GB2186736A 1 SPECIFICATION plurality of cycles of said high frequency so
that at the output of the drift region structure Ion beam arrangement the velocity modulated ion beam forms dis crete bunches of ions. Preferably there are This invention relates to an ion beam arrange- 70 means adjacent to the output of the drift ment and is specifically concerned with such structure for receiving said discrete bunches of an arrangement in which the ion beam pro- ions and for applying a high frequency alter duced thereby is in the form of a series of nating electric field to the bunches such that bunches or packets of energy. It is customary the ion bunches are accelerated in the same to generate ions in a continuous stream and 75 sense, the electric field applied to said to direct them to the point at which they are bunches being substantially greater than the required for utilisation. As the utilisation of the electric field applied to modulate the ion ions often depends on them having a suffici- beam.
ently high energy, it is necessary for some The drift structure is preferably in the form applications for the ion beam to be passed 80 of a conductive tube extending over the whole through an accelerator which greatly increases length of the drift region and within which the their velocity. The acceleration of the ions is net accelerating electric field acting on the achieved by passing them through a strong ions is substantially zero. However, the drift electric field, but because the voltage neces- structure may simply comprise an initial con sary to impart very high energies to the ions 85 ductive collar follow ' ed by an open region can be very large indeed, it has been pro- which is shielded from electric field variations posed to subject the ions to a high frequency so that it constitutes a substantially equipoten alternating voltage so that they are given a tial drift region. Preferably, the drift tube in series of accelerating -kicks- along the length cludes means for electrostatically focussing of the accelerator. During one half cycle of the 90 the ion beam so as to confine it to predeter alternating voltage the ions are accelerated, mined path and to prevent it spreading out but are shielded by drift tubes from half wards into the walls of the drift tube. Conve cycles of the wrong polarity so that the net niently the focussing means consists of a plu effect is to always accelerate them in the rality of separate stages to which different fo same sense. An example of this tvpe of linear 95 cussing potentials can be applied. These fo accelerator has been described by Sloan and cussing potentials do not however, signifi Lawrence; D.H.Sloan and E.O.Lawrence, Physi- cantly affect the longitudinal potential, and cal Review 38 2021 (1931). However, there hence velocity, of the ions along the drift are other methods by which ions may be ac- tube.
celerated using multiples of low voltage accel- 100 The amplitude of the alternating electric field erating stages. The effect of the high frewhich is applied to the incident steady ion quency alternating voltage is to convert the beam is such as to produce a relatively small steady stream of ions into separate bunches velocity variation of ions within the beam so but in a very inefficient manner as many of that the beam as a whole is substantially the ions will initially be out of phase with 105 mono-energetic. Because the velocity modula accelerating voltage. This will, of course, re- tion is small, the drift tube must be fairly long sult in a de-acceleration force causing partial to gi-ve the ions sufficient time to separate loss of the ion beam. into discrete bunches. The point at which the The present invention seeks to provide an ions form the most distinct bunches is fairly improved ion beam arrangement in which a 110 critical, as beyond that point the bunches will relatively steady ion beam is converted in an again tend to merge. At the point where the efficient manner into a bunched beam. Such a bunches are most distinct, the relatively large bunched beam can be used directly if re- high frequency field is applied to accelerate quired, or it can be very effectively and effici- each bunch as a whole, and to impart a mean ently accelerated by including in the ion beam 115 velocity which is high in relation to the velo arrangement a high frequency accelerator city modulation. In this way, the bunch of ions which accelerates the bunched beam to a re- can be regarded as being essentially mono quired energy level. energetic.
According to this invention an ion beam ar- In a preferred example of the invention the rangement includes means for receiving a 120 ion beam apparatus includes the ion buncher steady ion beam having a predetermined mean in combination with a linear accelerator which velocity and for applying a high frequency al- accelerates the ion bunches to very high velo ternating electric field to the beam, so as to cities using a sequence of high frequency speed up or to slow down ions by an amount stages to each of which an alternating high which is small compared to the mean velocity 125 voltage V is applied. Each stage imparts an of the ions to produce a velocity modulated energy of approximately eV to the ions, and if ion beam; an e;ongate drift structure arranged N stages are provided in the sequence, the to receive the velocity modulated beam and final energy is approximately eV.N. By ensur dimensioned such that the transit time of the ing that the ions are in discrete bunches be beam through the drift region is in excess of a 130 fore the high accelerating voltage is applied 2 GB2186736A 2 and by ensuring that the ions within a bunch tube 4 such that the phase of the HF signal are essentially monoenergetic; the separate thereon is in a sense which accelerates each bunches are preserved as the ions are acceler- bunch as a whole. The increased energy im ated, allowing an extremely efficient ion beam parted by this electrode 4 is significantly transfer to take place through the ion beam 70 greater than the energy distribution within a arrangement. bunch.
The ion bunches can of course be used for In order to provide some degree of control purposes other than feeding a linear accelera- over the relative phases of the HF signals ap tor. Once the steady ion beam has been div- plied to the electrode tubes 2 and 4, the cir- ided into a stream of discrete ion pulses, each 75 cuit shown in Figure 1 is utilised. The HF sig pulse can be separately deflected and/or util- nal originates with an HF drive source 5 which ised as required. The invention is further de- is fed via a power splitter 6 to the respective scribed by way of example with reference to electrode tubes 2 and 4, via respective phase the accompanying drawings in which: adjusters 7, 8 and HF amplifiers 9, 10. The Figure 1 shows an ion beam buncher in 80 action of the amplifiers 9 and 10 is to in schematic form, crease the level of the signal necessary for Figure 2 shows a modified ion beam bun- application to the electrodes 2 and 4 and to cher, and provide any degree of impedance matching Figure 3 shows an ion beam arrangement which might be necessary. The output of the including a linear accelerator and an ion beam 85 signals from each HF amplifier is compared buncher. with the input signal at respective phase dis Referring to Figure 1 - a source of ions (not criminators 11, 12 and any phase discrepancy shown) produces a steady beam of ions in corrected by means of the phase adjuster 7 8.
which the charge density is substantially con- This compensates for any undesired phase dif stant and the mean energy of the ions varies 90 ferences introduced by the operation of the only slightly from a nominal value. Such a amplifiers 9 and 10. The two phase discrimi steady ion beam can be produced in accor- nators 11 and 12 are coupled together so dance with known techniques. In order to that any phase adjustment produced by one is convert the steady ion beam into a sequence imparted to the other so that the phase of the of ion bunches, the beam is passed through 95 HF signal at electrode 4 is known relative to the ion buncher which consists of a first short that on electrode 2. Ideally, the HF signals on earthed tube 1, an HF electrode tube 2 and a these two electrodes are in phase, but minor relatively long earthed drift tube 3 the interior adjustments may be necessary in dependence of which represents an equipotential drift re- on the input energy of the steady ion beam, gion. At the output of the drift tube 3 the ion 100 and on the length of the drift tube 3.
bunches are passed towards a further HF elec- Referring to Figure 2, a modified ion bun- trode tube 4 which acts to accelerate them. cher is shown, in which the phase and voltage The purpose of the tube 1 is to shield the ion control to the beam bunch is provided by an source and any previous stages from the ac- alternative method than in Figure 1. In this tion of the HF signal on the HF electrode 2. 105 instance the level of the alternating voltage The action of the HF signal applied by the fed to electrode tube 2 is taken directly from electrode tube 2 to the ion beam is to set up a common HF drive source 14 via an attenua an oscillating electric field which causes the tor 15, the necessary drive amplifier not being ions to either be accelerated or retarded with illustrated. It is necessary that the level of the respect to their initial velocity depending on 110 alternating HF voltage applied to electrode their position relative to the electrode. Thus tube 2 is substantially less than that applied the ions entering the long drift tube 3 will to the electrode tube 4, so that only a rela have slightly differing velocities, and the ampli- tively small velocity modulation is initially im tude of the HF electric field is chosen such parted to the ion beam. Whereas typically the that the velocity distribution is relatively small. 115 AC voltage applied to electrode tube 4 is of The HF signal applied to the electrodes is in the order of 50 KV, that applied to the elec the range 2 MHz to 30 MHz. The length of trode tube 2 is of the order of only 2-1/2 KV, the drift tube 3 is chosen such that the transit and the actual value is adjusted by means of time of the ions through the drift region is the variable attenuator 15 so as to produce significantly longer than the repetition period 120 the best separation of the bunches when they of the HF frequency. Thus typically, the length arrive at the electrode tube 4. In this instance, of the drift tube 3 is such that the ions take a the long drift tube 3 is provided with internal time to travel from one end of the drift tube focussing elements 20, 21, 22, 23 which act to the other in a time corresponding to ap- to confine the ion beam to the central axis of proximately six cycles of the HF signal. During 125 the ion beam buncher. These focussing elec this time, the effect of the initial variation of trodes are electrostatic in nature end operate velocity of the individual ions is to cause them in well known manner. Although relatively high to separate out into separate bunches. These voltages may be applied to these electrodes separate and discrete bunches of ions are ar- so as to cause transverse acceleration of the ranged to arrive at the gap of the HF drift 130 ions with respect to the longitudinal axis of 3 GB2186736A 3 the drift tube 3, they do not materially influ- put of this ion beam buncher 40 is a series of ence the longitudinal velocity of the ions. discrete bunches or packets of ions having a Phase variations between the HF tubes 2 relatively small energy distribution superim and 4 may be compensated by applying a dc posed on a mean energy of about 50 KeV as voltage from a source 32, to tube 3. If a 70 previously described. These bunches of ions positive voltage is applied, the positive ions in are then accelerated by means of the linear the gap between tubes 2 and 3 will not gain accelerator 41 and assuming that fifty stages as much energy as in the case of tube 3 are provided, an output energy of approxi being at earth potential. These same ions on mately 2.4 million electron volts is obtained at leaving drift tube 3 will gain additional energy. 75 the output port 42. It is likely that there will in The net result is a zero energy gain due to fact be a significant but small spread of ener tube 3. However, the time a particular ion gies at this output port and if the precise passes through tube 3 has increased, com- energy distribution of the ion beam is impor pared to the situation when tube 3 is at earth tant it is fed through an electrostatic separator potential. Consequently, a negative voltage 80 43 which in known manner imparts a predet would decrease the time ions pass through ermined curve to the individual ions depending tube 3. Thus by applying a de voltage to tube on their energy. By selecting just those ions at 3, one can compensate for phase variations port 44 having a predetermined narrow band between HF tubes 2 and 4 due to the attenu- energy, only the required ions can be directed ator 15. Thus the d.c. bias source 32 is ad- 85 to the point of utilisation 45.
justable in magnitude and polarity of its vol- By means of the ion buncher in accordance tage to permit the best phase relationship be- with this invention the transport efficiency of tween the ion bunches at the output end of such an ion beam arrangement can be in the drift tube 3 and the HF signal on the creased very significantly indeed. In a conven- electrode tube 4 to be achieved. 90 tional ion beam system in which a steady ion In Figure 2, the ion beam buncher is ar- beam is applied to a linear accelerator, the ranged to feed a linear accelerator in which beam transport efficiency might be only of the just the first two stages 24 and 25 are illus- order of 16 to 17%. Use of the ion beam trated. The linear accelerator consists of a buncher in accordance with the invention can large number of separate HF stages, at each 95 raise the transport efficiency up to 70%, stage of which an accelerating voltage of 50