A pulsed negative ion beam injector

NUCLEAR

INSTRUMENTS

AND

METHODS

30 (I964) 23-33;

© N O R T H - H O L L A N D P U B L I S H I N G CO.

A PUI~ED NEGATIVE ION BEAM INJECTOR D. D A N D Y and D.P. H A M M O N D

United Kingdom Atomic Energy Authority, A.IV.R.E., Aldermaston, Berkshire Received 2 April 1964

The design requirements of a flexible injector for a tandem electrostatic accelerator are discussed. In order to produce a beam burst of 2 ns duration for neutron time of flight experiments a system incorporating chopping and klystron bunching

of an ion beam from a duoplasmatron source was adopted. In addition, provision has beenmade for the use o f radio frequency sources for the production of heavy negative ion beams. Results of d.c. and pulsed beam tests are presented.

1. lnlroduetion The tandem electrostatic generator at this laboratoryZ,~) produces high energy d.c. beams of all hydrogen isotopes and also heavy ion beams. Interest in neutron scattering by time of flight methods has led to a requirement for high intensity pulsed beams of protons and deuterons, with pulse lengths of a few ns and repetition frequencies of a few megacycles. At the same time, it is desired to retain the heavy ion facilities of the present machine and if possible, to extend the range of heavy ions available. An injector meeting these requirements has recently been built and tested.

should be as large as possible for the given duration of the pulse. This peak current is limited by the negative ion source, the current loading of the machine and by the bunching efficiency. For most experimental arrangements to date, pulsed beam has been produced by deflection across an aperture. The efffciency of this method is low, of the order of 1 or 2~o. Bunching improves this efficiency. The compression of an ion beam to form short bursts has been achieved by two completely independent methods. The first is by velocity modulation of a low energy beamS,S). The second depends on the variation of path length introduced into a fixed energy beam by deflection in a wide aperture magnet6,7). Because the injector of a tandem generator is readily accessible at low energy, velocity modulation was chosen in this instance. A further advantage of velocity modulation is that the divergence of the beam at the target is essentially unaffected by bunching; an important beam handling consideration.

2. Design requirements 2.1. PULSED OPERATION The present state of negative ion source technology is such that it is difficult to obtain an increase of a factor of ten or more in yield within the velocity-space acceptance of the accelerator. It becomes attractive to employ a "bunching" system to raise the peak ion intensity and shorten the pulse length without increasing either the source yield or the accelerator loading. For neutron time of flight experiments the duration of the beam burst should be comparable with the time resolution of the scintillation detector, that is of the order of one or two nanoseconds. The repetition frequency depends on the range of neutron flight times to be measured and values of 2.5 and 5 Mc/s were chosen. In seeking to produce a beam burst it is necessary to consider first the requirements that this beam burst has to satisfy. This list of requirements can be very long but the more important items are : peak current, time duration At, repetition rate, angular divergence of the beam at the target and energy spread in the ion burst. The rigidity of some of these requirements depends upon the particular experiment being performed; however in almost every case the peak current

The velocity modulation method does however introduce an energy spread into the beam, which gives rise to ion-optical defects, comparable to chromatic aberration, in the focussing elements subsequent to the buncher. By careful design of the system, these effects can be reduced to an acceptable level. It is important to reduce to a minimum the energy spread introduced into the beam from all causes because this energy spread will show itself as a time spread in the bunched pulse obtained at the target. Such energy spread may be inherent in the source itself or may be derived from the charge exchange process of negative ion formation. The inherent energy spread of a duoplasmatron source is low e.g. < 50 eV s) and its relatively high yield renders it very suitable for use in this injector. It is common practice to use the H~" component of the total ion beam from such a source to provide negative ions in a separate charge exchange cell z0). The first stage of this process employs 23

24

D. D A N D Y

AND

the molecular break-up : Hg~H~-+

H0--9eV.

D.P.

HAMMOND

and stopping the gas supply to the exchange chamber, the same r.f. source can be used to produce negative ions with high total gas economy.

The molecular binding energy is distributed between the particles and if the initial H~" beam is at 60 keV, as a typical figure, the energies of the resultant Hi~ ions will be

7ION SOURCE 2'

EINZEL

[(30 × 103)j ± (9)J]2 ~ 30 keV q- 1 keV. This can be expected to give a maximum time spread at the target of -t- 10 ns. Pulse trimming at high energy would therefore become necessary if a pulse length of 2 ns was required and the intensity of the beam would be considerably reduced. This difficulty is greatly reduced at the expense of intensity by using the atomic beam from the source. The energy spread arising from the charge exchange process is then due mainly to the energy loss caused by collisions with the charge exchange gas, which for 30 keV protons in hydrogen amounts to approximately 28eV. The energy spread is therefore significantly less. It has been found that about 35~o of the source yield can be atomicg). Hydrogen has been chosen as the charge exchange gas because of the high equilibrium fraction of negative ions obtainable1°). 2.2. D.c. OPERATION In addition to the pulsed requirements, the injector must also produce d.c. beams of the hydrogen isotopes and certain heavy negative ions. In the past the accelerator has used a standard radio frequency negative ion source to produce beams of 016, O is and F 19 in addition to protons, deuterons and tritons. The absence of metallic surfaces and .high gas efficiency make this type of source a reliable and adaptable device. Its main disadvantage is that charge exchange in the canal takes place at the energy of extraction (up to 10 keV), and so is not an independent variable. Consequently the source will not produce significant quantities of negative ions of those materials for which the electron attachment cross section does not become appreciable until higher energies are reached. Attempts to produce usable quantities of C lz and He 4 have not been successful. Recent work by Burn et al. 11) shows the results of high energy charge exchange. This injector has been designed such that an r.f. source may easily be substituted for the duoplasmatron. The short canal r.f. source produces positive ions which are charge exchanged in the separate exchange chamber over a wide range of energies. By reversing the first acceleration polarity, substituting a long canal in the r.f. source

LENS

EXCHANGE

!

CANAL

CHOPPER

2'4" 20 °

ANALYSING

MAGNET

5' EINZEL

~-

~.J

LENS

BUNCHER

PRE

ACCELERATION

STACK

I

PRE INSTALLATION 3' 6"

TARGET

TANDEM

IS I

4-

POSITION

TOP

TUBE

m m m m

I' 6"

t2'

STRIPPER

± Fig. 1. Schematic of i~ector and tandem.

A PULSED

25

N E G A T I V E I O N BEAM I N J E C T O R

2.3. TItE ION-OPTICAL SYSTEM A schematic of the complete system is shown in fig. 1. For efficient transmission through the whole accelerator and in particular through the stripper, the emittance of the injector ion beam must match the admittance of the accelerating tube stripper system. The injector must present a cross-over of the correct size and divergence at the correct energy, in order that the first acceleration tube will focus the beam at the centre terminal stripper. It is also necessary for there to be another crossover in the system to allow the exchange chamber, fast chopper and analysing magnet to be located at positions of small beam diameter. The provision of adequate flight path for the buncher is a problem when related to the chromatic aberrations of the tubes. The bunching length increases linearly with reduction in bunching voltage, but increases with the 1.5 power of the particle energy. So, to achieve a short flight path, it is necessary to bunch a low energy beam with high-voltage modulation. This is just the condition to give maximum defocussing in the accelerator tube. A compromise has been achieved by bunching at low energy, then accelerating prior to injection to as high a voltage as practicable by means of an accelerator insensitive to energy spread, viz. tube I. In this way the percentage energy spread seen by tube II is considerably reduced. The advantage of this system is that most of the apparatus associated with the injector is then at earth potential. However the level to which the injection voltage can be raised is limited by the space available inside the pressure vessel. Calculations have been made on the system shown in fig. 1 using Elkind's analysis lz) of the optics of long accelerator tubes and his notation (fig. 3) with appropriate suffixes is used throughout. Tube I needs to be operated in the condition when

L1/K1 is about --1 so that it has least sensitivity to changes in NI (fig. 3). From this aspect best results would be obtained by making P1 zero. However as the magnification of a tube is proportional to K/P, such a system would have infinite magnification. For tube II the value Lz/K~ is fixed at about 0.2 so that the chromatic aberration of the tube can only be reduced for a fixed range of energy spread, by reducing the value of Nz. The system has been designed so that tube II works with a constant value of N2 over the normal range of terminal voltage (3-6 MV) ensuring a constant beam profile, in a similar way to the present injectorlS).

The distance between tubes I and I1 is 3.5 feet and the voltages then necessary on tube I for an initial ion energy of 30 keV at terminal voltages of 6 MV and 3 MV are approximately 120 kV and 85 kV respectively. 6 6MV

3MV

S-

/ / eI

ENERGY

4

--

/

i,/

SPREAD IN

kV

3

s' oe

2

I

-3

-2

-I

2

3

IMAGE --I

SHIFT

AT

STF~IPPER

IN

FEET

-2

/

St

,/

-4

I

/

-

-S

"~- - 6 Fig. 2. Chromatic aberration o f two tube system with 30 keV initial ion energy.

The overall magnification in the system is 0.57 and 0.94 at 6 MV and 3 MV. The size of the image at the object point for tube I must be 0.42 cm at 6 MV and 0.25era at 3 MV, with respective divergences of 0.032 and 0.054 radians respectively for 30 kV ion energy and a ~6 inch diameter stripper. Fig. 2 shows the calculated behaviour of the system under bunching conditions. 3. Simplified bunching and chopping t h e o r y

14)

3.1. BUNCHING Consider the simple system in which a steady monoenergetic beam is velocity modulated at a single plane in such a way that ions travelling at different velocities in the drift tube arrive at the target at the same time. It is easily shown that for perfect bunching the modulating waveform must follow the law

26

D. D A N D Y

AND

where = s/uo = transit time of ions of energy E0

s = length of drift tube u0 ----velocity of ions of energy E0 Eo = ion energy before modulation eta = modulation energy t = time at which ions enter the drift tube. The power requirements make the use of this type of energy waveform across a single gap impracticable.

VI

~----- p

V° |~

K- =~L.-d

i|i|i|iiii,iii

a!B

i|

N~ Y e VI

D.P.

HAMMOND

this represents a considerable saving in power over the single gap case it is still extremely difficult to generate such a waveform at the required power. The application of a sine wave modulating potential is similar i n principle to the ideal waveform but because it is further departing from the theoretical best it follows that the target waveform will be inferior and a greater percentage of ions will arrive outside the required time bracket. These ions will have to be removed before striking the target. However sine waves are relatively easy to generate at high voltages since the capacitive load can form part of a tuned circuit and the energy required to charge the capacitance each cycle can be largely recirculated. The target current waveform would be given by the expressions

I

A cos wt

iT = i0 1

(1 + ~ n w t ) { . ]

1 -~

'

(1)

~ ' - = IO L K

/r ---- t + (1 + A sin w t ) i '

(2)

where /1 = ½ S ~ w .

uo

.•_.•20•

40~ ~60

2

4 6

80

K "it = =

/ ~°

8 Fig. 3. F o c u s s i n g c h a r a c t e r i s t i c s o f a n a c c e l e r a t o r t u b e .

If however an ion utilisation factor of say 50 % can be tolerated an inserted buncher tube, that is two modulating gaps, can be used. The length of the bunching tube must be chosen such that the transit time of the ions between the two gaps is equal to one half of the period of the bunching waveform. In this way the modulating potential can be applied to the inserted buncher tube and consequently the waveform generator is loaded only by the capacitance of the buncher tube and not by the whole injector. Whilst

Em

E0"

iT = target current at time tT io = mean beam current f = w/2~ = frequency of sine wave modulating potential em = E m sin wt = modulation energy and the other symbols as before. It can be seen from eq. (1) that infinite peaks occur in iT, when A cos wt = (1 + = sin wt) ¢.

(3)

The conditions for a single infinite peak are easily calculated by putting two of the roots of eq. (3) equal. As the value of A is increased beyond the value for a single infinite current peak, two current peaks will occur at the target with ever increasing phase separation. In order to ensure that the maximum number of ions arrive within a given time bracket, then neglecting debunching effects the optimum separation of the peaks will correspond to that time bracket. The phase angle of peak separation is defined as ACT which is given by A ~ T = w ( h - - tl) + 2A I( 1 + = sin

wtz)-t

(l + = sin

wtx)-i1

(4)

when the two peaks arrive at times t = tx and t = t~.

A P U L S E D N E G A T I V E ION BEAM I N J E C T O R

For values of ~ < 0.1 the expressions can be simplified and are summarised below iT = i0 (1 -- A cos w t ) - l

(5)

27

it becomes necessary to use high energy chopping to trim the beam burst.

4. Description ol the injector This section will be concerned with a description of infinite peak at A cos wt = 1 (6) the various items malting up the injector, viz. ion ACT = 2 [cos -1 1/A ( A s -- 1)j] (7) source, chopper, b u n c h e r , analysing magnet, einzel lenses and target assembly used. For the sake of percentage separation completeness a block diagram of the electronics is = (100/=) [(As - 1) ~ cos-~ (llA)l. (8) shown in fig. 4 but for detailed circuitry the reader is In practice the use of a sine wave modulating poten- referred to the accompanying paper by Anderson and tial can result in the utilisation of nearly 30% of the Swann18). A complete schematic diagram of the injecbeam and as a waveform of this type is relatively easy tor is shown in fig. 1. The ion source can be one of three interchangeable to generate it was the particular one chosen on the types, a duoplasmatronS), fig. 5; a negative ion extracnew injector. tion radio frequency source, fig. 6; or a positive 3.2. DEBUNCHING EFFECTS ion extraction r.f. source with a separate charge The first debunching effect is due to energy spread exchange canal. In the first case extraction by means in the beam from the source and the corresponding of a Pierce system is at 30 keV. For the r.f. source, extraction is at probe potential, that is 7-10 keV and time spread at the target is given by the Pierce extraction electrode forms an additional A t T "~ AEs/wF_.m accelerating gap at the end of the canal. The first einzel lens, which matches the source output where AE8 is the ion energy spread and the other to the exchange chamber, is a simple two inch diameter symbols as before. This time spread must be made small in comparison three cylinder lens, with water cooling on the centre to the beam burst duration at the target and hence electrode. The whole source and lens assembly is it is obviously best to choose a source whose inherent optically aligned to ensure concentricity. This is energy spread is small. Typical values are 1 keV for extremely important because if the lens components are not concentric the ion beam wiU be deflected and an r.f. source and 50 eV for a duoplasmatron. The second debunching effect is due to space charge lost. The only external adjustment provided is cenrepulsionXS). For peak currents of 1 mA or less this tering the source and anode plate with respect to the is likely to have very little effect on the systems consi- extractor. The exchange chamber is shaped as shown in fig. 5, dered. and is designed to be a collimator to stop those particles 3.3. CHOPPING which lie outside the velocity space admittance o f the It is necessary to use at least one chopper to remove accelerator, thus reducing tube loading and also ions which will arrive at the target outside the required background radiation. Particles with high divergence time bracket. In theory this chopper may be inserted off axis can still pass through the exchange chamber, anywhere in the system, from the ion source to the but are stopped on a 4 in. aperture in front of the second einzel lens. For an equilibrium population target. For the tandem Van de Graaff an additional requirement is that the unwanted beam must be remo- density Of negative ions in the beam, a gas target ved before the final acceleration to avoid extra loading thickness of 200 micron is required in the chamber, of the high voltage generator. It thus becomes attrac- corresponding to a flow of 45 ccs/hour at NTP of tive to insert the chopper immediately prior to the hydrogen. The ion source, lens and exchange chamber buncher, so that it will be required to pass the ions are mounted within a light alloy casting, which is for a predetermined period only. F o r a percentage pumped by a 12 in. oil diffusion pump with refriutilisation of approximately 30~o at a repetition rate gerated baffle. Operating pressure inside this casting of 5 Mc/s this period is of the order of 60 ns. The is about 10 # torr. disadvantage o f inserting the buncher in this position The chopper and the buncher are considered together is that debunehing may occur subsequent to the as they are essentially complementary in function. The chopper with a resulting increase in the duration of chopping waveforms are applied to a travelling wave the target pulse. If significant debunching takes place deflector system in which their velocity of propagation _

28

D. D A N D Y

~

f

A N D D . P . HAMMOND

ION BEAM

!

..J'-I

1"1_. WAVEFORM GENERATOR

ANALYSING MAGNET

I

BUNCHING WAVEFORM GENERATOR r" .....

BUNCHING TUBE

.<

I

POWER SUPPLY AND

. . . . . . .

I

(

DISTRIBUTION -

UNIT

-1

T ......

CHOPPING DRIVE SIGNAL

I I I

ZERO PULSE FOR TIME OF FLIGHT MEASUREMENTS q~

I I I I

] SAMPL I NG OSCILLOSCOPE

~l(

s M,/s

l

CRYSTAL OSCILLATORI "

O- 3 6 0 e PHASE SHIFT

O-360

e

PHASE SHIFT

TARGET BUNCHED WAVEFORM

PULSE SYNC [ SIGNAL

SHAPER

_/L Jr_ F i g . 4.

Block diagram of electronics.

is arranged to match the velocity of the ions in the beam. A d.c. bias is provided to cut off the beam during intervals between pulses. Because the beam is diverging as it passes through the chopper, the chopper plates are shaped to follow the beam profile. The buncher is designed such that the distance between the two accelerating gaps is variable between 15 cm and 26 cm and because the beam diameter within the buncher is large, of the order of 10 em, the electrodes on either side of the gaps are gridded. It was found that in order to minimise defocussing of the beam due to the grids under bunched conditions it was necessary to use a parallel wire grid structure made of 0.003" phosphor bronze wire spaced thirty to the inch. Using these grids with a modulating gap of 0.28 in. a 4 mm spot on the target increased to 7 mm

diameter under bunching conditions. This is discussed more fully in appendix 1. The beam transmission through the grids is expected to be of the order of 65~o. In the 20 ° analysing magnet the path length of particles in the field has been made long, 30 cm on a 100 em radius of curvature, so that the focussing action of the magnet is weak. The usual entrance and exit dees are provided, to permit adjustment of the fringe field. The maximum field strength is 2.5 kG, which is sufficient to deflect 100 keV chlorine ions. The second einzel lens is a simple three cylinder lens which has to transfer the object defined by the exchange chamber to the required object point for the tandem, that is, the entrance to the pre-acceleration tube, with unit magnification. Because of the long focal length, the aperture has been opened to 6 in. to reduce spheri-

A PULSED

NEGATIVE

29

ION BEAM INJECTOR

cal aberration without requiring excessive volts on the lens. Provision has been made for gridding the electrodes of this lens if required. The target assembly shown in fig. 7 is situated at the image point of the second einzel lens and consists of an 8 mm diameter insulated target which is matched direct into a 50 ohm cable. An insulated electron suppression ring can be fitted above the target if required. 5. Results The results can be conveniently divided into two sections, namely d.c. operation and pulsed operation. 5.1. D . C . OPERATION

-L Fig. 5. The duoplasmatron ion source, the first einzel lens and

the exchange chamber.

The d.c. operation of the injector was tested using three different types of ion source, a negative ion extraction r.f. source, a positive ion extraction r.f. source with a separate charge exchange canal and a duoplasmatron ion source using positive ion extraction and a separate charge exchange canal. In all cases the charge exchange gas used was hydrogen. Both types of r.f. source were tested using hydrogen deuterium and oxygen beams and in the duoplasmatron source hydrogen and deuteron beams were used. The results obtained are summarised in table 1. The negative ion target currents shown in table 1 w e r e measured with the electron suppression ring at 300 V negative with respect to the target, and with

TAaLE I Beam current in ~tA

Source type

Source gas

Exchange gas

At final target

Beam A t Faraday

Charge exchange

cup Direct

Negative extraction r.f.

Positive extraction r.f.

Duoplasmatron

With 4 mm aperture

Hydrogen

H-

28

Deuterium

D-

8

4.5

4

Oxygen

O-

3

1.7

i.5

12

11

Hydrogen

Hydrogen

H-

4

2.3

2

1%

Deuterium

~

D-

8

4.6

4

3%

Oxygen

~

O-

7.5

4

3.5

Hydrogen

D

H~" "--* H -

10

5.4

3.9

H+ - . . H -

23

10.5

7.5

12 %

30

D. D A N D Y

AND D.P.

HAMMOND

f--

ANODE

!

I

QUARTZ BUTTON~

I

./ I. . . .

/

f_l

. ~ . l

.-m

_.-ll,

•

;CHANGE C A N A L

Fig. 6. The negative r.f. ion source. a 4 mm diameter aperture inserted into the electron suppression ring. The 4 mm aperture has been calculated on the basis that any beam outside 4 mm diameter at the target position would, by the time it had reached the centre terminal of the tandem, be outside the admittance of the stripper, and hence would be unusable. In practice when using either of the two r.f. sources the beam was so well focussed on the target that the transmission through the 4 mm hole was of the order of 90%. On changing over to the duoplasmatron source this figure was reduced to about 70% indicating an increase in the best focussed spot size at the target. It was thought this was due to the fact that in order to obtain at the target, 4 ~,A of negative hydrogen ions from the atomic beam, there is in the region of the first einzel lens 4 mA of positive ions. At this concentration of ions the focussing action of this lens is impaired and it is also entering the region in which effects due to space charge repulsion are becoming appreciable.

More recent calculations 17) have shown however that a 4 mm aperture at the target position is not a true image of the stripper. On the basis of these calculations a new aperture was designed consisting of an 8 mm bore 12 cm long tube. This was inserted in place of the 4 mm aperture and the current transmission was increased by 8%. The corresponding currents measured on the Faraday cup situated just after the analysing magnet are also given in table 1. In between this position and the target position the beam passes through six grids, four in the buncher and two in the Einz¢l lens. Each grid has a calculated transmission of approximately 90%. The expected transmission through this portion of the injector is of the order of 55~o. In all cases the measured transmissions are in good agreement with this figure. A comparison of the two types of r.f. ion source for the three different gases used shows that a. The proton yield from the direct extraction source is better than that from the source using a separate charge exchange chamber,

A PULSED NEGATIVE ION BEAM INJECTOR MOUNTEO ON INSULAIIN~ STANDS

/

~

Q6A~TZ

r-7

CLA~P:N~ RING

SO J r

MArCHJNG

T£AM,NATION

Fig. 7. The target assembly. b. for deuterium, the yields of the two sources are identical, c. on going to a heavier ion such as oxygen the source with the separate charge exchange canal is superior. This is due principally to the fact that the charge exchanging process is taking place at 30 keV, where the electron attachment cross section is appreciably greater than at extract potential, normally (5--10) keV s).

31

For the positive ion extraction r.f. source, by turning off the exchange chamber gas and reversing the polarity of the analysing magnet, positive ion currents in the Faraday cup were also measured and hence the percentage of beam which charge exchanged was easily calculated. These figures are given in table 1 in the column marked charge exchange. The negative hydrogen ion target currents are also tabulated for the duoplasmatron ion source. The current obtainable from the molecular beam is approximately twice as much as that obtained from the atomic beam. 5.2. PULSED OPERATION In this case target current was observed directly on a Hewlett Packard Type 187 B sampling oscilloscope. As the cable impedance is fifty ohm and the lowest amplitude range on the oscilloscope is 1 mV/cm, in order to observe pulses above the oscilloscope noise level it is necessary for the d.c. target current to be greater than about 15/zA. This assumes a bunching efficiency of (20-+25)%. As the negative ion target currents were in all cases less than 15/zA the pulsing system was tested using positive ion beams. The results are shown in table 2. TABLE

2 i I

Source type

Source gas

Ion Pulsewidth at IBunching beam half height efficiency I

/

Duoplasmatron

Positive extraction r.f.

2.1 i 0.2

yOro on/-Deuterium

D+

Hydrogen

H~

I

2, 22

2.2 ± 0.2

21

I

5-6

r

Fig. 8. Bunched and chopped photographs for H+.

Because of the inherently small energy spread in the duoplasmatron source and the fact that the aim was for an optimum timing resolution of 2 ns the majority of measurements were taken using this type of ion source. For comparison purposes a set of results taken with the positive ion extraction r.f. source are also included. All the pulsed target current waveforms were photographed on time scales of 2 ns/cm and 10 ns/cm with the target current adjusted to give about 6 cm deflection on an amplitude scale of 10 mV/cm. For the positive ion atomic and molecular beams from hydrogen and also the D~ beam from deuterium, pulse widths at half height of 2 + 0.2 ns and bunching effi-

32

D. D A N D Y AND D . P . HAMMOND

ciencies in the region of ( 2 0 - + 25) % were obtained. The r.f. source gave a pulse width at half height of 5--->6 ns and this corresponded approximately to 600 eV energy spread in the source. Because the results for different beams were so similar for the duoplasmatron source we will consider only one beam in detail, namely the proton beam. Fig. 8, abed, show a typical set of optimised pulsed photographs. Fig. 8, ab were taken on 2 ns/cm time scale and 8, cd on a 10 ns/cm scale. The amplitude scale in all cases was 10 mV/cm. Fig. 8, ac show the bunched pulse superimposed above the earth line with the bunching volts Vm optimised to 1.65 kV. The mean d.c. level as measured on a microammeter was 57/~A and the pulse widths at half height ~ and at 10% height zx0./, were 2.1 + 0.2 ns and 7.2:t:0.8 ns respectively. Fig. 8, bd show the corresponding photographs obtained with the chopper operating. In this case it is seen that, between bunched pulses the trace falls to the earth line and the measured values for z½ and T10% are 2 . 0 + 0 . 2 n s and 6 . 0 + 0 . 6 n s . The chopper is therefore effective in rejecting the mean d.c. level background and also to a limited extent in trimming the skirts off the bunched pulse. It is not however 100% effective in trimming off all the skirts and possible reasons for this are discussed in the paper by Anderson16).

!i "~,~x././" j-. a 6

"\.

4

Ip

"-<.

a6~ [a4> ~ 2 p~

I6 m

>

~.

.,...___. ~

Q

~

p

rt,"

I I 1.4

6. Conclusions and future developments The injector described here has fulfilled the design specifications and produces beam bursts of 2 ns width at half height. The presence of skirts on the pulse is however a little disturbing and further work will have to be carried out in order to understand and if possible eliminate this effect. Work is going ahead to provide a post acceleration chopping system giving beam bursts of 0.5 ns. With the consequent loss in current which this would entail it is necessary to improve the performance of the duoplasmatron ion source to give an increased current yield and a more refined optical system is being developed which will handle this extra current.

IO - t

•

O.

/

/ ° ~ °

graph and the effects of varying other parameters are discussed in detail by Anderson16). A check on the beam burst width was also made by inserting a solid zirconium tritide target and observing the flight time of neutrons from the reaction t(d, n) He 4 over a two metre flight path. The time of flight system consisted simply of a 2 " × 4 " NE213 liquid scintillator optically coupled to a 5 " diameter Mullard 58 AVP photomultiplier, whose anode output was fed direct into the time to pulse height converter. The time zero reference signal was derived from the bunching waveform generator. The pulse width at half height, obtained after suitable corrections had been made for other time uncertainties, mainly electronics and detector size, was 2.4 + 0.3 ns. On using the duoplasmatron source and separate charge exchange canal to give H - from H~, the maximum electron suppressed current obtainable through the 4 mm aperture was 0.9/~A.

I *.5

| t.t

/

I

t.t

t.

BUNCHING VOLTAGE Vu

kV

Fig. 9. Variation of buncher volt vs "f~ and the bunching efficiency.

It was also necessary to compare experimental and theoretical results when various parameters were changed. Fig. 9 shows the effects, for a proton beam, on T, and on the bunching efficiency when Vm was varied. It is seen that there was a definite optimum value of Vm a minimum ~½ and maximum bunching efficiency. This occurred at Vm = 1.70 kV. This

giving

The authors wish to thank Prof. K.W. Allen for his encouragement and advice. Thanks are also due to Dr. L.E. Collins for much of the design work on the optics of the system, Messrs. F.E. Whiteway, J.H. Anderson and D. Swarm for the design and building of the electronics associated with the pulsing, and Mr. N.J. Cole, who was responsible for the detailed engineering design work. We are indebted also to Mr. D. Swarm and Mr. P. Collins for their assistance during the experimental work. Appendix BEAM DEFOCUSSING EFFECTS DUE TO BUNCHING This section is devoted to a rather intriguing problem which arose in the early stages of the development

A P U L S E D N E G A T I V E ION BEAM I N J E C T O R

work. Originally the buncher grids consisted of 0.003" tungsten wire arranged in a mesh of twenty to the inch. The modulating gap was 0.14 inch. When optimum bunching volts were applied two detrimental effects were experienced, the first being that superimposed upon the bunching voltage I'm was a periodic ripple. This was interpreted as being caused by the buncher grids "bouncing", thus changing the modulating gap and hence the capacity in the tank circuit of the buncher drive unit. This effect was easily cured by increasing the modulating gap size. The second effect was far more serious and showed up as an increase in target spot size as Vm was increased. At I'm equal to zero the target spot diameter was easily focussed to 4.5 ram. This increased linearly to 21 mm when Vm was increased to 1.68 kV, the calculated optimum value for protons of 30 keV energy. This effect was obviously extremely serious as the usable portion of the beam was limited to the centre 4 mm only. As this effect was thought to be caused principally by the separate lens actions of each individual grid aperture, due to field penetration, two remedies were suggested is) to ease the problem. a. The deviation produced by the grid apertures is directly proportional to the grid spacing. Hence increasing the modulating gap should give a reduced image size. There is however a practical limit on the maximum modulating gap which can be used. This is determined by the transit time across the gap which must be small in comparison with the time spent in traversing the buncher. The best results were obtained with a modulating gap of 0.28 inch. With this gap and the original mesh grids the spot size at optimum V~, reduced to 11 mm. The reduction to 11 mm is significant but is still not enough. b. If it were possible to align the grids accurately the total defocussing would be reduced because any con-

33

vergence at the first grid would be partially eliminated by divergence at the second grid. Hence parallel wire grids were constructed, consisting of 0.003" phosphor bronze wires at a spacing of thirty to the inch. It was found experimentally that the combination which gave the best results was when the two outer grids were parallel to each other but at 90 ° to the two inner grids. With this configuration, under optimum bunching volts the spot size could be focussed to 6.5 mm. Any other grid combination gave multiple spots at the target. These effects are not fully understood at the present time and it is hoped to make this the subject of a full scale experiment in the near future.

References i) K.W. Allen and F.A. Julian, Nature 184 (1959) 303. 2) K.W. Allen, Nucl. Instr. and Meth. U (1961) 93. a) K.R. Spangenberg, Vacuum Tubes (McGraw Hill, NewYork, 1948). 4) Ashby, Harris, Klein and Nakada, University of California Radiation Laboratory, Report No. UCRL-4641 (1955). 5) N.N. Flerov and E.A. Temanov. J. Nucl. En. $ (1958) 91. 6) R.C. Mobley, Phys. Rev. 88, (1952) 360. 7) R.C. Mobley, Rev. Sci. Instr. 34 (1963) 256. 8) M. Von Ardenne, Exp. Tech. der Physik. 9 (1961) 227. 9) p.H. Rose, R.P. Bastide and A.B. Wittkower. Rev. Sci. Instr. 32 (1961) 581. x0) P.H. Rose, Nucl. Instr. and Meth. U (1961) 49. tl) N. Burn, P. Ashbaugh and A. E. Litherland, American Phys. Soc. Bull. Series 2, 8 (1963) 377. lz) M.M. Elkind, Rev. Sci. Instr. 24 (1953) 129. t3) L.E. Collins and A.C. Riviere, Nucl. Instr. and Meth. 4 (1959) 121. 14) F.E. Whiteway, A.W.R.E. Report No. 0-12/61 (1961). i5) A. Garren. Space charge expansion of ion bunches drifting down a conducting pipe. Report No. U C R L 1394 (1951). 16) J.H. Anderson and D. Swann, Nucl. Instr. and Meth. 30 (1964) I. L.E. Collins, private communication. 18) L.E. Collins, private communication. 19) H.W. Lefevre, R.C. Borehers and C. H. Poppe, Rev. Sci. Instr. 33 (1962) 1231.