logic reasoning, established a relatively complete discharge physics image of Penning ion source, by three aspects of theory, simulation and experiment launched a study of the ion source, propose a meaningful upgrade plan for the further optimization of the ion source.