Method and apparatus for controlling the acceleration energy of a radiofrequency multipole linear accelerator

Abstract

 A radio-frequency multipole linear accelerator (10) is made to vary its particle acceleration energy without varying the frequency of a radio-frequency power supplied to the accelerator (10) and therefore, without providing to the accelerator any means for varying the effective resonance frequency of the accelerator itself. For this purpose the voltage of the radio-frequency accleration fields developed in the accelerator is varied irrespective of the normal acceleration condition conventionally taken into consideration. To achieve the purpose the radio-frequency accelerator (10) is power-supplied by an amplification-variable radio-frequncy power amplifier (2) whose amplification is automatically controlled so as to keep the acceleration voltage in the accelerator constant by a controller (4). The controller (4) outputs a amplification control signal by comparing a small power picked up from the accelerator by a pick-up loop (5) with adesired acceleration voltage determined by a voltage setting apparatus (3). With the accelerator (10) operated at an acceleration voltage lower than the value complying with the normal acceleration condition, the particle acceleration energy shows a plurality of spectra, resulting in a change in the average acceleration energy. Further, with a magnet (6) provided at the outlet of the accelerator (10), a particle beam having a specific acceleration energy can be also taken out.

Description

Background of the Invention

[0001] The present invention relates to a method and apparatus for controlling the particle acceleration energy of a radio-­frequency multipole linear accelerator.

[0002] The particle acceleration condition of a radio-frequency linear accelerator is uniquely restricted by the relationship among the incident speed of particles to be accelerated, the the voltage, frequency and disposition intervals of the radio-frequency acceleration fields. The disposition invervals of the acceleration fields correspond, in the case of a drift tube type accelerator, to the disposition intervals of the tubular acceleration electrodes and, in the case of a radio-frequency multipole linear accelerator, to the pitches of wave forms formed on the ridge portion of each of the electrodes.

[0003] In conjunction with the above, it has hitherto been concluded that the particle acceleration energy of a radio-­frequency quadrupole linear accelerator can not be varied only by varying the radio-frequency acceleration voltage unless the frequency of the acceleration voltage is varied (see, e.g., Nuclear Instrum./Method in Phys. Research, B37(1989), pp. 253-­255; K. Tokiguchi et al, "Ion Beam Acceleration Using FQR"). In such an application field as that where a radio-frequency quadrupole linear accelerator is applied as an ion implanter for use in a semiconductor device manufacturing process, it is one of the essential requirements for the accelerator to have its particle acceleration energy made variable. A radio-­frequency quadrupole linear accelerator devised so as to have its particle acceleration energy varied by varying the frequency of the radio frequency power supplied to the accelerator is disclosed, for example, in Japanes Laid-open Patent Application No. 60-115199.

[0004] However, beause a radio-frequency linear accelerator, which is of a resonant cavity type, has a proper resonance frequency determined by the geometry of the cavity constituting the accelerator, it is necessary for the accelerator to be provided additionally with some means for enabling the accelerator to be tuned with varied frequencies. One of such means is a dummy resonator externally connected in parallel to the accelerator. The dummy resonator can be practiced either in the form of a L-C tank circuit made up of a constant reactance and a variable capacitance, as is employed in the above patent application, or in the form of a resonant cavity with an end wall constituted as a plunger. The resonance frequency can otherwise be varied by inserting in the cavity constituting the resonator a ferrite or dielectric substance for retarding the velocity of electromagnetic waves.

[0005] Of the above exemplified means for tuning a cavity type accelerator, the L-C tank circuit, whose Q-value is at least about one-tenth as low as that of the accelerator, consumes a large amount of radio-frequency power uselessly, while the cavity type dummy resonator with the resonance frequency made variable by constituting one end wall in the form of plunger causes the entire construction cost to be very expensive because such resonator has a somewhat complicated constitution and, in addition, needs precision machining. Further, the method of inserting a ferrite or dielectric substance needs any electrical or mechanical means for adjusting the effect of such substance on the electromagnetic field within the accelerator, and, therefore, makes the constitution of the accelerator itself very complicated.

Objects and Summary of the Invention

[0006] Accordingly, the present invention aims at eliminating the above difficulties and disadvantages involved in making variable the particle acceleration energy of a radio-frequency multipole linear accelerator, and makes it an object of the present invention to provide a new method for varying the particle acceleration energy of a radio-frequency multipole linear accelerator without employing any additional means for varying the resonance frequency of the accelerator itself.

[0007] Another object of the present invention is to provide, according to the above new method, an apparatus for varying the average particle acceleration energy of a radio-frequency multipole linear accelerator.

[0008] A further object of the present invention is to provide an apparatus for making a radio-frequency multipole linear accelerator produce selectively a plurality of particle beams having different acceleration energies.

[0009] The above new method according to the present invention consists only of varying the voltage of the radio-frequency particle acceleration fields in a radio-frequency multipole linear accelerator.

[0010] An apparatus according to the present invention consists of a system for varying the particle acceleration voltage developed in a radio-frequency linear accelerator.

[0011] Another apparatus according to the present invention consists of such a system and an electromagnet providing various magnetic fields for the selection of a particle beam having a specified acceleration energy.

[0012] The present invention is based on the inventors' very recent findings that a radio-frequency quadrupole linear accelerator, if operated with an acceleration voltage lower than the value satisfying the conventional acceleration condition, shows a plurality of energy spectra with respect to the accelerated particles. The spectra are, of course, distributed in a region of energy lower than that given under the normal acceleration condition. The principle or theory of the invention has not analysed satisfactorily yet. However, the following conjecture may well be given.

[0013] In a radio-frequency quadrupole linear accelerator, because the acceleration fields developed between the wave forms provided on the vanes (constituting the quadrupole) not only accelerate particles but also have, at the same time, a strong beam bunching effect throughout the entire length of acceleration, and accordingly, the particles can safely be accelerated without being scattered even if the acceleration voltage does not satisfy the normal acceleration condition. The distribution of acceleration energy spectra in a lower energy region may well be reasoned by the abnormal acceleration voltage lower than a value satisfying the normal acceleration condition.

[0014] In contrast with such a multipole type linear accelerator, a drift tube type accelerator can not be expected to accelerate particles safely under an acceleration voltage not satisfying the normal acceleration condition, because the particles to be accelerated are scattered owing to the weak bunching effect of the accelerator. In a drift tube type accelerator, the particles being accelerated are made coverged into a beam only within the drift tubes (throughout the entire acceleration length in the case of a quadrupole type accelerator) by means of electrostatic or magnetic lenses provided at the drift tubes. The acceleration fields themselves only accelerate particles and does not show any bunching effect at all owing to the distribution form of the fields.

Drief Description of the Drawings

[0015] In the following the present invention is described in futher detail in reference to the accompanying drawings, in which:

Fig. 1 shows blockdiagrammatically the constitution of an embodiment of the present invention applied to a radio-­frequency quadrupole linear accelerator;

Fig. 2 shows blockdiagrammatically the constitution of another embodiment of the present invention applied to a radio-­frequency quadrupole linear accelerator; and

Figs. 3 to 8 are examples of acceleration energy spectra shown by a radio-frequency quadrupole linear accelerator to which the the present invention is applied.

Detailed Description of the Invention

[0016] In Fig. 1, which shows an embodiment of the present invention, only a chain-line enclosure 1 represents the embodi­ment, while another enclosure 7 shows an experimental apparatus (Rutherford backscattering spectroscope) used to prove the effect of the invention. Further, a reference number 10 shows a radio-frequency quadrupole linear accelerator to which is applied the invention. A dotted line 11 represents a particle beam accelerated by the radio-frequency quadrupole linear accelerator 10.

[0017] The present embodiment 1 itself is made up of a radio-­frequency power source 2, a voltage setting apparatus 3 and a controller 4. The power source 2, which supplies a radio-­frequency power to the accelerator 10, consists essentially of a quartz-controlled oscillator and a power amplifier. In such a constitution, a small power is picked from the accelerator 10 by a pick-up loop 5 inserted therein, and fed to the controller 4 as a signal reflecting the acceleration voltage developed in the accelerator 10. The controller 4 compares the signal with an output from the voltage setting apparatus 3, and outputs a control signal toward the radio-frequency power source 2. This control signal controls the gain of the power amplifier constituting the radio-frequency power source 2, so that the acceleration voltage may be automatically controlled at a value set by the voltage setting apparatus 3. The acceleration voltage is thus varied through the operation of the voltage setting apparatus 3.

[0018] By varying the particle acceleration voltage in the accelerator 10 with the above embodiment applied, the particle acceleration energy showed such spectra as shown in Figs. 3 to 8, in which the abscissas and the ordinates respectively represent energy and the number of particles. All the spectra were obtained with the above mentioned Rutherford backscatter­ing spectroscope 7, which consists essentially of a target 8 and a surface barrier detector 9. The target 8 is made of a 100 A to 200 A thin gold film evaporated on a graphite plate. The detector 9 counts the number of particles (positive ions) having different acceleration energies. Of Figs. 3 to 8, Fig. 3 shows a mono-peak spectrum for ¹⁴N⁺ accelerated by a 100% voltage determined from the exact acceleration condition, while Figs. 4, 5 and 6 show spectra for the same ions accelerated by a 87%, 84% and 78% voltage, respectively. Throughout this series of experiments, ¹⁴N⁺ ions having an incident energy of 84 keV were injected to a radio-frequency quadrupole linear accelerator having a proper resonace frequency of 70.300 MHz. An acceleration voltage of about 54.8 kV complies with the normal acceleration condition of this accelerator. On the other hand Figs. 7 and 8 show spectra for ¹¹B⁺ accelerated by a 100% and 88% voltage, respectively. In these experiments, the incident energy of the ¹¹B⁺ ions was 66 keV, while the proper resonance frequency of the accelerator used was 70.340 MHz. The acceleration voltage complying with the normal condition is about 43 kC for this accelerator.

[0019] As is understood from the experimental spectral patterns as shown in Figs. 3 to 8, this embodiment of the invention makes it possible to vary average acceleration energy.

[0020] Another embodiment of the present invention is shown in Fig. 2 with the embodiment itself indicated by a reference number 1a. In Fig. 2, the constituents corresponding to those in Fig. 1 are indicated by the same reference number as those used in Fig. 1. This embodiment comprises an electromagnet 6 to be located at the beam outlet of the accelerator 10. With a magnetic field applied vertically to the beam 11 by this magnet 6, any one of the particle beams having different energies can selectively be taken out.

[0021] It is needless to say that the present invention is applied to any radio-frequency multipole linear accelerator not restricted only to such a quadrupole type accelerator as exemplified in the above description of the present invention.

Claims

1. A method of varying the particle acceleration energy of a radio-frequency multipole linear accelerator operated with a radio-frequency power supplied, the radio-frequency power causing particle acceleration voltages to be developed in the accelerator, the method comprising a step of:
varying the radio-frequency power to be supplied to the accelerator so as to vary the particle acceleration voltages developed in the accelerator.

2. A method of varying the particle acceleration energy of a radio-frequency multipole linear accelerator operated with a radio-frequency power supplied, the radio-frequency power causing particle acceleration voltages to be developed in the accelerator, the method comprising steps of:
varying the radio-frequency power to be supplied to the accelerator so as to vary the particle acceleration voltages developed in the accelerator; and
electromagnetically warping particle beams accelerated by said radio-frequency multipole linear accelerator.

3. An apparatus for varying the particle acceleration energy of a radio-frequency multipole linear accelerator, the apparatus comprising:
a radio-frequency power amplifier for supplying a radio-frequency power to said radio-frequency multipole linear accelerator;
a radio-frequency oscillator for exciting said radio-­frequency power amplifier;
a voltage setting means for setting a desired particle acceleration voltage to be developed in said radio-frequency multipole linear accelerator; and
a controlling means for controlling the gain of said radio-frequency power amplifier according to a signal picked up from the radio-frequency multipole linear accelerator and an output from said votage setting means.

4. An apparatus for varying the particle acceleration energy of a radio-frequency multipole linear accelerator, the apparatus comprising:
a radio-frequency power amplifier for supplying a radio-frequency power to said radio-frequency multipole linear accelerator;
a radio-frequency oscillator for exciting said radio-­frequency power amplifier;
a voltage setting means for setting a desired particle acceleration voltage to be developed in said radio-frequency multipole linear accelerator;
a controlling means for controlling the gain of said radio-frequency power amplifier according to a signal picked up from the radio-frequency multipole linear accelerator and an output from said votage setting mans; and
a magnet for providing a magnetic field to warp particle beams accelerated by said radio-frequency multipole linear accelerator, said magnetic field being for selecting out a particle beam having a specific acceleration energy.

Drawing