Method For The Acceleration Of Ions In Linear Accelerators And A Linear Accelerator For The Realization Of This Method

Abstract

The present invention relates to methods for acceleration of ions in linear accelerator and to a linear accelerator realizing this method. The method for the acceleration of ions in linear accelerators consisting of a cavity resonator 1 and drift tubes 2 employing a standing r.f. electromagnetic wave, according to the invention, is characterized in that the resonator is excited in the E.sub.011 mode enabling the energy of the accelerated-ion beam to be controlled continuously by establishing a region with a uniform distrubution of the accelerating field and by varying the extent of that tregion. This method can be realized by a linear accelerator comprising a cavity resonator 1 with drift tubes 2; tuners 3 arranged on the side wall of the resonator 1; an additional tuning means made in the form of a conducting post 4 installed in an end wall of the resonator 1 near its side wall parallel with the axis of the resonator and capable of being moved along that axis.

Description

The present invention relates to accelerators, and more specifically to methods for the acceleration of ions in linear accelerators and to linear accelerators for their realization.

In the prior art, there is a method for the acceleration of ions in linear accelerators which are cylindrical cavity resonators with drift tubes. This method is based on the excitation of an r.f. electromagnetic standing wave in the E.sub.010 mode, in which case the electric field is uniformly distributed along the axis of the resonator. In this case, because of the requirement for synchronism, acceleration can be effected only at a single value of the accelerating field strength.

A disadvantage of this method is that the accelerated-particle beam at the output from the accelerator can only have one value of energy for which the accelerator has been designed. On the other hand, most research and development programmes may be markedly extended, if the energy of the accelerated particles can be adjusted within broad limits.

Intermediate energies have been obtained on the heavy-ion linear accelerators at Berkeley, U.S.A., and Manchester, Great Britain, by varyin the tilt of the accelerating field excited in the E.sub.010 mode.

Among the disadvantages of this method are the impossibility of continuously controlling the energy of the accelerated particles, more than a two-fold loss of beam intensity, increased energy spread, and impaired stability of accelerator operation.

Another method for obtaining accelerated particles with intermediate energies is based on the use of a chain of single resonators uncoupled for the r.f. supply. By cutting off the r.f. supply to a certain number of the final resonators, it is possible to obtain beams with the energy attained at the excited resonators.

Among the disadvantages of this method are the difficulty of feeding the chain of uncoupled resonators with r.f. power in phase, the complex design and the low efficiency of the accelerator because of considerable additional losses of r.f. power on the many end walls in the chain of resonators.

Neither of these methods offers a means for the continuous control of the energy of accelerated particles.

An object of the present invention is to avoid the above-mentioned disadvantages.

The present invention is aimed at providing a method for the acceleration of ions in linear accelerators ensuring the continuous control of the energy of accelerated particles without any impairment in the quality of the beam, owing to a change in the mode of excitation, and at developing a linear-accelerator which realizes this method in the simples and most efficient way.

The present invention resides in that in the method disclosed herein ions are accelerated by an r.f. field excited in the E.sub.011 mode, enabling the energy of the accelerated particles to be controlled continuously by establishing region in which the accelerating field strength is distributed uniformly and by varying the extent of this uniform region, while the linear accelerator intended to realize this method, consisting of a cavity resonator with drift tubes and with tuners arranged on its side wall, has an additional tuning means made in the form of a conducting post installed in an end wall of the resonator near its side wall in parallel with the axis of the resonator and capable of being moved along this axis.

The fundamentally new method for the acceleration of particles disclosed herein enables the energy of the accelerated particles to be controlled continuously within broad limits (from one-third to the maximum energy of the accelerator) without any loss in the intensity of the accelerated beam. This acceleration method, in conjunction with the additional tuning means made in the form of a conducting post installed in an end wall of the resonator near its side wall in parallel with the axis of the resonator and capable of being moved along this axis, when used in a linear accelerator intended to realize this method, ensures high monochromaticity and stability of the characteristics of the beam with time without posing stringent requirements for the accuracy of the elements of the accelerating structure.

It is important that any linear accelerator can be switched over to controlled beam energy operation with ease and with only minor changes in its construction.

The additional tuning means for beam-energy control disclosed herein is very simple in design. The principle underlying the method for energy control and the construction of the tuning element make it possible to accomplish this control automatically to a pre-determined program.

A further advantage of the acceleration method disclosed herein is that it can be realized on the prior-art linear accelerators without major changes in construction and, as a consequence, without additional capital outlays.

The invention will be more fully understood from the following description of preferred embodiments when read in connection with the accompanying drawings wherein:

FIG. 1 is a plot showing the distribution of the accelerating field along the resonator excited in the E.sub.011 mode;

FIG. 2 shows the construction of a linear accelerator according to the invention;

FIG. 3 is a plot showing the frequency of the E.sub.011 mode and the frequencies of the nearby modes as functions of the length of the post immersed into the resonator;

FIG. 4 shows the spectra of some of the intermediate beam energies obtained in an experiment on a linear accelerator designed for a maximum proton energy of 9 MeV.

The method for the acceleration of ions disclosed herein consists in that acceleration is accomplished by an r.f. field excited in the E.sub.011 mode, by establishing a region in which the accelerated field is distributed uniformly and the extent of which can be controlled at will. Ordinarily, when the E.sub.011 mode is excited in the resonator of a linear accelerator with drift tubes all cells of which are tuned to resonate at the same frequency, the distribution of the r.f. electric field along its axis obeys the cosine law (curve a in FIG. 1).

The region in which the accelerating field is distributed uniformly and the extent of which can be controlled at will is realized in the linear accelerator of FIG. 2 which is a cylindrical cavity resonator 1 with drift tubes 2, tuners 3 arranged on its side wall, and an additional tuning means made in the form of a conducting post 4 installed on an end wall of the resonator near its side wall so that it can be moved along the axis of the resonator. By varying the length to which the post 4 is immersed into the resonator, one can shift the node of the electric field along the axis of the resonator and, as a result, reduce the field strength represented by the right-hand branch of curve b in FIG. 1, which by varying the depth of immersion into the resonator for the tuners 3, one can produce a region with a uniform distribution and a steep downward jump of the accelerating field.

If the extent of the region with the uniform distribution and an ideally steep downward jump of the accelerating field is changed, the energy of the accelerated particles will also change in a jamp, in step with the change of energy across one accelerating period. In practice, the downward jump is gradual rather than abrupt. Therefore, it is possible, by varying the slope of the jump, to control energy continuously within the limits corresponding to the change of energy across one accelerating period.

In the limiting case, by adjusting the depth of immersion into the resonator for the tuning post 4 and the tuners 3, it is an easy matter to obtain a uniform distribution for the accelerating field strength along the entire resonator owing to the said deformation of the field excited in the E.sub.011 mode.

Within certain limits, regions with a uniform distribution of field strength in the resonator excited in the E.sub.011 mode can be controlled solely by means of the tuners 3. However, in this case, the control is more difficult, since it involves a considerable increase in the size of the tuning means. Besides there remains the right-hand branch of the field (represented by curve c in FIG. 1) which results in an increased energy spread of the beam of accelerated particles.

In actual operation of the accelerator, the continuous control of the energy of accelerated ions is accomplished by moving the post 4 and the tuners 3 to a pre-determined program under which the length of post 4 immersed into the resonator and the depth of immersion into the resonator for the tuners 3 are calibrated in advance according to the required energy of the accelerated particles. This control, can be accomplished automatically. Furthermore, the length of the post 4 immersed into the resonator 1 and the depth of immersion for the tuners 3 can be varied from the outside, without any impairment of the vacuum in the resonator.

The stability of the characteristics of the accelerated beam in the face of changes in the energy of the accelerated particles is ensured by increasing the separation between the resonant frequency f.sub.011 of the resonator, corresponding to the E.sub.011 mode, and the frequencies f.sub.010 and f.sub.012 (FIG. 3) of the nearby modes, as the length 1 of the post 4 immersed into the resonator 1 (FIG. 2) is increased. The stability of the accelerating field is enhanced accordingly.

As an example of an embodiment of the acceleration method disclosed herein, we may quote the results obtained on a linear proton accelerator with a maximum design energy of 9 MeV. This accelerator operated in an experimental program, with the continuous control of the energy of accelerated particles. Energy control was accomplished remotely. Some of the spectra of the beams accelerated under conditions of continuous energy control are shown in FIG. 4. The energy of accelerated particles is laid off as abscissa and the relative intensity of the beams at the output of the accelerator is laid off as ordinate.

The slight decrease in beam intensity with decreasing energy of the accelerated particles seen in the spectra of FIG. 4 does not steam from the essence of the acceleration method disclosed herein, but is due to the type of beam focusing used in acceleration. In the case on hand, use was made of grid focusing, with the result that for some length of the accelerator where the accelerating field was not applied the beam was not focused either, and some of the accelerated particles were lost. No loss of beam intensity occurs when the accelerated beam is focused by quadrupole magnetic lenses installed in the drift tubes.

Claims

What is claimed is :

1. A method for the acceleration of ions in linear accelerators of the cavity resonator type with drift tubes, employing a standing r.f. electromagnetic wave in the E.sub.011 mode, enabling the energy of the accelerated-ion beam to be controlled continuously by establishing a region with a uniform distribution of the accelerating field and by varying the extent of that region.

2. A linear accelerator to realize the method for the acceleration of ions, comprising a cavity resonator with drift tubes; tuners arranged on the side wall of said resonator; an additional tuning means ensuring control of the extent of the region with a uniform distribution of the accelerating field and made in the form of a conducting post installed in an end wall of said resonator, near its side wall parallel with the axis of said resonator and capable of being moved along that axis.