The present invention relates to a radio frequency multipole linear accelerator.
The basic constitution and function of radio frequency multipole linear accelerators can be briefed as follows by exemplifying a quadrupole linear accelerator, which is the sole type that has hitherto been practiced of the radio frequency multipole type linear accelerators. The radio frequency quadrupole linear accelerator is substantially equivalent to a radio frequency resonant cavity of a specific type. The resonant cavity fundamentally consists of a cylindrical cavity drum and four electrodes enclosed therein. The four electrodes, which are prolonged in the direction parallel to the axis of the cavity drum, have their root portions fixed (both mechanically and electrically) to the inner surface of the cavity drum at angular intervals of 90.degree., so that their tip portions, viewed cross-sectionally, may form a quadrupole configuration with the axis put in accordance with that of the cavity drum. Further, the four electrodes have their tips wave-formed along the length direction. With an electromagnetic standing wave, for example, of TE210 mode made developed in the cavity having such a constitution as described above, there is produced in the space around and in the vicinity of the cavity axis an electric field having two components: one component, which is axial along the cavity axis, acts so as to accelerate charged particles along the cavity axis; and the other, which is radial, acts so as to make the particles converge around the axis. Thus the cavity, with traveling charged particles introduced thereto through the particle inlet hole on one of the cavity walls, accelerates the charged particles with the same kept bunched.
In such a principle the effectual performance of bunched acceleration of particles largely depends, in practice, on the precision of the relative arrangement of the electrodes, especially with respect to their tip portions. Therefore, it is practical to provide some means for fine adjusting the position of the tip portion of electrode. Such adjusting means has conventionally been practiced, for example, by a mechanism as illustrated in FIG. 7.
According to FIG. 7, which shows a partial cross-sectional view (in which only one electrode is shown) of a conventional resonant cavity of the quadrupole type radio frequency linear accelerator, each one of the four electrodes 72 is mounted in a cavity drum 71 through a electrode supporting element 75 fixed directly to the cavity drum 71 by means of a positioning pin 76 and a screw bolt 78. The electrode 72 is secured to the electrode supporting element 75 by a positioning pin 77 and an adjusting screw bolt 73. The electric contact between the electrode 72 and the cavity drum 71 is kept predominantly through contact elements 74 interposed between the root portions on both sides of the electrodes 72 and the inner surface of the cavity drum 71. The contact elements are made of an electroconductive elastic materials. In such a constitution of electrode mounting, the fine positional adjustment of the electrode tip portion is effected by adjusting the tilt of the electrode 72 through driving the adjusting screw bolt 73.
The above method of adjusting the position of the electrode tip portion is, however, accompanied by an important disadvantage that the tilting of the electrode 72 influences the contact pressure on the contact elements 74, resulting in the variation of the contact resistance between the electrode 72 and the cavity drum 71 and, therefore, causing the quality factor (Q-value) of the cavity to vary. It is essentially important that the cavity should be operated at a constant high Q-value without being affected by any fine positional adjustment of the electrodes.