Times The Pressure Type Movable Accelerator Beam Optics Research

The beam optical system is the most important part of the compact Cockcroft-Walton accelerator used for neutron generation. It is a combine of the RF ion source&ion extractor, the pre-focusing lens, the accelerating tube, the drift space and the secondary electron prohibiting device. The deuteron beam is extracted from plasma, which is generated by a RF ion source, and then transported to the target through the pre-focusing lens, the accelerating tube and the drift space. Neutrons are generated by D(d, n)3He or T(d, n)4He reactions. When transporting, many secondary electrons can be generated from the target and the drift space under the bombardment of deuterons. The electrons entering the accelerating tube can seriously influence the load capacity and breakdown characteristics of the accelerating tube. As a result, secondary electron prohibiting device is needed to prohibit electrons into the accelerating tube.The beam optics study of the compact Cockcroft-Walton accelerator is mostly divided into the following three parts:(1) Chapter2. A method for approximating the meniscus of a plasma ion source was introduced to the numerical codes of beam extraction. It is based on a hypothesis that the meniscus is coincident with the surface where the electric field strength is equal to Ep. This method can improve the convergence time of numerical codes by providing an initial guess. The extraction characteristics of the RF ion source we used have been studied by experiments and simulations. The extracted beam firstly increases and then decreases as the extracting voltage increases. The maximum extracted beam at different extracting voltages increases as the magnetic induction increases, while it firstly increases and then decreases as the pressure increases.(2) Chapter3. A computer program, ACCE, which is used for the calculation of the beam transport in axial symmetrical electrostatic systems, was developed using transfer matrices. ACCE can be used in the beam transport calculation of most complex axial symmetrical electrostatic fields, once the non-linear effect could be ignored. The required computing time of ACCE is very short. ACCE program has been used to design the beam transport of the compact Cockcroft-Walton accelerator. The pre-focusing lens of the compact Cockcroft-Walton accelerator is a double-cylinder immersion lens with different diameters, and the accelerating tube is a triple-cylinder accelerating tube. The simulation shows that the beam matching and the beam spot<10mm on target could be achieved by adjusting the voltage of the pre-focusing lens.(3) Chapter4. The secondary electron yield of titanium deuteride surface induced by deuterons was measured in the80keV to254keV energy range. The clean surface was produced by removing the oxide layers and absorbed gases on the target by self-sputtering with deuterons. The maximum yield of the secondary electron is about1.37at143keV. The secondary electron yield at254keV is about1.20. Two methods can be used to prohibiting the electrons entering the accelerating tube:a transverse magnetic field or a prohibiting electrostatic field. The secondary electrons can be prohibited if the voltage on the prohibiting electrode is lower than-250Volts, or if B·L is approximately equal to5×105m·T, where B is the magnetic induction and L is the length of the transverse magnetic field.The theoretical and simulative and experimental results in this paper are the base of the design and the setting up of the compact Cockcroft-Walton accelerator. Moreover, the results can be used for reference to the setting up of other similar accelerators.