Ion beam arrangement

Abstract

 An ion beam arrangement accepts a steady beam of ions from an ion source and forms a stream of ion bunches from it, so that the bunches or packets of ions can be subsequently utilised. The ion beam buncher consists of an electrode (2) to which a high frequency alternating potential (14) is applied to impart a velocity modulation to ions within the steady beam. This electrode is followed by a drift space (3) in which the velocity modulation results in the formation of the discrete bunches of ions, but without any overall increase in ion energy. These ion bunches can then be accelerated to high energy levels using a multistage alternating voltage linear accelerator (24, 25).

Description

[0001] This invention relates to an ion beam arrangement and is specifically concerned with such an arrangement in which the ion beam produced thereby is in the form of a series of bunches or packets of energy. It is customary to generate ions in a continuous stream and to direct them to the point at which they are required for utilisation. As the utilisation of the ions often depends on them having a sufficiently high energy, it is necessary for some applications for the ion beams to be passed through an accelerator which greatly increases their velocity. The acceleration of the ions is achieved by passing them through a strong electric field , but because the voltage necessary to impart very high energies to the ions can be very large indeed, it has been proposed to subject the ions to a high frequency alternating voltage so that they are given a series of accelerating "kicks" along the length of the accelerator. During one half cycle of the alternating voltage the ions are accelerated, but are shielded by drift tubes from half cycles of the wrong polarity so that the net effect is to always accelerate them in the same sense. An example of this type of linear accelerator has been described by Sloan and Lawrence; D.H.Sloan and E.O.Lawrence, Physical Review 38 2021 (1931). However, there are other methods by which ions may be accelerated using multiples of low voltage accelerating stages. The effect of the high frequency alternating voltage is to convert the steady stream of ions into separate bunches but in a very inefficient manner as many of the ions will initially be out of phase with accelerating voltage. This will, of course, result in a de-acceleration force causing partial loss of the ion beam.

[0002] The present invention seeks to provide an improved ion beam arrangement in which a relatively steady ion beam is converted in an efficient manner into a bunched beam. Such a bunched beam can be used directly if required, or it can be very effectively and efficiently accelerated by including in the ion beam arrangement a high frequency accelerator which accelerates the bunched beam to a required energy level.

[0003] According to this invention an ion beam arrangement includes means for receiving a steady ion beam having a predetermined mean velocity and for applying a high frequency alternating electric field to the beam, so as to speed up or to slow down ions by an amount which is small compared to the mean velocity of the ions to produce a velocity modulated ion beam; an elongate drift structure arranged to receive the velocity modulated beam and dimensioned such that the transit time of the beam through the drift region is in excess of a plurality of cycles of said high frequency so that as the output of the drift region structure the velocity modulated ion beam forms discrete bunches of ions. Preferably there are means adjacent to the output of the drift structure for receiving said discrete bunches of ions and for applying a high frequency alternating electric field to the bunches such that the ion bunches are accelerated in the same sense, the electric field applied to said bunches being substantially greater than the electric field applied to modulate the ion beam.

[0004] The drift structure is preferably in the form of a conductive tube extending over the whole length of the drift region and within which the net accelerating electric field acting on the ions is substantially zero. However, the drift structure may simply comprise an initial conductive collar followed by an open region which is shielded from electric field variations so that it constitutes a substantially equipotential drift region. Preferably, the drift tube includes means for electrostatically focussing the ion beam so as to confine it to predetermined path and to prevent it spreading outwards into the walls of the drift tube. Conveniently the focussing means consists of a plurality of separate stages to which different focussing potentials can be applied. These focussing potentials do not however, significantly affect the longitudinal potential, and hence velocity, of the ions along the drift tube.

[0005] The amplitude of the alternating electric field which is applied to the incident steady ion beam is such as to produce a relatively small velocity variation of ions within the beam so that the beam as a whole is substantially mono-energetic. Because the velocity modulation is small, the drift tube must be fairly long to give the ions sufficient time to separate into discrete bunches. The point at which the ions form the most distinct bunches is fairly critical, as beyond that point the bunches will again tend to merge. At the point where the bunches are most distinct. the relatively large high frequency field is applied to accelerate each bunch as a whole, and to impart a mean velocity which is high in relation to the velocity modulation. In this way, the bunch of ions can be regarded as being essentially mono-energetic.

[0006] In a preferred example of the invention the ion beam apparatus includes the ion buncher in combination with a linear accelerator which accelerates the ion bunches to very high velocities using a sequence of high frequency stages to each of which an alternating high voltage V is applied. Each stage imparts an energy of approximately eV to the ions, and if N stages are provided in the sequence, the final energy is approximately eV.N. By ensuring that the ions are in discrete bunches before the high accelerating voltage is applied and by ensuring that the ions within a bunch are essentially mono-­energetic; the separate bunches are preserved as the ions are accelerated, allowing an extremely efficient ion beam transfer to take place through the ion beam arrangement.

[0007] The ion bunches can of course be used for purposes other than feeding a linear accelerator. Once the steady ion beam has been divided into a stream of discrete ion pulses, each pulse can be separately deflected and/or utilised as required. The invention is further described by way of example with reference to the accompanying drawings in which:

Figure 1 shows an ion beam buncher in schematic form,

Figure 2 shows a modified ion beam buncher, and

Figure 3 shows an ion beam arrangement including a linear accelerator and an ion beam buncher.

[0008] Referring to Figure 1, a source of ions (not shown) produces a steady beam of ions in which the charge density is substantially constant and the mean energy of the ions varies only slightly from a nominal value. Such a steady ion beam can be produced in accordance with known techniques. In order to convert the steady ion beam into a sequence of ion bunches, the beam is passed through the ion buncher which consists of a first short earthed tube 1, an HF electrode tube 2 and a relatively long earthed drift tube 3 the interior of which represents an equipotential drift region. At the output of the drift tube 3 the ion bunches are passed towards a further HF electrode tube 4 which acts to accelerate them. The purpose of the tube 1 is to shield the ion source and any previous stages from the action of the HF signal on the HF electrode 2. The action of the HF signal applied by the electrode tube 2 to the ion beam is to set up an oscillating electric field which causes the ions to either be accelerated or retarded with respect to their initial velocity depending on their position relative to the electrode. Thus the ions entering the long drift tube 3 will have slightly differing velocities, and the amplitude of the HF electric field is chosen such that the velocity distribution is relatively small. The HF signal applied to the electrodes is in the range 2 MHz to 30 MHz. The length of the drift tube 3 is chosen such that the transit time of the ions through the drift region is significantly longer than the repetition period of the HF frequency. Thus typically, the length of the drift tube 3 is such that the ions take a time to travel from one end of the drift tube to the other in a time corresponding to approximately six cycles of the HF signal. During this time, the effect of the initial variation of velocity of the individual ions is to cause them to separate out into separate bunches. These separate and discrete bunches of ions are arranged to arrive at the gap of the HF drift tube 4 such that the phase of the HF signal thereon is in a sense which accelerates each bunch as a whole. The increased energy imparted by this electrode 4 is significantly greater than the energy distribution within a bunch.

[0009] In order to provide some degree of control over the relative phases of the HF signals applied to the electrode tubes 2 and 4, the circuit shown in Figure 1 is utilised. The HF signal originates with an HF drive source 5 which is fed via a power splitter 6 to the respective electrode tubes 2 and 4,via respective phase adjusters 7, 8 and HF amplifiers 9, 10. The action of the amplifiers 9 and 10 is to increase the level of the signal necessary for application to the electrodes 2 and 4 and to provide any degree of impedance matching which might be necessary. The output of the signals from each HF amplifier is compared with the input signal at respective phase discriminators 11, 12 and any phase discrepancy corrected by means of the phase adjuster 7, 8. This compensates for any undesired phase differences introduced by the operation of the amplifiers 9 and 10. The two phase discriminators 11 and 12 are coupled together so that any phase adjustment produced by one is imparted to the other so that the phase of the HF signal at electrode 4 is known relative to that on electrode 2. Ideally, the HF signals on these two electrodes are in phase, but minor adjustments may be necessary in dependence on the input energy of the steady ion beam, and on the length of the drift tube 3.

[0010] Referring to Figure 2, a modified ion buncher is shown, in which the phase and voltage control to the beam bunch is provided by an alternative method than in Figure 1. In this instance, the level of the alternating voltage fed to electrode tube 2 is taken directly from a common HF drive source 14 via an attenuator 15, the necessary drive amplifier not being illustrated. It is necessary that the level of the alternating HF voltage applied to electrode tube 2 is substantially less than that applied to the electrode tube 4, so that only a relatively small velocity modulation is initially imparted to the ion beam. Whereas typically the AC voltage applied to electrode tube 4 is of the order of 50 KV, that applied to the electrode tube 2 is of the order of only 2½ KV, and the actual value is adjusted by means of the variable attenuator 15 so as to produce the best separation of the bunches when they arrive at the electrode tube 4. In this instance, the long drift tube 3 is provided with internal focussing elements 20, 21, 22, 23 which act to confine the ion beam to the central axis of the ion beam buncher. These focussing electrodes are electrostatic in nature and operate in well known manner. Although relatively high voltages may be applied to these electrodes so as to cause transverse acceleration of the ions with respect to the longitudinal axis of the drift tube 3, they do not materially influence the longitudinal velocity of the ions.

[0011] Phase variations between the HF tubes 2 and 4 may be compensated by applying a dc voltage from a source 32, to tube 3. If a positive voltage is applied, the positive ions in the gap between tubes 2 and 3 will not gain as much energy as in the case of tube 3 being at earth potential. These same ions on leaving drift tube 3 will gain additional energy. The net result is a zero energy gain due to tube 3. However, the time a particular ion passes through tube 3 has increased, compared to the situation when tube 3 is at earth potential. Consequently, a negative voltage would decrease the time ions pass through tube 3. Thus by applying a dc voltage to tube 3, one can compensate for phase variations between HF tubes 2 and 4 due to the attenuator 15. Thus the d.c. bias source 32 is adjustable in magnitude and polarity of its voltage to permit the best phase relationship between the ion bunches at the output end of the drift tube 3 and the HF signal on the electrode tube 4 to be achieved.

[0012] In Figure 2, the ion beam buncher is arranged to feed a linear accelerator in which just the first two stages 24 and 25 are illustrated. The linear accelerator consists of a large number of separate HF stages, at each stage of which an accelerating voltage of 50 KV is applied. In this way, the total accelerating voltage experienced by the ions is multiplied by the number of stages and it is not necessary to generate extremely high accelerating voltages in a single step. The principle of the linear accelerator utilising HF signals is that only ions which are correctly placed relative to the HF signal are accelerated. In the present invention, since the ions are fed to the first stage of the HF ion accelerator in discrete bunches, it is possible to arrange that all of the ions within a bunch are accelerated in an efficient manner. The spacing between the adjacent HF electrodes, i.e. between electrode 4 and electrode 26 and between electrode 26 and electrode 27, is chosen to correspond to the time taken for the ions to travel in one cycle of the HF signal. It will be appreciated that the electrode tube length is increased the further the ions travel into the accelerator, as the ion velocity progressively increases so that they travel further during each period of the HF signal. The action of the earthed drift tubes 28 and 29 is to provide a reference potential for the electric field and to shield the ions from the field reversal of the HF signal. Each earthed drift tube 28, 29 includes a focussing element 30.

[0013] Figure 3 shows a system in which the ion beam arrangement consists of a conventional ion source 39 which feeds a steady beam of ions typically having an energy of 50 KeV to the ion beam buncher 40 which takes the form as illustrated in Figure 1 and 2. The output of this ion beam buncher 40 is a series of discrete bunches or packets of ions having a relatively small energy distribution superimposed on a mean energy of about 50 KeV as previously described. These bunches of ions are then accelerated by means of the linear accelerator 41 and assuming that fifty stages are provided, an output energy of approximately 2.4 million electron volts is obtained at the output port 42. It is likely that there will in fact be a significant but small spread of energies at this output port and if the precise energy distribution of the ion beam is important it is fed through an electrostatic separator 43 which in known manner imparts a predetermined curve to the individual ions depending on their energy. By selecting just those ions at port 44 having a predetermined narrow band energy, only the required ions can be directed to the point of utilisation 45.

[0014] By means of the ion buncher in accordance with this invention the transport efficiency of such an ion beam arrangement can be increased very significantly indeed. In a conventional ion beam system in which a steady ion beam is applied to a linear accelerator, the beam transport efficiency might be only of the order of 16 to 17%. Use of the ion beam buncher in accordance with the invention can raise the transport efficiency up to 70%.

Claims

1. An ion beam arrangement including means for receiving a steady ion beam having a predetermined mean velocity and for applying a high frequency alternating electric field to the beam, so as to speed up and slow down ions by an amount which is small compared to the mean velocity of the ions to produce a velocity modulated ion beam; an elongate drift structure arranged to receive the velocity modulated beam and dimensioned such that the transit time of the beam through the drift region is in excess of a plurality of cycles of said high frequency so that at the output of the drift region the velocity modulated ion beam forms discrete bunches of ions.

2. An arrangement as claimed in claim 1 and wherein there is provided means adjacent to an output end of the drift structure for receiving said discrete bunches of ions and for applying a high frequency alternating electric field to the bunches such that the ion bunches are accelerated in the same sense, the electric field applied to said bunches being substantially greater than the electric field applied to modulate the ion beam.

3. An arrangement as claimed in claim 1 or 2 and wherein the drift structure is in the form of a conductive tube.

4. An arrangement as claimed in claim 1, 2 or 3 and wherein the applied high frequency alternating electric field has a frequency in relation to the mean velocity of the ion beam passing through it, such that the transit time of the ions is at least approximately a plurality of periods of said high frequency.

5. An arrangement as claimed in any of the preceding claims and wherein the drift structure includes means for electrostatically focussing the ion beam so as to confine it to predetermined path and to prevent it spreading outwards into the drift structure itself.

6. An arrangement as claimed in claim 5 and wherein the focussing means consists of a plurality of separate stages positioned along the length of the drift structure and to which mutually different focussing potentials can be applied.

7. An arrangement as claimed in any of the preceding claims and wherein a common frequency source is arranged to apply the high frequency electric field to the steady ion beam and to apply the high frequency electric field to the ion bunches produced by said drift tube.

8. An arrangement as claimed in claim 7 and wherein the magnitude of the high frequency electric field applied to the steady ion beam is substantially less than that applied to the ion bunches.

9. An arrangement as claimed in any of claims 2 to 8, and wherein means are provided for applying a d.c. voltage bias to said drift structure relative to that of said means adjacent to the output of the drift structure and to which the high frequency signal is applied.

10. An arrangement as claimed in claim 9 and wherein the d.c. voltage bias is variable in magnitude and polarity.

11. An arrangement as claimed in any of the preceding claims and wherein a multi-stage high frequency linear accelerator is positioned to receive said ion bunches and to accelerate them under the action of an applied high frequency electric field which is synchronised to the position of the individual ion bunches.

Drawing