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Klystron bunching of the ion beam of a 5.5 MV Van de Graaff
NUCLEAR
INSTRUMENTS
KLYSTRON
AND
METHODS
BUNCHING
J. H. ANDERSON,
41 (1966)30-40;0 NORTH-HOLLAND
OF THE ION BEAM
R. BATCHELOR,
PUBLISHING
co.
OF A 5.5 MV VAN DE GRAAFF
F. A. HOWE, G. JAMES and J. H. TOWLE
Atomic Weapons Research Establishment, Aldermaston, Berks., England Received 22 November
1965
Modifications to the AWRE 5.5 MV Van de Graaff to improve its performance as a pulsed machine are described. A klystron bunching facility has been incorporated into the top terminal of
the machine and pulsed ion beams with the pulse width about 1.5 ns and analysed mean currents of 10 ,ILAare obtained at 5 MC/S repetition rate.
1. Introduction
paratus needs to be compact and reliable in an environment where electrical surges are frequent, the insulating gas pressure is high and the radiation level is intense.
For some years the AWRE 5.5 MV Van de Graaff * has been used as a pulsed machine for fast neutron spectroscopy. In the early work pulsing at 5 MC/S was accomplished by rf deflection of the accelerated ion beam’) but the neutron background was high and the beam intensity comparatively low. In the next stage’) the rf deflection was applied to the beam prior to acceleration and this reduced considerably the neutron background particularly when deuterons were accelerated. However the practical lower limit for the pulse duration was 3 ns and with this burst width the mean current on target never exceeded 1 PA. In a few applications’) post acceleration chopping was used to trim the beam bursts to less than 2 ns, but this resulted in a corresponding reduction in mean current. The next obvious step is to increase the pulse intensity and reduce the burst duration by beam bunching and this report gives details of how this has been accomplished by the “klystron” method. This method has had fairly wide application to Cockcroft Walton and Tandem accelerators4, “) but has not been greatly used in single stage Van de Graaff machines. Moak et a1.6) have recently used the method on the ORNL 3 MV Van de Graaff. The main reasons for choosing the klystron method rather than the variable magnetic path method7-9) were greater convenience and lower cost. The introduction of a compression magnet at the output of the machine would have necessitated a complete reorganisation of the target room facilities and would have involved the extra cost of a building extension. On the other hand klystron bunching prior to acceleration has meant that the experimental target facilities have remained undisturbed and all can be served with the pulsed beam. The main disadvantage of the klystron method is that it adds extra complexity to the top terminal of the machine and consequently the ap* Supplied by High Voltage Engineering Corporation,
2. General design considerations The general problem of klystron bunching of ion beams has been considered by Whiteway”). Power consumption limitations lead to the choice of an inserted bunching tube as the most convenient method of applying velocity modulation to an ion beam. In this method a sinewave bunching voltage is applied to a tube with a field gap at each end. The length of tube is chosen (in relation to the bunching frequency) so that an ion accelerated at the first gap receives an equal acceleration at the second gap. In this way the beam is subject to a sine wave energy modulation along axis. It can be shown”) that bunching occurs, and a waveform with a single infinite peak is obtained when +X&/u
= 1
(1) where c( is the ratio of the peak sine wave modulation energy E,,, to the mean ion energy, u is the velocity corresponding to the mean ion energy, D is the drift distance over which the beam of velocity u becomes compressed or bunched, o = 27ifwherefis the frequency of the bunching voltage applied to the tube. The bunching efficiency depends on the value of CI which should be less than 0.1. For this condition about 28% of the beam can be utilised in forming an ion burst within a time bracket of 1% of the interval between bursts. In a practical system the ion beam is subjected (over a distance of several metres) to a number of stages of acceleration and drift. However this complex system can be replaced5,i0) by a simple drift system with a single equivalent drift length and constant ion beam energy. Most of the beam compression takes place in
Burling-
ton, Mass.
30
KLYSTRON
BUNCHING
PROBE 1
ION
130
Mcls
SOURCE
20
cm I
>
CHOPPING PLATES
-BUhlCHlNG
WAVEFORM ‘5 Mc’r BUNCHING
GAP 64
DEFLECTOR
TUBE
LENS
;P
cm I
VIRTUAL
YIGH ENERGY ACCELER’N COLUMN 387 cln I
ELECTROSTATIC QUADRUPOLE LENS
STEERING PLATES
IMAGE
Fig. 1. Schematic layout of machine with bunching modifications.
the drift tubes at lower beam energies and hence the ion beam is substantially bunched prior to entry into the accelerating tube of the machine. For the system described in this paper (fig. 1 for the schematic layout of the Van de Graaff), the mean beam energy at the point where the bunching modulation is applied is about 30 keV. With an equivalent drift length at 30 keV of 31 cm (which is the figure corresponding to the system in fig. 1 when the machine voltage is 6 MV), the value of c(w for protons is 1.55 x 10’. A lower machine voltage or a higher ion mass leads to a lower value of s(w. We aim to produce
OF
AN
ION
BEAM
31
pulses at a repetition rate of 5 MC/S and hence if we bunch at this frequency, the maximum value of ix required is 0.5 which represents a peak energy modulation E,,, of 15 keV. This value is intolerably high, particularly in relation to the focussing of the beam through the accelerator. Since it is not possible to increase the equivalent drift length or decrease II by large factors, it becomes necessary to increase o and in the present design a bunching frequency of 15 MC/S is used. The pulse repetition rate can be retained by beam chopping at 5 MC/S but this must be synchronised with the 15 MC/S bunching frequency. The consequence of using a bunching frequency three times the pulse repetition frequency is to reduce the beam utilisation factor to 3 of the value which could be obtained if the two frequencies are equal. Hence the theoretical beam utilisation factor for the present system is about 97;. However this corresponds to a very useful increase over simple beam chopping where the maximum utilisation factor for production of 1 ns wide pulses at a repetition rate of 5 MC/S is 0.50,:. Prior to installing the klystron bunching facility the terminal layout was as follows. After extraction from the ion source, ions were focussed by means of an einzel lens on to an aperture 2.3 mm diameter situated about 32 cm above the top of the accelerating tube, the ion energy after the einzel lens being about 30 keV. Between the einzel lens and aperture the beam passed through two pairs of deflecting plates, one pair at right angles to the other, to which 5 MC/S rf deflection voltages were applied, thus causing the focussed spot to move in an ellipse. By applying a dc voltage to the second pair of deflector plates one edge of the ellipse was made to cross the aperture and ion bursts, 200 ns apart, were transmitted. The aperture was followed by a gap lens which matched the ion energy to the machine voltage. For klystron bunching it was decided to insert the bunching tube between the bottom pair of rf deflecting plates and the aperture, the rf deflection system, with some modifications (section 3) remaining to fulfill the requirement of chopping out the unbunched and unwanted sections of the beam. The bunching modulation is applied to ions of about 30 keV energy and this modulation is sufficiently low (table 1) not to impair the focus at the chopping aperture. The main bunching drift space becomes the section between the aperture and the top of the accelerating tube and it is necessary to increase this to about 64cm in order to keep the modulation amplitude down to a reasonable value. Doubling this distance over its previous value also has a beneficial effect on the focussing through the machine.
32
etal.
J. H. ANDERSON
TABLE 1
Machine settings
Ion mass
___~ EO
(kev) 30 30 30 30 30 30 30 30 30 30 30 30
El (kev)
L
-5.5 +17 +40.5 -5.5 +17 +40.5 -5.5 +17 +40.5 -5.5 +17 $40.5
EZ (MW 2 4 6 2 4 6 2 4 6 2 4 6
Fig. 2 shows theoretical plots of the image distance (L) measured from the bottom of the accelerating tube, against N, the ratio of output to input energy, for the accelerating tube, calculated from the formula due to Elkind”)
_
(N”-l-25)-45(K/P)(N”+l)-’ __~
4<(KIP)(3NS-1)-(&l)
(3N+-l-20
where K is the length of the accelerating tube, P is the object distance i.e. distance between the focussing aperture and the top of the accelerating tube, t is a correction factor pertaining to the end effects
Fig. 2. Theoretical plots of image distance against N, the ratio of output to input energy, for the accelerating tube.
Peak modulation energy E, (kev) 1.5 3.5 5.4 1.1 2.5 4.8 0.87 2.0 3.1 0.75 1.8 2.7
Bunching amplitude V,
Net modulation
&VI
WV)
0.82 1.9 2.9 0.54 1.2 2.4 0.49 1.1 1.8 0.54 1.3 2.0
energy Eb
+ 1.2
+ 2.7 f 4.2 + 0.85 + 1.9 If- 3.7 + 0.67 + 1.6 + 2.9 k 0.58 * 1.4 f 2.1
of the tube and may be taken as 1.345 in the present case1 “). The two curves in fig. 2 correspond to the two values of P before and after the bunching modifications. The horizontal bar lines indicate the range in N value which correspond to a modulation energy of f 3 keV about mean N values of 85 and 150 for output energies of 2 and 6 MeV. These curves show that focussing is rather less sensitive to energy modulation at N = 150 than at the normal value of N= 85 and that a higher value of P is required to make the sensitivity less for a given value of N. Either possibility leads to a negative value of L i.e. the beam is diverging at the output of the accelerator tube. We chose to continue to operate at the N value of 85 but at the increased value of 64 cm for P,which also satisfies the requirement of a longer drift space. Another focussing element, in the form of an electrostatic quadrupole lens pair, is installed at the base of the machine to converge the beam so that it can be easily analysed and focussed on to a target. Referring to fig. 1 the major modifications are the insertion of the buncher tube, increasing the distance from the focussing aperture to the entrance of the accelerating tube from 32 cm to 64 cm and the addition of the quadrupole lens pair underneath the accelerating tube. To accommodate the extra drift distance a new and larger terminal spinning is required and this in turn means that the pressure tank has to be increased in size. The latter is accomplished by adding a collar of height 2 ft between the base plate of the machine and the existing pressure tank. In the work described here we use an rf type ion source excited by a push pull oscillator operating at 130 MC/S and 80 W. Measurements on the extracted
KLYSTRON
BUNCHING
beam from this source showed the ion energy spread was less than 50 eV, ref. r3), a low value being required to keep the debunching effects small. With a canal diameter of 0.046” the gas consumption is about 3 cm3/h. Experience with terminal pulsing2) showed that this gas flow produced a pressure of approximately 4 x 10e4 mm in the einzel lens region, causing excessive scattering and defocussing of the beam and voltage breakdown across the lens elements. In the present system an improvement in operating pressure is obtained by paying attention to gas conductance in the design of the chopper, buncher and lens assemblies, and by the addition of a second tube, hereafter called the differential pumping tube. The measured conductance at the second gap lens is now 72.5 l/set for hydrogen, with an estimated working pressure in the einzel lens region of 3 x 1O-5 mm. In the following sections we give details of all these modifications, including design details of the additional
ION
BEAM-
(ft4fn~~
!s
OF AN
ION
BEAM
33
electronics for klystron bunching. giving some details of the machine tained so far.
We conclude performance
by ob-
3. Electronics With reference to fig. 1 it is seen that the bunching modulation voltage is applied to ions which, if singly charged, have an energy in eV equal to the sum of the ion source probe voltage and the first gap lens. We will call this energy E,. Immediately after the chopping aperture the ion energy is changed by an amount E, which is equal in eV to the second gap lens voltage. E, must be adjusted so that (E, +E, +E,)/(E, + E,) = N where E2 is the energy acquired in the accelerating tube. Through the tube the energy increases uniformly from (E, + E,) to (E, +E, + E2) and from the exit of the tube the ions travel a distance of up to 12 m to the target where optimum bunching should occur. Table 1, column 5, gives values of the peak modulation energy,
-
f0)
CERAHIC INSULATOR
TANTALUM
POWER
BUFFER
e-SHIFT
XTAL
AMP 5 Uch
AMP.
-430
OSC.5Hclr
Fig. 3. Bunching and chopping assembly and block diagram of electronics.
34
et a/.
J. H. ANDERSON
1
1111
1 “2
v,
!
5
II v,
cv4059
I
1
v5 cv3521
Fig. 4a. 5 Mc/sec beam chopping circuit.
E, required to achieve this bunching for various machine settings and ion masses. This modulation is obtained by applying rf of amplitude V,, to the bunching tube and if the length of the tube is such that the ion transit time through the tube is equal to 1/(2f) then V, = L/z. In fact the length of the tube is adjusted to effect a suitable compromise between the extreme cases of acceleration of mass 1 and mass 4 singly charged ions. For masses 1 and 4 at 30 keV the correct lengths should be 8 and 4 cm respectively; the compromise is 4x12 i.e. 5.66 cm. Column 6 of table 1 gives the corresponding rf amplitudes for this length of bunching tube. The net modulation energy, Ebr given to that part of the beam which is bunched is given in column 7. A scale drawing of the assembly of deflector plates and buncher tube, together with a block diagram of the associated electronics is shown in fig. 3. The circuit diagram of the unit feeding the deflector plates is shown in fig. 4a. A master 5 MC/S signal is derived from a crystal oscillator. The tuned output transformer T, feeds two
passive networks with phase shift +45” and -45” and is loosely coupled via a 2.2 pF capacitor to the output socket and the O”-360” phase shift unit. The two 5 MC/S signals of equal amplitude and 90” out of phase are fed through identical buffer amplifier valves V, and V, to the tuned output power amplifier valves V, and V,. The anodes of valves V, and V, are dc coupled, one to each pair of deflector plates which form the beam chopping assembly. The voltage supply for these output valves is derived from the common +300 V line via a 25 kG variable resistance, thus providing adjustment of the rf output voltages. Peak reading voltmeters register the rf output voltage. Bias voltages are applied to each of the two pairs of deflector plates in order to balance the bottoming potentials of the output valves. This system therefore provides the means for making the focussed spot move in a circle of radius up to 4 mm in the plane of the deflection aperture. Adjustment of the bias voltages moves the circle such that it crosses the aperture once per cycle thereby allowing a chopped pulse to be transmitted. Normally the rf amplitudes are
35
d
h
9
v, C”4014
“2 C”4014
Fig. 4b. O”-360” phase shift circuit.
Fig. 4c. Frequency
tripler and 15 MC/S power amplifier.
36
J. H. ANDERSON
chosen so that the width of the chopped pulse at half height is about 20 ns. ldeally this pulse should be rectangular in shape but since the focussed spot has a finite diameter-about 2 mm- the edges are rounded
et d. APERTURE
3.5 cm
Off.
The bunching tube is fed from a 15 MC/S power amplifier synchronised to the 5 MC/S chopping voltage by feeding a signal from the 5 MC/S master oscillator through a phase shifter, limiting amplifier and a frequency tripler to the amplifier. Fig. 4b,c give the circuit details of these units. In the phase shifter the 5 MC/S imput signal is fed to the amplifier valve V, having a balanced output transformer T,. This provides a balanced supply to a passive bridge network which feeds signals 90” out of phase to the segments of the quadrant capacitor VC,. The specially shaped rotor plate14) of this capacitor, which gives an output voltage with linear phase shift-angle characteristics, is connected to the tuned input of amplifier valve V,. The limiter amplifier valve V, delivers a 5 MC/S amplitude stabilised signal to the frequency tripler unit of 22 V peak to peak. In the frequency tripler, fig. 4c, the input signal feeds a tuned anode amplifier valve V, which is coupled via transformer T, to the tuned input of a push-pull frequency tripler stage (valves V, and V, in class C). Choice of the push-pull arrangement reduces the second harmonic content in the frequency tripled output waveform. The 15 MC/S output drives the power amplifier valve V,, which in practice is contained in a separate unit and the connections are made via a coaxial link. This valve is operated in class C and the output tank circuit, which includes the bunching tube capacitance, is shunt fed from the anode. Control of the output bunching voltage from O-5 kV peak sine wave is achieved by changing the screen grid potential from - 50 V to 200 V (at 10 mA) using a fixed anode supply of 1.2 kV. The power requirement of the chopping and bunching electronics is 200-400 W depending on the output voltage settings.
El
INSULATOR
4. The quadrupole lens It was shown in section 2 that with the modified design the beam would be divergent at the exit of the accelerator tube. This section describes the quadrupole lens pair which is fixed at the base of the machine in order to converge the beam. The lens assembly, fig. 5, replaces a simple drift pipe which was situated between the base of the accelerator tube and the base-plate of the machine. The overall length of the lens is thus restricted to 92 cm. The beam
Fig. 5. Electrostatic
quadrupole
lens pair.
KLYSTRON
BUNCHING
emerging from the tube appears to diverge from a point 265 cm above the tube exit, this distance being the (negative) value of L obtained by setting K = 387 cm, P = 64 cm and N= 85 into eq. (2). (The actual beam crossover occurs 218 cm above the tube exit aperture.) The object distance for the lens is therefore 265 cm and is independent of machine voltage if N is constant. It is required to focus the beam on to the entrance slit of a double focussing 90” bending magnet. This slit is situated 137 cm below the lens and this is therefore the image distance I. The two quadrupole lenses each of length (/) 41 cm are separated by a 10 cm gap. This layout is shown schematically in fig. 1. The pole-tip gap is determined by the diameter and divergence of the beam entering the lens and these are estimated as follows. Elkind”) gives the following expression for the ratio of the beam diameters at the exit and entrance of the accelerating tube (d, and d, respectively), G/d,
= 1 + (2i(@+l))
{(K/P)-(N-1)/(40),
(3)
which with K = 387 cm, P = 64 cm and N = 85 gives d,/d, = -0.88, the minus sign denoting a crossover within the tube. The value of d, can be estimated by considering the divergence of the injected beam. The maximum divergence above the pulsing aperture is limited by a previous aperture to 0.05 rad. With zero final gap voltage (as required for 2.5 MV machine voltage) the maximum input divergence is 0.05 rad and the maximum value of d, is 0.05 x 64 = 3.2 cm. Thus from eq. (3) we have d,= 2.8 cm and the maximum divergence 6 at the tube exit aperture is 2.8/265 = 0.0105 rad. We will refer to the plane in which thesecond lens is diverging as the diverging plane. We require that the object and image distances L and Zare the same in both planes (stigmatic case). In the diverging plane the beam will be widest in the first lens”). The lateral spread there will not be greater than (L+# N 3 cm. In the converging plane the lateral spread is greatest in the second lens and should not be greater than (I+#,)’ where 0’ is the output divergence from the lens pair. Using the graphs given in ref. i5) the angular magnification 0’/0 is about 3 in the converging plane, thus making (Z+#’ = 4.9 cm. Accordingly the pole-tip gap is fixed at 2” (= 5.08 cm) and the voltages required for the upper and lower lens are 3.2 x E2 keV and 3.6 x E, keV, respectively, for singly charged ions, where E2 is the ion energy in MeV. The stigmatic focussed spot is not round, but elongated in the diverging plane, the linear magnifications being 0.3 and I .4 in the converging and diverging planes, respectively. By arranging that
OF
AN
ION
BEAM
37
the diverging plane of the quadrupole lens coincides with the median plane of the double focussing bending magnet, the overall magnification of the lens magnet system is made roughly equal in the median and perpendicular planes. This leads to ease in beam handling. In fig. 5 the 3.5 cm diameter aperture made in a plate of 0.32 cm thick tantalum prevents stray beams from hitting the electrodes and insulators. Secondary electrons arising from the aperture are prevented from entering the accelerating field of the tube by applying a 20 kV negative potential to the last five electrodes of the tube. 5. The differential pumping tube The important requirement of this tube is to provide maximum possible gas conductance with minimum tube loading. Consequently the pumping apertures need to be as large as possible but of configuration and arrangement such that electrons present in the tube are collected before reaching energies in excess of about 500 keV.
Fig. 6. Differential pumping tube.
38
J. H. ANDERSON
et d.
In the present design the apertures are 15.3 cm diameter and each is sub-divided across a diameter by a 1.9 cm wide bar. The aluminium is dished, thus forming between electrodes an electrostatic field which directs electrons to the tubes axis for collection on the cross bars. Optical baffling down the tube is provided by displacing adjacent electrode apertures by 15” relative to each other. This gives complete baffling at each twelfth electrode and thus limits the energies attained by electrons moving parallel to the axis. The calculated conductance of the tube assuming a continuous smooth wall tube of section similar to the electrode aperture is 205 l/set for hydrogen. The measured conductance is 106 I/set, the difference being due to the finned nature of the internal structure and the limitation of the paths of the gas molecules due to the spiralling. A diagram of the differential pumping tube is shown in fig. 6. 6. Performance 6.1. TEST BENCH MEASUREMENTS Prior to installation in the accelerator the performance of the modified terminal was studied on a test bench. This bench included all the apparatus from the ion source to the pulsing aperture, and below that was placed a drift tube of length 31 cm which is the length required to simulate accelerator conditions when the machine voltage is 6 MV. The beam was collected by a 50 Q target assembly and the voltage signal transmitted via low-loss coaxial cable to a sampling oscilloscope, the trigger signal being obtained from the 5 MC/S crystal oscillator. The rise time of the oscilloscope and cable was estimated to be 0.3 ns. The target was 8 mm diameter and a secondary collector grid was mounted 3.6 mm in front of it, as shown in fig. 7. The waveform observed when the rf deflection voltages are adjusted to give a chopped pulse of duration 20 ns at half height and the bunching voltage and phase shifter adjusted for optimum bunching is shown in fig. 7. This waveform is understood as follows. The initial ramp AB represents the penetration time of the ion burst through the secondary collector grid and hence its width should be equal to the ion burst duration. The shelf BC is caused by displacement current during the flight time of the ion burst between grid and target and the main peak represents the current, mainly from secondary electron emission, when the burst reaches the target, the width of which also gives a measure of the burst duration. It can be seen that the burst duration is about 0.7 ns. The measurement was repeated with the drift tube of length 31 cm
I
GRIDi I I
3.6nn
Fig. I. Bunched and chopped waveform observed on the test bench with a 50R target assembly. The proton beam energy is 26 keV.
replaced by one of length 1 I1 cm and the burst duration in this case was measured to be 0.9 ns. This difference between the two measurements arises from the intrinsic energy spread in the ion beam. If we assume that a time spread of (0.9’ -0.72}f results from the ion pulse, mean energy 26 keV (the actual energy in this test) drifting a distance 80 cm, then the energy spread is approximately 80 eV. This figure is consistent with that expected from the intrinsic spread of ions emitted from the source”) and also the energy spread introduced by beam choppingr6), the latter being extremely small. Fig. 8a,b show the target waveforms obtained by bunching but without applying the rfdeflection voltages. In (a) the bunching voltage is adjusted for optimum and in (b) the HI + bunching of the Hz+ component component is optimised. In (b) the characteristic klystron double peaking obtained when the optimum bunching voltage has been exceeded, is easily seen for the H, and H, beams. 6.2. MACHINE The bunched
TESTS
pulses accelerated
by the machine
were
KLYSTRON
BUNCHING
analysed by a 90” bending magnet and focussed by a quadrupole lens pair on to the 50 52 target used for the bench tests. The pulse waveform observed with a 2.6 MeV H, beam is shown in fig. 9, indicating a width at half height of 1.5 ns. The additional spread over that observed on the test bench is due to debunching effects, during and after the acceleration process. It can be shown17) that the time dispersion introduced by the analysing magnet of this machine is about 0.5 ns for 2.6 MeV protons and hence this effect accounts for a significant fraction of the additional spread. For the case shown in fig. 9 the indicated peak current is about 200/1A, which is a factor 4 higher than the dc level before chopping and bunching is applied. We therefore conclude that the bunching factor is at least 4, because the true peak amplitude must be greater than that observed due to the bandwidth limitation of the measuring system. An upper limit for the bunching
I-
6 5 L-
DC BEAM
-LEVEL t
B
20
nS/Div
(a)
-
oc BEAM
OF
AN
ION
_
BEAM
39
2 nS/DiV
Fig. 9. Bunched and chopped waveform observed on the machine with a 50-Q target assembly. The proton beam energy is 2.6 MeV.
factor can be obtained from the ratio of the widths of the chopped and bunched pulses. This is equal to about 12, starting with a 19 ns wide pulse. The actual bunching factor is less than 12 because the shape of the chopped pulse is not rectangular. The system has also been tested by observing time of flight spectra from y-rays and neutrons emitted when protons bombard a LiF target and when deuterons bombard a Be target. In both cases the y-ray and the high energy neutron peaks had widths at half height of 2.0 ns. These were consistent with the pulse widths observed electronically, the extra width arising from the detection system. In these experiments a mean current of about IO PA was analysed. There was some indication that the pulse width increased slightly as the analysed pulsed current was increased above IOvA, possibly due to the effect of space charge repulsion. So far the maximum analysed proton beam observed is 20 /LA. In setting up the machine for time of flight experiments we have found it convenient to monitor the beam bursts in the following way. Between the analysing magnet and the target the beam passes through a tube of length 5 cm and diameter 1.9 cm. The voltage induced on the tube by the enclosed charge is fed via a wide band emitter follower and 50 R cable to the control panel. Here an amplifier with a gain of about IOOand rise time 1 ns, based on a design by Lunsford”), raises the signal to a level suitable for display by a sampling oscilloscope.
LEVEL
7. Conclusions ----tZOnSlDiv
(b) Fig. 8. (a) Bunched waveform optimised for a 26 keV Hz beam, observed on the test bench. (b) Bunched waveform optimised for a 26 keV HI beam, observed on the test bench.
The application of klystron bunching within the terminal of the AWRE 5.5 MV Van de Graaff has resulted in a much improved performance of that machine in pulsed operation. At a 5 MC/S repetition rate an order of magnitude increase in mean pulsed
40
J. H. ANDERSON
current has been obtained over that previously available with simple terminal pulsing. In addition the pulse width has been reduced to about 1.5 ns. Because of the higher intensity available the new system will be extremely valuable for fast neutron inelastic scattering studies to weakly excited levels and also for work on scarce, and hence small, samples e.g. separated isotopic targets. The system will also find application in high resolution time of flight studies on the neutrons emitted from charged particle induced reactions, using a long flight path. The system can easily be modified in one of several possible ways to give a lower pulse repetition rate. We wish to carrying out Swann for his Mr. T. Dyke, pressure tank
thank Mr. R. J. Prosser, for his help in the test bench measurements, Mr. D. help in the design of the electronics and who arranged the modification to the and terminal spinning.
References 1) R. Batchelor and J. H. Towle, AWRE Report No. NR/P12158.
et al.
2) J. H. Towle and W. B. Gilboy, Nucl. Physics 44 (1963) 256. 3) J. H. Towle and B. E. F. Macefield, Nucl. Physics 66 (1965) 65. M. P. Nakada, UCRL Report No. 4541 (1955). 5) J. H. Anderson and D. Swann, Nucl. Instr. and Meth. 30 (1964) 1. 6) C. D. Moak, R. F. King, J. W. Johnson, H. E. Banta, J. Judish and W. H. du Preez, Rev. Sci. Instr. 35 (1964) 672. 7) R. C. Mobley, Phys. Rev. 88 (1952) 360. 4)
8) L. G. Cranberg, R. A. Ferneld, F. S. Hahn and E. F. Shroder, Nucl. Instr. and Meth. 12 (1961) 335. 9)
R. C. Mobley, Rev. Sci. Instr. 34 (1963) 252.
10) F. E. Whiteway, AWRE Report No. O-12/61 (1961). it) M. M. Elkind, Rev. Sci. Instr. 24 (1953) 129. 12) A. C. Riviere, private communication. is) L. E. Collins, R. H. Gobbett and P. T. Stroud, Paper 4. Conf. Particle Accelerators, Washington USA. (March 1965). 14) F. E. Terman, Radio Engineers Handbook p. 949. 15) H. A. Enge, Rev. Sci. Instr. 30 (1959) 248. 16) T. K. Fowler and W. M. Good, Nucl. Instr. and Meth. 7 (1960) 245. 17) B. E. F. Macefield, AWRE Report No. NR/P-5/61. 1s) J. S. Lunsford, Rev. Sci. Instr. 35 (1964) 1483.
INSTRUMENTS
KLYSTRON
AND
METHODS
BUNCHING
J. H. ANDERSON,
41 (1966)30-40;0 NORTH-HOLLAND
OF THE ION BEAM
R. BATCHELOR,
PUBLISHING
co.
OF A 5.5 MV VAN DE GRAAFF
F. A. HOWE, G. JAMES and J. H. TOWLE
Atomic Weapons Research Establishment, Aldermaston, Berks., England Received 22 November
1965
Modifications to the AWRE 5.5 MV Van de Graaff to improve its performance as a pulsed machine are described. A klystron bunching facility has been incorporated into the top terminal of
the machine and pulsed ion beams with the pulse width about 1.5 ns and analysed mean currents of 10 ,ILAare obtained at 5 MC/S repetition rate.
1. Introduction
paratus needs to be compact and reliable in an environment where electrical surges are frequent, the insulating gas pressure is high and the radiation level is intense.
For some years the AWRE 5.5 MV Van de Graaff * has been used as a pulsed machine for fast neutron spectroscopy. In the early work pulsing at 5 MC/S was accomplished by rf deflection of the accelerated ion beam’) but the neutron background was high and the beam intensity comparatively low. In the next stage’) the rf deflection was applied to the beam prior to acceleration and this reduced considerably the neutron background particularly when deuterons were accelerated. However the practical lower limit for the pulse duration was 3 ns and with this burst width the mean current on target never exceeded 1 PA. In a few applications’) post acceleration chopping was used to trim the beam bursts to less than 2 ns, but this resulted in a corresponding reduction in mean current. The next obvious step is to increase the pulse intensity and reduce the burst duration by beam bunching and this report gives details of how this has been accomplished by the “klystron” method. This method has had fairly wide application to Cockcroft Walton and Tandem accelerators4, “) but has not been greatly used in single stage Van de Graaff machines. Moak et a1.6) have recently used the method on the ORNL 3 MV Van de Graaff. The main reasons for choosing the klystron method rather than the variable magnetic path method7-9) were greater convenience and lower cost. The introduction of a compression magnet at the output of the machine would have necessitated a complete reorganisation of the target room facilities and would have involved the extra cost of a building extension. On the other hand klystron bunching prior to acceleration has meant that the experimental target facilities have remained undisturbed and all can be served with the pulsed beam. The main disadvantage of the klystron method is that it adds extra complexity to the top terminal of the machine and consequently the ap* Supplied by High Voltage Engineering Corporation,
2. General design considerations The general problem of klystron bunching of ion beams has been considered by Whiteway”). Power consumption limitations lead to the choice of an inserted bunching tube as the most convenient method of applying velocity modulation to an ion beam. In this method a sinewave bunching voltage is applied to a tube with a field gap at each end. The length of tube is chosen (in relation to the bunching frequency) so that an ion accelerated at the first gap receives an equal acceleration at the second gap. In this way the beam is subject to a sine wave energy modulation along axis. It can be shown”) that bunching occurs, and a waveform with a single infinite peak is obtained when +X&/u
= 1
(1) where c( is the ratio of the peak sine wave modulation energy E,,, to the mean ion energy, u is the velocity corresponding to the mean ion energy, D is the drift distance over which the beam of velocity u becomes compressed or bunched, o = 27ifwherefis the frequency of the bunching voltage applied to the tube. The bunching efficiency depends on the value of CI which should be less than 0.1. For this condition about 28% of the beam can be utilised in forming an ion burst within a time bracket of 1% of the interval between bursts. In a practical system the ion beam is subjected (over a distance of several metres) to a number of stages of acceleration and drift. However this complex system can be replaced5,i0) by a simple drift system with a single equivalent drift length and constant ion beam energy. Most of the beam compression takes place in
Burling-
ton, Mass.
30
KLYSTRON
BUNCHING
PROBE 1
ION
130
Mcls
SOURCE
20
cm I
>
CHOPPING PLATES
-BUhlCHlNG
WAVEFORM ‘5 Mc’r BUNCHING
GAP 64
DEFLECTOR
TUBE
LENS
;P
cm I
VIRTUAL
YIGH ENERGY ACCELER’N COLUMN 387 cln I
ELECTROSTATIC QUADRUPOLE LENS
STEERING PLATES
IMAGE
Fig. 1. Schematic layout of machine with bunching modifications.
the drift tubes at lower beam energies and hence the ion beam is substantially bunched prior to entry into the accelerating tube of the machine. For the system described in this paper (fig. 1 for the schematic layout of the Van de Graaff), the mean beam energy at the point where the bunching modulation is applied is about 30 keV. With an equivalent drift length at 30 keV of 31 cm (which is the figure corresponding to the system in fig. 1 when the machine voltage is 6 MV), the value of c(w for protons is 1.55 x 10’. A lower machine voltage or a higher ion mass leads to a lower value of s(w. We aim to produce
OF
AN
ION
BEAM
31
pulses at a repetition rate of 5 MC/S and hence if we bunch at this frequency, the maximum value of ix required is 0.5 which represents a peak energy modulation E,,, of 15 keV. This value is intolerably high, particularly in relation to the focussing of the beam through the accelerator. Since it is not possible to increase the equivalent drift length or decrease II by large factors, it becomes necessary to increase o and in the present design a bunching frequency of 15 MC/S is used. The pulse repetition rate can be retained by beam chopping at 5 MC/S but this must be synchronised with the 15 MC/S bunching frequency. The consequence of using a bunching frequency three times the pulse repetition frequency is to reduce the beam utilisation factor to 3 of the value which could be obtained if the two frequencies are equal. Hence the theoretical beam utilisation factor for the present system is about 97;. However this corresponds to a very useful increase over simple beam chopping where the maximum utilisation factor for production of 1 ns wide pulses at a repetition rate of 5 MC/S is 0.50,:. Prior to installing the klystron bunching facility the terminal layout was as follows. After extraction from the ion source, ions were focussed by means of an einzel lens on to an aperture 2.3 mm diameter situated about 32 cm above the top of the accelerating tube, the ion energy after the einzel lens being about 30 keV. Between the einzel lens and aperture the beam passed through two pairs of deflecting plates, one pair at right angles to the other, to which 5 MC/S rf deflection voltages were applied, thus causing the focussed spot to move in an ellipse. By applying a dc voltage to the second pair of deflector plates one edge of the ellipse was made to cross the aperture and ion bursts, 200 ns apart, were transmitted. The aperture was followed by a gap lens which matched the ion energy to the machine voltage. For klystron bunching it was decided to insert the bunching tube between the bottom pair of rf deflecting plates and the aperture, the rf deflection system, with some modifications (section 3) remaining to fulfill the requirement of chopping out the unbunched and unwanted sections of the beam. The bunching modulation is applied to ions of about 30 keV energy and this modulation is sufficiently low (table 1) not to impair the focus at the chopping aperture. The main bunching drift space becomes the section between the aperture and the top of the accelerating tube and it is necessary to increase this to about 64cm in order to keep the modulation amplitude down to a reasonable value. Doubling this distance over its previous value also has a beneficial effect on the focussing through the machine.
32
etal.
J. H. ANDERSON
TABLE 1
Machine settings
Ion mass
___~ EO
(kev) 30 30 30 30 30 30 30 30 30 30 30 30
El (kev)
L
-5.5 +17 +40.5 -5.5 +17 +40.5 -5.5 +17 +40.5 -5.5 +17 $40.5
EZ (MW 2 4 6 2 4 6 2 4 6 2 4 6
Fig. 2 shows theoretical plots of the image distance (L) measured from the bottom of the accelerating tube, against N, the ratio of output to input energy, for the accelerating tube, calculated from the formula due to Elkind”)
_
(N”-l-25)-45(K/P)(N”+l)-’ __~
4<(KIP)(3NS-1)-(&l)
(3N+-l-20
where K is the length of the accelerating tube, P is the object distance i.e. distance between the focussing aperture and the top of the accelerating tube, t is a correction factor pertaining to the end effects
Fig. 2. Theoretical plots of image distance against N, the ratio of output to input energy, for the accelerating tube.
Peak modulation energy E, (kev) 1.5 3.5 5.4 1.1 2.5 4.8 0.87 2.0 3.1 0.75 1.8 2.7
Bunching amplitude V,
Net modulation
&VI
WV)
0.82 1.9 2.9 0.54 1.2 2.4 0.49 1.1 1.8 0.54 1.3 2.0
energy Eb
+ 1.2
+ 2.7 f 4.2 + 0.85 + 1.9 If- 3.7 + 0.67 + 1.6 + 2.9 k 0.58 * 1.4 f 2.1
of the tube and may be taken as 1.345 in the present case1 “). The two curves in fig. 2 correspond to the two values of P before and after the bunching modifications. The horizontal bar lines indicate the range in N value which correspond to a modulation energy of f 3 keV about mean N values of 85 and 150 for output energies of 2 and 6 MeV. These curves show that focussing is rather less sensitive to energy modulation at N = 150 than at the normal value of N= 85 and that a higher value of P is required to make the sensitivity less for a given value of N. Either possibility leads to a negative value of L i.e. the beam is diverging at the output of the accelerator tube. We chose to continue to operate at the N value of 85 but at the increased value of 64 cm for P,which also satisfies the requirement of a longer drift space. Another focussing element, in the form of an electrostatic quadrupole lens pair, is installed at the base of the machine to converge the beam so that it can be easily analysed and focussed on to a target. Referring to fig. 1 the major modifications are the insertion of the buncher tube, increasing the distance from the focussing aperture to the entrance of the accelerating tube from 32 cm to 64 cm and the addition of the quadrupole lens pair underneath the accelerating tube. To accommodate the extra drift distance a new and larger terminal spinning is required and this in turn means that the pressure tank has to be increased in size. The latter is accomplished by adding a collar of height 2 ft between the base plate of the machine and the existing pressure tank. In the work described here we use an rf type ion source excited by a push pull oscillator operating at 130 MC/S and 80 W. Measurements on the extracted
KLYSTRON
BUNCHING
beam from this source showed the ion energy spread was less than 50 eV, ref. r3), a low value being required to keep the debunching effects small. With a canal diameter of 0.046” the gas consumption is about 3 cm3/h. Experience with terminal pulsing2) showed that this gas flow produced a pressure of approximately 4 x 10e4 mm in the einzel lens region, causing excessive scattering and defocussing of the beam and voltage breakdown across the lens elements. In the present system an improvement in operating pressure is obtained by paying attention to gas conductance in the design of the chopper, buncher and lens assemblies, and by the addition of a second tube, hereafter called the differential pumping tube. The measured conductance at the second gap lens is now 72.5 l/set for hydrogen, with an estimated working pressure in the einzel lens region of 3 x 1O-5 mm. In the following sections we give details of all these modifications, including design details of the additional
ION
BEAM-
(ft4fn~~
!s
OF AN
ION
BEAM
33
electronics for klystron bunching. giving some details of the machine tained so far.
We conclude performance
by ob-
3. Electronics With reference to fig. 1 it is seen that the bunching modulation voltage is applied to ions which, if singly charged, have an energy in eV equal to the sum of the ion source probe voltage and the first gap lens. We will call this energy E,. Immediately after the chopping aperture the ion energy is changed by an amount E, which is equal in eV to the second gap lens voltage. E, must be adjusted so that (E, +E, +E,)/(E, + E,) = N where E2 is the energy acquired in the accelerating tube. Through the tube the energy increases uniformly from (E, + E,) to (E, +E, + E2) and from the exit of the tube the ions travel a distance of up to 12 m to the target where optimum bunching should occur. Table 1, column 5, gives values of the peak modulation energy,
-
f0)
CERAHIC INSULATOR
TANTALUM
POWER
BUFFER
e-SHIFT
XTAL
AMP 5 Uch
AMP.
-430
OSC.5Hclr
Fig. 3. Bunching and chopping assembly and block diagram of electronics.
34
et a/.
J. H. ANDERSON
1
1111
1 “2
v,
!
5
II v,
cv4059
I
1
v5 cv3521
Fig. 4a. 5 Mc/sec beam chopping circuit.
E, required to achieve this bunching for various machine settings and ion masses. This modulation is obtained by applying rf of amplitude V,, to the bunching tube and if the length of the tube is such that the ion transit time through the tube is equal to 1/(2f) then V, = L/z. In fact the length of the tube is adjusted to effect a suitable compromise between the extreme cases of acceleration of mass 1 and mass 4 singly charged ions. For masses 1 and 4 at 30 keV the correct lengths should be 8 and 4 cm respectively; the compromise is 4x12 i.e. 5.66 cm. Column 6 of table 1 gives the corresponding rf amplitudes for this length of bunching tube. The net modulation energy, Ebr given to that part of the beam which is bunched is given in column 7. A scale drawing of the assembly of deflector plates and buncher tube, together with a block diagram of the associated electronics is shown in fig. 3. The circuit diagram of the unit feeding the deflector plates is shown in fig. 4a. A master 5 MC/S signal is derived from a crystal oscillator. The tuned output transformer T, feeds two
passive networks with phase shift +45” and -45” and is loosely coupled via a 2.2 pF capacitor to the output socket and the O”-360” phase shift unit. The two 5 MC/S signals of equal amplitude and 90” out of phase are fed through identical buffer amplifier valves V, and V, to the tuned output power amplifier valves V, and V,. The anodes of valves V, and V, are dc coupled, one to each pair of deflector plates which form the beam chopping assembly. The voltage supply for these output valves is derived from the common +300 V line via a 25 kG variable resistance, thus providing adjustment of the rf output voltages. Peak reading voltmeters register the rf output voltage. Bias voltages are applied to each of the two pairs of deflector plates in order to balance the bottoming potentials of the output valves. This system therefore provides the means for making the focussed spot move in a circle of radius up to 4 mm in the plane of the deflection aperture. Adjustment of the bias voltages moves the circle such that it crosses the aperture once per cycle thereby allowing a chopped pulse to be transmitted. Normally the rf amplitudes are
35
d
h
9
v, C”4014
“2 C”4014
Fig. 4b. O”-360” phase shift circuit.
Fig. 4c. Frequency
tripler and 15 MC/S power amplifier.
36
J. H. ANDERSON
chosen so that the width of the chopped pulse at half height is about 20 ns. ldeally this pulse should be rectangular in shape but since the focussed spot has a finite diameter-about 2 mm- the edges are rounded
et d. APERTURE
3.5 cm
Off.
The bunching tube is fed from a 15 MC/S power amplifier synchronised to the 5 MC/S chopping voltage by feeding a signal from the 5 MC/S master oscillator through a phase shifter, limiting amplifier and a frequency tripler to the amplifier. Fig. 4b,c give the circuit details of these units. In the phase shifter the 5 MC/S imput signal is fed to the amplifier valve V, having a balanced output transformer T,. This provides a balanced supply to a passive bridge network which feeds signals 90” out of phase to the segments of the quadrant capacitor VC,. The specially shaped rotor plate14) of this capacitor, which gives an output voltage with linear phase shift-angle characteristics, is connected to the tuned input of amplifier valve V,. The limiter amplifier valve V, delivers a 5 MC/S amplitude stabilised signal to the frequency tripler unit of 22 V peak to peak. In the frequency tripler, fig. 4c, the input signal feeds a tuned anode amplifier valve V, which is coupled via transformer T, to the tuned input of a push-pull frequency tripler stage (valves V, and V, in class C). Choice of the push-pull arrangement reduces the second harmonic content in the frequency tripled output waveform. The 15 MC/S output drives the power amplifier valve V,, which in practice is contained in a separate unit and the connections are made via a coaxial link. This valve is operated in class C and the output tank circuit, which includes the bunching tube capacitance, is shunt fed from the anode. Control of the output bunching voltage from O-5 kV peak sine wave is achieved by changing the screen grid potential from - 50 V to 200 V (at 10 mA) using a fixed anode supply of 1.2 kV. The power requirement of the chopping and bunching electronics is 200-400 W depending on the output voltage settings.
El
INSULATOR
4. The quadrupole lens It was shown in section 2 that with the modified design the beam would be divergent at the exit of the accelerator tube. This section describes the quadrupole lens pair which is fixed at the base of the machine in order to converge the beam. The lens assembly, fig. 5, replaces a simple drift pipe which was situated between the base of the accelerator tube and the base-plate of the machine. The overall length of the lens is thus restricted to 92 cm. The beam
Fig. 5. Electrostatic
quadrupole
lens pair.
KLYSTRON
BUNCHING
emerging from the tube appears to diverge from a point 265 cm above the tube exit, this distance being the (negative) value of L obtained by setting K = 387 cm, P = 64 cm and N= 85 into eq. (2). (The actual beam crossover occurs 218 cm above the tube exit aperture.) The object distance for the lens is therefore 265 cm and is independent of machine voltage if N is constant. It is required to focus the beam on to the entrance slit of a double focussing 90” bending magnet. This slit is situated 137 cm below the lens and this is therefore the image distance I. The two quadrupole lenses each of length (/) 41 cm are separated by a 10 cm gap. This layout is shown schematically in fig. 1. The pole-tip gap is determined by the diameter and divergence of the beam entering the lens and these are estimated as follows. Elkind”) gives the following expression for the ratio of the beam diameters at the exit and entrance of the accelerating tube (d, and d, respectively), G/d,
= 1 + (2i(@+l))
{(K/P)-(N-1)/(40),
(3)
which with K = 387 cm, P = 64 cm and N = 85 gives d,/d, = -0.88, the minus sign denoting a crossover within the tube. The value of d, can be estimated by considering the divergence of the injected beam. The maximum divergence above the pulsing aperture is limited by a previous aperture to 0.05 rad. With zero final gap voltage (as required for 2.5 MV machine voltage) the maximum input divergence is 0.05 rad and the maximum value of d, is 0.05 x 64 = 3.2 cm. Thus from eq. (3) we have d,= 2.8 cm and the maximum divergence 6 at the tube exit aperture is 2.8/265 = 0.0105 rad. We will refer to the plane in which thesecond lens is diverging as the diverging plane. We require that the object and image distances L and Zare the same in both planes (stigmatic case). In the diverging plane the beam will be widest in the first lens”). The lateral spread there will not be greater than (L+# N 3 cm. In the converging plane the lateral spread is greatest in the second lens and should not be greater than (I+#,)’ where 0’ is the output divergence from the lens pair. Using the graphs given in ref. i5) the angular magnification 0’/0 is about 3 in the converging plane, thus making (Z+#’ = 4.9 cm. Accordingly the pole-tip gap is fixed at 2” (= 5.08 cm) and the voltages required for the upper and lower lens are 3.2 x E2 keV and 3.6 x E, keV, respectively, for singly charged ions, where E2 is the ion energy in MeV. The stigmatic focussed spot is not round, but elongated in the diverging plane, the linear magnifications being 0.3 and I .4 in the converging and diverging planes, respectively. By arranging that
OF
AN
ION
BEAM
37
the diverging plane of the quadrupole lens coincides with the median plane of the double focussing bending magnet, the overall magnification of the lens magnet system is made roughly equal in the median and perpendicular planes. This leads to ease in beam handling. In fig. 5 the 3.5 cm diameter aperture made in a plate of 0.32 cm thick tantalum prevents stray beams from hitting the electrodes and insulators. Secondary electrons arising from the aperture are prevented from entering the accelerating field of the tube by applying a 20 kV negative potential to the last five electrodes of the tube. 5. The differential pumping tube The important requirement of this tube is to provide maximum possible gas conductance with minimum tube loading. Consequently the pumping apertures need to be as large as possible but of configuration and arrangement such that electrons present in the tube are collected before reaching energies in excess of about 500 keV.
Fig. 6. Differential pumping tube.
38
J. H. ANDERSON
et d.
In the present design the apertures are 15.3 cm diameter and each is sub-divided across a diameter by a 1.9 cm wide bar. The aluminium is dished, thus forming between electrodes an electrostatic field which directs electrons to the tubes axis for collection on the cross bars. Optical baffling down the tube is provided by displacing adjacent electrode apertures by 15” relative to each other. This gives complete baffling at each twelfth electrode and thus limits the energies attained by electrons moving parallel to the axis. The calculated conductance of the tube assuming a continuous smooth wall tube of section similar to the electrode aperture is 205 l/set for hydrogen. The measured conductance is 106 I/set, the difference being due to the finned nature of the internal structure and the limitation of the paths of the gas molecules due to the spiralling. A diagram of the differential pumping tube is shown in fig. 6. 6. Performance 6.1. TEST BENCH MEASUREMENTS Prior to installation in the accelerator the performance of the modified terminal was studied on a test bench. This bench included all the apparatus from the ion source to the pulsing aperture, and below that was placed a drift tube of length 31 cm which is the length required to simulate accelerator conditions when the machine voltage is 6 MV. The beam was collected by a 50 Q target assembly and the voltage signal transmitted via low-loss coaxial cable to a sampling oscilloscope, the trigger signal being obtained from the 5 MC/S crystal oscillator. The rise time of the oscilloscope and cable was estimated to be 0.3 ns. The target was 8 mm diameter and a secondary collector grid was mounted 3.6 mm in front of it, as shown in fig. 7. The waveform observed when the rf deflection voltages are adjusted to give a chopped pulse of duration 20 ns at half height and the bunching voltage and phase shifter adjusted for optimum bunching is shown in fig. 7. This waveform is understood as follows. The initial ramp AB represents the penetration time of the ion burst through the secondary collector grid and hence its width should be equal to the ion burst duration. The shelf BC is caused by displacement current during the flight time of the ion burst between grid and target and the main peak represents the current, mainly from secondary electron emission, when the burst reaches the target, the width of which also gives a measure of the burst duration. It can be seen that the burst duration is about 0.7 ns. The measurement was repeated with the drift tube of length 31 cm
I
GRIDi I I
3.6nn
Fig. I. Bunched and chopped waveform observed on the test bench with a 50R target assembly. The proton beam energy is 26 keV.
replaced by one of length 1 I1 cm and the burst duration in this case was measured to be 0.9 ns. This difference between the two measurements arises from the intrinsic energy spread in the ion beam. If we assume that a time spread of (0.9’ -0.72}f results from the ion pulse, mean energy 26 keV (the actual energy in this test) drifting a distance 80 cm, then the energy spread is approximately 80 eV. This figure is consistent with that expected from the intrinsic spread of ions emitted from the source”) and also the energy spread introduced by beam choppingr6), the latter being extremely small. Fig. 8a,b show the target waveforms obtained by bunching but without applying the rfdeflection voltages. In (a) the bunching voltage is adjusted for optimum and in (b) the HI + bunching of the Hz+ component component is optimised. In (b) the characteristic klystron double peaking obtained when the optimum bunching voltage has been exceeded, is easily seen for the H, and H, beams. 6.2. MACHINE The bunched
TESTS
pulses accelerated
by the machine
were
KLYSTRON
BUNCHING
analysed by a 90” bending magnet and focussed by a quadrupole lens pair on to the 50 52 target used for the bench tests. The pulse waveform observed with a 2.6 MeV H, beam is shown in fig. 9, indicating a width at half height of 1.5 ns. The additional spread over that observed on the test bench is due to debunching effects, during and after the acceleration process. It can be shown17) that the time dispersion introduced by the analysing magnet of this machine is about 0.5 ns for 2.6 MeV protons and hence this effect accounts for a significant fraction of the additional spread. For the case shown in fig. 9 the indicated peak current is about 200/1A, which is a factor 4 higher than the dc level before chopping and bunching is applied. We therefore conclude that the bunching factor is at least 4, because the true peak amplitude must be greater than that observed due to the bandwidth limitation of the measuring system. An upper limit for the bunching
I-
6 5 L-
DC BEAM
-LEVEL t
B
20
nS/Div
(a)
-
oc BEAM
OF
AN
ION
_
BEAM
39
2 nS/DiV
Fig. 9. Bunched and chopped waveform observed on the machine with a 50-Q target assembly. The proton beam energy is 2.6 MeV.
factor can be obtained from the ratio of the widths of the chopped and bunched pulses. This is equal to about 12, starting with a 19 ns wide pulse. The actual bunching factor is less than 12 because the shape of the chopped pulse is not rectangular. The system has also been tested by observing time of flight spectra from y-rays and neutrons emitted when protons bombard a LiF target and when deuterons bombard a Be target. In both cases the y-ray and the high energy neutron peaks had widths at half height of 2.0 ns. These were consistent with the pulse widths observed electronically, the extra width arising from the detection system. In these experiments a mean current of about IO PA was analysed. There was some indication that the pulse width increased slightly as the analysed pulsed current was increased above IOvA, possibly due to the effect of space charge repulsion. So far the maximum analysed proton beam observed is 20 /LA. In setting up the machine for time of flight experiments we have found it convenient to monitor the beam bursts in the following way. Between the analysing magnet and the target the beam passes through a tube of length 5 cm and diameter 1.9 cm. The voltage induced on the tube by the enclosed charge is fed via a wide band emitter follower and 50 R cable to the control panel. Here an amplifier with a gain of about IOOand rise time 1 ns, based on a design by Lunsford”), raises the signal to a level suitable for display by a sampling oscilloscope.
LEVEL
7. Conclusions ----tZOnSlDiv
(b) Fig. 8. (a) Bunched waveform optimised for a 26 keV Hz beam, observed on the test bench. (b) Bunched waveform optimised for a 26 keV HI beam, observed on the test bench.
The application of klystron bunching within the terminal of the AWRE 5.5 MV Van de Graaff has resulted in a much improved performance of that machine in pulsed operation. At a 5 MC/S repetition rate an order of magnitude increase in mean pulsed
40
J. H. ANDERSON
current has been obtained over that previously available with simple terminal pulsing. In addition the pulse width has been reduced to about 1.5 ns. Because of the higher intensity available the new system will be extremely valuable for fast neutron inelastic scattering studies to weakly excited levels and also for work on scarce, and hence small, samples e.g. separated isotopic targets. The system will also find application in high resolution time of flight studies on the neutrons emitted from charged particle induced reactions, using a long flight path. The system can easily be modified in one of several possible ways to give a lower pulse repetition rate. We wish to carrying out Swann for his Mr. T. Dyke, pressure tank
thank Mr. R. J. Prosser, for his help in the test bench measurements, Mr. D. help in the design of the electronics and who arranged the modification to the and terminal spinning.
References 1) R. Batchelor and J. H. Towle, AWRE Report No. NR/P12158.
et al.
2) J. H. Towle and W. B. Gilboy, Nucl. Physics 44 (1963) 256. 3) J. H. Towle and B. E. F. Macefield, Nucl. Physics 66 (1965) 65. M. P. Nakada, UCRL Report No. 4541 (1955). 5) J. H. Anderson and D. Swann, Nucl. Instr. and Meth. 30 (1964) 1. 6) C. D. Moak, R. F. King, J. W. Johnson, H. E. Banta, J. Judish and W. H. du Preez, Rev. Sci. Instr. 35 (1964) 672. 7) R. C. Mobley, Phys. Rev. 88 (1952) 360. 4)
8) L. G. Cranberg, R. A. Ferneld, F. S. Hahn and E. F. Shroder, Nucl. Instr. and Meth. 12 (1961) 335. 9)
R. C. Mobley, Rev. Sci. Instr. 34 (1963) 252.
10) F. E. Whiteway, AWRE Report No. O-12/61 (1961). it) M. M. Elkind, Rev. Sci. Instr. 24 (1953) 129. 12) A. C. Riviere, private communication. is) L. E. Collins, R. H. Gobbett and P. T. Stroud, Paper 4. Conf. Particle Accelerators, Washington USA. (March 1965). 14) F. E. Terman, Radio Engineers Handbook p. 949. 15) H. A. Enge, Rev. Sci. Instr. 30 (1959) 248. 16) T. K. Fowler and W. M. Good, Nucl. Instr. and Meth. 7 (1960) 245. 17) B. E. F. Macefield, AWRE Report No. NR/P-5/61. 1s) J. S. Lunsford, Rev. Sci. Instr. 35 (1964) 1483.